Syntheses, Magnetic and Spectral Studies on the Coordination Compounds of the Polystyrene- anchored Thiazolidin-4-one

The reaction between polystyrene 3-formylsalicylate and thiophene-2-carboxylic acid hydrazide in DMF in the presence of ethyl acetate results in the formation of polystyrene N-(2-carbamoylthienyl)3'-carboxy-2'-hydroxybenzylideneimine (I). A benzene suspension of I reacts with mercaptoacetic acid and forms the polystyrene N-(2carbamoylthienyl)-C-(3'-carboxy-2'-hydroxyphenyl) thiazolidin-4-one, PSCH2–LH2 (II). A DMF suspension of II reacts with Zn(II), Co(II), Cu(II), Zr(OH)2(IV) and MoO2(VI) ions and forms the corresponding polystyrene-anchored coordination compounds, [PSCH2–LZn(DMF)] (III), [PSCH2–LCo(DMF)3] (IV), [PSCH2–LHCu(OAc)] (V), [PSCH2–LH2Zr(OH)2(OAc)2] (VI) and [PSCH2–LHMoO2(acac)] (VII) respectively. The polystyrene-anchored coordination compounds have been characterized on the basis of elemental analyses, spectral (IR, reflectance, ESR) studies and magnetic susceptibility measurements. II acts as a neutral tridentate ONS donor ligand in VI, a monobasic bidentate OS donor ligand in VII, a monobasic tridentate ONS donor ligand in V and a dibasic tridentate ONO donor ligand in III and IV. The acetato groups behave as monodentate ligands in V and VI. A square-planar structure for V, a tetrahedral structure for III, an octahedral structure for IV and VII and a pentagonal-bipyramidal structure for VI are suggested.


Introduction
In recent years there has been considerable interest in the syntheses and use of functionalized polymers having chelating abilities due to their practical convenience, operational flexibility and formation of coordination compounds with high metal to polymer bond energies 1 .A structural study of polymer-anchored compounds seems useful in view of their numerous applications in organic synthesis 2 , immobilization of enzymes 3 , biological systems 4 , water treatment 5 and as catalysts 6 etc. Thiazolidin-4-ones belong to an important group of heterocyclic compounds with carbonyl group at fourth position 7 .They show broad spectrum of biological activities due to their ready accessibility and diverse chemical reactivity 8 .They are involved in variety of applications such as antimicrobial, antibacterial, anticonvulsant, antifungal, anti-HIV, antiproliferative, anti-inflammatory, cystic fibrosis and antithyroid [9][10][11] etc.Many drugs possess modified pharmacological properties in the form of the metal complexes 12 .These facts prompted us to explore the coordination behavior of thiazolidin-4-one (II) derived from the Schiff base (I) (obtained from the condensation of polystyrene 3-formylsalicylate and thiophene-2-carboxylic acid hydrazide) towards Zn(II), Co(II), Cu(II), Zr(OH) 2 (IV) and MoO 2 (VI) ions.A perusal of the literature indicates that several polymer-anchored ligands containing O atom(s) like crown ethers 13 , acetylacetone 14 a n d iminodiacetic acid 15 have been reported, however there is no report on the coordination compounds of polymer-anchored ligand containing thiazolidin-4-one moiety.In this paper, we describe the syntheses and characterization of polystyrene-anchored thiazolidin-4-one, PSCH 2 -LH 2 (II) and its coordination compounds with above ions.
The elemental analyses, IR, reflectance spectral studies, ESR spectra and magnetic susceptibility measurements were carried out as described in our previous report 20 .

Synthesis of polystyrene N-(2-carbamoylthienyl)-3'-carboxy-2'-hydroxybenzylideneimine (I)
Polystyrene 3-formylsalicylate (1.0 g) was allowed to suspend and swell in DMF (100 mL) for 45 min.To this suspension, a DMF solution (60 mL) of thiophene-2-carboxylic acid hydrazide (0.66 g, 4.68 mmol) and ethyl acetate (100 mL) were added, while stirring magnetically.The mixture was refluxed for 8 h and then cooled to room temperature.The polystyrene-anchored Schiff base, I obtained was suction filtered, washed with DMF and ethyl acetate.It was dried in vacuo at room temperature.

