Indi an Journal of Chemistry Vol. 42A. March 2003, pp. 499-505

Synthesis and characterisation of polymer supported Schiff base metal chelates

K Sivadasan Chettiyart & K Sreekumar* Department of Applied Chemistry. Cochin University of Science & Technology, Kochi 682022, Kera la. Indi a

Received 6 December 2001; revised 17 October 2002

Di viny l benzene (DVB )-crosslinked polystyrene has been functio na li sed to introduce thi osemicarbazone ligand function. Thi s Schiff base thiosemicarbazone ligand is used to prepare chelates of Fe(lll ), Co(ll), Ni (ll ), Cu(ll ) and Z n(ll ). The factors influencing metal complexation like temperature, pH , influence of solvent etc are examined. The polymeri c metal complexes are well characterised by spectral techniques and assigned a proper geometry on the basis o f anal yti ca l. magnetic measurement and spectral data.

The functionalised polymers find wide use in organic synthesis especially as polymeri c reagents t

•3

,

polymeric catalysts4, supported metal complex

catalysts5.8

, supported chiral catalysts9.1O

, polymeric chelating ligands " -15 etc. Since functionali sed resins contain highly selective coordinating groups, they can be used as chelating agents for separation of metal ions '6.

17. Since the polymer anchored complexes of

the metal Ions are magnetically dilute, characterization of the complexes especially, structural elucidation is not straight forward as compared to simple metal complexes ' 8.

Among polymer supported metal chelates , Schiff base metal chelates are important in biological processes, preconcentration of metal ions, catalysis etc I9

.20

. The present work aims at the preparation of polymer anchored chelating ligands based on crosslinked polystyrene supported Schiff base thiosemicarbazone. The effect of time, pH, temperature, solvents etc, on complexing ability are studied and the optimum conditions for maximum complexation are determined. The polymeric Schiff base metal chelates are characterised by IR, UV and ESR spectral techniques. These data coupled with magnetic moment measurements help in assigning a defi nite geometry to the metal complexes. Comparison between chelating properties of polymers and the monomeric systems particularly, with respect

I . . d21 to structura aspects IS examllle .

Materials and Methods Fe(IlI) sulphate (Ferric alum), Co(Il) acetate,

t Department o f Chemi stry, DB College, Sasthamkottah, Koll am 690521. Kera la, India

CoOl) chloride, Ni(II) sulphate, Ni(ll ) nitrate, Cu(lI ) chloride, Cu(n) sulphate, Zn(H) nitrate etc were of Analar grade obtained from E Merck (Germany). The solvents were distilled and dried before use. Commercial grade styrene and DVB were washed with aqueous NaOH (1-2%) to remove the stabili ze r and were then washed with water and dri ed over anhydrous CaCh and distilled under reduced pressure. Microanalyses were done at RSIC, CDRI , Lucknow. IR spectra were recorded on a Perkin Elmer-397 Spectrometer using KBr pellets, UV spectra were recorded on a Shimadzu model l60A doubl e beam Spectrophotometer with reflectance attachment. ESR spectra were recorded with Varian X-band 104 spectrometer at liquid nitrogen temperature usi ng polycrystalline samples. Thermal analys is was carried out using a Du Pont-2000 Thermal Analys is-TGA thermobalance. DYB-crosslinked polystyrene res ins of varying degree of crosslinking ranging from 2 to 15% were prepared by free radical sLl spension polymerization. The differently cross linked resins were chloromethylated by adopting standard procedures. The chloromethylated res in was ox idi sed to aldehyde resin followed by condensation with thiosemicarbazone22

.23

.

Preparatioll of aldehyde polymer Chloromethylated resin (20 g), OM SO (300ml ) and

NaHC03 (19 g) were refluxed at 138-140°C for 14h. The aldehyde resin formed was filtered, washed with hot water several times and then with diox ane-water (1 : I v/v). The resin was finally washed witlI polar solvents like dioxane, ethanol and dichlo romethanc and with a non-polar solvent like benzene and dri ed in vacuo.

500 lNDlAN J CHEM., SEC. A, MARCH 2003

The a ldehyde capacity of the resin can be determined by making use of the addition reaction wi th known excess of NaHS03 solution24

. The excess of NaHSOJ was determined by titrating against standard 0 . 1 N lz solution . The aldehyde capacity of the res in was found to be 3.46 meq/g .

