Rare Earth Starting Materials and Methodologies for
Synthetic Chemistry
The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and “designer reagents” such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.
<!>Motivation for the Review<!>Scope of the Review<!><!>General Considerations on
the Reactivity of
REs<!>Salt Elimination/Metathesis<!><!>Metathesis<!>Protonolysis/Protolysis<!><!>Insertion/Oxidation<!>Metals
as Synthetic Precursors<!>Activation in Liquid Ammonia<!><!>Activation in Liquid Ammonia<!>Insertion into C–X Bonds<!>Direct Metalation<!>Redox
Transmetalation<!><!>Redox
Transmetalation<!>Metal Vapor Synthesis<!><!>Metal Vapor Synthesis<!>Halides<!>Divalent
Ln Halides<!><!>Divalent
Ln Halides<!><!>Divalent
Ln Halides<!><!>Divalent
Ln Halides<!><!>Divalent
Ln Halides<!>Trivalent RE Halides<!><!>Trivalent RE Halides<!><!>Trivalent RE Halides<!>Borohydrides<!><!>Divalent Ln Borohydrides<!>Trivalent RE Borohydrides<!>Nitrogen
Donors<!>Divalent
Ln Silylamides<!><!>Divalent
Ln Silylamides<!>Trivalent RE Silylamides<!>Tetravalent Ce Amides<!>Oxygen
Donors<!>Aryloxides<!><!>Aryloxides<!>Ce(IV)
Alkoxides and CAN<!>Triflates<!><!>Triflates<!>Organometallic
Reagents<!><!>Simple Hydrocarbyls (Me, nBu, Ar)<!>α-Silyl-alkyls<!>Benzyls<!>Organoaluminates<!>Conclusions and Future Perspectives<!>
<p>This paper was published
ASAP on January 31, 2022, with errors in Schemes 21 and 79. The corrected
version was reposted on February 2, 2022.</p><!><p>The organometallic
and coordination chemistry of the rare earth (RE) and lanthanoid (Ln)
metals (Sc, Y, La, and Ce–Lu) was overshadowed for the best
part of the 20th century by the popularity and wide applicability
of transition-metal (TM) complexes. This historical disparity finds
its roots in how research into the chemistry of REs and TMs developed
through the centuries. While the first TM organometallic complexes
were identified in the 19th century by pioneers of the ilk of Bunsen,
Frankland, and Mond, attempts to stabilize RE–C bonds were
largely unsuccessful until the 1950s.1 Finally,
in 1954 Wilkinson and Birmingham succeeded in the stabilization of
the first RE organometallic complexes, RE(Cp)3 (RE = Sc,
Y, La, Ce, Pr, Nd, Sm, and Gd; Cp = {C5H5}−),2 over a century after
Frankland's preparation of dialkylzinc species.3 Following Wilkinson and Birmingham's breakthrough,
scientists started closing this historical 100 year gap with TM chemistry,
and since then, RE organometallic and coordination chemistry has developed
from mere curiosity to a burgeoning research field.4−9 A key component of these discoveries is the use of ligand architectures
that can fulfill the electronic and steric demands of the large and
electropositive RE ions. This step-change in RE chemistry was also
propelled by the many advances in anaerobic manipulation techniques
and ligand design, which in turn led to discoveries that challenged
common assumptions and opened unexpected research avenues.7,10</p><p>Crucially, these advances have largely been supported by the
preparation of adequate starting materials for anaerobic synthesis
and the development of innovative synthetic strategies. In particular,
the number of RE starting materials is staggering, constituting an
immense library that synthetic chemists can access to design their
own unique methodologies. All this essential information is spread
over 100 years' worth of literature and laboratory work, carried
out by a huge number of researchers across the whole world. It is
not uncommon for different research teams to devise their own unique
methodologies and synthetic strategies, the nuances of which are often
passed on as word of mouth—sometimes constituting a religious
rite of passage for new starters. This review work provides a systematic
account of the most important synthons used in RE synthetic chemistry,
covering their preparations and applications as synthetic precursors.
Despite the publication of several reviews and book chapters on many
of the topics included in this work (vide infra, section 1.3), the last
comprehensive account on the preparation of RE starting materials
and their application in synthesis was provided by Edelmann in 1997.9 Since then the field has seen an immense amount
of progress and the number of new starting materials at the disposal
of synthetic RE chemists has increased significantly. The aim is to
compile an up-to-date account of the most important synthons and methodologies
that constitute the synthetic toolbox for any researcher approaching
RE chemistry, thus providing an essential point of reference especially
for those at the beginning of their journey into the f-block world.</p><!><p>Throughout the
review the terms "rare earth" and "lanthanoid"
will be used. The term "rare earth" will be used to
describe all group 3 and lanthanoid elements, with the exception of
the highly radioactive and rare promethium. The abbreviation "RE"
in chemical formulas is used to describe group 3 metals (Sc and Y)
and lanthanoids (La–Lu) collectively, whereas the abbreviation
Ln will be used whenever a specific reaction or methodology applies
exclusively to La–Lu or divalent species.</p><p>The aim of
this review is to illustrate essential aspects of synthetic methods
applied to RE chemistry, focusing primarily on the preparation of
starting materials and their most important applications as synthetic
precursors. In order to make this work more focused, the starting
materials and methods described in this review will primarily relate
to anaerobic synthesis. The review is subdivided into starting material
categories, and dedicated synthetic methods are discussed within each
section: metals as synthetic precursors (section 3), halides (section 4), borohydrides (section 5), amides (section 6), oxygen donors (section 7), triflates (section 8), and organometallics (section 9). With the exception of section 3, each section contains
tables listing selected starting materials for each individual category
of this review. In the case of organometallic reagents (section 9), Zimmermann and Anwander
published a very comprehensive account in 2010,11 so this review will provide a broad overview of the topic
and illustrate the progress achieved over the past decade. This review
work covers all relevant literature published up to December 2021.</p><!><p>Metals as synthetic precursors:12,13</p><p>Guo, Z.; Huo, R.; Tan,
Y. Q.; Blair, V.; Deacon, G.
B.; Junk, P. C. Synthesis of Reactive Rare Earth Complexes by Redox
Transmetalation/Protolysis Reactions-A Simple and Convenient Method;
2020 (ref (12))</p><p>Cloke, F. G. N. Zero Oxidation State Compounds
of Scandium,
Yttrium, and the Lanthanides; 1993 (ref (13))</p><p>Halides:(14,15)</p><p>Taylor, M. D. Preparation of Anhydrous
Lanthanon Halides;
1962 (ref (14))</p><p>Meyer, G. The Rare Earth Elements –
The Divalent
State in Solid Rare Earth Metal Halides; 2012 (ref (15))</p><p>Borohydrides:(16−21)</p><p>Marks, T. J.; Kolb, J.
R. Covalent Transition Metal,
Lanthanide, and Actinide Tetrahydroborate Complexes; 1977 (ref (16))</p><p>Ephritikhine, M. Synthesis, Structure, and Reactions
of Hydride, Borohydride, and Aluminohydride Compounds of the f-Elements;
1997 (ref (17))</p><p>Makhaev, V. D. Structural and Dynamic Properties
of
Tetrahydroborate Complexes; 2000 (ref (18))</p><p>Visseaux, M.;
Bonnet, F. Borohydride Complexes of Rare
Earths, and Their Applications in Various Organic Transformations;
2011 (ref (19))</p><p>Guillaume, S. M.; Maron, L.; Roesky, P.
W. Catalytic
Behavior of Rare-Earth Borohydride Complexes in Polymerization of
Polar Monomers; 2014 (ref (20))</p><p>Paskevicius, M.; Jepsen,
L. H.; Schouwink, P.; Černý,
R.; Ravnsbæk, D. B.; Filinchuk, Y.; Dornheim, M.; Besenbacher,
F.; Jensen, T. R. Metal Borohydrides and Derivatives–Synthesis,
Structure and Properties; 2017 (ref (21))</p><p>Amides:(22−24)</p><p>Anwander, R. Lanthanide Amides; 2005 (ref (22))</p><p>Lappert, M. F.; Protchenko, A.; Power, P. P.; Seeber,
A. Metal Amide Chemistry; 2009 (ref (23))</p><p>Goodwin, C. A.
P.; Mills, D. P. Silylamides: Toward
a Half-Century of Stabilizing Remarkable f-Element Chemistry; 2017
(ref (24))</p><p>Oxygen donors:(25−30)</p><p>Bradley, D. C.; Mehrotram
R. C.; Gaurm D. P. Metal Akoxides;
1978 (ref (25))</p><p>Mehrotra, R. C.; Singh, A.; Tripathi, U.
M. Recent Advances
in Alkoxo and Aryloxo Chemistry of Scandium, Yttrium and Lanthanides;
1991 (ref (26))</p><p>Bradley, D. C.; Mehrotra, R. C.; Rothwell,
I. P.; Singh,
A. Alkoxo and Aryloxo Derivatives of Metals; 2001 (ref (27))</p><p>Anwander, R. Routes to Monomeric Lanthanide Alkoxides;
2005 (ref (28))</p><p>Boyle, T. J.; Ottley, L. A. M. Advances
in Structurally
Characterized Lanthanide Alkoxide, Aryloxide and Silyloxide Compounds;
2008 (ref (29))</p><p>Parmar, V.; Mills, D. P.; Winpenny, R. E.
P. Mononuclear
Dysprosium Alkoxide and Aryloxide Single-Molecule Magnets; 2021 (ref (30))</p><p>Triflates:(31)</p><p>Lawrance, G.
A. Coordinated Trifluoromethanesulfonate
and Fluorosulfate; 1986 (ref (31))</p><p>Organometallics:(4,5,11,32−38)</p><p>Cotton, F. A. Alkyls and
Aryls of Transition Metals;
1955 (ref (4))</p><p>Davidson, P. J.; Lappert, M. F.; Pearce,
R. Metal σ-Hydrocarbyls,
MRn. Stoichiometry, Structures, Stabilities,
and Thermal Decomposition Pathways; 1976 (ref (32))</p><p>Bochkarev, M. N.; Kalinina, G. S.; Bochkarev, L. N.
Advances in the Chemistry of Organolanthanides; 1985 (ref (33))</p><p>Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L.
Synthesis, Structure, and Reactivity of Organometallic π-Complexes
of the Rare Earths in the Oxidation State Ln3+ with Aromatic
Ligands; 1995 (ref (34))</p><p>Cotton, S. A. Aspects of the Lanthanide-Carbon
σ-Bond;
1997 (ref (5))</p><p>Deacon, G. B.; Forsyth, C. M.; Nickel, S.
Bis(pentafluorophenyl)mercury–a
versatile synthon in organo-, organooxo-, and organoimido-lanthanoid
chemistry; 2002 (ref (35))</p><p>Edelmann, F. T.; Freckmann, D. M.
M.; Schumann, H. Synthesis
and Structural Chemistry of Non-Cyclopentadienyl Organolanthanide
Complexes; 2002 (ref (36))</p><p>Zimmermann, M.; Anwander, R. Homoleptic
Rare-Earth Metal
Complexes Containing Ln–C σ-Bonds; 2010 (ref (11))</p><p>Lyubov, D.; Trifonov, A. A. Ln(II) Alkyl Complexes:
From Elusive Exotics to Catalytic Applications; 2021 (ref (37))</p><p>Izod, K. Alkyl, Carbonyl and Cyanide Complexes of the
Group 3 Metals and Lanthanides; 2021 (ref (38))</p><p>Evans, W. J. The Organometallic
Chemistry of the Lanthanide
Elements in Low Oxidation States; 1987 (ref (40))</p><p>Edelmann, F. T. Scandium, Yttrium, and the Lanthanide
and Actinide Elements, Excluding their Zero Oxidation State Complexes;
1995 (ref (6))</p><p>Edelmann, F. T. Lanthanides and Actinides;
1997 (ref (9))</p><p>Anwander, R. Principles in Organolanthanide
Chemistry;
1999 (ref (8))</p><p>Anwander, R. Herrmann, W. A. Features of
Organolanthanide
Complexes; 2005 (ref (39))</p><p>Liddle, S. T. Lanthanides: Organometallic
Chemistry
Fundamental Properties; 2012 (ref (41))</p><p>Nicholas, H. M.;
Mills, D. P. Lanthanides: Divalent
Organometallic Chemistry; 2017 (ref (42))</p><p>Ortu, F.; Mills,
D. P. Low Coordinate Rare Earth and
Actinide Complexes; 2019 (ref (7))</p><p>Layfield, R. A. Lanthanides;
2021 (ref (43))</p><!><p>Unlike TMs, RE metals are not involved in classic two-electron
reactions like oxidative addition and reductive elimination. Therefore,
synthetic strategies are mostly limited to a handful of key reactions:
(1) salt elimination, (2) metathesis, (3) protonolysis (also referred to as protolysis), and (4) insertion/oxidation.9,44 This is not an exhaustive list,
as other more specialized methodologies have been also implemented
in RE chemistry and will be discussed in more detail in the various
sections of this review.</p><!><p>In salt
elimination/metathesis reactions, a RE salt precursor, LnX2 or REX3 (X = halide, borohydride, triflate, etc.), is reacted with a ligand transfer reagent M(L)n (n = 1, 2 depending on the oxidation state
of the metal and formal charge of the ligand), producing a new RE
complex and eliminating an inorganic salt (eq 1). Classic ligand transfer reagents are group
1 and group 2 salts of a variety of bases, such as hydrocarbyls, amides,
alkoxides, and aryloxides. Additionally, the methodology entails the
elimination of highly insoluble inorganic salts with very high lattice
energies, thus providing a strong driving force for the reaction.
For these reasons, this is one of the most popular methodologies used
in RE synthetic chemistry and can be applied to the synthesis of both
homoleptic and heteroleptic species, though the occurrence of salt
occlusion can prevent the isolation of clean products.8,411</p><p>The availability of different halide
sources offers several possibilities in terms of lattice energy and
solubility of the byproducts of salt elimination reactions. Estimates
of lattice energies (eq 2) can be obtained from simple electrostatic considerations:2where the lattice energy, U, is proportional to the number of ions (n) and
their charges (z+ and z–) and inversely proportional to their ionic radii
(r+ and r–), i.e., the distance between the two charges.45 Smaller ions will give the highest lattice energy
values (LiF, 1034 kJ mol–1), while larger ions will
have the smallest lattice enthalpies (CsI, 613 kJ mol–1) (Table 1).46 Additionally, the relative strength of the RE–X
bonds should also be taken into account: RE3+ ions are
hard Lewis acids, and the bond strength with halides decreases descending
the group. One additional consideration is the solubility of the alkali
metal salts. Often salt elimination reactions have to be carried out
in ethereal solvents because of the poor solubility of starting materials
and reagents. However, alkali metal salt can have some solubility
in certain organic solvents, which can have a detrimental effect on
the outcome of salt elimination protocols and make purification procedures
extremely challenging (Table 1).</p><!><p>Solubility in diethyl ether.50</p><!><p>Metathesis reactions produce
a ligand exchange between two metals (eq 3), based on differences in their affinities for different
donors. For example, this is a method that can be employed for the
preparation of RE organometallics complexes (i.e., transmetalation), such as the metathesis reaction between aryloxide
complexes and Li{CH(SiMe3)2} to give [RE{CH(SiMe3)2}3].51 This
method is particularly useful for the preparation of homoleptic complexes,
as the occurrence of ligand scrambling often prevents the isolation
of heteroleptic species.3</p><!><p>RE starting
materials can perform acid/base reactions with protic substrates,
accompanied by the highly favorable elimination of volatile gases
or liquids (eq 4). Such
reactions are classically driven by differences in pKa (Table 2), though other important factors can come into play (e.g., saturation of metal coordination sphere or use of chelating ligands).
However, simple acid/base considerations are often a good indication
when planning a methodology that makes use of these reactions.4</p><!><p>Value in DMSO.</p><!><p>RE metals can
sometimes react directly with substrates, either by inserting into
a polar bond (e.g., a carbon–halogen bond, eq 5) or by reducing the target
donor (eq 6). The former
method has been used very rarely, and it mainly applies to divalent
REs that are able to behave like pseudo-alkaline
earth (AE) metals (section 3.2). Oxidation reactions instead are more versatile, though
their main applications are limited to Sm, Eu, and Yb; these are usually
performed on protic substrates and accompanied by the elimination
of hydrogen (sections 3.1 and 3.3). Both insertion and oxidation
reactions can be sluggish with RE metals, and better results are often
achieved when metals have been activated. Oxidation reactions have
also been combined with transmetalation reactions by using redox transmetalating
reagents, such as organomercurials (section 3.4).56</p><!><p>The use of REs in their metallic
form as primary starting materials
for synthesis has been investigated for over a century. For example,
high-temperature reaction of RE powders with dry HCl or iodine is
an established methodology for the preparation of solvent-free RECl3, and most REs react readily with iodine or 1,2-dioodoethane
in ethereal solvents to afford di- and trihalide species (vide infra, section 4.1). REs can also react directly with organic substrates
(section 3.3), though
often activation strategies have to be employed which circumvent the
relative inertia of the metal. Examples of this approach are (1) activation
in liquid ammonia (section 3.1), (2) activation with Hg or HgCl2 (section 3.3), (3) redox
transmetalation (section 3.4), and (4) metal vapor synthesis (section 3.5). Additionally, in some cases REs can
also react directly with substrates in a fashion similar to the insertion
of Mg into C–X bonds (section 3.2).</p><!><p>At the
turn of the 20th century, the use of liquid ammonia as an alternative
nonaqueous solvent sparked the interest of the scientific community,53 leading to the development of new reaction apparatuses
and experimental techniques which set criteria still used today for
experimental work with ammoniacal solutions (Figure 1).54,55 Pioneering work by
Joannis,53 Mentrel,56 Roederer,57 and Moissan58−60 focused on the solubilization of alkali and AE metals and the properties
of resulting solutions. Kraus further developed this new research
field by studying various s-block metals in liquid ammonia and first
proposed the presence of solvated electrons is such media.61−63 Around the same time, Cottrel focused his efforts on the dissolution
of Mg and Ca and their reactivity with acetylene.64 The first ammoniacal solutions of any f-element were first
reported by Watt et al. in 1950, who described the
use of liquid ammonia on various U and Th salts (e.g., iodates, oxalates, peroxides).65 Warf
and Korst decided to extend this study to the classic divalent Lns,
owing to their resemblance to the heavy AE elements.66 The heavy AEs share many similarities with classic divalent
Lns Eu, Sm, and Yb, such as the stability of the +2 oxidation state
and ionic radii (Ca2+, 1.00 Å; Yb2+, 1.02
Å; Sm2+,1.17 Å; Eu2+, 1.22 Å;
Sr2+, 1.18 Å).67 Because
of this their coordination chemistry has often been developed in parallel.68 By using freshly distilled ammonia, Warf and
Korst were able to dissolve Eu and Yb at −78 °C, while
no reaction was observed with Sm (Scheme 1 and Figure 1).</p><!><p>Illustration of basic setup for the preparation of ammoniacal
solutions
of divalent Lns. Alternatively, modification of Schlenk line manifolds
can be implemented to optimize ammonia transfer and workup procedures.55</p><!><p>The nature of the cation–electron
ion pair was further corroborated
by EPR studies on Eu ammoniacal solutions performed by Catterall and
Symons in 1964;69,70 additionally, the electronic
spectra of Eu and Yb species were recorded a year later by Waugh and
co-workers.71 Interestingly, Peer and Lagowski
were able to dissolve various other Lns (Sm, Tb, Er, and Tm) in ammonia via co-condensation and recorded the electronic spectra
of the resulting solutions; both Sm and Tb were identified to be in
their divalent state, while definitive conclusions could not be drawn
for Er and Tm.72</p><p>Eu and Yb ammonia
solutions are relatively stable when stored under
anaerobic conditions. However, they decompose over time to generate
the parent amide Ln(NH2)2, similar to what is
observed with alkali and AE metals (Scheme 2).73 In the case
of Yb, depending on reaction conditions, degradation of ammoniacal
solutions can also produce oxidized trivalent amide Yb(NH2)3.73 Divalent Eu(NH2)2 is obtained also under ammonothermal reaction conditions
(up to 5000 atm of NH3), while for Yb the same reaction
conditions generate pure trivalent Yb amide or the salt Na[Yb(NH2)4].74 The use of ammonothermal
conditions with Lns has limited synthetic utility and has attracted
interest mostly for the preparation of new semiconductors;75 also, supercritical ammonia (160 °C) has
been used to prepare metal sulfide salts of Yb, [Yb(NH3)8][M(S4)2]·NH3 (M
= Cu, Ag), and La, [La(NH3)9][Cu(S4)2].76 Additionally, Müller-Buschbaum
and Quitmann reported the molecular structure of complex [Sm(NH3)9][Sm(Pyr)6] (Pyr = pyrrolide, {C4H4N}−), obtained from the direct
reaction of pyrrole with Sm metal and pyrrole under solvothermal conditions,
followed by treatment with liquid ammonia.77 Recently, Kraus and co-workers investigated the preparation of Eu(II),
Yb(II), and Ho(III) azides by reacting the pure metal with AgN3 in liquid ammonia, leading to the structural identification
of ammino-adducts [Ho2(μ-NH2)3(NH3)10](N3)3·(NH3)1.25 and [Yb(NH3)8](N3)2.78,79</p><p>The ready availability of soluble forms of divalent Lns gives access
to the preparation of simple binary species. Such was the observation
of Salot and Warf in 1968, who postulated the formation of YbI2 from the reaction of Yb metal and NH4I in liquid
NH3.80 Shortly after, Howell
and Pytlewski followed a similar synthetic protocol and reported the
preparation of six unsolvated LnX2 (Ln = Eu, Yb; X = Cl,
Br, I) species (Scheme 3);81 this methodology has been used extensively
by synthetic chemists since then (vide infra for
preparation and uses of LnX2 precursors in synthesis, section 4.1).82 When Sm is employed, this methodology leads
to the formation of trivalent species.80</p><p>Owing to the highly reducing nature of these ammoniacal Ln solutions,
their synthetic utility was first explored in the 1960s. Fischer and
Fischer reacted freshly distilled cyclopentadiene with an ammoniacal
solution of europium or ytterbium, followed by sublimation to yield
analytically pure metallocene Ln(Cp)2 (1-Ln; Ln = Eu, Yb; Cp = {C5H5}−) (Scheme 4).83 Similarly, Wayda et al. were
able to synthesize their Cp* (Cp* = {C5Me5}−) analogues Ln(Cp*)2(NH3)x (2), though only the ammoniate
metallocene derivative [Yb(Cp*)2(NH3)(THF)]
(3) was structurally authenticated (Scheme 4).84 A similar approach has also been used for the preparation of other
organometallic derivatives, such as Ln(COT) (COT = {C8H8}−)85 and Eu(II)
propynide Eu(C≡CCH3)2 (4).86 Wayda et al. followed the original Ln(COT) synthesis developed by Thomas and
Hayes85 and were able to crystallize the
Yb derivative by slow diffusion of pentane in a pyridine solution,
thus obtaining the piano-stool complex [Yb(COT)(py)3] (5·3py, py = pyridine).87 Around
the same time of Fischer and Fischer's metallocene synthesis,
Ln ammoniacal solutions were also used to for the reduction of 2,2′-bipyridine
and phenantroline (Scheme 5), leading to the isolation of Ln(bipy)4 (6-Ln; Ln = Eu, Yb; bipy = 2,2′-bipyridine) and Yb(phen)4 (7, phen = phenantroline);88 Pappalardo also reported the preparation of Yb(bipy)3 (8) using a similar methodology.89</p><p>Smith and co-workers also used Yb ammoniacal solutions to prepare
the bis-aryloxide Yb complex [Yb(ODbmp)2(THF)3] (9, Scheme 6) (ODbmp = OC6H2tBu2-2,6-Me-4),90 which was originally prepared via protonolysis or redox transmetalation (vide
infra, sections 3.4 and 7.1).91 Similarly, Evans et al. isolated the Eu2+ complex [Eu(ODb)2(NCMe)4] (10, ODb = OC6H3tBu2-2,6)
by dissolving a Eu ingot and 2,6-di-tert-butylphenol
in liquid ammonia, followed by extractions with acetonitrile (Scheme 6).92 However, when 2,6-diisopropylphenol (HODipp) is employed,
the hydroxo-bridged Eu(II) complex [Eu4(ODipp)2(μ-ODipp)4(μ3–OH)2(NCMe)6] is obtained; attempts by Evans et al. to use this methodology to obtain a hydroxo-free Eu(II) derivative
were not successful.92</p><p>This synthetic approach has been revisited more recently by Müller-Buschbaum
and co-workers, who have used similar methodologies for the activation
of a variety of nitrogen-containing heterocycles, e.g., indole (11), carbazole (12), pyrazole
(13), and pyrrole (14) (Scheme 7).77,93−98 These reactions in some cases are accompanied by concomitant formation
of {NH2}− from ammonolysis.95 In their work, Müller-Buschbaum and co-workers
carried out reactions in the absence of solvents and under solvothermal
conditions, though in some cases low-temperature reactivity was also
achieved, such as with the synthesis of [Yb(NH3)8][Yb(pyr)6] (14c, pyr = pyrrolide, {C4H4N}−).97 This approach is particularly interesting for modern RE synthetic
chemistry, as it can provide an access route to very challenging complexes
avoiding the use of coordinating solvents.98 In the case of the reaction between Yb and carbazole (C12H8NH, CarbH), N-phenylpiperazine (Phpip)
was added to aid crystallization of the target complex [Yb(Carb)2(NH3)4] (12, Carb = carbazolide,
{C12H8N}−), though this product
cannot be separated from cocrystallized CarbH·Phpip.94 It is also noteworthy that Müller-Buschbaum
and co-workers extended this work to various divalent and trivalent
REs by reacting metal powders with amines, either directly or in the
presence of trace amounts of Hg under solvothermal conditions (vide infra, sections 3.3 and 3.4).77,99 Remarkably, the use of ammoniacal Yb solutions in the presence of
2,2′-dipyridylamine afforded the isolation of the first RE
molecular nitride [Yb3N(dpa)6][Yb(dpa)3] (15, dpa = 2,2′-dipyridylamide, {(C6H4N)2N}−).100</p><!><p>Eu,
Sm, and Yb resemble the chemistry of Sr and Ca respectively, owing
in particular to the stability of the divalent state and ionic radii
(see section 3.1).
