The Role of Seven-Coordination in Ru-Catalyzed Water Oxidation

A family of Ru complexes based on the pentadentate ligand t5a(3-) ((2,5-bis(6-carboxylatopyridin-2-yl)pyrrol-1-ide) and pyridine (py) that includes {Ru-II(Ht5a-kappa-(NO)-O-2)(py)(3)} (1H(II)(kappa-(NO)-O-2)), {Ru-III(t5a-kappa-(NO1.5)-O-3)(py)(2)} (2(III)(kappa-(NO1.5)-O-3)), and {Ru-IV(t5a-kappa-(NO2)-O-3)(py)(2)}(+) ({2(IV)(kappa-(NO2)-O-3)}(+)) has been prepared and thoroughly characterized. Complexes 1H(II)(kappa-(NO)-O-2), 2(III)(kappa-(NO1.5)-O-3), and {2(IV)(kappa-(NO2)-O-3)}(+) have been investigated in solution by spectroscopic methods (NMR, UV-vis) and in the solid state by single-crystal X-ray diffraction analysis and complemented by density functional theory (DFT) calculations. The redox properties of complex 2(III)(kappa-(NO1.5)-O-3) have been studied by electrochemical methods (CV and DPV), showing its easy access to high oxidation states, thanks to the trianionic nature of the t5a(3-) ligand. Under neutral to basic conditions complex {2(IV)(kappa-(NO2)-O-3)}(+) undergoes aquation, generating {Ru-IV(OH)(t5a-kappa-(NO)-O-2)(py)(2)} (2(IV)(OH)(kappa-(NO)-O-2)). Further oxidation of the complex forms {Ru-V(O)(t5a-kappa-(NO)-O-2)(py)(2)} (2(V)(O)(kappa-(NO)-O-2)), which is a very efficient water oxidation catalyst, reaching a TOFMAX value of 9400 s(-1) at pH 7.0, as measured via foot of the wave analysis. The key to fast kinetics for the catalytic oxidation of water to dioxygen by 2(V)(O)(kappa-(NO)-O-2) is due not only to the easy access to high oxidation states but also to the intramolecular hydrogen bonding provided by the noncoordinated dangling carboxylate at the transition state, as corroborated by DFT calculations.