Preparation of Schiff base thiosemicarbazone polYlller

The a ldehyde po lymer (10 g) was swollen in a lcoho l (50 ml ). To this, thiosemicarbazide (15 g) was added and the mixture was refluxed on a water bath for 10 h. The thiosemicarbazone obtained was washed wi th a lcoho l, water and acetone. The Schiff base resin

was finally dried at 50°C and kept in a vacuum dessicator.

Preparation of metal chelates In all chelation studies, a defi nite amount of the

resin ( I g) was equilibrated at 27°C with an aqueous solution of suitable metal ion (0.02 M , 40 ml) for a defi nite period (6 h, 12 h, 18 h and 24 h) . The po lymer meta l complex was filtered, washed with

water (5 x 5ml) and dried in vacuo. The metal io n remaining after chelation was esti mated by adopting standard volumetric method (in the case of Cu, Co, Ni, Zn) and colorimetric method (in the case of Fe). The metal intake by the res in was determined in each case.

Results and Discussion Polystyrene of different crosslink densities (2, 5, 10

and 15% crosslinked with DYB) were used. The polystyrene beads were chloromethylated usi ng chloromethyl methyl ether employing Friedel-Crafts reac ti on in the presence of Lewis acid catalyst, anhydrous SnCI4 . A ldehyde polymer (2) was prepared by oxidising the chloromethylated resin (1) with DMSO in the presence of NaHC03. Condensatio n of the aldehyde with thiosemicarbazide in alcoholic medium resulted in the Schiff base, DVB-crosslinked polystyrene thiosemicarbazone (DYB-PST) (3). Scheme I gives the preparation of the thiosemicarbazone resin .

The polymeric thiosemicarbazone resins were equilibrated with aqueous metal ion solution (Fe(III ), Co(ll) , Ni(ll), Cu(II) and Zn(II)) at room temperature to g ive the polymer meta l che lates . Table 1 gives the magnetic moment and metal intake values of the different complexes.

LJ-\ CH, CI DMSO, 1400 C. a ~ NaHCO,

(I) (1)

~ -CHO + H , N-HN - C _ NH , ~~ ,, -s

to, CH=N-NH- C - H ,

- II s

(3 )

Scheme -1

Table l--Polymer-anchored Schi ff base complexes"

Complex Colour

[P-(TSChFe3+(H2O)CI1CI2 Brown [P-(TSChCo(H2OhlCI2 Reddish

brown [P-(TSC)2NilS04·2H20 Green ish

ye llow [P-(TSC)2CulCI22H20 Light

green [P-(TSC)2Znl(N03h .2H2O Light

ye llow

Magnet ic Metal moment binding

(BM ) capac ity

3.36 3.23

2.87

2.69

(6h stirring) (meq/g)

0.99 0. 19

0.16

0.20

0. 10

"Polymer supported thiosemicarbazone metal complex

The functionalised polystyrene resins with its large framework, will have its functional groups wide apart. This provides ambient space for the formation o f metal complex in a microenvironment in a macromolecular matrix. There may be only s li gh t changes in geometry due to the presence o f macromolecular environment with the overa ll structure remai ning the same as in low molecular weight species.

DYB-crosslinked polystyrene th iosemicarbazone Sch iff base ligand and its metal complexes show I R spectral bands similar to low mo lecular speci es . except for the very small shifts in spectra l bands, due to the presence of polymer support. A sharp band

observed at 1600 cm - I is asslgned to VC=N

(azomethine) of the Schiff base which is shifted to

1610 crn- I on complexation. The ligand shows a broad

CHETny AR el al. : POLYMER SUPPORTED SCHIFF BASE METAL CHELATES SO l

Table 2-Metal intake by 2% DVB-PST resin

Time of Metal intake caEacity" (meg/g) stirring (h) Fe(J II ) Co(Jl) Ni (II) Cu(II) Zn(H)

6 0.99 0.19 0.16 0.20 0.10 12 0.65 0.24 0.11 0.23 0.11 18 0.45 0.32 0.10 0.25 0.14 24 0. 17 0.35 0.05 0.28 0.19

