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 CoII ions of formula [Co2(bta)(H2O)4]n·2nH2O (1 and 2; H4bta = 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 bta4− anion acting as tetrakis-monodentate 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 CoII ions, with the strong decrease of χMT upon cooling being mainly due to the depopulation of the higher energy Kramers doublets of the six-coordinated CoII ions. The computed values of the exchange coupling between the CoII 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 Ĥ = −J∑i,jŜi·Ŝj). 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).