Protonated and deprotonated vanadyl imidazole tartrates for the mimics of the vanadium coordination in the FeV-cofactor of V-nitrogenase

Abstract

Chiral imidazole-based oxidovanadium tartrates (H₂im)₂[Δ,Λ-V^IV₂O₂(R,R-H₂tart)(R,R-tart)(Him)₂]·Him (1, H₄tart = tartaric acid, Him = imidazole) and [Λ,Λ-V^IV₂O₂(R,R-tart)(Him)₆]·4H₂O (2) and their corresponding enantiomers (H₂im)₂[Λ,Δ-V^IV₂O₂(S,S-H₂tart)(S,S-tart)(Him)₂]·Him (3) and [Δ,Δ-V^IV₂O₂(S,S-tart)(Him)₆]·4H₂O (4) were obtained in alkaline solutions. Interestingly, the tartrates chelate with vanadium bidentately through α-alkoxy/α-hydroxy and α-carboxy groups and imidazole coordinates monodentately through nitrogen atom. It is worth noting that complexes 1 and 3 contain both protonated α-hydroxy and deprotonated α-alkoxy groups simultaneously, which have short V–O_α-alkoxy distances [1.976(4)_av Å in 1–4] and long V–O_α-hydroxy distances [2.237(3)_av Å in 1 and 2.230(2)_av Å in 3]. There is an interesting strong intramolecular hydrogen bond [O(11)⋯O(1) 2.731(5) Å] between the two parts in 1 and 3. The protonated V–O distances are closer to the average bond distance in reported FeV-cofactors (FeV-cos, V–O_α-alkoxy 2.156_av Å) in VFe proteins, which corresponds to the feasible protonation of coordinated α-hydroxy in R-homocitrate in V-nitrogenase, showing the homocitrate in the mechanistic model for nitrogen reduction as a secondary proton donor. Furthermore, vibrational circular dichroism (VCD) and IR spectra of 1–4 pointed out the disparity between the characteristic vibrations of the C–O and C–OH groups clearly. EPR experiment and theoretical calculations support +4 oxidation states for vanadium in 1–4. Solution ¹³C {¹H} NMR spectra and CV analyses exhibited the solution properties for 1 and 2, respectively, which indicates that there should be a rapid exchange equilibrium between the protonated and deprotonated species in solutions.