Synthesis, structural characterization and properties of one-dimensional coordination polymers of cobalt(II)- and nickel(II)-phosphonate complexes with 2,2'-bipyridine as a secondary ligand component: observation of both cis and trans conformations of a diphosphonic acid

Abstract

Two new isomorphous cobalt and nickel phosphonates [M^II(2,2'-bipy)₂LH₄]_n[LH₂]_n, M = Co (compound 1), M = Ni (compound 2) were hydrothermally synthesized from p-xylylenediphosphonic acid (LH₄) and the corresponding metal salts with 2,2'-bipyridine as secondary ligand component. Both the compounds 1 and 2 are characterized by routine elemental analyses, IR-, electronic-spectral analyses, thermogravimetric studies and unambiguously characterized by single crystal X-ray crystallography. The structures were refined in monoclinic space group C2/c. The crystal structure consists of 1D [M(2,2'-bipy)₂LH₄]²⁺ chains and [LH₂]^2- anions. The flexibility of non-rigid ligand p-xylylenediphosphonic acid (LH₄) tends to adopt a rare cis conformation in the crystal structure to meet the coordination requirement of the metal center from the usual trans conformation. The hydrogen bonding in the crystal structure leads to cylindrical tubes that extend via p-xylylenediphosphonic acid resulting in a 2D supramolecular sheet throughout the crystal. Compounds 1 and 2 are additionally characterized by thermogravimetric studies.