Chiral and achiral vanadyl lactates with vibrational circular dichroism: Toward the chiral metal cluster in nitrogenase

The Phenomenon of Vibrational Circular Dichroism Enhancement: A Systematic Survey of Literature Data

While the intensity of vibrational circular dichroism (VCD) signals is commonly 104-105 times smaller than that of corresponding IR signals, several kinds of systems display enhanced VCD spectra with g-values (VCD/IR intensity ratio) above 10-3 and even reaching 5 × 10-2 in some exceptional cases. These systems include transition metal and lanthanide complexes, protein and peptide fibrils, short oligopeptide gels, crystalline compounds, gels and solution aggregates of organic compounds. We review the literature on VCD enhancement, focusing on collecting and analyzing data on enhanced g-values. Special attention is given to the mechanisms proposed to produce these effects.

Spontaneous conversions of glutamine, histidine and arginine into α-hydroxycarboxylates with NH4VO3 or V2O5

Glutamine gets transformed to 2-hydroxy-5-oxoproline with NH₄VO₃ in a neutral solution as a product of 2,2'-bipyridine oxidovanadium(v) 2-hydroxy-5-oxoproline [V^V₂O₃(hop)₂(bpy)₂]·7H₂O [1, H₂hop = 2-hydroxy-5-oxoproline] with yields of 65.6%. Similarly, histidine and arginine are converted into the corresponding α-hydroxycarboxylates as 2,2'-bipyridine oxidovanadium(iv) 3-(1H-imidazolyl-5-yl)-2-hydroxyacrylate [V^IV₂O₂(imha)₂(bpy)₂]·bpy [2, H₂imha = 3-(1H-imidazolyl-5-yl)-2-hydroxyacrylic acid] and guanidinium oxidovanadium(v) 1-(aminoiminomethyl)-2-hydroxyproline (CN₃H₆)[V^VO₂(Haimhp)₂]·2H₂O [3, H₂aimhp = 1-(aminoiminomethyl)-2-hydroxyproline] with V₂O₅ in low yields respectively, where an aggregate of oxidovanadium(v) arginine (H₂arg)_n(V^VO₃)_n·½nH₂O (4, Harg = arginine) has been isolated preferentially in an initial experiment for 3. α-Hydroxycarboxylates chelate bidentately with vanadium viaα-alkoxy and α-carboxy groups in 1-3, as observed from structural analyses. Their racemizations have been observed after the conversions. There is no coordination in 4 based on solid-state ¹³C NMR spectra, and only strong hydrogen bonds exist in the anion chains (V^VO₃)^- and protonated arginines. 1 and 4 were fully characterized by elemental analysis, UV-vis, IR and solid-state ¹³C NMR spectroscopies, TG and X-ray structural analyses, and theoretical bond valence calculations (BVS). The conversions of glutamine, histidine and arginine occur spontaneously in a solution under ambient conditions.

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

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.

Broad-Range Spectral Analysis for Chiral Metal Coordination Compounds: (Chiro)optical Superspectrum of Cobalt(II) Complexes

Chiroptical broad-range spectral analysis extending from UV to mid-IR was employed to study a family of Co(II) N-(1-(aryl)ethyl)salicylaldiminato Schiff base complexes with pseudotetrahedral geometry associated with chirality-at-metal of the Δ/Λ type. While common chiral organic compounds have well-separated absorption and circular dichroism spectra (CD) in the UV/vis and IR regions, chiral Co(II) complexes feature an almost unique continuum of absorption and CD bands, which cover in sequence the UV, visible, near-IR (NIR), and IR regions of the electromagnetic spectrum. They can be collected in a single (chiro)optical superspectrum ranging from the UV (230 nm, 5.4 eV) to the mid-IR (1000 cm^-1, 0.12 eV), which offers a fingerprint of the structure and stereochemistry of the metal complexes. Each region of the superspectrum contributes to one piece of information: the NIR-CD region, in combination with TDDFT calculations, allows a reliable assignment of the metal-centered chirality; the UV-CD region facilitates the analysis of the Δ/Λ diastereomeric equilibrium in solution; and the IR-VCD region contains a combination of low-lying metal-centered electronic states (LLES) and ligand-centered vibrations and displays characteristically enhanced and monosignate VCD bands. Circular dichroism in the NIR and IR regions is crucial to reveal the presence of d-d transitions of the Co(II) core which, due to the electric-dipole forbidden character, would be otherwise overlooked in the corresponding absorption spectra.