New water oxidation chemistry of a seven-coordinate ruthenium complex with a tetradentate polypyridyl ligand

The mononuclear ruthenium(II) complex [Ru]²⁺ (Ru = Ru(dpp)(pic)₂, where dpp is the tetradentate 2,9-dipyrid-2′-yl- 1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [²Ru ^III]³⁺ and [²Ru^V(O)]³⁺ to [¹Ru^IV(OOH)]³⁺ then [²Ru ^III(···³O₂)]³⁺ at pH 0, and through [³Ru^IV(O)]²⁺, [ ²Ru^V(O)]³⁺, and [¹Ru ^IV(OO)]²⁺ at pH 9 before reaching the same [ ²Ru^III(···³O ₂)]³⁺ species, from which the liberation of the weakly bound O₂ might require an additional oxidation to form [ ³Ru^IV(O)]²⁺ to initiate further cycles involving all seven-coordinate species.