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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.59 no.1 Concepción mar. 2014

http://dx.doi.org/10.4067/S0717-97072014000100004 

 

SYNTHESES AND STRUCTURAL STUDIES ON COORDINATION COMPOUNDS OF POLYSTYRENE-ANCHORED SCHIFF BASE WITH SOME METAL IONS

 

DINESH KUMARa, PRAVEEN KUMAR GUPTA*bAND ARUN SYAMALc

a Department of Chemistry, National Institute of Technology, Kurukshetra 136119, Haryana, India
b Department of Chemistry, Haryana College of Technology & Management, Kaithal, 136027, Haryana, India,
c School of Coordination Chemistry', D Wing, Ushanagar, Bhandup, Mumbai, 400 078, India
* e-mail: praveen_gemini@yahoo.com; dkumar_nitk@yahoo.com


ABSTRACT

Aminomethylated polystyrene (PSCH2-NH2) reacts with 3-ethoxysalicylaldehyde in equimolar amounts to produce monobasic bidentate (ON donor) polystyrene-anchored Schiff base (PS-LH). The latter upon refluxing with metal the ions in 1:2 molar ratios gives polystyrene-anchored coordination compounds of types: [PSCH2-LM(OAc)(DMF)] (where M = Cd, Co, Cu, Zn, UO2), [PSCH2-LNi(OAc)(DMF)3], [PSCH2-LFeCl2(DMF)2], [PSCH2-LMCl(CH3OH)] (where M = Cd, Zn) and [PSCH2-LMoO2(acac)]. The polystyrene-anchored compounds have been characterized on the basis of elemental analyses, spectral (IR, reflectance, ESR) and magnetic susceptibility measurements. The polystyrene-anchored Co(II), Cu(II), Ni(II) and Fe(III) compounds are paramagnetic, while others are diamagnetic. Co(II) and Cu(II) compounds are square-planar; Zn(II) and Cd(II) compounds are tetrahedral; Ni(II), Fe(III), MoO2(VI) and UO2(VI) compounds are octahedral.

Keywords: polystyrene-anchored, Schiff base, coordination compounds, diamagnetic, magnetically dilute.


 

1. INTRODUCTION

The polymer-anchored coordination compounds have a great interest due to there wide application field. Metal complexes on polymer support have shown various uses in organic synthesis1, curing agent for epoxy resin2, as catalyst3, as ion exchanger4 etc. This technique of immobilization on an inert support have drawn much attention due to their easy separation from the reaction mixture leading to operational flexibility, selectivity, efficiency, stability and ease of handling and economy in various industrial processes. Insoluble polymer supports are more frequently used as inert support for immobilizing the transition metal over cross-linked chloromethylated polystyrene5, poly(hydroxylethylmethacrylate), poly(glycidylmethacrylate)6,7 and silica8. Among organic polymers polystyrene has been extensively used as support with a wide range of functional groups incorporated in it to bind the metal into the polymer. Cross-linked polystyrene with specific properties are widely used as catalyst as they are inert, non-volatile, non-toxic and recyclable. Polymer-anchored metal catalysts are known to catalyze various reactions: epoxidation of alkanes, alkenes9,10, oxidation of aromatic alcohols11, hydrogenation of alkenes12, reduction of ketones and nitriles13 etc. In the present work, we report the synthesis and characterization of aminomethylated polystyrene-anchored Schiff base (I) and its coordination compounds with Cd(II), Co(II), Cu(II), Ni(II), Zn(II), Fe(III), MoO2(VI) and UO2(VI).

2. EXPERIMENTAL

2.1. Materials

Aminomethylated polystyrene beads (PSCH2-NH2) (containing 1.17 mmol of NH2 per g of resin and 1% crosslinked with divinylbenzene) [Sigma Chemical Co.(USA)], 3-ethoxysalicylaldehyde [Aldrich Chemical Co.(USA)] and cadmium chloride hemipentahydrate [SD's Fine Chemicals], iron(III) chloride(anhydrous), cobalt(II) acetate tetrahydrate, cadmium(II) acetate tetrahydrate, dioxouranium(VI) acetate tetrahydrate [BDH]; nickel(II) acetate tetrahydrate[Fluka AG(Switzerland)]; zinc(II) acetate dihydrate, zinc(II) chloride dihydrate[SD's Fine Chemicals]; copper(II) acetate monohydrate, Dimethylfomamide, methanol, acetone[Ranbaxy Laboratories Ltd.] were used for the synthesis. Bis(acetylacetonato)dioxomolybdenum(VI) was synthesized by following the published procedure14. The solvents were dried on molecular sieves prior to use.

