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

Abstract

The enantiopure Schiff bases ( R or S )- N -1-(X-C 6 H 4 )ethyl-2-hydroxy-1-naphthaldimine {X = H [( R or S )-HL1], p -CH 3 O [( 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-C 6 H 4 )ethyl-2-oxo-1-naphthaldiminato-κ 2 N , O ]-Λ/Δ-cobalt(II) {X = H [Λ/Δ-Co-( R or S )-L1], p -CH 3 O [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 C 2 -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 Co 2+ -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 C 2 point group) and distance from the Co 2+ core.

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

“…The fact that diastereomeric structures of the kind Λ‐Zn R L 1 vs. Δ‐Zn R L 1 lend almost mirror‐image ECD spectra signifies that they are dominated by the chirality atmetal rather than by the chirality at the carbon atoms, although it is this latter which dictate the former through diastereoselection. Similar results have been found before for different families of M II ‐(Ar)salicylaldiminato/naphthaldiminato complexes [17–26] . Molecular orbital analysis of the diagnostic bands confirms they are due to exciton‐coupled MLCT‐type transitions (Figure S12).…”

“…Overall, we observe a conservation of Λ or Δ‐chirality‐at‐metal induction in the R or S ‐ligated complexes (i. e., Λ‐M R L or Δ‐M S L) resulting from diastereoselection in solid and solution phases (or gas‐phase optimized structures), as observed for a series of analogues M II ‐( R or S )‐(Ar)salicylaldiminates/naphthaldiminates (M=Co, [22,25] Ni, [20] Cu, [18,19] and Zn [5c–d,17,21,26] ). In contrast, solid‐versus‐solution studies reveal solvation‐induced chirality‐at‐metal inversion in Cu II ‐( R )‐( p ‐CH 3 O)salicylaldiminates [19] and Cu II ‐( R or S )‐1‐ N ‐(phenyl)‐2,4‐dihalogen‐salicylaldiminates [23]…”

“…[5d] The structures of the analogous M II -(R or S)-(Ar)naphthaldiminates (M=Zn [17] and Ni [20] ) also contained two symmetry-independent molecules in the asymmetric unit with the same metal configuration (i. e., Λ-MRL or Δ-MSL diastereomer), while the Cu II -(R)-(Ar)naphthaldiminates (Ar = m-CH 3 OC 6 H 4 ) [18] show two symmetry-independent molecules in the asymmetric unit with opposite configuration atmetal (i. e., Λ/Δ-CuRL, diastereomeric pair). However, a consistent stereochemical chirality-at-metal induction is reported for the analogous M II -(R or S)-(Ar) salicylaldiminates/naphthaldiminates (M=Co, [22,25] Ni, [20] Cu, [18,23] and Zn [5c-d,17,21,26] ) to provide Λ-MRL or Δ-MSL diastereomers for R or S-HL ligands. Only in a single case, opposite chirality-at-metal induction has been reported to give Δ-CuRL or Λ-CuSL in the case of Cu II -(R or S)-(Ar)salicylaldiminates.…”

“…[34] The computational procedure employed was similar to that previously reported for several analogs. [18][19][20][21][22][23][24][25][26] Starting from the X-ray structure of ZnRL 1 , the geometries for all three pairs of complexes Λ-ZnRL 1À 3 /Δ-ZnRL 1À 3 were constructed. Conformational searches were run by varying all possible rotatable bonds and keeping OÀ Zn and NÀ Zn distances fixed.…”

“…[13][14] We have recently reported on the synthesis, characterization, chiroptical properties, supramolecular chemistry, (dia)stereoselectivity and X-ray molecular structures along with computational studies of four-coordinated chiral [M(R or S-N O) 2 ] complexes (N O = deprotonated Schiff bases, M=Co/Cu/Ni/Zn) with Λ/Δ-chirality-at-metal induction in tetrahedral or distorted tetrahedral/square-planar geometry. [15][16][17][18][19][20][21][22][23][24][25][26] Coordination of the enantiopure ligands (R or S-N O) to the metal leads to the formation of two diastereomers Λ-M-R-N O and Δ-M-R-N O (or Λ-M-S-N O and Δ-M-S-N O), while the racemic ligands (R/S-N O) give all four diastereomers. The noncovalent interactions acting within the metal-chiral ligands framework in the solid state, or solute-solvent interactions in solution, provide a difference in free energy between the two diastereomers and, hence, result in one thermodynamically favored diastereomer.…”