Synthesis and characterization of thermally stable Schiff base polymers and their copper(II), cobalt(II) and nickel(II) complexes

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REACTIVE & FUNCTIONAL POLYMERS

Reactive & Functional Polymers 68 (2008) 292–306

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Synthesis and characterization of thermally stable Schiff base polymers and their copper(II), cobalt(II) and nickel(II) complexes ¨ zbu¨lbu¨l b, S. Serı_n b M. Tuncßel a,*, A. O a

b

Faculty of Education, University of Nigde, Nigde, Turkey Faculty of Science and Arts, Department of Chemistry, C¸ukurova University, Balcali, Adana, Turkey Received 9 June 2007; received in revised form 9 August 2007; accepted 25 August 2007 Available online 6 September 2007

Abstract In this study, the Schiff base monomers [(M1; N,N0 -p-phenylenebis(salicylideneimine)) and ethylenediamine (M2; N, N -p-ethylenebis(salicylideneimine))] were synthesized by the condensation reaction between p-phenylenediamine and aromatic aldehydes. The Schiff base polymers (SBPs) having double azomethine groups were prepared by oxidative polycondensation (OP) reaction of monomers in aqueous alkaline medium with NaOCl [P1; poly-(N,N0 -p-phenylenebis (salicylideneimine)) and P2; poly-(N,N0 -p-ethylenebis(salicylideneimine))] as the oxidant at 90 °C. Average molecular weights of SBP were determined by gel permeation chromatography (GPC). Metal complexes of the SBP were synthesized by the reaction of polymers and metal salts. The monomers and SBP were characterized by elemental analyses, GPC, thermogravimetric analyses, UV–Vis, FT-IR, 1H and 13C NMR spectroscopic studies. Also the new Cu(II), Ni(II) and Co(II) complexes of SBP were prepared and characterized by elemental analyses, UV–Vis, FT-IR, atomic absorption spectroscopy (AAS), thermogravimetric analyses and magnetic susceptibility measurements. The results suggested that the SBP and metal ions in 1:1 molar ratio produced binuclear complexes with oxygen and nitrogen donor atoms. All synthesized complexes have dimeric structures by the polymeric ligand units. The weight losses of P1–Cu, P2–Ni and P1–Co complexes were found as 57%, 60% and 61%, at 900 °C, respectively. Thermal stability of P1 complexes is higher than that of P2 complexes. Magnetic moment studies showed that all complexes have various configurations. The metal ion uptake studies were done by batch technique. The polymer P1 was determined to be more effective in removing Cu(II) ions than the P2 polymer in batch technique. Ó 2007 Elsevier Ltd. All rights reserved. 0

Keywords: Schiff base polymer; Oxidative polymerization; Thermal stability; Metal complexes; Metal ion uptake

1. Introduction

*

Corresponding author. Tel.: +90 3882112826; fax: +90 3882112801. E-mail address: [email protected] (M. Tuncßel).

Schiff base polymers (SBPs) or polyazomethines are synthesized by polycondensation reaction between diamine and a dialdehyde or diketone. The major drawback of the SBP is their limited

1381-5148/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2007.08.012

M. Tuncßel et al. / Reactive & Functional Polymers 68 (2008) 292–306

solubility in most organic solvents due to their rigid chain structure. Some general methods have been used to overcome the limited solubility properties of SBP, for example, insertion of flexible bonds between main chain aromatic rings [1–3]. Due to the reagents used for the reaction process, the oxidative polycondensation (OP) may also be regarded as a good example of environmentally benign or green synthetic process of polymeric materials. SBP and their metal complexes have been prepared because of their interesting and important properties, such as their ability to bind toxic and heavy metal ions, thermal stability, and exhibition of catalyst and photoluminescence properties [4–16]. Synthesis of metal complexes of SBP has received the attention of various groups as the metal ions drastically alter the thermal stability as well as the functional properties of the polymer backbone. Metal complexes of polymers synthesized by the OP have useful properties such as paramagnetism, metal ion uptake, antimicrobial activity and antistatic agent [17–19]. The complex forming polymeric ligands are characterized by reactive functional groups containing O, N and S donor atoms and capable of coordinating to different heavy metal ions that have been usually studied [20]. These materials frequently show selectivity to certain metal ions facilitating their use for preconcentration and separation of trace metal ions from saline and non-saline water samples at an appropriate pH range [21]. It is known that solid phase extraction is an attractive technique based on the use of a sorbent that retains analytes. Incorporation of metal ions in polymers not only affects their physical characteristics like their strength, but also their chemical activity. In addition, they are also used as mechano-chemical systems. Some of the polymer metal complexes obtained from Ni(II), Co(II) and Ti(II) act as efficient heterogeneous catalyst for polymerization of butadiene [22]. Although Schiff base ligands and their corresponding metal complexes have been extensively studied, relatively few polymer metal complexes have been reported. For this reason, in our earlier work, we investigated the preparation and characterization of new SBP [23]. The obtained SBP metal complexes exhibited good thermal stability and metal ion uptake properties. For these reasons, in this paper, we aimed to extend our investigations. Therefore, we will report the synthesis and characterization of new SBP and their metal complexes with a double azomethine group and aromatic backbone in this paper.

