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

Chiral imidazole-based oxidovanadium tartrates (Him)[Δ,Λ-VIV2O(,-Htart)(,-tart)(Him)]·Him (1, Htart = tartaric acid, Him = imidazole) and [Λ,Λ-VIV2O(,-tart)(Him)]·4HO (2) and their corresponding enantiomers (Him)[Λ,Δ-VIV2O(,-Htart)(,-tart)(Him)]·Him (3) and [Δ,Δ-VIV2O(,-tart)(Him)]·4HO (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 distances [1.976(4) Å in 1-4] and long V-O distances [2.237(3) Å in 1 and 2.230(2) 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 2.156 Å) in VFe proteins, which corresponds to the feasible protonation of coordinated α-hydroxy in -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 C {H} 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.