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Syntheses and Characterization of the Coordination Compounds of N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one
A dry benzene solution of the Schiff base N-(2-hydroxymethylphenyl)-3′-carboxy-2′-hydroxybenzylideneimine upon reacting with mercaptoacetic acid undergoes cyclization and forms N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one, LH3 (I). A MeOH solution of I reacts with Co(II), Ni(II), Cd(II), Zr(OH)2(IV), and UO2(VI) ions and forms the monomeric coordination compounds, [M(LH)(MeOH)3] [where M = Co(II), Ni(II)], [M′(LH)(MeOH)] [here M′ = Cd(II), UO2(VI)] and [Zr(OH)2(LH)(MeOH)]. The coordination compounds have been characterized on the bases of elemental analyses, molar conductance, molecular weight, spectral (IR, NMR, and reflectance) studies, and magnetic susceptibility measurements. I behaves as a dibasic tridentate OOS donor ligand in these compounds. The compounds are nonelectrolytes ( = 3.8–8.9 mho cm2 mol−1) in DMF. A tetrahedral structure for [Cd(LH)(MeOH)] and an octahedral structure for the remaining compounds are suggested.
The chemistry of thiazolidin-4-ones is very well documented in the literature and has been studied extensively due to their versatile biological properties . Thiazolidin-4-ones, a saturated form of thiazole with carbonyl group on fourth carbon , have biological activities like those that are antipsychotic , antitubercular , antibacterial , anticonvulsant , antifungal , amoebicidal , antioxidant , antibiotic , and so forth.
Metal complexes play an important role in plant and animal life due to their physicochemical and biological properties. Metal ions are involved in specific interactions with antibiotics, proteins, nucleic acids, and other biomolecules . Most of the drugs have improved pharmacological properties in the form of their metal complexes. Transition metal ions play a very important role in the pharmacological action of metal-based drugs and these drugs are more effective against infectious microbes than the uncomplexed drugs .
These facts motivate us to explore the coordination behavior of a newly synthesized thiazolidin-4-one with some transition metal ions.
In this paper, we describe the syntheses and characterization of N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one, LH3 (I) and its coordination compounds with Co(II), Ni(II), Cd(II), Zr(OH)2(IV), and UO2(VI) ions. The structure of Schiff base and thiazolidin-4-one, LH3 (I) is shown in Figure 1.
o-Aminobenzylalcohol [Aldrich]; nickel(II) acetate tetrahydrate, cadmium(II) acetate dihydrate, dioxouranium(VI) acetate tetrahydrate (Sarabhai); cobalt(II) acetate tetrahydrate, hexadecaaquaoctahydroxotetrazirconium(IV) chloride (BDH); methanol, ethanol, mercaptoacetic acid, dry benzene, and sodium bicarbonate (Ranbaxy) were used as supplied for the syntheses. 3-Formylsalicylic acid  and hexadecaaquaoctahydroxotetrazirconium(IV) acetate  were synthesized by the following reported procedures.
2.2. Analyses and Physical Measurements
The organic skeleton of the respective coordination compounds was decomposed by the slow heating of ~0.1 g of the latter, with conc. HNO3. The residue was dissolved in a minimum amount of conc. HCl and the corresponding metal ions were estimated as follows: the Ni(II) contents of the respective coordination compound were estimated by complexometric titration method against standardized EDTA solution using murexide as an indicator. The Co(II) and Cd(II) contents in the respective coordination compounds were estimated by the complexometric titration method against standardized EDTA solution using xylenol orange as an indicator. The uranium content in [UO2(LH)(MeOH)] was estimated gravimetrically as U3O8 after decomposing the compound with a few drops of conc. HNO3 and then igniting the residue. The zirconium contents were estimated gravimetrically as ZrO2 after decomposing the corresponding compound with a few drops of conc. HNO3 and then igniting the residue. The C, H, and N contents of LH3 and its coordination compounds were determined by CHN Eager analyzer model-300. The S and Cl contents were estimated gravimetrically as BaSO4 and AgCl, respectively. The molecular weight measurements were carried out by the Rast method using diphenyl as the solvent . The molar conductances of the coordination compounds were measured in DMF with the help of a Toshniwal conductivity bridge (CL01-02A) and a dip-type cell calibrated with KCl solutions. The IR spectra were recorded in KBr pellets (4000–400 cm−1) on a Beckman 20 spectrophotometer. The reflectance spectra were recorded on a Beckmann DU spectrophotometer attached with a reflectance arrangement. The magnetic susceptibility measurements were carried out at room temperature, using Hg[Co(NCS)4] as the standard . The diamagnetic corrections were computed using Pascal’s constants. The magnetic susceptibilities were corrected for temperature-independent paramagnetism term (TIP)  using a value of units for Ni(II) and Co(II) ions.
