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-cpiaH3 (5-(2-carboxy-phenoxy)-isophthalic
acid). This ligand readily reacts with CoCl2·6H2O 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, {Co3(o-cpia)2(phen)­(H2O)·5H2O}n (1), {Co1.5(o-cpia)­(bpy)}n (2), {Co2(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.