Mechanistic Insights into Nitrile Activation by Cobalt(III)-Hydroperoxo Intermediates: The Influence of Ligand Basicity.

The versatile applications of nitrile have led to the widespread use of nitrile activation in the synthesis of pharmacologically and industrially valuable compounds. We reported the activation of nitriles using mononuclear cobalt(III)-hydroperoxo complexes, [Co III (Me 3 -TPADP)(O 2 H)(RCN)] 2+ [R = Me ( 2 ) and Ph ( 2 Ph )], to form cobalt(III)-peroxyimidato complexes, [Co III (Me 3 -TPADP)(R-C(=NH)O 2 )] 2+ [R = Me ( 3 ) and Ph ( 3 Ph )]. The independence of the rate on the nitrile concentration and the positive Hammett value of 3.2(2) indicated that the reactions occur via an intramolecular nucleophilic attack of the hydroperoxide ligand to the coordinated nitrile carbon atom. In contrast, the previously reported cobalt(III)-hydroperoxo complex, [Co III (TBDAP)(O 2 H)(CH 3 CN)] 2+ ( 2 TBDAP ), exhibited the deficiency of reactivity toward nitrile. The comparison of p K a values and redox potentials of 2 and 2 TBDAP showed that Me 3 -TPADP had a stronger ligand field strength than that of TBDAP. The density functional theory calculations for 2 and 2 TBDAP support that the strengthened ligand field in 2 is mainly due to the replacement of two tert -butyl amine donors in TBDAP with methyl groups in Me 3 -TPADP, resulting in the compression of the Co-N ax bond lengths. These results provide mechanistic evidence of nitrile activation by the cobalt(III)-hydroperoxo complex and indicate that the basicity dependent on the ligand framework contributes to the ability of nitrile activation.