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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-Oα-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 Å1 (FeMo-co 2.17 Å2), while deprotonated V-Oα-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.

Unlike the most of α-alkoxy coordination in α-hydroxycarboxylates to vanadium, novel α-hydroxy coordination to vanadium(IV) has been observed for a series of chiral and achiral monomeric α-hydroxycarboxylato vanadyl complexes [VO(H2cit)(bpy)]·2H2O (1), [VO(Hmal)(bpy)]·H2O (2), [VO(H2cit)(phen)]·1.5H2O (3), [VO(Hmal)(phen)]·H2O (4), and [(Δ)VO(S-Hcitmal)(bpy)]·2H2O (5), [VO(H2cit)(phen)]2·6.5H2O (6), which were isolated from the reactions of vanadyl sulfate with α-hydroxycarboxylates and N-heterocycle ligands in acidic solution. The complexes feature a tridentate citrate, malate or citramalate that chelates to vanadium atom through their α-hydroxy, α-carboxy and β-carboxy groups; while the other β-carboxylic acidic group of citrate is free to participate strong hydrogen bonds with lattice water molecule. The neutral α-hydroxy group also forms strong intermolecular hydrogen bonds with water molecule and the negatively-charged α-carboxy group in the environment. The inclusion of a hydrogen ion in α-alkoxy group results in the formation of a series of neutral complexes with one less positive charge. There are two different configurations of citrate with respect to the trans-position of axial oxo group, where the complex with trans-hydroxy configuration seems more stable with less hindrance. The average bond distances of V-Ohydroxy and V-Oα-carboxy are 2.196 and 2.003Å respectively, which are comparable to the VO distance (2.15Å) of homocitrate in FeV-cofactor of V-nitrogenase. A new structural model is suggested for R-homocitrato iron vanadium cofactor as VFe7S9C(R-Hhomocit) (H4homocit=homocitric acid) with one more proton in homocitrate ligand.

The electronic structures of FeFe-cofactors (FeFe-cos) in resting and turnover states, together with their PN clusters from iron-only nitrogenases, have been calculated using the bond valence method, and their crystallographic data were reported recently and deposited in the Protein Data Bank (PDB codes: 8BOQ and 8OIE). The calculated results have also been compared with those of their homologous Mo- and V-nitrogenases. For FeFe-cos in the resting state, Fe1/2/4/5/6/7/8 atoms are prone to Fe3+, while the Fe3 atom shows different degrees of mixed valences. The results support that the Fe8 atom at the terminal positions of FeFe-cos possesses the same oxidation states as the Mo3+/V3+ atoms of FeMo-/FeV-cos. In the turnover state, the overall oxidation state of FeFe-co is slightly reduced than those in the resting species, and its electronic configuration is rearranged after the substitution of S2B with OH, compatible with those found in CO-bound FeV-co. Moreover, the calculations give the formal oxidation states of 6Fe2+–2Fe3+ for the electronic structures of PN clusters in Fe-nitrogenases. By the comparison of Mo-, V- and Fe-nitrogenases, the overall oxidation levels of 7Fe atoms (Fe1–Fe7) for both FeFe- and FeMo-cos in resting states are found to be higher than that of FeV-co. For the PN clusters in MoFe-, VFe- and FeFe-proteins, they all exhibit a strong reductive character. [ABSTRACT FROM AUTHOR]

Neutral hexnuclear and octanuclear polymolybdate-based porous complexes [Mo6O6(μ2-O)9(Hbtz)6]·21H2O (1, Hbtz = 1H-benzotriazole) and [Mo8O8(μ2-O)12(Hdatrz)8]·28H2O (2, Hdatrz = 1H-1,2,4-triazole-3,5-diamine), dodecanuclear and hexadecnuclear complexes Na4[Mo12O12(μ2-O)4(μ3-O)12(3-trz)8]·14H2O (3, 3-trz = 1,2,4-triazol-3-amine) and Na8[Mo16O16(μ2-O)16(μ2-OH)16(3-trz)8]·29H2O (4) have been successfully obtained and well characterized respectively. As the pH values of the solutions increase, the central metallic nuclei of 1–4 decrease gradually. Under basic conditions, isolated intrinsic microporous materials 1 and 2 can be constructed as six- and eight-membered rings with pore diameters of 3.2 and 4.8 Å, respectively, while dodecanuclear molybdenum(V) cluster 3 aggregates as an isopolyacidic anion. Interestingly, two {Mo8} units in 4 are linked by strong intramolecular hydrogen bonds [2.692(5) and 2.691(5) Å] into a dimer between bridged hydroxy groups and bridged oxygen atoms. There are diversiform pores presented in 1–4, and the gas adsorption experiments show that 1–4 are obviously favorable for the adsorption of gases O2 and CO2 under different pressures, while little or no adsorption for gases N2, CH4 and H2 respectively. The CO2 adsorption capacities of 1–4 are attributed to the pore sizes and the functions of the amines and uncoordinated triazolate N-donor groups. The influences of neutral and anion complexes have been discussed on the skeletons and porous sizes. [ABSTRACT FROM AUTHOR]

Hexanuclear polyoxomolybdenum-based discrete supermolecules Nax[MoV6O6(μ2-O)9(Htrz)6−x(trz)x]·nH2O (x = 0, n = 15, 1; x = 1, n = 12, 2; x = 2, n = 10, 3; x = 2, n = 49, 4; Htrz = 1H-1,2,3-triazole) have been prepared and fully characterized with different amounts of sodium cations inside and outside the intrinsic holes. Structural analyses demonstrate that they all exist a triangular channel constructed by six molybdenum–oxygen groups with inner diameters of 2.86 (1), 2.48 (2), and 3.04 (3/4) Å, respectively. Zero, one, or two univalent enthetic guest Na+ have been hosted around the structural centers, which reflect the expansion and contraction effects at microscopic level. Water-soluble species can serve as crown ether-like metallacycles before and after the sodium binding. Diverse nanoscale pores are further formed through intermolecular accumulations with hydrogen bonding. Gas adsorption studies indicate that 2–4 can selectively adsorb CO2 and O2 but have little or even no affinities toward H2, N2, and CH4. Theoretical calculations corroborate the roles of Na+ and auxiliary ligand with different states in bond distances, molecular orbitals, electrostatic potentials, and lattice energies in these discrete clusters. The binding orders of sodium cations in 2–4 are similar with the classical crown ethers, where 2 is the strongest one with 2.226(4)av Å for sodium cation bonded to six O atoms. [ABSTRACT FROM AUTHOR]