Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents

Complexes of the type [Ru(bpy)2pyimH]2+ [bpy = 2,2′‐bipyridine; pyimH = 2‐(2‐pyridyl)imidazole] with various substituents on the bpy ligands can act as photoreductants. Their reducing power in the ground state and in the long‐lived 3MLCT excited state is increased significantly upon deprotonation, and they can undergo proton‐coupled electron transfer (PCET) in the ground and excited state. PCET with both the proton and electron originating from a single donor resembles hydrogen atom transfer (HAT) and can be described thermodynamically by formal bond dissociation free energies (BDFEs). Whereas the class of complexes studied herein has long been known, their N–H BDFEs have not been determined even though this is important in view of assessing their reactivity. Our study demonstrates that the N–H BDFEs in the 3MLCT excited states are between 34 and 52 kcal mol–1 depending on the chemical substituents at the bpy spectator ligands. Specifically, we report on the electrochemistry and PCET thermochemistry of three heteroleptic complexes in 1:1 (v/v) CH3CN/H2O with CF3, tBu, and NMe2 substituents on the bpy ligands.