Stable Mixed-Valent Complexes Formed by Electron Delocalization Across Hydrogen Bonds of Pyrimidinone-Linked Metal Clusters.

Electron transfer across a mixed-valent hydrogen-bonded self-dimer of oxo-centered triruthenium clusters bridged by a pair of 4(3 H)-pyrimidinones is reported. Spectroelectrochemical studies in methylene chloride reveal that 1 rapidly self-dimerizes upon one-electron reduction, forming the strongly coupled mixed-valent hydrogen-bonded dimer (1). In the mixed-valent state, significantly broadened, partially coalesced ν(CO) bands are observed, allowing estimation of the electron transfer rate ( k) by an optical Bloch line shape analysis. Simulation of the FTIR line shapes provides an estimate of k on the order of 10 s, indicating a highly delocalized electronic structure across the hydrogen bonds. These findings are supported by the determination of the formation constant ( K) for (1), which is found to be on the order of 10 M, or nearly 4 orders of magnitude higher than that for the neutral isovalent dimer (1). This represents a stabilization of approximately 5.7 kcal/mol (1980 cm) arising from electron exchange across the hydrogen bonds in the mixed-valent state. Significantly, an enormous intensity enhancement of the amide ν(NH) band (3300 cm) of (1) is observed, supporting strong mixing of the bridging ligand vibrational modes with the electronic wave function of the mixed-valent state. These findings demonstrate strong donor-bridge-acceptor coupling and that highly delocalized electronic structures can be attained in hydrogen-bonded systems, which are often considered to be too weakly bound to support strong electronic communication.