Spin exchange effects on the physicochemical properties of tetraoxolene-bridged bimetallic complexes

The synthesis, physical, and spectroscopic properties of a series of metal complexes bridged by the redox-active chloranilate ligand are described. Compounds containing the (CA(cat,cat))(4-) ligand, where (CA(cat,cat))(4-) represents the fully reduced aromatic form of chloranilate, have been prepared by two different routes from H(2)CA and H(4)CA starting materials; the corresponding (CA(sq,cat))(3-) analogue was obtained by one-electron oxidation with decamethylferrocenium tetrafluoroborate. Homo- and heterobimetallic complexes containing Cr-III and Ga-III with chloranilate have been prepared, yielding the following six complexes: [Ga-2(tren)(2)(CA(cat,cat))](BPh4)(2) (1), [Ga-2(tren)(2)(CA(sq,cat))](BPh4)(2)(BF4) (2), [GaCr(tren)(2)(CA(cat,cat))](BPh4)(2) (3), [GaCr(tren)(2)(CA(sq,cat))](BPh4)(2)(BF4) (4), [Cr-2(tren)(2)(CA(cat,cat))] (BPh4)(2) (5), and [Cr-2(tren)(2)(CA(sq,cat))](BPh4)(2)(BF4) (6) (where tren is tris(2-aminoethyl)amine). Single-crystal X-ray structures have been obtained for complexes 1, 3, and 5; nearly identical C-C bond distances within the quinoidal ligand confirm the aromatic character of the bridge in each case. Complex 2 exhibits a temperature-independent magnetic moment of mu(eff) = 1.64 +/- 0.04 mu(B) in the solid state between 4 and 350 K, consistent with the CA(sq,cat) formulation of the ligand and an S = 1/2 ground state for complex 2. Complex 3 exhibits a value of mu(eff) = 3.44 +/- 0.09 mu(B) that is also temperature-independent, indicating an S = 3/2 ground state. Complexes 4-6 are all influenced by Heisenberg spin exchange. The temperature-independent behavior of complexes 4 and 6 indicate the presence of strong antiferromagnetic exchange between the Cr-III and the (sq,cat) bridging radical yielding well-isolated ground states of S = 1 and 5/2 for 4 and 6, respectively. In contrast, complex 5 exhibits a weak intramolecular antiferromagnetic exchange interaction between the two Cr-III centers (J = -2 cm(-1) for = -2J(1).(2)) via superexchange through the diamagnetic CA(cat,cat) bridge. The absorption spectra of the CA(sq,cat)-containing complexes exhibit a number of sharp, relatively intense features in fluid solution. Group theoretical arguments coupled with a qualitative ligand-field analysis including the effects of Heisenberg spin exchange suggest that several of the observed transitions are a consequence of exchange interactions in both the ground- and excited-state manifolds of the compounds.