Experimental and Theoretical Biological Probing of Schiff Bases as Esterase Inhibitors: Structural, Spectral and Molecular Insights.

The present study was designed to evaluate the in vitro and in silico potential of the Schiff bases (Z)-4-ethoxy-N-((5-nitrothiophen-2-yl)methylene)benzenamine (1) and (Z)-2,4-diiodo-6-((2-methyl-3-nitrophenylimino)methyl)phenol (2). These Schiff bases were synthesized according to a reported method using ethanol as a solvent, and each reaction was monitored on a TLC until completion of the reaction. The structures of both compounds were elucidated using spectroscopic techniques such as UV-Vis, FTIR, 1H NMR and 13C NMR. Molecular structure was determined using single-crystal XRD, which revealed that compounds 1 and 2 were monoclinic and triclinic, respectively. Hirshfeld surface analysis (HS) and 2D fingerprint plots were used to determine the intermolecular interactions along the contact contribution in the crystalline molecules. The structures of both compounds were optimized through a hybrid functional method B3LYP using the 6-31G(d,p) basis set, and various structural parameters were studied. The experimental and theoretical parameters (bond angle and bond length) of the compounds were compared with each other and are in close agreement. The in vitro esterase potential of the synthesized compounds was checked using a spectrophotometric model, while in silico molecular docking studies were performed with AutoDock against two enzymes of the esterase family. The docking studies and the in vitro assessment predicted that such molecules could be used as enzyme inhibitors against the tested enzymes: acetylcholine esterase (AChE) and butyrylcholine esterase (BChE).

Crystal structure, Hirshfeld surface analysis and DFT studies of (E)-4-methyl-2-{[(2-methyl-3-nitro-phen-yl)imino]-meth-yl}phenol.

The title compound, C15H14N2O3, was prepared by condensation of 2-hy-droxy-5-methyl-benzaldehyde and 2-methyl-3-nitro-phenyl-amine in ethanol. The configuration of the C=N bond is E. An intra-molecular O-H⋯N hydrogen bond is present, forming an S(6) ring motif and inducing the phenol ring and the Schiff base to be nearly coplanar [C-C-N-C torsion angle of 178.53 (13)°]. In the crystal, mol-ecules are linked by C-H⋯O inter-actions, forming chains along the b-axis direction. The Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H⋯H (37.2%), C⋯H (30.7%) and O⋯H (24.9%) inter-actions. The gas phase density functional theory (DFT) optimized structure at the B3LYP/ 6-311 G(d,p) level is compared to the experimentally determined mol-ecular structure in the solid state. The HOMO-LUMO behaviour was elucidated to determine the energy gap.

Crystal structure and Hirshfeld surface analysis of (E)-2-{[(2-iodo-phen-yl)imino]-meth-yl}-6-methyl-phenol.

The title compound, C14H12INO, was synthesized by condensation of 2-hy-droxy-3-methyl-benzaldehyde and 2-iodo-aniline, and crystallizes in the ortho-rhom-bic space group P212121. The 2-iodo-phenyl and benzene rings are twisted with respect to each other, making a dihedral angle of 31.38 (2)°. The mol-ecular structure is stabilized by an O-H⋯N hydrogen bond, forming an S(6) ring motif. In the crystal, mol-ecules are linked by C-H⋯π inter-actions, resulting in the formation of sheets along the a-axis direction. Within the sheets, very weak π-π stacking inter-actions lead to additional stabilization. The Hirshfeld surface analysis and fingerprint plots reveal that the crystal structure is dominated by H⋯H (37.1%) and C⋯H (30.1%) contacts. Hydrogen bonding and van der Waals inter-actions are the dominant inter-actions in the crystal packing. The crystal studied was refined as a two-component inversion twin.

Synthesis, crystal structure and Hirshfeld surface analysis of 1,7-dimethyl-5a,6,11a,12-tetra-hydro-benzo[b]benzo[5,6][1,4]oxazino[2,3-e][1,4]oxazine.

