A broad view at the complexity involved in water oxidation catalysis based on Ru-bpn complexes

A new Ru complex with the formula [Ru(bpn)(pic)₂]Cl₂ (where bpn is 2,2′-bi(1,10-phenanthroline) and pic stands for picoline) (1Cl₂) is introduced to investigate a true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradenate equatorial ligands. A comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV−vis, NMR, resonance Raman,) and kinetic study is presented. 1²⁺ undergoes a substitution reaction when it is chemically (by using NaIO₄) or electrochemically oxidize to Ru^III, where picoline is replaced by hydroxido ligand to produce [Ru(bpn)(pic)(OH)]²⁺(2²⁺). The former complex is in an equilibrium with an oxo-bridged species {[Ru(bpn)(pic)]₂(µ-O)}⁴⁺ (3⁴⁺) that is the major form of the complex in the Ru^III oxidation state. The dimer formation is the rate determining step of the overall oxidation process (k_dimer= 1.35 M^-1s^-1), which is in line with electrochemical data at pH = 7 (k_dimer= 1.4 M^-1s^-1). 3⁴⁺ can be reduced to [Ru(bpn)(pic)(OH₂)]²⁺(4²⁺), showing a sort of square mechanism behavior. All species generated in situ at pH 7 have been thoroughly characterized by NMR, mass spectrometry, UV-Vis and electrochemical techniques. 1²⁺ and 4²⁺ are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 1²⁺ triggered by Ce^IV shows its capability to oxidize water to dioxygen.