Physico-Chemical Studies on the Coordination Compounds of Thiazolidin-4-One

A dry benzene solution of the Schiff base, N-(2-hydroxyphenyl)-3-carboxy-2-hydroxybenzylideneimine upon reacting with mercaptoacetic acid undergoes cyclization and forms N-(2-hydroxyphenyl)-C-(3-carboxy-2-hydroxyphenyl)thiazolidin-4-one, LH3 (I). A MeOH solution of I reacts with Mn , Cu , Zn, Fe and MoO 2 ions and forms the monomeric coordination compounds, [Mn(LH)(MeOH)3] (II), [Cu(LH)(MeOH)] (III), [Zn(LH)(MeOH)] (IV), [FeCl(LH)(MeOH)2] (V), and [MoO2(LH)(MeOH)](VI). e coordination compounds have been characterized on the basis of elemental analyses, molar conductance, molecular weight, spectral (IR, re�ectance, ESR) studies, and magnetic susceptibility measurements. I behaves as a dibasic tridentate OOS donor ligand in these compounds. e compounds are nonelectrolytes (ΛM = 6.2–13.8mho cm mol) in DMF. A square-planar structure for III; a tetrahedral structure for IV and an octahedral structure for II,V, andVI are suggested.


Introduction
Heterocyclic compounds of Schiff bases possessing thiazolidin-4-one skeleton (having carbonyl group at 4th position) are known for their versatile pharmacological and industrial importance [1].ey have been studied extensively because of their ready accessibility, diversed chemical reactivity, and broad spectrum of biological activities [2].
A perusal of the literature reveals that much has been reported on the syntheses and characterization [13,14] of a variety of thiazolidin-4-ones, but little is known about their coordination compounds [15][16][17].
Metal ions play a key role in the actions of drugs.ey are involved in speci�c interactions with antibiotics, proteins, membrane components, nucleic acids, and other biomolecules [18,19].Many drugs possess modi�ed pharmacological properties in the form of the metal complexes.Transition metal ions possess an important role in the design of metal-based drugs and such complexes are more effective against infectious diseases compared to the uncomplexed drugs [20,21].Metals complexes of Cu(II) has proved to be bene�cial in diseases such as tuberculosis, gastric ulcers, rheumatoid arthritis, and cancers [22].e complexes of Fe(III) and Mn(II) also have excellent medicinal values [23].
Hence these facts prompted us to explore the coordination behavior of a newly synthesized thiazolidin-4-one with transition metal ions.

Analyses and Physical
Measurements.e organic skeleton of the respective coordination compounds was decomposed by the slow heating of ∼0.1 g of the latter, with conc.HNO 3 .e residue was dissolved in minimum amount of conc.HCl and the corresponding metal ions were estimated as follows: the Mn(II) and Zn(II) contents of the respective coordination compounds were estimated by complexometric titration method against standardized EDTA solution using eriochrome black-T as the indicator.e Cu(II) contents was estimated iodometrically against a standard solution of sodium thiosulphate to the starch end point.e Fe(III) ions were reduced to Fe(II) ions with aqueous SnCl 2 and then estimated against standard K 2 Cr 2 O 7 solution using N-phenylanthranilic acid as an indicator.e molybdenum contents was estimated gravimetrically aer decomposing the given MoO 2 (VI) compound with a few drops of conc.HNO 3 and conc.H 2 SO 4 , and then igniting the residue in an electric Bunsen at 500 ∘ C. MoO 3 obtained was dissolved in 6N NaOH, and then molybdenum was estimated as bis(8hydroxyquinolinato) dioxomolybdenum(VI).
e C, H, and N contents of I and its coordination compounds were determined by CHN Eager analyzer model-300.e S and Cl contents of I and its coordination compounds were estimated gravimetrically as BaSO 4 and AgCl, respectively.e molecular weight measurements were carried out by the Rast method using diphenyl as the solvent [25].
e 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.e IR spectra were recorded in KBr pellets (4000-400 cm −1 ) on a Beckman-20 spectrophotometer.e re�ectance spectra were recorded on a Beckmann D� spectrophotometer attached with a re�ectance arrangement.e magnetic susceptibility measurements were carried out at room temperature, using Hg[Co(NCS) 4 ] as the standard [26].e diamagnetic corrections were computed using Pascal's constants.e magnetic susceptibilities were corrected for temperature independent paramagnetism term (TIP) [26] using value of 60 × 10 −6 cgs units for Cu(II) ions, zero for Mn(II), and Fe(III) ions.e ESR spectrum of III was recorded at liquid N 2 temperature in polycrystalline solids on a Varian V4502-12 X-band ESR spectrophotometer with 100 KHz modulation using diphenylpicrylhydrazide as a g-marker and monitoring the frequency with frequency meter.

Synthesis of the Schiff
Base. e Schiff base was prepared as per the reported procedure [27].

