Role of Hydrogen Bonding in Photoinduced Electron–Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

Electron–proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1­(T), can be described by a kinetic model that accounts for the H-bonding of 1­(T) to phenol, 1­(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1­(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either 1­(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1­(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.