Synthesis of polystyrene N-(2-carbamoylthienyl)-C-(3'-carboxy-2'-hydroxyphenyl) thiazolidin-4-one, PSCH 2 -LH 2 (II)
Mercaptoacetic acid (0.32 g, 3.51 mmol) was added to the swollen suspension of I (1.0 g) in benzene (100 mL).The mixture was refluxed for 12 h on a water bath and then cooled to room temperature.The solid product was filtered and washed with 10% sodium bicarbonate solution followed by chilled distilled water.The product was dried as mentioned above.IR bands (KBr):

Syntheses of coordination compounds of II
1.0 g of II was allowed to suspend and swell in DMF (100 mL) for 1 h.A DMF solution of appropriate metal salt (2.34 mmol) was added to the above suspension.The mixture was refluxed on water bath for 8-10 h and the products obtained were suction filtered, washed several times with ethyl acetate and DMF.The products were then dried as mentioned above.

Results and Discussion
The reaction between polystyrene 3-formylsalicylate and thiophene-2-carboxylic acid hydrazide in DMF in the presence of ethyl acetate results in the formation of polystyrene N-( 2

Infrared spectral studies
The infrared spectra of I-VII were recorded in KBr and the prominent peaks are shown in Table 2.The ν(C==N)(azomethine) stretch of I occurs at 1620 cm -1 .This band disappears and a new band appears in II at 1580 cm -1 due to the ν(C--N)(thiazolidinone ring) stretch 21 , indicating the formation of corresponding thiazolidin-4-one.The formation of II is further supported by the appearance of a new band at 830 cm -1 due to the ν(C--S) (thiazolidinone ring) stretch 22 .II occurs in the keto-form 23 as evident by the presence of a strong band due to the ν(C==O)(amide) stretch at 1652 cm -1 .This band remains almost at the same energy in V, VI and VII indicating the non-involvement of the keto O atom towards coordination.III and IV do not display the ν(C==O)(amide) stretch but show new band at 1260 and 1245 cm - 1 respectively suggesting the conversion of -C(O)NH-moiety (keto-form) into -C(OH)==N-moiety (enol-form) followed by the deprotonation of enolic OH group and subsequently involvement of enolic O atom towards coordination.The ν(C--O)ф stretch 23 of II occurs at 1532 cm -1 .This band remains unchanged in VI indicating the noninvolvement of phenolic O atom towards coordination.However, the shifting of this band towards higher energy (≤ 10 cm -1 ) in the remaining compounds favours the formation of a bond between phenolic O atom and the corresponding metal ions.The ν(C==O)(thiazolidinone) stretch 24 of II occurs at 1695 cm -1 .This band shows a negative shift by 45 cm -1 in VI indicating the coordination through O atom of the carbonyl group of thiazolidinone moiety.The occurrence of this band almost at the same energy in the remaining compounds indicates the non-involvement of the carbonyl O atom towards coordination.The [ν(C--N)(thiazolidinone ring)] stretch 21 of II shifts from 1580 cm -1 to lower energy by 35-42 cm -1 in the III, IV and V lending support for the involvement of ring N atom towards coordination.However, this band remains almost at the same energy in VI and VII.The [ν(C--S)(thiazolidinone ring)] stretch 22 of II occurring at 830 cm -1 shifts to lower energy by 29 cm -1 in VII, on the other hand, it remains unchanged in the remaining polystyrene-anchored coordination compounds.The ν(C--S)(thiophene ring) stretch 25 of II occurring at 648 cm -1 shifts to lower energy by 35 and 43 cm -1 in V and VI respectively.On the other hand, it remains unchanged in the remaining polystyrene-anchored coordination compounds.The ν as (COO) and ν s (COO) stretches of free acetate ions occur at 1560 and 1416 cm -1 respectively 26 .The ν as (COO) and the ν s (COO) stretches occur at 1585, 1350; and 1595, 1370 cm -1 in V and VI respectively.The magnitude of energy separation (ν = 225 and 235 cm -1 ) between ν as (COO) and ν s (COO) is > 144 cm -1 and it indicates the monodentate nature of acetato groups 26 , since in the event of bidentate coordination, the energy separation between ν as (COO) and ν s (COO) is < 144 cm -1 .DMF shows a band at 1680 cm -1 due to the ν(C==O) stretch 27 .This band shifts to lower energy by 40 and 35 cm -1 in III and IV indicating the involvement of O atom towards coordination 27 .The absence of a band between 835-955 cm -1 , characteristic of ν(Zr==O) stretch 28 in VI suggests its structures as . The appearance of a band at 1128 cm -1 due to the δ(Zr--OH) bending mode also supports the suggested structure of the compound 26 .VII exhibits the ν s (O==Mo==O) and ν as (O==Mo==O) stretches at 924 and 938 cm -1 respectively and these bands occur in the usual range (892-964 cm -1 ; 842-928 cm -1 ) reported for the majority of MoO 2 (VI) compounds 26 .The presence of ν s (O==Mo==O) and ν as (O==Mo==O) bands indicates a cis-MoO 2 structure as the compounds with trans-MoO 2 structure exhibit only the ν as (O==Mo==O) since the ν s (O==Mo==O) is IR inactive 29 .

