Syntheses and Vibrational Circular Dichroism Spectra of the Complete Series of [Ru((−)- or (+)-tfac)n(acac)3−n] (n = 0 ∼ 3, tfac = 3-Trifluoroacetylcamphorato and acac = Acetylacetonato)

Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)₃] (Ru-0), [Ru(acac)₂(trop)] (Ru-1), [Ru(acac)(trop)₂] (Ru-2), and [Ru(trop)₃] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Synthesis and Characterization of Bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) (X = H, p-CH3O, p-Br) with Symmetry- and Distance-Dependent Vibrational Circular Dichroism Enhancement and Sign Inversion

The enantiopure Schiff bases (R or S)-N-1-(X-C₆H₄)ethyl-2-hydroxy-1-naphthaldimine {X = H [(R or S)-HL1], p-CH₃O [(R or S)-HL2], and p-Br [(R- or S)-HL3]} react with cobalt(II) acetate to give bis[(R or S)-N-1-(X-C₆H₄)ethyl-2-oxo-1-naphthaldiminato-κ²N,O]-Λ/Δ-cobalt(II) {X = H [Λ/Δ-Co-(R or S)-L1], p-CH₃O [Λ/Δ-Co-(R or S)-L2], and p-Br [Λ/Δ-Co-(R or S)-L3]} (1–3), respectively. Induced Λ and Δ chirality originates at the metal center of the C₂-symmetric molecule in pseudotetrahedral geometry. Differential scanning calorimetry analyses explored the thermal stability of the complexes, which undergo reversible phase transformation from crystalline solid to isotropic liquid phase for 1 and 3 but irreversible phase transformation for 2. Like other cobalt(II) complexes, compounds 1–3 exhibit a continuous ensemble of absorption and circular dichroism bands, which span from the UV to IR region and can be collected into a superspectrum. Infrared vibrational circular dichroism (IR-VCD) spectra witness the coupling between Co²⁺-centered low-lying electronic states and ligand-centered vibrations. The coupling produces enhanced and almost monosignate VCD spectra, with both effects being mode-dependent in terms of the A or B symmetry (in the C₂ point group) and distance from the Co²⁺ core.

Chiroptical superspectra of naphthaldiminatocobalt(II) complexes 1−3 exhibit a continuous ensemble of absorption and circular dichroism bands from the UV to IR region, including peculiar and almost monosignate vibrational circular dichroism (VCD) spectra dominated by the coupling between Co²⁺-centered low-lying electronic states and ligand-centered vibrations, which depend on the normal-mode symmetry and distance from the Co²⁺ core

A new horizon for vibrational circular dichroism spectroscopy: a challenge for supramolecular chirality

The development of the solid state and time-step VCD methods opened a new horizon to reveal the mechanism of chirality amplification from microscopic to supramolecular scales.

Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes

Chiral coordination compounds of Co(II) and other open-shell metal complexes display enhanced vibrational circular dichroism (VCD) spectra associated with the existence of low-lying excited states (LLESs). In addition to the enhancement, a series of Co(II) salicylaldiminato complexes exhibits an almost monosignate pattern of VCD bands, a unique feature if compared with the usual alternation of positive and negative signals. Frequency and excited-state calculations reveal that VCD enhancement and sign reversal selectively affect the normal modes of B symmetry of the C₂-symmetric pseudotetrahedral species thanks to their combination with one or more LLES having the same B symmetry. This proves the strict relation between VCD enhancement and monosignate appearance and demonstrates an unprecedented symmetry dependence of the two phenomena.

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.

A vibrational circular dichroism (VCD) methodology for the measurement of enantiomeric excess in chiral compounds in the solid phase and for the complementary use of NMR and VCD techniques in solution: the camphor case

