Oxygen Atom Transfer as an Alternative Pathway for Oxygen-Oxygen Bond Formation.

Abstract

Fundamental understanding of catalytic mechanisms of water oxidation is a prerequisite for the design and development of efficient and rugged water oxidation catalysts. In this work, a detailed mechanistic study of the water oxidation mechanism of the [RuII(npm)(4-pic)2(H2O)]2+ (npm = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)-pyridine, pic = 4-picoline) complex, [RuII-OH2]2+, reveals oxygen atom transfer from highly reactive ruthenium oxo intermediates to noncoordinating nitrogen atoms of the ligand as a novel route for oxygen evolution via storage of oxidizing equivalents as N-oxide groups on the ligand framework. Theoretical calculations show that the initial complex, [RuII-OH2]2+, is transformed to a di-N-oxide [RuII-OH2,(-NO)2]2+ complex upon oxidation via facile OAT steps from RuV=O species and that [RuV=O,(-NO)2]3+ represents the most likely reactive species for the critical O-O bond formation. Furthermore, a new stepwise mechanism for oxygen evolution is introduced, which proceeds via coupling of Ru-O and N-O moieties producing a peroxide intermediate, [RuV-OO-N,(-NO)]3+, and can compete with the water nucleophilic attack pathway for the oxygen evolution reaction. In this mechanism, a water molecule is oxidatively activated to an "oxygen atom" which is "stored" at a noncoordinating pyridine. Oxidative activation of a second water molecule, facilitated by coordination expansion of the intermediate N-oxide, generates the second oxygen atom required to produce a dioxygen molecule.