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New approach to synthesis of functionalised silsesquioxanes via hydrosilylation
Catalysis Communications 24 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
New approach to synthesis of functionalised silsesquioxanes via hydrosilylation Hieronim Maciejewski a, b,⁎, Karol Szubert a, Bogdan Marciniec a, b a b
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland Poznań Science and Technology Park, Rubież 46, 61-612 Poznań, Poland
a r t i c l e
i n f o
Article history: Received 31 December 2011 Received in revised form 5 March 2012 Accepted 11 March 2012 Available online 18 March 2012 Keywords: Hydrosilylation Platinum complexes Ionic liquids Immobilisation Functionalised silsesquioxanes
a b s t r a c t Synthesis of functionalised silsesquioxanes was carried out in the process of hydrosilylation catalysed by platinum complexes immobilised in ionic liquids. Platinum complexes at different oxidation states in the medium of three ionic liquids. Results of the study indicate that activity of catalytic systems investigated strongly depends on the type of ionic liquid used for metal complex immobilisation as well as on the kind of olefin subjected to hydrosilylation. The most effective catalytic system for all reactions studied was PtCl4 in 1,2,3trimethylimmidazolium methylsulfate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Materials of unique properties, designed for specific functional applications, such as nanocomposites containing nanosized fillers, attracted great interest in recent years [1,2]. Currently, among the most popular nanofillers are polyhedral oligomeric silsequioxanes (POSS) of the general formula (RSiO3/2)n, which, due to the presence of organofunctional substituents (R) enable formation of covalent bonds between filler and matrix [3–7]. A particular attention is paid to cage derivatives, especially to those of cubic structure (T8) [4]. In spite of tremendous application potential of silsesquioxanes, their commercial use is still small, mainly because of high prices of these derivatives which are a result of technologically difficult, time-consuming and low yield methods of their synthesis. This is why a search for new effective methods of synthesis of organofunctional silsesquioxanes is carried out. A vast majority of the organosilsesquioxanes are prepared by hydrosilylation of an appropriate olefin with hydridosilsesquioxane [4,8,9]. Hydrosilylation is catalysed mainly in homogeneous systems by transition metal complexes, those of platinum in particular [8–10], but unfortunately, isolation of products from post-reaction mixtures often makes a serious problem, particularly in the case of polymeric products of high viscosity or solid products. Therefore efforts are made to apply heterogeneous catalysts or immobilised metal complexes. In recent years, the application of ionic liquids to immobilisation of metal complexes for hydrosilylation processes was reported [11–21],
however, biphasic catalysis in a liquid–liquid system was not employed as yet for the synthesis of functionalised silsesquioxanes. In this paper we report results of research on the application of platinum complexes immobilised in ionic liquids (IL) to synthesis of functionalised silsesquioxanes via hydrosilylation reactions. In these studies, octakis(hydridodimethylsiloxy)octasilsesquioxane (so-called spherosilicate) was used as a reaction substrate. 2. Experimental section 2.1. General methods and chemicals Octakis(hydridodimethylsiloxy)octasilsesquioxane (HMe2SiO)8 [SiO1.5]8 was synthesised following published procedures [22]. All olefins and solvents were purchased from Aldrich and used without further purification. Platinum complexes and ionic liquids were purchased from Aldrich and Strem, respectively. All IL were dried prior to use under vacuum at 60 °C for 8 h. The NMR spectra (1H, 13C, and 29Si) were recorded on Varian Gemini 300 VT and Varian Mercury 300 VT spectrometers. C6D6 was used as a solvent. FT-IR spectra were recorded on a Bruker Tensor 27 Fourier transform spectrometer equipped with a SPECAC Golden Gate diamond ATR unit. In all cases 16 scans at a resolution of 2 cm− 1 were performed to record the spectra. 2.2. General procedure for catalytic tests
⁎ Corresponding author at: Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland. Fax: +48 61 8291409. E-mail address: [email protected] (H. Maciejewski). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.03.011
All manipulations were carried out under argon using Schlenk techniques.
2
H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
Scheme 1. Model reaction of hydrosilylation of olefins by POSS.