Mol-ecules of the title compound, C16H16N2O2, occupy special positions on the twofold rotation axes. The heterocyclic ring adopts a slightly twisted envelope conformation with one of the two junction carbon atoms as the flap. The mean planes through the two halves of the mol-ecule form a dihedral angle of 72.01 (2)°. In the crystal, mol-ecules are linked by pairs of C-H⋯O and N-H⋯C contacts into layers parallel to (100). H⋯H contacts make the largest contribution to the Hirshfeld surface (58.9%).

Crystal structure, Hirshfeld surface analysis and DFT studies of 4-methyl-2-({[4-(tri-fluoro-meth-yl)phen-yl]imino}-meth-yl)phenol.

The title compound, C15H12F3NO, crystallizes with one mol-ecule in the asymmetric unit. The configuration of the C=N bond is E and there is an intra-molecular O-H⋯N hydrogen bond present, forming an S(6) ring motif. The dihedral angle between the mean planes of the phenol and the 4-tri-fluoro-methyl-phenyl rings is 44.77 (3)°. In the crystal, mol-ecules are linked by C-H⋯O inter-actions, forming polymeric chains extending along the a-axis direction. The Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from C⋯H/H⋯C (29.2%), H⋯H (28.6%), F⋯H/H⋯F (25.6%), O⋯H/H⋯O (5.7%) and F⋯F (4.6%) inter-actions. The density functional theory (DFT) optimized structure at the B3LYP/6-311 G(d,p) level is compared with the experimentally determined mol-ecular structure in the solid state. The HOMO-LUMO behaviour was elucidated to determine the energy gap. The crystal studied was refined as an inversion twin.

Crystal structure, Hirshfeld surface analysis and DFT studies of (E)-4-methyl-2-{[(4-methyl-phen-yl)imino]-meth-yl}phenol.

In the title compound, C15H15NO, the configuration of the C=N bond of the Schiff base is E, and an intra-molecular O-H⋯N hydrogen bond is observed, forming an intra-molecular S(6) ring motif. The phenol ring is inclined by 45.73 (2)° from the plane of the aniline ring. In the crystal, mol-ecules are linked along the b axis by O-H⋯N and C-H⋯O hydrogen bonds, forming polymeric chains. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the packing arrangement are from H⋯H (56.9%) and H⋯C/C⋯H (31.2%) inter-actions. The density functional theory (DFT) optimized structure at the B3LYP/ 6-311 G(d,p) level is compared with the experimentally determined mol-ecular structure, and the HOMO-LUMO energy gap is provided. The crystal studied was refined as an inversion twin.

Crystal structure and Hirshfeld surface analysis of 2-phenyl-1H-phenanthro[9,10-d]imidazol-3-ium benzoate.

In the title compound, C21H15N2 +·C7H5O2 -, 2-phenyl-1H-phenanthro[9,10-d]imidazole and benzoic acid form an ion pair complex. The system is consolidated by hydrogen bonds along with π-π inter-actions and N-H⋯π inter-actions between the constituent units. For a better understanding of the crystal structure and inter-molecular inter-actions, a Hirshfeld surface analysis was performed.

Crystal structure, DFT and MEP study of (E)-2-{[(3-chloro-phen-yl)imino]-meth-yl}-6-methyl-phenol.

In the crystal structure of the title compound, C14H12ClNO, the mol-ecules are linked through C-H⋯O hydrogen bonds and C-H⋯π inter-actions, forming chains parallel to the [010] direction. π-π inter-actions and intra-molecular hydrogen bonds are also observed. The mol-ecular geometry of the title compound in the ground state has been calculated using density functional theory at the B3LYP level with the 6-311++G(2d,2p) basis set. Additionally, frontier mol-ecular orbital and mol-ecular electrostatic potential map analyses were performed.

(E)-3-{[(2-Bromo-3-methyl-phen-yl)imino]-meth-yl}benzene-1,2-diol: crystal structure and Hirshfeld surface analysis.

The title compound, C14H12BrNO2, was synthesized by the condensation reaction of 2,3-di-hydroxy-benzaldehyde and 2-bromo-3-methyl-aniline. It crystallizes in the centrosymmetric triclinic space group P . The configuration about the C=N bond is E. The dihedral angle between the planes of the 5-(2-bromo-3-methyl-phenyl ring and the catechol ring is 2.80 (17)°. In the crystal, O-H⋯O hydrogen-bond inter-actions consolidate the crystal packing.

