Electron Transfer Mediator Effects in Water Oxidation Catalysis by Solution and Surface-Bound Ruthenium Bpy-Dicarboxylate Complexes

Abstract

Electrocatalytic water oxidation by the catalyst, ruthenium 2,2′bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru(bda)(isoq) 2 ], 1, was investigated at metal oxide electrodes surface-derivatized with electron transfer (ET) mediators. At indium-doped tin oxide (ITO) in pH 7.2 in H 2 PO 4 − / HPO 4 2− buffers in 0.5 M NaClO 4 with added acetonitrile (MeCN), the catalytic activity of 1 is enhanced by the surface-bound redox mediators [Ru (4,4′-PO 3 H 2bpy)(4,4′-R-bpy) 2 ] 2+ (RuPbpyR 2 2+ , R = Br, H, Me, or OMe, bpy = 2,2′bipyridine). Rate-limiting ET between the Ru 3+ form of the mediator and the Ru IV (O) form in the [Ru V/IV (O)] +/0 couple of 1 is observed at relatively high concentrations of HPO 4 2− buffer base under conditions where O•••O bond formation is facilitated by atom-proton transfer (APT). For the solution [Ru(bpy) 3 ] 3+/2+ mediator couple and 1 as the catalyst, catalytic currents vary systematically with the concentration of mediator and the HPO 4 2− buffer base concentration. Electron transfer mediation of water oxidation catalysis was also investigated on nanoparticle TiO 2 electrodes co-loaded with catalyst [Ru(bda)(py-4-O(CH 2 ) 3 -PO 3 H 2 ) 2 ], 2, (py = pyridine) and RuPbpyR 2 2+ (R = H, Me, or OMe) with an interplay between rate-limiting catalyst oxidation and rate-limiting O•••O bond formation by APT. Lastly, the co-loaded assembly RuPbpyR 2 2+ + 2 has been investigated in a dye-sensitized photoelectrosynthesis cell for water splitting.

Artificial Molecular Photosynthetic Systems: Towards Efficient Photoelectrochemical Water Oxidation

“…The ET from WOCs to the photooxidized RuP 2+ analogue was further accelerated in the presence of a synthetic mimic of a biological redox relay, the benzimidazole‐phenol ( 13 )‐mediated system shown in Figure , which is similar to the Tyr–His pair in the PSII; this led to significant photocurrent density enhancement. These results emphasize the importance of redox mediators for the catalysis on the surfaces …”

“…These results emphasize the importance of redox mediators for the catalysis on the surfaces. [98] Theoretical studies Ap hotoinduced multistep event in artificial photosynthesis must accompany the PT from the WOC to the proton acceptor (amino acid residues in photosynthesis,a nd typicallyw ater in artificial photosynthesis);t herefore, the multistep events hould be treated as aP CET process. In general,P CET can be treated by the Marcus theory if the ET process is nonadiabatic, [26,99,100] and the theoryp redicts the effects of the distance and driving force of ET on the rate of the reactiona se xpressed by Equations (4) and (5): [101,102] V ET r DA…”

Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes

“…Ruthenium complexes with the formula [(bda)Ru(L) 2 ], where L is a neutral pyridinebased donor, were first reported by the Sun group in 2009, and WOCs of this type remain among the fastest to date. 27,28 Since its initial report, [(bda)Ru(L) 2 ] complexes have attracted a great deal of interest from the Sun group 23 and others, [29][30][31][32][33][34] and related systems are an active area of research. 19,[35][36][37][38][39] Several mechanisms for water oxidation by [(bda)Ru(L) 2 ] have been proposed, which differ in the nature of the O-O bond-forming step, shown in Scheme 3.…”

“…Due to a faster bimolecular step, the isoquinoline complex [(bda)Ru(isq The Meyer group has proposed involvement of a water nucleophilic attack mechanism based on solution and surface electrochemical experiments, labelled B in Scheme 3 and corresponding to A in Scheme 1. [30][31][32] Depending on the conditions, the RDS was proposed to be the O-O bond formation step or proton-coupled oxidation of Ru IV -OH to Ru V =O. Evidence for this mechanism include an observed first-order dependence on catalyst concentration, rate enhancement with higher concentration of base, and slow electron transfer for the higher oxidation states of ruthenium.…”

O–O bond formation mechanisms during the oxygen evolution reaction over synthetic molecular catalysts

“…Meyer et al [42] investigated the electron transfer mediator effects during the water oxidation reaction using Ru(bda)(L)2 catalysts, where H2bda and L denote 2,2′-bipyridine-6,6′-dicarboxylic acid and the ligand, respectively, and redox mediators, and revealed the dependence on the redox potential of the mediator. In addition, the experimental results demonstrated the important role of the atom-proton transfer (APT) process and O-atom transfer to the O atom of a H2O molecule combined with the proton transfer to the added HPO4 2− buffer base during water oxidation over [Ru V (O)] + .…”