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

Abstract

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 4-picoline) (1Cl₂) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradentate equatorial ligands. Comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 1²⁺ undergoes a substitution reaction when it is chemically (by using NaIO₄) or electrochemically oxidized to Ru^III, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)]²⁺ (2²⁺). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)]₂(μ-O)}⁴⁺ (3⁴⁺) which 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^-1 s^-1), which is in line with the electrochemical data at pH = 7 (k_dimer = 1.4 M^-1 s^-1). 3⁴⁺ can be reduced to [Ru(bpn)(pic)(OH₂)]²⁺ (4²⁺), showing a sort of square mechanism. 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.