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

Abstract

Five mononuclear RuII complexes supported by two pentadentate polypyridyl ligands, N,N‐bis(2‐pyridylmethyl)‐N‐(bis‐2‐pyridylmethyl)amine (N4Py) {[Ru(N4Py)(Cl)](PF6), 1Cl; [Ru(N4Py)(OH2)](PF6)2, 1Aq} and newly designed N‐benzyl‐N‐((6‐(6‐methylpyridin‐2‐yl)pyridin‐2‐yl)methyl)dipyridin‐2‐yl‐methanamine (N2Py‐MeBpy‐Bz) {[Ru(N2Py‐MeBpy‐Bz)(Cl)](PF6)·MeCN, 2Cl·MeCN; [Ru(N2Py‐MeBpy‐Bz)(OH2)](PF6)2·3H2O·MeOH, 2Aq·3H2O·MeOH and [Ru(N2Py‐MeBpy‐Bz)(MeCN)](PF6)2·0.5MeCN·H2O, 2ACN·0.5MeCN·H2O} were synthesized and characterized using different spectroscopic techniques such as UV/Vis, IR, 1D and 2D NMR spectroscopy, and mass spectrometry. The physicochemical properties of complexes 1Cl and 1Aq, and structural analysis of 1Aq were reported by Kojima and co‐workers (Chem. Sci. 2012, 3, 3421–3431). Structural characterizations of 1Cl, 2Cl·MeCN, and 2ACN·0.5MeCN·H2O were done by using single‐crystal X‐ray diffraction analyses. Catalytic water oxidation activities of aqua‐ligated RuII complexes, using CeIV as sacrificial electron acceptor at pH 1, were examined. Complex 2Aq shows higher activity as compared to 1Aq. Electrochemical study suggests that a formal [RuVI=O]4+ species is the active species which triggers the oxidation of water. Mechanistic investigation reveals that O–O bond formation takes place via water nucleophilic attack (WNA) pathway. The deactivation pathway of catalyst 2Aq has also been investigated. It was observed that complex 2Aq lost its water oxidation activity primarily due to ligand degradation via oxidative N‐debenzylation pathway.

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

“…The initial rate of oxygen evolution was found to be proportional to the concentration of catalyst 2-Ru III , following a pseudo-first-order reaction with a first-order rate constant of 0.168 s –1 (Figure ). This first-order rate constant which is equal to the turnover frequency (TOF) is remarkably higher than the TOFs of reported mononuclear Ru-WOCs having neutral polypyridyl ligands. ,,, Although complex 2-Ru III catalyzes water oxidation rapidly with high TOF, the catalytic performance is limited to moderate turnovers. This suggests that the catalyst degrades in a side reaction under highly oxidizing condition.…”

“…The SWV curve exhibits four redox peaks at the potentials of −0.13, 0.66, 1.15, and 1.46 V. The first redox event at the potential of −0.13 V is tentatively assigned as Ru III /Ru II redox couple. Because of the presence of negatively charged ligand, the Ru III /Ru II redox couple appears at a markedly lower potential than those of WOCs containing neutral 10,16,17 as well as anionic donor ligands, 13,14 which implies a much enhanced donor ability of the anionic L 3− ligand. In order to know the number of electron transfers involved in the second and third redox events, controlled potential electrolysis (CPE) experiments were carried out at 0.80 and 1.25 V, respectively, at pH ∼1.…”

“…This first-order rate constant which is equal to the turnover frequency (TOF) is remarkably higher than the TOFs of reported mononuclear Ru-WOCs having neutral polypyridyl ligands. 1,10,16,17 Although complex 2-Ru III catalyzes water oxidation rapidly with high TOF, the catalytic performance is limited to moderate turnovers. This suggests that the catalyst degrades in a side reaction under highly oxidizing condition.…”

“…Considerable progress has been made during the past years with the development of molecular WOCs based on precious metals (Ru and Ir) and earth-abundant first-row transition metal ions; among these, Ru-based WOCs − have attracted much attention because of their versatile redox properties, superior catalytic performance, and higher stability. In recent years, focus has been diverted toward the design of single-site WOCs − because of their facile synthesis and less complexity in the mechanistic investigation. Over the past few years, tremendous efforts with both experimental and theoretical studies have been devoted in order to understand the mechanism of water oxidation mediated by molecular WOCs, particularly the critical O–O bond forming event.…”

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

“…The PCET phenomena with Ru-aquo complexes are well studied in the literature. 12,13 In this way, the hydrogen atoms (H + + e − ) from the water molecule can be removed. The PCET process helps to avoid the building-up of Coulombic charges and, thereby, decreases the energy barrier.…”