Preparation and Catalytic Properties of a Ru(II) Coordinated Polyimide Supported by a Ligand Containing Terpyridine Units
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- Seckin, T., Özdemir, I., Köytepe, S. et al. J Inorg Organomet Polym (2009) 19: 143. doi:10.1007/s10904-009-9262-z
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The synthesis of terpyridine-based polyimide catalysis for hydrosilylation reaction is outlined in this work. 5,5′′-Bis(bromomethyl)-2,2′:6′,2′′-terpyridine was polymerized with the corresponding diimide derivatives of dianhyrides to give polyimides utilizing the terpyridine unit in the main chain. The synthesis of polyimides containing Ru(II) complexes in the side chain is described. Condensation polymerization is used to synthesize the macromolecular backbone and, as a result, the Ru(II) complex was attached via coordination chemistry. The material design emphasizes the relationship between the molecular structure and supramolecular organization of these polymers. It demonstrates that terpyridyl complexes remain a versatile functionality for constructing supramolecular assemblies. The terpyridine unit may enhance the electron carrier mobility of the polymers, while the incorporation of ruthenium complexes into a conjugated polymer significantly changes the catalytic properties of the resulting polymers. The prepared polyimide-supported catalyst provides superior catalytic activity (70–79%), selectivity and stability in the hydrosilylation of acetophenone. The catalyst can be easily isolated from the reaction product, which benefits recycling. The catalysts were reused for four experiments.
KeywordsMetal complexesPolyimideHeterogeneous catalysisHydosilylation
The development of new materials based on supramolecular chemistry for the synthesis of novel chemical sensors is a field of current interest [1–3]. This can be achieved by designing new receptors containing a host center and a covalently attached suitable group, which could selectively undergo a change in redox, color or photo physical properties [4, 5] upon guest binding. Among potential receptors, polyamines, bipyridine and terpyridine are well-known host molecules able to display binding processes with anions and cations . On the other hand, polypyridyl units such as bipyridine and terpyridine can be used to prepare heterogeneous catalysts by coordinating suitable transition metal ions [5, 6]. A number of studies have been reported on the attachment of polypyridyl units with the immobilization of transition metals on polymer supports [7, 8]. Terpyridine-containing polymers, with their receptors containing a host center and a covalently attached suitable group are potential materials for heterogenization of valuable homogeneous catalysts . While several attempts have been made to heterogenized the soluble transition metal complex catalysts to extend the benefits of heterogeneous catalysis to homogeneous systems; viz., easier separation catalyst and reaction products, and reusability for the supported catalyst [10–12], there has been limited success in the case of heterogeneous catalysis [13, 14]. Moreover, homogeneous catalysts are characterized by high activity and selectivity, which are not generally achieved by the corresponding heterogeneous catalysts . Therefore, during heterogenization of homogeneous catalysts, it was a long-held view that any advantages with regard to work up of the reaction mixture, purity of the product prepared, catalyst recovery, and continuous reaction procedure can only be realized in conjunction with serious disadvantages with regard to activity and selectivity. However, recent studies have revealed that some heterogenized catalysts indeed can give equivalent or higher selectivity and yields compared to their homogeneous counterparts [16, 17]. Several papers have recently appeared on the heterogenization of transition metal complexes onto solid supports, including organic polymers and inorganic solids of varied porosity. Among organic polymers used for this purpose, polyimides have taken an important place [18–20].
Polyimides have been widely used in many different technological areas such as microelectronics, coating, composites, separation membranes, and fiber applications . Most of the polyimides reported to date are based on pure organic systems. Relatively little research effort has been paid to metal-containing polyimides. In most of the examples reported in the literature, studies on metal-containing polyimides have been focused on the polyimide–metal interaction and the adhesive properties of polymers on the metal surface [22–25] due to the importance of polyimides as interlayer dielectric materials in microelectronic industry [26–34]. Another application of metal-containing polyimides is the preparation of polyimide-supported transition metal complex catalysts for various reactions, including hydrogenation, hydroformylation hydrosilylation, oxidation and metathesis [35, 36]. Hydrosilylation catalyzed by transition–metal complexes offers the most straightforward and atom-economic route to carbon–silicon and oxygen–silicon bond formation, which is important for organic synthesis as well as dendrimer and polymer chemistry. Moreover, it is applied to the reduction of ketones to secondary alcohols . In general, the term hydrosilylation is used to describe an addition reaction of hydrosilanes to double and triple bonds. In addition, hydrosilylation is a very convenient method for the synthesis of a variety of organosilicon compounds [38–40].
