MOLECULAR GEOMETRY

. A series of 2-(4-substituted-phenylimino)thiazolidin-4-one compounds was synthesized via heterocyclizing the corresponding N -aryl-2-chloroacetamides with ammonium thiocyanate. Their chemical structures were elucidated based on an extensive analysis of their spectroscopic data, including infrared, 1 H NMR, 13 C NMR, and mass analyses. The possible tautomeric forms of synthesized thiazolidine-4-ones were studied. The tautomerization equilibrium parameters, ΔH, Δ G , and K eq were calculated using the DFT/B3LYP methodology, where it has been indicated that the tautomeric form, phenylimino, is more favourable than the phenylamino form. The antibacterial and antioxidant properties of the synthesized thiazolidin-4-ones were investigated. 2-(Chlorophenyl-imino)thiazolidin-4-one displayed potent antibacterial activity against E. coli (88.46%) and S. aureus (91.66%), and highest percent of inhibition (81.8%,


INTRODUCTION
Numerous nitrogen and/or sulfur containing heterocyclic compounds have been identified as promising drug candidates. The thiazolidinone ring has been proven as a privileged scaffold in a wide variety of known biologically active compounds. The thiazolidinone ring has a sulfur atom at position 1, a nitrogen atom at position 3, and a carbonyl group at the 2, 4, or 5 positions. The various derivatives are associated with numerous pharmacological properties, such as bactericidal, pesticidal, fungicidal, insecticidal, antiviral (anti-HIV), antidiabetic, anticonvulsant, tuberculostatic, antiinflammatory, anticancer, antithyroid, and immunostimulant properties [1][2][3][4][5]. It is worth noting that substituents in the 2-, 3-, and 5-positions can be changed, but the group attached to the C2-position exerts the greatest influence on structure and properties [6][7][8][9]. As a result, a few thiazolidin-4-one derivatives with C2 and N3 substituted positions, as well as the presence of an electron-withdrawing substitution on the aromatic ring at the C2 position, present good inhibition against Gram-positive and Gram-negative bacteria. On the other hand, a retrospective analysis of these compounds revealed that anticancer activity increases when the cycloalkyl moiety located in position C2 is replaced with a hetero(aryl) moiety [1]. Indeed, 2imino-4-thiazolidinone compounds have attracted great attention due to their diverse biological activities [10]. They display anticancer [11][12][13], kinase inhibitors [14,15], antimicrobial [16], and anti-inflammatory [17] activities ( Figure 1). A notable congener of this group are the 2arylaminothiazol-4-ones, which are effective growth inhibitors of HT29 adenocarcinoma cells [18] and lung cancer cells H460 and H460/TaxR [19,20]. Furthermore, thiazolidin-4-one compounds are efficient integrin avb3 antagonists [21] and inhibitors of CDK1/cyclin B [22]. Herein we report on the synthesis of five 2-(arylimino)thiazolidin-4-one derivatives where the aryl group is either unsubstituted or substituted with electron-donating (methyl and methoxy) or electron-attracting (nitro and chlorine) groups. Moreover, investigating the two possible tautomeric forms (phenylamino and phenylamino forms) in light of their thermodynamic parameters (ΔH and ΔG), and equilibrium constant (Keq). Finally, their antibacterial and antioxidant activities were evaluated.

Synthesis of 2-(arylimino)thiazolidin-4-ones
As a part of our continuous research program on the chemistry of N-aryl-2-chloroacetamide compounds [23][24][25], herein we report on the chemical reactivity of 2-chloroacetamide reagents towards the highly versatile sulfur donner, ammonium thiocyanate. The heterocyclization of Naryl-2-chloroacetamide derivatives 1 upon treatment with ammonium thiocyanate has been achieved by refluxing in ethanol for 4 hours to generate the corresponding 2-(arylimino) thiazolidin-4-ones 2a-e (Scheme 1). The chemical structures of these thiazolidine-4-ones were elucidated based on their spectral data. The infrared spectra exhibited absorptions bands of the N-H and carbonyl functionalities at 3225-3200 and 1686-1657 cm -1 , respectively. The 1 H NMR spectra displayed the singlet signal, integrated for two protons, corresponding to the methylene group in the range δ 3.96 to 4.05 ppm. While the proton of imino function (cyclic N-H) was resonated in the range from δ 11.03 to 12.01 ppm. The carbon signal of methylene group was recorded in the 13 C NMR spectra ranging from δ 32.68 to 33.37 ppm. Scheme 1. Synthesis of 2-(arylimino)thiazolidin-4-one compounds 2a-e.

