The morphological, optical, and structural changes in crystalline chiral imines derived from 2-naphthaldehyde as a result of changing the −F, −Cl, and −Br halogen (−X) atoms are reported. Scanning electron microscopy (SEM), optical absorption, photoluminescence (PL), and powder X-ray diffraction (XRD) studies were performed. Theoretical results of optical and structural properties were calculated using the PBE1PBE hybrid functional and compared with the experimental results. Differences in surface morphology, absorbance, XRD, and PL of crystals were due to the change of halogen atoms in the chiral moiety of the imine. Absorption spectra exhibited the typical bands of the naphthalene chromophore located in the ~200–350 nm range. Observed absorption bands in the UV region are associated with π→π* and n→π* electronic transitions. The band gap energy was calculated using the Tauc model. It showed a shift in the ~3.5–4.5 eV range and the crystals exhibited different electronic transitions associated with the results of absorbance in the UV region. XRD showed the monoclinic→orthorhombic crystalline phase transition. PL spectra displayed broad bands in the visible region and all the samples have an emission band (identified as a green emission band) in the ~400–750 nm range. This was associated with defects produced in the morphology, molecular packing, inductive effect and polarizability, crystalline phase transition, and increase in size of the corresponding halogen atoms; i.e., changes presumably induced by −C−X^…X−, −C−X^…N−, −C−N^…π, and −C−X^…π interactions in these crystalline materials were associated with morphological, optical, and structural changes.

Using a general solvent-free procedure for the synthesis of chiral Schiff bases, the following compounds were synthesized and their crystal structures determined: (S)-(+)-2-{[(1-phenylethyl)imino]methyl}naphthalene, C₁₉H₁₇N, (1), (S)-(+)-2-({[(4-methylphenyl)ethyl]imino}methyl)naphthalene, C₂₀H₁₉N, (2), (R)-(−)-2-({[(4-methoxylphenyl)ethyl]imino}methyl)naphthalene, C₂₀H₁₉NO, (3), (R)-(−)-2-({[(4-fluorophenyl)ethyl]imino}methyl)naphthalene, C₁₉H₁₆FN, (4), (S)-(+)-2-({[(4-chlorophenyl)ethyl]imino}methyl)naphthalene, C₁₉H₁₆ClN, (5), (S)-(+)-2-({[(4-bromophenyl)ethyl]imino}methyl)naphthalene, C₁₉H₁₆BrN, (6), (S)-(+)-2-({[1-(naphthalen-1-yl)ethyl]imino}methyl)naphthalene, C₂₃H₁₉N, (7), (S)-(+)-2-{[(1-cyclohexylethyl)imino]methyl}naphthalene, C₁₉H₂₃N, (8), (S)-(−)-2-{[(1,2,3,4-tetrahydronaphthalen-1-yl)imino]methyl}naphthalene, C₂₁H₁₉N, (9), and (+)-2-({[(1S,2S,3S,5R)-2,6,6-trimethylbicyclo[3.1.1]hept-3-yl]imino}methyl}naphthalene, C₂₁H₂₅N, (10). The moiety provided by the amine generates conformational flexibility for these imines. In the crystals, no strong intermolecular contacts are observed, in spite of the presence of aromatic groups.

Four mononuclear bioefficient zinc coordination complexes [Zn(NN)₃](ClO₄)₂ (A–D) involving chiral bidentate Schiff base ligands have been synthesized and characterized by IR, ¹H, and ¹³C NMR spectroscopy and mass spectrometry. X-ray crystal structures of three of the zinc complexes revealed that the zinc metal ion is hexacoordinated, exhibiting a distorted octahedral geometry where both the nitrogen atoms (NN = pyridyl and imine) of imines are coordinated to the central zinc ion. The isolated zinc complexes were evaluated for their antimicrobial activity in vitro against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus, displaying varying levels of growth inhibition. An acute toxicity test conducted using Artemia salina and Swiss albino mice showed that the zinc complexes A–D were non-toxic towards A. salina at concentrations below 414, 564, 350, and 385 µM, respectively, and did not affect liver biochemical parameters, although pyknosis was induced in hepatocytes of the treated mice.

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.

The aim of this review is to provide mainly an outlook of the synthesis and characterization of chiral mono- and α-diimines ligands and their Pd(II) complexes carried out in our group in the last few years. Some other issues with simple chiral imines synthesized in our lab are also outlined. The report includes details about their versatile coordination patterns, biological activity in cancer cell lines, and engaging properties in different fields, such as materials science.

The (S)-(+)-1-(4-bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine ligand, C₁₆H₁₆BrNO, (I), was synthesized through the reaction of 4-methoxyanisaldehyde with (S)-(−)-1-(4-bromophenyl)ethylamine. It crystallizes in the orthorhombic space group P2₁2₁2₁ belonging to the Sohncke group, featuring a single molecule in the asymmetric unit. The refinement converged successfully, achieving an R factor of 0.0508. The Pd^II complex bis{(S)-(+)-1-(4-bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine-κN}dichloridopalladium(II), [PdCl₂(C₁₆H₁₆BrNO)₂], (II), crystallizes in the monoclinic space group P2₁ belonging to the Sohncke group, with two molecules in the asymmetric unit. The central atom is tetracoordinated by two N atoms and two Cl atoms, resulting in a square-planar configuration. The imine moieties exhibit a trans configuration around the Pd^II centre, with average Cl—Pd—N angles of approximately 89.95 and 90°. The average distances within the palladium complex for the two molecules are ∼2.031 Å for Pd—N and ∼2.309 Å for Pd—Cl.