Mononuclear Ruthenium-Based Water Oxidation Catalyst Supported by Anionic, Redox-Non-Innocent Ligand: Heterometallic O–O Bond Formation via Radical Coupling Pathway

Abstract

Cerium(IV)-driven water oxidation catalysis mediated by a mononuclear ruthenium(III) complex, [Ru(L)(pic) 3 ] (H 3 L = 2,2′-iminodibenzoic acid, pic = 4-methylpyridine), has been demonstrated in this work. The mechanistic details of water oxidation have been investigated by the combined use of spectroscopy, electrochemistry, kinetic analysis, and computational studies. It was found that water oxidation proceeds via formal high-valent Ru VII species. The capability of accessing such a high-valent state is derived from the non-innocent behavior of the anionic tridentate ligand frame which helps in accumulation of oxidative equivalents in cooperation with metal center. This metal−ligand cooperation facilitates the multi-electron-transfer reaction such as water oxidation. Kinetic analysis suggests water oxidation at a single site of Ru where O−O bond formation occurs via radical−radical coupling pathway between the oxygen atom of ruthenium-oxo species and the oxygen atom of the hydroxocerium(IV) ion.

Synergic Reaction Kinetics over Adjacent Ruthenium Sites for Superb Hydrogen Generation in Alkaline Media

“…In Figure 4c, both Ru 1,n ‐NC and Ru/NC exhibited obvious peaks of underpotential deposition hydrogen (H UPD ), and no other peaks corresponding to the oxidation and reduction of Ru‐based oxides were observed. [ 37 ] The H UPD peak of Ru 1,n ‐NC was stronger than that of Ru/NC, implying that the H adsorption amount on Ru 1,n ‐NC was larger. To be specific, with the help of favorable H 2 O adsorption, large amounts of adsorbed hydrogen was formed on Ru 1,n ‐NC to accelerate its proton supply.…”

The impact of secondary coordination sphere engineering on water oxidation reactivity catalysed by molecular ruthenium complexes: a next-generation approach to develop advanced catalysts

“…Scheme 1 shows four possible O-O bond formation pathways namely (a) water nucleophilic attack (WNA), (b) interaction of two M-O units (I2M), (c) heterometallic radical coupling pathway (I2M-HC), and (d) oxide relay. The WNA and I2M are very familiar well-established pathways, 6,14 whereas I2M-HC 15,16 and oxide relay 17,18 are non-conventional pathways. The oxide relay and I2M-HC may be considered a subcategory of the WNA and I2M pathways, respectively.…”

Iron–N-heterocyclic carbene complexes as efficient electrocatalysts for water oxidation under acidic conditions

“…Artificial water splitting to produce hydrogen energy is the most promising technology to develop a carbon-free, environmentally friendly, and sustainable energy economy. [1][2][3] However, its overall reaction efficiency is still greatly hindered by the water oxidation process due to the thermodynamically unfavorable reaction process involving a complex mechanism of four-electron and four-proton transfer (2H 2 O → O 2 + 4H + + 4e − ) and the slow kinetics of oxygen-oxygen (O-O) bond formation. [4][5][6] In nature, the Mn 4 CaO 5 cluster and immediate protein surroundings make up the O 2 -evolving catalytic center under neutral conditions of photosystem II (PSII).…”