Oxalate- and Squarate-Biimidazole Supramolecular Synthons: Hydrogen-Bonded Networks Based on [Co(H2biimidazole)3]3+ Ce´dric Borel,† Krister Larsson,‡ Mikael Håkansson,§ Bjo¨rn E. Olsson,† Andrew D. Bond,| ¨ hrstro¨m*,† and Lars O
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2821–2827
Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, SE-41296 Gothenburg, Sweden, National Electron Accelerator Laboratory for Nuclear Physics and Synchrotron Radiation Research (MAX-laboratory), Lund UniVersity, SE-22100 Lund, Sweden, Department of Chemistry, UniVersity of Gothenburg, SE-41296 Gothenburg, Sweden, and Department of Physics and Chemistry, UniVersity of Southern Denmark, CampusVej 55, DK-5230 Odense M, Denmark ReceiVed January 21, 2009; ReVised Manuscript ReceiVed March 12, 2009
ABSTRACT: The utility of R22(9) biimidazole-carboxylate, R22(10) biimidazole-oxalate/squarate and R22(9) biimidazole-(tris-oxalate) synthons is considered for crystal engineering of hydrogen-bonded networks based on [CoIII(H2biim)3]3+ cations (H2biim ) 2,2biimidazole) and oxalate, squarate or [MIII(C2O4)3]3- anions. Syntheses and crystal structures are described for [CoIII(H2biim)3][MIII(C2O4)3] · 2H2O (M ) Cr, 1; M ) Co, 2), [CoIII(H2biim)3](HC4O4)3 · 2H2O, 3, and [CoIII(H2biim)3](C2O4)Cl · 5.5H2O, 4. Compounds 1 and 2 are isostructural and comprise [Co(H2biim)3]3+ cations bridged by [M(oxalate)3]3- anions in two directions and water molecules in the third direction to give a 3D H-bonded network. Both outer and inner O atoms of the coordinated oxalate ions act as H-bond acceptors, forming motifs closely related to the anticipated R22(9) biimidazole-(tris-oxalate) synthon. Compound 3 contains a more complex H-bond pattern in 3D, built from the intended R22(10) biimidazole-squarate synthon and additional H-bonds between protonated squarate molecules and water molecules. The structure of compound 4 (obtained with synchrotron radiation) contains layers in which stacked pairs of oxalate anions bridge between [CoIII(H2biim)3]3+ cations to form a dense 2D kgd-net, separated by layers of disordered chloride anions and H-bonded water molecules. Introduction Crystal engineering of three-dimensional nets connected via hydrogen bonds continues to be an important issue in the solidstate version of “self-assembly” or “supramolecular chemistry”.1 A large variety of H-bond motifs have been used in this area, and the H-bond can be supported by a charge-charge interaction if the species involved are ionized.2 A popular cationic building block is [M(H2biim)3]3+ (H2biim ) 2,2-biimidazole, especially with M ) Co; Scheme 1), which can be prepared in both racemic and enantiopure3 forms. This cation has been combined, by us and others, with simple anions such as carbonate4 and sulfate5 to give interpenetrated 3D ths-nets6 ((10,3)-b in Wells nomenclature7). In 2004, we extended this chemistry to the R22(9) carboxylate-biimidazole synthon (Scheme 2) with the continued intention to construct 3D nets. This approach proved to be only partly successful, however, because other H-bond motifs were obtained with the R22(9) motif when [Co(H2biim)3]3+ was combined with phthalate (benzene-1,4-dicarboxylate) anions.8 A survey of the Cambridge Structural Database (CSD)9 suggests that the R22(9) carboxylate-biimidazole synthon in Scheme 2 is relatively robust: out of 39 structures containing carboxylate and biimidazole fragments, the motif is observed in 34 cases. In [trans-Zn(H2biim)2(H2O)2]glutarate tetrahydrate (CSD refcode MEFPUJ) and the corresponding succinate hydrate (MEFQUE), for example, alkyldicarboxylic acids bridge between [trans-Zn(H2biim)2(H2O)2]2+ complexes via two R22(9) synthons to yield the anticipated 1D chains.10 Similar chains are formed in [trans-Co(H2biim)2(H2O)2]bis(isophthalate)-tet* Corresponding author. E-mail:
[email protected]. † Chalmers University of Technology. ‡ Lund University. § University of Gothenburg. | University of Southern Denmark.
