Physicochemical , Spectral , and Biological Studies of Mn ( II ) , Cu ( II ) , Cd ( II ) , Zr ( OH ) 2 ( IV ) , and UO 2 ( VI ) Compounds with Ligand Containing Thiazolidin-4-one Moiety

The Schiff base (I) upon reacting with mercaptoacetic acid in dry benzene undergoes cyclization and forms N-(2-carbamoylthienyl)-C-(3-carboxy-2-hydroxyphenyl)thiazolidin-4-one, LH 3 (II). A MeOH solution of II reacts with Mn(II), Cu(II), Cd(II), Zr(OH) 2 (IV), and UO 2 (VI) ions and forms the coordination compounds, [Mn(LH)(MeOH) 2 ], [Cu(LH)] 2 , [Cd(LH)], [Zr(OH) 2 (OAc) 2 (LH 3 )], and [UO 2 (NO 3 )(LH 2 )(MeOH)]. The compounds have been characterized on the basis of elemental analyses, molar conductance, molecular weight, spectral (IR, reflectance, and EPR) studies and magnetic susceptibility measurements. LH 3 behaves as a neutral tridentate ONS donor ligand in [Zr(OH) 2 (OAc) 2 (LH 3 )], monobasic tridentate ONS donor ligand in [UO 2 (NO 3 )(LH 2 )(MeOH)], dibasic tridentate OOS donor ligand in [Cu(LH)] 2 and dibasic tetradentate OONO donor ligand in [Mn(LH)(MeOH) 2 ] and [Cd(LH)]. [Cu(LH)] 2 is dimer, while all other compounds are monomers in diphenyl. A squareplanar structure for [Cu(LH)] 2 , a tetrahedral structure for [Cd(LH)], an octahedral structure for [Mn(LH)(MeOH) 2 ], a pentagonalbipyramidal structure for [Zr(OH) 2 (OAc) 2 (LH 3 )], and an eight-coordinate structure for [UO 2 (NO 3 )(LH 2 )(MeOH)] are proposed. The ligand (II) and its compounds show antibacterial activities towards E. coli. (Gram negative) and S. aureus (Gram positive).


Introduction
There are numerous biologically active molecules with five membered rings, containing two heteroatoms among which is the 4-thiazolidinone ring system which is a core structure in various synthetic compounds.
Thiazolidin-4-one, a saturated form of thiazole with carbonyl group on fourth carbon, has been considered as a magic moiety (wonder nucleus).It has played an important role in medicinal chemistry [1,2] and posseses almost all types of biological activities [3,4] like antimicrobial [5], antitubercular [6], antibacterial [7], anticonvulsant [8], antifungal [9], anticancer [10], septicidal [11], and antiviral [12].Much attention has been paid to biologically active metal complexes in recent years.Oxygen and nitrogen donor ligands [13] have been widely studied due to their high potential to coordinate with transition metals.Compounds containing carbonyl oxygen groups [14] have important position among organic reagents as potential donor ligands for the transition metal ions.Organic compounds and metal complexes [15] both display a wide range of pharmacological activities including anticancer, antibacterial, and fungi static effects.
In this paper, we describe the syntheses, characterization, and antibacterial studies of II and its coordination compounds.
The structures of Schiff base (I) and thiazolidin-4-one (II) are shown in Figures 1 and 2

