Electron transfer on the infrared vibrational time scale in the mixed valence state of 1,4-pyrazine- and 4,4'-bipyridine-bridged ruthenium cluster complexes

Intramolecular electron transfers within the mixed valence states of the ligand bridged hexaruthenium clusters Ru₃(μ₃-O)(μ-CH₃CO₂)₆(CO)(L)(μ- L')Ru₃(μ₃-O)(μ-CH₃CO₂)₆(CO)(L) (L' = 1,4-pyrazine; L = 4- dimethylaminopyridine (1), pyridine (2), 4-cyanopyridine (3), or L' = 4,4'- bipyridine; L = 4-dimethylaminopyridine (4), pyridine (5), 4-cyanopyridine (6)) were examined. Two discrete and reversible single electron reductions are evident by cyclic voltammetry in the redox chemistry of 1-5, and the intercluster charge-transfer complexes are well-defined. The splitting of the reduction waves, ΔE, is related to the electronic coupling H(AB) between the triruthenium clusters, and varies from 80 mV for 5 to 440 mV for 1. In the case of 6, the splitting of the reduction waves, ΔE, is <50 mV and the intercluster charge-transfer complex is not defined. The mixed valence states of 1-3 also exhibit intervalence charge transfer (ICT) bands in the region 12 100 (1) to 10 800 cm^-1 (3) which provide spectroscopic estimates of H(AB) in the range 2180 (1) to 1310 cm^-1 (3). The magnitude of the electronic coupling H(AB) is found to strongly influence the IR spectra of the singly reduced (-1) mixed valence states of 1-6 in the v(CO) region. In the case of relatively weak electronic coupling (4-6), two v(CO) bands are clearly resolved. In the cases of strong electronic coupling (1-3), these bands broaden to a single v(CO) absorption band. These data allow the rate constants, k(e), for electron transfer in the mixed valence states of 1, 2, and 3 to be estimated by simulating dynamical effects (Bloch-type equations) on v(CO) absorption band shape at 9 x 10¹¹, 5 x 10¹¹, and ca. 1 x 10¹¹ s^-1, respectively. The less strongly coupled 4,4'-bipyridine-bridged complexes 4-6 exhibit IR line shapes in the -1 mixed valence states that are not as strongly affected by electron-transfer dynamics. The rate constant for the -1 mixed valence state of 4 is close to the lower limit that can be estimated by this approach, between 1 x 10¹⁰ and 1 x 10¹¹ s^-1.