New Water Oxidation Chemistry of a Seven-Coordinate Ruthenium Complex with a Tetradentate Polypyridyl Ligand

The mononuclear ruthenium­(II) complex [<b>Ru</b>]²⁺ (<b>Ru</b> = 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 (<i>Inorg. Chem</i>. <b>2008</b>, 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 [^<b>2</b><b>Ru</b>^<b>III</b>]³⁺ and [^<b>2</b><b>Ru</b>^<b>V</b>(O)]³⁺ to [^<b>1</b><b>Ru</b>^<b>IV</b>(OOH)]³⁺ then [^<b>2</b><b>Ru</b>^<b>III</b>(···³O₂)]³⁺ at pH 0, and through [^<b>3</b><b>Ru</b>^<b>IV</b>(O)]²⁺, [^<b>2</b><b>Ru</b>^<b>V</b>(O)]³⁺, and [^<b>1</b><b>Ru</b>^<b>IV</b>(OO)]²⁺ at pH 9 before reaching the same [^<b>2</b><b>Ru</b>^<b>III</b>(···³O₂)]³⁺ species, from which the liberation of the weakly bound O₂ might require an additional oxidation to form [^<b>3</b><b>Ru</b>^<b>IV</b>(O)]²⁺ to initiate further cycles involving all seven-coordinate species