Unexpected Promotional Effects of Alkyl-Tailed Ligands and Anions on the Electrochemical Generation of Ruthenium(IV)-Oxo Complexes

Electro-generation of Ru-IV=O species from the Ru-II-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 Ru-II-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 Ru-IV=O species. The L with a longer alkyl chain in an acidic aqueous medium promotes the conversion of Ru-III-OH to Ru-IV=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.

Automated summary

The study reveals that ligands with longer alkyl chains can effectively promote the conversion of Ru-III-OH to Ru-IV=O, and the presence of perchlorate anions can further enhance this conversion. Additionally, the alkyl chain on the ligands also helps promote the adsorption of ruthenium complexes on the electrode surface.