Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru 2 Mn Complex

The heterotrinuclear complex  A {[Ru II (H 2 O)(tpy)] 2 (μ‐[Mn II (H 2 O) 2 (bpp) 2 ])} 4+ [tpy=2,2′:6′,2′′‐terpyridine, bpp=3,5‐bis(2‐pyridyl)pyrazolate] was found to catalyze water oxidation both electrochemically and photochemically with [Ru(bpy) 3 ] 3+ (bpy=2,2′‐bipyridine) as the photosensitizer and Na 2 S 2 O 8 as the electron acceptor in neutral phosphate buffer. The mechanism of water oxidation catalyzed by this unprecedented trinuclear complex was studied by 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 Ru1 IV Mn III Ru2 III ), 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 Ru1 IV Mn V Ru2 IV ), which 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 O 2 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 (Ru IV Mn IV Ru IV ).