Organocalcium "heavy Grignard" reagents, (R)CaX (R =
alkyl, aryl; X = halide), were originally investigated in the early
20th century as potential alternatives to classic organomagnesium
analogues;101−103 additionally, several structurally authenticated
examples have been reported by Westerhausen and co-workers over the
last two decades,104,105 and recently this chemistry
has also been extended to Sr and Ba.106 Consequently, classic divalent Lns (Eu, Sm and Yb) have also been
investigated for the preparation of pseudo-Grignard
reagents, i.e. (R)LnX (R = alkyl, aryl; X = halide).107,108 Unlike for Mg, heavy group 2 Grignard reagents readily undergo Schlenk-type
degradation affording dialkyl and dihalide species;104,105 therefore, owing to the similarities of classic divalent Lns with
the heavy AE metals, this type of degradation is also a likely occurrence
for a putative (R)LnX pseudo-Grignard derivative
(Scheme 8).</p><p>D. F. Evans et al. first reported the reaction
of Yb and Eu metal with alkyl and aryl iodides in THF at −20
°C (Scheme 9).107,108 In the case of Yb, magnetic susceptibility measurements revealed
the presence of small quantities of Yb3+ ion (Yb3+: [Xe]4f13, Î¼eff = 4.5
Î¼B), with Yb2+ as the predominant ion
(Yb2+: [Xe]4f14, Î¼eff = 0 Î¼B);108 in the case of europium, the magnetic moment of (Ph)EuI was measured
at 7.5 Î¼B, close to the value for free Eu2+ (Eu2+: [Xe]4f7, Î¼eff = 7.5 Î¼B; Eu3+: [Xe]4f6, Î¼eff = 1.5 Î¼B).108 In the
case of Sm metal, reactivity with iodobenzene and iodoethane is sluggish,
requiring higher temperatures compared to Eu and Yb and affording
a much higher proportion of trivalent species.108 Moreover, further reactivity tests with Gd and Er showed
no reactivity, while La and Ce are converted into mixtures of trivalent
species, i.e., (R)REI2, (R)2REI, RE(R)3, and REI3.108 Beletskaya and co-workers further investigated oxidative
addition of Ce, Eu, Sm, and Yb with iodothiophenes and bromopentafluorobenzene
in the presence of an initiator, e.g., diiodoethane
or dibromoethane (Scheme 9).109 The authors reported the
formation of organoderivatives (C4H3S)LnI (Ln
= Ce, Eu, Sm and Yb) and (C6F5)LnBr (Ln = Eu,
Sm and Yb), though their formation was confirmed only via reactivity with Ph3SnCl and detection of corresponding
Ph3SnAr derivative (Ar = C4H3S, C6F5).109 Ce has a very
unstable divalent state (Ce3+/Ce2+: E = −3.2
V);110,111 therefore, it cannot be excluded that reactivity
with iodothiophene produced trivalent (C4H3S)CeI2 or (C4H3S)2CeI rather than
divalent (C4H3S)2CeI, as both trivalent
species can in turn generate Ph3Sn(C4H3S) upon reaction with Ph3SnCl.</p><p>(Ph)LnI (Ln = Eu, Yb) performs well as protonolysis reagent toward
3,5-diphenylpyrazole (HPh2pz) to give [Ln(Ph2pz)(I)(THF)4] (16-Ln, Ln = Eu, Yb; Ph2pz = {3,5-Ph2C3HN2}−; Scheme 10);112 the same outcome was obtained also with freshly
prepared (Me)LnI.113 Ali et al. further exploited the utility of (Ph)LnI by performing protonolysis
reactions with a family of amidines, ArFormH (ArNCHNAr; Ar = C6H3Me2-2,6 – Xyl, C6H2Me3-2,4,6 – Mes, C6H3iPr2-2,6 – Dipp), leading to
the formation of divalent Ln complexes [Eu(DippForm)(I)(THF)4] (17), [Eu(XylForm)(I)(DME)2] (18), [Eu(XylForm)(I)(κ1-DME)(μ2:κ2O-DME)]∞ (19), [Yb(DippForm)(I)(THF)3] (20), [{Yb(MesForm)(μ-I)(THF)2}2] (21) and [{Yb(XylForm)(μ-I)(THF)2}2] (22) (Scheme 10); however, the authors also observed the
formation of several trivalent derivatives during the isolation of
these species.113 To gain a better understating
of the chemical behavior of these reagents, Junk and co-workers revisited
the synthesis of putative (Ph)YbI first described by D. F. Evans and
proposed the occurrence of Schlenk-type equilibria and disproportionation
reactions (Scheme 10).112,113 The authors attempted to identify the components
of these degradations and initially isolated [Yb(I)2(THF)4] and trivalent [Yb(Ph)3(THF)3]. Additionally,
when phenyl iodide is reacted with Yb in DME, fractional crystallization
affords [Yb(I)2(DME)3] and the mixed-valent
separated ion pair complex [Yb(DME)4][Yb(Ph)4(DME)]2.112 Subsequently, Junk
and co-workers ascribed the formation of the trivalent Yb formamidinate
species [Yb(DippForm)(I)2(THF)3] (23) and [Yb(XylForm)2(I)(DME)] (24), together
with the hydroxo-bridged Eu complex [{Eu(XylForm)(I)(μ-OH)(THF)2}2] (25), to the presence of other
trivalent Yb derivatives, e.g., (Ph)YbI2 and (Ph)2YbI (Scheme 11).113</p><p>The first structurally authenticated Ln pseudo-Grignard complexes [{Yb[C(SiMe3)3](μ-I)(OEt2)}2] (26) and [{Yb[C(SiMe2CH = CH2)3](μ-I)(OEt2)}2] (27) were reported by Smith in 1994114 and 1996,115 obtained
from the direct reaction of Yb powder with (SiMe2R)3CI (R = Me, CH = CH2) in diethyl ether (Scheme 12). Moreover, Niemeyer
and Heckmann isolated aryl complex [Yb(C6H3Ph2-2,6)(I)(THF)3] (28) from
the reaction between 2,6-Ph2C6H3I
and Yb powder, while use of Eu powder led to the isolation of bis-aryl
complex [Eu(C6H3Ph2-2,6)2(THF)2] (29) (Scheme 12).116 Such a
difference in reactivity between Yb and Eu is an indication of the
presence of a Schlenk-type equilibrium, which is more pronounced for
the larger Eu2+ cation. Confirmation of this observation
came also from the isolation of EuI2(THF)2 as
a byproduct of the reaction. Conversely, treatment of Yb powder with
neopentyl iodide, tBuCH2I, leads to further
oxidation to the trivalent Yb complex [Yb(CH2tBu)3(THF)2] (30) with concomitant
formation of solvated YbI2 (Scheme 12).117 Niemeyer
proposed that the divalent pseudo-Grignard species
(tBuCH2)YbI is initially formed, which then
reacts with a further equivalent of tBuCH2I
to yield (tBuCH2)YbI2; the latter
species undergoes Schlenk-type equilibrium yielding 30 and YbI3. Finally, YbI3 reacts with excess
Yb metal to form YbI2.117</p><!><p>RE metals can react
directly with protic substrates, thus providing a very efficient synthetic
route which is accompanied by the evolution of hydrogen as a byproduct
(Scheme 13).118 However, this is a relatively rare synthetic
approach with limited scope, especially compared to other widely used
synthetic methodologies such as salt metathesis, transamination, or
transmetalation reactions; additionally, in some cases harsh reaction
conditions and additives are required.</p><p>Evans and Pires de Matos reported independently the reactivity
of Ln metals with alcohols.119−121 The reaction of Eu metal with
2-methoxyethanol proceeds smoothly at room temperature and affords
polymeric [Eu(OCH2CH2OMe)2]n in good yields, which Evans and co-workers employed
as protonolysis reagent toward substituted phenols HOC6H4Me2-2,6 (HOXyl) and HOC6H3iPr2-2,6 (HODipp), affording the heteroleptic
aryloxide complexes [{Eu(μ3:η2-OCH2CH2OMe)(η2-OCH2CH2OMe)(OC6H3R2-2,6)-][H+]}4] (31a, R = Me; 31b, R = iPr) (Scheme 14).119 Pires de Matos and
co-workers obtained Eu(OMe)2, Eu(OEt)2, Eu(OiPr)2, and Yb(OMe)3 by treating metal
powders with methanol, ethanol, or isopropanol (Scheme 14);121 in the case of the reaction between Yb and isopropanol, activation
with ammonia was required and the only isolated product was the cluster
[Yb5O(OiPr)13] (32),
previously prepared by Bradley and co-workers by using catalytic amounts
of HgCl2 in the reaction (vide infra, Scheme 16).122 In the same report, Carretas et al. illustrated the use of metal vapor synthesis (MVS) techniques for
the preparation of the same alkoxides and reported an improvement
of the overall yields in all cases.121 Moreover,
Evans and Greci investigated the use of different solvents to facilitate
the direct reaction of Eu metal with HOXyl and HODipp. When HOXyl
was reacted with Eu ingots in N-methylimidazole (C3H3N2-Me), the bridged aryloxide complex
[Eu2(OXyl)(μ-OXyl)3(C3H3N2-Me)5] (33) was obtained
(Scheme 14); similarly,
treatment of HODipp with Eu ingots in acetonitrile yielded the aryloxide
complex [Eu2(ODipp)2(μ-ODipp)2(MeCN)4(μ-NCMe)] (34).120</p><p>Direct metalation reactions can also be facilitated by using small
quantities of Hg or HgCl2. The proposed mechanism involves
the amalgamation of metallic mercury with the RE metals, which are
readily oxidized to their trivalent or divalent state upon reaction
with protic substrates, accompanied by formation of hydrogen (Scheme 15).123 When HgCl2 is employed, metallic
mercury is likely formed from the direct reaction with the substrate
(usually amines or alcohols), leading to the formation of Hg(L)2 with the concomitant production of HCl. The transient Hg
amide or alkoxide then reacts with the RE metal powder via redox transmetalation (vide infra), thus producing
the desired RE derivative, RE(L)3 or Ln(L)2,
and regenerating metallic Hg (Scheme 15).</p><p>Mazdiyasni and co-workers were the first to report the direct activation
of isopropanol with RE metals in the presence of HgCl2 (occasionally
combined with Hg(OAc)2 or HgI2) under reflux,
and they discovered that the use of excess HgCl2 led to
the presence of RE chlorides in the final products.124,125 Deacon et al. subsequently demonstrated that solvated
RECl3 can indeed be prepared conveniently from metal powders
and HgCl2 (vide infra, section 4.2).126 In Mazdiyasni's original reports, the authors identified
the formation of the expected isopropoxide species RE(OiPr)3 for all metals but Eu, which formed the divalent
Eu(OiPr)2 instead.124,125 These results were later revisited by Hubert-Pfalzgraf, Caulton,
and Bradley, who studied the products of these direct reactions via single-crystal XRD studies and identified the formation
of the oxo-bridged pentametallic clusters [RE5O(OiPr)13] (32-RE; RE = Y, Pr, Yb) as the main
products, rather than the expected homoleptic isopropoxide species
(Scheme 16).122,127,128 Nief and Mathey used a similar method to synthesize [Ln(η5-C4H2PPh2-2,5)2(THF)2] (33-Ln; Ln = Sm, Yb) and bis-arsolyl [Ln(η5-C4H2AsPh2-2,5)2(THF)2] (34-Ln; Ln = Sm, Yb), obtained from the reaction of metal powders with bis-phospholyl or bis-arsolyl dimers in
the presence of catalytic amounts of HgCl2 (Scheme 16).129,130 Additionally, Edelman and Recknagel showed that this strategy can
be used in the reaction of dimethylfulvene with Ln metal powders (Ln
= Sm, Yb), generating ansa-metallocene complexes
of formula [Ln{(C5H4)2C2Me4}(THF)2] (35-Ln) (Scheme 16).131 Finally, Roesky and co-workers synthesized
the gallyl lanthanide complexes [Ln{Ga(Dipp-Bian)}2(THF)4] (36-Ln; Ln = Sm, Eu, Yb; Dipp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene)
from the reduction of dimeric precursor [{Ga(Dipp-Bian)}2] with Ln powders activated with Hg.132</p><p>This synthetic strategy was revisited by Deacon, Junk, and Müller-Buschbaum,
who studied the activation of a variety of protic substrates such
as substituted phenols and heterocyclic amines.98,133 In this new methodology, proligands are usually solids and the reactions
are carried out without solvents and in the presence of Hg or HgCl2, operating under solvothermal conditions, i.e., heating at temperatures higher than the melting point of the proligand.
These types of reactions are usually carried out in partially evacuated
sealed vessels (e.g., Carius tubes or sealed glass
ampules).98,133 Deacon and co-workers focused
their efforts on the use of sterically demanding aryloxides and substituted
pyrazolates, comprising all of the RE metals in their methodologies.
Because of the absence of coordinating solvents in these reactions,
homoleptic species are usually obtained with various degrees of nuclearity
depending on the ligands' steric features, ionic radii of the
RE metals employed, reaction times, and crystal packing interactions
in the solid state. When Eu or Yb are reacted with HOC6H4Ph2-2,6 (HODpp) directly, very little conversion
is observed even after prolonged reaction time and under forceful
conditions; when the same reactions are carried out in the presence
of Hg (Scheme 17),
the desired Ln(ODpp)2 species are generated, which can
then be recrystallized from toluene to give dimeric [{Eu(ODpp)}(μ-ODpp)3Eu] (37-Eu) and [{Yb(ODpp)}2(μ-ODpp)2] (37-Yb) (Scheme 18).134 Similarly,
when classic trivalent REs are employed, the expected tris-aryloxo
complexes, [RE(ODpp)3] (38-RE; RE = Y, La,
Ce, Pr, Nd, Gd, Ho, Er, Lu), are obtained (Scheme 17); also in the case of RE3+ ions,
direct reactivity in the absence of Hg leads to very little conversion
into the desired products.123 Deacon and
Junk successfully extended this methodology to other REs (including
Sc)135 and aryloxides comprising different
substitution patterns of the aryl groups,136−138 including buttressing substituents in 3- and 5-positions.135 Additionally, reactions can also be carried
out in the presence of fluxes (e.g., 1,3,5-tritert-butylbenzene and 1,2,4,5-tetramethylbenzene) which
increase the contact between metal and alcohol. This methodology has
proven to be particularly useful for the preparation of heterobimetallic
compounds and can be employed also for the synthesis of bimetallic
systems incorporating alkali139 and AE
metals (39 and 40, Scheme 18).140 One of
the great advantages of these methodologies is that resulting complexes
can spontaneously crystallize from the reaction mixtures in the Carius
tubes upon cooling, together with avoiding the preparation of tailored
starting materials and using coordinating solvents. However, in some
cases recrystallizations have to be carried out, and owing to the
insolubility of these species in nonpolar solvent media, these usually
have to be performed at very high temperatures (e.g., toluene at 190 °C).140</p><p>Reactivity of REs (Sc, La,
Nd, Eu, Sm, Yb, and Lu) with bis-tert-butylpyrazole
(tBu2pzH) in the
presence of Hg and under solvothermal conditions delivers monomeric,
[RE(tBu2pz)3] (41-RE; RE = Sc, Sm), and dimeric species, [RE2(tBu2pz)6] (42-RE; RE = Nd, Sm,
Lu).141 Similarly, 41-Sc can
also be obtained via direct synthesis in the presence
of mercury metal at temperatures between 270 and 300 °C.142 When divalent Eu and Yb are employed, the methodology
produces slightly different outcomes, leading for example to the isolation
of the mixed-valent Yb(II)/(III) complex [Yb2(tBu2pz)5] (43, Scheme 19).143 Interestingly, these methodologies are affected by subtle variations
in reaction conditions. For example, in the case of Eu heating of
the reaction with tBu2pzH at 220 °C for
24 h affords the monomeric complex [Eu(tBu2pz)2] (though no structural validation has been provided for this
conformation), while after heating for 15.5 h the tetrameric species
[Eu4(tBu2pz)8] (44) is obtained instead (Scheme 19).141 Similar
to the use of substituted phenols (vide supra, Scheme 17), this is a very
effective methodology for the synthesis of homoleptic solvent-free
RE complexes and has also been extended to other pyrazoles featuring
varying degrees of substitution.144 Kempe
and Deacon have also reported the direct reaction of Yb metal with
substituted aminopyridines, leading to the formation of homoleptic
divalent and trivalent Yb complexes depending on the ligand employed.145</p><p>Müller-Buschbaum and
co-workers extended this approach to
other heterocyclic N-donors including 1H-1,2,3-benzotriazolo[4,5-b]pyridine (45-RE, RE = La, Sm; 46-RE, RE = Y, Tb),146 benzimidazole (47),147 and unsubstituted pyrazole (48 and 49, Scheme 20).148 All the reactions were performed in evacuated
reaction vessels and under solvothermal conditions "in the
melt", with varying reaction temperatures depending on the
substrates. The methods employed by Müller-Buschbaum are perfectly
suited for the preparation of homoleptic derivatives with several
REs, though in some cases proligands are also included in the resulting
complexes, either trapped in the lattice through solid-state packing
interactions or by coordinating directly to the metal centers (Scheme 20).</p><p>Another activation strategy
consists of the use of iodine, which
is a methodology commonly employed for the activation of Mg in the
preparation of Grignard reagents;149 REs
can also react directly with I2 to give REI2 and REI3 salts (vide infra, sections 4.1 and 4.2). Small amounts of iodine have been used by Junk,
Deacon, and co-workers to activate a range of REs (Y, La, Nd, Sm,
Eu, Dy, and Yb) and react them with variously substituted phenols
and pyrazoles.150−153 The exact nature of the mechanism involved in this methodology has
not been fully identified; however, Junk and Deacon invoked the formation
of highly reactive divalent REI2 salts, obtained from the
comproportionation reaction between REI3 and RE.150 The transient divalent species is oxidized
by protic proligand substrates, followed by Schlenk-type rearrangement
to give homoleptic RE(L)3 (L = alkoxide, pyrazolate) complexes
together with the regeneration of REI3 (Scheme 21).</p><p>Junk and Deacon have
reported the reaction of Yb and Tb metal with
variously substituted pyrazoles, C3H2N2(CF3)-1-(C4H3S)-3 (HTtfpz) and C3H2N2-Ph-1-(C4H3S)-3 (HPhtpz) (Scheme 22).151,152 The former affords THF-solvated
complex [Tb(Ttfpz)3(THF)3] (50)
when reacted with Tb filings and few crystals of iodine (ca. 8%), though the product is isolated in rather low yields. In the
case of the reaction of HPhtpz with Yb, the intermediate [Yb(Phtpz)(I)(THF)4] (51) is first isolated, which upon treatment
with DME affords the bis-pyrazolate complex [Yb(Phtpz)2(DME)2] (52). Interestingly, this
behavior is not observed in the reactions involving substituted phenols,
where homoleptic species of the type [RE(OAr)3(Slv)n] (53Ar-RE; OAr = ODipp, OMes, ODpp, ODbmp; Slv = THF, DME,
diglyme, MeCN; n = 1–3)150,153 are usually obtained even with Yb.150 Very recently, Junk, Deacon, and co-workers have also used a similar
methodology for the preparation of homoleptic Ln(II) and heteroleptic
RE(III) formamidinates.154,155 Bochkarev and co-workers
have also shown that small amounts of preformed REI3 (ca. 5 mol %) can be used in conjunction with metallic REs
to give very similar results in the reaction with 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone
(PMIP), to give dimeric complexes [{RE(PMIP)(μ-PMIP)2}2] (54-RE; RE = Y, Nd, Gd, Tb, Er, Tm, Lu)
(Scheme 23).156 Mashima et al. have also employed
a similar methodology for the synthesis of RE(III) cyclooctatetraenyl
complexes; in their synthetic strategy, RE metals are reacted with
cyclooctatetraene in the presence of equimolar amounts of iodine,
1,2-dibromoethane, or Ph3PCl2 in hot THF, affording
heteroleptic COT complexes with the formula [RE(COT)(I)(THF)n] (RE = La, Ce, Pr, n = 3; RE =
Nd, n = 2; Ln = Sm, n = 1), [{Sm(COT)(μ-X)(THF)2}2] (X = Cl, Br).157</p><p>In some cases, Ln and RE metals can react directly without the
aid of an activating agent, though this is a rare occurrence. For
example, Müller-Buschbaum and Quitmann showed that carbazole
reacts directly with Yb under solvothermal conditions (255 °C,
carbazole mp 246 °C, Scheme 24) after a quick initiation at 280 °C, to form
the homoleptic coordination polymer [Yb(Carb)2]∞ (55).158 The outcome of
this reaction is analogous to that obtained with Yb ammoniacal solutions
of Yb, though requiring harsher conditions and long reaction times
(ca. 48 days).94 Anwander
and co-workers have also shown that 3,4-dimethylpyrazole reacts with
La powder at high temperatures (220 °C) under partial vacuum
(Scheme 24), leading
to the formation of coordination polymer [La(Me2pz)3]∞ (56).159 Similarly, some REs are also able to react directly with
pyridylbenzimidazoles under solvothermal conditions and in the absence
of Hg.147 Additionally, in 2005 Junk and
co-workers reported the direct reaction of La, Eu, and Yb with 2,6-dibenzylphenol
(HODbp) carried out at 170 °C (Scheme 24).136 The reactions
with La and Eu proceed smoothly: in the case of La, the trivalent
bimetallic complex [{La(ODbp)2(μ-ODbp)}2] (57, ODbp = {O–C6H3(CH2Ph)2-2,6}−) is isolated, whereas
the divalent species [{Eu(ODbp)(μ-ODbp)}2] (58) is obtained when Eu metal is employed.136 Eu can also react directly with N,N′-bis(aryl)formamidines (ArFormH, Ar = Dipp, DF;
DF = C6H3F2-2,6) in acetonitrile
to give complexes of formula [Eu(ArForm)2(CH3CN)2] (59, Ar = Dipp; 60, Ar
= DF), while Yb requires the presence of catalytic amounts of Hg to
obtain analogous complexes (Scheme 24).160 Nief and Mathey also
showed that Sm and Yb powders can react directly with biphospholyls
and biarsolyls in THF at room temperature (Scheme 24), breaking the E–E (E = P, As) bond
and forming bis-phospholyl and arsolyl complexes
[Ln(η5-C4Me4P)2(THF)2] (61-Ln) and [Ln(η5-C4Me4As)2(THF)2] (62-Ln Ln = Sm, Yb).129,130</p><!><p>Direct reactivity
of REs with proligands often requires prior activation of the metal
or some other activation strategy (see sections 3.1, 3.2 and 3.3). However, there are some examples of direct
reactivity in which the RE metal does not require activation and is
involved in a redox exchange with another metal-containing species.
