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 Ru 3 (μ 3 -O)(μ-CH 3 CO 2 ) 6 (CO)(L)(μ-L‘)Ru 3 (μ 3 -O)(μ-CH 3 CO 2 ) 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, Δ E, 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, Δ E, 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 × 10 11, 5 × 10 11, and ca. 1 × 10 11 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 × 10 10 and 1 × 10 11 s - 1 .