Synthesis, X-Ray Structure, and Characterization of a Complex Containing the Hexakis(urea)cobalt(II) Cation and Lattice Urea Molecules
The 12: 1 reaction of urea (U) with CoI2 in EtOH yielded the “clathrate-coordination” compound [CoU6]I2·4U (1). The complex crystallizes in the monoclinic space group P21/c. The lattice constants are a = 9.844(4), b = 7.268(3), c = 24.12(1) Å, and β=98.12(1)∘. The crystal structure determination demonstrates the existence of octahedral [CoU6]2+ cations, I- counterions, and two different types (two U1 and two U2) of hydrogen-bonded, lattice urea molecules. The [CoU6]2+ cations and the U1 lattice molecules form two-dimensional hydrogen-bonded layers which are parallel to the ab plane. The I- anions are placed above and below each layer, and are hydrogen bonded both to U1 molecules and [CoU6]2+ cations. Each U2 molecule is connected to a [CoU6]2+ cation through an N–H⋯O hydrogen bond resulting in a three-dimensional network. Room temperature magnetic susceptibility and spectroscopic (solid-state UV/Vis, IR, Raman) data of 1 are discussed in terms of the nature of bonding and the known structure.
1. INTRODUCTION<!>2. EXPERIMENTS<!>2.1. X-ray crystallography<!>3.1. Synthetic comments<!>3.2. Description of structure<!>3.3. Physical and spectroscopic characterization<!>4. CONCLUSIONS
<p>Urea,
H2NCONH2 (hereafter abbreviated as U), is a very old
compound, first isolated by Rouelle in 1773 and subsequently synthesized from
inorganic materials by Wöhler in 1828. There is a renewed interest in the
coordination chemistry of U and its substituted derivatives. These efforts are
driven by a number of considerations, including the solution of pure chemical
[1] and spectroscopic [2] problems, the desire to provide useful bioinorganic
models for the intermediates in the catalytic mechanism of the metalloenzyme
urease (for its active site see Scheme 1) [3, 4], and the goal to isolate
functional complexes with interesting supramolecular structures [5].
Single-crystal X-ray crystallography has revealed [6] that U normally
coordinates as a monodentate ligand through the oxygen atom (I in Scheme 2). In a very limited
number of cases [7, 8], U behaves as an N,O-bidentate bridging ligand (II in Scheme 2), while in [Hg2Cl4U2]
each U molecule bridges the two HgII atoms through only the oxygen
atom [9] (III in Scheme 2). Of
particular chemical/biological interest is the ability of U to undergo
metal-promoted deprotonation [3, 10]; the monoanionic ligand H2NCONH− adopts the μ2 (IV in Scheme 2) and μ3 (V in Scheme 2) coordination modes.</p><p>Free
ureas have been among the central players in organic crystal engineering [11].
In contrast, little is known about the supramolecular architectures created by
hydrogen bonding interactions between simple metal-urea complexes. By reacting
metal ions with ureas that contain both an efficient coordination site and two
hydrogen bonding functionalities, assembly can be dictated by
intermolecular/interionic hydrogen bonding interactions. We have relatively
recently [12–14] embarked on a program which has as a short-term goal the
creation of supramolecular structures based on hydrogen bonding interactions
between simple metal complexes with U
or substituted ureas as ligands. Literature [5] reveals the existence of
single-crystal X-ray and neutron structures for the impressive complex [CoU6]Br2
⋅4U, which has
been described as a "clathrate-coordination" compound. We were interested in
investigating whether a similar iodide complex could exist; the present paper
provides the answer to this question. Another goal of this work has been the
study of the vibrational spectra of metal ion-U complexes, especially in view
of the reassignment of the vibrational spectrum of free U [15].</p><!><p>All manipulations were performed under
aerobic conditions using materials and solvents as received. IR spectra were
recorded on a Perkin-Elmer PC16 FT-IR spectrometer with samples prepared as KBr
pellets. Far-IR spectra were recorded on a Bruker IFS 113v FT spectrometer with
samples prepared as polyethylene pellets. Solid-state (diffuse reflectance,
DRS) electronic spectra in the 350–850 nm range were
recorded on a Varian Cary 3 spectrometer equipped with an integration sphere.
