Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

Abstract

Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminate (R- or S-N^O), diastereoselectively provide Λ- or Δ-chiral-at-metal four-coordinated Zn(R- or S-N^O)2 {Ar = C6H5; Zn-1R or Zn-1S and p-C6H4OMe; Zn-2R or Zn-2S}. Two R- or S-N^O-chelate ligands coordinate to the zinc(II) in a tetrahedral mode and induce Λ- or Δ-configuration at the zinc metal center. In the solid state, the R- or S-ligand diastereoselectively gives Λ- or Δ-Zn configuration, respectively, and forms enantiopure crystals. Single crystal structure determinations show two symmetry-independent molecules (A and B) in each asymmetric unit to give Z' = 2 structures. Electronic circular dichroism (ECD) spectra show the expected mirror image relationship resulting from diastereomeric excess toward the Λ-Zn for R-ligands and Δ-Zn for S-ligands in solution. ECD spectra are well reproduced by TDDFT calculations, while the application of the exciton chirality method, in the common point-dipole approximation, predicts the wrong sign for the long-wavelength couplet. A dynamic diastereomeric equilibrium (Λ vs Δ) prevails for both R- and S-ligand-metal complexes in solution, respectively, evidenced by (1)H NMR spectroscopy. Variable temperature (1)H NMR spectra show a temperature-dependent shift of the diastereomeric equilibrium and confirm Δ-Zn configuration (for S-ligand) to be the most stable one and favored at low temperature. DSC analyses provide quantitative diastereomeric excess in the solid state for Zn-2R and Zn-2S, which is comparable to the results of solution studies.

Asymmetric construction of tetrahedral chiral zinc with high configurational stability and catalytic activity

“…5c). This very high stability is in stark contrast to the typical tetrahedral zinc complexes which have been reported to undergo an instantaneous stereoinversion 30,31 . This result demonstrates the effectiveness of the design of L 2− in slowing down stereoinversion, even when the tetrahedral four-coordinate structure has a labile coordination site.…”

A novel nano‐size lanthanum metal–organic framework based on 5‐amino‐isophthalic acid and phenylenediamine: Photoluminescence study and sensing applications

“…The main objective of XRD is to determine the suggested structure of the metal complex and considered a further evidence about the structure. Previous reports mentioned that the transition metal complexes commonly are crystalline in nature and/or amorphous in nature with tetrahedral, octahedral or square planar geometry . (Figure S5; supplementary material) shows the XRD patterns of La‐MOF ( M1 and M2) which recorded between 2θ ranging from 3‐80 o .…”

Exciton coupling between enones: Quassinoids revisited

“…Recently, we have discovered that chromophores with a “curved” conjugated aromatic system, such as BODIPY and phenanthrene, are generally unsuited for straightforward exciton chirality applications because of their large intrinsic magnetic transition dipole moments . Another possible source of error is the incorrect assumption that the exciton coupling potential is necessarily positive . Apart from these few specific issues, the application of ECM is safe and unambiguous when the molecular conformation is established, the chromophores are strong, and their transitions well known.…”

“…43,44 Another possible source of error is the incorrect assumption that the exciton coupling potential is necessarily positive. 45,46 Apart from these few specific issues, the application of ECM is safe and unambiguous when the molecular conformation is established, the chromophores are strong, and their transitions well known. This happens in fact for several chiroptical probes purposely developed for the so-called exciton chirality derivatization approach, such as benzoate, p-substituted benzoate, cinnamate, naphthoate, anthroate, and so on.…”