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Catalysts 2017, 7(2), 39; doi:10.3390/catal7020039

Mechanistic Analysis of Water Oxidation Catalyst cis-[Ru(bpy)2(H2O)2]2+: Effect of Dimerization

Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA
These authors contributed equally to this work.
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Academic Editors: Albert Demonceau, Ileana Dragutan and Valerian Dragutan
Received: 27 November 2016 / Revised: 9 January 2017 / Accepted: 18 January 2017 / Published: 25 January 2017
(This article belongs to the Special Issue Ruthenium Catalysts)
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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). View Full-Text
Keywords: Ru complexes; catalysis of water oxidation; density functional theory; X-band EPR spectroscopy; oxygen evolution; H-bond; dimer formation Ru complexes; catalysis of water oxidation; density functional theory; X-band EPR spectroscopy; oxygen evolution; H-bond; dimer formation
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Erdman, D.; Pineda-Galvan, Y.; Pushkar, Y. Mechanistic Analysis of Water Oxidation Catalyst cis-[Ru(bpy)2(H2O)2]2+: Effect of Dimerization. Catalysts 2017, 7, 39.

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