Room temperature magnetic susceptibility measurements were carried out by
Faraday's method using a Cahn-Ventron RM-2 balance standardized with HgCo(NCS)4;
diamagnetic corrections were estimated using Pascal's constants. C, H and N
elemental analyses were performed with a Carlo Erba EA 108 analyzer, [CoU6]I2
⋅4U (hereafter referred to
as 1).</p><p>To a stirred refluxing colorless solution of
U (0.72 g, 12 mmol) in EtOH (30 cm3) was added solid CoI2 (0.31 g, 1.0 mmol) in small portions. The obtained blue solution was refluxed
for further 15 minutes. A noticeable color change to pink occurred after
cooling down. The solution began to deposit X-ray quality, orange crystals of 1 after 24 hours.
When precipitation was judged to be complete, the product was collected by
filtration, washed with a little cold EtOH (1-2 cm3) and Et2O
(2×5 cm3), and dried in air. The yield was 77% (based on the metal);
found %: C, 13.22; H, 4.50; N, 30.48. Calc %
for C10H40N20O10CoI2:
C, 13.15; H, 4.42; N, 30.68; selected
IR data (KBr, cm−1): 3450 (sh), 3438 (s), 3346 (m), 3438 (sh), 1685 (sh),
1666 (s), 1648 (sh), 1622 (m), 1578 (m), 1478 (m), 1444 (m), 1156 (m), 1050 (w),
780 (m), 617 (sh), 600 (w), 570 (m), 532 (m), 366 (m).</p><!><p>X-ray data were collected at 298 K using a Crystal Logic Dual Goniometer diffractometer with graphite-monochromated Mo-Ka radiation (λ=0.71073⁢ Å). Lorentz, polarization, and Ψ-scan absorption corrections were applied
using Crystal Logic software. Symmetry equivalent data were averaged with Rint=0.0202, to give 3006
independent reflections from a total 3086 collected. The structure was solved
by direct methods and refined by full-matrix least-squares on F2,
using 3006 reflections and refining 276 parameters. All nonhydrogen atoms were
refined anisotropically. All hydrogen atoms bonded to nitrogen atoms were
located by difference maps and their positions were refined isotropically.
There were no significant residual peaks in the electron density map. Details
of the data collection and refinement are given in Table 1. Topological
analysis of the nets was performed using TOPOS program package [16, 17].</p><!><p>The CoI2/U reaction system was synthetically investigated in the past.
Depending on the reaction and crystallization conditions, the complexes [CoU6](I8)
[18], [CoU6](I3)2
⋅2U [19], [CoU2(H2O)4][CoI4]⋅H2O [20]
and [CoU4(H2O)2]I2 [20] were
isolated and structurally characterized. In these four complexes, the U : CoII ratio varies from 1 : 1 to 8 : 1. We wondered if complexes with a higher U to CoII ratio, that is, a higher urea percentage, would be capable of existence. Thus,
the 12 : 1 reaction of U and CoI2 in refluxing EtOH gave orange
crystals of compound [CoU6]I2
⋅4U (1) in very good yield (ca. 80%). The
reaction can be represented by the stoichiometric equation (1): (1)CoI2+10 U→TEtOH[CoU6]1I2⋅4U.</p><p>The
"wrong" stoichiometry employed, that is, U : CoI2 = 12 : 1 instead of
10 : 1 (required by (1)), is necessary for the precipitation of pure 1. The 10 : 1 reaction ratio in EtOH
under reflux leads to a mixture of 1 and [CoU6](I3)2
⋅2U [19)]; the
identity of the latter was confirmed by unit cell determination. The same
complex cannot be prepared in other solvents; use of MeCN leads to complexes
[CoU2(H2O)4][CoI4] [20] and [CoU4(H2O)2]I2 [20] mentioned above, and to a third product
(analyzed as [CoU6]I2) which has yet to be
structurally characterized.</p><!><p>The structure of 1 consists of octahedral [CoU6]2+ cations, I− anions and lattice urea molecules (four lattice urea
molecules per cation). The structure of the [CoU6]2+ cation
is shown in Figure 1, and selected bond lengths and angles are listed in Table 2. The CoII ion sits on an inversion centre and is surrounded by six
O-bonded urea ligands. The octahedral coordination around the CoII atom is slightly distorted, as evidenced by the Co–O bond distances and O–Co–O
bond angles. The Co–O bond distances in 1 are comparable to those in
other [CoU6]2+ complexes [18–20]. The urea ligands in 1 are coordinated in a bent fashion, with the C–O–Co angles ranging from
130.3(2)° to 133.6(2)°. This is the usual way of coordination of urea and its
derivatives [2, 12]. There are six strong intramolecular (intracationic)
hydrogen bonds with atoms N(1), N(11), and N(21) (and their symmetry
equivalents) as donors, and atoms O(1), O(11), and O(21) (and their symmetry
equivalents) as acceptors. These six intramolecular hydrogen bonds give a great
thermodynamic stability which is responsible for the formation of [CoU6]2+.</p><p>We
have up to now discussed aspects of the molecular structure of 1.
