Effect of Alkyl Groups on the Aerobic Peroxidation of Hydrocarbons Catalyzed by Cobalt(III) Alkylperoxo Complexes

Abstract

The synthesis of various cobalt(III) complexes bearing a quaterpyridine ligand with different alkylperoxo groups via an unconventional approach is reported. Under ambient conditions, the cobalt(III) tert-butylperoxo complex, [Co III -(OO t Bu)(qpy)(NCCH 3 )] 2 + (2) (qpy = 2,2':6',2'':6'',2'''-quaterpyridine) oxidizes various organic substrates (RH) and leads to the formation of other alkylperoxo complexes with the general formula, [Co III (OOR)(qpy2 (4), PhCHMe (5), PhCMe 2 (6), p-Br-PhCMe 2 (7), and p-NO 2 -PhCMe 2 (8)]. The reactivity of 2 is further tuned by incorporating electron-donating and -withdrawing substituents on the qpy ligand to afford [Co III (OO t Bu)(X-qpy)(NCCH 3 )] 2 + [X = Me 2 (9), Me 4 (10), and Cl 2 (11)]. All complexes have been characterized by different physicochemical methods. The structures of 8, 9, and 11 are further determined by X-ray crystallography. The kinetics of the aerobic peroxidation of cyclopentene with 2-11 indicate that the reaction rate depends on the R group with a descending order of tertiary > secondary > primary alkyl groups.In addition, the rate of reaction is also sensitive to the nature of substituents on qpy. Complexes with electron-withdrawing substituents react more rapidly than those with electrondonating substituents.