Analyses and Physical
Measurements.The organic skeleton of the respective coordination compounds were decomposed by the slow heating of ∼0.1 g of the latter, with conc.HNO 3 .The residue was dissolved in minimum amount of conc.HCl and the corresponding metal ions were estimated as follows.The Mn(II) and Cd(II) contents of the respective coordination compounds were estimated by complexometric titration method against standardized EDTA solution using eriochrome black-T and xylenol orange as the indicators, respectively.The Cu(II) content was estimated iodometrically against a standard solution of sodium thiosulphate to the starch end point.The zirconium content was estimated gravimetrically as ZrO 2 after decomposing the corresponding compound with a few drops of conc.HNO 3 and then igniting the residue.The uranium content in [UO 2 (LH)] was estimated gravimetrically as U 3 O 8 after decomposing the compound with a few drops of conc.HNO 3 and then igniting the residue.The C, H, and N contents of LH 3 and its coordination compounds were determined by CHN Eager analyzer model-300.The S and Cl contents of LH 3 and its coordination compounds were estimated gravimetrically as BaSO 4 and AgCl, respectively.The molecular weight measurements were carried out by the Rast method using diphenyl as the solvent [19] .The molar conductances (Λ  ) 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 [20].The diamagnetic corrections were computed using Pascal's constants.The magnetic susceptibilities were corrected for temperature-independent paramagnetism term (TIP) [20] using value of zero for Mn(II) ions and 60 × 10 −6 cgs units for Cu(II) ions.

Syntheses of the Coordination Compounds of II.
A MeOH solution (30-50 mL) of the appropriate metal salt (10 mmol) was added to a MeOH solution (50 mL) of II (3.64 g, 10 mmol) and the mixture was then refluxed for 3-4 h.The solid products formed were suction-filtered, washed with MeOH, and then dried as mentioned above.Yield = 35-55%.

Results and Discussion
A dry benzene solution of the Schiff base I reacts with mercaptoacetic acid and forms N-(2-carbamoylthienyl)-C-(3 carboxy-2  -hydroxyphenyl)thiazolidin-4-one, LH 3 (II).The reaction of the latter with appropriate metal salts in 1 : 1 molar ratio in MeOH produces the coordination compounds: The analytical data of II and its coordination compounds are given in Table 1.

