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

A new Ru complex with the formula [Ru(bpn)(pic) 2 ]Cl 2 (where bpn is 2,2′-bi(1,10-phenanthroline) and pic stands for 4-picoline) ( 1 Cl 2 ) 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. 1 2+ undergoes a substitution reaction when it is chemically (by using NaIO 4 ) or electrochemically oxidized to Ru III , in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)] 2+ ( 2 2+ ). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)] 2 (μ-O)} 4+ ( 3 4+ ) 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 ). 3 4+ can be reduced to [Ru(bpn)(pic)(OH 2 )] 2+ ( 4 2+ ), 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. 1 2+ and 4 2+ are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 1 2+ triggered by Ce IV shows its capability to oxidize water to dioxygen. The nature or the active species involved in the catalytic water oxidation by a new coordinatively saturated Ru complex with the formula [Ru(bpn)(pic) 2 ]Cl 2 (bpn: 2,2′-bi(1,10-phenanthroline), pic: 4-picoline) is investigated.