Insight into Water Oxidation by Mononuclear Polypyridyl Ru Catalysts

Abstract

A family of compounds based on the mononuclear coordination complex [Ru(tpy)(bpy)(OH(2))](2+) (1b; tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine) are shown to be competent catalysts in the Ce(IV)-driven oxidation of water in acidic media. The systematic installation of electron-withdrawing (e.g., -Cl, -COOH) and -donating (e.g., -OMe) groups at various positions about the periphery of the polypyridyl framework offers insight into how electronic parameters affect the properties of water oxidation catalysts. It is observed, in general, that electron-withdrawing groups (EWGs) on the bpy ligands suppress catalytic activity (k(obs)) and enhance catalytic turnover numbers (TONs); conversely, the presence of electron-donating groups (EDGs) accelerate catalytic rates while decreasing catalyst stability. We found that 2,2'-bipyridine N,N'-dioxide is produced when 1b is subject to excess Ce(IV) in acidic media, which suggests that dissociation of the bpy ligand is a source of catalyst deactivation and/or decomposition. Density functional theory (DFT) calculations corroborate these findings by showing that the Ru-N(bpy) bond trans to the O atom is weakened at higher oxidation levels while the other Ru-N bonds are affected to a lesser extent. We also show that the Ru-Cl bond is not robust in aqueous media, which has implications in studying the catalytic behavior of systems of this type.

CeIV‐ and Light‐Driven Water Oxidation by [Ru(terpy)(pic)3]2+ Analogues: Catalytic and Mechanistic Studies

“…suggested that Ru-Cl complexes act as precatalysts to the related Ru-aqua catalysts. [22] Compared with 4, the observed higher TOF of 5 serves as further proof for this mechanism.…”

“…This proposed mechanism is not surprising, since single-site ruthenium catalysts have been reported and extensively studied earlier. [21,22] To confirm our hypothesis, trans-[Ru(terpy)(pic) 2 (OH 2 )](ClO 4 ) 2 (5) was synthesized and examined as WOC. Not only did we observe the catalytic current as shown in the electrochemistry study, but also oxygen evolution in Ce IV -driven water oxidation (Figure 7).…”

Ab Initio Molecular Dynamics Study of Water Oxidation Reaction Pathways in Mono‐Ru Catalysts

“…A microscopic mechanistic understanding of the natural process is clearly relevant to support the design of artificial devices for fuel generation from solar energy. [2,3] The most successful artificial catalysts are based on Ru, [4,5] Co, [6][7][8] and more recently Ir complexes. [9] In particular, mononuclear molecular catalysts based on Ir III have shown to be highly active and robust.…”

New trinuclear dendritic complexes with [Ru(tpy)(bpy)X]n+ (X = Cl, H2O; n = 1, 2) for enhanced water oxidation and light-driven alcohol oxidation

“…Previous insights into the electron withdrawing and donating effects of functional groups on the [Ru(bpy)(tpy)(H 2 O)] 2+ motif have revealed that electron-donating groups can cause an increase in the π-back bonding of the tpy ligand to the bpy ligand and lower the oxidation potentials of the resulting complexes. This behavior eventually leads to an increase in catalytic activity but at a compromise to the catalytic stability [17]. It is therefore expected from the electrochemical data obtained that enhanced catalytic activity can be achieved, and are ascribed to the increase in the π-back bonding to the bpy ligand by the conjugated tristerpyridine ligand.…”

“…Grafting the complex onto a TiO 2 film which has been deposited onto a fluorine-doped tin oxide substrate was also demonstrated to be a viable approach for the preparation of effective photoanodes for achieving water splitting [13,15]. On the other hand, optimization of the water oxidation activity of [Ru(tpy)(bpy)X] n+ catalysts by modulating the electronic and steric effects through modification of the ligand scaffold have been extensively investigated [16][17][18], and the effects of a redox mediator on the water oxidation rate was reported [19]. In addition, many [Ru(tpy)(bpy)(H 2 O)] 2+ -based complexes have been applied as catalysts for light-driven alcohol oxidation reactions with the Ru(IV) = O species possessing excellent reactivity for the oxidation of organic substrates [20,21].…”