Pseudotetrahedral Zn(ii)-(R or S)-dihalogen-salicylaldiminato complexes with Λ- or Δ-chirality induction at-metal

Abstract

Reactions of enantiopure (S or R)-N-1-(phenyl)ethyl-2,4-X1,X2-salicylaldimine (S-H or R-H; X1, X2 = dihalogen) with Zn(II)-nitrate give bis[(S or R)-N-1-(phenyl)ethyl-2,4-X1,X2-salicylaldiminato-κ2N,O]-zinc(II), (Δ-ZnS or Λ-ZnR) with Δ/Λ-chirality induction at-metal in the C2-symmetric molecules....

Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)‐N‐1‐(Ar)Ethylsalicylaldiminato‐κ2N,O]zinc(II) with Λ/Δ‐Chirality‐At‐Metal Induction

“…Recalling that the estimated diastereomeric populations (used to generate Boltzmann‐averaged spectra) are dominated by Λ‐Zn R L 1−3 and Δ‐Zn S L 1−3 species, respectively, the results strongly suggest that the Zn R L 1−3 or Zn S L 1−3 complexes result in a diastereomeric excess of Λ‐Zn R L 1−3 or Δ‐Zn S L 1−3 (as major diastereomer) in solution. This result is in line with the X‐ray structures for Λ‐Zn R L 1−3 or Δ‐Zn S 1−3 in solid state, [5c–d,17,21,26] 1 H NMR evidence, VT‐ECD results, and DFT optimization findings discussed above.…”

“…As the complexes show two separate peaks for each proton, corresponding to the Δ‐ and Λ‐diastereomers in solution, there might be a thermodynamic equilibrium between the two diastereomers (Δ⇆Λ) which will be shifted by changing the temperature [21,26] . To check this hypothesis, 1 H NMR spectra for Zn S L 1 were run at variable temperatures starting from 20 °C to 0 °C, −20 °C and −40 °C in CDCl 3 (Figure 8).…”

“…The spectra feature a strong band below 300 nm and a moderately intense band at 300-350 nm with maxima at λ max = 320-325 nm, due to the intra-ligand π!π* and n!π* transitions, respectively. [17,21,26] The spectra for the complexes further show a strong broad band at 350-420 nm with λ max = 372-378 nm, assigned to the metal-ligand charge transfer (MLCT) transitions, which are obviously absent in the Schiff base ligands. Absorption spectra recorded at different time intervals suggest a rapid decomposition of the complex upon dissolution in chloroform (Figure 5): the intra-ligand n!π* band becomes more intense, while the MLCT band completely disappears over time.…”

“…1 H NMR spectra at different time intervals after dissolution might also show an equilibrium (Δ.Λ) shift provided that the equilibrium is slow enough with respect to the NMR time scale. [16,17,21,26] To check this equilibrium shift, we recorded 1 H NMR spectra for ZnSL 1 or ZnRL 1 in CDCl 3 at different time intervals after dissolution of the sample. The spectral analyses demonstrate that the diastereomeric ratios (Δ:Λ or Λ:Δ) remain almost unchanged (data not shown), suggesting no equilibrium shift in course of time in solution, in line with our previous findings for Zn-(R or S)-(Ar)naphthaldiminates.…”

“…Similar results have been found before for different families of M II -(Ar)salicylaldiminato/naphthaldiminato complexes. [17][18][19][20][21][22][23][24][25][26] Molecular orbital analysis of the diagnostic bands confirms they are due to exciton-coupled MLCT-type transitions (Figure S12). The occupied orbitals (HOMO and HOMO À 1) are a combination of metal d orbitals (d xy and d yz ) and ligand π orbitals, while the virtual (LUMO) orbital is a ligand π* orbital.…”