Scheme 1. The Cationic Building Block [Co(H2biim)3]3+ and the Oxalate, Squarate and [M(oxalate)3]3- Anions
Scheme 2. Possible H-Bond Motifs Using Biimidazole and 1,2-Diacids with Their Corresponding Graph-Set Descriptors
rahydrate (ITAXUW)11 and also in [trans-Zn(H2biim)2(H2O)2]bis(isophthalate)(2,2′-biimidazole)dihydrate (PAVZAO), although in the latter case the chains include three distinct components: {[Zn(H2biim)2(H2O)2]2+-(isophthalate)--(2,2′-biimidazole)-(isophthalate)--}n.12 Thus, the synthon appears to have reliable utility for constructing 1D supramolecular structures. Utilizing the R22(9) motif for crystal engineering of predictable 3D structures proves to be rather more difficult. In [Ni(2,2′-bibenzimidazole)3]benzene-1,4-dicarboxylate trihydrate
10.1021/cg900075j CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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(CEFGEE),13 the anticipated synthon is adopted for two of the three bibenzimidazole sites around the nickel complex, but water molecules intervene at the third site to give an entirely different H-bond motif. Similarly, in [trans-Co(H2biim)2(H2O)2](1,3,5benzenetricarboxylate)tetrahydrate (PAVZIW),13 two of the three carboxylate groups of the trimesic anion form the R22(9) motif while the third remains protonated, and water molecules again become involved in H-bonding at this latter site. One impressive example where the R22(9) motif operates entirely as intended is [Co(H2biim)3](1,3,5-benzenetricarboxylate)hydrate (REGHAN),14 in which [Co(H2biim)3]3+ cations and trimesic anions are linked into a (6,3) 2D net, and cations and anions stack on top of each other to create a honeycomb 3D structure. Curiously, the recently reported closely related compound [Ru(H2biim)3](1,3,5-benzenetricarboxylate)DMF · 9H2O retains the R22(9) motif but forms doubly interpenetrated 3D ths-nets.15 It is not clear whether it is the metal ion, the change of solvent or something else in the preparation that produces this striking structural difference. The starting point for the current study was our interest to investigate whether similar structural chemistry could be obtained using the R22(10) synthon (Scheme 2) derived from oxalate and squarate anions (i.e., the anions of squaric acid, 3,4-dihydroxycyclobut-3-ene-1,2-dione), and also from oxalate complexes. The oxalate anion is a versatile H-bond acceptor that has been used for more than 30 years in crystal engineering (and can of course on its own form the R22(9) motif),16 while squarate anions have also been used as building blocks for H-bonded networks.17 The tris-oxalate complexes are one of two classic complex anions that enable extended systems through H-bonds or as bridging ligands18 (the other being the hexacyananometalates, also recently combined with biimidazoles19). The use of tris-oxalates also provides the possibility to introduce a second metal ion into the system and might produce interesting structural variations with racemic or enantiopure20 tris-oxalates, although the latter are likely to be problematic in protic solvents since racemization can occur even for the “inert” Cr(III) and Co(III) complexes.21 We were also aware of the possibility to obtain H-bonds to the “inner” O atoms of the coordinated oxalate anion (forming another R22(9) synthon) since this has proven to be a reliable motif in the formation of 3D porous networks of the formula {MII[Co(ethylenediamine)(oxalato)2]2}n.22 An initial survey of the CSD gave limited, but encouraging, results: four relevant structures showed either the R22(10) motif or the related R22(7)R12(5) motif (Scheme 2).10,23 Thus, we set out to investigate experimentally the feasibility of using the R22(10) biimidazole-(1,2-carbonyl) synthon for the preparation of 3D H-bonded networks. Experimental Section Materials and Methods. All chemicals were reagent grade and used without further purification. 2,2′-Biimidazole, [Co(H2biim)3](NO3)3, K3[Cr(oxalate)3] and Na3[Co(oxalate)3] were prepared according to literature procedures.3,24 IR analyses were made with a Perkin-Elmer Instruments Spectrum One FT-IR spectrophotometer using KBr tablets. [CoIII(H2biim)3][CrIII(ox)3] · 2H2O 1. 0.0105 g (0.024 mmol) K3[Cr(oxalate)3] was dissolved in 6 mL of water, and 0.0162 g (0.025 mmol) of [Co(H2biim)3](NO3)3 was added. The solution was filtered and then diluted to 30 mL. The mixture was kept at 4 °C, and dark brown crystals appeared within two days. The yield was 0.010 g and 53%. [CoIII(H2biim)3][CoIII(ox)3] · 2H2O 2. 0.0100 g (0.025 mmol) Na3[Co(oxalate)3] was dissolved in 6 mL of water, and 0.0162 g (0.025 mmol) of Co(H2biim)3](NO3)3 was added. The solution was filtered
