Co(II) Coordination Polymers with Co-Ligand Dependent Dinuclear to Tetranuclear Core: Spin-Canting, Weak Ferromagnetic, and Antiferromagnetic Behavior

Four new Co(II) containing coordination polymers have been synthesized using an ether bridged tricarboxylic acid ligand, o-cpiaH(3) (5-(2-carboxy-phenoxy)-isophthalic acid). This ligand readily reacts with CoCl2 center dot 6H(2)O in the presence of different nitrogen donor ligands such as 1,10-phenanthroline (phen), 4,4'-bipyridyl (bpy), 4,4'-azopyridine (apy), and 1,4-bis(4-pyridinylmethyl)piperazine (bpmp) under hydrothermal conditions to afford three 3D and one 2D coordination polymers, {Co-3(o-cpia)(2)(phen)(H2O)center dot 5H(2)O}(n) (1), {Co-1.5(o-cpia)(bpy)}(n) (2), {Co-2(o-cpia)(OH)(apy)(0.5)}(n) (3), and {Co(o-cpiaH)(bpmp)(0.5)}(n) (4). Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric to tetrameric depending upon the co-ligand used. In complexes 1 and 2, Co(II) ions form an angular and linear trimeric unit that extends along all directions to generate an overall 3D structure. In contrast, 3 forms a 3D coordination polymer containing a tetranuclear Co(II) unit. When the distance between the two donor N atoms of the co-ligand is further increased as in 4, a (4,4) net connected 2D coordination polymer results where the Co(II) ions form a dimeric paddle wheel unit. In addition to single-crystal X-ray crystallography, the complexes are also characterized by IR spectroscopy, thermogravimetry, and elemental analysis. Variable temperature magnetic susceptibility measurements on the complexes were carried out over the temperature range 1.72-300 K. Complex 1 exhibits ferromagnetic interactions due to uncompensated magnetic moments of the system leading to spin-canted antiferromagnetic behavior, while 2 and 3 show predominantly antiferromagnetic interactions. Complex 4 exhibits weak ferromagnetic behavior below 9 K.