Crystal structure and DFT study of (E)-2-chloro-4-{[2-(2,4-di-nitro-phen-yl)hydrazin-1-yl-idene]meth-yl}phenol aceto-nitrile hemisolvate.

The title Schiff base compound, C13H9ClN4O5·0.5CH3CN, crystallizes as an aceto-nitrile hemisolvate; the solvent mol-ecule being located on a twofold rotation axis. The mol-ecule is nearly planar, with a dihedral angle between the two benzene rings of 3.7 (2)°. The configuration about the C=N bond is E, and there is an intra-molecular N-H⋯Onitro hydrogen bond present forming an S(6) ring motif. In the crystal, mol-ecules are linked by O-H⋯O and N-H⋯O hydrogen bonds, forming layers lying parallel to (10). The layers are linked by C-H⋯Cl hydrogen bonds, forming a supra-molecular framework. Within the framework there are offset π-π stacking inter-actions [inter-centroid distance = 3.833 (2) Å] present involving inversion-related mol-ecules. The DFT study shows that the HOMO and LUMO are localized in the plane extending from the phenol ring to the 2,4-di-nitro-benzene ring, and the HOMO-LUMO gap is found to be 0.13061 a.u.

(Z)-3-({[3-Meth-oxy-5-(tri-fluoro-meth-yl)phen-yl]imino}meth-yl)benzene-1,2-diol.

The title compound, C15H12F3NO3, crystallizes with one mol-ecule in the asymmetric unit. The mean planes of the two phenyl rings of the Schiff base moiety, bearing the OH groups and the imine group, respectively, are inclined to each other by 4.91 (1)°. In the crystal, mol-ecules are linked via pairs of bifurcated O-H⋯O hydrogen bonds between the phenol OH groups, forming inversion dimers with an R 1 2(5) ring motif. The structure exhibits also intra-molecular O-H⋯N and C-H⋯F hydrogen-bonding inter-actions. Hirshfeld surfaces analysis and two-dimensional fingerprint plots were applied to qu-antify the inter-molecular inter-actions. The three F atoms of the tri-fluoro-methyl group are disordered over two sets of sites, with occupancy factors of 0.578 (8) and 0.422 (8). The crystal studied was refined as an inversion twin.

Crystal structure and Hirshfeld surface analysis of a Schiff base: (Z)-6-[(5-chloro-2-meth-oxy-anilino)methyl-idene]-2-hy-droxy-cyclo-hexa-2,4-dien-1-one.

The title compound, C14H12ClNO3, is a Schiff base that exists in the keto-enamine tautomeric form and adopts a Z configuration. In the crystal, the dihedral angle between the planes of the benzene rings is 5.34 (15)°. The roughly planar geometry of the mol-ecule is stabilized by a strong intra-molecular N-H⋯O hydrogen bond. In the crystal, pairs of centrosymmetrically related mol-ecules are linked by O-H⋯O hydrogen bonds, forming R 2 2(10) rings. Besides this, the mol-ecules form stacks along the [001] direction with C-H⋯π and C-H⋯Cl contacts between the stacks. The inter-molecular inter-actions in the crystal were analysed using Hirshfeld surfaces. The most significant contribution to the crystal packing is from H⋯H contacts (30.8%).

Crystal structure of 2-{[(5-nitro-thio-phen-2-yl)methyl-idene]amino}-phenol.

The title compound, C11H8N2O3S, is roughly planar; the di-hedral angle between the planes of the thio-phene and benzene rings is 8.38 (10)°. An intra-molecular O-H⋯N hydrogen bond generates an S(5) ring motif. In the crystal, mol-ecules are linked into centrosymmetric dimers by pairs of O-H⋯O hydrogen bonds with an R 2 (2)(22) graph-set motif. Aromatic π-π stacking inter-actions [centroid-centroid sep-ar-ations = 3.653 (3) and 3.852 (3) Å] link the dimers into a three-dimensional network.