Electrostatic and conformational effects on the electronic structures of distortional isomers of a mixed-valence binuclear Cu complex.

The electronic structure of the binuclear copper complex [Cu2(L)] 3+ [L ) N(CH2CH2N(H)CH2CH2N(H)CH2CH2)3N] has been investigated by resonance Raman and electroabsorption spectroscopy. Crystallographic Cu2 distances of 2.364(1) and 2.415(1) A determined for the nitrate and acetate salts, respectively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the solid state and in solution by the ligand environment through coupling to ligand torsional modes that are, in turn, stabilized by hydrogen bonding. Electroabsorption data on the three major visible and near-infrared electronic transitions of Cu 2L, Imax (max) ) 1000 nm (1200 M -1 cm -1 ), 748 nm (5600 M -1 cm -1 ), and 622 nm (3350 M -1 cm -1 ), reveal a difference dipole moment between the ground and excited states (¢IA) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is more polarizable than the excited state. A general model to explain this behavior in terms of the proximity of accessible transitions involving copper d electrons is proposed to explain the larger polarizability of the ground state. Raman excitation profiles (REPs) provide evidence for multiple conformational states of [Cu2(L)] 3+ . Separate REPs were obtained for each of the components of the two major Raman bands for O1 (a Cu-Cu stretching mode) and O2 (a Cu-Cu-Neq bending mode). The Raman data along with quantum chemical ZINDO/S CI calculations provide evidence for isomeric forms of Cu2L with strong coupling between the conformation of L and the Cu-Cu bond length.