"Natural pH, Room temeErature (27°C)

band between 3300-3450 cm- I due to VNH and VNH2

This band remains unchanged on complexation . The intense band occurring at 830 cm- I due to Vc=s

stretching in the ligand is shifted to 815-820 cm- I in the metal complex . This is in agreement with the

lowering of Vc-s on coordination25. Metal-nitrogen and

metal-sulphur bonds were shown by bands observed at 550 cm- I

(VM-N) and 414 cm-I ( VFe-S). Similar

ass ignments to metal-sulphur bonds gave values 420, 418 and 416 cm- I in DYB-PST Cu, DYB-PST Ni and DYB-PST Co complexes respectively. The participation of azomethine nitrogen and thiocarbonyl groups in complexation is thus confirmed. The shift of Vc= to hi gher frequency by 5-15 cm - I clearly shows that nitrogen of the azomethine group is coordinated to the metal ion26

. Thus, coordination of azomethine nitrogen is same as in non-anchored complexes except for a s light shift.

The amount of metal ions bound to the polymer ligands of different crosslink densities were determined by quantitative analysis . The residual metal ion remaining after chelation was estimated . This gives a measure of the metal bound to the polymer complex. The values are given in Table 2 for 2% DYB-PST resin.

There is release of Fe(JII) and Ni(II) from DYB­PST anchored thiosemicarbazone complex into soluti on, as observed by change in metal intake values from 0.99 to 0.17 meq/g in the case of Fe(III) and 0.16 to 0.05 meq/g, in the case of Ni(II). These observations point to the fact that the stability of Fe(lII) and Ni(II) complexes in polymeric thiosemicarbazones in acidic metal salt solutions are considerably low. The decomposition increases with increase in the time of contact.

The time for maximum complexation in the case of Cu(ll), Co(II) and Zn(II) ions was found to be 24 h. Maximum complexation occurred at 6h of equilibration. In the case of Cu(II ) ion, the

Table 3--Effect of degree of crosslinking on metal chelation

Crosslink Metal intake caEaci ty" (meg/g) density Fe(III) Co(Jl) Ni(II) Cu(ll ) Zn(H)

2 0.99 0.19 0.16 0.20 0.10 5 0.38 0.08 0.04 0.19 0. 17 10 0.41 0.03 0.12 0.18 15 0.60 0.38 0.35

aNatural pH , Room temperature (27°C), Time 6h

1.20

1.00 .

~ i" 0 .80 .§. !l 0.60 S .5 g 0.40 GI ~

0.20

~Fe I

-.- Cu ___ N; I -.- Co

~ 0.00 +----~...---.:=----~'-------~ 2 5 10 15

Crosslink density

Fig. I-Plot of metal intake versus cross link density

complexation ability of the polymer remained more or less constant except for a gradual increase within the period of stirring, 24 h. This points to the stability of polymer metal chelate of Cu (II ) in aqueous solu tion . Co (II) also shows the same observation, as far as metal intake kinetics is considered . There tS a stepwi se increase in metal intake of Cu(ll).

Polymer crosslinking also has an effect on metal complexation. The complexing ability was found to decrease as a result of increase in degree of crosslinking (Table 3). 2% and 5% resins are essentially microporous, but as the degree of crosslinking increases, macroporous domain increases. This points to the increased rigidity of the support as a result of crosslinking. In the case of Fe(III) , Cu(lI) and Co(lI), 10 and 15 % cross lin ked resins showed greater metal intake than expected (Fig. 1). These res ins will have greater macroporous nature. There is gradual increase of macroporosity of the resins as the degree of crosslinking is increased. The enhanced porosi ty of the resin as a result of crosslinking may provide channels within the beads for the metal ion to penetrate into the network. Hence considerable amount of adsorption takes pl ace fo r

502 INDIAN J C HEM ., SEC. A, MARCH 2003

metals with favourable adsorption kinetics27 . The porosity is estimated quantitatively by different parameters like specific su rface area Ssp, total pore volume (Wo) and pore radius r. The values obtained are r= 4-200nm, Ssp = 590m2/g and Wo= 0.36 cm3/g.

There may also be a change of degree of cross linking on functionalisation of the res in . The strength of interaction between the basic functional groups of the polymer ligand and metal ion may also determine the ligand capacity for complexation .

Temperature, pH and solvent also influence complexation of metal ions by DYB-PST resin . (Table 4, 5 and 6) . These factors serve one another in forming a stable macromolecular metal chelate.