293

In this study, we present the preparation and characterization of Cu(II), Ni(II) and Co(II) chelates with new SBP (Fig. 3). The synthesized monomer and polymers have been characterized by 1 H(13C) NMR, FT-IR, UV–Vis spectroscopy, thermal analyses and analytical methods. Elemental analyses, atomic absorption spectroscopy (AAS), FT-IR, thermogravimetric analyses, UV–Vis spectroscopy, potentiometric titration and magnetic susceptibility were used for the characterization of the SBP metal complexes. The metal ion uptake studies of SBP were done by batch technique. The magnetic properties of the SBP metal complexes were determined by the Faraday method. The type of chelation of polymeric ligand and the geometry around the metal center were discussed. 2. Experimental 2.1. Materials and measurements p-Phenylenediamine, ethylenediamine, salicylaldehyde, metal salts (Cu(CH3COO)2  H2O, CoCl2  6H2O, NiCl2  6H2O) and other chemicals were obtained from Merck and Fluka (reagent grade). Reagent grade (Merck) neocuprion and dimethyl glyoxime were used as received. Double distilled water was used for the preparation of the solutions, titrations and metal ion uptake studies. The standard stock solutions were prepared by dissolving an appropriate amount of Cu(II) and Ni(II) salts in DMSO. After acidic decomposition of complexes, the rates of metal ion content were determined by potentiometric titrations. The infrared and UV–Vis spectra were measured by Jasco 300 FT-IR and Shimadzu UV-160, respectively. The FT-IR spectra were recorded using KBr discs (4000–400 cm1). UV–Vis spectra (200– 800 nm) of monomers, SBP and their metal complexes were determined by using DMF and DMSO. Monomers and SBP were characterized using 1H NMR and 13C NMR spectra (Bruker AC FT NMR 400 MHz spectrometer) recorded at 25 °C using deuterated DMSO-d6 as solvent. Elemental analyses (C, N, H and S) were performed using a CHNS-932 (Leco) elemental analyzer. Percentages of the metal ions of the complexes were determined using an Ati-Unicam model-929 atomic absorption spectrophotometer (AAS). DTA, TG and DSC analyses were performed with a Shimadzu DTA-TGA-50 and DSC thermal analyzer. The thermogravimetric measurements were made between 60 °C and

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1000 °C (in nitrogen, rate 10 °C/min). The average molecular weights of polymers were determined by gel permeation chromatography (GPC). The columns were calibrated using different molecular weight polystyrene standards. The flow rate of the THF was maintained at 1 ml throughout the experiments and 1 wt% (10 mg in 1 ml) of polymer solution was filtered and injected for obtaining the GPC chromatograms. Melting points were determined by a Gallenkap model melting point apparatus. Magnetic measurements were carried out at 25 °C using a Sherwood Scientific magnetic susceptibility balance with CuSO4  5H2O as the standard. Magnetic susceptibilities were calculated as Bohr pffiffiffiffiffiffiffiffiffiffiffiffiffi magneton (BM) using the leff ¼ 2:82 X m  T equation. The molar conductivities of the polymers and their complexes were determined in DMSO (103 M) at the room temperature using a Tacussion model conductivity meter. Potentiometric titrations and pH adjustments were carried out in a Selecta model equipped with a combined glass calomel electrode. 2.2. Synthesis of monomers The M1 was synthesized according to the literature by a common condensation method (Fig. 1)

[24]. p-Phenylenediamine (10 mmol) was dissolved in ethanol (50 ml). This mixture was added into the ethanolic solution of salicyladehyde (20 mmol) at 20 °C. After the salicylaldehyde was added the mixture was stirred magnetically and refluxed for 2 h. The solvent was evaporated under reduced pressure and the unreacted phenylenediamine was separated from the mixture by extraction with toluene. The monomer was dried in vacuum desiccator over CaCl2. M2 was synthesized by the same method. Structure of M2 is given in Fig. 2. 2.3. Characterization of M1 Color: yellow, m.p.: 135 °C, yield: 83%; UV–Vis in DMSO (nm): 238, 278, 374; FT-IR (KBr, cm1): 3435 (–OH str), 1641 (–CH@N– str), 2850 (Hydrogen bonded –OH), 1586 (C@C aromatic), 1286 (phenolic C–O bending), 1042–984 (CH, aromatic), 862–759 (substituted benzene rings); 1H NMR (DMSO, d, ppm): 6.9–7.7 (m, Ar-H), 13.1 (s, o-OH), 9.0 (CH@N, azomethine H); 13C NMR (DMSO, ppm): 117, 119, 123, 132, 133 and 146 (aromatic ring), 164 (azomethine carbon). Elemental analyses (%), calculated for C20H16N2O2 (Mw = 316.3): C, 75.9; N, 8.9; H, 5.1. Found: C, 75.5; N, 9.0; H, 5.0%. H3CO

OH

HC

CH

OP

OP

OCH3

CH N

O

OCH3

NaOCl KOH, 90 oC

NaOCl KOH, 90 oC H3CO

OH

HC

N N

N HO

OH

CH N O

HO

N

HO

OH

*

OCH3

N

N

N

HO

H3CO

HC N

O

N

O

N

n

Fig. 1. Synthesis and the proposed structure of P1, structure of M1.

n

Fig. 2. Synthesis of P2 and proposed structures of P2 and M1.