Borel et al. and then diluted to 30 mL. The mixture was kept at 4 °C, and orange crystals appeared within two days. Yield about 50%. [CoIII(H2biim)3](Hsquarate)3 · 2H2O 3. 0.0100 g (0.015 mmol) Co(H2biim)3](NO3)3 was dissolved in 4 mL of water with 1 mL of 5 M HCl(aq). To the solution was added 53 mg of H2squarate (0.46 mmol). The mixture was filtered, and the orange solution was kept at 4 °C for several weeks before the appearance of a few orange crystals. The yield in this reaction is thus very low. [Co(H2biim)3][C2O4]Cl · 5.5H2O 4. 0.046 g (0.07 mmol) [Co(H2biim)3](NO3)3 was dissolved in 10 mL of water, 0.019 g (0.14 mmol) of disodiumoxalate was added and the mixture heated, a small amount of ammonium chloride was added, and finally 5 drops of 1 M HCl(aq) was added until the solution became clear orange colored with pH ca. 2. The solution was stored at 4 °C. After a few days pale orange crystals were obtained. Mp >300 °C, loss of crystallinity at 180 °C. The identity of the bulk phase was confirmed by powder X-ray diffraction. Yield 70-80% Anal. Found (calcd for C20H29ClCoN12O9.5): C, 34.53 (35.12); H, 4.24 (4.27); N, 24.80 (24.57). TGA analysis showed weight loss of 12.0% (calcd 14.5%) up to 200 °C and a final weight reduction of 85.1% (calcd for CoO 89.1%, CoCl2 81.1%) at 600 °C. The presence of Cl was confirmed by qualitative analysis. Concomitant precipitation of the starting material [Co(H2biim)3](NO3)3 in some samples, as revealed by powder X-ray diffraction, may account to some extent for the discrepancy of the calculated water content and the water content indicated by the TGA measurement. X-ray Crystallography. Compound 1 was analyzed using a Rigaku R-AXIS IIc image plate system with graphite-monochromated Mo KR radiation (λ ) 0.7107 Å) from an RU-H3R rotating anode operated at 50 kV, 90 mA. Ninety oscillation photographs with a rotation angle of 2° were collected and processed using the CrystalClear software package.25 Empirical corrections were applied for the effects of absorption using the REQAB program within CrystalClear. Measurements on compound 2 were made with a Bruker SMART CCD diffractometer and Mo KR radiation. CCD data were integrated with the SAINT+ package,26 and a multiscan absorption correction was applied using SADABS.27 Data for compound 3 were collected with a Bruker Nonius X8-APEXII CCD diffractometer with graphite-monochromated Mo KR radiation. Data were integrated using SAINT26 within APEX2,28 and a multiscan absorption correction was applied using SADABS.27 Crystals of compound 4 were small and weakly diffracting, and were analyzed with a MARCCD/MARDTB Phi-Axis goniometer using synchrotron radiation (λ ) 0.9070 Å) at MAXLAB II, Lund, Sweden.29 CCD data were integrated and corrected for absorption using TWINSOLVE.30 All structures were solved by direct methods (SIR9731) and refined against all F2 data by full-matrix least-squares (SHELXL9732), including anisotropic displacement parameters for all non-H atoms. H atoms were placed in calculated positions (unless otherwise stated) and allowed to ride during subsequent refinement. The crystallographic data are summarized in Table 1. The structure of compound 4 is problematic and could only be solved with the help of synchrotron data. It contains layers of hydrogen-bonded biimidazole complexes and oxalate anions that are clearly resolved, but with electron density between these layers that is more difficult to interpret. Several independent crystal structure