AbstractThe 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.