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 [(RuIII)-Ru-2](3+) and [Ru-2(V)(O)](3+) to [Ru-1(IV)(OOH)](3+)then [Ru-2(III)(center dot center dot center dot O-3(2))](3+) at pH 0, and through [Ru-3(IV)(O)](2+), [Ru-2(V)(O)](3+), and [Ru-1(IV)(OO)](2+) at pH 9 before reaching the same [Ru-2(III)(center dot center dot center dot O-3(2))](3+) species, from which the liberation of the weakly bound O-2 might require an additional oxidation to form [Ru-3(IV)(O)](2+) to initiate further cycles involving all seven-coordinate species.