determinations were made on crystals from different batches, but the situation persisted. The asymmetric unit contains two half [Co(H2biim)3]3+ complexes (both sited on crystallographic 2-fold axes), one whole oxalate dianion, and eight clear peaks in the electron density between the layers (Figure 1). Six of the eight peaks refine to give acceptable anisotropic displacement parameters as fully occupied O atoms. The other two lie close to each other and close to an inversion center, forming unacceptable contacts, which necessitates a disordered model: both sites refine acceptably as half-occupied O atoms. The peaks form an approximately planar hexagonal arrangement with most interpeak distances consistent with H-bonded water molecules (in the approximate range 2.65-2.98 Å). In the region of the two disordered peaks, one site forms interpeak distances greater than 3.3 Å, consistent with Cl- forming H-bonds to neighboring water molecules. Including this site as half-occupied Clproduces a somewhat enlarged and oblate anisotropic displacement ellipsoid, but this was considered to be acceptable under the circumstances. The neighboring half-occupied O site completes the H-bonded network where Cl- is considered to be absent. Since the Cl- ions must balance the charge of the {[Co(H2biim)3](oxalate)}+ layers, there must be a total of one Cl- per asymmetric unit (8 Cl- ions per unit cell), which requires at least one other site to contain some Cl- occupancy.
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Table 1. Crystallographic Data for 1-4
chemical formula formula weight measurement temp/K crystal system λ/Å space group a/Å b/Å c/Å R/deg β/deg γ/deg vol/Å3 Z Fcalc/Mg m-3 µ/mm-1 reflns collected indep reflns R(int) obsd reflns (I > 2σ(I)) parameters/restraints goodness-of-fit R1 (I > 2σ(I)) wR2 (all data) largest diff peak/e · Å-3 largest hole/e · Å-3
1
2
3
4
C24H22Co2N12O14 820.40 298(2) monoclinic 0.7107 C2/c 12.004(3) 18.280(5) 13.625(3) 90 96.839(8) 90 2968.5(13) 4 1.836 1.212 9239 2449 0.079 2331 245/0 1.19 0.0670 0.1437 1.186 -0.946
C24H22CoCrN12O14 813.47 173(2) monoclinic 0.7107 C2/c 12.0595(4) 18.4539(6) 13.5995(5) 90 97.710(1) 90 2999.14(18) 4 1.802 1.010 24999 4585 0.035 3773 236/0 1.01 0.0386 0.1133 1.427 -0.667
C30H25CoN12O14 836.55 298(2) triclinic 0.7107 P1j 10.1255(9) 13.9169(5) 14.6328(5) 118.386(1) 100.462(1) 99.392(1) 1708.10(17) 2 1.627 0.593 37915 9476 0.021 8216 542/9 1.09 0.0313 0.0892 0.397 -0.404
C20H29ClCoN12O9.5 683.93 100(2) monoclinic 0.9070 C2/c 14.0103(8) 20.9727(10) 21.3754(11) 90 106.979(3) 90 6007.1(6) 8 1.512 0.730 9194 4960 0.033 4332 407/0 1.07 0.0715 0.2061 1.118 -0.951
The most acceptable in terms of intermolecular contacts was modeled as half-occupied Cl-, which provided an acceptable anisotropic displacement ellipsoid. H atoms were not included on the remaining water molecules. The refined model represents a reasonable approximation without excessive disorder modeling. The resulting empirical formula [Co(H2biim)3][C2O4]Cl · 5.5H2O is consistent with elemental analysis and weight loss observed by TGA, and it is also reasonable on the basis of molecular volume: analysis of the structure with empty layers (using CALC SOLV in PLATON33) indicates 1864.5 Å3 solvent accessible volume per unit cell, and the model contains 8 Cl- and 44 H2O molecules with a total volume of ca. 1950 Å3 (assuming the occupied volume of H2O to be 40 Å3 and that of Cl- to be 23 Å3). Attempts to employ a continuous solvent area model for the disordered layers (using SQUEEZE in PLATON) did not yield any sensible result. It was not possible to resolve any of the disorder problems by refinement in a lower-symmetry space group. The crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supple-
Figure 1. Distribution of atomic sites in the disordered water-chloride layers of 4. The red atoms are consistent with water molecules, while the green atoms are more consistent with Cl-. The atoms labeled (1) and (2) correspond to two disorder components. A crystallographic center of inversion exists at the center of the diagram.
