Variations of structures on changing the ratios of metal ions in rare Ca(II)–Zn(II) hetero-metallic self-assembled coordination polymers of hexamethylenetetramine and benzoate

Abstract

Two rare hetero-metallic calcium(II)-zinc(II) complexes [CaZn4(OBz)10(μ2-hmt)2]n (1) and [Ca2Zn4(OBz)12(μ2-hmt)2]n (2) have been synthesized using basic zinc carbonate, benzoic acid (HOBz), hydrated calcium chloride and hexamethylenetetramine (hmt) by varying the molar ratio of the reactants. Both the complexes have been analyzed by elemental analysis, IR spectroscopy and X-ray crystallography. The complex 1 is a 1D polymer which contains one calcium ion and four zinc atoms in the asymmetric unit together with ten benzoates and two hmts. The polymer has been constructed by the alternate joining of paddle-wheel Zn2(OBz)4 units and Zn2Ca trinuclear species by μ2-hmt bridging molecules connecting Zn2+ ions. Zinc atoms have five coordinate square pyramidal geometries and four coordinate tetrahedral geometries in Zn2(OBz)4 and Zn2Ca moieties, respectively, whereas calcium atoms have six-coordinate distorted octahedral geometry. Complex 2 is also a 1D polymer but unlike complex 1, it contains four independent zinc and two independent calcium atoms in the asymmetric unit together with twelve benzoates and two hmts. By contrast, the polymeric structure of complex 2 has been formed by the connection of Zn2Ca trinuclear species via μ2 hmt bridging molecules at Zn centers. Complex 2 is also a 1D polymer but unlike complex 1, it contains four independent zinc and three independent calcium atoms in the asymmetric unit together with twelve benzoates and two hmts. All four zinc atoms are four coordinate with tetrahedral environments and the calcium atoms are six coordinated (two are located on a center of symmetry) exhibiting a distorted octahedral geometry.