Bipyridine complexes of E<sup>3+</sup>(E = P, As, Sb, Bi): strong Lewis acids, sources of E(OTf)<sub>3</sub>and synthons for E<sup>I</sup>and E<sup>V</sup>cations
Triflate salts of trications [(bipy) 2 E] 3+ ([6E][OTf] 3 ) and [(tbbipy) 2 E] 3+ ([6 0 E][OTf] 3 ) (bipy ¼ 2,2 0 -bipyridine, tbbipy ¼ 4,4 0 -di-t butyl-2,2 0 -bipyridine; E ¼ P, As, Sb, Bi) have been synthesized and comprehensively characterized. The unique molecular and electronic structures of this new class of complexes involving pnictogen Lewis acids has been assessed in the solid, solution and gas phases to reveal systematic variations in metric parameters, ligand lability and charge concentration. While the Lewis acidity of E 3+ has the trend E ¼ Bi < Sb < As < P as determined by gas-phase calculations and 1 H NMR spectroscopy, the Lewis acidity of [6E] 3+ has the trend E ¼ P < As < Sb < Bi according to gas-phase calculations. Derivatives of [6As) are latent sources of E(OTf) 3 as demonstrated by their reactions with dmap, which give the corresponding derivatives of [(dmap) 3 E][OTf] 3 . The highly oxidizing nature of P(OTf) 3 and As(OTf) 3 is evidenced in reactions of [6 0 E][OTf] 3 (E ¼ P, As) with phosphines, which give E Icontaining monocations [(R 3 P) 2 E] 1+ and oxidatively coupled dications [R 3 PPR 3 ] 2+ , illustrating new P-P and P-As bond forming strategies. Cations [6 0 E] 3+ (E ¼ P, As) are C-H bond activating agents that dehydrogenate 1,4-cyclohexadiene, with higher activity observed for E ¼ P. Combinations of [6 0 E] 3+ and t Bu 3 P activate H 2 and D 2 under mild conditions, evidencing frustrated Lewis pair activity. Oxidation of [6 0 P][OTf] 3 with SO 2 Cl 2 gives [(tbbipy) 2 PCl 2 ][OTf] 3 , containing a P V -trication, but there is no evidence of the analogous reaction with [6 0 As][OTf] 3 . The observations highlight new directions in the chemistry of highly charged cations and reveal a rich reactivity for p-block triflates E(OTf) 3 , which can be accessed through derivatives of [6E][OTf] 3 and [6 0 E][OTf] 3 .
Introduction<!>Results and discussion<!>Gas-phase structures, bonding, and Lewis acidity of [6E] 3+<!>NMR characterization of [6E][OTf]
<p>Numerous monocationic and dicationic p-block element centered complexes are known, 1 but structurally authenticated salts containing trications are rare, because the charge concentration oen results in oxidation of the ligands. For example, the trisphosphine-antimony trication 1 (Chart 1) undergoes reductive elimination of a diphosphonium dication below room temperature. 2,3 In this context, the pyridine ligands in 2, 4 the tris-pyrazole based ligands in 3a,b, 5 the carbene based ligands in 4a-c, 6 and the crown ether ligands in 5E 7 illustrate types of oxidatively resistant donors that enable studies of such reactive coordination centers.</p><p>As a prototypical ligand for transition metal acceptors in a variety of oxidation states, 2,2 0 -bipyridine (bipy) offers relatively high basicity and oxidative resistance, which we have now exploited to enable a comprehensive study of a series of compounds of generic formulae [(bipy) 2 E][OTf] 3 , [6E][OTf] 3 , and [(tbbipy) 2 E][OTf] 3 , [6 0 E][OTf] 3 (E ¼ P, As, Sb, Bi; tbbipy ¼ 4,4 0di-t butyl-2,2 0 -bipyridine). 8 The compounds are characterized as salts containing trications that represent bipy or tbbipy complexes of E 3+ . A diverse reactivity is evident for these complexes, including ligand exchange, which provides access to the dmap complexes [7E][OTf] 3 (E ¼ P, As). Element triates, E(OTf) n , are widely employed as Lewis acids, 9 oxidizing agents [10][11][12] and latent sources of E n+ . [13][14][15] Interesting examples of small-molecule activation and catalysis effected by p-block element triate salts are also well documented, [16][17][18] including those involving Sb(OTf) 3 19 and Bi(OTf) 3 , 20 which can be isolated on preparative scales, 21,22 enabling their ubiquitous use. Lighter congeners featuring the more electronegative P and As centers have not yet been reported, precluding assessment of their reactivity. In this context, we demonstrate that derivatives of</p><!