mentary publication no. CCDC 717493-717496. Copies can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or on application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax, +44 1223-336-033; e-mail,
[email protected]). Network Analysis. The short (Schla¨fli) symbol gives the number of smallest rings found in the net and also the stoichiometry and the connectivity (p) of the nodes, through the relation p ) 1/2 + (1/4 + 2 · (sum of superscripts))1/2, and thus gives a rough idea of the type of network in question. The short (Schla¨fli) symbol, the vertex (or long) symbol, and the C10 (TD10) value were calculated using TOPOS.34
Results and Discussion Synthesis. Compounds 1 and 2, [CoIII(H2biim)3][MIII(ox)3] · H2O (M ) Co, Cr), were prepared by mixing aqueous solutions of [Co(H2biim)3](NO3)3 and [M(ox)3] · H2O. Compound 3, [CoIII(H2biim)3](Hsq)3 · 2H2O, was obtained in low yield by dissolving [Co(H2biim)3](NO3)3 and squaric acid in water and adjusting the pH to 6. Compound 4, [Co(H2biim)3](C2O4)Cl · 5.5H2O, was obtained by mixing aqueous solutions of the biimidazole complex and disodiumoxalate, with subsequent addition of NH4Cl(s) and 1 M HCl(aq). Attempts at pH adjustment using oxalic acid did not yield any crystalline products. All metal complexes used were racemic; to date, it has not been possible to obtain crystals using enantiomerically pure materials. Crystal Structures. [CoIII(H2biim)3][MIII(ox)3] · 2H2O, M ) Co, Cr. These two compounds are isostructural. [CoIII(H2biim)3][CrIII(ox)3] · 2H2O 2 is specifically discussed because this was obtained at low temperature and is more precise. Additional data for [CoIII(H2biim)3][CoIII(ox)3] · 2H2O 1 can be found in the Supporting Information. No unusual features are present in the molecular units of either compound, and a displacement ellipsoid plot of 2 is given in Figure 2. There are three major H-bond motifs in the structure. First, two of the biimidazole ligands in [CoIII(H2biim)3]3+ form multiple H-bonds to the oxalate ligands in [CrIII(ox)3]3- (Figure 3), linking the complexes into chains along the crystallographic c axis. The H-bond motif is related to the second R22(9) synthon in Scheme 2, although it incorporates one H-bond to the noncoordinated O atom of one oxalate ligand, and a bifurcated
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Figure 2. Displacement ellipsoid plot (50% probability for non-H atoms) of [CoIII(H2biim)3][CrIII(ox)3] · 2H2O 2 showing H-bonds between the complexes and lattice water molecules. Both complexes lie on crystallographic 2-fold axes. As it was not possible to unequivocally determine the positions of the water protons, these were excluded from the refinement.
Borel et al.
Figure 4. The structure of 2 can be described as a (628)(66)(698)-net; gray nodes are tris-oxalate complexes, red nodes are waters clusters and pink nodes are biimidazole complexes.
Figure 5. Displacement ellipsoid plot (50% probability for non-H atoms) for [Co(H2biim)3](Hsquarate)3 · 2H2O 3.
Figure 3. H-bonds around the two lattice water molecules in 2 defining a four-connected node. Only the acidic H atoms are shown. H atoms on the water molecules could not be resolved and were excluded from the refinement.