Such species are typically transmetalating reagents, with organomercurials
as the most popular choices (e.g., HgPh2, Hg(C6F5)2, and Hg(CCPh)2);12,35,161−165 however, these methodologies have also been extended to include
other redox active metals, such as Sn,90,166,167 Tl,91,168−174 Bi,175,176 and Ag.177 The
first reaction of an RE with an organomercurial reagent was reported
in 1945 by Gilman and Jones, who reacted La metal with HgPh2 though they were not able to identify the reactivity products.178 Broadly speaking, these methodologies can be
divided into two categories: (1) redox transmetalation (RT) reactions
and (2) redox transmetalation combined with ligand exchange, also
termed redox transmetalation protonolysis or protolysis (RTP) (Scheme 25).12 Though initially developed for classic divalent Lns (Yb
and Eu), RT and RTP reactions can also be used for the synthesis of
trivalent RE derivatives.12 RT/RTP reactions
have been pioneered by Deacon and co-workers since the 1970s, who
developed the first applications in RE coordination and organometallic
chemistry and also engineered bespoke reaction apparatuses (Figure 2) for facile removal
of hazardous byproducts (i.e., Hg and Tl).179 Their implementation in synthetic protocols
has given access to complexes supported by a wide array of ligands
such as cyclopentadienyl,168,169,172,173,180 carboranes,181 monodentate amides,90,182−185 bidentate amides,160,186−191 pyrazolates,166,174,175 aryloxides,91,170,171,183,192 thiolates193,194 and N-heterocyclic
carbenes (NHCs).195,196 These methodologies have been
covered extensively in other review articles;12,35 therefore, this section will focus on key examples that best demonstrate
their applications in RE chemistry.</p><!><p>Sketch of the apparatus originally developed by Deacon and co-workers
for RT and RTP reactions with Hg and Tl reagents. Note: All joints are kept grease-free by using PTFE sleeves. Reproduced
with permission from ref (179). Copyright 1990 Wiley.</p><!><p>In general, RT and RTP methodologies are very useful for obtaining
homoleptic complexes, though it should be noted that coordinating
solvents are usually required for these reactions and solvated derivatives
are often isolated as a result. In some cases activation of the metal
is required, which can be achieved by one of the methods illustrated
in previous sections (e.g., addition of small quantities
of Hg, I2, and REI3). Deacon pioneered the use
of Hg reagents in RT and RTP reactions and reported the preparation
of several organoytterbium and organoeuropium complexes, Ln(R)2 (Ln = Eu, Yb; 64-Ln, R = CCPh; 65-Ln, CCtBu; 66-Ln, Ph; 67-Ln C6F5; 68-Ln o-HC6F4; 68-Ln p-HC6F4), by reacting
metal powder with the corresponding organomercurial reagent in ethereal
solvents (Scheme 26).161−165 While Yb reacts smoothly with organomercurials, in the case of Eu,
addition of Hg metal is necessary for the reaction to take place.
Interestingly, reactivity of Sm powder with Hg(R)2 reagents
is less straightforward; when Hg(C6F5)2 is employed, Sm(C6F5)2 or Sm(C6F5)3 cannot be isolated and various
decomposition products are obtained instead, e.g., SmF2/SmF3, Sm(C6F5)(F)2, and fluorohydrocarbons,164,197 and similar
issues are encountered when using other trivalent Lns. Bochkarev and
co-workers were the first to report the synthesis of [Er(Ph)3(THF)3] (69-Er) and [Tm(Ph)3(THF)3] (69-Tm) from the reaction of metal powders
and HgPh2 in the presence of small quantities corresponding
triiodides (ErI3 and TmI3), though the isolation
of such species is extremely challenging.176 Despite the difficulties in isolating putative RE(R)3 (R = CCPh, Ph, C6F5) via RT,
reactivity in situ of trivalent REs with organomercurials
is used extensively as a first step in RTP reactions with various
protic substrates (vide infra). Nonetheless, RT reactions
with fluorinated thiols proceed smoothly to give fluorinated thiolate
complexes [Ln(S–C6F5)3(Slv)x] (70a-Ln; Ln = Ho, Er, Yb;
Slv = THF, DME, C5H5N; x =
2–3) and [{Ln(S–C6F5)2(μ–S-C6F5)(THF)x}2] (70b-Ln; Ln = Ce, Sm; x = 1, 2), which can also be obtained using RTP methodologies involving
Hg(C6F5)2 (Scheme 26).193,194</p><p>Deacon and co-workers have
shown that Hg-mediated RTP reactions
are extremely effective for the preparation of RE(II) and RE(III)
complexes with various aryloxide (OAr) ligands (Scheme 27).91,112,123,135,153,183,198,199 These reactions are typically carried out at room temperature with
relative short reaction times, and the resulting complexes are usually
crystallized as THF adducts, [RE(OAr)n(THF)m] (71Ar-RE, n = 2, RE = Sm, Eu, Yb; 72Ar-RE, n = 3, RE = Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu; m = 0–3), though in some cases solvent-free monomeric complexes
can also be isolated, e.g. [Nd(ODpp)3]
(72Dipp-Nd, Scheme 27).200 The outcome of these RTP reactions can be affected
by the nature of the RE involved, the choice of organomercurial reagents,
and aryloxide ligand. In the case of Yb and Eu, the corresponding
divalent complexes 71Ar-RE are usually obtained, which can be crystallized as THF
adducts.200 Interestingly, the use of different
Hg(R)2 reagents can selectively switch oxidation of Yb
metal between Yb(II) and Yb(III); this was demonstrated by Deacon et al. in the reaction with 3,5-substituted 2,6-diphenylphenols:
reactivity between Yb powder, 2,6-diphenyl-3,5-dimethylphenol and
HgPh2 leads to the Yb(II) complex 71Ar-Yb, whereas when Hg(C6F5)2 is used the trivalent complex 72Ar-RE is isolated instead.135 In some cases isolating pure divalent or trivalent
complexes can be problematic, like in the case of RTP reactions with
Hg(C6F5)2 and the phenol HOC6H2-2,6-tBu-4-OMe (HOArOMe). When Yb is employed, the divalent complex [Yb(OArOMe)2(THF)3] (71OMe-RE) is obtained in very good yields, while in
the case of Sm the corresponding divalent and trivalent complexes
[Sm(OArOMe)2(THF)3] (71OMe-RE) and [Sm(OArOMe)3(THF)] (72OMe-RE) are obtained, with the latter as the major product.199 RE-fluoride species can also be obtained because
of the decomposition of RE-(C6F5) intermediates
with certain supporting aryloxides.199 To
solve this problem, Deacon and co-workers replaced Hg(C6F5)2 with Hg(CCPh)2 in their methodology
and obtained the target homoleptic species 71Ar-RE or 72Ar-RE.199</p><p>Some of the earliest applications
of RTP methodologies were the
preparation of Cp derivatives, comprising both divalent Ln(Cp)2 and trivalent Ln(Cp)3 complexes (Scheme 28).197 These methodologies have also been extended to substituted Cp ligands,
such as {C5H4Me}− (CpMe),197 Cp*,180 and {C5H4PPh2}− (CpPPh2).170 Deacon and co-workers
found that when Yb(C6F5)2 was used
as protonolysis reagent with HCpMe the reaction afforded
an explosive solid, likely because of the formation of unstable fluorinated
species.197 Therefore, the use of fluorine-free
organomercurials is often preferable for these methodologies. RTP
reactions are also capable of producing homoleptic Ln(Cp)2 complexes with extremely bulky ligands, such as {C5HPh4}− (CpPh4) and {C5Ph5}− (CpPh5), e.g. [Ln(CpPh5)2] (73-Ln; RE = Sm,
Eu, Yb),201,202 [Yb(CpPh4)2(THF)].202,203 This is a particularly remarkable
application as classic metathetical reactivity between alkali metal
Cp salts and RE halides is often not suitable to obtain Cp derivatives
with high steric congestions. Interestingly, Jaroschik, Deacon, and
co-workers occasionally used also heteroleptic organomercurials, Hg(Ph)(C6F5) and Hg(Ph)(CCPh), for some of these reactions.201,204,205</p><p>Another successful application of RTP reactions with REs
and organomercurials
involves the use of mono- and multidentate N-donors.
A large number of protic substrates have been employed, with silylamines,
arylamines, formamidines, and pyrroles among the most popular.12 This is a particularly important use of RTP
methodologies owing to the prominent role played by N-donors in RE coordination chemistry.7,24 Also in this
case, the main products of these reactions are homoleptic bis-amide
complexes, both with mono- and multidentate donors. Deacon and co-workers
have applied this methodology to several substitute pyrazolates and
showed that this approach is applicable to both divalent Lns (Eu and
Yb) and trivalent REs (Scheme 29).174,206−208 The choice of mercurial reagent can be crucial for the outcome of
these reactions. When HgPh2 is reacted with Yb and proligand
in a 1:2 stoichiometry, the target bis-pyrazolates Yb(R2pz)2 (74R-Yb) are usually obtained.174 However, when
Hg(C6F5)2 is employed, the reaction
leads to oxidation to Yb(III) and formation of Yb(R2pz)3 (75R-Yb);174,209,210 it is noteworthy
that short reaction times tend to favor the formation of 74R-Yb over 75R-Yb.35 Interestingly, direct protonolysis between Yb(C6F5)2 and Ph2pzH proceeds smoothly to give
Yb(Ph2pz)2 (75Ph2-Yb).174 The use of
RTP reactions is exemplified by the extensive work done by Junk, Deacon,
and co-workers in the synthesis of RE formamidinate (ArForm; Ar =
Dipp, p-Tol, o-(CF3)C6H4, DF, 2,3,4,5-F4C6H –
TF, o-FC6H4 – F) complexes
(Scheme 29).160,186,188−191,211 For these ligand systems, reactivity
conditions and stoichiometries can be tailored to obtain either divalent
or trivalent formamidinate complexes, "RE(ArForm)2" (76Ar-RE) and "RE(ArForm)3" (77Ar-RE), irrespective of the choice
of organomercurial agent, although Hg(C6F5)2 is usually the oxidant of choice. It should be noted that
a large excess of metal filings is used for Yb to afford clean conversion
to divalent 76Ar-Yb. Reactions are usually carried out in THF, and the resulting complexes
are normally isolated as ethereal adducts (e.g.,
THF, DME, diglyme) upon recrystallization, with varying coordinated
solvents depending on steric congestion around the metal center and
differences in ionic radii. As previously discussed, fluorinated mercurial
reagents can lead to the formation of undesired byproducts through
fluoride abstraction and oxidation of the RE metal. Junk and co-workers
have been able to take advantage of this type of reactivity to selectively
synthesize heteroleptic complexes of general formula RE(ArForm)2(X) (X = F, Br), such as [La(DippForm)2(F)(THF)]211 and [RE(DippForm)2(Br)(THF)] (RE
= La, Nd, Yb),188 the latter obtained by
employing Hg(2-BrC5F4)2.</p><p>Deacon and co-workers demonstrated that RTP methodologies can be
of great utility for the stabilization of complexes with sterically
demanding amides. Amines, HN(SiMe3)2, and HN(SiMe3)(Dipp) react smoothly with Sm, Eu, and Yb in THF in the presence
of HgPh2, to give Ln(II) bis-amide complexes [Ln{N(SiMe3)2}2(THF)2] (78-Ln·2THF) and [Ln{N(SiMe3)(Dipp)}2(THF)2] (79-Ln).212 Deacon,
Jones, and co-workers have also shown that RTP methodologies can afford
complexes with very low coordination numbers, reporting the synthesis
of 3-coordinate complex [Yb{N(Dipp)(Mes)}2(THF)] (80) from Yb metal, HgPh2, and HN(Dipp)(Mes) (Scheme 30).182 Niemeyer and Hauber reacted Eu and Yb powders
with triazene HN3(Dmp)(Tph) (Dmp = C6H3Mes2-2,6; Tph = C6H4Tripp-2; Tripp
= C6H2iPr3-2,4,6) and
Hg(C6F5)2, obtaining the heteroleptic
3-coordinate complexes [Ln{N3(Dmp)(Tph)}(C6F5)] (81-Ln; Ln = Eu, Yb).213 Additionally, Jones and co-workers showed that heteroleptic
organomercurials of the type "Hg(L)(X)" (L = ligand,
X = halide) can be employed as direct RT reagents (Scheme 30).182 Reaction of Hg(L†)(I) (L† =
N(Ar)(SiMe3); Ar = C6H2iPr{C(H)Ph2}2-4,2,6) with metal powders yields
two different results: with Yb, the homoleptic 2-coordinate complex
[Yb(L†)2] (82) is obtained,
with concomitant formation of [Yb(I)2(THF)2];
in the case of Eu, the halide-bridged dimer [{Eu(L†)(μ-I)(THF)}2] (83) is obtained instead.
Analogous reactions with Sm and Tm were also attempted, but without
success.</p><p>The use of Tl reagent in RT methodologies is
also very well-established.
Tl+ is not as oxidizing as Hg2+ (ETl+/Tl = −0.34 V; EHg2+/Hg = 0.85 V),214 thus allowing for more control of the reactivity particularly in
the case of divalent Lns. Additionally, suitable transmetalation reagents
"Tl(L)" are easily accessible by reacting Tl(OEt) with
protic substrates such as cyclopentadienes,168,169,172,173,215 pyrazoles,174,206 and phenols.91,170,198 Methodologies are similar to those used for organomercurial RT and
RTP reactions, requiring usually ethereal solvents (THF, DME), though
pyridine can also be used in some cases.179 Additionally, small aliquots of mercury can also be added to these
reactions to aid reactivity.179 The first
compounds to be synthesized with these Tl(I) transmetalation reagents
were Cp complexes, Ln(CpR)2 (1-Ln, CpR = Cp; 84-Ln, CpR = CpMe; 85-Ln, CpR = CpPPh2;
RE = Eu, Yb) and Ln(CpR)3 (86-Ln, CpR = Cp; 87-Ln, CpR = CpMe; RE = Ce, Nd, Sm, Gd, Er, Yb) (Scheme 31).168,169,172,173,215 Despite the toxicity of Tl, Tl(I) reagents offer some advantages
with respect to organomercurials, owing to their higher chemical and
thermal stability and tolerance toward a wider variety of solvents
(e.g., pyridine and MeCN). It is noteworthy that
the synthesis of divalent derivatives with Yb, 1-Yb and 84-Yb, often requires large excess of metal powder to avoid
formation of trivalent 86-Yb or 87-Yb; however,
in the case of Sm, only trivalent complexes 86-Sm and 87-Sm are obtained even in the presence of excess metal.169,215 Deacon and co-workers monitored the reaction between Yb powder and
Tl(Cp) by IR spectroscopy, which revealed that the reaction proceeds
first with the formation of 84-Yb followed by reduction
with the excess Yb metal to yield divalent 1-Yb.179 Deacon170,198 and Lappert91 reported also the application of Tl(I) RT reactions
to the synthesis of sterically encumbered aryloxides, [Yb(OAr)2(THF)3] (53Ar-Yb) and Yb(OAr)3 (88-Yb) (Ar
= C6H3tBu2-2,6, C6H2tBu2-2,6-Me-4, C6H2tBu3-2,4,6) (Scheme 31).91 Similar to what was reported for the synthesis of Cp complexes,
a large excess of Yb is required to access divalent derivatives. In
addition, pyrazolate complexes, RE(R2pz)2 (74R-Ln, R = Me, Ph),
are readily accessible for Eu and Yb, while reaction of Sm powder
with Tl(Ph2pz) affords trivalent Sm(Ph2pz)3 (75Ph2-Sm) even under strict stoichiometric control (Scheme 31).174,206</p><p>Bi and Ag reagents, BiPh3, [Bi(C6F5)3], and [Ag(C6F5)(py)], have also
been used as alternatives to Hg and Tl in transmetalation reactions,
though their application so far has been more limited compared to
Hg and Tl reagents.175−177 BiPh3 was used by Bochkarev et al. to prepare [Er(Ph)3(THF)3]
in good yields from Er powder and with a small quantity of ErI3 (4%).176 More recently, Deacon,
Junk, and co-workers have demonstrated that [Bi(C6F5)3]175 and [Ag(C6F5)(py)]177 can be used
in RTP reactions for the synthesis of RE pyrazolate and formamidinate
complexes,154 thus proving that these species
could provide an attractive alternative to the more toxic Hg and Tl
regents. Roesky and co-workers have also shown that RT reactions with
Ag(I)-NHC reagents can be employed for the synthesis of heteroleptic
Ln-NHC complexes (Scheme 32).195,196 In these methodologies, Ag salts
"(NHC)Ag(I)" [NHC = 1,3-bis(R)imidazolin-2-ylidene;
R = Me (IMe2), Mes (IMes), and Dipp (IDipp)] are reacted
with Sm, Eu or Yb powders in THF at room temperature, and the outcome
of these reactions is dictated by the steric demands of the NHC ligand
employed and the ionic radii of the metal centers. With the most sterically
demanding IDipp, the products of this reaction are free carbene and
[Ln(I)2(THF)n] (RE = Eu, Yb).195 On the other hand, when the smaller IMes ligand
is employed, the heteroleptic NHC complexes [Ln(IMes)(I)2(THF)3] (89-Ln; Ln = Eu, Yb) are obtained.195 Finally, the reaction between (IMe2)Ag(I) and Yb affords the bis-NHC adduct [Yb(IMe2)2(I)2(THF)] (90), while the tetra-NHC
adducts [Ln(IMe2)4(I)2] (91-Ln; Ln = Sm, Eu) are obtained with the larger divalent lanthanoids.196</p><p>Sn reagents have also been employed in RT reactions. This methodology
was first introduced into RE chemistry by Lappert and co-workers who
reacted Yb powder with [Sn{N(SiMe3)2}2] in THF under reflux and obtained the bis-silylamide complex [Yb{N(SiMe3)2}2(DME)] (78-Yb·DME)
upon crystallization from DME (Scheme 33).90 Lappert's
methodology involved a Sn(II)-to-Sn(0) reduction, whereas Deacon and
co-workers later employed Sn(IV) reagents—SnMe3(Ph2pz), SnMe3(OAr) (Ar = C6H2tBu2-2,6-Me-4), and SnMe2(Ph2pz)2—for the synthesis of RE pyrazolate
and aryloxides, i.e., RE(Ph2pz)2 (74Ph2-RE), RE(Ph2pz)3 (75Ph2-RE), RE(OAr)2 (53tBuMe-Ln), and RE(OAr3) (88tBuMe-Ln, Scheme 33).166,167 While the first two reagents afford a one-electron reduction, Sn(IV)/(III),167 forming Sn2Me6 as byproduct,
the latter behaves as a two-electron oxidizing agent generating putative
"SnMe2" (Scheme 33).166</p><!><p>The use of
MVS techniques is not a very common methodology in RE coordination
and organometallic chemistry and has been employed by very few research
teams across the world.13 However, its
application has led to some remarkable results such as the first synthesis
of [Sm(Cp*)2(THF)2] (probably one of the most
iconic and well-studied f-element complexes ever reported),216 the isolation of the first zerovalent RE molecular
species [RE(3C6H3tBu3-1,3,5)2] (RE = Y, La, Pr, Nd, Sm, Gd, Tb, Dy,
Ho, Er, Lu),217−219 and the first examples of Sc(II) and Sc(I)
complexes, [RE(C6H3tBu3-1,3,5){1-CH2C(Me)23,5-tBu2C6H3}(H)]220 and [{Sc(η5-tBu2C2P3)}2(μ-η6:η6-tBu3C3P3)].221 With this technique,
metal vapor is generated at low temperatures (−196 °C)
under high vacuum and condensed with a substrate (Figure 3), and the resulting reaction
mixture is then extracted in an organic solvent for recrystallization.13,40,222</p><!><p>Examples of MVS reactors employed by Cloke
(left) and DeKock (right).
