LONG-RANGE ANTIFERROMAGNETIC EXCHANGE IN DINUCLEAR COPPER(II) COMPLEXES CONTAINING 1,4-DICYANAMIDOBENZENE DIANION BRIDGING LIGANDS

Six Cu(II) dimers, [{Cu(dien)}(2)(mu-dicyd)][CF3SO3](2) (1), [{Cu(dien)}(2)(mu-Me(2)dicyd)][CF3SO3](2) (2), [{Cu(dien)}(2)(mu-Cl(2)dicyd)][CF3SO3](2) (3), [{Cu(L)}(2)(mu-dicyd)] (4), [{Cu(L)}(2)(mu-Me(2)dicyd)] (5), and [{Cu(L)}(2)(mu-Cl(2)dicyd)] (6), where dicyd(2-), Me(2)dicyd(2-) and Cl(2)dicyd(2-) are unsubstituted, 2,5-dimethyl- and 2,5-dichloro-1,4-dicyanamidobenzene dianions, respectively, dien is diethylenetriamine, and L(-) = 1,3-bis(2-pyridylimino)isoindolinato, have been synthesized and characterized by elemental analysis, IR, UV-vis, and EPR spectroscopy, and magnetic studies. Temperature-dependent magnetic susceptibility measurements of the complexes 1-6 from 5 to 300 K are reported. The data for 2, 3, 5, and 6 have been fitted to a dimer model with a modified Bleaney-Bowers expression which derived antiferromagnetic exchange constants -J = 10.6, 4.5, 5.2, and 3.0 cm(-1), respectively (where the Hamiltonian is of the form H = -2JS(a) . S-b). For 4, an approach to a maximum in chi(m) with decreasing temperature gave an estimated -J < 3.5 cm(-1). Only complex 1 showed Curie-Weiss behavior. This is far weaker antiferromagnetic exchange compared to that observed for dinuclear Ru(III) complexes incorporating the dicyd(2-) bridging ligands (J. Am. Chem Sec. 114, 5130 (1992)) and is attributed to a symmetry and energy mismatch between Cu(II) sigma* magnetic orbitals and the pi(nb) molecular orbitals of the bridging ligand which are important for superexchange.