Two-step binding of O ₂ to a vanadium(III) trisanilide complex to form a non-vanadyl vanadium(V) peroxo complex

Treatment of V(N[ ^tBu]Ar) ₃ (1) (Ar = 3,5-Me ₂C ₆H ₃) with O ₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η ¹-O ₂)V(N[ ^tBu]Ar) ₃ (2) (δH ^‡ = 3.3 ± 0.2 kcal/mol and δS ^‡ = -22 ± 1 cal mol ^-1 K ^-1), which subsequently isomerizes to (η ²-O ₂)V(N[ ^tBu]Ar) ₃ (3) (δH ^‡ = 10.3 ± 0.9 kcal/mol and δS ^‡ = -6 ± 4 cal mol ^-1 K ^-1). The enthalpy of binding of O ₂ to form 3 is -75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of 3 and 1 to form 2 equiv of O≡V(N[ ^tBu]Ar) ₃ (4) occurs by initial isomerization of 3 to 2. The results of computational studies of this rearrangement (δH = 4.2 kcal/mol; δH ^‡ = 16 kcal/mol) are in accord with experimental data (δH = 4 ± 3 kcal/mol; δH ^‡ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of 4, the reaction of O ₂ with 1 in the presence of ^tBuCN was studied. At -45 °C, the principal products of this reaction are 3 and ^tBuC(=O) N≡V(N[ ^tBu]Ar) ₃ (5), in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.