O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling

Abstract

In this review we discuss at the mechanistic level the different steps involved in water oxidation catalysis with ruthenium-based molecular catalysts. We have chosen to focus on ruthenium-based catalysts to provide a more coherent discussion and because of the availability of detailed mechanistic studies for these systems but many of the aspects presented in this review are applicable to other systems as well. The water oxidation cycle has been divided in four major steps: water oxidative activation, O-O bond formation, oxidative activation of peroxide intermediates, and O evolution. A significant portion of the review is dedicated to the O-O bond formation step as the key step in water oxidation catalysis. The two main pathways to accomplish this step, single-site water nucleophilic attack and O-O radical coupling, are discussed in detail and compared in terms of their potential use in photoelectrochemical cells for solar fuels generation.

Synthesis and Physicochemical Properties of Ruthenium(II) Complexes Having Pentadentate Scaffolds: Water Oxidation Activity and Deactivation Pathway

“…The key step involved in water oxidation catalysis is the O–O bond formation. The following two pathways are well accepted: (i) water nucleophilic attack (WNA) at electron deficient high‐valent metal‐oxo group, and (ii) bimolecular radical coupling between two metal‐oxyl groups (I2M) . To know which pathway O–O bond formation occurred, the kinetic studies were carried out under excess Ce IV condition at pH 1 CF 3 SO 3 H solution with both the Ru II complexes.…”

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…The lower oxidation state transitions from A to B and B to C are not observed experimentally probably owing to the low quality of the electrochemical response for this complex. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations...…”

The Art of Splitting Water: Storing Energy in a Readily Available and Convenient Form

“…Thanks to the low activity of these catalysts, most of the intermediates in the catalytic cycle of water oxidation could be observed and studied by electrochemical and spectroscopic techniques. The primary mechanism for the critical O–O bond formation step was then identified to be water nucleophilic attack (WNA) on an electrophilic metal‐oxo species (Scheme ) . This step was shown to be rate‐limiting for most of the studied WOCs and could be facilitated by introduction of a base to the reaction mixture or to the ligand framework (atom–proton transfer mechanism, see Scheme ).…”