A Strongly Coupled Mixed Valence State Between Ru3 Clusters. Intramolecular Electron Transfer on the Infrared Vibrational Time Scale in a Pyrazine (pz) Bridged Dimer of Triruthenium Clusters, [{Ru3(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco = 1-azabicyclo[2,2,2]octane)

A pyrazine bridged dimer of triruthenium clusters [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco =1-azabicyclo[2,2,2]octane, pz = pyrazine) gives an inter-cluster mixed valence state upon one electron reduction. The splitting of the reduction waves in cyclic voltammetry, ΔE, was 470 mV. This corresponds to a comproportionation constant for formation of the mixed valence state of 9.0 × 107. The IR spectrum of the mixed valence species in the ν(CO) region, measured by reflectance IR spectroelectrochemistry, showed a completely coalesced ν(CO) band. This represents delocalized redox behavior on the infrared vibrational timescale. The rate constant for intramolecular electron transfer was estimated to be 1 × 1012 s-1 at -18 °C by simulating the ν(CO) absorption bandshape. The mixed valence complex showed an intervalence charge transfer (ICT) band at 12500 cm-1. Hush analysis provides a spectroscopic estimate of HAB to be 2490 cm-1. Crystal structures of [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)]·2CH2Cl2 and its precursor, [RuIII2RuII(μ3-O)(CH3CO2)6(CO)(abco)2]·CH2Cl2, were determined by X-ray crystallography. Trends in Ru-O(oxo) and Ru-O(acetate) distances in these compounds show that the divalent site is localized on the Ru center attached to the carbonyl ligand.