Increase in temperature provides additional segmental motion to the macromolecular network with the incorporated functional groups. The metal ions come into close contact with reactive sites, by penetrating through the beads. The thermal stability of the polymer support is important in studying temperature effect. The DYB-PST resin is stable for study within 30°C and 80°C. The metal intake capaci ty shows a s light increase, but at higher temperature, a sli ght decrease is also noted .

The effect of pH (Table 5) is important, as very low pH causes the Schiff base linkage to break . At higher p H values, there is a chance of precipitation of metal ions as hydrox ide. Hence, an optimum pH value is selected fo r study eg; complexation study involving Co, Ni , Cu and Zn is better within a range of pH 4 to 5.5. For Fe, the study is limited to pHd.

Effect of solvent on complexation, is dependent on the avai lability of reaction si tes for reaction. The reactive species in solution must gai n access to the reactive sites in the polymer beads. This can be ach ieved by swelling of the polymeric resin in a suitable solvent. Swelling generates pores and molecular penetration occur by diffusion. The active sites in polymer immobilized complexes are confined to a region in space defined by the dimension of the polymer. The concept of phase boundary problem involved in reactions at solid-liquid interphase may be introduced. The mathematical formalism applied to heterogeneous systems cou ld be used to interpret mass transfer with reactions in polymer support. Stabi lity, suface area, porosity and nature of active s ites are important. Ligands based on DYB-PST resin , a hydrophobic support, gives beads with pore diameter most suited for large scale application.

Table 4--Effect of temperature on the ex tent of compl ex ation. Metal intake by 2% DYB-PST

Metal ion Metal intake capacity" (meg/g)

Fe(lII ) Co(ll ) Ni(lI) Cu( lI) Zn(lI)

"Natural pH; Ti me, 6h

0.99 0. 19 0.16 0.2 0. 1

1.01 0.2

0 . 12 0 .2 1 0 . 18

0.88 0 .22 0.06 0 .17 0.2

Table 5--Effect of pH on meta l intake by DYB-PST res in

pH Metal intake capacitt (meg/g) f e(l ll ) Co(ll ) Ni(II) C u(II ) Zn(l l)

2 1.02 0. 15 0 .08 0.1 I 0.08 3 0.35 0.19 0.16 0 .20 0. 10 4 0.24 0.18 0.44 0. 15 5 0.29 0.21 0.78 0. 18 5.5 0.31 0.22 0.66 0 .20

Time of st irring, 6h; Room temperature (27°e)

Table 6-Extent of swelling of DYB-PST Sch iffs base po lymer

Degree of Swelling rati o" cross- Methanol Elhanol AcelOn i tri Ie THF linking

2 1.65 2.00 1.05 DO 5 2.26 2. 15 1.20 ].60 10 1.60 1.20 0 .80 D O 15 1.00 1.20 0.70 1.70

"Room temperature (27°C)

The exten t of swelling in a definite so lvent , expressed in terms of swelling ratio, is g iven in Table 6. Swelling ratio is defined as the ratio of the increase in weight of the polymer ro the orig inal weight on swelling in a solvent of defin ite volume and density (for a definite period), the weight of the polymer and volume of solvent taken uni formly for comparison.

A definite weight of the polymer (0.5 g) was allowed to s'Vvell in a solvent (5 mL 24 h). After definite period, the solvent was carefully drained off ard the weight of the polymer was determin ed . From the increase in weight, the swelling ratio was calcu lated .

The swelling rat io shows that so lvents li ke methanol, ethanol , acetonitrile and THF are good solvents for DYB-PST resin. Thi s is cl ear frol11

CHETIIY AR ef al.: POLYMER SUPPORTED SCHIFF BASE METAL CHELATES 503

Table 7-Effect of solvents on metal complexation with DYB-PST Schiffs base resin

Metal ion Degree of crosslinking, % Acetonitrile

2 1.06 5 0.96

Cu(lI ) 10 0.90 15 0.60 2. 0.43 5 0.28

Co(l l) 10 0.25 15 0.29 2 5

Zn(lI) 10 0.09 15 0.10

"Natura l pH, Room temperature (27°C)

chelate formation with metals like Cu(II) and Co(ll) in these solvents. Table 7 shows the effect of solvent on metal complexation. 40 ml 0.02 M metal ion was used for studying solvent effect. The solution was prepared by dissolving the metal salt in minimum amount of water and making upto the required concentration by adding the respective solvent (by noting the solubility in the selected solvent).