><p>Complexes of E(OTf) 3 with bipy or tbbipy were prepared according to Scheme 1 and isolated as crystalline solids. While all derivatives decompose to give the protonated ligand on exposure to ambient atmosphere, they can be stored indenitely under inert atmosphere at room temperature. Derivatives of [6E][OTf] 3 are less soluble in MeCN and CH 2 Cl 2 than derivatives of [6 0 E][OTf] 3 , due to the presence of four t butyl groups in the latter. Interestingly, while the phosphorus derivatives are yellow due to a HOMO-LUMO transition centered around 300 nm, 23  The bond angles for a given E in [6E][OTf] 3 and [6 0 E][OTf] 3 are expected to be very similar because the divergent planes dened by N 1 -E-N 2 and N 3 -E-N 4 result in the t Bu groups facing away from each other, so that steric repulsion between them is minimal. For example, the quaternary carbon centers in the t Bu groups that are para to N 2 and N 4 in [6 0 P][OTf] 3 are separated by nearly 10 Å, and the bond angles within the disphenoidal frames of [6P] 3+ and [6 0 P] 3+ are essentially identical, as they are for [6Sb] 3+ and [6 0 Sb] 3+ . The inductive effect of a t Bu group para to the nitrogen atoms is expected to make the tbbipy ligands more basic compared to bipy and lead to stronger E-N interactions. Consistently, the 31  The trans congured E-N bonds in all derivatives are ca. 0.1 Å longer than the E-N bonds in equatorial positions due to the mutual trans inuence of the E-N 1 and E-N 3 interactions (Fig. 2). For the equatorial positions, the trend in E-N bond lengths, [6P] 3+ z [6 0 P] 3+ < [6 0 As] 3+ < [6Sb] 3+ z [6 0 Sb] 3+ < [6Bi] 3+ , reects the sum of the respective covalent radii ( P r,cov ). The trend in the axial (N 1 -E-N 3 ) and equatorial interligand angles 3+ , is consistent and is attributed to the extent of triate Table 1 Selected bond lengths (Å) and angles ( ) in the solid-state structures of and [6Bi][OTf] 3 , calculated (gas phase, PBE0/def2-TZVP) values for cations [6As] 3+ and [6 0 Bi] 3+ , and sums of covalent ( P r,cov ) 25 and van der Waals ( P r,vdW ) 26,27 radii for selected atom pairs anion association, which is greater for an atom with a larger atomic radius. The chelation angles (</p><p>as the N-E-N interaction subtends smaller angles for longer E-N bonds.</p><p>Infrared spectra for derivatives of [6 0 E][OTf] 3 enable quanti-cation of the interion coordination in the solid state. The symmetric SO 3 stretch, n s (SO 3 ), in several triate salts has been studied previously and appears as a characteristically sharp absorbance in the 1020-1050 cm À1 range. 28 The portions of the infrared spectra of [6 0 E][OTf] 3 shown in Fig. 3 illustrate a trend in n(SO 3 ) of E ¼ P > As > Sb > Bi, which we attribute to the degree of charge transfer from the anion to the pnictogen centre, which inuences the S-O bond order. The broader bands for the heavier homologues are attributed to the loss of C 3v symmetry due to cation-anion interaction. The spectra for ligand-free Sb(OTf) 3 and Bi(OTf) 3 , for which extensive Sb-O and Bi-O interactions are predicted, exhibit similarly broad S-O stretching bands (see Fig. S2, ESI †) that are shied to lower frequencies (958 cm À1 and 1000 cm À1 , respectively) than the analogous value in [Bu 4 N][OTf] (1032 cm À1 ), 28 which features a weakly coordinating cation, and the calculated value for an isolated triate anion in the gas phase (1035 cm À1 , PBE0/aug-cc-pVTZ).</p><!