In this paper, we describe the synthesis, characterization, and catalytic properties of new polyimides that are obtained from a 5,5′′-bis(bromomethyl)-2,2′:6′,2′′-terpyridine coordinated to Ru(II). The improved reactivity was evidenced by the formation of high molecular weight polyimides that are obtained via a two-stage process. Once the model compound, 5,5′′-bis(bromomethyl)-2,2′:6′,2′′-terpyridine was obtained, the monomer was reacted with various diimides to form a novel assembly of coordination polyimides with Ru(II) that showed outstanding thermal and dielectric properties. Excellent selectivity is observed favoring hydrosilylation of acetophenone using dichloro-5,5′′-dimethyl-2,2′:6′,2′′-terpyridine ruthenium(II) complex and its polymers as the catalyst. The catalysts can also be recycled and reused without significant loss of selectivity or activity. The results are comparable to those for the homogeneous system. The performance of the catalysts is retained on reuse in four experiments.
All synthetic experiments were performed under a nitrogen atmosphere in a MBraun glove box (Labmaster 130) or by using standard Schlenk techniques. Size exclusion chromatography (SEC) versus poly(styrene) (PS) standards was carried out in N-methyl-2-pyrrolidinone (NMP) on PLGel 10 μm mixed Bed LS columns (Polymer Laboratories) using a 2414 differential refractometer and a UV-diode array detector (Waters Corp.). The flow was set to 0.7 mL/min prior to SEC; samples were filtered through a 0.22 μm Teflon filter (Millipore) in order to remove particles. SEC columns were calibrated versus polystyrene standards (Polymer Standards Service (PSS), molecular weights 347–2.7 × 106 g/mol). UV–vis spectra were recorded on a Hitachi U 2000 spectrophotometer in the range 300–800 nm using quartz cuvettes. NMR data were obtained at 300.13 MHz (1H) and are listed in parts per million downfield from tetramethylsilane. Differential scanning calorimeter (DSC), differential thermal analysis (DTA) and thermogravimetry (TG) were performed with Shimadzu DSC-60, DTA-50 and TGA-50 thermal analyzers, respectively, Inherent viscosities (ηinh = ln ηr/c at polymer concentration of 0.5 g/dL) were measured with an Ubbelohde suspended-level viscometer at 30 °C using NMP as the solvent. Deionized water was used throughout. All chemicals were purchased from Aldrich and used after purification. NMP was distilled over CaH2 under reduced pressure and stored over 4 Å molecular sieves. Reagent grade aromatic dianhyrides such as pyromellitic dianhyrides (PMDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhyrides (BPDA), 4,4′-oxydiphthalic anhydride (ODPA) and 3,3′,4,4′-biphenyltetracarboxylic anhydride (BTDA) that was sublimed at 250 °C under reduced pressure, and all the dianhyrides were dried under vacuum at 120 °C prior to use.
2.1 Synthesis of Diimides
Elemental analysis of the diimides
2.2 2-Bromo-5-Methylpyridine (2) (Scheme 2)
1H NMR (CDCl3), δ ppm (integration, multiplicity, assignment): 2.30 (3 H, s, H-7), 7.38 (2 H, m, H-3, 4), 8.20 (1 H, s, H-6). 13C NMR (CDCl3): δ ppm, (assignment) 17.66 (C-7), 127.4 (C-3), 132.4 (C-5), 138.9 (C-2), 139.35 (C-4), 150.62 (C-6). MS (EI, 70 eV): m/z (%) = 171 (44) [M +], 92 (100) [M ± 79]. Elemental Anal. calcd. for C6H6BrN (172.0): C, 41.86; H, 3.49, Br, 46.45, N, 8.14; Found: C, 41.76; H, 3.50; Br, 46.55; N, 8.19.
