Synthesis, characterization and magnetism of binuclear cobalt(II) complexes bridged by tetracarboxylato groups

Three new binuclear cobalt(II) complexes with extended tetracarboxylato- bridge have been synthesized and characterized, namely [Co-2 (PMTA) (bpy)(4)] (1), [CO2 (PBMTA)-(phen)(4)] (2) and [Co2(PMTA) (NO(2)phen)(4)] (3), where PMTA represents the tetraanion of pyromellitic acid, and bpy, phen, NO2-phen denote 2,2'-bipyridine, 1,10-phenanthroline; 5-nirto-1,10-phenanthroline, respectively. Based on elemental analyses, molar conductivity measurements, IR and electronic spectra studies, it is proposed that these complexes have PMTA-bridged structures and consist of two cobalt(II) ions, each in a distorted octahedral environment. These complexes were further characterized by variable temperature magnetic susceptibility measurements (4-300 K) and the observed data were successfully simulated by the equation based on the spin Hamiltonian operator, (H) over cap = - 2J (S) over cap(1) . (S) over cap(2), giving the exchange integral J = -1.02 cm(-1) for 1, J = - 1.21 cm(-1) for 2 and J = -1.18 cm(-1) for 3, respectively. These results revealed the operation of antiferromagnetic spin-exchange interaction between the two Co(II) ions within each molecule.