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

A family
of Ru complexes based on the pentadentate ligand t5a3– ((2,5-bis­(6-carboxylatopyridin-2-yl)­pyrrol-1-ide)
and pyridine (py) that includes {RuII(Ht5a-κ-N2O)­(py)3} (1HII(κ-N2O)), {RuIII(t5a-κ-N3O1.5)­(py)2} (2III(κ-N3O1.5)), and {RuIV(t5a-κ-N3O2)­(py)2}+ ({2IV(κ-N3O2)}+) has been prepared and thoroughly characterized.
Complexes 1HII(κ-N2O), 2III(κ-N3O1.5), and {2IV(κ-N3O2)}+ 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 2III(κ-N3O1.5) have been studied by electrochemical methods
(CV and DPV), showing its easy access to high oxidation states, thanks
to the trianionic nature of the t5a3– ligand. Under
neutral to basic conditions complex {2IV(κ-N3O2)}+ undergoes
aquation, generating {RuIV(OH)­(t5a-κ-N2O)­(py)2} (2IV(OH)­(κ-N2O)). Further oxidation of the complex forms {RuV(O)­(t5a-κ-N2O)­(py)2} (2V(O)­(κ-N2O)), 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 2V(O)­(κ-N2O) 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.