Energetic and thermodynamic parameters
The synthesized derivatives have two possible tautomeric forms, i.e., phenylamino, 2, and phenylimino, 2', as shown in Scheme 1. To estimate the tautomerization equilibrium and so the predominated form, the reaction thermodynamic parameters (ΔH and ΔG), and equilibrium constant (Keq) for the tautomeric pathways I (2 → 2') and II (2' → 2) were calculated at P = 1 bar and 298 K and summarized in Table 1. The tautomeric reaction I in all derivatives, 2a-e, exhibited negative enthalpies (ΔH298K) and free energy (ΔG298K) change ranging from -15.34 to -23.80 and -9.78 to -26.53 kcal mol −1 , respectively, and thus it is a spontaneous exothermic process. On contrary, the tautomeric reaction II displayed positive values for both enthalpies and free energy change which indicate its non-spontaneous endothermic nature. Furthermore, the equilibrium constant (Keq) calculation of both I and II pathways showed that I has high equilibrium constant with positive power, range 5.19×10 1 -4.45×10 4 , while II has low values with negative power, range 2.25×10 -5 -1.93×10 -2 , and thus it discloses that the tautomer 2', phenylimino, is favourable more than the phenylamino form, 2 [26]. In addition, the data showed that the nitro derivative has the dramatically lower equilibrium constant than other derivatives where the chloro displayed the higher value, according to the following order 2d < 2b < 2c < 2a < 2e, which may be attributed to that the electron withdrawing effect of the nitro group hindered the transformation toward the phenylimino tautomer whereas the other electron donating groups facilitate tautomerization. Molecular geometry Figure 2 displayed the DFT optimized structures of the phenylimino tautomers of the compounds 2a-e. The dihedral angle revealed that the imino nitrogen atom were almost coplanar with the thiazole ring where the Nam-Th 2 -NTh-Th 4 and Nam-Th 2 -S-Th 5 ≈ 177.5°. The investigated derivatives were non-planar where the phenyl ring tilted on the thiazole ring, i.e., Ph 2 -Ph 1 -Nam-Th 2 ≈ 56.0-61.3°. The different substituents on the phenyl ring were coplanar with it in compounds 2b-e. The bond length and angles, comparing to those of analogous compounds X-ray single crystal [27], exhibited significant matching with 0.00-0.09 Å and 0.0-7.7° divergences, RMSD 3.64-3.96×10 -2 Å and 2.2-2.8°, respectively, which may be arisen from that in the practical, there are interaction between molecules forming the solid crystal lattice, while in theoretical calculations, no intermolecular columbic interactions present as sole molecule in gaseous status was considered [28].

Frontier molecular orbitals (FMO's)
The energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) were computed ( Table 2). The HOMO-LUMO gap, ΔE = EH-EL, illustrates the molecule's chemical stability and charge transfer [29][30]. The frontier orbitals plot of the investigated compounds ( Figure 3) showed that: (i) The HOMO for compounds 2a-e was consisted mainly from the π-orbital of the 4-substituted phenylimino moiety and heteroatoms lone pair of electrons of the thiazole ring. The LUMO was constructed mainly from the π*-orbital of the whole molecule without contribution of the substituents except in 2d where the nitro group was strongly involved in formation. (ii) The EHOMO data clears the effect of substitution participation where the derivative 2c has the lowest value, -5.88 eV, while the highest one is 2d, Figure 3. The HOMO and LUMO parts of the investigated compounds.  [31]. Hence, the studied derivatives may be sorted owing to their ΔE values as 2d < 2c < 2b < 2e < 2a.
Hence, compound 2d has the maximum electronegativity and compound 2c has the minimum, 5.04 and 3.51 eV, respectively, and so all compounds have Lewis's acids character (Table 2). Soft molecules, low ΔE, have higher reactivity than hard ones since they can easily donate electrons. Therefore, the data demonstrated that derivatives 2a and 2d were the hardest and softest, respectively. Also, compounds 2c and 2d exhibited the lowest and highest electrophilicity index.

