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 [Cu-2(L)](3+) [L = N(CH2CH2N(H)CH2CH2N(H)CH2CH2)(3)N] has been investigated by resonance Rama n and electroabsorption spectroscopy. Crystallographic Cu-2 distances of 2. 364(1) and 2.415(1) A determined for the nitrate and acetate salts, respect ively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the soli d state and in solution by the ligand environment through coupling to ligan d torsional modes that are, in turn, stabilized by hydrogen bonding. Electr oabsorption data on the three major visible and near-infrared electronic tr ansitions Of Cu2L, lambda (max) (epsilon (max)) = 1000 nm (similar to 1200 M-1 cm(-1)), 748 nm (5600 M-1 cm(-1)), and 622 nm (3350 M-1 cm(-1).), revea l a difference dipole moment between the ground and excited states (Delta m u (A)) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is m ore polarizable than the excited state. A general model to explain this beh avior 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 confo rmational states of [Cu-2(L)](3+). Separate REPs were obtained for each of the components of the two major Raman bands for nu (1) (a Cu-Cu stretching mode) and nu (2) (a Cu-Cu-N-eq bending mode). The Raman data along with qua ntum chemical ZINDO/S CI calculations provide evidence for isomeric forms o f Cu2L with strong coupling between the conformation of L and the Cu-Cu bon d length.