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2.4. Characterization of M2 Color: yellow, m.p.: 121 °C, yield: 80%; UV–Vis in DMSO (nm): 235, 271, 368; FT-IR (KBr, cm1): 3442 (–OH str), 1631 (–CH@N– str), 2870 (Hydrogen bonded –OH), 1584 (C@C aromatic), 1286 (phenolic C–O bending), 1035–991 (CH, aromatic), 851–766 (substituted benzene rings); 1H NMR (DMSO, d, ppm): 6.8–7.3 (m, Ar-H), 13.1 (s, o-OH), 8.4 (CH@N, azomethine H); 13C NMR (DMSO, ppm): 117, 119, 132, 133 and 162 (aromatic ring), 167 (azomethine carbon). Elemental analyses (%), calculated for C16H16N2O2 (Mw = 268.3): C, 71.6; N, 10.4; H, 6.0. Found: C, 71.9; N, 10.2; H, 6.1%. 2.5. Synthesis of SBP by OP SBP was synthesized through the oxidative polycondensation of monomers in aqueous solutions with NaOCl (Figs. 1 and 2). The Schiff base M1 (0.015 mol) was dissolved in an aqueous solution of KOH (10%, 0.015 mol) and placed in a 50 ml three-necked round-bottomed flask. It was fitted with a condenser, a thermometer and a stirrer, in addition to a funnel containing NaOCl. After heating to 65 °C, NaOCl (0.015 mol) was added dropwise for about 30 min. The reaction mixture was stirred at 90 °C for 10 h. The mixture was neutral-

295

ized with 0.015 mol diluted HCl (37%) at room temperature. Then, the unreacted monomer was extracted with CHCl3, filtered and dried in a vacuum oven at 105 °C. The crude product was then separated by column chromatography on efficient supports which were separately chosen for polymers. The product was then dissolved in THF and passed through a silica gel column with a mixture of acetone and toluene as eluent. The other polymer (P2) was synthesized by the same method. According to spectroscopic and thermal analyses, whereas the similar molecular weights were observed for P1 and P2 polymers, different thermal and metal ion uptake behaviors were observed for these polymers. The decomposition points, FT-IR, UV–Vis, GPC and analytical data of the SBP are given in Tables 1–4. 2.6. Synthesis of polymer metal complexes Polymer metal complexes were synthesized by the addition of the appropriate metal salts (1 mmol, in 25 ml absolute EtOH) to a hot solution of the polymers (1 mmol, in 30 ml THF). The pH was adjusted to 7.0 using alcoholic ammonia (0.01 M). The resulting solutions were stirred and heated in a water bath at 80 °C for 12 h. The resulting product was collected by filtration, washed with EtOH, diethyl ether and hot water, and finally dried under vacuum at

Table 1 Some analytical data and physical properties of the SBP and their complexes D.p. (°C)a

Compounds

Yield (%)

C P1 (C20H16N2O2)n [Cu2(C20H16N2O2)2]  2.9H2O [Ni2(C20H16N2O2)2]  8H2O [Co2(C20H16N2O2)2]  2.5H2O P2 (C16H14N2O2)n [Cu2(C16H14N2O2)2]  1.5H2O [Ni2(C16H14N2O2)2]  H2O [Co2(C16H14N2O2)2]  1.8H2O a

235 268 265 261 233 252 256 268

60 54 48 41 55 56 42 45

KM (X1 cm2 mol1)

Found (calculated) (%)

75.3 62.8 64.6 63.6 71.3 57.3 58.0 58.3

H (75.9) (63.2) (64.1) (64.1) (71.9) (57.9) (58.7) (58.7)

4.8 4.4 4.3 4.5 5.2 4.8 4.9 5.0

N (5.1) (4.0) (4.0) (4.0) (5.1) (4.5) (4.6) (4.6)

8.5 6.8 7.1 7.0 8.9 8.8 8.0 8.2

Metal (8.8) (7.4) (7.5) (7.5) (8.4) (8.4) (8.6) (8.6)

– 16.1(16.7) 15.1 (15.7) 15.6(15.7) – 22.9 (23.7) 21.1 (21.9) 21.6 (22.0)

6 4 4 5 8 3 5 3

Decomposition point.

Table 2 Reaction conditions, GPC molecular weights and Tg of polymers Polymer

Time (h)

Yield (%)

Mn (g mol1)

Mw (g mol1)

Mw/Mna

Tgb (°C)

P1 P2

20 14

60 55

42,000 48,000

40,000 39,000

1.01 1.16

270 225

a b

Determined by GPC using polystyrene standards as reference in THF. Determined by DSC at 5 °C/min heating range.

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Table 3 UV–Vis and mass spectral data of the free ligands and their complexes and magnetic moments of the metal complexes Compound

leff (BM)a

Kmax/nm (emax/M1 cm1)

P1 (C20H16N2O2)n [Cu2(C20H16N2O2)2]  2.9H2O [Ni2(C20H16N2O2)2]  8H2O [Co2(C20H16N2O2)2]  2.5H2O P2 (C16H14N2O2)n [Cu2(C16H14N2O2)2]  1.5H2O [Ni2(C16H14N2O2)2]  H2O [Co2(C16H14N2O2)2]  1.8H2O

– 1.63 3.11 3.71 – 1.71 3.18 4.18

278 261 264 266 270 262 261 268

a

(81,000), (83,200), (81,800), (85,600), (74,600), (74,400), (68,400), (70,500),

360 436 440 438 340 399 405 412

(2400), 390 (8520), 410 (41,200) (44,100), 386 (32,900), 610 (968) (38,500), 385 (23,500), 614 (834) (35,700), 396 (26,400), 695 (885) (4130), 405 (38,700) (36,350), 636 (880), 435 (795) (34,650), 618 (884), 430 (795) (36,400), 720 (752), 433 (962)

Per metal ion at room temperature.

