Two-Step Binding of O₂ 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-Me2C6H3) with O2 was shown by stopped-flow kinetic studies to result in the rapid formation of (η1-O2)­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 (η2-O2)­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 O2 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 O2 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.