The effect of perchlorate and dodecyl sulfate on the electrochemical generation of ruthenium (IV) oxo species

Abstract

Ruthenium oxo (Ru=O) complexes are the potential catalysts and electrocatalysts for various oxidation reactions. It can be synthesized chemically or electrochemically. However, the electrochemical oxidation of Ru(III)-OH to Ru(IV)=O is a kinetically sluggish proton coupled electron transfer process which limits their applications. To have a better understanding on the mechanism of the electrochemical oxidation of Ru(III)-OH to Ru(IV)=O and the effect of strong electrode adsorbates on the rate of Ru(IV)=O formation, the effect of perchlorate and dodecyl sulphate on the cyclic voltammograms of three ruthenium complexes, namely [Ru(tpy)(dpa)OH₂]²⁺, [Ru(tpy)(dppa)OH₂]²⁺, and [Ru(tpy)(dpha)OH₂]²⁺ (tpy: 2,2’:6’,2-terpyridine, dpa: 2,2’-dipyridylamine, dppa: (2,2’-dipyridyl)-n-propylamine, dpha: (2,2’-dipyridyl)-n-hexylamine) were studied. These complexes contain different alkyl chain on the dipyridylamine ligands. It was found that perchlorate anion can promote the electrochemical generation of Ru(IV)=O species in [Ru(tpy)(dppa)OH₂]²⁺ and [Ru(tpy)(dpha)OH₂]²⁺ on a glassy carbon electrode. The occurrence of adsorption was confirmed by chronocoulometric measurements and rotating disk voltametry. The electrochemical activation of the glassy carbon electrode surface can further enhance the oxidation of Ru(III)-OH and Ru(IV)=O. Dodecyl sulphate anion, another strong adsorbate, had similar effect as perchlorate in promoting the electrochemical oxidation of Ru(III)-OH to Ru(IV)=O on both a glassy carbon and a platinum electrode. It appears that both the alkyl chain on the ligands and the presence of strong anionic adsorbate such as perchlorate or SDS could facilitate the adsorption of the ruethnium complexes onto the electrode surface, which in turn promoted the oxidation of Ru(III)-OH to Ru(IV)=O. A mechanism for the promotion of the electrochemical generation of ruthenium oxo species is proposed. Anions such as perchlorate and dodecyl sulphate can adsorb strongly on the surface of a glassy carbon (perchlorate only) and a platinum (both perchlorate and platinum) electrode. The adsorption of these anions draw the cations of ruthenium complexes to the electrode surface. The presence of an alkyl chain on the ligand of the ruthenium complexes further enhances the adsorption of the complexes on the electrodes. These adsorbed ruthenium complexes can interact with functional groups such as quinone-like moieties on a glassy carbon electrode or platinum oxide on a platinum electrode through the hydrogen-bonding with the aquo or hydroxo ligand. This assembly would eventually facilitate the concerted proton-electron transfer process in the oxidation of Ru(III)-OH to Ru(IV)=O. The results from this study will lead to the development of better electro-catalysts for the oxidation processes based on ruthenium oxo complexes.