Table 4 Infrared spectral data of the SBP and their complexes (cm1)a Compound

m(HO)b

m(HO)b

m(CH@N)

m(C–O–C)

m(–C@C–)

m(M–N)

m(M–O)

P1 (C20H16N2O2)n [Cu2(C20H16N2O2)2]  2.9H2O [Ni2(C20H16N2O2)2]  8H2O [Co2(C20H16N2O2)2]  2.5H2O P2 (C16H14N2O2)n [Cu2(C16H14N2O2)2]  1.5H2O [Ni2(C16H14N2O2)2]  H2O [Co2(C16H14N2O2)2]  1.8H2O

3320 3130 3250 3390 3390 3360 3340 3250

– 870 840 820 – 840 865 870

1630 1590 1620 1610 1615 1610 1600 1605

1330 1310 1280 1250 1320 1280 1350 1300

1520 1505 1490 1500 1505 1495 1490 1500

– 515 m 570 m 590 w – 530 w 575 w 520w

– 460 – – – 455 450 460

a b

br br br br br br br br

w w w w w w

s s s s s s s s

m m m m m m m w

s s m m s s m w

m

m m w

Keys: sh (shoulder), m (medium), s (strong), w (weak), br (broad). Hydrated water.

90 °C. All metal complexes of SBP were prepared by the same method and isolated as powdered material. The purity of all complexes was evaluated by thin layer chromatography. The complexes were then dissolved in DMSO and passed through TLC paper with acetone as eluent. The geometry of the complexes was confirmed by magnetic moment measurements and absorption spectra. Elemental analyses, thermogravimetric analysis, FT-IR, UV–Vis, magnetic measurements and AAS confirmed the compositions of the complexes. Analytical and spectral data of all complexes are given in Tables 1 and 4, respectively. Analytical data, FT-IR and TG indicate that metal complexes have coordinated and uncoordinated water molecules. The decomposition points, FT-IR, UV–Vis, analytical and magnetic moment data of the SBP metal complexes are given in Tables 1, 3 and 4. 2.7. Metal ion uptake studies The metal ion uptake studies were done by batch technique. In the batch technique, a suspension of the polymer on the metal solution of known volume and concentration was agitated for a 12-h period over hot plate/magnetic stirrer. The pH of solutions

was adjusted using standard buffers (4, 7 and 9). The dried SBP was powdered, sieved (100 mesh, ASTM) and suspended over the water at pH 4 for one day. The SBP was filtered off, and washed two times with distilled water. The metal ion concentration in the filtrate and the washing collected was estimated following the neocuprion method for Cu(II), and the dimethyl glyoxime method [25] for Ni(II) from which the percentage of metal ion uptake by the SBP and distribution coefficient values (Kad) were calculated. Percentage of Co(II) ions of complexes was determined by potentiometric titration after acidic decomposition. 2.8. Crystallization We were not able to obtain single crystals of metal complexes suitable for X-ray diffraction studies; however, useful information regarding the structures of polymer metal complexes was derived from spectroscopic and magnetic measurements. 3. Results and discussion SBP interacts with metal ions to form binuclear complexes and their suggested structures are shown

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in Fig. 3. The decomposition points, yields, analytical and magnetic moment data of the complexes are given in Tables 1 and 3. The polymers P1 and P2 with Cu(II), Co(II) and Ni(II) salts were reacted to yield the complexes. All metal complexes of SBP are black colored compounds. Elemental analyses and AAS showed that the metal:ligand ratio of the polymer metal complexes is 1:1. According to the FT-IR, potentiometric titration, 13C NMR and 1H NMR data, while polymerization of P1 occurs dominantly in the form of C–O–C (77%) and C–C (23%) type coupling, polymerization of P2 occurs with both C–O–C (55%) and C–C (45%) type couplings. Polymers containing C–O–C type coupling have two donor nitrogen atoms whereas polymers containing C–C type coupling have two donor nitrogen atoms and two oxygen atoms. These groups are known as suitable electron donors to prepare polymer metal complexes. Therefore, the coordination may be constructed as bidentate (C–O–C type, two nitrogen donors) or tetradentate (C–C type, two nitrogen and two oxygen donors) for every polymeric ligand unit (see Fig. 3). As a result, P1 and P2 may bind metal ions such as Cu(II), Ni(II)

H3CO

O

O

297

and Co(II) to form a variety of complex structures. The special conformation of the polymeric ligand units with nitrogen atoms in para position on the aromatic ring facilitates the formation of dimer complexes since the polymeric ligand units act like a bridge. SBP metal complexes have the satisfactory analytical data and the instrumental studies suggest that the complexes are of the general formula [(M)n(Sal-p-phen)n]  xH2O or [(M)n(sal-ethyldiam)n]  xH2O (x = between 1 and 8) where M is copper(II), nickel(II) or cobalt(II). The solubility properties of polymer metal complexes were completely different from that of the corresponding SBP. The obtained SBPs were soluble in several polar solvents such as DMF, DMSO, THF and chloroform. However, their solubilities in nonpolar solvents (benzene, toluene) and hydroxylated solvents (water, methanol, etc.) were almost negligible. Polymer metal complexes were insoluble in ethanol, 1,4-dioxane, chloroform and acetone, whereas complexes are insoluble in DMSO and DMF. Conductivity of solutions of the complexes in DMSO (103 M) is shown in Table 1. All polymers and complexes are non-electrolytes because their conductivity values ranged from 3 to 8 X1 cm2 mol1.