2.3. Synthesis of the N-(2-hydroxymethylphenyl)-3′-carboxy-2′-hydroxybenzylide-neimine (Schiff Base)
A MeOH solution (30 mL) of o-aminobenzylalcohol (1.23 g, 10 mmol) was added to a MeOH solution (30 mL) of 3-formylsalicylic acid (1.66 g, 10 mmol) and the mixture was then refluxed for 1 h. The precipitates formed were suction filtered, washed with MeOH, and dried in vacuo at room temperature over silica gel for 24 h. Yield = 58%. The elemental analyses of the compound gave the satisfactory results.
2.4. Synthesis of N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one (I)
A dry benzene solution of the Schiff base (2.71 g, 10 mmol) and mercaptoacetic acid (0.92 g, 10 mmol) were refluxed for 12 h in a water bath. The mixture was cooled to room temperature and then was washed with 10% sodium bicarbonate solution. The benzene layer was separated using a separating funnel. The partial evaporation of the benzene layer gave a solid product, which was filtered, washed with petroleum ether and recrystallized from petroleum ether. The compounds were dried as mentioned above. Yield = 14%. Anal: (I, C17H15NO5S) (obsd: C, 58.91%; H, 4.37%; N, 4.12%; S, 9.11%. calc.: C, 59.13%; H, 4.35%; N, 4.06%; S, 9.28%); IR bands (KBr): 2860 cm−1 [(O–H)(intramolecular H-bonding)], 1710 cm−1 [(C=O)(thiazolidinone ring)], 1675 cm−1 [(C=O)(carboxylic)], 1570 cm−1 [(C–N)(thiazolidinone ring)], 1520 cm−1 [(C–O)(phenolic)], 1225 cm−1 [(C–O)(alcoholic)], and 830 cm−1 [(C–S)(thiazolidinone ring)].
2.5. Syntheses of Coordination Compounds of I
A MeOH solution (30–50 mL) of the appropriate metal salt (10 mmol) was added to a MeOH solution (50 mL) of I (3.45 g, 10 mmol) and the mixture was then refluxed for 3-4 h. The solid products formed were suction filtered, washed with MeOH and recrystallized from MeOH and were then dried as mentioned above. Yield = 50–75%.
3. Results and Discussion
A dry benzene solution of the Schiff base reacts with mercaptoacetic acid and forms N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one, LH3 (I). The reaction of I with appropriate metal salt in 1 : 1 molar ratio in MeOH produces the coordination compounds, [M(LH)(MeOH)3] [where M = Co(II), Ni(II)], [M′(LH)(MeOH)] [where M′ = Cd(II), UO2(VI)], and [Zr(OH)2(LH)(MeOH)]. The formations of I from the Schiff base and the coordination compounds of I take place according to Schemes 1 and 2.
