Unexpected promotional effects of alkyl-tailed ligands and anions on the electrochemical generation of ruthenium(IV)-oxo complexes

Abstract

Electro-generation of RuIV=O species from the RuII-aqua complex is a crucial step to excel the electrocatalytic water oxidation performance of ruthenium oxo complexes. We report herein the synthesis and X-ray crystal structural characterizations of new RuII-aqua complexes containing N-substituted 2,2′-dipyridylamine ligands (L) tagged with an alkyl chain of various lengths, [Ru(tpy)(L)(OH2)]2+. Cyclic voltammetric analyses show that the length of the alkyl chain exerts great influence on the electro-generation of RuIV=O species. The L with a longer alkyl chain in an acidic aqueous medium promotes the conversion of RuIII−OH to RuIV=O efficiently. Surprisingly, this conversion can be further enhanced by the presence of perchlorate anions. Chronocoulometric data reveal that the alkyl chain on L promotes the adsorption of the ruthenium complex on the electrode surface. Bulk electrolysis results indicate that the [Ru(tpy)(dppa)(OH2)]2+ (dppa=(2,2′-dipyridyl)-n-propylamine) gives the most active electrocatalytic water oxidation activity among the ruthenium-aqua complexes investigated in this study.