><p>Optimized structures for [6E] 3+ in the gas phase adopt a disphenoidal C 2 symmetry for all derivatives, consistent with the observed solid-state structures. Selected calculated bond lengths and angles are given in Table 2. As observed experimentally in the solid state, the computed structures reveal axial E-N distances that are longer by ca. 0.1 Å than the equatorial E-N distances and average E-N bond distances that are primarily determined by the respective covalent radii. For pnictogen centers with a larger covalent radius, the bite angle N 1/3 -E-N 2/4 and the equatorial and the axial interligand angles are smaller. The equatorial interligand angle in the solid-state structures of Sb and Bi derivatives are signicantly smaller than in the anion-free gas-phase structures of [6Sb] 3+ and [6Bi] 3+ , suggesting that the steric pressure of the interion contacts present in the solid state inuence this angle. By comparison the axial interligand angle N 1 -E-N 3 in the experimental and calculated structures are essentially identical, implying minimal distortion due to interion contacts.</p><p>Natural Bond Orbital (NBO) partial charges and Wiberg Bond Indices (WBI) for the gas-phase cations are listed in Table 3, evidencing a high positive charge for the central pnictogen centre, which is greater for heavier elements, as expected on the basis of relative electronegativities. Consistently, the WBIs for the N-E interaction has the trend P > As > Sb > Bi, implying a more ionic E-N bond for the heavier pnictogens. For a given derivative, the WBI value for the axial E-N interactions is smaller than the equatorial interactions, indicating less effective bonding along the N 1 -E-N 3 axis than in the N 2 -E-N 4 plane. Noting that the equatorial interligand angles range from 95 to 100 in all cases, we surmise that of the three mutually perpendicular p-orbitals that serve as acceptor orbitals at E 3+ , two are engaged by N 2 and N 4 in the equatorial plane, while the third accommodates two strained trans interactions involving N 1 and N 3 .</p><p>To assess the relative Lewis acidities of E 3+ , we have calculated the enthalpies for the heterolytic removal of both bipy ligands from [6E] 3+ . Scheme 2a represents removal of the ligands and relaxation of their geometries to the C 2h minimum for free bipy, and Scheme 2b represents removal of the ligands with retention of the geometry observed in [6E] 3+ . The difference between the two enthalpies represents the energy required for two non-interacting bipy molecules (C 2h ) to adopt the (bipy) 2 geometry in each complex (Scheme 2c). The DH rxn values in Table 4 show that the enthalpic requirement for ligand dissociation from E 3+ has the trend E ¼ Bi < Sb < As < P, irrespective of whether or not steric factors are considered. Values for ligand strain show a parallel trend, but the range (98-181 kJ mol À1 ) is small compared to the range for the overall ligand dissociation process (2302-3575 kJ mol À1 ). We therefore conclude that steric Table 2 Select bond lengths and angles in the calculated (gas-phase, PBE0/def2-TZVP) structures of cations [6E] 3+ . See Fig. 2 for numbering scheme effects have a minor inuence on the calculated enthalpies of ligand dissociation in Scheme 2a, which are dominated by electronic effects. We rationalize the calculated trend in dissociation enthalpies on the basis of atomic size, with a smaller atom having a higher charge concentration and the best orbital match in the N(sp 3 )/E(np) HOMO-LUMO interaction (cf. N(sp 3 )/P(3p) vs. As(4p) vs. Sb(5p) vs. Bi(6p)). The electrostatic and orbital interactions are both expected to weaken as atomic radii and the number of nodes in the acceptor p-orbitals increase. The trend in ligand strain is presumably related to the N 1 -E-N 3 angle, which shows the most dramatic variation amongst all parameters in the calculated structures of [6E] 3+ , and decreases over a 22 range from phosphorus (175.68 ) to bismuth (153.59 ). We propose that the strained ligand geometry in [6P] 3+ is enforced by orbital interactions involving three mutually perpendicular 3p acceptor orbitals at the P 3+ centre. By comparison, in [6Bi] 3+ , where E-N bonding is calculated to be more ionic (Table 3), the preference for an N 1 -E-N 3 angle of 180 is lowest.