DYB-PST metal complexes are assigned proper geometry on the basis of a clear knowledge of magnetic properties and spectral data. Spectral data mainly involved UY /Vis, IR and ESR. The magnetic susceptibilities were determined at room temperature by the Guoy method. The azomethine group of thiosemicarbazone shows an n. n* transition around 29,000 cm- I

. This is shifted to higher energy on complex formation (33,900-32,800 cm- I

) . A second n n* transition originating from thioamide portion of thiosemicarbazone moiety is found at some what lower energies (24,000-28,100 cm- I

). This also is shifted to higher energies on complex formation (27,000-29,000 cm- I

) .

DYB-PST Fe (Ill) complex shows a magnet ic moment of 3.36 BM. Thi s low value indicates a lowspin-highspin (spin crossover) octahedral geometry for Fe(I1I) complex. This is in agreement wi th the magnetically dilute nature of polymer supported metal complexes. Usualiy, Fe(IIl) complexes are high-spin with hi glt value of magnetic moment. Spin-pairing can be induced in d5 Fe(llI) by the thiosemicarbazone ligands.

The electron ic spectra shows three bands in the range 46,600-40,000 cm- I

, 33900 cm- I-32800 cm- I,

Metal intake in different solvents" (meg/g) THF Methanol Ethanol

0.79 0.68 1.0 0.47 0.39 0.77 0.35 0.27 0.42 0.34 0.14 0.40 0.2 1 0.50 0.56 0.25 0.28 0.34 0.27 0.19 0.28 0.26 0.22 0.31

0.10 0.06 0.12 0.03

23200-22,200 cm·1 corresponding to the transItI ons, 6A1g -7 4T1g , 111= 23,200-22,200 cm- I

. , IiA1g -7 .J T2g - I 6 4£ d 6A .JA 112=33 ,900-32,800 cm ., Alg-7 g an I;; -7 Ig ,

113 = 46,600-40,000 cm- I

The first one is due to d-d-transition extending to the visible region obscuring the high energy band . The second and third transitions are due to a hi gh energy and low energy charge transfer band.

The IR spectral data show that DYB-PST Schiff base acts as a bidentate ligand co-ordinating th rough azomethine nitrogen and sulphur. Thiosemicarbaz ide may adopt cis or trans configuration in complexes. In majority of complexes, TSC molecule is in the cis configuration . The metal-sulphur bond is usuall y shorter in ci s (vFe-S 414cm- l

, vCo-S 416 cm- I) since

sulphur exerts a stronger trans effect28. ESR spectral

data show that DYB-PST Fe(llI) complex have g

values (gil = 2.3, g.l =2.23, gay = 2.25 ) where gil > g~ .

Fe(IIl) complex shows g values where gil > g.l . The observations agreed with a high spin-low spin crossover arrangement. Two chelate ri ngs are formed in each case occupying four sites . The remaining sites are occupied by water molec llles and/or anions. The geometry assigned is in agreement with the elemental analysis . The TG curve of the complex shows a weight loss of about 6.6% upto 230°C. Thi s may be due to removal of water of coordination. Thus on the basis of IR and UV spectral data, magnetic moment measurements. and ESR measurements, a low spin octahedral geometry may be ass igned to DYB-PST Fe complex .

DYB-PST Co complex shows a )..lell va lue of 3.23 BM, which is lower than the magnetic moment of an

504 INDIAN 1 CHEM., SEC. A, MARCH 2003

octahedral complex. This is close to a distorted octahedral geometry. The UY!Vis spectra contain bands at 33,333 cm- I corresponding to the transition 4TI g (F) -1

4Tl g (P). Another transition corresponds to ~T,g (F) -1 4Azg at 40,983 em-I which is essentially a two-electron process. There is one more spin allowed transition which is generally in the range 24,390-25,000 cm- I which may be attributed to 4Tlg(F) -1

~T2iF) transition. There is a tendency to acquire some intensity due to spin-orbit coupling. Hence a distorted octahedral geometry may be assigned. ESR spectral data (g ll= 2.17, g.1 = 2.14, gav=2.15) show that g value deviate from free electron value. The weight loss within the temperature 196°C is about 13% as observed in TG analysis.

