Polymer-anchoring Schiff base ligands and their metal complexes: Investigation of their electrochemical, photoluminescence, thermal and catalytic properties

1 Introduction

The oxidation of the organic compounds is one of the most important cycle reactions in industrial chemistry. The immobilization of the homogeneous catalysts onto the solid supports supplies potential for enlarging the utilities of the heterogeneous catalysts to the homogeneous systems. In addition to inorganic supports (Das and Clark, 2000; Kockritz et al., 2006), the polymeric supports have attained care because they are inert, nontoxic, nonvolatile, insoluble, and often recyclable. Chloromethylated polystyrene crosslinked with divinylbenzene is one of the most extensively used supports. Disadvantages of its use cover the possible leaching of the active metal and a decrease in the activity onto recycling. The leaching problem can be solved by using chelating ligands (Antony et al., 2000; Wang et al., 2000). Ruthenium- and cobalt-immobilized complexes have the advantage of better distanciation from the product (Lei, 2000). Several polymer-anchored Schiff base catalysts have been reported (De et al., 1994). For the oxidants, many types have been used, such as molecular oxygen (Gauli et al., 2005), iodosylbenzene (Che et al., 1987) and oxone (Khan et al., 1991).

The polymer-anchored coordination complexes have large-scale interest due to their extensive application field. Transition metal complexes on polymer support have indicated various uses in organic synthesis (Roice et al., 2000; Ficht et al., 2001), curing agent for epoxy resin (Otaigbe et al., 1998), as catalyst (Maurya et al., 2008), as ion exchanger (Phillips and Fritz, 1982), etc. This technique of immobilization on an inert support has ground much care due to their simple separation from the reaction mixture arriving to operational flexibility, selectivity, efficiency, stability and ease of handling and economy in various industrial processes. Insoluble polymer supports are more typically used as inert support for immobilizing the transition metal over cross-linked chloromethylated polystyrene (Merrifield, 1963), poly(hydroxylethylmethacrylate), poly(glycidylmethacrylate) (Sasaki and Matsunaga, 1968) and silica (Liu et al., 2001). Among organic polymers polystyrene has been prevalently used as support within a wide variety of functional groups coupled in it to bind the metal into the polymer. Cross-linked polystyrene with specific properties is rather 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 and alkenes (Patel et al., 2003; Antony et al., 2000), oxidation of aromatic alcohols (Valodkar et al., 2004), hydrogenation of alkenes (Takahashi et al., 2005), reduction of ketones and nitriles (Valodkar et al., 2003), etc.

In this study, we prepared three Schiff base ligands H₂L¹–H₂L³ from the reaction of the 2,4-dihydroxybenzaldehyde with the 1,4-diaminobutane, 1,4-diaminobenzene and trans-1,4-diaminocyclohexane. We obtained the polymer supported Schiff base ligands from the reactions of the Schiff bases and Merrifield’s peptide resin. Then, we obtained their Co(II), Cu(II) and Ni(II) complexes. All compounds were characterized by the spectroscopic and analytical methods. We did alkane oxidation and alkene epoxidation reactions of the polymer transition metal complexes. The morphologies of the polymer supported ligands and their metal complexes were investigated by the scanning electron microscopy.

2 Experimental

3 Results and discussions

In this study, the Schiff base ligands were synthesized from the reaction of the diamines 1,4-diamino butane, 1,4-diaminobenzene and trans-1,4-diaminocyclohexane with 1,4-dihydroxy benzaldehyde in the ethanol solution. The proposed structures of the Schiff base ligands and their polymer-anchored derivatives are given in Fig. 1. The imines prepared are formed in nearly quantitative yields and are of high purity. All compounds are very stable at room temperature in the solid state. The yields of the complexes (Fig. 2) are lower than those of the ligands. Further stirring and heating did not increase the yield of the complexes containing polymer-anchoring groups. The low yields may be due to the steric hindrance around the coordination center. All the ligands are soluble in common organic solvents such as CHCl₃, EtOH, MeOH, THF. The results of the elemental analyses are in consistent with the composition suggested for the ligands and their metal complexes.

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Figure 1

Proposed structures of the synthesized Schiff base ligands.

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Figure 2

The proposed structures of the metal complexes.

