文摘
The catalytic water oxidation mechanism proposed for many single-site ruthenium complexes proceeds via the nucleophilic attack of a water molecule on the RuV鈺怬 species. In contrast, Ru(II) complexes containing 4-t-butyl-2,6-di-1鈥?8鈥?(naphthyrid-2鈥?yl)-pyridine (and its bisbenzo-derivative), an equatorial water, and two axial 4-picolines follow the thermodynamically more favorable 鈥渄irect pathway鈥?via [RuIV鈺怬]2+, which avoids the higher oxidation state [RuV鈺怬]3+ in neutral and basic media. Our experimental and theoretical results that focus on the pH-dependent onset catalytic potentials indicative of a PCET driven low-energy pathway for the formation of products with an O鈥揙 bond (such as [RuIII鈥揙OH]2+ and [RuIV鈥揙O]2+) at an applied potential below the RuV鈺怬/RuIV鈺怬 couple clearly support such a mechanism. However, in the cases of [Ru(tpy)(bpy)(OH2)]2+ and [Ru(tpy)(bpm)(OH2)]2+, the formation of the RuV鈺怬 species appears to be required before O鈥揙 bond formation. The complexes under discussion provide a unique functional model for water oxidation that proceeds by four consecutive PCET steps in neutral and alkaline media.