Proton coupled electron transfer from the excited state of a ruthenium(II) pyridylimidazole complex

Proton coupled electron transfer (PCET) from the excited state of [Ru(bpy)(2)pyimH](2+) (bpy = 2,2'-bipyridine; pyimH = 2-(2'-pyridyl)imidazole) to N-methyl-4,4'-bipyridinium (monoquat, MQ(+)) was studied. While this complex has been investigated previously, our study is the first to show that the formal bond dissociation free energy (BDFE) of the imidazole-N-H bond decreases from (91 +/- 1) kcal mol(-1) in the electronic ground state to (43 +/- 5) kcal mol(-1) in the lowest-energetic (MLCT)-M-3 excited state. This makes the [Ru(bpy)(2)pyimH](2+) complex a very strong (formal) hydrogen atom donor even when compared to metal hydride complexes, and this is interesting for light-driven (formal) hydrogen atom transfer (HAT) reactions with a variety of different substrates. Mechanistically, formal HAT between (MLCT)-M-3 excited [Ru(bpy)(2)pyimH](2+) and monoquat in buffered 1:1 (v:v) CH3CN/H2O was found to occur via a sequence of reaction steps involving electron transfer from Ru(II) to MQ(+) coupled to release of the N-H proton to buffer base, followed by protonation of reduced MQ(+) by buffer acid. Our study is relevant in the larger contexts of photoredox catalysis and light-to-chemical energy conversion.