Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

The heterotrinuclear complex A {[RuII(H2O)(trpy)]2(μ-[MnII(H2O)2(bpp)2])}4+ was disclosed to catalyze water oxidation both electrochemically and photochemically with [Ru(bpy)3]3+ as the photosensitizer and Na2S2O8 as the electron acceptor in neutral phosphate buffer. The mechanism of water oxidation catalyzed by this unprecedented trinuclear complex was studied via density functional calculations. The calculations showed that a series of oxidation and deprotonation events take place from A, leading to the formation of complex 1 (formal oxidation state of Ru1IVMnIIIRu2III), which is the starting species for the catalytic cycle. Three sequential oxidations of 1 result in the generation of the catalytically competing species 4 (formal oxidation state of Ru1IVMnVRu2IV) that triggers the O-O bond formation. The direct coupling of two adjacent oxo ligands bound to Ru and Mn leads to the production of a superoxide intermediate Int1. This step was calculated to have a barrier of 7.2 kcal mol-1 at the B3LYP*-D3 level. Subsequent O2 release from Int1 turns out to be quite facile. Other possible pathways were found to be much less favorable, including water nucleophilic attack, the coupling of an oxo and a hydroxide, and the direct coupling pathway at a lower oxidation state (RuIVMnIVRuIV).