Structural Identification of a Novel Axially Chiral Binaphthyl Fluorene Based Salen Ligand in Solution Using Electronic Circular Dichroism: A Theoretical–Experimental Analysis

Abstract

A novel axially chiral binaphthyl fluorene based salen ligand, AFX-155 [2,2'-(1E,1'E)-(R)-1,1'-binaphthyl-2,2'-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-l)ethynyl)phenol)], with potential applications in homogeneous catalysis, biophotonics, and sensing was synthesized. A full comparative theoretical-experimental analysis of the UV-vis and electronic circular dichroism (ECD) spectra of the 10 primary isomers, comprising stereoisomers and optical isomers, revealed the presence of the unique structure in tetrahydrofuran (THF) solution, the trans-R-intra//trans-R-extra. A proposed route of attack of the (R)-(+)-2,2'-diamino-1,1'-binapthalene onto a salicaldehyde 5-(2-(2-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)ethynyl)-2-hydroxybenzaldehyde followed by a consecutive attack of the resulting species onto another salicaldehyde, both via Burgi:Dunitz trajectory, validates the unambiguous formation of the established isomer. Steric hindrances seem to be the determinant factor that defines the 3D structural conformation of this particular stereoisomer of AFX-155 with triple axial chirality. The determination of every optimal structure and the dominant conformers of AFX-155 were calculated evaluating, in CONFLEX, their steric energies using force fields at MMFF94S (2006-11-24HGTEMP) level in gas phase. The geometry of the conformers was optimized in THF (using PCM) using Gaussian 09 at the DFT/B3LYP level of theory and 6-31G* basis set. The first 100 electronic excited states were calculated using the same level of theory and basis set.

Diastereomeric preference of a triply axial chiral binaphthyl based molecule: a concentration dependent study by chiroptical spectroscopies

“…For comparison to the experimental measurements, the half-width at half height (HWHH) of 0.2 eV with a Gaussian line-shaped function was used to simulate ECD spectra. We noted that in the previous work, 15 a broader Gaussian line-shaped function with 0.35 eV HWHH was used on the truncated systems where four -C 6 H 13 groups were replaced by four methyl groups. 15 3 Results and discussion Fig.…”

“…We noted that in the previous work, 15 a broader Gaussian line-shaped function with 0.35 eV HWHH was used on the truncated systems where four -C 6 H 13 groups were replaced by four methyl groups. 15 3 Results and discussion Fig. 1.…”

“…CDCl 3 was chosen as it offers sufficient solubility and does not interfere in the IR window from 1700-1100 cm À1 . Since the previous ECD study 15,16 was carried out in deuterated THF solvent, i.e. THF-d 8 , we also perform further VA and VCD measurements in THF-d 8 for comparison.…”

“…This molecule was recently synthesized for its potential applications in homogeneous catalysis, biophotonics, and biosensing. 15,16 It was also characterized using ECD and twophoton CD (TPCD) spectroscopy by Hernández and co-workers. 15,16 One of our goals is to utilize VCD spectroscopy and DFT calculations to clearly establish the axial chirality of AFX-155 in solution.…”

“…15,16 It was also characterized using ECD and twophoton CD (TPCD) spectroscopy by Hernández and co-workers. 15,16 One of our goals is to utilize VCD spectroscopy and DFT calculations to clearly establish the axial chirality of AFX-155 in solution. A second goal of our study is to examine the effects of environmental perturbations, i.e.…”