interaction from one N-H group to the two coordinated O atoms of the oxalate ligands, giving a formal graph-set descriptor of R22(9) + R12(4). Each tris-oxalato complex is also connected to two other tris-oxalato complexes via an H-bond bridge comprising two water molecules. Finally, the water molecules also form H-bonds to the remaining biimidazole ligand (Figure 4). The overall structure consists of an H-bonded 3D net, and it can be described as a net formed by five-connected nodes (the tris-oxalato complexes), four-connected nodes (the pair of lattice water molecules), and three-connected nodes (the biimidazole complexes).35 The resulting 3-, 4- and 5-connected net is shown in Figure 4. Topology analysis showed this 3D-net to have short (Schla¨fli) symbol (628)(66)(698), TD10 ) 1529, and
vertex symbols 63 · 63 · 89, 6 · 62 · 6 · 62 · 6 · 62, and 6 · 6 · 6 · 6 · 6 · 6 · 6 · 6 · 6 · 83 for the three nodes respectively. In addition to H-bond motifs, one-dimensional chains can be envisaged in the structure, running essentially along the diagonal of the ac unit cell face, formed by “face-to-face” interactions between imidazole rings. The same motif, albeit restricted to a pairwise interaction, can be found between the H-bonded helices of optically pure [Co(Hbiim)3].36 [Co(H2biim)3](Hsquarate)3 · 2H2O 3. As for 1 and 2, the molecular building blocks in this compound (Figure 5) do not display any remarkable features and the bond geometry data can be found in the Supporting Information. Major H-bond motifs in the structure are the anticipated R22(10) squarate-biimidazole synthon for one biimidazole ligand of [Co(H2biim)3]3+, plus a second R22(10) motif within (Hsquarate)22- dimers. The two remaining biimidazoles form H-bonds to two (Hsquarate)- anions with an almost perpendicular arrangement between the ligand and squarate planes. In addition, two water molecules form H-bonds between (Hsquarate)- anions. The complete hydrogen-bonded assembly around the complex cation is shown in Figure 6. In this case, the complexity of the H-bond pattern defies any network
H-Bonded Networks Based on [Co(H2biimidazole)3]3+
Figure 6. The H-bonded assembly around the complex cation in [Co(H2biim)3](Hsquarate)3 · 2H2O 3.
interpretation since there are simply too many interactions. Instead, this structure is best described as parallel H-bonded anionic bands of squarates and water (Figure 7), cross-linked by H-bonds to the [Co(H2biim)3]3+ cations. In addition to the H-bonds, 1D chains formed by “face-to-face” interactions between imidazole rings can be envisaged, closely comparable to those in 1 and 2. In this case, our intended crystal engineering strategy is hampered by the fact that the desired state of deprotonation is not achieved for squaric acid. This is a delicate synthetic problem: not only must the pH be carefully controlled to obtain the doubly deprotonated species in solution (in some cases incompatible pKa values will even make this impossible), but we may also be faced with particular H-bonding circumstances in the solid state which could create mixtures of species not possible in solution. In the latter case, intermolecular interactions in the crystalline structure may take precedence over solution thermodynamics. A remarkable example of this is the H-bonded network structure of trimesic acid, C6H3(COOH)3, which contains no fewer than four different states of protonation of the triacid.37 We have also recently commented on the coexistence of oxalic acid and the oxalate dianion in Ba(C2O4)(H2C2O4) (H2O)2.38 [Co(H2biim)3](C2O4)Cl · 5.5H2O 4. The molecular building blocks of [Co(H2biim)3](C2O4)Cl · 5.5H2O, 4, are shown in Figure 8. The oxalate dianions deviate somewhat from the expected flat structure, with a dihedral angle of 7.8(1)° between the planes defined by the two carboxylate groups. The C-O bond lengths lie in the range 1.222(7)-1.259(7) Å (compared to a mean value from the CSD9 of 1.291 Å for protonated oxalate O atoms), and the surroundings of the oxalate anion clearly cannot accommodate any in-plane H atom. The Co-N bond lengths, 1.930(4)-1.950(4) Å, are also within the expected range for Co(III). The only feature that seems unusual in the structure is a relatively short interplanar separation between oxalate ions, with C(20) · · · C(20) ) 3.193(8) Å and C(20) · · · C(21) ) 3.166(8) Å. Although scrutiny of the CSD reveals numerous examples of oxalates with C · · · C separations in this range, pairwise interactions shorter than 3.3 Å are rare (a total of seven structures). Moreover, these cases are all found between coordinated oxalates, all have a slight lateral offset, and all have a C-C · · · C-C torsion angle close to zero (i.e., there is no