OCH3

3.1. Molecular weight determination N N M

M

N N H3CO

O

O

OCH3

Bidentate Structure

N N H3CO H3CO

M O

O M

O

OCH3 O

OCH3

N N

Tetradentate Structure

Fig. 3. Bidentate and tetradentate structure of P2 complexes.

The number average molecular weight (Mn) and weight average molecular weight (Mw) of the polymers (P1 and P2) were determined by Gel Permeation Chromatography (GPC) using polystyrene standards (PS). GPC data are given in Table 2. The molecular weights of SBP are given in Table 2. The Mn and Mw values of P1 and P2 were found to be 42,000, 48,000 g mol1 and 40,000, 39,000 g mol1, respectively. The formation of high molecular weights of polymers suggests that the OP conditions used for the synthesis are highly effective. The Mn, Mw and PS values of P2 are higher than those of P1. The chromatogram of SBP showed a bimodal distribution for solution of polymers. These results corroborate the assumptions made on the basis of 1H NMR spectra. 3.2. Electronic spectra The electronic spectra provide more information about the electronic structure of the SBP and their metal complexes. The electronic spectra of SBP

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and their metal complexes were measured in DMSO and DMF at room temperature. The wavelength of maximum absorbance and molar extinction coefficients are reported in Table 3. The energy of the p–p* transition of the chelate ring monomer is characteristic of Schiff bases which showed delocalized systems with intramolecular hydrogen bonding [26]. The existence of a n–p* transition in the monomer of the electronic spectra indicates that the monomer molecule has a planar conformation in solution, with no rotation of the chelate ring around the bonding imine and hydroxyl group. The UV–Vis spectra of the complexes provide additional evidence for the transformations that come from complexation when compared to those of the SBP. Clear differences are observed from the electronic absorption spectra of SBP and their metal complexes. The bathochromic shifts of the UV–Vis spectra are observed in all metal complexes. The absorption peak of SBP at around 410 nm slightly shifted to longer wavelength in P1 and P2 metal complexes. This means that the length of pelectron conjugation is changed by complexation. The bands at 352–388 nm were assigned to the imine p–p* transition in the SBP. The imine p–p* transitions of the metal complexes were red-shifted to some extent since the imine nitrogen was coordinated to the metal ion. These complexes showed broad and intense visible bands between 360 and 460 nm due to the binding of metal to the polymeric ligand units. All synthesized SBP and their metal complexes containing imino groups and aromatic rings exhibited dye character since the molar extinction coefficients (e) were over 40,000 m1 cm1 (maximum absorption 0.50) [27]. The electronic spectra of all complexes show two major bands, which are more intense than the corresponding SBP in the region of 420–500 nm, which could be assigned to the charge transfer transitions. Therefore, we assumed that the SBP is coordinated to the metal ion through the oxygen and the imine nitrogen. Also, electronic spectra of all the complexes were characterized by strong-intensity absorptions with shoulders around 580–670 nm, which can be associated to d–d transitions [28]. The UV–Vis spectra of the copper complex of P1 showed two bands in the regions 590 and 472 nm which may be assigned to the 2B1g and 2Eg and 2 B1g and 2Eg transitions, respectively, corresponding to the pseudo-tetrahedral geometry around the copper ions. However, the UV–Vis spectra of the copper complex of P2 showed a band at 636 nm

corresponding to the square planar geometry around the copper ions. 3.3. Infrared spectra FT-IR spectral data of the SBP and their metal complexes are given in Table 4. FT-IR spectra of SBP and their metal complexes are shown in Figs. 4 and 5. The shift and presence or absence of certain bands in the polymer spectra gives an idea about the nature of polymerization. The infrared spectra of the Schiff base monomer displayed a shoulder of a broad band of weak intensity around 2730–2988 cm1. This band was ascribed to the O–H stretching vibration, which is known to shift significantly to lower frequencies because of OH  N intramolecular hydrogen bonding [29]. M1 showed a peak in the 3260–3450 cm1 range corresponding to the phenolic OH and crystal water molecules. This band slimmed and shifted in the polymers due to polymerization by the oxygen. The infrared spectrum of P2 showed bands at 1215 cm1 and 1230 cm1 assignable to the phenolic C–O vibration. However, in the spectrum of P1, the C–O band shifted to the higher region because of the polymerization via oxygen atom. The strong bands at around 1500 cm1 are due to the C@C stretching of the aromatic ring. The weak bands at 980 cm1 correspond to CH deformations. The bands in the 1610–1637 cm1 range were assigned to the –CH@N– group in monomers and polymers. The imine peak in the metal complexes showed red-shifts of ca. 10–25 cm1 compared to the polymers, indicating coordination of the imine nitrogen to the metal ions [30]. This feature can be explained by the withdrawing of electrons from the nitrogen atom to the metal ion due to coordination. The infrared spectra of all the metal complexes exhibited broad bands in the 3365–3442 cm1 range that are attributed to OH group of the crystal water molecules [22]. The C–O–Cu frequency of the coordinated phenolate group of P1 was detected as a strong band at 1281 cm1 shifted +26 cm1 from free polymer (Fig. 4). This feature can prove the involvement of the phenolate oxygen atom in the coordination. These similar frequencies correspond to those observed in Ni and Co complexes. The most obvious transformations after the complexation reactions were found in the 500–600 cm1 range. In the metal complex spectra, there were increased absorptions at this range. More signals were observed for metal complexes than that of

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299

Fig. 4. IR spectrum of M1, P1 and P1 metal complexes.

corresponding SBP. On the other hand, in the infrared spectra of the complexes new absorption bands at 515–590 cm1 and 450–460 cm1 were observed, which indicated the M–N and M–O bonds, respectively [31]. Infrared spectra of the P1–Ni and P1– Co complexes did not show any frequency of the M–O band, which may be explained by their low participation in complex formation via oxygen atoms because of polymerization of the P1 dominant C–O–C coupling types. 3.4. 1H and

13

C NMR spectra

The 1H NMR spectra of the monomers and polymers were recorded by using DMSO-d6 as the solvent. As a result of the 1H and 13C NMR spectral studies, important clues concerning the geometry of SBP were obtained. 1H NMR spectra of monomers and SBP are shown in Figs. 6 and 7.

