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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

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journal contribution
posted on 1999-04-29, 00:00 authored by Tasuku Ito, Tomohiko Hamaguchi, Haruko Nagino, Tadashi Yamaguchi, Hiroaki Kido, Igor S. Zavarine, Todd Richmond, John Washington, Clifford P. Kubiak
Intramolecular electron transfers within the mixed valence states of the ligand bridged hexaruthenium clusters Ru33-O)(μ-CH3CO2)6(CO)(L)(μ-L‘)Ru33-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 15, and the intercluster charge-transfer complexes are well-defined. The splitting of the reduction waves, ΔE, is related to the electronic coupling HAB 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 13 also exhibit intervalence charge transfer (ICT) bands in the region 12 100 (1) to 10 800 cm-1 (3) which provide spectroscopic estimates of HAB in the range 2180 (1) to 1310 cm-1 (3). The magnitude of the electronic coupling HAB is found to strongly influence the IR spectra of the singly reduced (−1) mixed valence states of 16 in the ν(CO) region. In the case of relatively weak electronic coupling (46), two ν(CO) bands are clearly resolved. In the cases of strong electronic coupling (13), these bands broaden to a single ν(CO) absorption band. These data allow the rate constants, ke, 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 46 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.

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