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relative twist about the normal to the molecular plane). In compound 4, the two oxalates (arranged around a crystallographic 2-fold axis) are more or less directly on top of each other and twisted with respect to each other (C-C · · · C-C torsion angle 26°). This geometry appears to be imposed by the H-bond interactions of the two oxalate ions to the same biimidazole ligand and appears to be of little practical significance; the interplanar distances are much closer to normal π-π interactions than the intradimer C · · · C separation of 2.89((0.6) Å in dimers of the tetracyanoethylene radical anion, [TCNE]22-, for example, in which some orbital based bonding has been suggested.39 The packing features two different types of layers, one comprising a 2D net of [Co(H2biim)3]3+ cations and oxalate anions and the second containing disordered chloride anions and water molecules. The first layer type contains both enantiomers of [Co(H2biim)3]3+ H-bonded to the stacked pairs of oxalates with N · · · O distances in the range 2.609(5)-2.760(5) Å and N-H · · · O angles of 145-168°. Interestingly, although frustrating from a crystal engineering perspective, the structure does not contain the anticipated H-bond motifs of Scheme 2. Instead, one biimidazole ligand of each [Co(H2biim)3]3+ cation binds to both stacked oxalates, with the planes of the oxalates lying approximately perpendicular to the plane of the biimidazole ligand, similar to the arrangement observed for squarate in compound 3. The result is a 2D sheet with trigonal (Co) and hexagonal (oxalate dimers) nodes, giving a (6,3) kgd-net as shown in Figure 9. The structure is clearly reminiscent of the intended H-bonded net, but the oxalate anions serve to link between several biimidazole ligands in a manner more complex than that anticipated. It is noteworthy perhaps that there are no “face-to-face” interactions between imidazole rings in 4, in contrast to the structures of 1-3. Instead, the biimidazole ligands approach each other in a manner something akin to a “phenyl embrace”.40 Tentatively, these observations might be attributed to the much higher ratio of N-H bonds to O acceptors in 4, 2:1 compared to approximately 1:1 in 1-3, causing the complexes to “crowd around” the small oxalate anions. It is possible that the intended crystal design strategy might be more successful if chloride anions could be excluded, thereby forcing a different ratio of [Co(H2biim)3]3+ to oxalate. However, so far such experiments have proven unsuccessful. The approximately planar, hexagonal water-chloride motif in 4 is not especially unusual. Interrogation of the CSD using the motif search feature in Materials Mercury (based on the 3DSEARCH algorithm of Chisholm41) identified numerous cases of similar layered structures with planar motifs comprising only water molecules and chloride anions, for example PEHWOO42 and QOCYIQ.43 Conclusions The reported compounds are the results of our first attempts to employ the R22(10) biimidazole-(1,2-carbonyl) synthon for the preparation of 3D H-bonded networks. The structures form an illustrative series with increasing complexity of their H-bond assemblies. Compounds 1 and 2, [CoIII(H2biim)3][MIII(ox)3] · 2H2O, utilize the inner O atoms of the coordinated oxalate anions to form motifs closely related to the R22(9) synthon that has been observed on numerous previous occasions. The structures of 1 and 2 can be interpreted adequately as a 3-, 4- and 5- connected 3D-nets. Compound 3, [Co(H2biim)3](Hsquarate)3 · 2H2O, contains the anticipated R22(10) synthon, but application of the crystal engineering strategy exactly as intended is hampered by a synthetic factor, namely that the
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Borel et al.
Figure 7. The (Hsquarate)-/H2O network in 3. Blue stubs are biimidazole ligands coordinated to Co(III).
Figure 9. The 2D network structure in [Co(H2biim)3](C2O4)Cl · 5.5H2O 4 results in a (6,3) kgd-net displayed in green. H atoms are omitted.
case, the structure is clearly reminiscent of the intended structure, but the oxalate anions link between [Co(H2biim)3]3+ cations in a manner more complex than anticipated. This can be tentatively attributed to the relatively high ratio of N-H donors to O atom acceptors, and a more predictable result might be obtained if chloride can be excluded from the system. Acknowledgment. We thank the Swedish Research Council and the Nordforsk network in Crystal Engineering and Supramolecular Materials (Grant No. 060062) for financial support. ¨ . thanks the Hasselblad Foundation for a writer’s stipend to L.O Grez sur Loing. Supporting Information Available: Displacement ellipsoid plot of 1 and crystallographic information files (CIF) for 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.
References Figure 8. A displacement ellipsoid drawing (50% probability for non-H atoms) of the biimidazole complexes and oxalate H-bonded bridges in [Co(H2biim)3][C2O4]Cl · 5.5H2O 4. Stacked oxalate anions are visible at the center of the diagram.
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