DYB-PST Ni(II) thiosemicarbazone complex showed a magnetic moment of 2.87 BM. This is sl ightly lower than that of a distorted tetrahedral geometry .

Ni(ll) complex exhibited absorption bands in the region 21,000-25,000 cm- I. This is an allowed d-d transition corresponding to JT, (F) -1 3 A2• The charge transfer band at 33,000 cm- I can be assigned to 3TI (F)

-1 3TI (P) transition. The tetrahedral species is paramagnetic with unpaired electrons. This agrees with ESR spectral data where gll>g.1>ge (gll=2.36, 3.1 =2.19, gav = 2.25). The covalency parameter shows a value of 0.129 showing covalent character for the complex . The TG data show a region corresponding to dehydration within the temperature 162°C due to the removal of hydrated water molecules. A system of axial symmetry with comparably high paramagnetism can be possible to the complex, if we assign a distorted tetrahedral geometry .

CU(Il) complex exhibited an effective magnetic moment of 2.69 BM. This value is higher than that of sq uare planar geometry, but lower than that for a regular tetrahedral geometry. The electronic spectra of Cu(l!) complex showed absorption bands at 22,320 cm- I, 33,444 cm- I and 42,152 cm·-I. The first band is assumed to be due to "Big -1 2AIg transition. The other two are charge transfer transi tions . The band at 4 1,152 cm- I may be due to S -1 Cu charge transfer band. Usually distorted tetrahedral species is found in some Schiff bases with bulky substituents on nitrogen. All these are in agreement with ESR spectral data, (311= 2.38, g.1 = 2.14, gay = 2),gll>g.1). The covalency parameter is 0.305 . The higher g value of the complex indicate a more planar bonding due to the

M = Fe(III). Co(lI) r. x = cr, CH,COO', NO,' or H,O

t-< )-H?">~~.~~ -< >i HI'f__ ,;' ,':, '/

\C=-=-=. S' :' s =-=-=-c /- , "-H,N , N~

X

x = CI-. CH3COO-, N0

3- or H

20

(I)

//7//////

HC II

" / ' , , ,'/ ,

" / , ,'/ '

',/\' , ,

H~\~' _ / ",-IM----~~ , /, _$

-- / "" -- ---- :1 , :.;-:.$ -:- - - C

/c 'j "-...

I NH2 ~N ,

N~NH

II He

/ M = Ni(II), Cu(lI)

"'\"""~\:-<:\:-.;:\+~~:"'C,""'"

(II)

presence of two sulphur donors_ The TG data show a weight loss of 13% within the temperature range or 150°C showing the removal of hydrated water molecules_ A distorted tetrahedral geometry is assigned to the complex.

Zn2+ belongs to iO configuration and has no crystal

field stabilization effect. Magnetic momen t measurements show that the complex is diamagnetic . Electronic spectra show no characreristic bands. Thermal data show expUlsion of hydrated water molecules resulting in a weight loss of 12%. The complex may be assigned a tetrahed ra l geometry in the absence of coordinated water molecules.

DYB-PST Schiff base thiosemicarbazone acts as a neutra l bidentate ligand co-ordinating through azomethine nitrogen and thione sulphur, as found from the experimental observations. Two

CHETIIY AR et af.: POLYMER SUPPORTED SCHIFF BASE METAL CHELATES 505

thiosemicarbazone units are involved in complex formation. Two chelate rings are formed. The proposed structure of DYB-PST Fe(III) and Co (II) complexes are given in Structure I.

The two ligands occupy the four coordination sites. The fifth and sixth positions are occupied by anions like chloride, acetate, nitrate or H20. In the case of CoOl) complex, the 5th and 6th positions may be occupied by two water molecules in two sites of coordination as evident by a weight loss of 13% at 196°C in TG studies. In the case of Fe(III) complex, one coordination site is occupied by coordinated water molecule, as evident by weight loss of 6.6% at 230°C in TG studies.

DYB-PST Ni and Cu complexes are assigned a distorted tetrahedral geometry (Structure II).

The four coordination sites are occupied by the two bidentate ligands. The presence of polymer support in the complex leads to distortion of geometry from normal octahedral to tetrahedral. Usually a distorted tetrahedral geometry is assigned.

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