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

Abstract

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, [CoIII(Me3-TPADP)­(O2H)­(RCN)]2+ [R = Me (2) and Ph (2 Ph)], to form cobalt­(III)–peroxyimidato complexes, [CoIII(Me3-TPADP)­(R–C­(NH)­O2)]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, [CoIII(TBDAP)­(O2H)­(CH3CN)]2+ (2 TBDAP), exhibited the deficiency of reactivity toward nitrile. The comparison of pK a values and redox potentials of 2 and 2 TBDAP showed that Me3-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 Me3-TPADP, resulting in the compression of the Co–Nax 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.

Controlling Reactivity through Spin Manipulation: Steric Bulkiness of Peroxocobalt(III) Complexes

“…Similarly, the S = 0 ground state of the peroxocobalt(III) complex, [Co III (Me 3 -TPADP)(O 2 )] + , which is structurally similar to 2 Me , exhibits an inability to activate nitriles. 36 However, upon the addition of acid, the resulting hydroperoxocobalt(III) species, [Co III (Me…”

“…The DFT results demonstrate the influence of the ligand field, controlled by the steric bulk of Nsubstituents on the ligand, in relation to the spin state of the peroxocobalt(III) complexes and their reactivity to nitrile activation. Similarly, the S = 0 ground state of the peroxocobalt(III) complex, [Co III (Me 3 -TPADP)(O 2 )] + , which is structurally similar to 2 Me , exhibits an inability to activate nitriles 36. However, upon the addition of acid, the resulting hydroperoxocobalt(III) species, [Co III (Me 3 -T P A D P ) ( O 2 H ) ] 2 + , a c t i v a t e s n i t r i l e s t o f o r m peroxyimidatocobalt(III) complexes, [Co III (Me 3 -TPADP)-(R−C(�NH)O 2 )] 2+ , through a nucleophilic pathway with a positive ρ value (3.2) obtained from the Hammett plot using para-substituted benzonitrile derivatives 36.…”

“…Similarly, the S = 0 ground state of the peroxocobalt(III) complex, [Co III (Me 3 -TPADP)(O 2 )] + , which is structurally similar to 2 Me , exhibits an inability to activate nitriles 36. However, upon the addition of acid, the resulting hydroperoxocobalt(III) species, [Co III (Me 3 -T P A D P ) ( O 2 H ) ] 2 + , a c t i v a t e s n i t r i l e s t o f o r m peroxyimidatocobalt(III) complexes, [Co III (Me 3 -TPADP)-(R−C(�NH)O 2 )] 2+ , through a nucleophilic pathway with a positive ρ value (3.2) obtained from the Hammett plot using para-substituted benzonitrile derivatives 36. Therefore, side-on S = 0 peroxocobalt(III) complexes, 2 Me and [Co III (Me 3 -TPADP)(O 2 )] + , undergo difficulty in nitrile activation due to the hindrance of Co−O bond dissociation.…”