Infrared Spectral Studies.
The infrared spectra of I, II and the coordination compounds of the latter were recorded in KBr and the prominent peaks (in cm −1 ) are shown in Table 2.I exhibits the ](C=N) (azomethine) stretch at 1635 cm −1 .This band disappears in II and a new band appears at 1570 cm −1 due to the ](C-N) (thiazolidinone ring) stretch [21] indicating the conversion of I into II.The formation of II is further supported by the appearance of a new band at 820 cm −1 due to the ](C-S) (thiazolidinone ring) stretch [22].The ](C-N) (thiazolidinone ring) stretch of II shifts to lower energy by 55 and 65 cm −1 in [Mn(LH)(MeOH) 2 ] and [Cd(LH)], respectively, suggesting the involvement of the thiazolidinone ring N atom towards coordination [22].However, the existence of this band at the same energy in the remaining coordination compounds indicates the noninvolvement of the thiazolidinone ring N atom towards coordination.II exhibits a strong band at 2855 cm −1 due to the intramolecular H-bonded OH groups of phenolic and/or carboxylic acid moieties [23].The appearance of this band almost at the same energy in the coordination compounds of the Zr(OH) 2 (IV) and UO 2 (VI) ions is indicative of the noninvolvement of the ](C-S) (thiophene) OH groups of the above moieties towards coordination.On the other hand, the coordination compounds of the rest of the metal ions do not show this band indicating the breakdown of H-bonding and subsequent deprotonation of the OH groups followed by the involvement of phenolic and carboxylic O atoms towards coordination.The ](C=O) (thiazolidinone ring) stretch [24] of II occurs at 1690 cm −1 .This band shifts to 1645 and 1660 cm −1 in the coordination compounds of Zr(OH) 2 (IV) and UO 2 (VI) ions indicating the coordination through O atom of the carbonyl group of thiazolidinone moiety.However, the above band remains unchanged in the rest of the coordination compounds suggesting the noninvolvement of the above O atom towards coordination.The ](C=O) (carboxylic) stretch [25] of II occurs at 1675 cm −1 .This band remains unchanged in the coordination compounds of Zr (OH) 2 (IV) and UO 2 (VI) ions indicating the noninvolvement of the O atom of carboxylic group towards coordination.However, the remaining coordination compounds display two new bands between 1550-1575 cm −1 and 1340-1365 cm −1 .These bands are assigned to the ]  (COO) and ]  (COO) stretches of the carboxylato ligand.The energy separation (Δ] = 210-218 cm −1 ) between the ]  (COO) and ]  (COO) stretches is indicative of the monodentate nature of the carboxylato ligand [26].The presence of a strong band at 1648 cm −1 due to the ](C=O) (amide) stretch in II indicates that it occurs in the keto form [27].This band shifts to lower energy by 20 and 35 cm −1 in [Mn(LH)(MeOH) 2 ] and [Cd(LH)], respectively, suggesting the involvement of the keto O atom towards coordination.However, the existence of this band at the same energy in the remaining coordination compounds indicates the noninvolvement of the keto O atom towards coordination.The ](C-O)Φ stretch [28] (1530 cm −1 ) of II undergoes a positive shift by ≤10 cm −1 in the coordination compounds of Mn(II) and Cd(II) ions, while it shifts to 20 cm −1 in [Cu(LH)] 2 .This band remains unchanged in the remaining compounds indicating the noninvolvement of phenolic O towards coordination.The ](C-S) (thiazolidinone ring) stretch [21] of II shifts from 820 cm −1 to lower energy by 35 cm −1 in [Cu(LH)] 2 , suggesting the formation of bond between the metal ions and the S atom.However, it remains unchanged in the rest of the coordination compounds indicating the noninvolvement of S atom towards coordination.The ](C-S) (thiophene moiety) stretch [21] of II occurs at 640 cm −1 .The appearance of a band at the same energy in the coordination compounds of Mn(II), Cu(II), and Cd(II) ions favours the noninvolvement of S atom of thiophene moiety towards coordination.On the other hand, the negative shift by 60 and 45 cm −1 of this band in [Zr(OH) 2 (OAc) 2 (LH 3 )] and [UO 2 (NO 3 )(LH 2 )(MeOH)], respectively, indicates the coordination through S atom of thiophene moiety.The presence of a broadband between 3300 and 3400 cm −1 due to ](O-H)(MeOH) stretch and the decrease of ] (C-O)(MeOH) stretch [28] from 1034 cm −1 to lower energy by 35 and 49 cm −1 in [Mn(LH)(MeOH) 2 ] and [UO 2 (NO 3 ) (LH 2 )(MeOH)], respectively, indicate the involvement of the O atom of MeOH towards coordination [28].
The absence of a band in the region 835-955 cm −1 due to the ](Zr=O) stretch [29] in the present Zr(OH) 2 (IV) compound suggests its formulation as [Zr(OH) 2 (OAc) 2 (LH 3 )] and not as [ZrO(H 2 O)(OAc) 2 (LH 3 )] [25].The presence of a broadband at 3445 cm −1 due to coordinated OH group and the appearance of a new medium intense band at 1125 cm −1 due to the (Zr-OH) bending mode [25] also support the proposed structure of the present Zr(IV) compound.The acetato ligand is coordinated to the metal ion in a monodentate fashion [30] as evident by the appearance of two new bands at 1580 and 1365 cm −1 due to the ]  (acetate) and ]  (acetate) stretches, respectively, in the spectrum of [Zr(OH) exhibits the ]  (O=U=O) stretch [31] at 920 cm −1 .The force constant ( U-O ) and U-O bond length in the present compound are 7.03 mdyn/ Å and 1.73 Å, respectively [25].