2.3 5-Methyl-2-Tributylstannyl Pyridine (3) (Scheme 2)
A solution of 2 (32.14 g, 0.187 mol) in THF (300 mL) was cooled to −78 °C and BuLi (122 mL, 1.6 M in hexane, 0.195 mol) was added dropwise for 30 min. After stirring for another 90 min, Bu3SnCl (60.5 mL, 0.224 mol) was added. The mixture was stirred for 8 h at −78 °C and then allowed to warm to room temperature. After adding H2O (100 mL), the aqueous layer was extracted 4-times with Et2O. The combined organic fractions were dried over anhydrous Na2SO4 and the solvent was evaporated under vacuum. The resulting liquid product was purified by Kugelrohr distillation to yield 70.59 g (98%) of 3; b.p. 130 °C/0.0038 torr. 1H NMR (CDCl3), δ ppm (integration, multiplicity, assignment): 0.89 (9 H, t, J = 7.3 Hz, H-4′), 1.11 (6 H, t, J = 8.0 Hz, H-1′), 1.34 (6 H, tq, J = 7.35 Hz, H-3′), 1.58 (6 H, m, H-2′), 2.38 (3 H, s, H-7), 7.40 (2 H, m, H-3,4), 8.59 (1 H, s, H-6). 13C NMR (CDCl3) δ ppm (assignment): 9.71 (C-1′), 13.63 (C-4′), 18.46 (C-7), 27.30 (C-3′), 29.06 (C-2′), 131.19 (C-5), 131.78 (C-3), 133.92 (C-4), 151.27 (C-6), 169.57 (C-2). MS (EI, 70 eV): m/z (%) = 326 (50) [M ± 56], 268 (45) [M ± 114], 212 (100) [M ± 170]. Elemental Anal. calcd. for C18H33NSn (382.2): C, 56.56; H, 8.64; N, 3.67; Sn, 31.08; Found: C, 56.29; H, 8.84; N, 3.78; Sn, 31.09.
2.4 5,5′′-Dimethyl-2,2′:6′,2′′-Terpyridine (4) (Scheme 3)
2.5 5,5′′-Bis(Bromomethyl)-2,2′:6′,2′′-Terpyridine (5) (Scheme 3)
A mixture of 4 (2.27 g, 8.69 mmol), N-bromosuccinimide (NBS) (7.75 g, 43.5 mmol) and azobisisobutyronitrile (AIBN) (221 mg, 1.35 mmol) in CCl4 (120 mL) was refluxed under N2 for 32 min and the precipitated succinimide was removed immediately from the hot mixture by filtration. The precipitate was washed with CCl4, the combined CCl4 phases were reduced to 50 mL under vacuum. The resulting precipitate was removed by filtration. The solid was dissolved in CH2Cl2 (100 mL) and extracted with 0.5 M Na2S2O3 solution (2 × 150 mL). The combined Na2S2O3 fractions were extracted with CH2Cl2 (50 mL) and the combined CH2Cl2 layers were dried over anhydrous Na2SO4 to yielding 870 mg (24%) of 5 after recrystallization from CHCl3; mp 195–196 °C. 1H NMR (CDCl3), δ ppm (integration, multiplicity, assignment): 4.58 (4 H, s, H-7,7′′), 7.91 (2 H, dd, J = 8.43, 2.21 Hz, H-4,4′′), 7.91 (1 H, t, J = 8.04 Hz, H-4′), 8.47 (2 H, d, J = 8.02 Hz, H-3′,5′), 8.61 (2 H, d, J = 8.01 Hz, H-3,3′′), 8.74 (2 H, d, J = 2.39 Hz, H-6,6′′). 13C NMR (CDCl3), δ (assignment): 30.54 (C-7,7′′), 122.24 (C-3,3′′), 121.15 (C-3′,5′), 133.15 (C-5,5′′), 137.79 (C-4,4′′), 138.08 (C-4′), 149.06 (C-6,6′′), 154.61 (C-2,2′′), 155.78 (C-2′,6′). MS (EI, 70 eV), m/z (%) = 419 (20) [M+]. Elemental Anal. calcd. for C17H13Br2N3 (419.1): C, 48.72; H, 3.13; N, 10.03; Found: C, 48.63; H, 2.68; N, 10.05.
2.6 Synthesis of Dichloro-5,5′′-Bis(Bromomethyl)-2,2′:6′,2′′-Terpyridine Ruthenium(II) (6)
A mixture of 5 (0.87 g, 2.08 mmol), RuCl3.xH2O and ethanol (50 mL) were heated under reflux for 6 h. The precipitated product was filtered off and washed with hexane (3 × 10 mL) and dried in vacuum. A brown product was obtained in 82% yield.