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 (3.31 g, 10 mmol) and the mixture was then re�uxed for 3-4 h.e solid products formed were suction �ltered, washed with, and recrystallized from MeOH, and were then dried as mentioned above.Yield = 50-80%.e coordination compounds are stable at room temperature.ey are insoluble in H 2 O, partially soluble in MeOH, EtOH, and completely soluble in DMSO and DMF.eir molar conductance measurements (Λ M = 6.2 − 13.8 mho cm 2 mol −1 ) in DMF indicate their nonelectrolytic nature.e analytical data of I and its coordination compounds are presented in Table 1.  2. e Schiff base exhibits the (C=N)(azomethine) stretch at 1630 cm −1 .is band disappears in I and a new band appears at 1575 cm −1 due to the (C-N)(thiazolidinone ring) stretch [28] indicating the conversion of the Schiff base into I. e formation of I is further supported by the appearance of a new band at 835 cm −1 due to the (C-S)(thiazolidinone ring) stretch [29], and it shows a negative shi by 15-35 cm −1 in the coordination compounds indicating the involvement of the S atom of the thiazolidinone moiety towards coordination [30].I shows the (C=O)(thiazolidinone ring) stretch [31] at 1700 cm −1 .is band remains unchanged in the coordination compounds indicating the noninvolvement of O atom towards the coordination.I exhibits a strong band at 2860 cm −1 due to the intramolecular H-bonded [32,33] OH group of phenolic and/or carboxylic acid moieties.is 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 towards coordination.e presence of a broad band 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 40-64 cm −1 in the coordination compounds of I indicate the involvement of the O atom of MeOH towards coordination [34].e appearance of two new bands between 1558-1575 cm −1 ,  as (COO) and 1333-1360 cm −1  (Δ     cm  ) between these stretches is >210 cm  which indicates the monodentate nature of the carboxylate moiety [35].e (C-O) stretch [34] of I occurs at 1525 cm  .is band shis to higher energy by 5-10 cm  in the coordination compounds indicating the involvement of phenolic O atom of either 3-aldehydo-2-hydroxybenzoic acid or 2-aminopohenol moieties towards coordination.On the basis of steric grounds, we suggest the noninvolvement of phenolic (2-aminophenol moiety) O atom towards coordination.e absence of a band between 820-860 cm  in V precludes the presence of the (Fe-O-Fe) bridged structure [36].e presence of a peak at 357 cm  in V con�rms the presence of Fe-Cl linkage [37] in the compound.VI exhibits the  s (O=Mo=O) and  as (O=Mo=O) stretches at 946 and 910 cm  , respectively [38].ese bands occur in the usual ranges:  s (O=Mo=O) stretch, 892-964 cm  and  as (O=Mo=O) stretch, 842-928 cm  , reported for the majority of MoO  VI compounds [38].e presence of two bands due to the (O=Mo=O) stretch is indicative of a cis-MoO  con�guration as the compound with trans-MoO  structure shows only  as (O=Mo=O) stretch since the  s (O=Mo=O) stretch is IR inactive [39].e absence of a band at ∼775 cm  in the MoO  VI compound indicates the absence of an oligomeric chain with ⋯Mo⋯Mo⋯Mo⋯interaction [40].e new nonligand bands in the present coordination compounds in the low frequency region are assigned to the (M-O)(550-570 cm  ) and the (M-S)(345-370 cm  ) and these bands [41] are in the expected order of increasing energy: (M-S) < (M-O).On the basis of analytical data (Table 1), valence requirements, and the infrared spectral studies, it is proposed that I behaves as a dibasic tridentate OOS donor ligand in the coordination compounds.

3.�. Re�ectance S�ectra� Studies
. II exhibits three bands at 17450, 22320, and 25300 cm  due to 6   g → 4   g ( 6   g → 4   g (, and 6   g → 4   g ( transitions, respectively, in an octahedral environment [42].e presence of an asymmetric broad band at 17230 cm  due to the    g →    g     g , and   g transitions in III suggests a square-planar arrangement of I around Cu II ion [43].e absence of a band in the range 8000-10000 cm  precludes the presence of a tetrahedral structure [44].V exhibits three bands at 12700, 16100, and 24900 cm  due to 6   g → 4   g ( 6   g → 4   g (, and 6   g → 4   g ( transitions, respectively, in an octahedral environment [45].

Magnetic
Measurements.e room temperature magnetic moments of the coordination compounds of I are presented in Table 2. e magnetic moments of II, III, and V are 5.90, 2.02, and 5.95 B.M., respectively.ese values are indicative of magnetically dilute high-spin octahedral coordination compounds of Mn II and Fe III ions [46].e IV and VI are diamagnetic as expected.[47] and the ground state [48,49] of Cu(II) is predominantly      (  B  as the ground state).Here  || and  ⟂ values denote the e�ective g-values when the externally applied �C �eld is parallel ( || ) and perpendicular ( ⟂ ) to the symmetry axis of the crystalline �eld around the paramagnetic centre, respectively.e value of geometric parameter (G), a measure of the exchange interaction, is 2.93, and this value lies within the range [50] 2.1-3.8,consistent with the ground state [47]      .Further the value of  || / || for the complex is 117.95 cm which lies in the range (90-140) cm as reported for square planar Cu(II) complexes [26].For ionic environments,  || is normally ≥ 2.3 and is < 2.3 for covalent environments.Cu , the more covalent is the bonding;  2 Cu = 1 indicates completely ionic bonding, while  2 Cu = 0.5 indicates completely covalent bonding [26].e larger the value of ( � ) 2 , the more covalent is the bonding; ( � ) 2 = 0 suggests a complete ionic bonding [26].e symbol   represents the Fermi contact contribution (A) to the coupling, where   is the dipolar contribution.e value of   and  have been calculated using the relation [48,49]:   = −( || −  ⟂ )/0.78 and  = − 0.48 − ( || /   ).e positive value of  suggests that  || should be greater than  ⟂ and it has also been observed by us.e lower value of   in comparison to that of the free ion value (3.5 × 10 −2 cm −1 ) indicates the presence of covalent character between the metal-ligand bonding.e absence of any band ∼1500 G due to the Δ  2 transition rules out the presence of M-M interaction.
T 1: Analytical, molar conductance (Λ M ) and molecular weight data of I and its coordination compounds.
T 2: I�, re�ectance spectral data (cm −1 ), and magnetic moments of coordination compounds of I. Compound  as (COO)  s (COO) (C-O)(phenolic) (C-S) (C-O)( ,  s (COO) stretches indicate the presence of the coordinated carboxylate group in the coordination compounds.e energy difference Preparative scheme of LH 3 (I).