Electron transfer in dinuclear cobalt complexes with ''non-innocent'' bridging ligands

Reaction of tripod cobalt(II) templates [{CH3C(CH2PAr2)(3)}Co-II] with potentially bridging ligands L generates the dinuclear compounds [(tripod)-Co-L-Co(tripod)](2+). With L = oxalate (C2O42-) a biscobalt(II) complex (1) is Formed, while with L = C6H2O42-, the dianion derived from 2,5-dihydroxy-1,4-benzoquinone (anilic acid), two-electron transfer within the dimetallic unit occurs and a biscobalt(III) charge distribution results (2a), as shown by X-ray structural analyses of 1 and 2 a, NMR spectroscopy, and theoretical investigations by the INDO method, Complex 2a exhibits an unusually intense, low-energy absorption in its electronic spectrum; this is explained with a simple MO model. One-electron reduction of 2 a generates the corresponding mixed-valence complex, which is highly stabilised through extensive electron delocalisation. Substituents at the 3,6 posi tions of the bridging ligand (Cl, Br, I, NO2, Me, iPr. Ph: 2b-h) as well as alkyl substitution at the aromatic rings of the tripod ligands (3,4) influence the optical and electrochemical properties consistent with the proposed model of charge distribution, Formal replacement of one [(tripod)Co-III](3+) moiety by [CH2](2+) leads to the mononuclear complex 6, which is shown to be a typical [(tripod)Co-III-catecholato)](+) complex, Therefore the substantially different optical and electrochemical properties of the dinuclear complexes with respect to those of 6 result from strong metal-metal interactions mediated by the bridging ligand.