Preliminary Assignment of Protonated and Deprotonated Homocitrates in Extracted FeMo-Cofactors by Comparisons with Molybdenum(IV) Lactates and Oxidovanadium Glycolates

Abstract

A similar pair of protonated and deprotonated mononuclear oxidovanadium glycolates [VO(Hglyc)(phen)(H₂O)]Cl·2H₂O (1) and [VO(glyc)(bpy)(H₂O)] (2) and a mixed-(de)protonated oxidovanadium triglycolate (NH₄)₂[VO(Hglyc)₂(glyc)]·H₂O (3) were isolated and examined. The ≡C-O(H) (≡C-OH or ≡C-O) groups coordinated to vanadium were spectroscopically and structurally identified. The glycolate in 1 features a bidentate chelation through protonated α-hydroxy and α-carboxy groups, whereas the glycolate in 2 coordinates through deprotonated α-alkoxy and α-carboxy groups. The glycolates in 3 coordinate to vanadium through α-alkoxy or α-hydroxy and α-carboxy groups and thus have both protonated ≡C-OH and deprotonated ≡C-O bonds simultaneously. Structural investigations revealed that the longer protonated V-O_α-hydroxy bonds [2.234(2) Å and 2.244(2) Å] in 1 and 3 are close to those of FeV-cofactor (FeV-co) 2.17 Ź (FeMo-co 2.17 Ų), while deprotonated V-O_α-alkoxy bonds [2, 1.930(2); 3, 1.927(2) Å] were obviously shorter. This shows a similar elongated trend as the Mo-O distances in the previously reported deprotonated vs protonated molybdenum lactates (Wang, S. Y. et al. Dalton Trans. 2018, 47, 7412-7421) and these vanadium and molybdenum complexes have the same local V/Mo-homocitrate structures as those of FeV/Mo-cos of nitrogenases. The IR spectra of these oxidovanadium and the previously synthesized molybdenum complexes including different substituted ≡C-O(H) model compounds show red-shifts for ≡C-OH vs ≡C-O alternation, which further assign the two IR bands of extracted FeMo-co at 1084 and 1031 cm^-1 to ≡C-O and ≡C-OH vibrations, respectively. Although the structural data or IR spectra for some of the previously synthesized Mo/V complexes and extracted FeMo-co were measured earlier, this is the first time that the ≡C-O(H) coordinated peaks are assigned. The overall structural and IR results well suggest the coexistence of homocitrates coordinated with α-alkoxy (deprotonated) and α-hydroxy (protonated) groups in the extracted FeMo-co.

Figure 1.

The environment of [VO(Hglyc)(phen)(H₂O)]Cl·2H₂O (1) showing hydrogen bonds between α-hydroxy groups, water molecules and α-carboxy groups; [VO(glyc)(bpy)(H₂O)] (2) showing hydrogen bonds between water molecules and α-alkoxy groups; (NH₄)₂[VO(Hglyc)₂(glyc)]·H₂O (3) showing hydrogen bonds between α-hydroxy groups and α-alkoxy groups. Ammonium ions and lattice water molecules were omitted for clarity.

Figure 2.

The structures of model vanadium/molybdenum hydroxycarboxylates with low oxidation states.

Figure 3.

Solution ¹³C NMR spectrum of [VO(Hglyc)(phen)(H₂O)]Cl·2H₂O (1) in D₂O.

Figure 4.

FT-IR spectra of [VO(Hglyc)(phen)(H₂O)]Cl·2H₂O (1), [VO(phen)₂Cl]Cl·H₂O, [VO(glyc)(bpy)(H₂O)] (2) and [VO₂(bpy)₂]Cl·4H₂O in the regions of 1800–400 cm⁻¹.

Figure 5.

FT-IR spectra of extracted FeMo-co and tetraethylammonium chloride (TEAC) in the region of 1800‒560 cm⁻¹.