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

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^tBu)(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, [Co^III(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 [Co^III(OO^tBu)(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 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 electron-donating substituents.