Parts in the reactor used by Cloke are A, coolant level; B, glass reaction vessel; C, gasket; D, insulated container for coolant; E, ground flange
seating for Viton O-ring; F, coolant drain; G, ligand vapor inlet, "gas-ring"; H, electron-beam
furnace; I, outlet to trap and diffusion pump; J, metal sample; K, gutter for collection of
products; L, vessel for product receipt. Panel on the
left reproduced with permission from ref (217). Copyright 1981 Royal Society of Chemistry
Publishing. Panel on the right reproduced from ref (222). Copyright 1978 American
Chemical Society.</p><!><p>The first reports on
the reaction between RE metal vapors and organic
substrates (e.g., 1-hexyne, 3-hexyne, 1,3-butadiene,
2,4-pentanedione, and cyclooctatetraene) were presented by Blackborow,223 Evans,224,225 and DeKock.222,226 By reacting RE metal atoms (RE = La, Ce, Nd, Er) with C8H8 (Scheme 34), DeKock and co-workers isolated a family of asymmetric complexes
of formula [RE(COT)(THF)2][RE(COT)2] (92-RE; RE = La, Ce, Nd, Er), while reactivity with Yb afforded
"Yb(COT)" (5), previously obtained from
reaction in liquid NH3 (vide supra, Scheme 4).222,226 Evans and co-workers successfully applied this technique to the
preparation of [Sm(Cp*)2(THF)2] (93·2THF) and [Sm(CpMe4Et)2(THF)2] (94·2THF; CpMe4Et = {C5EtMe4}−) by reacting Sm atoms with functionalized
cyclopentadienes (Scheme 34).216,227 In general, products obtained
from reactivity of RE atoms with unsaturated substrates show magnetic
properties that are in agreement with the presence of oxidized RE
ions (either +2 or +3) despite the formal 0 oxidation state,13 such as in the diazabutadiene complexes RE[{N(tBu)CH}2]3 (95-RE; RE =
Y, Nd, Sm, Yb) reported by Cloke and co-workers (Scheme 34).228 On the other hand, reaction of RE atoms with 1,3,5-tri-tertbutylbenzene, 1,3,5-tBu3C6H3, generates sandwich complexes of general formula [RE(C6H3tBu3-1,3,5)2] (96-RE; RE = Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho,
Er, Lu) where magnetic characterization reveals that the RE metals
are unequivocally in a zerovalent state (Scheme 34).217−219 When the same reaction is performed
with Sc atoms, together with the Sc(0) complex [Sc(C6H3tBu3-1,3,5)2] (96-Sc) a C–H activation product is also isolated, [Sc(C6H3tBu3-1,3,5){1-CH2C(Me)2–3,5-tBu2C6H3}(H)] (97), in which the metal center is formally in the +2 oxidation state
(Scheme 34).220 Cloke and co-workers extended this synthetic
methodology to cyclization reactions involving phospalkyne tBuCP.221,229 Trimerization of tBuCP promoted
by Sc atoms generated the triphosphabenzene ring C3P3tBu3, together with formation of phospholyl
ligand {C2P3tBu2}− and assembling of the double-decker Sc(I) complex
[{Sc(η5-C2P3tBu2)}2(μ–η6:η6-C3P3tBu3)] (98, Scheme 34).221 A byproduct of the same reaction
is also the scandocene complex [Sc(η5-C3P2tBu3)2} (99), which was at the time only the second Sc(II) complex ever reported.229</p><!><p>RE halides are very common starting materials in RE coordination
and organometallic chemistry. Divalent salts used in synthetic chemistry
are usually diiodides of classic divalent Lns (Sm, Eu, and Yb), with
the addition of Nd, Dy, and Tm. Both divalent and trivalent salts,
LnI2 and REX3 are classically employed in salt
metathesis reactions and with a variety of ligand sets. In this section
some examples of their reactivity will be covered.</p><!><p>Divalent halides
of the Lns, LnX2 (Table 3), have been known for over a century, obtained primarily
as diiodides.230 However, fluorides, chlorides,
and bromides are also reported for classic divalent Lns (Sm, Eu, and
Yb) with the addition in some cases of Nd, Dy, and Tm.6,9 Noticeably, ScI2 has also been reported, though it contains
impurities of ScI3 and is best described as Sc0.9I2.15,231,232 Out of all these halides, only a handful of them are of some synthetic
utility in coordination and organometallic chemistry. This is due
to the different physicochemical behavior of binary halides: for dihalides
of Nd, Sm, Eu, Dy, Tm, and Yb the metal is in a true divalent state
similar to group 2 metals (RE2+), while in the case of
the other REs these could be best described as trivalent metals with
an electron residing in the 5d band, i.e., RE3+(e–) (3d band in the case of Sc and 4d
band in the case of Y).15 Nonetheless,
NdI2 DyI2 and TmI2 are very reducing
and difficult to handle, which poses challenges for their application
in synthetic chemistry. In this section we will cover dihalides employed
in synthetic chemistry, focusing primarily on the iodides as they
are the ones that have found most widespread applications in synthetic
laboratories.42</p><!><p>Compounds in
italics have not
been isolated.</p><!><p>Solvent-free
dihalides can be obtained using a variety of methodologies
which usually entail solid-state high-temperature methods.15,233−236 These are convenient starting materials whenever ethereal solvents
are avoided, either for stabilization of complexes with low coordination
numbers or because of potential side reactivity.7,42 Solid-state
methodologies to obtain solvent-free REX2 salts comprise
(1) metallothermic reduction (Wohler method, Scheme 35, A);15,237 (2) comproportionation of trivalent halides with metallic RE (Scheme 35, B);238,239 (3) reduction of trivalent halides with
hydrogen (Scheme 35, C);230,240 (4) reaction of RE metal with
HgX2, where X is usually an iodide (Scheme 35, D);239 (5) thermal decomposition (Scheme 35, E); and (6) direct reaction between
elemental halogen and RE metal (Scheme 35, F).15,236,241 Additionally, solvent-free EuI2 and YbI2 can be obtained from the reaction of
metal powders with NH4I in liquid ammonia (Scheme 35, G).82,241</p><p>With the exception of liquid ammonia synthesis (method G, Scheme 35), these
methods usually require specialized equipment classically used for
solid-state synthesis and are unsuitable for standard synthetic laboratories.15 Bochkarev and Evans were able to adapt solid-state
methods originally developed by Corbett, Kruse, and others and devised
an approach which can be used in normal laboratory conditions (method F, Scheme 35).242−244 Bochkarev and Fagin initially reported the
synthesis of NdI2, DyI2, and TmI2 using a simple apparatus consisting of two thick-walled glass ampules
connected to a vacuum manifold (Figure 4).242,243 In this methodology, after charging
ampule A with metal shavings and iodine, the system is
thoroughly evacuated and then sealed off. Following this, the starting
materials are gradually dispensed from ampule A to ampule B (while A is kept at room temperature throughout
the operation, B has been preheated to 200–300
°C with a gas burner), followed by heating at 400–500
°C for a maximum of 5 min, generating the desired LnI2 with no further purifications. The authors also noted that after
initiation molten metal was formed, thus indicating that the reaction
core reached temperatures around 1500 °C.242,243 In Bochkarev's original method the ampules are reported to
be made of thick-walled borosilicate glass; however, owing to the
high temperatures necessary for the initiation of the reaction and
the heat subsequently generated, it is recommended that quartz reactors
be used for this setup instead.</p><!><p>Schematic representation of the apparatus
for the synthesis of
solvent-free LnI2 salts used by Bochkarev and co-workers. A and B, thick-walled ampules; C, rubber tubing; D, rubber tubing to vacuum line.242,243 Adapted with permission from ref (243). Copyright 2003 Elsevier.</p><!><p>Evans and co-workers developed a more sophisticated apparatus which
can produce LnI2 salts in large quantities (up to 50 g)
and with more controlled conditions (Figure 5).244 In Evans'
method, Ln metal powders (40 mesh) and iodine are placed in separate
addition funnels (F) and the whole system is kept under
static vacuum. The reactor is then heated to 450 °C inside a
furnace, and the starting materials are added gradually into a quartz
crucible (E) placed at the bottom of the reactor. After
an initiation period, the temperature is raised above the melting
point of the relative LnI2 (Ln = Nd 562 °C, Sm 520
°C, Eu 510 °C, Dy 659 °C, Tm 756 °C, Yb 772 °C),9 and the reagents are added portion-wise over
a period of 2 h, giving the desired LnI2 material.</p><!><p>Apparatus for
the synthesis of solvent-free LnI2 salts
used by Evans and co-workers. A, quartz tube; B, O-ring joint; C, connection to vacuum line; D, quartz addition tubes; E, quartz crucible; F, addition funnel; G, furnace. Reproduced from
ref (244). Copyright
2003 American Chemical Society.</p><!><p>Solvated iodide salts, [Ln(I)2(Slv)x] (100-Ln; Slv = THF, DME; x =
2–5) of divalent Lns can also be obtained via standard solution methods (Scheme 36). For the most stable divalent Lns (Sm, Eu, and Yb),
the main methodology was developed by Kagan and co-workers for the
synthesis of [Sm(I)2(THF)2] (100-Sm·2THF),245 which consists of direct
reaction of Ln metal powder or chips with 1,2-diiodoethane in THF.246−248 Evans and co-workers crystallographically characterized the 7-coordinate
THF adduct [Sm(I)2(THF)5],249 but the bis-THF adduct 100-Sm·2THF is
the predominant species after standard workups following Kagan's
methodology. In a similar fashion, Watson et al. reported
the synthesis of [Eu(Br)2(THF)2] (101-Eu·2THF) and [Yb(Br)2(THF)2] (101-Yb·2THF), obtained from the reaction between Ln metal powders
and 1,2-dibromoethane;247 this methodology
cannot be applied to the synthesis of solvated SmBr2, which
is obtained from the reduction of SmBr3 or substitution
of SmI2.250 Solvated SmI2 can also be obtained from the comproportionation reaction
of SmI3 with Sm metal;251 this
method has also been successfully applied to the preparation of solvated
TmI2, which can be then isolated as either THF or DME adduct,
[Tm(I)2(THF)5] (100-Tm·5THF)
and [Tm(I)2(DME)3] (100-Tm·3DME).252,253 Solvent-free NdI2 and DyI2 can also be converted
into THF or DME adducts, [Ln(I)2(THF)5] (100-Nd·5THF) and [Dy(I)2(DME)3]
(100-Dy·3DME), though manipulations have to be carried
out at low temperature owing to the propensity of these species to
react with ethereal solvents.254 Adducts
of SmI2 and YbI2 with N-heterocycles
(pyridine, lutidine, and 4-tert-butylpyridine) have
also been reported by Sella and Maunder,255 while Wakatsuki and Hou isolated also the hexamethylphoshoramide
(HMPA) adducts [Sm(I)2(HMPA)4] and [I]2[Yb(HMPA)4(THF)2].256 Finally, crown ether adducts of various LnI2 salts have
also been reported: Xémard et al. reacted 100-Tm·3DME with 18-crown-6 to obtain the adduct [Tm(I)2(18-crown-6)],257 while Huh et al. successfully attempted the encapsulation of LnI2 salts with 2.2.2-cryptand in DMF to give cationic adducts
[I]2[Ln(crypt)(DMF)n] (Ln =
Sm, Eu, n = 2; Ln = Yb, n = 1; crypt
= 2.2.2-cryptand).258</p><p>LnI2 salts and their adducts are commonly used
in salt
metathesis reactions and are particularly effective for the synthesis
of divalent Sm, Eu, and Yb complexes with a variety of supporting
ligands (vide infra, Figure 6). Most of the complexes obtained with these
starting materials are homoleptic derivatives, with varying coordination
numbers depending on the presence of coordinated solvents, ligand
denticity, and steric properties. Salt elimination reactions have
been employed for several decades in the preparation of cyclopentadienyl
derivatives, such as the archetypal [Ln(Cp*)2(THF)2] (93-Ln; Ln = Sm, Yb; Slv = Et2O,
THF).247,271 Metallocene-type Ln(Cp)2 complexes
can be obtained by reacting group 1 cyclopentadienyl salts with SmI2, EuI2, and YbI2 (93-Yb can also be obtained from YbCl2)272 and are isolated either as solvent-free or solvated adducts
of formula [Ln(CpR)2(Slv)x] (x = 0–2; 1, CpR = Cp; 73-Ln, CpR = CpiPr5; 93-Ln CpR = Cp*; 102R-Ln, Cp′, Cp″, Cp‴,
CpNaph, CpSiPh3, CpSiPh2Me, Cptt, Cpttt, CpiPr4, CpBn5,)273−279 (Cp′ = {C5H3SiMe3}−, Cp″ = {C5H3(SiMe3)2-1,3}−, Cp‴ = {C5H3(SiMe3)3-1,2,4}−,
Cptt = {C5H3tBu2-1,3}−, Cpttt = {C5H3tBu3-1,3}−, CpBn5 = {C5(CH2Ph)5}−) (Scheme 37). These
methodologies are more problematic with the highly reducing salts
NdI2, DyI2, and TmI2. When TmI2 or DyI2 are reacted with KCp* or KCp″ under
nitrogen, N2-activation products [{Tm(Cp*)2}2(μ-N2)] (103)280 and [{Dy(Cp′)2}2(μ-N2)] (104)277 are obtained,
highlighting the reducing nature of these starting materials. The
direct reaction of NdI2, DyI2, and TmI2 with cyclopentadiene (Scheme 37) was also reported by Bochkarev and co-workers, who
observed oxidation of the metal to Ln3+ and formation of
monoring complexes [Ln(Cp)(I)2(THF)3] (105-Ln) with Nd and Dy, whereas the metallocene-type complex
[Tm(Cp)2(I)(THF)2] (106) was obtained
from the reaction with TmI3, with concomitant formation
of LnI3.281 When HCp* was reacted
with NdI2 and DyI2, products of the reaction
were metallocenes [Ln(Cp*)2(I)(THF)] (107-Ln, Ln = −Nd, Dy) (Scheme 37).281 Nief and Evans were
able to obtain Tm metallocenes [Tm(CpR)(THF)x] (108R, CpR = Cp″, Cp‴, Cptt, Cpttt; x = 0, 1) via salt elimination
reaction between TmI2 and group 1 salts of sterically demanding
Cp ligands.277,282,283</p><!><p>Selected examples of ligands used in salt
elimination reactions
with Ln diiodides.</p><!><p>The same methodologies
have also been extended to the synthesis
of phospholyl and arsolyl derivatives. Nief and co-workers were able
to obtain several Tm(II) metallocene-type complexes, with varying
degrees of substitution on the phosphorus or arsenic heterocycles
(Scheme 38), i.e., [Tm{η5-C4H2PtBu2-2,5}2(THF)] (109),284 [Tm{η5-C4H2P(SiMe3)2-2,5}2(THF)]
(110),284 [Tm{η5-C4PtBu2-2,5-Me2-3,4}2(THF)x] (111,x = 0, 1),285,286 [Tm{η5-C4E(SiMe3)2-2,5-Me2-3,4}2] (112E, E = P, As),285 and [Tm{η5-C4PMe4}2] (113). SmI2 and YbI2 have also been used for the preparation of bis-phospholyl
(114P-116P) and arsolyl complexes (114As-116As);283,286 interestingly, in some cases with Sm and Tm the resulting phospholyl
complexes are obtained as dimeric structures in which the lone pair
of the phosphorus atom within the phospholyl ring bridges between
two metallocene moieties, i.e., [Ln{η5-C4P(R)}{μ:η5-C4H2P(R)}]2 (117, Ln = Sm, R = tBu2Me2; 118, Ln = Sm, R = (SiMe3)2Me2; 119, Ln = Tm, R
= tBu2).283,286</p><p>The discussion on cyclopentadienyl, phospholyl, and arsolyl
derivatives
of Ln(II) metals provides a good snapshot of the possibilities offered
by the species; classic divalent Lns (with the addition of Tm) are
able to preserve the divalent state in the final products, while NdI2 and DyI2 are too reducing. As mentioned above,
LnI2 salts can also be used in salt elimination reactions
with a variety of ligand transfer reagents. Nitrogen donors are among
the most popular ligand choices in RE chemistry, particularly monodentate
silylamide {N(SiMe3)2}− (Scheme 39). The homoleptic
complexes [Yb{N(SiMe3)2}2(Et2O)2] (78-Yb·2OEt2)
and [Eu{N(SiMe3)2}2(DME)2] (78-Eu·2DME) were first obtained by Andersen
and co-workers from the reaction between solvent-free LnI2 and two equivalents of Na[N(SiMe3)2] in ethereal
solvents (Et2O, THF or DME).272,287 Similarly, Evans et al. reported the synthesis
of [Sm{N(SiMe3)2}2(THF)2] (78-Sm·2THF) from [Sm(I)2(THF)2] and Na[N(SiMe3)2] in THF.288 These salt elimination protocols cannot be
applied to Tm(II), Dy(II), and Nd(II) because of their high reducing
nature. In the case of Dy and Tm, dinitrogen activation products [{Ln{N(SiMe3)2}2(THF)}2(μ-N2)] (120-Ln; Ln = Dy, Tm) are obtained;289 when TmI2 is employed, Evans and
co-workers isolated a purple solid which they assigned as putative
Tm{N(SiMe3)2}2(THF)x (78-Tm); however, no structural information has
yet been reported.289 DyI2 is
much more reducing than TmI2, and as a result, the solutions
obtained from the reaction with Na[N(SiMe3)2] are highly temperature-unstable, affording dinitrogen activation
product 120-Dy even at −78 °C. It is noteworthy
that the analogous Nd complex [{Nd{N(SiMe3)2}2(THF)}2(μ-N2)] (120-Nd) has been obtained only from the reduction of Ln/group 1 "ate"
complexes, rather than through a transient Nd(II) complex, "Nd{N(SiMe3)2}2".290 Additionally, Evans and co-workers applied a similar synthetic strategy
to the synthesis of aryloxide complexes [{Ln(OC6H2tBu2-2,6)2(THF)2}2(μ-N2)] (121-Ln; Ln = Nd, Dy),
which were isolated from the reaction between LnI2 and
K(OC6H2tBu2-2,6).289,291 Mills and co-workers were able to isolate the first Tm(II) bis-amide
complex, [Tm{N(SiiPr3)2}2] (122-Tm), which was obtained from the reaction between
TmI2 and K[N(SiiPr3)2]
in benzene;292 similar protocols were also
used by the same authors for the synthesis of the analogous [Sm{N(SiiPr3)2}2] (122-Sm),293 [Eu{N(SiiPr3)2}2] (122-Eu), and [Yb{N(SiiPr3)2}2] (122-Yb).292 When the less sterically demanding
silylamide ligand {N(SitBuMe2}− is employed, the solvated species [Ln{N(SitBuMe2)2}2(THF)2] (123-Ln; Ln = Sm, Yb) are obtained instead (Scheme 39). LnI2 salts can also be used
for the preparation of heterobimetallic "ate" complexes
of general formula LnM{N(SiMe3)2}3 (124M-Ln; M =
alkali metal; Ln = Eu, Sm, Yb); the methodology normally consists
of the reaction of three equivalents of ligand transfer reagent with
LnI2 (Scheme 39) and has also been used to synthesize separated ion pair
complexes [K(L)n][Ln{N(SitBuMe2)2}3] (125-Ln, Ln = Sm,
Eu, Tm, Yb, L = 2.2.2-cryptand, n = 1; 126-Ln, Ln = Sm, Eu, L = C7H8, n = 2; 127 Ln = Sm, L = DME, n = 3).294</p><p>A multitude of other ligands have also been employed in salt elimination
reactions with LnI2 salts, which include aromatic ligands
(Figure 6, A, B),295,296 mono- and multidentate dentate
alkyls (C–H),114,191,297−304 multidentate N-donors (I–M),299,305−310 phosphides (N),311 silanides
(O),312,313 and gallyls.132,314 A major challenge in Ln(II) chemistry is the stabilization of heteroleptic
complexes of the type "Ln(L)(I)" directly from salt
elimination reactions, owing to the large steric demands of divalent
Lns and the tendency in some cases to rearrange to homoleptic Ln(L)2 and LnI2. This interest originates from the possibility
of further functionalizing the complexes by substituting the iodide
ligand via metathetical reactivity. Multidentate
donors such as amidinates (I), guanidinates (J), β-diketiminates (BDI, K, M), tris-pyrazolylborates
(Tp, L), and bis-iminophosphorano-methanide (E) have been successfully employed to deliver such species.</p><p>Lappert and co-workers reported the preparation of the heteroleptic
complexes [{Yb(R′BDIR)(η-I)(THF)}2] (128, BDI = β-diketiminate; R = Dipp,
R′ = Me; 129, R = SiMe3; R′ = Ph) from K(R′BDIR) and
YbI2 in THF (Scheme 40);315 Jones et al. also reported the analogous complex [{Yb(tBuBDIDipp)(η-I)(THF)}2] (130) obtained with
similar methodologies.316 Chen and co-workers
reacted potassium salts of tethered BDI ligands, {(R)C(NDipp)CHC(R)NCH2CH2N(Me)CH2CH2NMe2}− (RBDIN2, R = Me, tBu), with [Yb(I)2(THF)2] and [Sm(I)2(THF)2] to give dimeric heteroleptic complexes
[{Ln(RBDIN2)(η-I)(THF)}2].317−320 Amidinate and guanidinate potassium salts afford similar results;
interestingly, the flanking aryl groups can also interact with the
metal center, thus supporting a switch in the coordination mode from
bidentate to monodentate, such as in the dimeric complexes [{Yb(Giso)(η-I)}2] (131, Giso = {(DippN)2CN(C6H11)2}−),321 [{Sm(Piso)(η-I)}2] (132,
Piso = {(DippN)2CNtBu}−),322 [{Yb(Priso)(η-I)(THF)}2] (133, Priso = {(DippN)2CNiPr2}−),321 and monomeric
Sm complex [Sm(Piso)(I)(THF)2] (134, Scheme 40).323 Furthermore, Roesky and co-workers reacted
the potassium salt K(H-BIPM) (H-BIPM = {CH(PPh2NSiMe3)2}−) with [Ln(I)2(THF)2] (Ln = Sm, Eu) in THF (Scheme 40) to give dimeric [{Ln(H-BIPM)(μ-I)(THF)}2] (135-Ln, Ln = Sm, Eu) and monomeric [Yb(H-BIPM)(I)(THF)2] (136).324,325 Finally, Takats and
co-workers were able to isolate the heteroleptic complexes [Ln(TptBu,Me)(I)] (137-Ln, Ln = Sm, Yb) from facile
reaction between K(TptBu,Me) and LnI2 in THF
at room temperature;309 remarkably, the
authors were also able to extend this chemistry to Tm(II) and isolated
complex [Tm(TptBu,Me)(I)] (137-Tm), which
is a very rare example of a heteroleptic Tm(II) species (Scheme 40).326</p><!><p>Trivalent halides
(Table 4) are the most
common starting materials used in RE synthetic chemistry, particularly
for salt elimination reactions; the only exceptions are trifluorides,
which are very rarely employed in organometallic and coordination
chemistry methodologies.327,328 These materials are
usually commercially available, but the cost of anhydrous salts can
be significant; additionally, a high degree of purity is required
for these starting materials, which cannot always be guaranteed from
commercial sources. There are also some special circumstances in which
a knowledge of these basic methodologies can be extremely useful,
as shown by Chilton and co-workers in the preparation of single molecule
magnets with isotopically pure 164Dy;329,330 the only commercially available starting material containing pure 164Dy is 164Dy2O3, which has
to be converted into 164DyCl3 for the preparation
of organometallic complexes.329,330 Additionally, owing
to the similarities between early Lns and trivalent actinides (Ans),
knowledge of these preparative methods can be used to develop synthetic
protocols with RE surrogates that can be transferred to the An family,
which is particularly important when working with scarce and highly
hazardous transuranic elements.331−333</p><!><p>Solvated salts
in italics have
not been structurally authenticated.</p><!><p>Trivalent salts can be obtained from the direct reaction
of the
metal with either halide or hydrogen halides (A and B, Scheme 41), though this type of approach can be challenging for at least one
of the following factors: (1) reactions need to be carried out at
high temperatures above the melting point of the REX3 salt
(usually above 700 °C); (2) the halide source needs to be of
very high purity (particularly if using HCl); (3) anhydrous halides
can react with the reaction vessels at high temperatures.235 This method can be problematic particularly
for RECl3, while the preparation of tribromides334 and triiodides335via direct reaction of the metal with bromine or
iodine under inert atmosphere is more straightforward, as both materials
can be obtained with very high purity.9,233 A more convenient
method consists of the reaction of the RE oxides, RE2O3, with NH4X (X = Cl, Br, I) at high temperatures.