The 1H NMR spectrum of M1 (Fig. 6) shows sharp peaks in the 6.9–7.7, 9.0 ppm range and at 13.1 ppm. These peaks can be ascribed to the aromatic ring, azomethine (–CH@N–) and hydrogen bonded azomethine nitrogen with –OH protons of monomer, respectively. The peak at 3.4 ppm is assigned to the H2O impurity in the DMSO-d6 reagent. According to the literature, the P1 obtained by solution polycondensation exhibited signals in the 8.1–8.8 ppm range due to the azomethine proton with broadening [32]. The 1H NMR spectrum of P1 (Fig. 6) also showed the broad signal at 8.8 ppm corresponding to the azomethine proton, the multiplet broad signal in the 6.6–7.8 ppm range indicating the aromatic protons. The disappearance of ohydroxyl proton of P1 can be attributed to the dominant participation of polymerization reaction via hydroxyl groups. The broadening of the peaks in the spectrum is attributed to the polymerization.

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Fig. 5. IR spectrum of M2, P2 and P2 metal complexes.

This indicates that aromatic ring protons participate in the coupling reaction at low rates. In the 1H NMR spectrum of M1, the integral ratio of the aromatic ring protons to azomethine proton is about 4.3, whereas it is 3.3 for the polymer, indicating that aromatic ring protons participate in the coupling reaction at low rates. In addition, the integral ratio of the azomethine protons to hydroxyl protons for monomer and polymer is 1.18 and 4.91, respectively. This indicates that hydroxyl group participates in the polymerization reaction at the highest rate. The detailed analysis of 1H NMR spectral data suggests that there are 77% C–O–C (oxyphenylene) and 23% C–C-type coupling types in the polymer (P1) structure. Also, coupling types and rates of polymers were determined by the potentiometric titration. The 1H NMR spectrum of P2 showed a singlet at 3.6 ppm that was assigned to the protons of the

methoxy group, and multiplets in the 6.5–7.8 ppm range were assigned to the aromatic protons. The other peak at 3.4 ppm is assigned to the H2O impurity in the DMSO-d6 reagent. The sharp singlet at 8.3 ppm was attributed to the proton of the azomethine group. The signal of the OH proton in the vanillin ring was observed at 10.5 ppm because of intramolecular hydrogen bonding with the nitrogen atom on the ethylene moiety (Fig. 6). In the 1H NMR spectrum of M2, the integral ratio of the aromatic ring protons to the azomethine proton is about 4.10, whereas it is 2.42 for P2, indicating that aromatic ring protons participate in the coupling reaction. In addition, the integral ratios of the azomethine protons to hydroxyl protons for M2 and P2 are 1.24 and 2.80, respectively. This indicates that a hydroxyl group also participates in the polymerization reaction. The detailed analysis of 1H

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Fig. 6. 1H NMR spectrum of M1 and P1.

NMR spectral data suggests that there are 55% C– O–C (oxyphenylene) and 45% C–C-type coupling types in the polymer (P2) structure. Also, coupling types and rates of polymers were determined by the potentiometric titration. The 13C NMR spectrum of M1 showed signals at 119, 122, 128, 133, 134, 151, 156, 164 and 167 ppm. A signal at 167 ppm is assigned to the azomethine carbon atom of M1. The signal at 164 ppm is attributed to hydroxyl group carbon atom. The 13C NMR spectrum of P1 indicated broad signals at 120, 123, 125, 130 and 152 ppm, respectively. The chemical shift of carbon that is bonded to hydroxyl group is 164 ppm for M1, whereas it is 152 for P1, indicating that hydroxyl oxygens participate in the polymerization reaction. 13 C NMR spectrum of P2 showed peaks different from that of the corresponding monomer. The 13C NMR spectrum of P2 showed signals at 118, 121, 126, 132, 133, 150 and 165 ppm. A signal at 165 ppm is assigned to the azomethine carbon atom of P2. The chemical shift of carbon that is bonded to hydroxyl group is 164 ppm for M2, whereas it is 150 for P2, indicating that hydroxyl oxygens participate in the polymerization reaction. These similarities of 13 C NMR spectra of M2 and P2 can be attributed to

the protection of all structures of monomers during the OP reaction except hydroxyl groups. According to the FT-IR, 13C NMR and 1H NMR data, polymerization of P2 occurs in the form of C–O–C type coupling and C–C type coupling. The proposed structure of polymer is given in Fig. 2. 3.5. Magnetic moments The magnetic moments of the polymer metal chelates were measured by the Gouy method and the diamagnetic corrections were made using Pascal’s constants. The square planar (d8) systems exhibit zero magnetic moment while the octahedral complexes can exhibit values as high as 3.5 BM. However, in the case of polynuclear systems the magnetic moment values do not have a fixed range of values, which is characteristic of a particular geometry. It has been reported [33] that the exchange interactions arising from the intermetallic bond and through ligands result in intermediate values which do not indicate a specific geometry type for the polynuclear systems. The magnetic moments values of synthesized polymer metal complexes are given in Table 3. It was determined that all complexes were paramag-

