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.
A critical step in creating an artificial photosynthesis system for energy storage is designing catalysts that can thrive in an assembled device. Single-site catalysts have an advantage over bimolecular catalysts because they remain effective when immobilized. Hybrid water oxidation catalysts described here, combining the features of single-site bis-phosphonate catalysts and fast bimolecular bis-carboxylate catalysts, have reached turnover frequencies over 100 s, faster than both related catalysts under identical conditions. The new [(bpHc)Ru(L)] (bpHcH = 2,2'-bipyridine-6-phosphonic acid-6'-carboxylic acid, L = 4-picoline or isoquinoline) catalysts proceed through a single-site water nucleophilic attack pathway. The pendant phosphonate base mediates O-O bond formation via intramolecular atom-proton transfer with a calculated barrier of only 9.1 kcal/mol. Additionally, the labile carboxylate group allows water to bind early in the catalytic cycle, allowing intramolecular proton-coupled electron transfer to lower the potentials for oxidation steps and catalysis. That a single-site catalyst can be this fast lends credence to the possibility that the oxygen evolving complex adopts a similar mechanism.
The preparation and characterization
of new Ru(II) polypyridyl-based chromophore–catalyst assemblies,
[(4,4′-PO3H2-bpy)2Ru(4-Mebpy-4′-epic)Ru(bda)(pic)]2+ (1, bpy = 2,2′-bipyridine; 4-Mebpy-4′-epic
= 4-(4-methylbipyridin-4′-yl-ethyl)-pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate;
pic = 4-picoline), and [(bpy)2Ru(4-Mebpy-4′-epic)Ru(bda)(pic)]2+ (1′) are described, as is the application
of 1 in a dye-sensitized photoelectrosynthesis cell (DSPEC)
for solar water splitting. On SnO2/TiO2 core–shell
electrodes in a DSPEC configuration with a Pt cathode, the chromophore–catalyst
assembly undergoes light-driven water oxidation at pH 5.7 in a 0.1
M acetate buffer, 0.5 M in NaClO4. With illumination by
a 100 mW cm–2 white light source, photocurrents
of ∼0.85 mA cm–2 were observed after 30 s
under a 0.1 V vs Ag/AgCl applied bias with a faradaic efficiency for
O2 production of 74% measured over a 5 min illumination
period.
A deeper mechanistic understanding of the key O-O bond formation step of water oxidation by the [Ru(bda)(L)] (bdaH = 2,2'-bipyridine-6,6'-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electron-donating substituents in [Ru(bda)(4-X-py)] and [Ru(bda)(6-X-isq)] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O-O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π-π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest Δ G for the O-O coupling step to be [Ru(bda)(4-I-py)] and [Ru(bda)(6,7-(OMe)-isq)] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru(bda)(4-I-py)] (330 s) is a 10-fold enhancement with respect to that of [Ru(bda)(py)], and the turnover frequency for [Ru(bda)(6-OMe-isq)] reaches 1270 s, two times faster than [Ru(bda)(isq)].
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