The appropriate amount of a catalyst (in the ratio of 10− 5 mol per 1 mol Si\H) and the ionic liquid (1% based on total weight of combined substrates) were placed into the reaction vessel and heated to 120 °C for 0.5 h. The system was heated for 0.5 h to dissolve the catalyst in ionic liquid, especially if the ionic liquid was solid at room temperature. Then the reaction system was cooled down and the mixture of octakis(hydridodimethylsiloxy)octasilsesquioxane, 1.2 equiv of olefin (calculated per each Si\H group) and toluene (in the amount indispensable for dissolving silsesquioxane) were added and the reaction vessel was heated again to 120 °C. After 1 h, the reaction vessel was cooled to room temperature and then the reaction mixture was separated from the catalytic system by decantation. The mixture was analysed by FT-IR spectroscopy and then, after the evaporation of toluene, the formation of desired products was verified by NMR analysis. The recovered catalytic system (catalyst in ionic liquid) was reused in the next reaction run. 3. Results and discussion Hydrosilylation was carried out using three olefins: hexadecene, allyl glycidyl ether and 4-allyl-1,2-dimethoxybenzene, as shown in Scheme 1. The above reactions were catalysed by systems based on four platinum complexes PtCl4, K2[PtCl4], K2[PtCl6], and [Pt2{(CH2_CHSiMe2)O}3] (Karstedt's catalyst). The complexes were immobilised in the following ionic liquids (IL), shown in Figs. 1–3. Our earlier studies have shown that the effectiveness of a catalytic system for hydrosilylation processes depends on the kind of ionic liquid. The latter should be a good solvent for a metal complex and firmly immobilise the complex while enabling to maintain its high catalytic activity, should form biphasic systems with parent substances and products and should be stable in the presence of reactive parent substances while not interacting with products. All these properties of ionic liquids are affected by the choice of cation and anion present in an ionic liquid. In our earlier studies we employed a dozen of different IL where anions were, among others, halogen ions (Cl−, Br−) or fluorine derivatives (BF4−, PF6−, SO3CF3−) [18,19]. However, these liquids appeared to be ineffective due to either poor solubility of metal complexes in them, difficulties in separation of catalytic system from reaction products (most of halogen anion-containing liquids) or drastic decrease in catalytic activity in subsequent reaction runs (fluorine derivatives). In the latter case, one of the reasons for catalyst
CH3 H3C N
N CH3 MeSO4
I Fig. 1. 1,2,3-Trimethylimidazolium methylsulphate [TriMIM][MeSO4] (I).
deactivation is the decomposition of ionic liquid (caused by the access of moisture) and the release of free fluoride ions that poison the catalyst [19]. Despite drying parent materials and ionic liquid prior to reaction, traces of moisture can access the system during isolation of product from catalytic system. On the other hand, we have found that IL that contain SO2 group in their anion (MeSO4−, (CF3SO2)2N−, Ace−, Sac−) are characterised by good stability and dissolve metal complexes well [20]. This is why the ionic liquids selected for the present study meet the above criteria. However, it has to be added that all our earlier studies on hydrosilylation in the presence of IL were carried out without solvents, but in this study the necessity appeared of using a solvent, because octakis(hydridodimethylsiloxy)octasilsesquioxane is a solid poorly soluble in olefins involved in the reaction. Therefore we applied toluene which dissolves the starting silsesquioxane, and the solution obtained in such a way was mixed with a particular olefin at an appropriate stoichiometric ratio. The mixture of parent materials was added to the catalytic system that was prepared earlier by dissolving a metal complex in an ionic liquid. As it was mentioned earlier, the ionic liquid made 1% based on total weight of combined substrates. Such an amount of ionic liquid secured complete solubility of metal complex and facilitated isolation of reaction product from catalytic system. Too large amount of ionic liquid caused a great dilution of catalyst (that was used at a low concentration of 10− 5 mol per 1 mol of Si\H), which resulted in slowing down the reaction, whereas too small amount of ionic liquid led to difficulties in accurate separation of reaction products from catalytic system. As already mentioned, the choice of ionic liquids (based on our earlier experience) was performed in such a way that complete solubility of employed metal complexes was secured. To obtain a fully homogeneous system, ionic liquid together with a metal complex was heated at 120 °C for half an hour. After cooling down, the catalytic system formed in such a way was homogeneous and no precipitation of metal complex occurred even after long time of storage. In all cases, the catalytic system did not dissolve in the reaction mixture, therefore the process was performed in a biphasic system and, after the reaction completion, the mixture was decanted and a next portion of reactants was introduced into the catalytic system to continue the process. All reactions were carried out for 1 h followed by FT-IR analysis of the post-reaction mixture (after its separation from the catalytic system) and Si\H conversion was determined on the ground of a decrease in the intensity of the band at about 2100 cm− 1. In this way the progress in the reaction carried out in the presence of the same portion of catalyst in subsequent catalytic runs was determined. The formation of a desirable compound was verified
(CH2)3CH3 H3C(H2C)3 P CH3 (CH2)3CH3
MeSO4
II Fig. 2. Tributyl(methyl)phosphonium methylsulphate [TriBMP][MeSO4] (II).
