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

A series of oxidovanadium(V) complexes [VO(L-κ4 O,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 H2 L (salans) containing different para-substituents on the phenoxo group [CMe3 CH2 CMe2 , 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 by1 H and51 V NMR investigations, the structures of complexes 1-3 are consistent with their solid state. The reactions of 1b, 2, and 3 with NH2 NMePh in n-hexane afforded the oxidovanadium(IV) [VO(L-κ4 O,N,N,O)] (4, L1 ; 5, L2 ; 6, L3 ) and 1,4-dimethyl-1,4-diphenyl-2-tetrazene (Me2 Ph2 N4 ) (7) as the main products together with a small amount of hydrazido(2-) vanadium(V) [V(L3 -κ4 O,N,N,O)(NNMePh)(OMe)] (8). Proposed reaction course for the oxidation of NH2 NMePh 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.