Solvent-Free Synthesis of Chiral Substituted N‑Benzylideneanilines Imines: X‑ray Structure, and DFT Study

TL;DR

This paper presents a solvent-free method to synthesize chiral imines and studies their structure and properties using experiments and theory.

Contribution

The study introduces a solvent-free synthesis method and explores the impact of substituents on electronic and optical properties of imines.

Findings

Steric effects, not halogen substituents, mainly influence crystal packing.

Imines with delocalized orbitals show enhanced charge-transfer ability and lower hardness.

Halogen interactions affect supramolecular packing and chiroptical activity.

Abstract

Solvent-free synthesis
of five chiral-substituted N-benzylideneanilines
(I–V) was reported in high
yields, and all these imines were characterized by various physicochemical
analysis techniques. Crystallographic analysis revealed that variations
in crystal packing were primarily influenced by steric effects rather
than halogen substituents. The contribution of halogen bonding to
the chromic properties of these compounds was systematically evaluated.
Their electronic and optical properties were also investigated theoretically
using density functional theory (DFT) calculations at the B3LYP and
M06–2X levels of theory with the 6–311+G­(d,p) basis
set, which reproduced the experimental structures with good accuracy.
Using frontier molecular orbital analysis, two regimes were identified:
imines I, II, and IV have localized
HOMOs on the imine moiety that lead to larger HOMO–LUMO gaps
and higher chemical hardness, while imines III and V have orbitals that are delocalized over extended π-systems
with smaller gaps, lower softness, and higher polarizability. Trends
were corroborated by global reactivity parameters which indicated
that the charge-transfer ability of III and V is enhanced and TD-DFT calculations reproduced the experimental
ultraviolet–visible (UV–vis) with red-shifted absorptions.
ECD spectra showed stronger Cotton effects in III and V being attributed to their electronic delocalization and
lower hardness. NBO analysis revealed stabilization via π­(C–C)
→ π*­(N–C) and n­(O/Cl) → π*­(C–C)
interactions; NCI plots highlighted dispersive and C–H···π
contacts. Complementary QTAIM analysis identified two weak Cl···Cl
interactions in imine IV, indicating the effect of halogen
contacts on supramolecular packing. This integrated experimental–theoretical
study showed that substituents regulate electronic structure, charge-transfer
capacity, and chiroptical activity, guiding future designs of imine-based
optoelectronic and functional chiral materials.

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