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In order to characterize of the Schiff base ligands, as different from the analytical methods, the spectroscopic techniques were also used. The ¹H(¹³C)-nmr, FTIR, electronic and mass spectral data are given in the experimental section. The ¹H NMR and ¹³C NMR spectra of the H₂L¹ and H₂L³ Schiff base ligands are shown in Fig. 2. The OH groups on the salicylidene moiety increase the electron density of the aromatic rings, due to the mesomeric effect. In Schiff base ligands H₂L¹–H₂L³, there is the proton donor OH group and one proton acceptor group in the ortho position. Due to the presence of the OH groups in ortho position to imine group, formation of a few intramolecular hydrogen bonds is possible. It is important to emphasize the ¹H resonance of the O—H group in the 13.89–13.97 ppm range. The signal due to the OH proton disappears in D₂O solution. The imine protons of the ligands were shown in the 8.84–8.32 ppm range. The signals in the 6.14–7.45 ppm range may be assigned to the protons of the aromatic rings. The multiplets in the 1.04–3.70 ppm range can be attributed to the protons of the aliphatic CH₂ groups. In the ¹³C NMR spectra of the ligands, the azomethine carbon atoms were shown in the 163.51–165.40 ppm range. The signals in the 102.86–165.17 ppm range can be attributed to the aromatic ring carbon atoms. The aliphatic carbon atoms of the ligands H₂L¹ and H₂L³ were shown in the 28.51–64.31 ppm range.

The formulation of the ligands is deduced from analytical data, ¹H and ¹³C NMR and further supported by mass spectroscopy. Mass spectra of the Schiff base ligands H₂L¹ and H₂L² are shown in Fig. 3. The molecular ion peaks, [M]⁺, of the Schiff base ligands were not shown. In place of the molecular ion peaks of the ligands, the fragmentation peaks [M + 1]⁺ were shown at m/e 329 (100%), 349 (100%) and 355 (100%). In the ligand H₂L¹, the decomposition products at m/e 249 and 250 can be assigned to the [C₁₃H₁₇N₂O₃]⁺ and [C₁₃H₁₈N₂O₃]⁺ ions, respectively. All the ligands decompose in a similar way.

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Figure 3

(a and b) The ¹H NMR (a) and ¹³C NMR spectra (b) of the H₂L³ and H₂L¹ Schiff base ligands, respectively.

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The infrared spectral data of the ligands H₂L¹–H₂L³ and their polymer-anchored Schiff base ligands are given in the experimental section. In the ligands, the bands in the 3500–3250 cm⁻¹ range may be assigned to ν(O—H) stretching. In the spectra of the polymer-anchored Schiff base ligands PS–H₂L¹–PS–H₂L³ (Fig. 4), the ν(O—H) stretching bands were shown in the 3218–3024 cm⁻¹ range. In the spectra of the free Schiff base ligands, the broad bands in the 2800–2700 cm⁻¹ range are assigned to the OH group vibration (ortho position) associated intramolecularly with the nitrogen atom of the CH⚌N group (Dolaz and Tümer, 2004). These bands disappear in the complexes, as a result of proton substitution by cation coordination to oxygen atom. For the ligands, the strong bands observed in the 1643–1633 cm⁻¹ range are assigned to the azomethine group vibration. These bands are slightly shifted toward lower frequencies in the complexes, and this change in the frequencies shows that the imine nitrogen atom coordinated to the Co(II), Cu(II) and Ni(II) ions. The medium intensity bands observed for all ligands in the 1357–1353 cm⁻¹ range can be attributed to the phenolic stretch. These bands are observed for the complexes at lower or higher wave numbers by ca. 10–45 cm⁻¹ relative to the free ligands suggesting involvement of the oxygen atom of the C—O moiety in coordination (Demirelli et al., 2006). In all of the present complexes, a medium and/or weak bands observed in the 548–535 and 453–438 cm⁻¹ range can be attributed to the ν(M—O) and ν(M—N) (Tümer, 2007) modes, respectively.

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Figure 4

(a and b) Mass spectra of the Schiff base ligands H₂L¹ (a) and H₂L² (b).

