Supramolecular Structures Constructed by Connection of One-Dimensional Polymer Chains Through the Dimeric Water Clusters in Coordination Polymers of Pyromellitate Ligand

Abstract

Three mixed-ligand coordination polymers [Cu(H(2)pm)(na)(2)(H2O)(2)] (n) center dot 4nH(2)O (1), [Ni(H(2)pm)(pa)(2)] (n) center dot 4nH(2)O (2), [Co(H(2)pm)(pa)(2)] (n) center dot 4nH(2)O (3) (H(2)pm = dianion of pyromellitic acid, na = nicotinamide, pa = picolinamide) were investigated mainly from the point of supramolecular architecture in their respective crystal packings. The structural properties of 1-3 were characterized by X-ray diffraction technique and Fourier transform infrared (FT-IR) spectroscopy. In all complexes, the metal centers are bridged by pyromellitate ligands to form 1D polymer chains of 2,2-TT type. However, these chains propagate in a different way as linear or zig-zag fashions. The polymer chains are connected by N-H center dot center dot center dot O hydrogen bonds to form sheet structures which include dimeric water clusters. The complexes have different supramolecular networks with respect to the behavior of dimeric water clusters between the sheet structures. The FT-IR investigation of the complexes was performed within the mid-IR region, mainly focusing on the characteristic vibrations of pyromellitic acid, nicotinamide and picolinamide moieties by considering their free states and ligand behaviours in the case of complex formation. The magnetic properties of 1-3 were investigated by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. EPR studies of 1 and 2 show that the paramagnetic centers have axially symmetric g values and unpaired electrons are dominantly in ground state. The magnetic susceptibility results indicate possible antiferromagnetic (1 and 2) and ferromagnetic (3) couplings mediated by the carboxylate bridges between the metal centers. The adjacent supramolecular sheets are linked by dimeric water clusters.