Abstract
While the catalytic activity of some Ru-based polypyridine complexes in water oxidation is well established, the relationship between their chemical structure and activity is less known. In this work, the single site Ru complex [Ru(bpy)2(H2O)2]2+ (bpy = 2,2′-bipyridine)—which can exist as either a cis isomer or a trans isomer—is investigated. While a difference in the catalytic activity of these two isomers is well established, with cis-[Ru(bpy)2(H2O)2]2+ being much more active, no mechanistic explanation of this fact has been presented. The oxygen evolving capability of both isomers at multiple concentrations has been investigated, with cis-[Ru(bpy)2(H2O)2]2+ showing a second-order dependence of O2 evolution activity with increased catalyst concentration. Measurement of the electron paramagnetic resonance (EPR) spectrum of cis-[Ru(bpy)2(H2O)2]2+, shortly after oxidation with CeIV, showed the presence of a signal matching that of cis,cis-[RuIII(bpy)2(H2O)ORuIV(bpy)2(OH)]4+, also known as “blue dimer”. The formation of dimers is a concentration-dependent process, which could serve to explain the greater than first order increase in catalytic activity. The trans isomer showed a first-order dependence of O2 evolution on catalyst concentration. Behavior of [Ru(bpy)2(H2O)2]2+ isomers is compared with other Ru-based catalysts, in particular [Ru(tpy)(bpy)(H2O)]2+ (tpy = 2,2′;6,2′′-terpyridine).
Highlights
The oxidation of water is a vital reaction in nature, occurring during photosynthesis and as a key step in solar energy conversion schemes centered around artificial photosynthesis
There are numerous water oxidation catalysts (WOC’s) that have been reported, many of which are based on transition metals, such as Ir [1,2] and Ru
The initial report indicated that cis-[Ru(bpy)2 (H2 O)2 ]2+ is unstable, and water oxidation is preceded by ligand loss and the formation of RuO2 [17]
Summary
The oxidation of water is a vital reaction in nature, occurring during photosynthesis and as a key step in solar energy conversion schemes centered around artificial photosynthesis. This work is focused on the study of water oxidation by the single-site complex cis[Ru(bpy) (H2 O)2 ]2+ and electron paramagnetic resonance (EPR) characterization of the products. Both cis- and trans-[Ru(bpy) (H2 O)2 ]2+ were reported and characterized by Meyer et al [16,17]. The catalytic mechanism of cis‐[Ru(bpy)2(H2O)2]2+ is and various products of its oxidation have been studied by density functional theory (DFT), EPR, believed to involve a water nucleophilic attack on the RuV=O species produced in a series of proton‐. It was found that for the more catalytically active cis isomer, the structure rate of O2Here, evolution increases non-linearly with complex This effect2is to the we report our study of water oxidation at acidicconcentration. On a much slower time scale, dimer formation during catalysis has previously been reported for [Ru(tpy)(bpy)(H2O)]2+ [13,14] and so a comparison of the two compounds is provided
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