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Cyclic voltammograms of the Schiff base ligands H₂L¹–H₂L³ were run in DMF-0.1 M Bu₄NBF₄ as supporting electrolyte at 293 K. All potentials quoted refer to measurements run at scan rates in the 100–1000 mV s⁻¹ range and against an internal ferrocene–ferrocenium standard, unless otherwise stated. The electrochemical studies were done in the 1 × 10⁻³ and 1 × 10⁻⁴ M solutions and obtained data are given in Table 1. The selective voltammograms of the Schiff base ligands H₂L¹ (a) and H₂L² (b) are shown in Fig. 5a and b. The voltammograms were recorded in the range from −2.0 to 2.0 V vs Ag⁺/AgCl. In 1 × 10⁻³ M concentration solution of the free Schiff base ligands, at 100–1000 mV/s scan rates, the ligands have the anodic peak potentials in the −0.84 to 0.95 V range. The ligand H₂L¹ has by difference anodic peaks (one or two). On the other hand, the ligands H₂L² and H₂L³ have only one anodic peak potential at all scan rates. The anodic peak values of the ligand H₂L¹ shift to the higher regions depending upon the increase of the scan rates. The ligands have two or three cathodic peak potentials in the −1.54 to 0.84 V region. By increasing the scan rate, the cathodic peak values shifted to the negative regions. The ligands H₂L² and H₂L³ show the reversible redox processes at −0.51 and −0.57 V at 500 mV/s scan rate, respectively. At the other all scan rates, the ligands have the irreversible redox processes. In 1 × 10⁻⁴ M solution, while the ligands H₂L¹ and H₂L² show one or two anodic peak potentials in the −0.84 to 0.38 V range, the ligand H₂L³ shows only one anodic peak in the range −0.51 to 0.07 V. On the other hand, the ligands show two or three cathodic peak potentials in the −1.51 to 0.67 V range, and all redox processes are irreversible. When the concentration of the solution of the ligands decreases to 1 × 10⁻⁴ M, the anodic and cathodic peak potentials’ data were reduced.

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Figure 5

(a and b) Infrared spectra of the free Schiff bases H₂L¹ and H₂L³ with their polymer anchored-ligands and their transition metal complexes.

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In order to investigate the effects of the different concentrations on the photoluminescence properties of the Schiff base ligands H₂L¹–H₂L³, DMF, CH₃CN, EtOH and MeOH solutions in 1.0 × 10⁻³–1.0 × 10⁻⁷ M range were used. The emission and excitation spectra of the Schiff base compounds H₂L¹–H₂L³ in 1.0 × 10⁻³ M solutions are given in Fig. 6a and b and the obtained data are given in Table 2. In the emission spectra of the free ligands, one emission band in the 678–610 nm range was observed. In 1.0 × 10⁻³ M DMF solution, the ligand H₂L¹ has the longest wavelength (LW) emission band in the visible region. In the CH₃CN solutions of the ligand H₂L¹, the emission spectral data are shown in the 662–660 nm range, and the wavelength was not changed depending upon the concentration changes. The emission data of the ligand H₂L² were changed by decreasing the solution concentration and, in the low concentrations, the bands were shifted to the lower wavelengths. In the same solutions, the emission data of the H₂L³ were not changed. In DMF solutions, the emission bands of the ligands were observed in the 678–639 nm range, and the values were shifted to the shorter wavelengths by decreasing the solution concentration. In the EtOH and MeOH solutions, the bands were not shifted to lower or higher wavelengths. As a result, while the low and high concentrations of the solutions in the DMF and CH₃CN solvents affect the emission bands of the ligands, the concentration differences in the EtOH and MeOH solvents do not affect the emission bands. Polarity differences of the solvents also affect the photophysical properties. By increasing the solvent polarity, the emission and excitation bands were shifted to the lower wavelengths. This situation may be due to the hydrogen bonding. The Schiff base ligands have hydroxy groups on the benzene rings. These groups have electron donating properties by mesomeric effect. As a result of this situation, emission and excitation bands of the ligands shift to the red region. However, while the ligand H₂L² has π-hyper conjugation effect, other ligands H₂L¹ and H₂L³ have π-conjugation effect on the benzenoid rings.

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Figure 6

(a and b) Cyclic voltammograms of the Schiff base ligands H₂L¹ (a) and H₂L³ (b) at different scan rates and in 1.0 × 10⁻³ M and 1.0 × 10⁻⁴ M DMF solution.

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In the excitation spectra of the free ligands, one excitation band in the 574–535 nm range was observed. In 1.0 × 10⁻³–1.0 × 10⁻⁶ M CH₃CN solutions, the ligand H₂L¹ has one excitation band at ∼560 nm, but in 1.0 × 10⁻⁷ M solution, this band was shifted to 574 nm. In a similar way, the ligand H₂L² has one excitation band at 574 nm in 1.0 × 10⁻³ M CH₃CN solution.