Reed et al. originally developed this methodology
as a solid-state synthesis ("dry method") for RECl3 and REBr3,336,337 which was later applied
for the preparation of REI3 by Young and Hastings (B, Scheme 41).338 However, this approach can also
be implemented in synthetic laboratories as a "wet method"
by dissolving the RE oxides in acid (HCl and HBr) and in the presence
of NH4X (X = Cl, Br). This methodology has been refined
over the years and has now become a routine approach for the preparation
of anhydrous RE chlorides and bromides in many synthetic laboratories.339,340 The synthesis of RE tribromides334 and
triiodides335 is also easily achieved via direct reaction of the metal with bromine or iodine
under inert atmosphere in ethereal solvents to yield solvated adducts.334,335 This is a particularly efficient method for the synthesis of triiodides,
which can be prepared in very large scales as ethereal adducts [RE(I)3(Slv)x] (Slv = Et2O,
THF; x = 3, 3.5, 4);335,341 Evans and
co-workers have also reported the preparation of pyridine adducts,342 while Deacon and Petricěk have obtained
adducts of various halides with DME and glyme.343,344 An alternative approach is based on the use of HgX2 salts
with metallic REs (D, Scheme 40), analogous to the method described previously
for divalent Lns (vide supra, section 4.1).233,345,346</p><p>In Reed's original methodology, RE2O3 and NH4Cl are mixed by heating with a Bunsen burner,
forming RE chlorides as ammonium salts adducts, (NH4)2RECl5 or (NH4)3RECl6,340 dispersed in an NH4Cl
matrix;336 the quantity of ammonium salt
is crucial for the success of this methodology, as incorrect stoichiometries
could lead to the formation of decomposition products such as oxychlorides
"REOCl". The resulting mixture is then transferred into
the bulb (A) of the apparatus shown in Figure 7.347A is connected to a bent delivery tube (Figure 7, C) equipped
with a collection bulb at the end (Figure 7, E); the apparatus is connected
to a high-vacuum line via a trap (Figure 7, F) positioned
at the end of the apparatus. Once the reactor has been charged with
the RECl3/NH4Cl mixture, the furnace is heated
to 300 °C for up to 30 h, affording anhydrous RECl3 upon sublimation of NH4Cl. It is important to avoid higher
temperatures at least at the initial stages of the final purification
step, as unreacted RE2O3 can react with (NH4)3RECl6 at 320 °C to form YOCl,
NH3, and H2O.337 A
similar apparatus was developed by Taylor and Carter, and later refined
by Kutscher and Schneider, for drying hydrated salts REX3(H2O)x (X = Cl, Br, I) with
ammonium halides NH4X (X = Cl, Br, I).14,348 Other iterations have also been reported by Corbett, Meyer, and
Edelmann, and the method has also been applied to the synthesis of
REBr3.240,349</p><!><p>Apparatus for the "dry method"
synthesis of anhydrous
trivalent RE chlorides.347A, pyrex flask; B, furnace; C, delivery
tube (internal diameter 28 mm); D, ground glass connection; E, ammonium chloride receiving bulb (500 mL); F, trap (connected via rubber stopper); G–G′, galvanized-iron cans; H, asbestos; I,
asbestos boards; J, asbestos inner cover; K, heating unit (six Westinghouse No. 299-425 space heaters, 110 V
and 220 W capacity); L, thermometer; M,
pyrex thermometer cover; N, high-vacuum pump connection.
Reproduced with permission from ref (347). Copyright 1939 Wiley.</p><!><p>A more convenient approach toward the preparation of REX3 salts (X = Cl, Br) is the so-called "wet method".
Different from the previously described "dry method",
RE2O3 (oxides containing tetravalent metals
can also be used, e.g., CeO2, Pr2O7, and Tb4O7) is first dissolved
in a concentrated HCl or HBr solution containing excess NH4X (Scheme 42).339,340,350,351 The solution is boiled to dryness (125 °C) under a stream of
dry air or nitrogen, and the fumes are passed through a trap containing
a 10% NaOH solution. The resulting white material, (NH4)2REX5(H2O)x, is finely ground and transferred into a sublimation apparatus
analogous to those illustrated by Meyer and Corbett.240,349 The final stage of this protocol is analogous to the one previously
described in the "dry method", which entails drying
the crude material under reduced pressure. Different temperature programs
are reported in the literature for this procedure; according to Marks
and Diaconescu, the raw material should be heated at 200 °C maximum
to remove excess water first, followed by removal of excess NH4X at higher temperatures (T > 300 °C).339,351</p><p>Anhydrous RECl3 can also be obtained by dehydration
of RECl3(H2O)x.
One method is briefly described above, which uses NH4X
(X = Cl, Br, I) as a drying agent and can be applied to other halides.352 Another typical methodology employed for drying
RECl3(H2O)x involves
the use of SOCl2 and was originally reported by Freeman
and Smith.352 In their methodology, RECl3(H2O)x is converted
into a fine powder, transferred into a flask with excess SOCl2, and then refluxed gently until dehydration is complete;
particular care should be taken to avoid overheating, as decomposition
of SOCl2 could lead to the formation of undesired oxychlorides.
The time required to achieve complete dehydration increases going
across the Ln family, with 1 h required for LaCl3 and 110
h required for ErCl3; this is likely due to the increased
Lewis acidity of the metals as a consequence of the higher charge
density of the smaller Lns.9,352 It should be noted
that anhydrous RE halides are not particularly soluble, and in many
instances these have to be converted into solvated forms. Such procedures
can be very time-consuming; therefore, simple and quick methodologies
that deliver these species as ethereal adducts can be particularly
desirable (vide infra).</p><p>High-temperature methods
are used for the preparation of unsolvated
REX3 with Hg(II) halides, but these methodologies have
several drawbacks (e.g., bespoke solid-state reactors,
removal of Hg by distillation, and purification of REX3 by sublimation) which make it impractical for most synthetic laboratories.
However, Deacon and co-workers showed that standard solution synthesis
can also be used for laboratory-scale preparation of solvated RECl3; this methodology can also be adapted to other halides and
is carried out using the same apparatus employed for other RT and
RTP reactions (vide supra, Figure 2).126,345 In Deacon's
method, a solution of REX3 in THF is produced which allows
removal of Hg residues via filtration (Scheme 43). The compounds
obtained through this method are solvated species, which were originally
reported by the authors as YbCl3(THF)3, YbBr3(THF)3, YbI3(THF)3, SmI3(THF)3 and ErCl3(THF)3.5.345 An alternative method was also reported by
Deacon et al. in which RE powders are treated with
hexachloroethane in THF, to give THF adducts [RE(Cl)3(THF)n] (137-RE; RE = La, Nd, Sm, n = 2; RE = Gd, Yb, n = 3; RE = Er, n = 3.5) (Scheme 43);171 additionally, the authors
reported the structure of dimeric [{Yb(Cl)2(μ-Cl)(THF)2}2] (138), which was obtained upon
treatment of [Yb(Cl)3(THF)3] (137-Yb·3THF) with pentane over several months. It is noteworthy that
there are several different reports in the literature regarding the
number of THF molecules present in solvated RECl3 salts.
This is due to the variety of methods and conditions employed for
their preparation, which lead to the isolation of these adducts as
either neutral molecular complexes—monomeric [RE(Cl)3(THF)3] (137-RE·3THF) or polymeric [RE(Cl)3(THF)2]n (137-RE·2THF)—or separated ion pair species [RE(Cl)2(THF)4][RE(Cl)4(THF)2], which are
usually simplified as [RE(Cl)3(THF)3.5] (137-RE·3.5THF). The adducts reported by Deacon and co-workers
were obtained by Soxhlet extraction,171 while other THF adducts are obtained by heating RECl3 in THF followed by simple removal of solvent, and their formula
is usually reported as RECl3(THF)2.339 Finally, Wu et al. reported
the direct reaction of RE powders with trimethylsilyl chloride and
methanol at room temperature (Scheme 43).354 These reactions produce
the desired RE chlorides in quantitative yields for all the metals
tried in this protocol (Y, La, Ce, Pr, Sm, Dy, and Yb); additionally,
different silanes (dimethylchlorosilane and silicon tetrachloride)
and alcohols can be employed (ethanol, propanol, and 1-pentanol).</p><p>Solvated RE triiodides can be obtained via various
solution methods, including (1) RT reaction of RE metal with HgI2 (vide supra, Scheme 43);345 (2) reaction
of RE metal with iodoethane or diiodoethane in THF;343,355 (3) reaction of RE metal with iodine in isopropanol;335 and (4) reaction of RE metal with iodine in
THF.335 The last method was developed by
Izod et al. and provides a very clean and consistent
route toward obtaining triiodides of all REs (Scheme 44).335 In this
method, iodine is added slowly to RE metal chips in THF, and the crude
material is then baked to remove excess iodine, followed by Soxhlet
extraction to afford solvated [RE(I)3(THF)x]
(138-RE; x = 3.5 or 4 for La–Nd; Table 1).335 La Pierre and co-workers reported an analogous methodology
in which the initial step is performed in diethyl ether; resulting
diethyl ether adducts [RE(I)3(Et2O)3] (139-RE; RE = La, Ce, Nd, Pr, Sm, Gd, Tb, Dy, Ho,
Er, Tm) were also isolated, and several of them were structurally
authenticated by the authors (Scheme 44).341 It is noteworthy that
these etherates easily lose solvent under reduced pressure, so the
final solvent content and composition can vary.341</p><p>In principle, salt elimination
reactions with REX3 salts
can afford both homoleptic, "RE(L)3", and
heteroleptic complexes, "RE(L)2X" and "RE(L)X2" (Scheme 45). The outcome of these reactions is largely dictated by the
electronic and steric features of the ligands employed, and the alkali
metal and halide source chosen for the reaction (vide supra, section 2.1).
The ligands used in salt elimination reactions with LnX2 (vide supra, Figure 6) have also been used in analogous chemistry with trivalent
REs, for which the ligand scope is far greater owing to the greater
stability of most RE3+ ions.</p><p>Because of all the possible combinations of
RE trihalides, ligand
transfer reagents, stoichiometric ratios and reaction conditions,
the number of salt elimination applications of REX3 salts
is enormous, and a full account is beyond the scope of this work.
Herein, some key examples are illustrated of their use with cyclopentadienyl
salts to form homoleptic, RE(CpR)3, and heteroleptic
complexes, RE(CpR)2X and RE(CpR)X2 (Scheme 46). In 1954 Wilkinson and Birmingham used anhydrous RECl3 (RE = Sc, T, La, Ce, Pr, Nd, Sm, and Gd) to synthesize RE(Cp)3 (140-RE) complexes via salt
elimination with Na(Cp), thus pioneering a synthetic route which quickly
became a gold-standard for RE synthetic chemists.2,402 Group 1 salts of a variety of substituted Cp ligands have been used
in salt metathesis reactions with RE halides; in some cases, group
2 (Be, Mg) and Tl(I) reagents have also been employed.327 Salt elimination reactions between RECl3 and Na(Cp) or K(Cp) salts are usually carried out in THF
or Et2O and under reflux conditions.2,402 The same strategy is applicable to substituted Cps, though it becomes
increasingly difficult to displace all three halides as the steric
demands of the ligands increase; RECl3 are the salts that
usually give the best results when attempting to isolate homoleptic
RE(CpR)3 compounds with the smaller Cp ligands, e.g., Cp′ (141-RE),403−408 Cp″ (142-RE),404 CpMe (143-RE),404,409−411 Cpt (144-RE),412,413 and Cptt (145-RE).414 Heteroleptic species can be obtained by altering the stoichiometric
ratio of these reactions, thereby using a 2:1 ratio for metallocene-type
complexes, RE(CpR)2X,282,283,415−419 and 1:1 ratio for monoring complexes, RE(CpR)X2 (Scheme 46).355,420,421 Metallocene-type complexes are
particularly desirable as they provide a fine control of the coordination
sphere of the metal center, with the additional possibility of functionalizing
the complexes by displacing the halide ligand.282,283,415−419 However, often alkali metal salts are occluded in the resulting
complexes; this occurrence can sometimes be avoided by using different
combinations of alkali metal salts. Lappert et al. reported the reactivity of RECl3 (RE = Sc, Y, La, Ce,
Pr, Nd, Yb) with two equivalents of LiCp″, in the attempt to
obtain metallocene type complexes RE(Cp″)2Cl.422 However, the reactivity resulted in the formation
of LiCl adducts of formula [RE(Cp″)2(μ-Cl)2Li(THF)2], which can be converted into the desired
species, in this case dimers [{RE(Cp″)2(μ-Cl)}2], upon recrystallization or sublimation.422 Xie and co-workers showed that by using NaCp″ the
alkali metal-free compounds [{RE(Cp″)2(μ-Cl)}2] are obtained instead.423 Nonetheless,
it is not always possible to avoid alkali metal occlusion in salt
elimination reaction, as shown with the synthesis of heteroleptic
RE(III) metallocenes with Cp* as supporting ligand. With the exception
of [Sc(Cp*)2(Cl)] (146-Sc)424 and [Lu(Cp*)2(Cl)(THF)] (146-Lu),425 all the complexes obtained from
salt elimination reactions between RECl3 and M(Cp*) afford
MCl adducts [RE(Cp*)2(μ-Cl)2M(THF)2] (147M-RE; Slv = Et2O, THF, DME; M = Li, Na, K; RE = Y,426 Ce,427,428 La,429 Pr,429 Nd,430 Sm,431 Gd,429 Tb,429 Dy,429,432 Ho,429 Er,429 Tm,429,433 Yb,429,434 Lu435) (Scheme 46). Despite the drawbacks of these salt elimination reactions, "ate"
complexes are still excellent starting materials which can be further
functionalized and converted into very useful reagents. Nonetheless,
Meng et al. demonstrated that with the use of different
RE starting materials it is possible to avoid alkali metal occlusion
and were able to obtain [Dy(Cp*)2(X)(THF)] (148X; X = Br, I) from the reaction between KCp*
and DyBr3 or DyI3 in THF.432 It is also possible to laboriously convert 147M-RE into monomeric complexes
[RE(Cp*)2(Cl)(THF)] or solvent-free [RE(Cp*)2(Cl)]n, usually via sublimation
under high vacuum and subsequent recrystallization.428,433 Monomeric solvent-free RE(III) metallocenes [RE(CpR)X]
can be obtained via salt metathesis by employing
bulkier Cp ligands, such as Cpttt (148X-RE; X = Cl, Br, I; RE = Y, La–Lu
(except Eu and Tb)) and CpiPr4R (149R-RE; R = H, Me, Et, iPr), with
anhydrous REX3 (X = Cl, Br, I) (Scheme 46).282,283,415−419</p><!><p>RE borohydrides (Table 5) have gained increasing popularity
over the last three decades.17−19 These synthons have found numerous
applications in salt metathesis
reactions, especially for the stabilization of heteroleptic compounds
of formulas Ln(L)(BH4), RE(L)(BH4)2, and RE(L)2(BH4). Part of this interest is
due to their accessibility and ease of preparation, combined with
their versatile coordination chemistry. The borohydride ligand displays
various binding modes, mainly acting as mono-, bi-, or tridentate
donor (Figure 8), and
provides a certain degree of flexibility in salt elimination reactions
compared to RE halides.16,17 Though the BH4– ligand can act as a pseudo-halide,
it can also occupy multiple coordination sites when binding in a k2- or k3-fashion.19 Furthermore, BH4– is effectively a masked hydride which can readily decompose into
H– and "BH3",19 or can be removed with hydride-abstracting agents.436−439 It is noteworthy that borohydride compounds have more covalent character
compared to halides; for this reason, Ln(BH4)2 and RE(BH4)3 have a relatively higher solubility
than their halide counterparts.19 Crucially,
the formation of higher aggregates and "ate" complexes
are less likely when these materials are employed in salt elimination
reactions instead of REX3, thus making them excellent candidates
for obtaining discrete molecular entities.19,20</p><!><p>Typical
binding modes of the borohydride ligand in metal complexes.</p><p>Compounds in italics have not
been structurally authenticated.</p><p>CSD entry 1198893.</p><!><p>The syntheses
of divalent Ln borohydrides [Ln(BH4)2(MeCN)n] (Ln = Eu, n = 2; Ln =
Yb, n = 4) and [Ln(BH4)2(py)n] (Ln = Yb, n = 4; Ln =
Eu, n = 1.8) were first reported by Shore and co-workers
in 1991.440 To obtain these materials,
the authors first prepared ammoniacal solutions of LnCl2 by reacting metal powders with NH4Cl in liquid ammonia;
removal of ammonia afforded solid ammoniacal salts LnCl2(NH3)x which were converted
into MeCN or pyridine solvates, LnCl2(Slv)x (Slv = MeCN, pyridine), and then reacted with two
equivalents of NaBH4 (Scheme 47, A).440 Makhaev and Borisov were able to obtain Sm(BH4)2, Eu(BH4)2, and Yb(BH4)2 from the thermal decomposition of the corresponding NaLn(BH4)4 precursor (Scheme 47, B);441 interestingly, in the case of Yb it was also possible to obtain
the divalent species from thermal decomposition of Yb(BH4)3(THF)2.441 A more
convenient approach was devised by Visseaux and co-workers, who showed
that treatment of [Sm(BH4)3(THF)3] with Sm metal in THF at room temperature can afford [Sm(BH4)2(THF)2] (150-Sm·2THF) in very good yields (Scheme 47, C).19 The
same method was applied by Roesky and co-workers to synthesize the
Yb analogue [Yb(BH4)2(THF)2] (150-Yb). [Eu(BH4)2(THF)2]∞ (150-Eu·2THF) was also
prepared by Roesky and co-workers either via salt
metathesis reaction between [Eu(I)2(THF)2] and
NaBH4 or from the reaction between EuCl3 and
NaBH4 with the concomitant formation of H2 and
"BH3" (Scheme 47, D and E).442 Finally, Visseaux, Nief, and co-workers obtained
[Tm(BH4)2(DME)2] (150-Yb·2THF) from direct salt metathesis between [Tm(I)2(DME)3] and KBH4 (Scheme 47, F) or by reduction of Tm(III)
precursor [Tm(BH4)3(THF)3] with KC8 (Scheme 47, G).443 Interestingly, the
authors noted that comproportionation reaction between [Tm(BH4)3(THF)3] and Tm did not yield a reduced
Tm(II) species.</p><p>Divalent borohydrides have
found application as salt metathesis
precursors with various ligands, including BDIs (151-Ln; Ln = Sm, Eu, Yb),444 bis-iminophosphorano-methanides,442,445 tris-pyrazolylborate,443 substituted
pyrrolyls,446 and cyclopentadienyls.447 Roesky and co-workers reacted K(H-BIPM) with
[Ln(BH4)2(THF)2] (Ln = Eu, Yb) to
give heteroleptic borohydride complexes of formula [Ln(H-BIPM)(BH4)(THF)2] (152-Ln; Ln = Eu, Yb) (Scheme 48).442 The dual behavior of these reagents (pseudo-halide and masked hydride) was highlighted by the
same authors who synthesized cationic [Eu(BH4)(THF)5][BPh4]∞ (153) and
[Yb(BH4)(THF)5][BPh4] (154) by reacting [Ln(BH4)2(THF)2] with
[Me3NH][BPh4] (Scheme 48);442 in this
reaction the hydride is abstracted to form H2 with concomitant
formation of "BH3" and NEt3. Finally,
Momin et al. used salt metathesis to synthesize the
dimeric Sm(II) monoring complex [{Sm(Cp*)2(μ-BH4)(THF)2}2] (155) and [Tm(TptBu,Me)(BH4)] (156) (Scheme 48), which have both been used
as initiators for the polymerization of ε-caprolactone.443,447</p><!><p>Trivalent
RE borohydrides, RE(BH4)3, are known for all
the RE metals (Table 5 and Scheme 49).
