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 ν(CO) region. In the case of relatively weak electronic coupling (4−6), two ν(CO) bands are clearly resolved. In the cases of strong electronic coupling (1−3), these bands broaden to a single ν(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 ν(CO) absorption band shape at 9 × 10¹¹, 5 × 10¹¹, and ca. 1 × 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 × 10¹⁰ and 1 × 10¹¹ s^-1.