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 condi- tions, the cobalt(III) tert-butylperoxo complex, [Co-III- ((OOBu)-Bu-t)(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)(cipy)(NCCH3)](2+) ER -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((OOBu)-Bu-t)(X-qpy)(NCCH3)](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 electron-donating substituents.