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 ).
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