With the exception of Sc, Ce, and Eu, these species were first obtained
as THF solvated salts RE(BH4)3(THF)3 from the reaction of RE(OMe)3 with B2H6 in THF at room temperature (Scheme 49, A).448,449 A more convenient approach is based on simple metathetical reactivity
between RECl3 and MBH4 (M = Li, Na, K) in THF.
This method was first attempted by Rossmanith and Muckenhuber in 1959,
but in their methodology they could only obtain partial conversion
to mixed borohydride-chloride salts RE(BH4)2Cl.450,451 Moreover, Andersen reported that reactivity
of LiBH4 with NdCl3 was sluggish and did not
afford the desired Nd(BH4)3 product.452 However, when these reactions are carried out
under reflux for at least 48 h a good conversion to solvated RE(BH4)3 complexes is obtained, as demonstrated by Ephritikhine
and co-workers with the preparation of [Nd(BH4)3(THF)3] (157-Nd·3THF, Scheme 49, B),453 and this method has been successfully applied
to the synthesis of tris-borohydrides with all REs.444,453−458 Traces of halides are often present in the final products despite
very long reaction times and use of large excess of MBH4.456 Mills and co-workers developed this
methodology further by using triiodide RE salts with a large excess
of KBH4 (Scheme 49, C) and were able to identify halide impurities via X-ray studies;436−438 the percentage of iodide present
in [La(BH4)3(THF)4]436 (157-La·4THF) was measured at 3%, whereas
the halide content in [Y(BH4)3(THF)3] (157-Y·3THF) and [Tb(BH4)3(THF)3] (157-Yb·3THF) was significantly
higher (ca. 15%).437 Adducts
with other Lewis bases are also reported, including pyridine and NH3. Finally, solvent-free RE(BH4)3 of
several REs (Sm, Gd, Tb, Dy, Er, Tm, and Yb) have been obtained by
Hauback, Jensen, and co-workers via ball-milling
of RECl3 with LiBH4,459−462 including divalent Sm(BH4)2 and Yb(BH4)2 (Scheme 49, D). Solvent-free RE(BH4)3 can also be obtained from the reaction of REH3 with H3B·SMe2, both with dry (ball-milling) and solution
methods.463</p><p>Much like REX3 salts,
trivalent RE borohydrides are
very efficient reagents for salt metathesis reactions, and they have
been used extensively in cyclopentadienyl chemistry for the synthesis
of monoring and metallocene-type complexes (Scheme 50). It is noteworthy that RE(III) metallocenes
have also bene synthesized by reacting Cp salts with RE(BH4)3 generated in situ from RECl3 and NBH4.464 Monoring complexes,
RE(Cp)X2, can be particularly challenging to isolate when
using halide starting materials, while they are usually readily accessible
with borohydride reagents owing to the stabilizing properties of the
borohydride ligand and its flexible coordination modes.465 This is exemplified by the variety of coordination
motifs exhibited by this family of complexes: (1) monoring complexes, e.g. [RE(CpR)(BH4)2(THF)]
(158, CpR = Cp*, RE = Sc;466159, CpR = CpiPr5; RE
= Dy439) and [RE(CpR)(BH4)2(THF)2] (160-RE, CpR = Cp*, RE = La,467 Nd;453,468161, CpR = Cpttt, RE = La436); (2) borohydride-bridged dimers, e.g. [{Ce(Cpttt)(μ-BH4)2(THF)}2] (162);436 (3) clusters, e.g. [{La(Cpttt)(BH4)2}6] (163)436 (Scheme 50). Metallocene-type
complexes, [RE(CpR)2(BH4)], can be
obtained with several supporting Cp ligands (164-RE,
CpR = CpiPr4, RE = Nd, Sm;469165, CpR = Cp‴, RE = La;436166-RE, CpR = Cpttt, RE = La, Dy, Tm);436 in a similar
vein to heteroleptic halide-Cp RE complexes, the formation of dimeric
species is observed when steric hindrance of the ligands is reduced, e.g., [{RE(Cptt)2(μ-BH4)}2] (167-RE; RE = La, Ce).436 Unlike with halide-based starting materials, "ate"
complex formation is usually not an issue in these methodologies.
However, when similar reactions were carried out by Liu et
al. between phospholyl K(Htp) (Htp = {C4H2PtBu2-2,5}−) and various
RE borohydrides (RE = La, Ce, Nd, Sm), the leading to the isolation
of "ate" complexes [{RE(Htp)2(μ-BH4)2K(μ-DME)2}2] (168-RE; RE = La, Ce) and [La(Htp)(μ-Htp)(μ-BH4)2K(OEt2)(THF)]∞ (169) (Scheme 50) were isolated.438 Nonetheless, the authors
were able to avoid salt occlusion by performing the reactions in nBu2O under reflux, which gave borohydride-bridged
dimers [{RE(Htp)2(μ-BH4)}2]
(170-RE; RE = La, Ce, Nd, Sm).438</p><p>While RE halides can be converted into alkyls
and allyls from the
reaction with organolithium and Grignard reagents, analogous reactivity
of RE borohydrides is not as straightforward. Visseaux and co-workers
reacted Nd(BH4)3 and Sm(BH4)3 with Grignard reagents and could not obtain pure products.470 Nonetheless, reactions with half an equivalent
of Mg(C3H5)2 generated the heteroleptic
allyl derivatives [RE(BH4)2(C3H5)(THF)3] (171-RE; RE = Nd, Sm) in
crystalline form and with excellent yields (Scheme 51).470 In addition
to this, RE borohydrides can be used in protonolysis reactions, similar
to their divalent analogues (vide supra, Scheme 50). This particular
application was pioneered by Ephritikhine and co-workers, inspired
by their previous work with An borohydrides, which can form cationic
species upon treatment with ammonium salts.471 Accordingly, Guillaume et al. synthesized the dimeric
COT complex [{Nd(COT)(μ-BH4)(THF)}2] (172) via salt elimination reaction between
K2(COT) and Nd(BH4)3(THF)3 and then
reacted it with [Et3NH][BPh4] to give the cationic
complex [Nd(COT)(THF)4][BPh4] (173) (Scheme 51).472 Furthermore, RE(BH4)3 starting materials can also be converted cleanly into cationic separated
ion pair complexes [RE(BH4)2(THF)5][BPh4] (174-RE; RE = Y,473 La,473 Ce,474 Nd,473,474 Sm,473 Dy)475,476 upon reaction with [Et3NH][BPh4] (Scheme 51). Visseaux and co-workers obtained also the analogous complex [Nd(BH4)2(THF)5][B(C6F5)4] (175) from the reaction between [Me2PhNH][B(C6F5)4] and [Nd(BH4)3(THF)3].457 Other reagents that have been used with borohydride complexes for
the formation of cations are highly electrophilic hydride abstracting
reagents [Ph3C][B(C6F5)4]437 and [(Me3Si)2H][B(C6F5)4].439,477,478</p><!><p>Amide ligands are ubiquitous in RE chemistry and have
found a multitude
of applications.22−24 Among these, silylamides are probably the most popular
ligand class; Bradley et al. pioneered the use of
the bis(trimethylsilyl)amide ligand {N(SiMe3)2}− in RE coordination chemistry in the early 1970s,496,497 and their work became a centerpiece of modern RE and f-element synthetic
chemistry.24 More recently, the conjugated
base of tetramethylsilazane, {N(SiHMe2)2}−, has also become a very important ligand in RE chemistry.498 Together with possessing a rich coordination
chemistry, silylamides are also excellent Brønsted bases and
are therefore extremely useful for protonolysis reactivity.22−24 Because of this, RE silylamides have now become essential starting
materials for synthetic chemistry. Other RE amides have also been
used in protonolysis reactions (e.g., Ln(NH2)2 and RE(NH2)3,499 RE(NMe2)3, RE(NiPr2)3, and RE(NCy2)3),22−24 but these will
not be covered in this review as their uses in RE chemistry are limited
compared to the vast applications of silylamides.</p><!><p>The first
Ln(II) silylamides, Ln{N(SiMe3)2}2 (Ln = Yb, Eu), were originally synthesized by Tilley et
al. via salt elimination reactions between Na[N(SiMe3)2] and EuI2 or YbI2 (divalent
salts were both been obtained from reactions in liquid ammonia) and
were isolated as Et2O or DME adducts, [Ln{N(SiMe3)2}2(Slv)2] (78-Ln;
Ln = Yb, Eu; Slv = Et2O, DME) (Scheme 52 and Table 6).500−502 It is noteworthy that 78-Eu·2THF was originally obtained from the reduction of Eu{N(SiMe3)2}2Cl with sodium napthalenide; however,
current salt elimination methodologies are far more convenient.503 The Sm analogue [Sm{N(SiMe3)2}2(THF)2] (78-Sm·2THF)
was obtained by Evans and co-workers with similar methodologies using
[Sm(I)2(THF)2] and Na[N(SiMe3)2].28878-Yb can also
be synthesized via (1) RT reaction of Yb metal with
[Sn{N(SiMe3)2}2],90 (2) RTP reaction of Yb metal with HgPh2 and
HN(SiMe3)2,162 and
(3) protonolysis between Yb(Bn)2 (Bn = CH2Ph)
and HN(SiMe3)2504 (Scheme 52; see section 3.4 for details
on RT and RTP methods). All these alternative methods are highly desirable
to avoid the presence of alkali metal or halide impurities. Additionally,
the protonolysis route affords 78-Yb as a solvent-free
species, [Yb{N(SiMe3)2}{μ-N(SiMe3)2}]2, without the need to use labor-intensive
desolvation protocols.505 The Tm analogue
Tm{N(SiMe3)2}2 has been obtained
as a transient species by Evans and co-workers, but its isolation
and full characterization have not been achieved to date.290</p><!><p>Compounds in italics
have not been
structrally autheticated.</p><p>CSD entry 2056064.</p><!><p>Anwander
and co-workers were also able to introduce the smaller
silylamide {N(SiHMe2)2}− to
divalent Ln chemistry.506 In their work,
they observed that salt metathesis reaction between [Sm(I)2(THF)2] and Li[N(SiHMe2)2] could
not deliver clean products, so they decided to explore a protonolysis
route.50678-Sm·2THF
was an ideal candidate as precursor for this strategy, owing to the
favorable difference in pKa between HN(SiHMe2)2 and HN(SiMe3)2 (22.6 and
25.8 respectively).506 The reaction of 78-Sm·2THF with HN(SiHMe2)2 afforded
oligomeric silylamide [Sm3{N(SiHMe2)2}6(THF)2] (176-Sm),506 and the same method has been used for the synthesis
of [Yb3{N(SiHMe2)2}6(THF)]507 (176-Yb) and [Eu3{N(SiHMe2)2}6(THF)2] (176-Eu) (Scheme 52 and Table 6).502</p><p>Amides 78-Ln have been employed extensively
as protonolysis
reagents with various ligand systems, particularly multidentate amines
and alcohols. 78-Yb reacts with MeBDIDipp-H in toluene under reflux (Scheme 53) to give the heteroleptic tetrameric complex
[{Yb(MeBDIDipp){N(SiMe3)2}2}4] (177), which was further
converted into the parent dimeric hydride [{Yb(MeBDIDipp)(μ-H)}2] upon treatment with PhSiH3.508 Junk and Cole followed a similar
approach with the synthesis of the sterically hindered bis-formamidinate
complex [Sm(DippForm)2(THF)2] (76Dipp-Sm), which in addition
was obtained via salt elimination and RTP methodologies
(vide supra, Scheme 29).191 Moreover, Shi et al. treated the terphenyl-aniline H2N(C6H3Xyl-2,6) with 78-Yb·2THF (Scheme 53), yielding the
dimeric heteroleptic amide complex [{Yb{NH(C6H3Xyl-2,6)}{μ-N(SiMe3)2}] (178).718 Another very effective use of these
silylamides is the synthesis of aryloxide complexes. This methodology
can provide an excellent route toward low-coordinate complexes, especially
because of the possibility of operating in the absence of ethereal
solvents. Lappert and co-workers isolated the dimeric complex [{Yb(OAr)(μ-OAr)}2] (179; Ar = C6H2(tBu)2-2,6-Me-4) from the reaction between 78-Yb and two equivalents of HOC6H2(tBu)2-2,6-Me-4 in hexane (Scheme 53).509 By using
the highly sterically demanding ligand {OC6H3Tripp2-2,6}−, Zhao et al. stabilized the bis-aryloxide complex [Sm(OC6H3Tripp2-2,6)2] (180) from the reaction
between the parent phenol, HOC6H3Tripp2-2,6, and 78-Sm·2THF (Scheme 53).510 The ability
of silylamides to act as Brønsted bases has also been exploited
by Deacon and co-workers in the reaction of 78-Yb·2THF
with the ammonium salt [Me3NH][BPh4] (Scheme 53), which generates
the pseudo-metallocene cationic complex [Yb{N(SiMe3)2}{(η6-Ph)2BPh2}(THF)n] (181; n = 0, 1).511181 is itself an excellent synthetic precursor for protonolysis reactivity,
as demonstrated by the same authors in the reaction with tBu2pzH and subsequent formation of the pyrazolate complex
[Yb(tBu2pz){(η6-Ph)2BPh2}(THF)].511</p><p>Another interesting application of 78-Ln was demonstrated
by Anwander and co-workers with the synthesis of heteroleptic mono-Cp
complexes using the lead reagent Pb(Cp*)2 (Scheme 54).512 In this methodology, 78-Ln·2THF and 176-Ln are reacted with 0.5 equiv of Pb(Cp*)2, causing oxidation
of the Ln(II) center to Ln(III) and formation of piano-stool complexes
[Ln(Cp*){N(SiMe3)2}2] (182-Ln; Ln = Sm, Yb) and [Ln(Cp*){N(SiHMe2)2}2(THF)] (183-Ln; Ln = Sm, Yb), with concomitant
formation of metallic Pb.512 Conversely,
when 78-Eu·2THF is employed, the metallocene complex
[Eu(Cp*)2{N(SiMe3)2}(THF)] (184) is isolated.</p><!><p>RE(III)
silylamides [RE{N(SiMe3)2}3] (185-RE) were originally synthesized by Bradley et
al. in 1972 via salt elimination reactions
between RE trichlorides and Li[N(SiMe3)2] in
THF (Scheme 55 and Table 6).496,497 The methodology developed by Bradley is still widely used, but it
can suffer from salt occlusion or "ate" complex formation.
However, the use of heavier alkali metal salts and RE triiodides can
suppress salt occlusion, and recrystallization or sublimation can
be used for further purifications.24,342 Alternative
procedures have also been used, such as (1) salt elimination reaction
between RE(OTf)3 and Na[N(SiMe3)2]513−515 and (2) salt elimination reaction between
benzyl-potassium reagents and RECl3 followed by protonolysis
with HN(SiMe3)2 (Scheme 55).513,516 The dimethylsilyl
analogues [RE{N(SiHMe2)2}3(THF)n] (186-RE·nTHF; RE = Sc, n = 1; RE = Y, La–Lu, n = 2) can be prepared following similar salt elimination
methods using RE chlorides and group 1 transfer reagents M[N(SiHMe2)2] (Scheme 55), though reactions involving chloride salts and K[N(SiHMe2)2] lead to the isolation of products containing
halide impurities.498185-La can also be used as a starting material for the preparation of solvent-free 186-Lavia protonolysis, similar to the strategy
used for obtaining their divalent congeners 176-Ln (Scheme 55).502,517−519 Moreover, Dietrich et al. obtained 186-Y from the protonolysis reaction between
[Y(Me)3]n and HN(SiHMe2)2 (Scheme 55; vide infra, section 9.1).520 Finally,
Anwander and co-workers attempted to prepare 186-RE using
RE(OTf)3 and M[N(SiHMe2)2] (M = Na,
K); however, they could not obtain the desired products in pure form
because of the contamination of M(OTf).498</p><p>The applications of RE silylamides in protonolysis reactivity are
numerous, and a full account is beyond the scope of this review.22−24 Some representative examples will be listed here to outline their
main applications. Lappert and co-workers used 185-RE (RE = Sc, Y, La Pr, Nd, Dy, Ho, Er, Yb) in protonolysis reactions
with HODbmp, obtaining homoleptic aryloxo complexes [RE(ODbmp)3] (187-RE; RE = Sc, Y, La Pr, Nd, Dy, Ho, Er,
Yb) (Scheme 56).521 This method has found many applications, particularly
for obtaining RE aryloxides (especially whenever targeting solvent-free
derivatives) and to exclude the possibility of halide and alkali metal
contamination.522−524 Additionally, these protonolysis methodologies
can be employed toward the synthesis of complexes supported by multidentate
oxygen donors. Examples of this synthetic approach have been shown
by Schelter and co-workers with the preparation of [RE(TriNOxOMe)(THF)] (188-RE; RE = Nd, Dy; TriNOxOMe = {[(2-tBuNO)(5-OMe)C6H3CH2]3N}3–) from the deprotonation
of parent hydroxylamine H3TriNOxOMe with 185-Nd and 185-Dy (Scheme 56),525 or by Dong
and Robinson with the use of 186-RE (RE = Y, La) for
the synthesis of the heteroleptic complexes [La{κ3N,O,O′-N(C6H2tBu2-3,5-O-2)2(CH2Ph)}{N(SiMe3)2}(THF)2] (189) and [Y{μ:κ3N,O,O′-N(C6H2tBu2-3,5-O-2)2(CH2Ph)}{N(SiMe3)2}]2 (190) (Scheme 56).526 Similarly, 185-RE can also be
used for the deprotonation of amines to target the isolation of solvent-free
amides. Evans and co-workers reacted H2NDipp with 185-RE (RE = Y, Yb), which led to the deprotonation of the
aniline and formation of the target anilido complexes RE(HNDipp)3 (191-RE; RE = Y, Yb), though only 191-Y was structurally authenticated, revealing a dimeric arrangement
in the solid state (Scheme 56).527 Anwander et al. have also shown that 186-Y and 185-La are
very effective deprotonating agents toward cyclopentadienes, obtaining
a range of heteroleptic metallocene-silylamide complexes, e.g. [RE(Cp*){N(SiHMe2)2}2] (183-RE; RE = Y, Lu), [Y(Cp*)2{N(SiHMe2)2}] (192), [Y(Cptet)2{N(SiHMe2)2}] (193; Cptet = {C5HMe4}−), and
[La(CpPh4)2{N(SiHMe2)2}] (194) (Scheme 56).528,529 Interestingly, Teuben and co-workers
could not obtain clean products when reacting 185-RE (RE
= Y, La, Ce) with HCp*, thus highlighting an advantage in using {N(HSiMe2)2}− over {N(SiMe3)2}− as a base for this type of reaction.530</p><p>This strategy is also very
effective for obtaining heteroleptic
complexes supported by multidentate amides. Vitanova et al. reacted MeBDIMes-H with 185-La and 186-RE (RE = Y, La) and obtained the heteroleptic
complexes [La(MeBDIMes){N(SiMe3)2}2] (195) and [RE(MeBDIMes){N(SiHMe2)2}2] (196-RE; RE = Y, La) (Scheme 57).531 A similar strategy
was adopted by Luo and co-workers to synthesize the heteroleptic species
[Y{(NDipp)2CPh}{N(SiHMe2)2}2(THF)] (197) and [RE{(NXyl)2CPh}{N(SiHMe2)2}2(THF)n] (198-RE; RE = Sc, Y; n = 0, 1) (Scheme 57).532 Moreover, Roesky and co-workers deprotonated
the chiral amidine (S,S)-N,N-bis(1-phenylethyl)benzamidine, (S)-HPEBA-H, with 185-Lu, affording the bis-amidinate
complex [Lu{(S)-PEBA}2{N(SiMe3)2}] (199, Scheme 57).533 Another
approach toward the stabilization of heteroleptic silylamide complexes
was demonstrated by Schelter and co-workers with the preparation of
heteroleptic guanidinate complexes [Ce{(NR)2CN(SiMe3)2}{N(SiMe3)2}] (200R; R = iPr, Cy); in their study,
they reacted 185-RE with carbodiimides R–N=C=N–R
(R = iPr, Cy) leading to insertion into one of the C–N
bonds (Scheme 57).524,534</p><!><p>Amide ligands
can function as supporting ligands for Ce(IV) complexes, and some
of these species are now routinely used as Ce(IV) starting materials
in protonolysis reactions. Two of the most common Ce(IV) amide starting
materials are complexes [Ce{N(SiHMe2)2}4] (201)535 and [Ce(NiPr4)4] (202).536 Both of these species are obtained from oxidation of Ce(III)
precursors, 186-Ce and [Ce(NiPr4)4Li], respectively, using PhICl2, Ph3CCl, or
C2Cl6 (Scheme 58). Both 201 and 202 can react
with protic substrates to generate new Ce(IV) complexes (Scheme 58), as shown by
Kim et al. with the synthesis of the 8-coordinate
complexes [Ce{Ph[C6H4N(tBu)(O)-2]2}2] (203)537 and [Ce{O[CH2–C6H4N(tBu)(O)-2]2}2] (204)538 and by Schneider et al. with
the stabilization of the tetramethylguanidinate (TMG) complex [{Ce(TMG)3(μ-TMG)}2] (205) (Scheme 58).536</p><!><p>The RE metals are very electropositive and highly
oxophilic and
therefore form very stable complexes with O-based
donors.26,28,44,559 The coordination chemistry of aliphatic alkoxides
and aryloxides with the REs is extremely well-developed, and throughout
this review various methodologies which are employed for the synthesis
of Ln(OR)2 and RE(OR)3 derivatives are discussed
(see section 3). Alkoxides
have attracted a lot of interest because of their high volatility,
which makes them very desirable precursors for the fabrication of
new materials.29 In principle, these derivatives
can be used as starting materials for protonolysis reactivity by matching
them with substrates with higher pKa (vide supra, Table 2); however, their applications are limited compared to other
protonolysis reagents illustrated in this review (e.g., amides and alkyls). Nonetheless, aryloxide RE complexes have found
applications as protonolysis or metathesis reagents for the synthesis
of challenging coordination complexes and organometallic derivatives,
and Ce(IV) alkoxides have also been used in protonolysis and metathesis
reactions.560−562 Additionally, RE inorganic derivatives containing
oxygen donors have also found several applications in synthesis, particularly
triflates and nitrates. For the purpose of this review we will focus
on the following reagents: (1) aryloxides (section 7.1) and (2) Ce(IV) alkoxides and CAN (section 7.2). Triflates
are discussed in a separate section (section 8).</p><!><p>The
synthesis of RE aryloxides
[RE(ODbmp)3] (72Dbmp-RE) was first reported by Lappert and co-workers in
1983, based on protonolysis of substituted phenol HODbmp with tris-silyalamide
precursors 185-RE (A, Scheme 59).521 This method is still widely applied to RE aryloxide synthesis and
has been used also for the synthesis of very sterically congested
systems which cannot be obtained using other synthetic strategies, e.g. [Y(OC6H2Ad2-2,6-tBu-4)3].563 Alcoholysis
with Ln(II) silylamides can also be used to obtain divalent Ln alkoxides,
and by using solvent-free 78-Yb it was possible to obtain
the homoleptic aryloxide 179 (A, Scheme 59; vide
supra, Scheme 53).505,509 Other approaches, some of which
are illustrated in detail in other sections of this review, include
(1) salt elimination between REX3 and MOAr (B, Scheme 59);270,522,564 (2) ammoniacal synthesis (C, Scheme 59; vide supra, Scheme 9);90,92,565 (3) direct reaction with metals (D, Scheme 59; vide supra, Scheme 14);120 (4) direct reaction with metals in the presence
of activators, e.g. Hg, HgCl2, I2 (E, Scheme 59; vide supra, Schemes 17 and 23);134,135,150,153,566 (5) RTP with organomercurials
and HOAr (F, Scheme 59; vide supra, Scheme 27);112,123,135,153,183,198,199 and (6) RT with Tl(OAr) reagents (G, Scheme 59; vide supra, Scheme 31).91,170,198</p><p>RE aryloxides (Table 7) have found extensive
usage in protonolysis reactions with various
substrates, particularly with multidentate donors containing aryloxide
functionalities.567−570 An example of this is the alcoholysis reaction of β-ketoimines
(Scheme 60) with 72Dbmp-RE (RE = Y, La,
Nd, Sm, Yb).567−569 β-Ketoimine 1-phenyl-3-N-(p-methoxyphenylimino)-1-butanone reacts smoothly in a 2:1
fashion with 72Dbmp-RE (RE = Y, Nd) in THF at room temperature to give heteroleptic complexes
of formula [RE{N(C6H4OMe-3)C(Me)CHC(Ph)O}2(ODbmp)] (206-RE; RE = Y, Nd).567 In a similar fashion, Shen and co-workers were able to
obtain a series of bimetallic RE complexes supported by a phenyl-bridged
BDI ligand (207-RE; RE = Y, La, Nd, Sm, Yb)568 and a tridentate β-ketoiminate containing
an additional aryloxo functionality (208-RE; RE = Y,
Nd, Sm, Yb).569</p><!><p>Compounds in italics have not been
structurally authenticated.</p><!><p>A particularly important application of 72Dbmp-RE is their use as synthetic precursors
for the preparation of new RE organometallic complexes. In this type
of methodology, the aryloxide donor is reacted with an organolithium
reagent, resulting in transmetalation and formation of insoluble Li(OAr)
as a byproduct. This strategy was applied successfully to the synthesis
of heteroleptic aryloxo-Cp RE complexes by Teuben and Watkin, who
reacted 72Db-RE (RE
= Ce, Sm) with one equivalent of Li(Cp*) to give the monoring complex
[RE(Cp*)(ODb)2(THF)n] (209-RE; RE = Ce, n = 0; RE = Sm, n = 1) (Scheme 61).571,572 Additionally, several σ-bonded
alkyl complexes have been obtained with this methodology, using alkyl
ligands stabilized by silyl substituents on the β-position.
