Spectroelectrochemical investigation of electrocatalytic water oxidation by a mononuclear ruthenium complex in a homogeneous solution

Abstract

investigation of electrocatalytic water oxidation by a mononuclear ruthenium complex in a homogeneous solution, Journal of Photochemistry and Photobiology A: Chemistry http://dx.2 Highlight • Electrocatalytic water oxidation was studied by Potential-step chronocoulospectrometry. • [Ru(EtOtpy)(bpy)OH 2 ] 2+ can work for electrocatalytic water oxidation under weakly acidic conditions. • The rate-determining step for the electrocatalysis is oxidation of Ru IV =O to Ru V =O species. Abstract Electrochemical water oxidation by [Ru(EtOtpy)(bpy)OH 2 ] 2+ (1) (EtOtpy = 4'-ethoxy-2,2':6'2''-terpyridine, bpy = 2,2'-bipydidine) was investigated in a homogeneous solution under weakly acidic conditions (pH = 5.3). The cyclic voltammogram of a 1 aqueous solution showed that successive proton-coupled electron transfer reactions of Ru II -OH 2 / Ru III -OH and Ru III -OH / Ru IV =O redox pairs and high anodic current above 1.1 V vs SCE. Electrocatalytic water oxidation was corroborated by the bulk electrolysis at 1.5 V; the significant amount of O 2 was evolved compared with the blank during the electrolysis. Potential-step chronocoulospectrometry (PSCCS) from 0.0 V to 1.52 V vs SCE was conducted to observe the change of 1 in solution during the electrocatalysis. The in situ UV visible spectral change showed oxidation of 1 (Ru II -OH 2 ) to Ru III -OH and to further oxidation of Ru III -OH to Ru IV =O, and that the Ru IV =O species mainly exists in a steady state after 200 s in the electrocatalysis. The in situ UV visible spectral change in a reverse potential step from 1.52 V to 0.22 V vs SCE exhibited that Ru II -OH 2 completely recovers by 2 electron re-reduction process from the steady state in the electrocatalysis. The observation of Ru IV =O in a steady state suggests that a rate determining step in the catalytic cycle is oxidation of Ru IV =O to Ru V =O rather than the O-O bonding formation by nucleophilic attack of water to Ru V =O in the 3 electrocatalysis in a homogeneous solution.

Effect of Quinoline Substitution on Water Oxidation by [Ru(Ql-tpy)(bpy)(OH2 )](PF6 )2

“…Berlinguette and Yagi groups have reported a series of mononuclear ruthenium complexes having [Ru II (tpy)(bpy)(H 2 O)] 2+ scaffold, wherein they revealed that electron‐withdrawing group suppresses the catalytic activity (e. g., TOF decreases), however increases the catalyst stability (TON) . Similar effects were also observed in a family of [Ru II (tpy‐R)(phen‐X)Cl] + complexes, where tpy‐R is the tridentate 4’‐(4‐methylmercaptophenyl)‐2,2’:6’,2’’‐terpyridine ligand, “phen” is 1,10‐phenanthroline, X is 5‐NO 2 or 5,6‐dimethyl or 3,4,7,8‐tetramethyl substituents …”

Mass Spectrometric and Vibrational Characterization of Reaction Intermediates in [Ru(bpy)(tpy)(H2O)]2+ Catalyzed Water Oxidation

“…Comparisons between experimental vibrational spectra and density functional theory calculations confirmed the identity and structure of these two complexes. [4,5,22,[25][26][27][28][29][30] We show that we can selectively detectt he first two oxidized complexes in the proposed catalytic cycle by controlling the applied EC potential. However, such key reaction species are typically presenti no nly minor concentrations, necessitating as ensitive and selectivep robe, such as electrospray ionization mass spectrometry (ESI-MS).…”

“…[1][2][3][4][5][6][7][8] MS identifies ions by their mass-to-charge (m/z)r atio, and with additional characterization tools such as collision-induced dissociation, can also provides ome structural information.C oupling spectroscopicc haracterization with MS, by schemes such as infraredm ultiple photon dissociation (IRMPD), [9] UV + IR resonant photodissociation, [10] and cryogenic ion vibrational predissociation (CIVP), [11] can further provide rich and detailed structural information as well as benchmarks fort heoretical methods used to characterizer eaction processes. [4,5,22,[25][26][27][28][29][30] We show that we can selectively detectt he first two oxidized complexes in the proposed catalytic cycle by controlling the applied EC potential. [12][13][14][15][16][17] Particularly,a ni n-line electrochemical (EC) cell can be coupled to an ESI source,s uch that intermediates and products formed under specific EC potentials can be sampled.…”

In operando studies on the electrochemical oxidation of water mediated by molecular catalysts

“…Reversibility of the redox couple and the accompanied changes in the UV-vis spectrum in combination with the observation of clear isobestic points are a good indication that the electrochemically formed species are stable. 52,[59][60][61][62] In situ UV-vis spectroscopy of the bulk solution has also been employed to show the stability of the catalytic material. This, however, can be tricky.…”