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Supramolecular architectures of novel chromium(III) oxalate complexes: steric effects of the ligand size and building-blocks approach

Five new oxalate complexes of chromium(III), [Hphen][Cr(phen)(C2O4)2]·2H2O (1), [Cr(phen)2(C2O4)][Cr(phen)(C2O4)2]·3H2O (2), [Cr(phen)2(C2O4)]NO3·H2C2O4·H2O (3), [Cr(bpy)2(C2O4)][Cr(bpy)(C2O4)2]·3H2O (4) and [Cr(bpy)2(C2O4)]NO3·1/2H2C2O4·4H2O (5) (phen = 1, 10-phenanthroline, bpy = 2, 2′-bipyridine), were prepared by using an (oxalato)tantalate(V) solution as a source of oxalate ligands. The compounds contain either the discrete mononuclear [Cr(L)2(C2O4)]+ cation [L = phen (3) ; L = bpy (5)] or the discrete mononuclear [Cr(L)(C2O4)2]− anion [L = phen (1)], or both types of mononuclear ions [L = phen (2) ; L = bpy (4)]. The crystal structures are dominated by the hydrogen-bonding and π⋯π-stacking interactions that give rise to the overall two- (compounds 1, 2, 4, 5) or three-dimensional (compound 3) architectures. Compounds 2 and 4 represent a borderline case between isostructurality and non-isostructurality ; they exhibit an analogous packing of the cation and the anion units, but the crystallization water molecules occupy different positions – due to a difference in size between the phen and bpy ligands. The influence of steric factors is evident also in the case of 3 and 5, which, despite very similar chemical formulae, exert a completely different packing of the constituents. By the self-assembling of 1 and 4, used as building blocks in the reaction with calcium(II) cations, the heterobimetallic polymeric compounds {;[CaCr2(phen)2(C2O4)4]·5H2O};n (6) and {;[CaCr2(bpy)2(C2O4)4]·0.83H2O};n (7) were obtained. The crystal structure of 7 is reported: the [Cr(bpy)(C2O4)2]− unit, through the two oxalate groups, acts as a chelating ligand towards Ca cations, resulting in heterometallic one-dimensional double zigzag chains, formed of diamond-shaped units. The characterization of the compounds obtained was accomplished by the spectroscopy and thermal analysis methods.

chromium(III) oxalate complexes ; crystal structures ; building-blocks chemistry

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