Radical reactions of diamine bis(phenolate) vanadium(iii) complexes. Solid state binding of O2 to form a vanadium(v) peroxo complex

Abstract

[VCl₃(THF)₃] reacted with the sodium salt of a tripodal diamine bisphenolate ligand precursor Na₂L2 to give a paramagnetic d² complex [V(L2)Cl] (2). The reaction of 2 with oxygen is strongly dependent on the experimental conditions, affording [VO(L2)Cl] (6) or [V(η²-O₂)(L2)Cl] (7). The formation of 7 involves the direct addition of O₂ to V(III) in the solid state with oxidation to V(V) without significantly disturbing the structure of 2. DFT calculations showed that compound 7 is an intermediate in the formation of 6 from 2. The reaction involves the cleavage of the η¹-O–O bond in a proposed dimeric species. The overall reaction of 2 moles of vanadium(III), complex 2, and one mole of O₂ to yield two moles of product 6 is a favourable process with ΔG₂₉₈ = −38.3 kcal mol⁻¹. 7 is the first non-oxido peroxidovanadium(V) complex obtained directly from the reaction of a crystal and the second example of a structurally characterized complex of that type. Reactions of V(L1)Cl (1) and V(L2)Cl (2) with one equivalent of the nitroxyl radical TEMPO in toluene also result in the formation of oxido V(V) complexes, VO(L1)Cl (5) and VO(L2)Cl (6). The reaction of VO(OⁱPr)₃ with Na₂L2 afforded [VO(L2)(OⁱPr)] (8) in high yield. A major isomer having the VO moiety in the equatorial plane was characterised by X-ray diffraction although solution NMR data showed the presence of a minor species with the oxido ligand trans to the tripodal nitrogen, as in 6. Complexes 6 and 8 are very active and selective sulfoxidation catalysts of thioanisole, but no enantiomeric excess was obtained.