The coordination compounds are air-stable at room temperature. They are insoluble in H2O, partially soluble in MeOH, EtOH, and completely soluble in DMSO and DMF. Their molar conductance measurements = 3.8–8.9 mho cm2 mol−1) in DMF indicate their nonelectrolytic nature. The analytical data of I and its coordination compounds are presented in Table 1.
aMass spectral data. bRast method data.|
3.1. Infrared Spectral Studies
The infrared spectra of I and its coordination compounds were recorded in KBr and the prominent peaks (in cm−1) are shown in Table 2. The Schiff base exhibits the (C=N)(azomethine) stretch at 1640 cm−1. This band disappears in I and a new band appears at 1570 cm−1 due to the (C–N)(thiazolidinone ring) stretch  indicating the conversion of the Schiff base into I. The formation of I is further supported by the appearance of a new band at 830 cm−1 due to the (C–S)(thiazolidinone ring) stretch . A negative shift of 15–35 cm−1 of the (C–S)(thiazolidinone ring) stretch in the coordination compounds indicates the involvement of the S atom of the thiazolidinone moiety in coordination . I shows the (C=O)(thiazolidinone ring) stretch  at 1710 cm−1. This band remains unchanged in the coordination compounds indicating the noninvolvement of O atom in the coordination. The (C–O)(alcoholic) stretch  of I occurs at 1225 cm−1 which remains unchanged in the complexes. I exhibits a strong band at 2860 cm−1 due to the intramolecular H-bonded OH group of phenolic and/or carboxylic acid moieties . This band disappears in the coordination compounds indicating the breakdown of H-bonding and subsequent deprotonation of the OH group followed by the involvement of phenolic and carboxylic acid O atoms in coordination. The presence of a broadband at ~3400 cm−1 due to (O–H)(MeOH) and the decrease of (C–O)(MeOH) stretch from 1034 cm−1 to lower energy by 45–60 cm−1 in the coordination compounds of I indicate the involvement of the O atom of MeOH in coordination . The appearance of two new bands between 1560–1572 cm−1, (COO) and 1337–1358 cm−1, (COO) stretches indicates the presence of the coordinated carboxylate group in the coordination compounds. The energy difference (Δ = 213–225 cm−1) between these stretches is >210 cm−1 which indicates the monodentate nature of the carboxylate moiety . The (C–O) stretch of I occurs at 1520 cm−1. This band shifts to higher energy by 6–10 cm−1 in the coordination compounds indicating the involvement of phenolic O atom of 3-formylsalicylic acid moiety in coordination . The absence of a band between 835–955 cm−1 due to the (Zr=O) stretch  in the present Zr(OH)2(IV) compound suggests its formulation as [Zr(OH)2(LH)(MeOH)] and not as [ZrO(H2O)(LH)(MeOH)] . The presence of a broadband at 3440 cm−1 due to (OH) stretch and the appearance of a new medium intense band at 1135 cm−1 due to the (Zr–OH) bending mode  also support the proposed structure of the present Zr(OH)2(IV) compound. [UO2(LH)(MeOH)] exhibits the (O=U=O) stretch at 898 cm−1 and this band occurs in the usual range (870–950 cm−1) observed for the majority of trans-UO2(VI) compounds . The force constant and U–O bond length in the present dioxouranium(VI) compound are 6.70 mdyn/Å and 1.74 Å, respectively. These values are in the expected range (6.58–7.03 mdyn/Å and 1.60–1.92 Å) reported for the majority of UO2(VI) compounds . The new nonligand bands in the present coordination compounds in the low-frequency region are assigned to the (M–O) (550–570 cm−1) and the (M–S) (345–375 cm−1) and these bands  are in the expected order of increasing energy: (M–S) < (M–O).