Lappert originally reported the reaction of 72Db-La and 72Db-Sm with Li{CH(SiMe3)2},
which gave homoleptic tris-alkyls [RE{CH(SiMe3)2}3] (210-RE; RE = La, Sm).51 This method was then extended to the synthesis of mid-
and late-Lns and is now the main methodology used for the preparation
of this class of compounds (Scheme 61).573,574</p><!><p>The coordination
chemistry of tetravalent cerium has attracted a lot interest, especially
because of the use of CAN as a selective oxidizing agent in organic
transformations.575 However, advances in
the coordination chemistry of Ce(IV) have been hampered by the lack
of well-defined and hydrocarbon-soluble starting materials, especially
compared to the ample variety of compounds available for trivalent
REs. A strong electron-donating environment is usually required to
stabilize Ce4+, and for this reason there has been a lot
of progress in developing Ce synthons containing oxygen donors.559 Bradley and co-workers reported the preparation
of Ce(OR)4 (Et, iPr, nPr, nBu) compounds by passing ammonia in an alcoholic solution of dipyridinium
Ce hexachloride, [C5H5N–H]2[CeCl6] ("ammonia method", Scheme 62, A).576−578 Gardeff and co-workers showed that CAN can also be used as a starting
material in a very similar methodology for the preparation of Ce(OMe)4 ("modified ammonia method", Scheme 62, B).579 Alternatively, CAN is dissolved in an alcohol
of choice and then reacted with NaOMe, thus allowing for an easier
control of reaction conditions and stoichiometry ("CAN method", Scheme 62, C); the authors also noted that Ce(OiPr)4 obtained
with the original "ammonia method" is more difficult
to purify, in comparison with the "CAN method" which
allows for quick reaction times and easier purification procedures.
Additionally, the reactions can be carried out in ethereal solvents,
such as THF or DME.579,580 Evans et al. used Gardeff's method to obtain Ce(OtBu)4, together with other mixed nitrate-alkoxide Ce(IV) salts, e.g. Ce(OtBu)n (NO3)4–n (n = 1–4).581 More recently Anwander
and co-workers have used the CAN method to obtain Ce(OCH2tBu)4, which can also be synthesized from alcoholysis
with silylamide 201 and crystallizes as the trimer [{Ce(OCH2tBu)4}3] (211).582 Finally, Schelter and Anwander converted
Ce(IV) silylamide 201 into [Ce(OtBu)4(py)2] (212) via fast alcoholysis
with tBuOH (Scheme 62, D).583</p><p>Ce(IV) alkoxides (Table 7) have been applied as protonolysis reagents for the synthesis
of Ce(IV) complexes, as demonstrated by Kim et al. with the synthesis of 8-coordinate complex [Ce(Harene-TriNOx)2] (213, HareneTriNOx = {[(2-tBuNO)C6H4]2[(2-tBuNOH)C6H4]C6H3}2–) (Scheme 63).560 Gulino et al. attempted to
use Ce(OiPr)4 in salt elimination reactions
with Tl(Cp) or Mg(Cp)2 without success, observing reduction
to Ce(III) instead.561 Nonetheless, when
the tin reagent R3SnCp (R = Me, nBu) was employed,
they were able to isolate Ce(Cp)3(OiPr) (214), though structural authentication was not obtained (Scheme 63).561 A different approach was followed by Evans et al., who reacted mixed Ce(IV) nitrate-alkoxides Ce(OtBu)2(NO3)2 and Ce(OtBu)(NO3)3 with Na(Cp). In the case of the reaction
between Ce(OtBu)2(NO3)2 and two equivalents of Na(Cp), the metallocene Ce(IV) complex Cp(Cp)2(OtBu)2 (215) was obtained
in excellent yields but could not be structurally authenticated (Scheme 63).562 However, when Ce(OtBu)(NO3)3 was reacted with three equivalents of Na(Cp), a mixture
containing 215 and [Cp(Cp)3(OtBu)]
(216) was obtained, and the latter was structurally characterized
(Scheme 63).562 Finally, Gordon and co-workers used the same
methodology to isolate metallocene derivatives [Ce(Cp′)2(OtBu)2] (217) and [Ce(Cp″)2(OtBu)2] (218) (Scheme 63).584</p><!><p>Triflate (OTf, {CF3SO3}−) salts are a very popular choice of starting
materials in RE synthetic
chemistry (Table 8).
These salts are used as more soluble replacements of halides in salt
elimination reactions.9,31 Additionally, triflates are better
leaving groups and can be easily displaced from the metal coordination
sphere.9,31 Similar to RECl3, RE(OTf)3 salts are usually commercially available both in anhydrous
and hydrate form. However, they can also be conveniently made from
the direct reaction of the oxide (RE2O3) with
triflic acid (HOTf), followed by thorough desiccation under reduced
pressure; reports vary from a minimum of 4 h to several days (Scheme 64, A).9,603−606 This method was originally reported
by Massaux and is the most common strategy for obtaining anhydrous
RE(OTf)3 salts.605 Triflate
salts of divalent Sm, Eu, and Yb are obtained via various methodologies. Sm(OTf)2 can be prepared from
direct salt elimination reaction of K(OTf) with SmI2 (Scheme 64, B)607 or by reducing Sm(OTf)3 with Sm metal in the presence of iodine608 or mercury609 (Scheme 64, C). Nief and co-workers obtained
poor results when performing this reaction in THF, whereas they achieved
excellent conversions when using DME, ultimately leading to the isolation
of the DME adduct Sm(OTf)2(DME)2.609 Another method for the preparation of Ln(OTf)2 is the reduction of Ln(OTf)3 with Grignard reagents,
which has also been successfully employed for the preparation of Yb(OTf)2(THF)3 (Scheme 64, D).610 The
DME-solvate analogue, Yb(OTf)2(DME), has also been obtained
from the reaction of HOTf with [Yb{N(SiMe3)2}2(OEt2)2].611</p><!><p>Compounds in
italics have not
been isolated.</p><!><p>Eu(OTf)2 can be obtained by treating EuCO3 with excess
triflic acid, affording monohydrate Eu(OTf)2(H2O) upon drying in vacuo at 90 °C
overnight (Scheme 64, E).606 Alternatively, water-free
Eu(OTf)2 can be obtained with the method described by Krossing
and co-workers, which starts from hydrated Eu(OTf)3(H2O)x.612 Eu(III) triflate is dissolved in triethylorthoformiate and heated
at 50 °C for 1 h, followed by treatment with MeCN and subsequent
crystallization of Eu(OTf)3(MeCN)2 at room temperature.
The MeCN solution of Eu(OTf)3(MeCN)2 is then
passed through a column filled with Zn/Hg amalgam to give the final
product Eu(OTf)2(MeCN)2 (Scheme 64, F).612 Outside the triad of classic Ln2+ ions, only Tm(II) forms
a stable triflate salt, [Tm(OTf)2(DME)2]∞.613 This was reported by
Nocton and co-workers in 2017 and was obtained via reduction of Tm(OTf)3 with KC8 in DME at −35
°C (Scheme 64, G).613 Finally, tetravalent
Ce(OTf)4 can be obtained from the reaction between CAN,
K2CO3, and HOTf.614 However, Berthet and co-workers reported that anhydrous material
cannot be obtained following this methodology because drying above
100 °C results in reduction to Ce(III).615 The authors were able to dry commercial Ce(OTf)4(H2O)x by treating it with trifluoromethanesulfonic
anhydride, TfOTf, followed by drying at 100 °C for 15 h.615</p><p>RE(III) triflates have numerous applications
as salt metathesis
reagents, with a scope very similar to RE halides including numerous
ligands, such as cyclopentadienyls, allyls, and a variety of mono-
and multidentate donors.616−621 Therefore, only some selected applications are listed in this section
to give an overview of the type of coordination chemistry accessible
with these synthons. Because triflates are very good leaving groups,
they have been used for the synthesis of sterically congested RE(CpR)3 systems, such as [La(Cp″)3] (219) and [RE(Cptt)3] (220-RE; RE = La, Ce, Pr),622 whose
synthesis is typically more challenging when using halide precursors.414 For example, 219 can be obtained
from the reaction between LaCl3 and K(Cp″) requiring
first treatment in THF at room temperature for 24 h (to form transient
La(Cp′′)2Cl), followed by reflux in toluene
for an additional 24 h; whereas with La(OTf)3 and Mg(Cp″)2 the reaction only requires stirring at room temperature in
THF overnight.622 Triflate salts have also
been used for the synthesis of heteroleptic open metallocene complexes
(e.g., [Ce(Cpttt)2(OTf)], 221), though in some cases alkali metal salts are occluded
in the products resulting in "ate" complex formation,
as observed by Lappert and co-workers with the isolation of [{Nd(Cp″)2(μ-OTf)2Li}2] (222).623 From the reaction of K2COT with Y(OTf)3, Edelmann and Kilimann obtained dimeric
[{Y(COT)(μ-OTf)(THF)}2] (223) (Scheme 65),624 which can then be functionalized further by
substituting the triflate anion with various ancillary ligands via salt elimination. Moreover, John and co-workers have
shown that triflate salts of various REs (Ce, Nd, Eu, Tb, and Yb)
can react with the potassium salts of substituted allyls (Scheme 65) to form homoleptic
allyl derivatives [RE{C3H3(SiMe3)2-1,3}3(THF)] (224-RE; RE = Ce, Nd,
Tb).625 The same metathetical protocol
is also effective for Yb(OTf)2 (Scheme 65) and the authors were able to obtain the
bis-allyl complexes [Yb{C3H3(SiMe3)-1-R-3̅}2(THF)n] (225R; R = H, SiMe3, SiPh3; n = 1, 2).626 This is noteworthy, as there are very few applications in the literature
of Ln(II) triflates as synthetic precursors in anaerobic synthesis.</p><!><p>Organometallic compounds of the REs are generally
very reactive
because of the high polarization of metal–carbon interactions.
This is a problem typical of σ-bonded organometallic compounds
but is also true for π-bonded organometallics, such as Cp, allyl,
and COT derivatives. Additionally, RE organometallics can easily undergo
β-hydride elimination and intramolecular C–H activation
reactions. All these issues, together with the high electropositive
character and large ionic radii of the REs, make the stabilization
of organometallic species extremely challenging. However, over the
years there has been a lot of progress in the stabilization of relatively
simple RE organometallics which can function as synthons for the preparation
of new compounds, particularly as reagents in protonolysis reactions.
Simple σ-bonded RE alkyls (Me, nBu) and aryls (Ph,
C6F5) have been well-investigated (section 9.1 and Table 9), though such compounds
tend to be extremely reactive and often thermally sensitive. In this
section Ln and RE aryls will not be discussed in detail because an
extensive account of their preparation and application is provided
in section 3 (vide supra). Alkyl complexes can be further stabilized by
substituting the central carbon donor with silyl substituents, and
this strategy has been used very effectively for the preparation of
RE complexes α-silyl ligands {CH2SiMe2R}− (R = Me, Ph) and {CH(SiMe3)2}−; such complexes are now common synthetic
precursors in RE organometallic chemistry (section 9.2 and Table 10). Moreover, RE benzyl (Bn, {CH2Ph}−) complexes constitute another important class
of organometallic starting materials (section 9.3 and Table 11); benzyl ligands are nominally alkyl-type
donors, though they ligate the metal center in a multihapto fashion.
Finally, organoaluminates, Ln(AlR4)2 and RE(AlR4)3 (R = Me, Et), are special types of organometallic
reagents which can be effectively considered as masked alkyls stabilized
by the formation of an adduct with AlR3 (section 9.4 and Table 12). Most of these synthetic precursors have
been the subject of a recent comprehensive review by Zimmermann and
Anwander;11 therefore, this section will
aim to provide a broad overview of these reagents, with some highlights
on the emergence of new reagents and new applications.</p><!><p>Compounds in italics have not
been structurally authenticated.</p><p>Compounds in
italics have not
been structurally authenticated.</p><p>Compounds in
italics have not
been structurally authenticated.</p><p>Compounds in
italics have not
been structurally authenticated.</p><!><p>Salt metathesis
reactivity is the most common methodology employed
for the synthesis of binary alkyl and aryl complexes (Scheme 66, A), though
there are some notable exceptions such as (1) RT reactions with organomercurials
and organobismuth reagents for the synthesis of aryl complexes (Scheme 66, B; vide supra, section 3.4),12 (2) donor
cleavage reactions from organoaluminate precursors (Scheme 66, C),631 and (3) transmetalation reactions between amide
precursors and MeLi (Scheme 66, D).504 Saran and
co-workers reported the reaction of MeLi and PhLi with RECl3 (Sc, Y, La, and Pr) in 1970.632 In the
case of reactivity with PhLi, the authors identified Sc(Ph)3 and Y(Ph)3via IR spectroscopy and elemental
analysis, though with La and Pr they identified the formation of "ate"
complex LiLa(Ph)4 from analogous reactivity. However, the
formulas of the methyl derivatives "RE(Me)3"
(226-RE; RE = Sc, Y, La) obtained from the
reactions between RECl3 and MeLi could not be unequivocally
identified.632 Neutral methyl complexes
have been obtained with Ho, Lu, and Y by Anwander and co-workers,
isolated as polymeric [RE(Me)3]n (226-RE; RE = Sc,633 Y,631 Ho,634 Lu631) from the donor-cleavage reaction of aluminate
precursors (Scheme 66; vide infra, section 9.4). Schumann reacted RECl3 with
six equivalents of MeLi in the presence of TMEDA and obtained a series
of hexamethyl "ate" complexes of formula [RE(μ-Me)6Li3(TMEDA)3] (227-RE; RE
= Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu).635−638 Furthermore, Okuda and co-workers carried out an in-depth study
on the stability and reactivity of 227-RE and were also
able to obtain the pentametallic complexes [RE2(μ-Me)6Li3(Et2O)n(THF)m] (RE = Sc, n =
3, m = 2; RE = Y, Tb, n = 2, m = 3).639 Anwander and co-workers
recently developed analogous chemistry with Ce, and in addition they
extend this approach to the preparation of butyl derivatives [RE(μ-nBu)4Li3(THF)4] (228-RE; RE = Sc, Y, La, Ce, Lu) and [RE(μ-nBu)4Li2(TMEDA)2] (229-RE; RE = Ce,
Lu).640 Finally, Anwander and co-workers
reported the synthesis and reactivity of the neutral methyl complex
[Yb(Me)2]n (230), which is the only example of methyl derivative with a divalent
Ln; 230 was obtained via transmetalation
reaction between bis-silylamide precursors and MeLi.504</p><p>RE alkyl complexes (Table 9) are excellent protonolysis
reagents owing to the very favorable
pKa (vide supra, Table 2) and react promptly
with Brønsted acids. Anwander and co-workers treated 226-Y with HN(SiMe3)2, HN(SiHMe2)2, and HOCHtBu2 (Scheme 67), obtaining the tris-amide
complexes 185-Y and 186-Y and alkoxide derivative
Y(OCHtBu2)3 (231).542,631 Interestingly, reactivity of 226-RE with HN(SiMe3)(Dipp) leads to the deprotonation of one methyl ligand and
formation of methylidene bridged cluster [RE3{N(SiMe3)(Dipp)}3(μ2-Me)3(μ3-Me)(THF)3] (232-RE; RE = Y, Ho, Lu)
(Scheme 67).634 In a similar fashion, Berger et al. investigated the reactivity of 229-Ce with HOCHtBu2, which led to the isolation of the cluster
[Ce2Li3(OCH2tBu)9(HOCH2tBu)2(THF)].640 Kramer et al. reacted the
THF adducts of hexamethyl complexes 227-RE with five
equivalents [Et3NH][BPh4] (Scheme 68), affording cationic complexes
[RE(Me)(THF)n][BPh4]2 (233-RE; RE = Sc, Tm, n = 5; RE =
Y, Gd–Er, Yb, Lu, n = 6).639 As would be expected, 230 is also an excellent
protonolysis reagent and reacts smoothly with HTptBu,Me (Scheme 68) to give
the terminal methyl complex [Yb(TptBu,Me)(Me)(THF)] (234).504</p><!><p>RE(III) complexes
with α-silyl-alkyl ligands (Table 10) were first reported by Lappert and Pearce;
in their seminal report, the authors obtained [RE(CH2SiMe3)3(THF)2] (235-RE; RE =
Sc, Y) from the reaction of RECl3(THF)x with Li{CH2SiMe3} in a mixture of hexane
and ether (Scheme 69, A).641 This salt elimination
method has since been applied to obtain analogous complexes with most
of the trivalent REs,642−645 with the exception of the larger Lns La–Nd. The larger {CH(SiMe3)2}− ligand was originally introduced
in RE chemistry by Lappert and Barker in 1974, who obtained Sc and
Y complexes [RE{CH(SiMe3)2}3] (210-RE) via salt elimination reactions, though
no structural authentication was provided (Scheme 69, A).646 However, it has been amply demonstrated that salt elimination reactions
can often lead to the formation of "ate" complexes with
these systems.647 Nonetheless, salt occlusion
can be avoided by following the transmetalation route devised by Lappert
and co-workers, in which aryloxide complexes [RE(ODb)3]
(72Db-RE) are treated
with Li{CH(SiMe3)2} to give neutral tris-alkyl complexes 210-RE (Scheme 69, B).51 It is important to note that these ligands are
not ideal for stabilizing neutral bis-alkyl Ln(II)
complexes; Lappert and co-workers obtained [Yb{CH(SiMe3)2}2(OEt2)2] (236) via both analogous salt metathesis and transmetalation
methodologies, but no structural information has been reported to
validate these results (Scheme 69, C and D).299 It is also noteworthy that other ligand variations have
been introduced, particularly the very sterically demanding {C(SiMe3)3}−, which has enabled the stabilization
and structural authentication of neutral homoleptic bis-alkyl complexes [Ln{C(SiMe3)3}3]
(Ln = Sm, Eu, Yb), which were obtained via salt elimination
reactions.114</p><p>The applications of both 235-RE and 210-RE as synthetic precursors in protonolysis reactions are numerous and
well-established.11235-RE is a particularly useful reagent owing to the production of volatile
SiMe4 as byproduct upon treatment with Brønsted acids.
