Remarkably fast oxidation of alkanes by hydrogen peroxide catalyzed by a tetracopper(II) triethanolaminate complex: Promoting effects of acid co-catalysts and water, kinetic and mechanistic features

Abstract

The kinetic and mechanistic features of alkane oxidations to the corresponding alkyl hydroperoxides (main primary products), alcohols and ketones (secondary products) in the systems composed of tetracopper(II) triethanolaminate catalyst [O⊂Cu 4{N(CH 2CH 2O) 3} 4(BOH) 4][BF 4] 2 ( 1), aqueous hydrogen peroxide, acetonitrile solvent and an acid promoter (co-catalyst), have been investigated based on the combination of experimental kinetic, selectivity, ESR and UV–vis methods. The nature of acid promoter (hydrochloric, sulfuric, nitric and trifluoroacetic acid) is shown to be a key factor affecting significantly the rate of alkane oxidation. Although all these acids exhibit noticeable promoting effect, it has been observed that in the presence of HCl the reaction proceeds extremely rapidly, being one order faster than those promoted by the other acids, and allowing to achieve the remarkably high turnover frequencies (TOFs) of ca. 600 h −1. The unusual rate-accelerating role of water has also been disclosed in the oxidation of cyclohexane catalyzed by 1 + HCl or 1 + CF 3COOH systems. Furthermore, uncommon second-order reaction kinetics with respect to the catalyst have been found. A mechanism of the alkane oxygenation has been proposed, which includes the formation of hydroxyl radicals attacking the alkane molecule. Hydroxyl radicals are formed via the interaction between H 2O 2 and catalytically active Cu(I) species, the latter being reversibly generated from 1 under the action of an acid, H 2O 2 and water.