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

Abstract

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.