Two-Step Binding of O₂ to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Abstract

Treatment of V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>1</b>) (Ar = 3,5-Me₂C₆H₃) with O₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η¹-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>2</b>) (Δ<i>H</i>^⧧ = 3.3 ± 0.2 kcal/mol and Δ<i>S</i>^⧧ = −22 ± 1 cal mol^–1 K^–1), which subsequently isomerizes to (η²-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>3</b>) (Δ<i>H</i>^⧧ = 10.3 ± 0.9 kcal/mol and Δ<i>S</i>^⧧ = −6 ± 4 cal mol^–1 K^–1). The enthalpy of binding of O₂ to form <b>3</b> is −75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of <b>3</b> and <b>1</b> to form 2 equiv of OV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>4</b>) occurs by initial isomerization of <b>3</b> to <b>2</b>. The results of computational studies of this rearrangement (Δ<i>H</i> = 4.2 kcal/mol; Δ<i>H</i>^⧧ = 16 kcal/mol) are in accord with experimental data (Δ<i>H</i> = 4 ± 3 kcal/mol; Δ<i>H</i>^⧧ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of <b>4</b>, the reaction of O₂ with <b>1</b> in the presence of ^<i>t</i>BuCN was studied. At −45 °C, the principal products of this reaction are <b>3</b> and ^<i>t</i>BuC­(O)­NV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>5</b>), in which the bound nitrile has been oxidized. Crystal structures of <b>3</b> and <b>5</b> are reported