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Spin Exchange Effects on the Physicochemical Properties of Tetraoxolene-Bridged Bimetallic Complexes

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posted on 16.04.2007, 00:00 by Dong Guo, James K. McCusker
The synthesis, physical, and spectroscopic properties of a series of metal complexes bridged by the redox-active chloranilate ligand are described. Compounds containing the (CAcat,cat)4- ligand, where (CAcat,cat)4- represents the fully reduced aromatic form of chloranilate, have been prepared by two different routes from H2CA and H4CA starting materials; the corresponding (CAsq,cat)3- analogue was obtained by one-electron oxidation with decamethylferrocenium tetrafluoroborate. Homo- and heterobimetallic complexes containing CrIII and GaIII with chloranilate have been prepared, yielding the following six complexes:  [Ga2(tren)2(CAcat,cat)](BPh4)2 (1), [Ga2(tren)2(CAsq,cat)](BPh4)2(BF4) (2), [GaCr(tren)2(CAcat,cat)](BPh4)2 (3), [GaCr(tren)2(CAsq,cat)](BPh4)2(BF4) (4), [Cr2(tren)2(CAcat,cat)] (BPh4)2 (5), and [Cr2(tren)2(CAsq,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 μeff = 1.64 ± 0.04 μB in the solid state between 4 and 350 K, consistent with the CAsq,cat formulation of the ligand and an S = 1/2 ground state for complex 2. Complex 3 exhibits a value of μeff = 3.44 ± 0.09 μB that is also temperature-independent, indicating an S = 3/2 ground state. Complexes 46 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 CrIII 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 CrIII centers (J = −2 cm-1 for Ĥ = −2JŜ1·Ŝ2) via superexchange through the diamagnetic CAcat,cat bridge. The absorption spectra of the CAsq,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.