Synthesis and electrochemical studies of some ruthenium aquo complexes with N-substituted dipyridylamines

Abstract

Ruthenium oxo complexes are potential electrocatalysts for a number of oxidation reactions. However, the electro-generation of ruthenium oxo species is kinetically slow on electrode surface which limits their applications. In this study, we investigated some factors associated with the promotion of the electro-generation of ruthenium oxo species on electrodes. The synthesis and characterization of [RuII(tpy)(MBP)(H2O)]3+, [RuII(tpy)(MBHP)(H20)]3+, [RuII(tpy)(dppa)(H2O)]2+ and [RuII(tpy)(dpha)(H2O)]2+ (tpy= 2,2':6,'2"-terpyridine, MBP= 1 -methyl-1 -(3-N, N'-bis(2-pyridyl)propylammo) pyrrolidinium, MBHP= 1 -methyl-l-(6-N, N'-bis(2-pyridyl)hexylamino)pyrrolidinium, dppa= (2,2'-dipyridyl)-n-propylamine and dpha= (2,2'-dipyridyl)-n-hexylamine) and the electrochemical properties of these complexes are the focuses of study in this project. The structures of [RuII(tpy)(MBP)(H2O)]3+ and [RuII(tpy)(dppa)(H2O)]2+ have been determined by X-ray crystallography. The effect of anions (including perchlorate, trifluoroacetate, trifluoromethanesulfonate, sulfate, tetrafluoroborate and hexafluorophosphate) on the rate of electro-generation of ruthenium oxo species was investigated. Among the anions investigated, perchlorate was found to promote the electro-generation of ruthenium oxo species. However, only the perchlorate anions alone is not sufficient to promote the oxidation of RuIII-OH to RuIV=O. It was found that in addition to perchlorate, the presence of an alkyl chain on the amine nitrogen of the 2,2'-dipyridylamine (dpa) is essential to speed up the electrochemical formation of RuIV=O species. The perchlorate anions have no effect in promoting the electrochemical oxidation of [RuIII(tpy)(dpa)(OH)]2+ which contains no alkyl tail on the amine nitrogen. The effect of the length and nature of the tail tagged on the 2,2'-dipyridylamine ligand on the rate of electro-generation of ruthenium oxo species was therefore also investigated. All the complexes [RuII(tpy)(MBP)(H2O)]3+, [RuII(tpy)(MBHP)(H2O)]3+, [RuII(tpy)(dppa)(H2O)]2+ and [Run(tpy)(dpha)(H2O)]2+containing either a cationic or a neutral tail on the 2,2'-dipyridylamine ligand showed an enhancement of the rate of oxo formation. A comparison of the cyclic voltammograms of [RuII(tpy)(dpa)(H2O)]2+, [RuII(tpy)(dppa)(H2O)]2+ and [RuII(tpy)(dpha)(H2O)]2+ suggested that a longer alkyl chain length can promote better formation of RuIV=O species. A mechanism involving the association of ruthenium complexes through hydrogen bonding between the perchlorate anion and the C-H hydrgoens on the alkyl chain, which can promote the formation of pre-associated [RuIII-OH...HO-RuIII] species, was proposed. Hydrogen bonding between the perchlorate anion and the C-H hydrogens on the alkyl chain were confirmed by the X-ray structure of the ruthenium complexes with the perchlorate anions.