3.2. NMR Spectral Studies
The NMR spectra of I and its coordination compounds were recorded in DMSO-. The chemical shifts () are expressed in ppm downfield from TMS. The prominent resonance signals of these compounds were compared with the reported peaks . I exhibits a singlet at 17.5 ppm due to the carboxylic proton, a sharp singlet at 13.60 ppm due to phenolic proton, a singlet at 2.35 ppm due to alcoholic proton, multiplets due to methylene protons at 4.70–4.79 ppm, and multiplets between 7.34 and 7.50 ppm due to the aromatic protons. The occurrence of the resonance signal at the same frequency ( 2.35 ppm) due to alcoholic proton (CH2OH) indicates the noninvolvement of the alcoholic group in coordination. The absence of the signal at 17.5 ppm due to the COOH proton in the coordination compounds indicates the deprotonation of the COOH group, followed by the involvement of its O atom in coordination. The absence of the resonance signal at 13.60 ppm due to the phenolic proton in the coordination compounds indicates the deprotonation of the phenolic OH group followed by its involvement in coordination . The appearance of resonance signals at 2.81–2.85 ppm due to alcoholic proton and at 3.0–3.1 ppm due to methyl protons in the coordination compounds supports the presence of MeOH in these compounds.
3.3. Reflectance Spectral Studies
The presence of three bands at 8700, 13000, and 18550 cm−1 due to →, → , and → transitions, respectively, suggests an octahedral arrangement of I around Co(II) ions in [Co(LH)(MeOH)3] . The value of / (2.13) lies in the range 2.00–2.80, reported for most of the octahedral Co(II) compounds . The spectral parameters [33–35] for [Co(LH)(MeOH)3] are cm−1, cm−1, , , and CFSE = −93.57 kJ mol−1. Reduction of Racah parameter from 971 cm−1 (free ion value) to 729 cm−1 and the value (25%) indicate the covalent nature of the compound and strong field nature of I . [Ni(LH)(MeOH)3] shows three bands at 8980, 15980, and 24900 cm−1 due to → , → , and → transitions, respectively, in an octahedral symmetry . The value of / is 1.78 which lies in the usual range (1.60–1.82), reported for the majority of octahedral Ni(II) compounds. The spectral parameters [34, 35] for [Ni(LH)(MeOH)3] are cm−1, cm−1, , , and CFSE = −128.75 kJ mole−1. The reduction of the Racah parameter from the free ion value (1030 cm−1) to 843 cm−1 and the value (18%) are indicative of the presence of covalent nature of the compound and the strong field nature of the tridentate ligand (I) . For a given ligand and given stereochemistry, the covalent character of the corresponding Co(II) and Ni(II) coordination compounds is comparable since Co(II) and Ni(II) occupy the adjacent positions in the nephelauxetic metal ion series (i.e., Co(II) ~ Ni(II)) . In the present Co(II) and Ni(II) coordination compounds, the values are quite comparable: [Co(LH)(MeOH)3]: 25%; [Ni(LH)(MeOH)3]: 18%. For a given ligand and given stereochemistry, the spectrochemical series of metal ions on the basis of increasing 10 values is Ni(II) < Co(II). Our calculated 10 values indicate that the 10 values are in the expected order: Ni(II) < Co(II). The greater negative CFSE value (−128.75 kJ mol−1) for [Ni(LH)(MeOH)3] in comparison to that of [Co(LH)(MeOH)3] (CFSE = −93.57 kJ mol−1) is as expected .
3.4. Magnetic Measurements
The room temperature magnetic moments of the coordination compounds of I are presented in Table 2. The magnetic moments of [Co(LH)(MeOH)3] and [Ni(LH)(MeOH)3] are 4.75 and 3.14 B.M., respectively. These values are indicative of the magnetically dilute high-spin octahedral coordination compounds of Co(II) and Ni(II) ions . The coordination compounds of other ions are diamagnetic.
Thus, on the basis of analytical data, valence requirements, molecular weight, spectral and the magnetic studies, it is proposed that I behaves as a dibasic tridentate OOS donor ligand in the tetrahedral compound, [Cd (LH)(MeOH)] (II) and in the octahedral compounds, [M(LH)(MeOH)3] [where M = Co(II), Ni(II)] (III), [Zr(OH)2(LH)(MeOH)] (IV) and for [UO2(LH)(MeOH)] (V), as shown in Figure 2.
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