Driving force and isotope dependence of the kinetics of proton-coupled electron transfer in oxoruthenium(IV) polypyridyl complexes

The kinetics of the comproportionation reaction of Ru(tpy)(bpy)O2+ and Ru(t py)(bpy)OH22+ to produce Ru(tpy)(bpy)OH2+ were evaluated in a series of com plexes that were substituted on the 4' position of the tpy ligand or the 4 and 4' positions of the bpy ligand (tpy = 2,2':6',2 "-terpyridine, bpy = 2, 2'-bipyridine). These substitutions did not change the steric or coordinati on environments about the Ru-O linkage but did modulate the driving force ( -Delta G degrees) for comproportionation over a range of similar to 8 kJ/mo l. The comproportionation reaction, which involves a net hydrogen atom tran sfer between the metal complexes, showed a linear dependence of its rate co nstant on the driving force across the range studied, with a slope of 0.66 +/- 0.06 for H2O and 0.64 +/- 0.05 for D2O. Thus, the slopes were in reason ably goad agreement with the value of 0.5 predicted by Marcus theory and, a s also expected, showed no effect of the driving force an the isotope effec t. The isotope effect for the Ru(tpy)(bpy)O2+ complex (11.5) was significan tly lower than that for Ru(bpy)(2)(py)O2+ (16.1) at the same driving force. The Ru(bpy)(2)(py)O2+ complex is more sterically crowded at the oxo ligand , so the likely origin of the isotope effect is the distance of transfer fo r the proton in the reaction.