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

Abstract

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.

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

“…Solution studies further explored the existence of a dynamic diastereomeric equilibrium between the two diastereomers (Λ ⇆ Δ), controlled by the temperature, as examined by variable-temperature 1 H NMR and ECD spectra. 19 , 26 …”

“…In our previous work on the salicylaldiminato Schiff base analogues of 1 – 3 , we introduced the concept of a (chiro)optical superspectrum to describe a continuous set of optical and chiroptical (CD) spectra spanning the aforementioned regions. 19 The superspectrum is rich in several bands and offers a distinctive fingerprint of the structure and stereochemistry of the metal complexes. The (chiro)optical superspectrum only manifests in the presence of the Co II core, which justifies the extension to cobalt(II) compounds 1 – 3 of previous studies concerning the homoleptic series of copper(II), nickel(II), and zinc(II).…”

The Synthesis of a Bis(thiosemicarbazone) Macrocyclic Ligand and the Mn(II), Co(II), Zn(II) and 68Ga(III) Complexes

“…The standard Ahlrichs valence triple-ξ including polarization functions (TZVP) basis set was used for the high-spin Co 2+ complexes and the DGDZVP basis set for the Zn 2+ complexes. These combinations of functional and basis sets were chosen because they have shown good agreement with experimental values for similar combinations of ligands and metals [87,88]. For comparison with the XRD data, values for selected bond lengths of the optimised structures are shown in DFT analysis of the [ZnHL] + complexes in the presence of water indicated four energy minima corresponding to the following diastereomeric pairs: ∆(δ,δ,δ,δ) and Λ(λ,λ,λ,λ) for [Zn1HL] + , and ∆(λ,λ,λ,λ) and Λ(δ,δ,δ,δ) for [Zn2HL] + .…”

Benchmarking computational methods and influence of guest conformation on chirogenesis in zinc porphyrin complexes

“…All these eight functionals have been successfully applied for TD-DFT simulations 44,45,73,[84][85][86][87][88] including simulations of the CD spectra of metal complexes. [89][90][91][92][93][94][95][96][97][98][99] In addition to the functionals, two basis sets of different sizes: cc-pVDZ [100][101][102] and cc-pVTZ 42,91,[100][101][102] were considered. As reported previously, for UV and CD spectra simulations the double-zeta basis set is considered to be accurate.…”