Theoretical study of catalytic mechanism for single-site water oxidation process

Water oxidation is a linchpin in solar fuels formation, and catalysis by single-site ruthenium complexes has generated significant interest in this area. Combining several theoretical tools, we have studied the entire catalytic cycle of water oxidation for a single-site catalyst starting with [Ru-II(tpy)(bpm)(OH2)](2+) (i.e., [Ru-II-OH2](2+); tpy is 2,2': 6',2 ''-terpyridine and bpm is 2,2'-bypyrimidine) as a representative example of a new class of single-site catalysts. The redox potentials and pK(a) calculations for the first two proton-coupled electron transfers (PCETs) from [Ru-II-OH2](2+) to [Ru-IV = O](2+) and the following electron-transfer process to [Ru-V = O](3+) suggest that these processes can proceed readily in acidic or weakly basic conditions. The subsequent water splitting process involves two water molecules, [Ru-V = O](3+) to generate [Ru-III-OOH](2+), and H3O+ with a low activation barrier (similar to 10 kcal/mol). After the key O-O bond forming step in the single-site Ru catalysis, another PECT process oxidizes [Ru-III-OOH](2+) to [Ru-IV-OO](2+) when the pH is lower than 3.7. Two possible forms of [Ru-IV-OO](2+), open and closed, can exist and interconvert with a low activation barrier (<7 kcal/mol) due to strong spin-orbital coupling effects. In Pathway 1 at pH = 1.0, oxygen release is rate-limiting with an activation barrier similar to 12 kcal/mol while the electron-transfer step from [Ru-IV-OO](2+) to [Ru-V-OO](3+) becomes rate-determining at pH = 0 (Pathway 2) with Ce(IV) as oxidant. The results of these theoretical studies with atomistic details have revealed subtle details of reaction mechanisms at several stages during the catalytic cycle. This understanding is helpful in the design of new catalysts for water oxidation.