Intramolecular electron transfer on the vibrational timescale in mixed valence ruthenium clusters

The thermodynamic stability of the mixed valence (one electron reduced) state between linked Ru3 units was studied by means of electrochemical methods for the series of the ligand-bridged triruthenium cluster dimer, [Ru₃(μ-O)(n-CH₃CO₂) ₆(CO)(L)(n-BL)Ru₃(μ-O)(μ-CH₃CO ₂)₆(CO)(L)] (BL = 1,4-pyrazine: L = 4-dimethylaminopyridine (dmap) (1a), pyridine (py) (1b), 4-cyanopyridine (cpy) (1e), 1-azabicyclo[2.2.2]octane (1d); BL = 4,4′-bipyridine: L = dmap (2a), py (2b), cpy (2c); BL = 2,7-diazapyrene: L = dmap (3a); BL = 1,4-diazabicyclo[2.2.2]octane: L = dmap (4a), py (4b), cpy (4c)). The mixed valence states undergoing rapid intramolecular electron transfers were observed by ER spectroelectrochemistry. By simulating dynamical effects on the observed v(CO) absorption bandshapes, the rate constants k_e for electron transfer in the mixed valence states of la, Ib, le and Id were estimated to be 9×10¹¹ s^-1 (at room temperature (rt)), 5×10¹¹ s^-1 (at it), c.a,1×10¹¹ s^-1 (at it), and 1×10¹² s^-1 (at-18 °C), respectively. Possible applications of this approach to asymmetric mixed valence systems were discusssed.