Oxalate- and Squarate-Biimidazole Supramolecular Synthons: Hydrogen-Bonded Networks Based on [Co(H₂biimidazole)₃]³⁺

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 = 2,2-biimidazole) 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.