For the first time, the success of a methodology for the determination of enantiomeric excess (% ee) in chiral solid samples by vibrational circular dichroism (VCD) spectroscopy is reported. We have used camphor to determine the % ee in a blind sample constituted by a mixture of its two enantiomers as a test for the validity of our approach. IR and VCD spectra of different enantiomeric mixtures of R/S-camphor in Nujol mulls were recorded and linear regressions of VCD intensities (ΔAbs.) vs. % ee for selected bands were found. Finally, the VCD intensities of a blind sample were interpolated in these linear regressions, obtaining its % ee with a rms of 2.4. These results in the solid phase were complemented with the determination of % ee in the liquid phase by VCD and NMR techniques, which are proved to be complementary techniques to carry out this kind of analysis. In the same way as in the VCD solid phase, linear regressions of ΔAbs. vs. % ee for selected bands were established, obtaining a rms of 1.1 in the % ee determination of a blind sample. ¹H NMR experiments at 600 MHz using the chiral solvating agent, (S,S)-ABTE, allow the determination of the proportions of enantiomers in CD₂Cl₂ solution with great accuracy. ¹³C CPMAS NMR spectra prove that this technique cannot be used for conglomerates and/or solid solutions.

Solid state vibrational circular dichroism towards molecular recognition: chiral metal complexes intercalated in a clay mineral

The solid state VCD method revealed chirality effects on the intermolecular interaction between Δ- or Λ-[Ru(phen)₃]²⁺ and R or S-BINOL intercalated in a montmorillonite clay.

Stereoselective interactions as manifested by vibrational circular dichroism spectra: the interplay between chiral metal complexes co-adsorbed in a montmorillonite clay

Solid state VCD is applied for intercalated metal complexes.

VCD spectroscopy as an excellent probe of chiral metal complexes containing a carbon monoxide vibrational chromophore

η⁵-Menthyl-C₅H₄Ru(ii) carbonyl complexes show VCD of CO; the stretching is very sensitive and diagnostic to the (R)_Ru/(S)_Rumetal chirality.

VCD studies on chiral characters of metal complex oligomers

The present article reviews the results on the application of vibrational circular dichroism (VCD) spectroscopy to the study of stereochemical properties of chiral metal complexes in solution. The chiral characters reflecting on the vibrational properties of metal complexes are revealed by measurements of a series of β-diketonato complexes with the help of theoretical calculation. Attention is paid to the effects of electronic properties of a central metal ion on vibrational energy levels or low-lying electronic states. The investigation is further extended to the oligomers of β-diketonato complex units. The induction of chiral structures is confirmed by the VCD spectra when chiral inert moieties are connected with labile metal ions. These results have demonstrated how VCD spectroscopy is efficient in revealing the static and dynamic properties of mononuclear and multinuclear chiral metal complexes, which are difficult to clarify by means of other spectroscopes.

Chirality and diastereoselection of Δ/Λ-configured tetrahedral zinc complexes through enantiopure Schiff base complexes: combined vibrational circular dichroism, density functional theory, 1H NMR, and X-ray structural studies

The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A(^)B)(2) complexes is correlated to the chirality of the bidentate enantiopure (R)-A(^)B or (S)-A(^)B Schiff base building blocks [A(^)B = (R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ(2)N,O with X = OCH(3), Cl, Br]. In the solid-state the (R) ligand chirality induces a Λ-M configuration and the (S) ligand chirality quantitatively gives the Δ-M configuration upon crystallization as deduced from X-ray single crystal studies. The diastereoselections of the pseudotetrahedral zinc-Schiff base complexes in CDCl(3) solution were investigated by (1)H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base -CH═N- imine proton in (1)H NMR indicates an equilibrium of both Δ- and Λ-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/Λ-chirality because of a characteristic band representing coupled vibrations of the two ligand's C═N stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density Functional Theory calculations.

Vibrational circular dichroism spectroscopy of chiral molecules

In this chapter, new developments and main applications of vibrational circular dichroism (VCD) spectroscopy reported in the last 5 years are described. This includes the determinations of absolute configurations of chiral molecules, understanding solvent effects and modeling solvent-solute explicit hydrogen bonding networks using induced solvent chirality, studies of transition metal complexes and their peculiar and enormous intensity enhancements in VCD spectra, investigations of conformational preference of chiral ligands bound to gold nano particles, and two new advances in applying matrix isolation VCD spectroscopy to flexible, multi-conformational chiral molecules and complexes, and in development of femtosecond laser based VCD instruments for transient VCD monitoring. A brief review of the experimental techniques and theoretical methods is also given. The purpose of this chapter is to provide an up-to-date perspective on the capability of VCD to solve significant problems about chiral molecules in solution, in thin film states, or on surfaces.