2.2. Analyses and physical measurements

IR spectra were recorded using KBr pellets(4000-400 cm-1) on a Nicolet Fourier transform infrared spectrophotometer calibrated with polystyrene. Reflectance spectra were recorded on a Beckmann DU spectrophotometer attached with a reflectance arrangement. Magnetic susceptibility measurements were carried out at room temperature, using double ended one sided sealed Gouy tube and Hg[Co(NCS)4] as the standard21. The diamagnetic corrections were computed using the procedure specially developed for the polymer supported coordination compounds. The magnetic susceptibilities for diamagentism of ligand and metal ions were corrected for temperature independent paramagnetism term(TIP). The TIP values were taken as 200 x 10-6 cgs units for Co(II) and Ni(II) and 60 x 10-6 cgs units for Cu(II). The TIP value for Fe(III) was taken as zero as the high spin d5 system has a 6A1g ground state.

2.3 Synthesis of polystyrene-anchored ligand(PSCH2—LH)(I)

PSCH2-NH2 (1.0 g) was suspended in DMF (20 mL) for 45 min. To this suspension, a DMF solution (60 mL) of 3-ethoxysalicylaldehyde (LH) (0.58 g, 3.51 mmol) was added. The mixture was refluxed for 8 h and then cooled to room temperature. The cream coloured product obtained was suction filtered, washed several times with DMF, MeOH, EtOH and petroleum ether and dried in vacuo.

2.4. Syntheses of polystyrene-anchored coordination compounds, [PSCH2—LM(OAc)(DMF)] (where M = Co Cd, Cu, Zn, UO2) and [PSCH2— LNi(OAc)(DMF)3]

PSCH2-LH (I) (0.5 g, 0.58 mmol) was suspended in DMF (15 mL) for 1 h. To this suspension, a DMF solution (30-60 mL) of appropriate metal acetate (1.17 mmol) was added. The mixture was heated under reflux, while stirring for 5-7 h. The products obtained were cooled to room temperature and then suction filtered, washed several times with DMF, MeOH, EtOH and Me2CO. The compounds were dried as mentioned above.

2.5. Synthesis of [PSCH2—LFeCl2(DMF)2]

PSCH2-LH (I) (0.5 g, 0.58 mmol) was suspended in DMF (15 mL) for 1 h. To this suspension, a DMF solution (30 mL) of anhydrous iron(III) chloride (0.20 g, 1.17 mmol) was added. The mixture was refluxed under anhydrous conditions, while stirring magnetically for 8 h. The reddish brown product obtained was cooled to room temperature and suction filtered, washed several times with DMF, MeOH, EtOH and Me2CO. The compound was dried as mentioned above.

2.6. Syntheses of [PSCH2—LMCl(MeOH)] (where M = Zn and Cd)

PSCH2-LH (I) (0.5 g, 0.58 mmol) was suspended in MeOH (15 mL) for 1 h. To this suspension, a MeOH solution (30 mL) of appropriate metal(II) chloride (1.17 mmol) was added. The mixture was heated under reflux, while stirring magnetically for 5 h. The products obtained were cooled to room temperature and then suction filtered, washed several times with DMF, MeOH, EtOH and Me2CO. The compounds were dried as mentioned above.

2.7. Synthesis of [PSCH2-LMoO2(acac)]

PSCH2-LH (I) (0.5 g, 0.58 mmol) was suspended in DMF (15 mL) for 1 h. To this suspension, a DMF solution (30 mL) of bis(acetylacetonato) dioxomolybdenum(VI) (0.38 g, 1.17 mmol) was added. The mixture was heated under reflux, while stirring magnetically for 7 h. The yellow coloured product obtained was cooled to room temperature and then suction filtered, washed several times with DMF, MeOH, EtOH and Me2CO. The compound was dried as mentioned above.

3. RESULTS AND DISCUSSION.

The nucleophilic addition reaction followed by the elimination of one H2O molecule between PSCH2-NH2 and LH in DMF results in the formation of PSCH2-LH (I)

PSCH2-LH reacts with metal complex/metal salts in 1:2 ratio for 5-8 h in DMF and forms polystyrene-anchored coordination complexes as shown below:

The compounds are insoluble in water as well as other organic solvents. The percent reaction conversion (PRC) of the polystyrene-anchored coordination compounds varies from 33.0 to 98.0 and the metal binding capacity17 (MBC) {MBC = [M% (observed) x 10]/(atomic weight of metal)} of PSCH2-LH (I) in the range 0.28-0.75 mmol/g of resin (Table 1). The metal ions in polystyrene-anchored coordination compounds are leached with hot diluted OAc/HCl at pH 4 - 5.5 and the leached resin can be used for the formation of other coordination compounds.