H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
(CH2)5CH3 N H3C(H2C)5 P (CH2)13CH3 F3CO2S SO2CF 3 (CH2)5CH3
Fig. 3. Trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide [TriHTP] [(CF3SO2)2N] (III).
by means of NMR analysis which was performed for the compound obtained after solvent evaporation. In all runs one product was obtained. In most cases, experiments were discontinued after 8 catalytic runs carried out in the presence of the same portion of catalyst, in spite of the fact that catalytic activity was still maintained. All catalytic systems showed high activity in the first run, whereas some activity differences between them appeared in next runs carried out with the use of the same portion of the catalyst. We have listed TON (turnover number) values which is a suitable tool for comparing catalytic activity for all catalytic systems used. Table 1 shows TON values obtained for hydrosilylation of olefins in the presence of platinum complexes immobilised in the ionic liquid I ([TriMIM][MeSO4]). On the ground of the obtained results, it can be concluded that the kind of olefin significantly influences TON values, and for this reason also affects the possibility of multiple use of the same portion of catalyst. In most cases, TON values decrease beginning from non-polar (hydrophobic) hexadecene to more polar and hydrophilic allyl glycidyl ether. The above conjecture was confirmed by determining Pt content in the post-reaction mixtures. Measurements performed by using ICP technique have shown that the post-reaction mixture after 8 runs contained from 5 to 30% of the initial platinum amount (for PtCl4, depending on the kind of olefin). Such a tendency is maintained, except for K2[PtCl4] and [Pt2{(CH2_CHSiMe2)O}3]. A similar situation was observed for hydrosilylation carried out in the presence of the ionic liquid II ([TriBMP][MeSO4]), as it results from data shown in Table 2. Also in this case the best results were obtained for hydrosilylation of hexadecene. It is worth of mentioning that in the presence of ionic liquid I and ionic liquid II, the highest catalytic activity was observed for PtCl4. Nevertheless, it should be emphasised that results obtained in the ionic liquid I are better than those in ionic liquid II, which suggests a better immobilisation of platinum complexes in the ionic liquid I, while maintaining high catalytic activity. These differences were even more clear when olefin hydrosilylation was performed in the presence of the ionic liquid III ([TriHTP][(CF3SO2)2N]) (Table 3). While comparing results discussed above one can notice that the effectiveness of the ionic liquid III in immobilising platinum complex and maintaining its catalytic activity is definitely the lowest one as well as that PtCl4 shows the highest catalytic activity also in the latter liquid. It is worth to add that Karstedt's catalyst, although it was not the most active one, showed comparable activity in all cases studied, i.e. it was not influenced by the kind of olefin or ionic liquid. The effect of the type of ionic liquid on the extent of leaching is particularly clear in the case of metal complexes at higher oxidation states, whereas in Table 1 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid I. Catalyst
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
Table 2 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid II. Catalyst
III
TON
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
87 600 35 200 67 000 32 400
62 500 55 900 22 000 34 300
45 900 45 500 3900 41 700
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
TON Hexadecene
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
74 700 32 100 66 400 32 500
49 800 48 900 44 800 17 800
37 000 29 200 39 300 31 800
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
that of Pt(0) complex the leaching is similar for all systems studied and equals to 5–10%. Results of our study bring into conclusion that the ionic liquid III is the least effective one and that the catalytic activity in the presence of this liquid decreases with each subsequent reaction run. One of reasons for such a situation can be too weak bonding between metal complexes and the liquid III, which can result in a gradual leaching of metal complex by reaction products (particularly by those of more hydrophilic nature) and by toluene solvent. ICP analyses have shown that contents of the platinum in the post-reaction mixture increased after each subsequent reaction run carried out in the presence of the ionic liquid III to reach after 8 runs the value of 45–65% of initial platinum amount (in the case of PtCl4) and even up to 90% (in that of K2 [PtCl6]). It is also possible that the low effectiveness of the ionic liquid III can be a result of catalyst deactivation caused by poisoning with fluoride ions formed during decomposition