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)](2+) , [Ru(III) (OH2 )(Py2 (Me) tacn)](3+) , [Ru(III) (OH)(Py2 (Me) tacn)](2+) and [Ru(IV) (O)(Py2 (Me) tacn)](2+) , 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)](2+) 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)](2+) 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)](2+) is oxidised to [Ru(V) (O)(Py2 (Me) tacn)](2+) prior to attack by a water molecule to give [Ru(III) (OOH)(Py2 (Me) tacn)](2+) . Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH2 )(Py2 (Me) tacn)](2+) (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.
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007, Tarragona, Spain.
Carla
C
Casadevall
Institut de Química Computacional i Catàlisi (IQCC) and, Departament de Química, Universitat de Girona Campus Montilivi, 17071, Girona, Spain.
Zoel
Z
Codolà
Institut de Química Computacional i Catàlisi (IQCC) and, Departament de Química, Universitat de Girona Campus Montilivi, 17071, Girona, Spain.
Miquel
M
Costas
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007, Tarragona, Spain. jlloret@iciq.es.|Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23, 08010, Barcelona, Spain. jlloret@iciq.es.
Julio
J
Lloret-Fillol
density functional calculations; labeling studies; mechanistic studies; ruthenium; water oxidation