Unusual Redox State Dependent Fermi Resonances in the Infrared Spectra of Trinuclear Ruthenium Clusters with Isocyanide Ligands

Four different redox states of the trinuclear ruthenium clusters of general formula [Ru3(μ3-O)(μ-CH3CO2)6(L)3] (L = xylyl isocyanide (1), tert-butyl isocyanide (2)) were generated electrochemically in solution and studied by reflectance infrared spectroelectrochemistry. The IR spectra in the ν(C⋮N) region show multiple-band structure in the RuIIIRuII2 (−1) and RuIII2RuII (0) states but a sharp single band in the RuIII3 (+1) or RuIVRuIII2 (+2) state. Density functional theory (DFT) ab initio calculations were performed for free xylyl isocyanide and tert-butyl isocyanide. The results suggest that the C⋮N−C bending mode interacts through Fermi resonance with the ν(C⋮N) fundamental for the 0 and −1 states of both 1 and 2. The extensive redox chemistry of the Ru3(μ3-O) clusters 1 and 2 offers a convenient means of effecting shifts in the energy of ν(C⋮N) bands of coordinated isocyanide ligands, in and out of Fermi resonance with the C⋮N−C bending overtone.