of ionic liquid anion, as already mentioned. The PtCl4 catalyst showed the highest activity for all reactions of olefin hydrosilylation studied in this work by using octakis(hydridodimethylsiloxy)octasilsesquioxane. At the same time, its immobilisation in the ionic liquids employed in this study was the best one, which points to its stronger interaction with ionic liquids. In most cases, platinum compounds at higher oxidation state (PtCl4 and K2[PtCl6]) showed higher catalytic activity, however, at the same time they were more susceptible to the influence of the kind of olefin as well as the kind of cation and anion present in ionic liquid. This fact suggests the occurrence of ionic interactions that are the stronger the higher platinum oxidation state. It is worth to add that catalytic activities for reactions catalysed by Pt(0) complex (Karstedt's catalyst) are comparable irrespective of the kind of olefin and ionic liquid, which suggests that in this case the effect of ionic interactions is considerably smaller. In order to determine what is the actual catalyst for hydrosilylation reactions, a study is underway on stoichiometric reactions of platinum complexes with ionic liquids and results of this study will be the subject of a separate paper. 4. Conclusions The developed catalytic systems enable highly efficient synthesis of organofunctional silsesquioxanes, their easy isolation from postTable 3 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid III. Catalyst
Hexadecene
3
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
TON Hexadecene
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
48 700 300 4400 30 100
40 700 800 3900 32 800
37 900 4900 3700 25 000
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
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H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
reaction mixtures as well as multiple use of catalyst which certainly will affect prices and availability of this desirable class of organosilicon compounds. The most active catalyst for all reactions studied in the presence of ionic liquids I, II and III was PtCl4. From among ionic liquids, the best immobilising properties were shown by the ionic liquid I, to which most of the metal complexes were bonded well without a loss of their catalytic activity. Acknowledgements The authors gratefully acknowledge financial support from the Ministry of Science and Higher Education (Poland), grant No. NN 204 165 436 and from the European Development Fund, Project No. UDAPOIG.01.03.01-30-173/09 co-financed from and carried out within the confines of Operational Programme Innovative Economy 2007–2013, Priority 1, Action 1.3. References [1] S. Thomas, S.V. Valsaraj, A.P. Meera, G.E. Zaikov (Eds.), Recent Advances in Polymer Nanocomposites: Synthesis and Characterisation, VSP, Leiden, 2010. [2] K. Rurack, R. Martinez-Manez (Eds.), The Supramolecular Chemistry of Organic– Inorganic Hybrid Materials, J. Wiley & Sons, Inc., Hoboken, 2010. [3] C. Hartmann-Thompson, Applications of Polyhedral Oligomeric Silsesquioxanes, Springer, 2011. [4] D.B. Cordes, P.D. Lickiss, F. Rataboul, Chemical Reviews 110 (2010) 2081–2173. [5] P.D. Lickiss, F. Rataboul, in: A.F. Hill, M.J. Fink (Eds.), Adv. Organometal. Chem., vol. 57, 2008, chap.1.
[6] M. Joshi, B. Singh Butola, Journal of Macromolecular Science 44 (2004) 389–410. [7] R.H. Baney, M. Itoh, A. Sakakibara, T. Suzukit, Chemical Reviews 95 (1995) 1409–1430. [8] B. Marciniec, H. Maciejewski, C. Pietraszuk, P. Pawluc, Hydrosilylation. A Comprehensive Review on Recent Advances, Springer, 2009. [9] A.K. Roy, Advances in Organometallic Chemistry 55 (2008) 1–59. [10] B. Marciniec, in: B. Cornils, W.A. Hermann (Eds.), Applied Homogeneous Catalysis with Organometallic Compounds, Wiley-VCH, Weinheim, 2002, chap. 2.6. [11] S. Aubin, F. Le Floch, D. Carrie, J.P. Guegan, M. Vaultier, Ionic Liquids. Industrial Applications for Green Chemistry, in: R.D. Rogers, K.R. Seddon (Eds.), ACS, Washinghton, 2002. [12] J. Van den Broecke, F. Winter, B.J. Deelman, G. Van Koten, Organic Letters 4 (2002) 3851–3854. [13] J. Peng, J. Li, Y. Bai, W. Gao, H. Qiu, H. Wu, Y. Deng, G. Lai, Journal of Molecular Catalysis A 278 (2007) 97–101. [14] J. Peng, J. Li, Y. Bai, H. Qiu, K. Jiang, J. Jiang, G. Lai, Catalysis Communications 9 (2008) 2236–2238. [15] B. Weyershausen, K. Hell, U. Hesse, Green Chemistry 7 (2005) 283–287. [16] T.J. Geldbach, D. Zhao, N.C. Castillo, G. Laurenczy, B. Weyershausen, P.J. Dyson, Journal of the American Chemical Society 128 (2006) 9773–9780. [17] N. Hofmann, A. Bauer, T. Frey, M. Auer, V. Stanjek, P.S. Schulz, N. Taccardi, P. Wasserscheid, Advanced Synthesis and Catalysis 350 (2008) 2599–2609. [18] H. Maciejewski, A. Wawrzynczak, M. Dutkiewicz, R. Fiedorow, Journal of Molecular Catalysis A 257 (2006) 141–148. [19] B. Marciniec, H. Maciejewski, K. Szubert, M. Kurdykowska, Monatshefte fur Chemie 137 (2006) 605–611. [20] H. Maciejewski, K. Szubert, B. Marciniec, J. Pernak, Green Chemistry 11 (2009) 1045–1051. [21] J. Pernak, A. Świerczyńska, M. Kot, F. Walkiewicz, H. Maciejewski, Tetrahedron Letters 52 (33) (2011) 4342–4345. [22] N.L.D. Filho, H.A. de Aguino, G. Pires, L. Caetano, Journal of the Brazilian Chemical Society 17 (2006) 533–541.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
