Spectroscopic, Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [Ru(V) (O)(Py2 (Me) tacn)] Intermediate.

A new family of ruthenium complexes based on the N-pentadentate ligand Py2 (Me) tacn (N-methyl-N',N''-bis(2-picolyl)-1,4,7-triazacyclononane) has been synthesised and its catalytic activity has been studied in the water-oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO4 ) to generate the WO intermediates Ru(II) (OH2 )(Py2 (Me) tacn) , Ru(III) (OH2 )(Py2 (Me) tacn) , Ru(III) (OH)(Py2 (Me) tacn) and Ru(IV) (O)(Py2 (Me) tacn) , which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. Ru(IV) (O)(Py2 (Me) tacn) has a long half-life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, (18) O-labelling and theoretical studies, and the conclusion is that the rate-determining step is a single-site water nucleophilic attack on a metal-oxo species. Moreover, Ru(IV) (O)(Py2 (Me) tacn) is proposed to be the resting state under catalytic conditions. By monitoring Ce(IV) consumption, we found that the O2 evolution rate is redox-controlled and independent of the initial concentration of Ce(IV) . Based on these facts, we propose herein that Ru(IV) (O)(Py2 (Me) tacn) is oxidised to Ru(V) (O)(Py2 (Me) tacn) prior to attack by a water molecule to give Ru(III) (OOH)(Py2 (Me) tacn) . Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium M(OH2 )(Py2 (Me) tacn) (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M(V) (O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.