</p><p>While reaction enthalpies for Scheme 2a and b represent the Lewis acidity of monoatomic trications E 3+ , DH rxn for Scheme 2d assesses the Lewis acidities of complexes [6E] 3+ by measuring the energy required for removal of a prototypical ligand, OPMe 3 , from hypothetical complexes [(bipy) 2 E(OPMe 3 )] 3+ . The enthalpies for this process indicate that the Lewis acidity of complexes [6E] 3+ has the trend E ¼ P < As < Sb < Bi, which is the opposite trend to that of monoatomic E 3+ , and is rationalised on steric grounds acknowledging the trend in atomic radii and consequential coordination sphere. Consistently, the range of enthalpy values calculated for Scheme 2d (178-255 kJ mol À1 ) is much smaller than that observed for Scheme 2a (2302-3575 kJ mol À1 ) and is comparable to the ligand strain enthalpies calculated for Scheme 2c (98-181 kJ mol À1 ). In addition, comparison of the optimized structures for [6E] 3+ and [(bipy) 2 E(OPMe 3 )] 3+ shows that the greatest geometric deformation upon complexation with OPMe 3 is compression of the interligand angle N 2 -E-N 4 (see Fig. 2 for denition). The magnitude of this geometric adjustment, which leads to steric clash between the bipy ligands, is greatest for E ¼ P (15 ) and least for E ¼ Bi (7 ), consistent with the calculated trend for Scheme 2d.     3). We propose that the more covalent N-P and N-As bonds are conformationally rigid due to the directional requirements of efficient orbital overlap to make a covalent bond (three mutually perpendicular porbitals), whereas the more ionic N-Sb and N-Bi interactions have a smaller barrier to motion due to the absence of a directional component for electrostatic interactions (point charges). The difference between the 31 P NMR chemical shi of free Et 3 PO and that of its adduct with a Lewis acid has been correlated with the strength of the Lewis acid (Gutmann-Beckett method). 31,32 No systematic trend was observed in the chemical shis observed (see Fig. S4, ESI †) in 31 P NMR assays of solutions containing equimolar amounts of Et 3 PO and [6 0 E][OTf] 3 . 33 Moreover, in the case of [6 0 P][OTf] 3 , a complex spectrum showing a mixture of products was obtained, none of which could be assigned to the phosphine oxide adduct. Deoxygenation of Et 3 PO by electrophilic phosphorus cations has been reported recently and may be operative. 34 Moreover, as a wide range in covalent radii (1.11-1.51 Å) 25 is spanned going from P to Bi, the steric inuence on 31 P chemical shis may be greater than those due to differing Lewis acidities, confounding a straightforward assessment due to steric factors, as highlighted recently for borane Lewis acids. 35 Reactivity of [6 0 P][OTf]  42 or bipy 43 ligands have been reported as well as two-coordinate arsenium monocations. [44][45][46] However, [6 0 As][OTf] 3 is the rst structurally authenticated example of an arsenic-centred trication.</p><!><p>The reactivity of Sb(OTf) 3 and Bi(OTf) 3 has been studied previously, leading to their widespread use as Lewis acid catalysts, 19,20 but the absence of synthetic routes to P(OTf) 3 and As(OTf) 3 has precluded investigations of these potential synthetic reagents. Phosphorus polycations have been used as a precursors to cationic bicyclophosphines and cyclic phosphorus oxides, 47 and derivatives of 4 (Chart 1) have been shown to bind transition metal centers via the lone pair at the phosphorus(III) center to give highly effective precatalysts for C-C bond forming reactions. 