Long range antiferromagnetic exchange in dinuclear copper(II) complexes containing 1,4-dicyanamidobenzene dianion bridging ligands

Six Cu(II)dimers, [{Cu(dien)}2(μ-dicyd)][CF3SO3]2 (1), [{Cu(dien)}2(μ-Me2dicyd)][CF3SO3]2 (2), [{Cu(dien)}2(μ-Cl2dicyd)][CF3SO3]2 (3), [{Cu(L)}2(μ-dicyd)] (4), [{Cu(L)}2(μ-Me2dicyd)] (5), and [{Cu(L)}2(μ-Cl2dicyd)] (6), where dicyd2−, Me2dicyd2− and Cl2dicyd2− 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 [Formula: see text]) For 4, an approach to a maximum in χm 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 dicyd2− bridging ligands (J. Am. Chem Soc. 114, 5130 (1992)) and is attributed to a symmetry and energy mismatch between Cu(II) σ* magnetic orbitals and the πnb molecular orbitals of the bridging ligand which are important for superexchange. Keywords: superexchange, copper dimer, dicyanamidobenzene.