1H NMR (CDCl3), δ ppm (integration, multiplicity, assignment): 4.61 (4 H, s, H-7,7′′), 7.78 (2 H, m, H-4,4′′), 7.92 (1 H, t, J = 8.00 Hz, H-4′), 8.50 (2 H, d, J = 8.00 Hz, H-3′,5′), 8.20(2 H, d, J = 8.00 Hz, H-3,3′′), 8.75 (2 H, d, J = 2.25 Hz, H-6,6′′). 13C NMR (CDCl3): δ ppm (assignment): 38.45 (C-7,7′′), 121.62 (C-3,3′′), 119.65 (C-3′,5′), 129.68 (C-5,5′′), 137.19 (C-4, 4′′), 138.08 (C-4′), 152.06 (C-6,6′′), 155.12 (C-2,2′′), 157.18 (C-2′,6′). Elemental Anal. calcd. for C17H13Br2N3RuCl2 (591.1): C, 34.54; H, 2.22; N, 7.11; Found: C, 34.61; H, 2.35; N, 7.06.
2.7 Synthesis of Polyimides
2.8 Catalytic Reactions
Hydrosilylation experiments were carried out in a 10 mL Schlenk tube equipped with a magnetic stirring. The Schlenk tube was evacuated and flushed with argon before the reaction mixture was introduced. Acetophenenone (10 mmol) and an appropriate amount of the catalyst (a catalyst loading of 0.6% w/w) were mixed and the mixture was stirred in an oil bath at 80 °C. The progress of the reaction was monitored by gas chromatography (GC). After purification, the product was characterized by 1H and 13C-NMR spectra. The analyses were performed quantitatively on a capillary GC equipped with a thermal conductivity detector and a 6-foot column of 10% = V-101 on chromosorp W-HP, 100-120 mesh.
3 Results and Discussion
Yields and some properties of the Ru(II) coordinated polyimides
Mw × 10−4
Mn × 10−4
Thermal properties of the polyimides
Surprisingly, the minimal weight loss shown by the high molecular weight DTP-Ru-PI-11 system under these conditions was comparable to that of Kapton (PMDA/ODA). This preliminary test suggests that this material may be able to withstand the harsh thermal environment required for certain electronic applications, despite the partially aliphatic nature of the polymer backbone.
The DTA of the sample treated with Ru(II) is shown in Fig. 5 shows a one-step weight loss exactly like that of the weight loss seen in the monomer. The weight loss can be explained as the Ru(II) component of the sample. Typical TGA curves for the polymers are shown in Fig. 4. TGA analysis of the polyimides under nitrogen indicate that polymer decomposition commences between 410–500 °C. This result may be attributed to the flexibility, polarity and associations of the chains in the polymeric backbones. A more stable polymer is obtained for DTP-Ru-PI-14 where DI-14 is used as condensing agent in polymerization. Small variations in thermal decomposition temperature are probably due to differences in molecular weight and chain stiffness. The glass transition temperatures (Tg) as measured by DSC are given in Table 3. The polyimide prepared from DI-11 has a relatively high Tg, again attributed to its rigid structure.
The dichloro-5,5′′-bis(bromomethyl)-2,2′:6′,2′′-terpyridine ruthenium(II) ligand is ideally suited for the construction of coordination polymers. The three chelating pyridine units offer an additional increase in the binding constants, and the formation of octahedral 2:1 ligand–metal complexes does not give rise to enantiomers.
The ruthenium coordination polymers were prepared not by metal complexation polymerization but rather by a polyimidization reaction of the preformed kinetically inert ruthenium complexes.
Metal complexes able to catalyze hydrosilylation reaction of organic substrate under mild conditions are very attractive for many processes. We have observed that dichloro-5,5′′-dimethyl-2,2′:6′,2′′-terpyridine ruthenium(II) can be used as effective catalysts for hydrosilylation reaction of acetophenone.
A new polyimide-derived sorbent based on a polydichloro-(5,5′′-bis(bromomethyl)-2, 2′:6′,2′′-terpyridine) ruthenium(II) was developed and successfully used for the selective catalysis of the hiydosilylation of acetophenone. The material provides sufficient catalytic capabilities of >6 mg metal ion/g sorbent over a broad range of pH and allows quantitative recoveries with no leaching. The catalyst may be reused. Elution is simply achieved by treatment with diluted CH2Cl2.