(4,4) Rectangular Lattices of Cobalt(II) with 1,2,4,5-Benzenetetracarboxylic Acid: Influence of the Packing in the Crystal Structure

The preparation, structural characterization and magnetic investigation of three new cobalt(II) complexes of formula [Co-2(bta)(H2O)(8)](n)center dot 4nH(2)O (1 and 2) and [Co-2(bta)(H2O)(8)](n)center dot 2nH(2)O (3) (H(4)bta = 1,2,4,5-benzenetetracarboxylic acid) are reported. The crystal structures of 1-3 consist of neutral square-grids of high-spin cobalt(II) ions with the fully deprotonated bta(4-) anion acting as a 4-fold connector. The interpenetration of two of these layers in 3 leads to a three-dimensional network. Each cobalt atom in 1-3 is six-coordinated with two carboxylate-oxygen atoms from two bta ligands in trans positions and four water molecules building somewhat distorted octahedral surroundings. Extensive hydrogen bonding involving the noncoordinated carboxylate-oxygens and the coordinated and crystallization water molecules afford three-dimensional supramolecular networks in all three compounds. The values of the intralayer cobalt-cobalt separations through cis- and trans-coordinated carboxylate groups are 6.6831(16) and 11339(2) angstrom for 1, and 6.682(4) and 11.329(5) angstrom for 2 and 6.7801(5)/6.7705(3) and 11.3923(4)/11.2952(3) angstrom for (3). Magnetic susceptibility measurements of polycrystalline samples of 1-3 in the temperature range 1.9-295 K show the occurrence of very weak antiferromagnetic interactions between the high-spin cobalt(II) ions, the strong decrease of chi T-M upon cooling being mainly due to the depopulation of the higher Kramers doublets of the six-coordinated cobalt(II) ions.