Figures 2 and 3 provide views of the hydrogen-bonded network of [CoU6]I2
⋅4U.
Metric parameters for the intermolecular hydrogen bonds present in the crystal
structure of 1 have been included in Table 3. The asymmetric unit of 1 contains five crystallographically independent urea molecules; three of them
are coordinated to the CoII atom while the other two (hereafter
termed U1 and U2) are lattice molecules. The [CoU6]2+ cations and the U1 lattice molecules form two-dimensional (2D)
by about 12 Å along the c axis (Figure 2). Each [CoU6]2+ cation is hydrogen bonded to six U1 molecules through the
hydrogen-bonded layers which are parallel to the ab plane and separated
N(1)–H(1A)⋯O(31)b (b 1−x, 0.5 + y, 0.5−z), N(22)–H(22A)⋯O(31)d (d x, 0.5−y,
−0.5 + z) and N(22)–H(22B)⋯O(31)e (e −x, 0.5 + y, 0.5−z)
(and their symmetry equivalents) hydrogen bonds, within a layer, with each U1 acting as hydrogen bond acceptor through the O(31) atom and connecting three
different [CoU6]2+ cations. The I− anions are
placed above and below each layer and are hydrogen bonded both to U1 molecules and [CoU6]2+ cations. Each I− accepts five hydrogen bonds connecting two [CoU6]2+ cations through the N(2)–H(2B)⋯Ic (c x, y, −1 + z and N(21)–H(21B)⋯Ih (h −1 + x, y,
−1 + z) hydrogen bonds and two U1 molecules through the
N(31)–H(31A)⋯If (f 1−x, 0.5−y, 1.5−z), N(31)–H(31B)⋯Ig
(g 1−x, 0.5 + y, 1.5−z) and N(32)–H(32A)⋯Ig hydrogen
bonds. In this arrangement, each [CoU6]2+ is hydrogen
bonded to four I− anions while each U1 molecule to two I− anions.</p><p>The
U2 molecules are hydrogen bonded to each other through the
N(41)–H(41A)⋯O(41)f and
N(42)–H(42A)⋯O(41)g hydrogen bonds to form one dimensional tapes that run
parallel to the b axis. The U2 tapes are parallel to the "[CoU6]I2
⋅2U1"
layers and the ab plane, and are separated by 9.844 Å along the a axis. Each U2 molecule is connected to a [CoU6]2+ cation through the N(2)–H(2A)⋯O(41)c hydrogen bond
(Figure 3) resulting in a three-dimensional (3D) hydrogen-bonded network; the U2 tapes are placed within the "[CoU6]I2
⋅2U1"
layers and connect them to the third dimension.</p><p>From
the topological point of view, the [CoU6]2+ cations and
the U1 molecules form a 2D framework, with a (4,4)-topology and two
different types of 4-connected nodes. Each [CoU6]2+ serves as a 4-connected node within the 2D net where the other type of 4-connected
node is situated on the centre of the R42(8) ring formed by two [CoU6]2+ and two U1 molecules (Figure 2).
