Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

• Published in: Angewandte Chemie International Edition 2016-07, Vol.55 (28), p.8067-8071
• Main Authors: Xie, Yan, Shaffer, David W., Lewandowska-Andralojc, Anna, Szalda, David J., Concepcion, Javier J.
• Format: Article
• Language: English
• Subjects: Activation; catalysis; Catalysts; Compensation; Electrochemistry; Leveling; Ligands; Mathematical analysis; Oxidation; pH effects; phosphonates; Protons; Ruthenium; water splitting; Yield
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201601943

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox‐potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH‐dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six‐coordinate [RuIV] center to yield an anionic seven‐coordinate [RuIV−OH]− intermediate is one of the key steps of a single‐site mechanism in which all species are anionic or neutral. These complexes are among the fastest single‐site catalysts reported to date. Breaking down barriers: Multifunctional bipyridyl diphosphonate ligands provide redox‐potential leveling through charge compensation and σ donation to allow easy access to high oxidation states of ruthenium‐based water oxidation catalysts (see structure; L=picoline or isoquinoline). They also shuttle protons in and out of the catalytic site, thus reducing activation barriers for proton‐coupled processes.