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

Abstract

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. © 2012 American Chemical Society.