EPR Studies. The EPR spectrum of [Cu(LH)]
2 in DMSO at 77 K has been recorded in -band, using 100 kHz field modulation, and the  values are relative to the standard marker tetracyanoethylene (TCNE) ( = 2.0028).The spectrum shows seven hyperfine lines (2nI + 1 = 2 × 2 × (3/2) + 1 = 7) in parallel components due to the intramolecular exchange interaction in the dimer with no superhyperfine lines.It exhibits the half-filled line at ∼1600 gauss due to ΔMs = 2 transition and conclusively proves the presence of the magnetic exchange interaction in this coordination compound.
The EPR spectral parameters of [Cu(LH)] 2 are as follows:  || = 2.25,  ⊥ = 2.07,  || = 92 × 10 −4 cm −1 ,  = 3.65,  2 cu = 0.57, and  av = 2.13.The values of  || ,  || , and  ⊥ obtained are close to the values reported for other dimetallic Cu coordination compounds [41].For ionic environments,  || is normally ≥2.3 and is <2.3 for covalent environments.The  || value (2.25) indicates that the metal-ligand bonding in the compound is covalent.The in-plane covalence parameter ( 2 cu ) has been calculated using the relation [20]: where  is related to the overlap integral () according to the relation: The smaller the value of  2 cu , the more covalent the bonding;  2 cu = 1 indicates complete ionic bonding, while  2 cu = 0.5 indicates complete covalent bonding.On the other hand, the larger the value of (  ) 2 , the more covalent the bonding; (  ) 2 = 0 suggests a complete ionic bonding.The observed value ( 2 cu ) of [Cu(LH)] 2 is less than unity and this indicates that this compound possesses significant covalent character in the M-L bonding [42].

Magnetic Measurements.
The magnetic moment of [Mn(LH)(MeOH) 2 ] is 5.92 B.M.This value is within the normal ranges reported for the magnetically dilute octahedral compounds of Mn(II) ions [43].The Cu(II) ion belongs to the  = 1/2 system and since its spin-orbit coupling constant is negative [20], the magnetically dilute Cu(II) coordination compound is expected to exhibit magnetic moment higher than the spin-only value of 1.73 B.M. due to the presence of orbital contribution [44].The magnetic moment (1.55 B.M.) of [Cu(LH)] 2 is substantially less than the above range of magnetic moment and this indicates the presence of antiferromagnetic exchange [20].The coordination compounds of Cd(II), Zr(OH) 2 (IV), and UO 2 (VI) of II are diamagnetic as expected.

Antibacterial Studies.
The antibacterial activities of ligand (II) and its complexes were tested against bacteria, E. coli (Gram negative) and S. aureus (Gram positive), by using disc diffusion method.Stock solution was prepared by dissolving compounds in DMSO.Under aseptic conditions, plain sterilised discs were soaked in solution of compounds for overnight.Test culture was spread over the plates containing Mueller Hinton Agar (MHA) E. coli and S. aureus by using sterile swab.Inoculated plates were dried for 30 minutes and discs were placed on inoculated plates.The plates were left for 30 minutes at room temperature to allow diffusion.The plates were then incubated at 37 ∘ C for 24 hours for E. coli and S. aureus.After incubation, diameter of zone of inhibition was noted for each disc as shown in Table 3.

Determination of Minimum Inhibitory Concentration (MIC).
The stock solution of compounds were prepared using distilled water as diluent.In a set of test tubes having 2 mL of Mueller Hinton Broth, compounds was serially diluted. 2 mL of the test culture was added to all tubes and tubes were incubated at 37 ∘ C for 24 hr.Lack of turbidity was noted for the determination of MIC (Table 4).

Conclusions
From the ongoing discussion, it may be stated that II behaves as a neutral tridentate ONS donor ligand in [Zr(OH) 2 (OAc)

Disclosure
This paper is the authors' own work, and the research is original.The results have not been published (in any language or medium), and the paper is not considered and will not be offered elsewhere.Both the authors have read and approved the paper, and due care has been taken to ensure the integrity of the work. respectively.

Table 1 :
Analytical, molar conductance (Λ  ), and molecular weight data of II and its coordination compounds.
a Mass spectral data, b Rast's method data.

Table 2 :
IR, reflectance spectral data (cm −1 ), and magnetic moments of coordination compounds of II.

Table 3 :
Antibacterial activity of ligand (I) and its coordination compounds (zone of inhibition in mm).