Table 1. Colour and Analytical Data of Polystyrene-Anchored Coordination Compounds.

 

The prominent IR bands of polystyrene-anchored coordination compounds are shown in Table 2. The v(C=N)(azomethine) stretch of PSCH2-LH(I) occurs at 1630 cm-1 and this band shifts to lower energy by 5-20 cm-1 in polystyrene-anchored coordination compounds. The positive shift of v(C-O) (phenolic) stretch of I from 1510 cm-1 to higher energy by <10 cm-1 in the coordination compounds indicates the involvement of phenolic O atom towards coordination15. The data preclude the presence of dimetallic structure, as in such a case v(C-O) (phenolic) stretch usually shifts to higher energy by ≤10 cm-1. Thus, PSCH2-LH acts as a bidentate ligand coordinating through the phenolic O and azomethine N atoms. The vsy(OAc) and vasy(OAc) stretches of the non-coordinated acetate ion occurs at 1416 and 1560 cm-1 respectively16. The present coordination compounds containing coordinated acetato group show these bands in the regions: 1370-1395 cm-1 and 1575-1595 cm-1 respectively. The energy difference (205-215 cm-1) between vsy(OAc) and vasy(OAc) is an indicative of monodentate nature of coordinated acetato group, since in case of bidentate coordination16, the energy difference is <144 cm-1. MeOH exhibits a band at 1034 cm-1 due to the v(C-O) stretch and this band shifts to lower energy by 65 cm-1 in [PSCH2-LMCl(MeOH)] (where M = Zn and Cd) indicating MeOH coordination15. DMF shows a band at 1680 cm-1 assigned to the v(C=O) stretch and this band shifts to lower energy by 20-50 cm-1 in the coordination compounds indicating the O coordination of DMF17. [PSCH2-LMoO2(acac)] exhibits two bands, due to vasy(O=Mo=O) and vsy(O=Mo=O) stretches occurring at 910 and 940 cm-1 respectively characteristics of a cis-MoO2 moiety. These bands lie in the usual ranges [840-925 cm-1, vbsy(O=Mo=O) and 892-964 cm-1, v (O=Mo=O) stretches] respectively, reported for the majority of MoO2(VI) compounds18. [PSCH2-LUO2(OAc)(DMF)] shows a band at 900 cm-1 assigned to vasy(O=U=O) stretch and this band lies within the usual range (870-950 cm-1) reported for the majority of UO2(VI) compounds17. The observation of only the va (O=U=O) band suggests the trans-UO2 structure. The force constant (fU-O)17 is 6.73 mdyn/Å. The U-O bond distance is 1.74 Å and lies in the usual range (1.60-1.92 Å) reported for the majority of the UO2(VI) compounds.

Table 2. IR Spectral Data (cm-1) of Polystyrene-Anchored Ligand and its Coordination Compounds.

 

The magnetic susceptibility measurements at room temperature for polystyrene-anchored compounds are presented in Table 3. The magnetic moment (2.53 B.M.) of Co(II) compound suggests its square planar-geometry19. The magnetic moment (1.92 B.M.) of Cu(II) compound indicates its magnetically dilute nature19. The magnetic moments of Ni(II) and Fe(III) compounds are 3.20 and 5.94 B.M. respectively suggesting their octahedral geometries19. The Cd(II), Zn(II), MoO2(VI) and UO2(VI) coordination compounds are diamagnetic. The data suggest an octahedral structure for MoO2(VI) and UO2(VI) compounds and tetrahedral structure for Zn(II) and Cd(II) coordination compounds.

Table 3. Magnetic Susceptibility Measurements and Reflectance Spectral Data of Polystyrene-Anchored Coordination Compounds.

 

The solid state reflectance spectral data of the polystyrene-anchored compounds are presented in Table 3. [PSCH2-LCo(OAc)(DMF)] exhibits two bands at 8800 and 25000 cm-1 assigned to 1A1g1B2g and 1A1g1B1g transitions respectively suggesting square planar symmetry20. [PSCH2-LCu(OAc)(DMF)] displays an asymmetric band at 17900 cm-1 due to 2B1g2A1g, 2B2g and 2Eg transitions, characteristic of square- planar symmetry20. [PSCH2-LNi(OAc)(DMF)3] displays two bands at 9250 and 16200 cm-1 due to 3A2g3T2g (v1) and 3A2g3T1g (F)(v2) transitions respectively, suggesting an octahedral structure20. The v2: v1 value of the compound is 1.75 which agrees well with the reported range (1.60-1.82) observed for the majority of the Ni(II) octahedral compounds20. Another band due 3A2g3T1g (P)(v3) transition could not be located since it is probably merged with the strong charge transfer band. [PSCH2-LFeCl2(DMF)2] (II) shows three spectral bands at 13000, 15800 and 20000 cm-1 due to 6A1g4T1g(G), 6A1g4T2g(G) and 6A1g4A1g(G) transitions respectively, suggesting an octahedral structure20.