This strategy has been applied to produce complexes with a plethora
of supporting ligands, and their utility as synthetic precursors has
been extensively reviewed before.11 Therefore,
only some representative examples are discussed herein. Okuda and
co-workers showed that by treating 235-RE (RE = Sc, Y,
Lu) with [Me2PhNH][B(C6F5)4] or [Et3NH][BPh4] the cationic complexes [RE(CH2SiMe3)2(THF)n][BR4] (237BPh4-RE: R = BPh4, RE = Sc, Y, Lu; 237BArF-RE: R = C6F5, RE = Y, Lu; n = 3, 4) are produced (Scheme 70).648,649 When at least two equivalents of Brønsted acid are employed,
the dicationic complexes [RE(CH2SiMe3)(THF)n][BPh4]2 (238-RE: RE = Y, n = 5; RE = Lu, n = 4)
are obtained (Scheme 70).648,649 Interestingly, 235-RE react
also with Lewis acids (e.g., BPh3, B(C6F5)3, AlR3) to give analogous
cationic complexes upon abstraction of one or more alkyl ligands.648,649 The efficacy of 235-RE as a protonolysis reagent has
also been demonstrated by Li et al. with the reaction
of 235-Sc with a series of substituted cyclopentadienes
(HCp, HCpMe, HCptet, HCp*, and HCp*′)
to produce half-sandwich dialkyl complexes [Sc(CpR)(CH2SiMe3)2(THF)] [CpR = Cp (239), CpMe (240), Cptet (241), Cp* (242), Cp*′ (243) - Cptet = {C5HMe4}−, Cp*′ = {C5Me4SiMe3}−] (Scheme 70).650 This approach can be extended
to different REs, as demonstrated by Hou and co-workers with the preparation
of 244-RE (RE = Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).651 The use of 210-RE in similar alkane
elimination reactions has also been explored, but these applications
are more limited compared to 235-RE owing to the kinetic
inertia provided by the more sterically demanding {CH(SiMe3)2}− ligand.11 Nonetheless, an important application of these reagents has been
demonstrated by Kempe with the preparation of heterobimetallic RE-TM
species via alkane elimination reactions between
RE alkyls and TM hydride precursors.652−654210-RE (RE = La, Sm, and Lu) reacts with [RE(Cp)2(H)] in benzene
at room temperature to give the tetrametallic complexes [RE{Re(Cp)2}3] (244-RE; RE = La, Sm, Lu);655 these species are extremely sensitive, which
according to the authors is the reason behind the low yields of these
reactions. The choice of reagent and solvent conditions in Kempe's
methodology is crucial, as the presence of THF leads to undesired
side-reactivity. Because of this, only unsolvated 210-RE can be employed.</p><!><p>A range of benzyl (Bn, {CH2Ar}−) complexes have been reported with
trivalent REs, which comprise (1) unsubstituted benzyls, (2) substituted
benzyls, and (3) bidentate benzyls (Table 11). Compared to other hydrocarbyls, benzyls
offer the advantage of acting as multihapto ligands and can delocalize
the negative charge from the benzylic position into the phenyl ring;
because of this they impart greater stability to resulting RE complexes
while still preserving the basicity (or nucleophilicity) of alkyl
fragments and are therefore very useful synthetic precursors for protonolysis
reactivity. Ln(Bn)2 and RE(Bn)3 are obtained via salt elimination reactions between LnI2 or
REX3 and benzylpotassium salts, though occasionally Li
salts are also employed. That was indeed the case when Mazer first
reported in 1978 the reaction between ScCl3 and three equivalents
of Li(BnNMe2) (BnNMe2 = {CH2[C6H4(NMe2)-2]}−), leading
to the formation of tris-benzyl complex Sc(BnNMe2)3 (245-Sc) (Scheme 71, A).670 Workup of the reaction following Mazer's
method can be frustrated by the solubility of the byproduct (LiCl),
so more convenient routes entail the use of benzylpotassium salts.
Harder671−673 and then Hou674 obtained benzyl complexes 245-RE (RE = Y, La, Pr, Nd,
Sm, Gd, Dy, Ho, Lu) by employing K(BnNMe2) with either
RECl3 (RE = Y, Nd, Gd, Sm, Dy, Ho, Yb, Lu—both anhydrous
and solvated)671−673 or [RE(Br)3(THF)4]
(RE = La, Pr, Nd) (Scheme 71, A).674 RE complexes
with monodentate benzyl ligands were first reported by Hessen and
co-workers, who prepared [La(Bn)3(THF)3] (246-La) and [La(BnMe)3(THF)3] (247; BnMe = {CH2(C6H4Me-4)}−) from [La(Br)3(THF)4] and KBnR in THF at 0 °C (Scheme 71, B).675 In a similar vein, Harder reacted [La(Br)3(THF)4] with K(BntBu) (BntBu = {CH2(C6H4tBu-4)}−) in THF at −50 °C to give [La(BntBu)3(THF)3] (248).676 RECl3 precursors are also a good match with
benzylpotassium for the preparation of 246-REvia salt metathesis, as demonstrated by Harder,676 Roesky,677 and Diaconescu.339 Interestingly, Diaconescu and co-workers observed
that 246-Ho and 246-Er could be obtained
in good yields starting from corresponding RECl3; however,
bromide salts were required to obtain benzyl complexes of larger metals, i.e., Nd and Gd.339 Liddle and
co-workers reported the convenient synthesis of 246-RE (RE = Y, La–Sm, Gd, Dy, Er) from [RE(I)3(THF)n] (138-RE) and K(Bn) in THF
at 0 °C (Scheme 71, C).678 It is noteworthy
that attempts to obtain a Yb(III) analogue led to the isolation of
the mixed-valent Yb(II)/Yb(III) complex [Yb(Bn)(THF)5][Yb(Bn)4(THF)2].678 Additionally,
Liddle and co-workers reported the heteroleptic benzyl complexes [RE(Bn)2(I)(THF)3] (249-RE; RE = Y, Er) from
the reaction of 138-Y and 138-Er with two
equivalents of K(Bn) (Scheme 71, C).679 Finally,
another class of benzyl complexes has been reported by Schmidt and
Behrle using N,N-dimethylbenzylamine
(DBA) metalated in the α-position.516 Analogously to the other methodologies employed for the synthesis
of 245-RE and 246-RE, Schmidt and Behrle
reacted three equivalents of K(DBA) with RECl3 (RE = Y,
La, Ce, Nd, Sm, Gd) in THF at −50 °C (Scheme 71, D), obtaining
[RE(DBA)3] (250-RE; RE = Y, La, Ce, Nd, Sm,
Gd).516 X-ray studies of 250-RE reveal that DBA ligands coordinate the metal centers in an η4-fashion.516</p><p>Benzyl derivatives
of divalent Lns have been reported by Harder
and Anwander. Harder and co-workers obtained Sm, Eu, and Yb benzyls
[Ln{CH(C6H4NMe2)(SiMe3)}2(THF)2] (251-Ln; Ln = Sm, Eu,
Yb) by reacting LnI2 with K{CH(C6H4NMe2)(SiMe3)} (Scheme 72).680 This bidentate
benzyl ligand imparts greater stability to the complexes owing to
the stabilization of the carbanion by the α-silyl substituent.
Recently, Anwander and co-workers have reported the synthesis of [Ln(Bn)2]n (252-Ln; Ln =
Sm, Eu, Yb) from [Ln(I)2(THF)2] and two equivalents
of K(Bn) (Scheme 72).681 The amorphous 252-Ln can be recrystallyzed in the presence of donors to give discrete
molecular species, i.e., [Eu4(Bn)8(THF)2] (253), [Sm2(Bn)4(THF)(THP)2]∞ (254), [Yb(Bn)2(THP)4] (252-Yb·4THP), and [Yb(Bn)2(DME)2] (252-Yb·2DME).681</p><p>RE benzyl complexes have been used as synthetic precursors in protonolysis
reactions with a range of substrates. Hessen and co-workers treated 246 and 247-La with either one or two equivalents
of [MePh2NH][BPh4] and obtained the corresponding
monocationic or dicationic complex, i.e., [La(Bn)2(THF)4][BPh4] (255), [La(BnMe)2(THF)4][BPh4], [La(Bn)(THF)6][BPh4]2 (256), and [La(BnMe)(THF)6][BPh4]2 (Scheme 73).675 These starting materials can also be used to
generate heteroleptic alkyl complexes with bidentate nitrogen donors,
such as amidinates (257),675 guanidinates (258-RE; RE = Y, La, Dy, Lu),682,683 and ferrocenyl bis-amides (259-RE;
RE = Nd, Gd, Ho, Er) (Scheme 73).339</p><p>Furthermore, RE benzyls can
be employed to generate new organometallic
complexes. Harder first showed that 245-RE can deprotonate
HCpBIG (CpBIG = {C5Ar5}−; Ar = C6H4R-4; R = Et, iPr, nBu) to afford the heteroleptic Cp derivatives
[RE(CpBIG)(BnNMe2)2] (260-RE; RE = Y, Nd, Dy, Tm).672,684 Interestingly, with 245-Sm and 245-Yb the metals are reduced to their
divalent state and the homoleptic metallocenes [RE(CpBIG)2] (261-RE; RE = Sm, Yb) are formed, with
concomitant formation of byproduct 1,2-di(2-Me2N-phenyl)ethane
as a result of the coupling of two benzyl radicals (Scheme 74).672,684 In addition to this, Liddle and co-workers used 246-RE and 249-RE to perform a double-deprotonation of H2–BIPM and generate alkylidene complexes [RE(BIPM)(Bn)]
(262-RE; RE = Y, Dy, Er),678,685 [RE(BIPM)(H-BIPM)]
(263-RE; RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy),678,686,687 and [RE(BIPM)(I)(THF)2] (264-RE; RE = Y, Er) (Scheme 74).679,688 Finally, Schmidt and
Behrle screened a series of protic substrates (i.e., HODb, H2NDipp, HN(SiMe3)2) with
the DBA complexes 250-Y and 250-La and obtained
clean conversion to [RE(ODb)3] (72Db-RE; RE = Y, La), RE(HNDipp)3, and [RE{N(SiMe3)2}3] (185-RE; RE = Y, La).516</p><p>Applications of Ln(II) benzyl complexes are more scarce
compared
to their trivalent counterparts. Harder obtained Sm(II) metallocene 261-Sm from the direct deprotonation of HCpBIG with 251-Sm (Scheme 75).689 Moreover, Anwander and co-workers
reacted 252-Eu and 252-Yb with H2NDipp (Scheme 75),
obtaining a cubane cluster with bridging imido ligands, [{Ln(η3-NDipp)(THF)}4] (265-Ln; Ln = Eu,
Yb).681</p><!><p>Since the first
report of [Y(AlMe4)3] (266-Y) and
[Nd(AlMe4)3] (266-Nd) by Evans et al. in 1995,691 RE tetraalkylaluminates
have emerged as a very interesting class of synthetic precursors in
RE and f-element chemistry.11 One of the
intriguing aspects of organoaluminates is their dual chemical behavior:
on the one hand they can be deemed "masked-alkyl" complexes
formed as adducts of AlR3, which can be used for protonolysis
reactivity; on the other hand they could be regarded as ionic complexes
of the {AlR4}− ligand, which can act
as a pseudo-halide in salt elimination reactions.
Their preparation and chemical properties were incorporated in the
detailed account on RE alkyl chemistry by Anwander and Zimmermann
in 2010;11 therefore, this section aims
to give a broad overview of their preparation and synthetic applications.</p><p>Evans et al. prepared [RE(AlMe4)3] (266-RE; RE = Y, Nd) by treating amide precursor
RE(NMe2)3(LiCl)3 with excess AlMe3 (Scheme 76, A).691 This synthetic approach
has since been optimized by generating the amide precursors in situ from RECl3(THF)x and Li(NMe2), and the methodology has been extended
to most of the REs.518,692,693 The overall reaction can be viewed as an amide-methyl metathesis
generating Me2NAlMe2 and putative methyl complex
"RE(Me)3", with the latter converted into 266-RE upon adduct formation with excess AlMe3.11 The Sc analogue 266-Sc cannot be
obtained via amide-methyl exchange (Scheme 76, B); nonetheless,
treatment of the hexamethyl Sc complex Sc(μ-Me)6Li3(THF)1.2639 with more
than six equivalents of AlMe3 affords [Sc(AlMe4)3]·Al2Me6 (266-Sc·Al2Me6). Organoaluminates obtained with
these methods can cocrystallize with Al2Me6,
which can be removed upon recrystallization.691,692,694 Most of these species are also
relatively thermally robust, especially if compared to alkyl derivatives,
and can also be sublimed. However, an important aspect of the chemistry
of 266-RE is the ability of polar solvents to trigger
their degradation via donor-induced cleavage of the
aluminate ligand (vide infra).11,695 It is possible to obtain 266-RE also by exchanging
other ligands with AlMe3, such as different amides and
alkoxides, though the purification of the desired organoaluminates
is complicated by the low volatility of the Me2Al(L) byproducts
(L = N(SiHMe2)2, OCH2tBu).692 Additionally, synthesis of 266-Yvia direct salt elimination reaction
between YCl3 and Li(AlMe4) has also been reported
(Scheme 74, C).692 Alternatively, homoleptic
trimethyl precursors [Y(Me)3]n (226-Y) and [Lu(Me)3]n (226-Lu) can be converted into the corresponding
organoaluminates by adduct formation with AlMe3 (Scheme 74, D).631,692</p><p>Amide-alkyl or alkoxide-alkyl exchange reactions can be used to
access divalent Ln organoaluminates [Ln(AlMe4)2]n (267-Ln; Ln = Sm, Yb)
and [Ln(AlEt4)2]n (268-Ln; Ln = Sm, Yb);556,696 these are
achieved by reacting bis-silylamide precursors 185-Sm and 185-Yb (Scheme 77, A) or bis-alkoxides Ln(ODipp)2(THF)x with AlR3 (R = Me, Et) (Scheme 77, B).556,601,696 Interestingly, 267-Ln and 268-Ln do not degrade in the presence of donors
or polar solvents, and the solid-state structures of THF adducts [Ln(AlEt4)2(THF)2] (269-Ln·2THF;
Ln = Sm, Yb) have also been reported.696267-Yb can also be obtained via thermally
induced self-reduction of 266-Yb, with concomitant formation
of Al2Me6 and C2H6 (Scheme 77, C).518 Finally, Eu(III) tris-amide 186-Eu·2THF can be converted into [Eu(AlR4)2]n (267-Eu – R = Me; 268-Eu – R = Et) upon treatment
with an excess of AlR3 (Scheme 77, D).518</p><p>RE organoaluminates (Table 12) are excellent
synthetic precursors in protonolysis
reactivity with a range of substrates. Anwander demonstrated this
with the preparation of Cp derivatives [RE(CpR)(AlMe4)2] (272-RE; CpR = Cp*,
RE = Y, La, Nd, and Lu; 273-RE; CpR = Cp*′;
RE = Y, La, Nd, Sm, Gd, and Lu) via deprotonation
of the corresponding cyclopentadiene in hexane or pentane under mild
conditions (Scheme 78).697,698 Lappert and co-workers first showed that
heteroleptic Cp-alkylaluminate complexes can undergo donor-induced
cleavage of AlMe3 upon treatment with a Lewis base.695 In a similar vein, 272-RE can
be converted to the "unmasked" alkyl analogues [RE(Cp*)(Me)2]3 (RE = Y, Lu) by addition of THF; the reaction
is fully reversible, and 272-RE is reobtained via donor addition reaction with AlMe3 at −35
°C.698 Furthermore, the difference
in the bonding character between divalent Ln and trivalent RE aluminates
is exemplified by their divergent reactivity with cyclopentadienyls.
When divalent 267-Yb is reacted with HCp*, the {AlMe4}− ligand acts as a weakly coordinating
anion and the separated ion pair complex [Yb(Cp*)(THF)4][AlMe4] (274) is isolated from the reaction.556 Interestingly, complex 274 is
obtained even when two equivalents of HCp* are used in the reaction,
and the authors detected unreacted pentamethylcyclopentadiene in the
reaction mixture, together with AlMe3(THF) and traces of
Cp*AlMe2.556 Zimmermann et al. have also shown that pro-ligand C6NH3[CH2NH(Dipp)]2-2,6 can be deprotonated
by 266-RE (RE = Y, La, Lu), yielding heteroleptic aluminate
complex [RE(BDPPpyr)(AlMe4)] (275; BDPPpyr
= {C6NH3[CH2N(Dipp)]2-2,6)}−) (Scheme 78).699</p><p>Anwander and co-workers have
also demonstrated that RE organoaluminates
can be employed for salt elimination reactivity with various ligand
transfer reagents.11 Piano-stool derivative 272-Sc is obtained from the 1:1 reaction between K(Cp*) and 266-Sc, while the metallocene complex [Sc(Cp*)2(AlMe4)] (276) is obtained when two equivalents
of K(Cp*) are used (Scheme 79).633 When K(TptBu,Me) is reacted with 266-Y, K(AlMe3)4 is eliminated together with one equivalent of AlMe3 and
the mixed alkyaluminate/alkyl complex [Y(TptBu,Me)(AlMe4)(Me)] (277) is isolated (Scheme 79),701 which can
also be obtained via protonolysis reactivity between
HTptBu,Me and 266-Y.700 However, when 266-La is used as a starting material,
the reaction produces the "Tebbe-like" methylidene complex
[La(TptBu,Me)(μ3-CH2)(AlMe3)2] (278) (Scheme 79).701 Salt elimination
protocols were also employed by Le Roux et al. to
produce the piano-stool phospholyl complex [RE{C4PMe2(SiMe3)2}(AlMe4)2] (279-RE; RE = La, Nd) (Scheme 79).702 Another
example of the interesting reactivity of RE organoaluminates has been
shown recently by Barisic et al. with the salt metathesis
reaction between 266-RE (RE = Y, Lu) and two equivalents
of K(2,4-dtbp) (2,4-dtbp = {CH2C(tBu)CHC(tBu)CH2}−). At room temperature
the reaction affords the aluminabenzene complexes [RE(2,4-dtbp)(C5H3AlMe-1-tBu2-3,5)] (280-RE; RE = Y, Lu) (Scheme 75).703 However, when the
same reaction is carried out at −40 °C, the pseudo-metallocenes [RE(2,4-dtbp)2(AlMe4)] (281-RE; RE = Y, Lu) are obtained instead, which can undergo
thermal decomposition to give aluminabenzene derivatives 280-RE.703</p><!><p>The landscape of RE and Ln synthetic chemistry has grown immensely
since the first adventurous steps taken more than half a century ago
by various pioneers of the discipline. This impressive growth has
been supported by the enormous expansion of the RE synthetic toolbox
and the opening of a myriad of synthetic possibilities, the potential
of which is still far from being fulfilled. Nonetheless, there is
still progress to be made in the development of synthetic precursors
and methodologies applicable to divalent Lns outside the Sm, Eu, and
Yb triad, which is exemplified by the relatively small number of synthetic
applications compared to the trivalent counterparts.42 Similarly, the landscape of molecular Ln(IV) chemistry—historically
limited to Ce(IV)—has been recently expanded with the stabilization
of the first molecular Pr(IV) and Tb(IV) species;708−713 as a result, new advances will likely emerge for the preparation
of specific starting materials and methodologies applicable to tetravalent
Ln chemistry. Recently, Evans and Daly have demonstrated that mechanochemical
synthesis can be used for the preparation of RE derivatives using
salt elimination methodologies, thus offering a viable alternative
to standard solution methods.714−716 Additionally, direct activation
of metals is a synthetic technique that is still used by only a few
groups around the world, largely because of the historic involvement
of toxic Hg and Tl reagents; however, these methods have now been
extended to more benign metals (Bi and Ag),175 which should encourage research teams to incorporate these methods
into their synthetic repertoire. Many of the limitations that historically
frustrated the progress of RE synthetic chemistry are gradually being
challenged and overcome; an example of this is the successful stabilization
and identification of the once elusive "RE(Me)3"
species by Anwander and co-workers,504,631,633 though this still has not been achieved with the
larger Lns.717 Finally, several of the
synthetic precursors presented in this work have emerged over the
past decade; therefore, because of these recent discoveries, there
will certainly be many new and exciting synthetic avenues that will
be opened-up in the years to come.</p><!><p>The author declares no
competing financial interest.</p><p>Fabrizio Ortu is from Sassari (Sardinia,
IT) and obtained his Ph.D.
from the University of Nottingham in 2014, where he studied s-block
organometallic chemistry under the supervision of Prof. Deborah Kays.
After his Ph.D. studies he moved to the University of Manchester to
work with Prof. David Mills as a postdoctoral researcher, covering
a range of topics in rare earth and actinide synthetic chemistry.
In June 2019 he was appointed to a lectureship at the University of
Leicester. His research interests lie in the coordination and organometallic
chemistry of alkaline earth and rare earth metals.</p>