The "[CoU6]2+
⋅2U1" layers are connected to the
third dimension only through the [CoU6]2+ cations, which therefore
serve as 6-connected nodes within the 3D framework. Each U2 molecule
is hydrogen bonded to two other U2 molecules and at the same time to
one [CoU6]2+. In this arrangement, each U2 molecule serves as a 3-connected node within the 3D framework. Therefore, the
3D framework is a trinodal net with 3-, 4-, and 6-connected nodes and a unique
(63)2
(44⋅62)
(44⋅610⋅8)
topology (Figure 4(a)). If we consider that the U2 molecules simply
connect the 2D layers and merge them to the [CoU6]2+ nodes, then we can simplify the 3D framework to a binodal net with 4- and
8-connected nodes and a unique (44⋅62)
(416⋅612)
topology (Figure 4(b)). The latter network can be simplified further to a
uninodal net by merging the 4-connected nodes to the 8-connected nodes resulting
in a 12-connected network with a unique 318⋅444⋅54 topology (Figure 4(c)). Interestingly, the new 12-connected network has the same
coordination sequence with the fcu net [21].</p><p>Complex 1 joins a handful of structurally
characterized complexes containing the octahedral cation [CoU6]2+ [5, 18, 19, 22]; it is isostructural with [CoU6]Br2
⋅4U [5] and [NiU6]I2
⋅4U [23].</p><!><p>The room
temperature value of the effective magnetic moment (Î¼eff)
for 1 is 4.93 BM per metal ion, to
be compared with the spin-only (g=2) μeff value of 3.87 BM. This value is within the range observed for six-coordinate,
high-spin cobalt(II) complexes [24]. Because of the intrinsic orbital angular
momentum in the octahedral ground state T41g(F), there is
consistently a considerable orbital contribution and μeff values for such compounds around room temperature are between 4.7 and 5.2 BM.</p><p>The
solid-state electronic spectral data of 1 also indicate an octahedral stereochemistry around cobalt(II). A multiple
structured bond, assigned to T41g(F)→T41g(P), is seen in the visible region near 530 nm with a clear shoulder
at 474 nm [25]. The multiple structure arises from the admixture of spin
forbidden transitions to doublet states mainly derived from G2 and H2. The T41g(F)→A42g transition appears as an
ill-defined shoulder at ~665 nm [25]. The calculated ligand field parameters
from the two transitions in the visible region are 10Dq = 8050 cm−1 and B = 865 cm−1 [25]; these values are typical for a CoIIO6 chromophore.</p><p>The
full vibrational analysis of crystalline U has been published [15]. Table 4
gives diagnostic IR bands of the free ligand and 1. Assignments have been given in comparison with the data obtained
for the free, that is, uncoordinated, U [15] and its manganese(II) complexes
[2]. The bands with ν(CN) character are situated at higher
wavenumbers in the spectrum of 1 than
for free U, whereas the ν(CO) band shows a frequency decrease.
These shifts are consistent with oxygen coordination, suggesting the presence
of +N=C–O− resonant forms [15], see Scheme 3. Upon
coordination via oxygen, the
positively charged metal ion stabilizes the negative charge on the oxygen atom;
the NCO group now occurs in its polar resonance form and the double bond
character of the CN bond increases, while the double bond character of the CO
bond decreases, resulting in an increase of the CN stretching frequency with a
simultaneous decrease in the CO stretching frequency [2, 12–14]. The appearance
of two bands for each of the Î´as(NH2) and Î´s(NH2)
modes in 1 may indicate the existence of two types of U
molecules, coordinated and uncoordinated (lattice). However, the appearance of
one band for each of the other modes suggests that the coordinated and lattice
(but hydrogen bonded) U molecules of 1 cannot, in general,
be differentiated in the vibrational spectrum. This is not unexpected, bearing
in mind that the hydrogen bonds have an effect similar to that of coordination
on the shifts of the CO and CN stretching vibrations. This can be explained
[15] by regarding the hydrogen bond as a donor-acceptor "complex," with the oxygen atom as the donor and the hydrogen atom as the acceptor. The appearance
of one IR-active ν(CoO) vibration at 366 cm−1 (F
1u under Oh) in the low-frequency region of 1 reflects the trans octahedral
stereochemistry of [CoU6]2+ [13, 26].</p><!><p>This
work has shown that the hexakis(urea)cobalt(II) cation can act as a hydrogen bonding
building block with multi-fold connectivity linking I− anions and U
molecules to generate a 3D architecture. We are presently pursuing our
prediction that this cation will form hydrogen bonding contacts to a variety of
inorganic and organic anions to generate a rich diversity of networks. Complex 1, which is isostructural with its
bromide analogue [5], is becoming the fifth structurally characterized member
of the CoII/I−/U family of complexes [18–20], emphasizing
the rich molecular and supramolecular chemistry of this system.</p><p>The
role of metal ions in supramolecular systems may simply be to act as
coordination centers providing a template for the formation of a rigid
framework of remote hydrogen bonding sites. Alternatively, the metal ion may
exert an electronic effect on the individual proton and acceptor sites, and
influence hydrogen bonding in a more subtle manner. The latter effect lets us
believe that the reactions of other metal ions with urea may lead to the
formation of complexes with novel supramolecular structures.</p>