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 (H2pm)(na)2(H2O)2)n� 4nH2 O( 1), (Ni(H2pm)(pa)2)n� 4nH2O (2), (Co(H2pm)(pa)2)n� 4nH2 O( 3 )( H 2pm = 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��� 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 com- plexes was performed within the mid-IR region, mainly focusing on the characteristic vibrations of pyromellitic acid, nicotinamide and picolinamide moieties by consid- ering 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 dx2� y2 ground state. The magnetic suscepti- bility results indicate possible antiferromagnetic (1 and 2) and ferromagnetic (3) couplings mediated by the carbox- ylate bridges between the metal centers.