ESR spectrum of [PSCH2-LCu(OAc)(DMF)] was recorded at liquid nitrogen temperature in polycrystalline solids using DPPH as a field marker. Hyperfine coupling due to copper is resolved in both the parallel and perpendicular regions. The spectrum shows well resolved four hyperfine lines with no super-hyperfine lines. However, the half field signal (ΔM = ±2, ~1600 gauss) corresponding to the Cu-Cu interaction was not observed and conclusively proves the absence of the magnetic exchange interaction in [PSCH2-LCu(OAc)(DMF)]. The spin-Hamiltonian parameters are: g|| = 2.25, g= 2.11, A|| = 1.68 x 10-2 cm-1, A= 3.9 x 10-3 cm-1, G = 2.3, αCu2 = 0.80, (α')2 = 0.27, κ = 0.54 and Pd = 1.66 x 10-2 cm-1. The g tensor values of Cu(II) complex can be used to derive the ground state. In square planar complexes, the unpaired electron lies in the dx2-y2 orbital giving 2B1g as the ground state with g|| > g. From the observed values, it is clear that g|| > g, which suggests that the complex is square planar. Also it is supported by the fact that the unpaired electron lies predominantly in the dx2-y2 orbital21. The geometric parameter (G), a measure of the exchange interaction22, was found to be 2.3 and this value lies within the range 2.1-3.8, consistent with the ground state dx2-y2. The molecular orbital coefficient, i.e. covalency parameter αCu2 (a measure of covalency of the in-plane σ-bonding) has been calculated using the equation21: αCu2 = - (A||/0.036) + (g||- 2.0023) + 3/7(g- 2.0023) + 0.04, where α is related to the overlap integral (S) between the metal dx2-y2 and ligand orbitals according to the relation: α2 - 2αα'S + (α')2 = 1. A value αCu2 = 0.5 indicates the complete covalent bonding, while the value αCu2 = 1.0 suggests complete ionic bonding. The observed value (αCu2 = 0.80) of the Cu(II) compound is less than unity and this indicates that this compound possesses significant covalent character in the M-L bonding21. The larger the value of (α')2, the more covalent is the bonding and a value of (α')2 = 0 indicates a complete ionic bonding. The (α')2 values of 0.52 for the Cu(II) compound indicates the covalent nature of the compound. The symbol κPd represents the Fermi contact contribution (A) to the coupling, where Pd is the dipolar contribution. The value of Pd and κ were calculated using the relation23: Pd = - (A||- A/0.78 and κ = - 0.48 - (A||/Pd). The lower value of Pd in comparison to that of the free ion value (3.5 x 10-2 cm-1) indicates the covalent interaction in the metal ligand system. A positive value of κ predicts that A|| should be greater than Aand this too has been observed by us.

CONCLUSION

The Schiff base has been successfully anchored to aminomethylated polystyrene leading to the formation of polymer-anchored ligand. The percent reaction conversion of the polystyrene-anchored coordination compounds varies from 33.0 to 98.0 and the metal binding capacity of PSCH2-LH is in the range 0.28-0.75 mmol/g of resin. The data suggest a square-planar structure (III) for [PSCH2-LM(OAc)(DMF)] (M = Co, Cu); tetrahedral structures (IV and V) for [PSCH2-LM(OAcXDMF)] and [PSCH2-LMCl(MeOH)] (M = Zn, Cd); octahedral structures (VI, VII, VIII and IX) for [PSCH2-LNi(OAc) (DMF)3], [PSCH2-LFeCl2/DMF)2], [PSCH2-LMoO2(acac)] and [PSCH2-LUO2(OAc)(DMF)] respectively. The polystyrene-anchored Co(II), Cu(II), Ni(II) and Fe(III) compounds are paramagnetic, while others are diamagnetic. All coordination compounds are magnetically dilute in nature which is supported by the fact that the reactive functional groups in PSCH2-LH(I), are situated every four to five styrene units apart which restricts the two ligands attached to the polymer chain coordinating to the same metal ions.

ACKNOWLEDGEMENTS

One of the author (P.K. Gupta) is grateful to Director of his Institute for his encouragement.

 

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(Received: November 22, 2012 - Accepted: December 26, 2013)