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Fig. 7. 1H NMR spectrum of M2 and P2.

netic in character. The magnetic moments of the copper complex of P1 are 1.63 BM at room temperature. These values can be assigned to one unpaired electron and offer evidence for the square planar geometry of copper complexes. Also, P2–Cu complex has a magnetic moment of 1.71 BM indicating a square planar configuration. The magnetic moments of the P1–Ni and P2–Ni complexes were calculated to have 3.11 and 3.18 BM ranges, respectively, at room temperature, corresponding to two unpaired electrons. These values are attributed to distorted octahedral nickel complexes [34]. The paramagnetic behavior of the P1–Co(II) complex and magnetic moment of 3.71 BM suggest that Co(II) complexes exhibit distorted octahedral geometry. P2–Co(II) complex was found to have 4.18 BM at room temperature. This value suggests a distorted planar geometry for this complex. Prob-

ably the coordination sphere is completed by the coordination of Cl ions to the metal ions. Infrared spectra and thermogravimetric studies showed the presence of crystal water molecules. The magnetic susceptibility measurements showed the existence of weak antiferromagnetic interactions for SBP with Cu(II), Ni(II) and Co(II). Molecular models indicate that there are no severe steric strains as a result of the proposed geometries for the complexes. 3.6. Metal ion uptake studies Metal ion uptake data of the polymers are given in Table 5. The saturation time was obtained by plotting the percentage of metal ion uptake versus contact time, keeping initial metal ion concentration fixed (2500 lg per 20 ml). The effect of the metal ion concentration on the uptake behavior of the SBP

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Table 5 Effect of pH on the adsorption behavior of the polymers for Cu(II), Ni(II) and Co(II) in batch technique Polymer

P1

P2

a

Metal ion

Cu(II) Ni(II) Co(II) Cu(II) Ni(II) Co(II)

Metal ion uptake (%) at different pHa 3.5

4.0

5.0

5.6

6.5

7.0

8.1

12.1 3.5 4.7 14.4 8.1 5.8

14.5 6.7 8.8 18.5 17.3 11.3

35.6 8.8 12.4 41.8 24.6 18.2

55.7 21.6 16.5 59.3 26.9 22.7

65.4 60.8 20.7 75.5 64.5 36.1

68.1 68.1 29.4 74.2 66.8 44.1

30.4 22.1 15.4 36.2 25.8 19.1

Metal ion concentration, 2500 mg per 20 ml; polymer quantity, polymer size, 100 mesh; temperature 25 °C; contact time, 12 h.

was studied at the concentration range (500–2500 lg per 20 ml) of metal ions. In our work, the optimum pH of the adsorption of Cu(II), Ni(II) and Co(II) ions were 5.6, 6.5 and 7.0, respectively. A high concentration of metal ion enhanced the percentage of loading. However, a leveling effect was not observed at higher concentration because of the available saturation coordination sites. In both polymers, the rate of Cu(II) adsorption is higher than that of Ni(II) and Co(II). On the other hand, the metal ion uptake percentage of P1 is higher than that of P2. The reason could be attributed to the fact that in an ethylene diamine condensed polymer, the presence of a benzene ring in the C–O–C moiety presumably led to an efficient molecular packing for Cu(II). Also the presence of hydroxyl groups in P1 was responsible for a higher uptake percentage of metal ion. However, in the case of P2, the reason for low concentration metal ion uptake of P2 could be the connection of C–O–C groups leading to a rigid polymeric backbone. Water insoluble SBP uptake of transition metal ions was slow due to the lower activity of the ligands placed inside the polymer chain. The metal complexing nature of the SBP depends not only an electron donor atoms of the ligand groups, but also their accessibility to the metal ions. Hence, steric hindrance by the polymeric matrix and the hydrophobic nature of the polymeric ligand units can limit the chelating reaction. Synthesized SBP could be used conveniently in heavy metal chelation for environmental applications. The high stability of P1 and P2 towards Cu(II) is remarkable, the fact that it may be of technological interest. 3.7. Thermal studies The thermal behavior of the SBPs and their metal complexes was studied by using thermogravimetric

techniques in the range of 30–915 °C in a nitrogen atmosphere. The TGA curves of SBP and metal complexes are given in Figs. 8–10. The results obtained from the thermogravimetric analyses indicated that the decomposition of the SBP proceeds in two steps. The mass loss of P1 in the range of 20– 110 °C is due to the water molecules in the structure. After the removal of hydrated water molecules, the weight loss of P1 began at 200 °C, which is lower than 300 °C for M1. An interesting observation is that the first decomposition point of the SBP has a lower stability than the corresponding monomers. The behavior indicates that polymers in the present study, which provide C–O–C type coupling, exhibit frail chemical C–O–C bonds to the phenyl rings. This behavior is derived from the conjugated structure of the main chain. The samples were heated up to 280 °C at 10 °C/ min to record the thermal transitions. M2 was completely decomposed at 370 °C whereas M1 was completely decomposed at 330 °C. 50% weight losses of M2 and P2 were observed at 350 °C and 640 °C, whereas 50% weight losses of M1 and P1 were observed at 320 °C and 540 °C, respectively. The first decomposition temperatures of P1 and P2 were observed at 235 °C and 270 °C, respectively. The DSC heating curve of P2 showed an endotherm at 270 and 360 °C followed by the decomposition of the polymer. This may be due to a phase transition. The glass transition temperatures of P1 and P2 were observed at 270 °C and 225 °C, respectively. The melting point of SBP was not observed. However, as expected, SBP had greater thermal stabilities at higher temperatures compared to the parent monomer. It is probably due to the formation of the C–C coupling and in the polymer backbone. The studied SBP metal complexes, after the elimination of the crystal water molecules at the range of 30–110 °C, decompose in two or three stages. P1–Ni

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Fig. 8. TG curves of monomers and polymers.

