Crystal structure and magnetic properties of two isomeric three-dimensional pyromellitate-containing cobalt(II) complexes

The hydrothermal preparation, crystal structure determination, and magnetic study of two isomers made up of 1,2,4,5-benzenetetracarboxylate and high-spin Co-II ions of formula [Co-2(bta)(H2O)(4)]n center dot 2nH(2)O (1 and 2; H(4)bta = 1,2,4,5-benzenetetracarboxylic acid) are reported. 1 and 2 are three-dimensional compounds whose structures can be described as (4,4) rectangular layers of trans-diaquacobalt(II) units with the bta(4-) anion acting as tetrakis-moncidentate ligand through the four carboxylate groups, which are further connected through other trans-[Co(H2O)(2)](2+) (1) and planar [Co(H2O)(4)](2+) (2) entities, with the bridging units being a carboxylate group in either the anti-syn (1) or syn-syn (2) conformations and a water molecule (2). The study of the magnetic properties of 1 and 2 in the temperature range 1.9-300 K shows the occurrence of weak antiferromagnetic interactions between the high-spin Coll ions, with the strong decrease of chi T-M upon cooling being mainly due to the depopulation of the higher energy Kramers doublets of the six-coordinated Coll ions. The computed values of the exchange coupling between the Coll ions across anti-syn carboxylate (1) and syn-syn carboxylate/water (2) bridges are J = -0.060 (1) and -1.90 (2) cm(-1) (with the Hamiltonian being defined as (H) over cap = -J Sigma(i,j)(S) over cap (i)center dot(S) over cap (j)). These values follow the different conformations of the carboxylate bridge in 1 (anti-syn) and 2 (syn-syn) with the occurrence of a double bridge in 2 (water/carboxylate).