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

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A unique protocol for the preparation of cobalt(III) alkylperoxo complexes [CoIII(OOR)(qpy)(NCCH3)]2+ with various alkyl (R) groups has been developed. It is found that the reaction rate for the aerobic peroxidation of cyclopentene (as a model substrate of hydrocarbons) is susceptible to the R group with 3°>2°>1°.


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, [CoIII(OOtBu)(qpy)(NCCH3)]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, [CoIII(OOR)(qpy)(NCCH3)]2+ [R=c-C5H7 (3), PhCH2 (4), PhCHMe (5), PhCMe2 (6), p-Br-PhCMe2 (7), and p-NO2-PhCMe2 (8)]. The reactivity of 2 is further tuned by incorporating electron-donating and -withdrawing substituents on the qpy ligand to afford [CoIII(OOtBu)(X-qpy)(NCCH3)]2+ [X=Me2 (9), Me4 (10), and Cl2 (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 211 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 electron-donating substituents.

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