Oxidation of 1-Methyl-1-phenylhydrazine with Oxidovanadium(V)-Salan Complexes: Insight into the Pathway to the Formation of Hydrazine by Vanadium Nitrogenase

Abstract

A series of oxidovanadium(V) complexes [VO(L-κ⁴O,N,N,O)(OR)] (1a, R = Et, L = L¹; 1b, R = Me, L = L¹; 2, R = Me, L = L²; 3, R = Me, L = L³) were synthesized by the σ-bond metathesis reaction between [VO(OR)₃] and the linear diaminebis(phenol) derivatives H₂L (salans) containing different para-substituents on the phenoxo group [CMe₃CH₂CMe₂, L¹; Me, L²; Cl, L³]. As shown by X-ray crystallography complexes 1a, 1b, and 2 exhibit cis-α geometry, do have a stereogenic vanadium center, and exist as a racemic mixture of the Δ cis-α and Λ cis-α enantiomers. In solution, as demonstrated by ¹H and ⁵¹V NMR investigations, the structures of complexes 1-3 are consistent with their solid state. The reactions of 1b, 2, and 3 with NH₂NMePh in n-hexane afforded the oxidovanadium(IV) [VO(L-κ⁴O,N,N,O)] (4, L¹; 5, L²; 6, L³) and 1,4-dimethyl-1,4-diphenyl-2-tetrazene (Me₂Ph₂N₄) (7) as the main products together with a small amount of hydrazido(2-) vanadium(V) [V(L³-κ⁴O,N,N,O)(NNMePh)(OMe)] (8). Proposed reaction course for the oxidation of NH₂NMePh by 1b-3 is discussed. Compounds 4-8 were characterized by chemical and physical techniques including the X-ray crystallography for 4, 7, and 8. The solid-state (powder) electron paramagnetic resonance spectra and magnetic features strongly indicate that complexes 4-6 are formed as a mixture of a mononuclear (S = 1/2) and a dinuclear (S = 1) species.