48 Intrigued by the unique intersection of molecular and electronic structures represented by the trications in [6 0 P][OTf] 3 and [6 0 As][OTf] 3 , and envisioning these salts as in situ equivalents of E(OTf) 3 (E ¼ P, As), we have conducted an initial survey of their reactivity.</p><p>[6 0 E][OTf] 3 as E(OTf) 3 transfer reagents Reactions of [6 0 E][OTf] 3 (E ¼ P, As) with three equivalents of 4dimethylaminopyridine (dmap) quantitatively (by 31 P and 1 H NMR) yield [(dmap</p><p>, (E ¼ P, As) and free tbbipy (Scheme 3a). Neither dmap complexes could be isolated from the reaction mixtures but their identities were denitively established by independent syntheses (Scheme 3b) and structural elucidation (Fig. 5). While the [7P] 3+ ion has previously been detected spectroscopically in mixtures of PCl 3 and dmap, 49,50 the 31 P NMR chemical shi attributed to the trichloride salt was reported to vary widely (d ¼ 79-114 ppm) depending upon concentration, suggesting a dynamic process. 51 By comparison, [7P][OTf] 3 exhibits a 31 P NMR chemical shi (d ¼ 101.7 ppm) for the redissolved crystals that does not vary over a broad concentration range.</p><p>The solid-state structures of [7E][OTf] 3 reveal three dmap ligands bound to the pnictogen centers giving a trigonal pyramidal geometry at the pnictogen centre for the [7E] 3+ ions. 52 Three weak contacts with the triate anions are evident, giving a six-coordinate geometry that is distorted by the  presence of a stereochemically active lone pair in each case.</p><p>The three weak E/O contacts are trans congured with respect to the three E-N bonds. The E-N bond lengths (Table 5) reect the relative atomic radii of the phosphorus and arsenic atoms and are 0.  5).</p><p>[6 0 E] 3+ as synthons for E I cations</p><p>In contrast to ligand exchange with dmap, reaction of [6 0 P]</p><p>[OTf] 3 with PMe 3 yields products due to redox chemistry (Scheme 4a). The previously reported P I containing reduction product, [(Me 3 P) 2 P] 1+ (d ¼ 15.0 and À156.3 ppm, 1 J PP ¼ 438 Hz) 53 and the P IV containing oxidation product, [Me 3 PPMe 3 ] 2+ (d ¼ 28.4 ppm), 54 have been denitively identied by 31 P NMR spectroscopy (Fig. S5, ESI †). The analogous reaction with PPh 3 yielded [(Ph 3 P) 2 P] 1+ (d ¼ 30, À174, 1 J PP ¼ 502 Hz) 55 as the major product, but a complex mixture of oxidation products was obtained, suggesting that [Ph 3 PPPh 3 ] 2+ , which is isoelectronic with the metastable hexaphenylethane molecule, 56 may also be unstable relative to its constitutional isomers (Scheme 4b).</p><p>Similarly, a 31 61 Moreover, the singlet at À146.9 ppm is also observed in the 31 P NMR spectrum of a 2 : 1 : 3 mixture of tbbipy, PCl 5 and TMSOTf.</p><p>Two congurational outcomes are envisioned for the octahedral structure of [(tbbipy) 2 PCl 2 ] 3+ , with a cis or trans arrangement of the chlorine atoms. The 1 H NMR spectrum of the cation shows six resonances in the aromatic region (Fig. 6) and two resonances for the t Bu groups, consistent with C 2 symmetry, precluding a trans conguration of chlorine centres. Gas-phase calculations using bipy ligands revealed that both isomers are true energy minima (no negative vibrational frequencies), but a 64 kJ mol À1 preference for the cis isomer was calculated, arising from signicant steric clash between the ortho hydrogen atoms of the ligands when a trans conguration is imposed (Fig. S10, ESI †). No cis/trans isomerism was detected experimentally upon heating a sample to 80 C for an hour, consistent with the rigidity of the disphenoidal frame inferred for [6P] 3+ from 1 H NMR spectroscopy (Fig. 4).</p><p>Addition of excess Cl 2 gas to MeCN solution of [6 0 P][OTf] 3 yields a product with identical spectral features as those assigned to [(tbbipy) 2 PCl 2 ] 3+ , as well as a number of unidenti-ed byproducts. Interestingly, equimolar mixtures of [6 0 As] [OTf] 3 and SO 2 Cl 2 showed no evidence of reaction even aer heating to 80 C for 2 hours.</p>