Magnetic measurements
The room temperature magnetic moments of the polystyrene-anchored coordination compounds of II are presented in Table 2.The magnetic moment of V is 1.88 B.M.This value lies within the range (1.70-2.20 B.M.) reported for the magnetically dilute Cu(II) compounds 30 .The magnetic moment of IV is 4.89 B.M and this value lies within the range reported for the high-spin octahedral Co(II) compounds 31 .III, VI and VII are diamagnetic.

Reflectance spectral studies
The electronic spectra of the compounds could not be recorded in the nujol mull as the polystyrene-anchored compounds do not form a good mull.Therefore, their reflectance spectra were recorded (Table 2).The compounds being insoluble in common solvents, their solution electronic spectra also could not be recorded.IV exhibits three bands at 8910, 13100 and 20100 cm -1 due to the 4 T 1g (F) → 4 T 2g (ν 1 ), 4 T 1g (F) → 4 A 2g (ν 2 ) and 4 T 1g (F) → 4 T 1g (ν 3 ) transitions, respectively suggesting an octahedral structure 32 .The ν 3 /ν 1 value is 2.26 which lies in the usual range (2.00-2.80)reported for the majority of octahedral Co(II) coordination compounds 32 .The parameters are: 10Dq = 10080 cm -1 , B' = 824 cm -1 , β = B'/B = 0.85, β 0 = 15% and CFSE = −96.4kJ mol -1 .The reduction of Racah parameter from the free ion value of 971 cm -1 to 824 cm -1 and the β 0 value (15%) indicate the covalent nature of the compound and the strong field nature of the ligand respectively.V exhibits a band at 17220 cm -1 which is assigned to 2 B 1g → 2 A 1g , 2 B 2g and 2 E g transitions for square-planar arrangement of ligand around Cu(II) ions 33 .The absence of a band in the range: 8000-10000 cm -1 precludes the presence of a tetrahedral structure 34 .

ESR studies
The ESR spectrum of V exhibits g || = 2.26 and g  = 2.09 indicating the presence of a tetragonal type symmetry about the Cu(II) ion 35 .The spectral parameters 35  (1.92  10 -2 cm -1 ) of P d 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.The spectrum does not show any band ~1500 G due to the ∆Ms = 2 transition and this precludes the presence of M--M interaction.

Table 1 .
). Analytical, MBC and PRC values of polystyrene-anchored coordination compounds of II.

Table 2 .
IR, reflectance spectral data (cm -1 ) a nd m a g n e t i c mo m e n t s o f polystyreneanchored coordination compounds.