A computational study of the mechanism for water oxidation by (bpc)(bpy)RuIIOH2

Abstract

A mechanistic study on the catalytic cycle water oxidation with 1 [(bpc)(bpy)Ru^IIOH₂]⁺ (Hbpc = 2,2′-bipyridine-6-carboxylic acid, bpy = 2,2′-bipyridine) is described in this paper. Stepwise oxidation via proton-coupled electron transfer gives 3 [(bpc)(bpy)Ru^IVO]⁺. An active 4 [(bpc)(bpy)Ru^VO]²⁺, which is involved in the OO bond formation is generated from further 1e⁻ oxidation of 3. Another different possible reaction at 4 was investigated and new destructive paths involving overoxidation of the metal were identified. The most viable path for OO bond formation via a water nucleophilic attack at the oxo of 4 is found to be the rate-determining step in this water oxidation catalytic cycle, and the hydro-peroxo 6 [(bpc)(bpy)Ru^IIIOOH]⁺ is generated accompanied with a proton transfer. The super-oxo 7_side-on [(bpc)(bpy)Ru^IVOO]⁺ and 8_side-on [(bpc)(bpy)Ru^VOO]²⁺, both low spin species, are generated by further oxidations of 6. Through an intersystem crossing they can transform to their high spin states, 9_end-on [(bpc)(bpy)Ru^IVOO]⁺ and 12_end-on [(bpc)(bpy)Ru^VOO]²⁺, respectively. Following a dissociative pathway O₂ is readily generated from both 9_end-on and 12_end-on.