An unusual μ-1,2,3-squarato-bridged two dimensional coordination polymer: Crystal structure, thermal, photoluminescence and magnetic studies

A new 2D coordination polymer, {[Ni-II(squarate)(2,2 '-bipy)(H2O)]center dot H2O}(n), [squarate = 3,4-dihydroxycyclo-but-3-ene-1,2-dionate, 2,2 '-bipy = 2,2 '-bipyridine], has been synthesized by using multi-oxygen donor squaric acid ligand and characterized by single crystal X-ray crystallographic and various spectroscopic studies. The structural analysis has revealed that the complex crystallizes in orthorhombic P-bca space group and it has 3D supramolecular structure. Within the complex, the squarato-dianion exhibits unusual mu-(1,2,3) bridging mode with Ni(II) ions and 2D coordination sheets are formed through bridging the metal ions by squarato-anions. The 2D coordination sheets are packed along crystallographic b-axis and the 2,2 '-bipyridyl (blocking ligand) moieties are hanging in the interlamellar spaces between the 2D coordination sheets. These 2D coordination sheets are further bridged by supramolecular pi center dot center dot center dot pi interactions using 2,2 '-bipyridyl ligands leading to the formation of 3D supramolecular framework which acts as a metal-organic supramolecular host (MOSH). During formation of 3D supramolecular structure, 1D supramolecular channels are formed along the crystallographic c-axis. The guest water molecules get stability within such supramolecular channels through hydrogen bonding interactions with free oxygen atoms of bridging squarate ions. The thermal study indicates that the complex decomposes in three steps. The variable temperature magnetic measurements suggest that the complex is antiferromagnetic in nature. The complex exhibits solid-state photoluminescence spectra at room temperature due to pi-pi*/n-pi* transition of the squarate and 2,2 '-bipyridine ligands. The present study points to the squarato-bridged metal complexes as unique model system to carry out the study on different bridging modes exhibited by the ligand. (C) 2013 Elsevier B. V. All rights reserved.