Construction of coordination polymers of cadmium (II) with mixed hexamethylenetetramine and terephthalate or thiocyanate ligands

Abstract

Three new polymeric complexes [Cd(hmt)(SCN)₂(H₂O)₂]_n (1), [Cd₃(μ₂-hmt)₂(SCN)₆(H₂O)₂]_n (2) and [Cd₂(hmt)₂(tp)₂(H₂O)₆]_n (3), [hmt = hexamethylenetetramine, tp = terephthalate] have been synthesized and characterized by single crystal X-ray diffraction. Both the compounds 1 and 2 are 1-D polymers where Cd units are linked by double end-to-end thiocyanate bridges but in 2 the chain is wider containing three cadmium atoms instead of one as found in 1. In both compounds the coordination environment around cadmium atom is distorted octahedral. Compound 3 is a three-dimensional polymer having water-filled microporous channels. Both tp and hmt are μ₂-bridged. One of the acid groups of tp is coordinated in chelating bidentate and the other in monodentate fashion. Half of its Cd atoms are hexa-coordinated and the rest are hepta-coordinated. Thermogravimetric analysis and X-ray diffraction study of 3 show that its framework remains intact upon removal of water molecules. The flexibility of coordination number around cadmium atoms (six or seven) probably plays an important role in establishing the rigidity of the framework.