Influence of biphenyl spacer appended to the flexible phosphonate arms in modulating the dimensionality of the coordination polymers: Synthesis, structural chemistry and magnetic properties

Two new compounds [Co (2,2′-bipy) (H2dbp)]n (1) and [Ni (2,2′-bipy)2(H2dbp)(H2O)]·H2O (2) based on the flexible ligand 4,4′-dimethylenebiphenyldiphosphonic acid (H4dbp) with 2,2′-bipyridine as secondary ligand have been synthesized under hydrothermal conditions. Both the compounds are well characterized by routine elemental analysis, IR, electronic spectroscopies, thermogravimetric analysis and finally by single crystal X-ray diffraction analysis. Compound 1 is a 1D extended coordination polymer and 2 is a discrete molecular compound. A comparative study between the geometries of H4dbp ligand (in compounds 1 and 2, present study) and p-xylylenediphosphonic acid (H4pxp) ligand (in previously reported compounds [Cu(2,2′-bipy)(H2pxp)]·nH2O (1A) and Ni(2,2′-bipy)2H4pxp]n[H2pxp]n (2A), see text) demonstrate the effect of the twisting in the benzene rings in changing higher dimensional Hxpxp (x refers to number of protonated hydroxyl groups) compounds to lower dimensional Hxdbp compounds. The eight membered Co-dimer rings formed in compound 1 represents the simple and isolated Co-dimer, exhibiting weak antiferromagnetic exchange between metal centers through OPO bridges.