Dinitrogenvanadates(–I): synthesis, reactions and conditions for their stability

Abstract

Reduction of [VCl₃(thf)₃] by Na or Li (M) in the presence of phosphines (L^m) and N₂ yielded the complexes Na[V(N₂)_n(PMe₂Ph)_{6 –n}](n= 1 or 2), M[V(N₂)_nL² _{4 –n}][n= 1 or 2, L²= Me₂PCH₂CH₂-PMe₂(dmpe) or Ph₂PCH₂CH₂PPh₂], Na[V(N₂)(dmpe)L³]{L³= PhP[(CH₂)₂PPh₂]₂, MeP[(CH₂)₃PMe₂]₂(bdmpm) or MeP[(CH₂)₃PEt₂]₂} and Na[V(N₂)₂L⁴]{L⁴= P[(CH₂)₂PPh₂]₃, P[(CH₂)₃PMe₂]₃ or P[(CH₂)₃PEt₂]₃}. Generally, the complexes are labile, with a maximum stability for cis- and trans-M[V(N₂)₂(dmpe)₂]. In the presence of CO or CNBu^t, N₂ is replaced by these ligands. The compounds have been characterized by their ν(NN) pattern and by ⁵¹V, ²³Na and ⁷Li NMR spectroscopy. The ⁵¹V NMR spectra exhibit well resolved multiplets. All spectroscopic data, in particular ⁷Li NMR of cis-and trans-[Li(thf)_x][V(N₂)₂(dmpe)₂] in tetrahydrofuran, are indicative of close contact ion-pair interaction of the kind V–NN ⋯ M⁺. The potassium ion and large cations such as [Net₄]⁺ do not provide stable complexes, and complexation of Li⁺ and Na⁺ by crown ethers leads to immediate decomposition of the N₂ complexes. The reductive protonation of part of the co-ordinated N₂ to NH₄ ⁺(plus small amounts of N₂H₅ ⁺) is discussed in terms of similar reactivity features to the analogous complexes, and in the context of these N₂ complexes as functional models for nitrogenases. The compound [V₂(bdmpm)₂Cl₃], isolated as an intermediate product of the reduction of VCl₃, reacted with LiNHNH₂ to yield ammonia.