Fig. 9. TG curves P1 and P1 metal complexes.

complex has the maximum water content of the all complexes. The first decomposition of P1–Ni complex started at 300 °C after the removal of water molecules. The decomposition was completed at 900 °C for all complexes. The initial decomposition

temperature (IDT) for P1 begins at 235 °C which is nearly 40–60 °C less than that of metal complexes. 50% weight loss of P1 complexes is nearly 100– 150 °C higher than that of P1. This can be explained by the higher thermal stability of metal complexes

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Fig. 10. TG curves P2 and P2 metal complexes.

than the corresponding SBP. P2–Cu complex had 39% carbon and metal oxide residue whereas P1– Cu complex had 43% carbon and metal oxide residue at 900 °C. The copper complexes have the maximum thermal stability in each system, which is closely followed by the Ni(II) polymer complexes. P1 metal complexes had 50% P1–Cu(II) and 50% carbon residue at 700 °C (Figs. 9 and 10) whereas P2–Cu had 50% carbon residue at 600 °C. Thermal stability of P1–Cu complexes is higher than P2–Cu complexes. The order of stability observed is P1–Cu > P1– Ni > P1–Co. This behavior indicates that SBPs in the present study, which provide NO donor sites, exhibit a greater affinity towards copper than nickel and cobalt do. The temperature corresponding to the different percentage weight losses does not show a uniform variation for different systems. The 50% weight loss occurs in the region 500–700 °C for all the systems except P1–Cu(II), where it is observed at an elevated temperature (650 °C). The final residues in Ni(II), Cu(II) and Co(II) polymer complexes are NiO, CuO and Co3O4, respectively. On the other hand, thermal stability of all complexes at higher temperatures (500–900 °C) is higher than corresponding SBPs. This stability can be explained by the insertion of metal ions into the polymer molecule. At 900 °C, all SBP-complexes

lose about 60% of their weight. Weight losses of P1–Cu, P1–Ni and P1–Co compounds were found to be 57%, 60% and 61%, respectively, at 900 °C. According to the DTA, Tmax values of P1–Cu, P1– Ni and P1–Co complexes were observed at 700 °C, 500 °C and 670 °C, respectively. The SBP metal complexes derived from p-phenylenediamine have higher thermal stability than the SBP metal complexes derived from ethylenediamine. The results corroborate some of the assumptions made on the basis of FT-IR and elemental analyses. The residues of SBPs and their metal complexes are supposed to be graphite-like polymer materials. Therefore, it is important to select suitable materials having the appropriate thermal shock resistance. This polymer residue may be used as a candidate material for the thermally resistant materials. For this reason the properties of SBP and their metal complexes may be useful for graphite materials technologies. It was found that metal complexes of SBP containing double azomethine groups possess thermal stability and can be used for technological applications. 4. Conclusion We prepared polymers (P1 and P2) containing double azomethine groups via oxidative polycon-

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densation reactions with Schiff base monomers (M1 and M2) with NaClO at 90 °C. Metal complexes of the SBP were synthesized by the reaction of polymers (P1 and P2) with metal salts. According to the FT-IR, 13C NMR and 1H NMR data, while polymerization of P1 occurs dominantly in the form of C–O–C (77%) and C–C (23%) type coupling, polymerization of P2 occurs with both C–O–C (55%) and C–C (45%) type couplings. These groups are known as suitable electron donors to prepare polymer metal complexes. The synthesized polymers were soluble in common solvents such as THF, DMF, and DMSO. These properties of the SBP are promising for their environmental and analytical usage. The satisfactory analytical data and the instrumental studies suggest that the complexes are of the general formula [(M)n(Sal-p-phen)n]  xH2O or [(M)n(sal-ethyldiam)n]  xH2O (x = between 1 and 8) where M is copper(II), nickel(II) or cobalt(II). Magnetic moment studies showed that all complexes have various configurations. Some important information was observed in this study. At first, all synthesized compounds have dye character because of the molar extinction coefficients (e) over 40,000 m1 cm1 (maximum absorption 0.50). Secondly, all complexes are thermally stable at high temperatures. An interesting observation is that the metal complexes have a higher stability than that of corresponding SBP. The introduction of metal ion coordination in the polymer backbone results in an increase in thermal stability. It was found that metal complexes of SBP possess high thermal stability and can be used for technological applications. At last, polymers are highly effective for metal ion extraction from waters. These properties of such complexes are important for their technological and environmental usage. Acknowledgement The authors thank to The Scientific and Technological Research Council for financial support (Project Number: TBAG 13571). References [1] M. Grigoras, C.O. Catanescu, J. Macromol. Sci. C 44 (2004) 131. [2] M.Y. Khuhawar, A.H. Channar, S.W. Shah, Eur. Polym. J. 34 (1997) 133.

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