In order to investigate the thermal stability of the polymer-anchoring compounds at the high temperatures, thermal studies were performed in the 20–1000 °C temperature range under the nitrogen atmosphere. Thermal curves of the PS-H₂L³, PS-L¹-Co and PS-L²-Ni complexes are shown in Fig. 7a–c. When thermal stabilities of the free Schiff base ligands H₂L¹–H₂L³ were investigated, decomposition of the ligands started at ∼200 °C. Polymer anchoring Schiff base ligands are stable up to 310 °C temperature. In the TGA curves of the polymer supported Schiff base ligands, the decomposition process occurs in the 390–500 °C temperature range. The ligands decompose in two steps. In the first step, the organic part of the polymer support ligands moves away from the molecules in the 390–394 °C temperature range. They are decomposed to the CO₂, H₂O and nitrogen oxide molecules. In the second step, the Merrifield resin is decomposed up to the 500 °C temperature. In the DTA curves, the losses of the ligand and solid support part were shown as the endothermic peaks. In the TG curves of the complexes, different decomposition processes were shown. The complexes of the ligand PS-H₂L¹ decompose in three steps. The TGA curves of the complexes show that decomposition of the organic structure mainly occured in two steps from 350 to 430 °C temperature range, which are related to the main weight loss. In the last step, the metal oxides occured. In the DTA curves of the complexes, decomposition processes are shown as the endothermic peaks. As distinct from the complexes of the ligand H₂L¹, TGA curves of the complexes of the ligands PS-H₂L² and PSH₂L³ show the loss of the coordinated water molecules and chloride ions at the lower temperatures. All these suggest that the supported catalysts were very stable and could be used within a broad temperature scale.

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Figure 7

(a and b) The fluorescence emission (a) and excitation (b) spectra of the Schiff base ligands in different solvents.

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Scanning electron micrograph (SEM) was also recorded to investigate the surface of the polymer-anchoring Schiff bases and their transition metal complexes. SEM images of the free PS, PS-L³-Co, PS-L³-Ni and PS-L³-Cu complexes are shown in Figs. 8a–d. When the polymeric support (PS) and Schiff base ligands interact, their surface morphologies change. The Schiff base ligands accumulate on the polymer surface. If the required metal complexes of the polymer anchoring Schiff base ligands are obtained, the metal salts and the polymer support ligands interact. When the obtained polymeric complexes are investigated, the appearance of the pores on surface of the metal complexes proves that the imprinting of the cobalt, copper and nickel ions on polymer anchoring Schiff base ligands could leave their footprints which increase the number of pores in the surface. This behavior may be attributed to the coordination of the metal ions to the active sites of the ligands.

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Figure 8

(a–c) Thermal curves of the polymer-anchored ligand PS-H₂L³ (a), PS-L¹-Co (b) and PS-L²-Ni (c) complexes.

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In order to investigate the catalytic activity of our catalysts, a set of additions of the alkane to alcohol and ketone (or alkene to epoxide) under several conditions were carried out (Tables 3 and 4). Solvent, reaction time, temperature, and catalyst loading were investigated. Influence of the complexes in cyclohexane, cyclooctane, cyclohexene and styrene oxidation under microwave irradiation is shown in Fig. 9. The binuclear PS-L²-Cu and PS-L³-Cu complexes showed the higher catalytic activity than the mononuclear PS-L¹-Cu complex on cyclohexane and cyclohexene substrates. This situation may be due to the structure of the ligands. In the complexes, the π–π^∗ electron delocalization phenomena may occur. This situation may affect the oxidation reaction. The best selectivities of the desired products Cy—OH and Cy⚌O were obtained in the Cu(II) complex (Table 3 and Fig. 9). Although the turnover of the cyclooctane is very high for the complexes, the yields of desired products (alcohol and ketone) are very low. This situation was also shown in the epoxidation of the styrene. The catalytic activity of the complexes in the alkane oxidation and alkene epoxidation was determined as low (see Fig. 10).

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Figure 9

(a–d) Scanning electron micrograph (SEM) images of the polymeric support (a), polymer-supported PS-L³-Co complex (b), PS-L³-Cu (c) and PS-L³-Ni catalyst (d).

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Figure 10

(a–d) Influence of the complexes in cyclohexane, cyclooctane, cyclohexene and styrene oxidation under microwave irradiation. 0.02 mmol catalyst:2 mmol alkane (alkene):4 mmol hydrogen peroxide (1:100:200) and 5 mL acetonitrile were used for each reaction. 300 W power was applied for 30 min. The reaction temperature and pressure were held at around 100 °C and 30 bar in closed DAP60 vessels.

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4 Conclusion

In this study, we obtained polymer-anchoring Schiff base compounds PS-H₂Lⁿ (n: 1, 2, 3) and their Co(II), Cu(II) and Ni(II) transition metal complexes. Electrochemical and photoluminescence properties of the free Schiff base ligands H₂L¹–H₂L³ were investigated and their interesting features were found. In the photophysical studies, the emission and excitation data at the higher wavelengths were obtained. Mostly, the ligands have irreversible redox properties. From the SEM images of the polymer-anchoring Schiff base ligands and their metal complexes, we determined the interaction between the metal ions and solid support ligands. In the catalytic studies, we found that the selectivity of the catalysts is low.