Mulliken atomic charges
The electrophilic or nucleophilic reaction reactivity may affect mostly by the Mulliken atomic charge. The atomic charges of the sulfur and carbon atoms of thiazole ring at the position 1 and 4 in all 2a-e derivatives, have positive charges, 0.027-0.116 and 0.237-0.259, respectively, while those at 2-, 3-and 5-positions were negatively charged. Furthermore, the azomethine nitrogen's have the positive charges in all derivatives while all oxygen atom of carbonyl groups on thiazole have negative charges following the order 2d < 2e > 2a = 2b ≈ 2c (Table 3).

Antibacterial and antioxidant activity
The antibacterial properties of the synthesized 2-(arylimino)thiazolidin-4-one compounds 2a-e were explored against Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus) according to the previously published methodology [27]. The % activity index for the explored thiazolidine-4-one compounds was calculated by the formula: A survey of the results (Table 4) shows that the synthesized 2-(arylimino)thiazolidin-4-one compounds inhibit the growth of E. coli with activity indices ranging from 53.84% to 88.46%. The presence of substituent on the phenyl group promoted the inhibition activity from 53.84% (no substituent on the phenyl group) to maximum activity when chlorine is substituted on the phenyl group (88.46%). Compound 2e, 2-(chlorophenyl-imino)thiazolidin-4-one, achieved very close activity to that displayed by the reference drug, Ampicillin, which inhibits the growth of the E. coli bacterium with an inhibition zone of 26 mm. By exploring the activity of the tested 2-(4substituted-phenylimino)thiazolidin-4-one compounds 2b, 2c, 2d, and 2e against the S. aureus bacterium, they displayed excellent antibacterial properties with diameter zones (19, 21, and 22 mm) and activity indices ranging from 79.16 to 91.66%. Fortunately, the best antibacterial activity was displayed by the targeting compound 2-(chlorophenyl-imino)thiazolidin-4-one (2e) with an inhibition zone of 22 mm and an activity index of 91.66% when compared with Ampicillin, which inhibits the growth of the S. aureus bacterium with an inhibition zone of 24 mm.

Compound
No.  Quadrupole GC/MS Thermo Scientific Focus/DSQII has been utilized to perform the mass analyses. Perkin-Elmer 2400 analyzer has been utilized to measure the elemental analyses (C, H, and N).

Synthesis of 2-(arylimino)thiazolidin-4-ones 2a-e.
To a solution of N-aryl chloroacetamide derivatives (1) (0.002 mol) in ethanol (40 mL), ammonium thiocyanate (0.003 mol, 0.22 g) was added and then the mixture was refluxed for 4 hours. The mixture was cooled to 20°C and the solid that obtained in each case was separated by filtration and dried. The solid was crystallized from ethanol to yield the thiazolidine-4-one products 2a-e.

Computational studies
The Gaussian 09W software [35] was employed for geometrical optimization and frequency calculations using DFT/B3LYP/6-311 ++ G(d,p) methodology [36][37][38]. All compounds calculated positive frequencies confirmed the stability of the optimized geometries. Thermodynamic parameters of the tautomeric reactions were performed using the kinetic and statistical thermodynamic package (KiSThelP) under atmospheric pressure and at T = 298 K [39]. In DMSO, the 1 H NMR chemical shifts were computed by the gauge-invariant atomic orbital (GIAO) method [40]. The Gauss View program was used in assignment of NMR spectral data [41]. The Materials Studio package DMol3 module has been utilized in estimating the Fukui indices [42] using gradient-corrected (GGA) and B3LYP functional with a double numeric plus polarization (DNP 3.5) basis set [43].