New approach to synthesis of functionalised silsesquioxanes via hydrosilylation Hieronim Maciejewski a, b,⁎, Karol Szubert a, Bogdan Marciniec a, b a b
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland Poznań Science and Technology Park, Rubież 46, 61-612 Poznań, Poland
a r t i c l e
i n f o
Article history: Received 31 December 2011 Received in revised form 5 March 2012 Accepted 11 March 2012 Available online 18 March 2012 Keywords: Hydrosilylation Platinum complexes Ionic liquids Immobilisation Functionalised silsesquioxanes
a b s t r a c t Synthesis of functionalised silsesquioxanes was carried out in the process of hydrosilylation catalysed by platinum complexes immobilised in ionic liquids. Platinum complexes at different oxidation states in the medium of three ionic liquids. Results of the study indicate that activity of catalytic systems investigated strongly depends on the type of ionic liquid used for metal complex immobilisation as well as on the kind of olefin subjected to hydrosilylation. The most effective catalytic system for all reactions studied was PtCl4 in 1,2,3trimethylimmidazolium methylsulfate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Materials of unique properties, designed for specific functional applications, such as nanocomposites containing nanosized fillers, attracted great interest in recent years [1,2]. Currently, among the most popular nanofillers are polyhedral oligomeric silsequioxanes (POSS) of the general formula (RSiO3/2)n, which, due to the presence of organofunctional substituents (R) enable formation of covalent bonds between filler and matrix [3–7]. A particular attention is paid to cage derivatives, especially to those of cubic structure (T8) [4]. In spite of tremendous application potential of silsesquioxanes, their commercial use is still small, mainly because of high prices of these derivatives which are a result of technologically difficult, time-consuming and low yield methods of their synthesis. This is why a search for new effective methods of synthesis of organofunctional silsesquioxanes is carried out. A vast majority of the organosilsesquioxanes are prepared by hydrosilylation of an appropriate olefin with hydridosilsesquioxane [4,8,9]. Hydrosilylation is catalysed mainly in homogeneous systems by transition metal complexes, those of platinum in particular [8–10], but unfortunately, isolation of products from post-reaction mixtures often makes a serious problem, particularly in the case of polymeric products of high viscosity or solid products. Therefore efforts are made to apply heterogeneous catalysts or immobilised metal complexes. In recent years, the application of ionic liquids to immobilisation of metal complexes for hydrosilylation processes was reported [11–21],
however, biphasic catalysis in a liquid–liquid system was not employed as yet for the synthesis of functionalised silsesquioxanes. In this paper we report results of research on the application of platinum complexes immobilised in ionic liquids (IL) to synthesis of functionalised silsesquioxanes via hydrosilylation reactions. In these studies, octakis(hydridodimethylsiloxy)octasilsesquioxane (so-called spherosilicate) was used as a reaction substrate. 2. Experimental section 2.1. General methods and chemicals Octakis(hydridodimethylsiloxy)octasilsesquioxane (HMe2SiO)8 [SiO1.5]8 was synthesised following published procedures [22]. All olefins and solvents were purchased from Aldrich and used without further purification. Platinum complexes and ionic liquids were purchased from Aldrich and Strem, respectively. All IL were dried prior to use under vacuum at 60 °C for 8 h. The NMR spectra (1H, 13C, and 29Si) were recorded on Varian Gemini 300 VT and Varian Mercury 300 VT spectrometers. C6D6 was used as a solvent. FT-IR spectra were recorded on a Bruker Tensor 27 Fourier transform spectrometer equipped with a SPECAC Golden Gate diamond ATR unit. In all cases 16 scans at a resolution of 2 cm− 1 were performed to record the spectra. 2.2. General procedure for catalytic tests
⁎ Corresponding author at: Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland. Fax: +48 61 8291409. E-mail address: [email protected] (H. Maciejewski). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.03.011
All manipulations were carried out under argon using Schlenk techniques.
2
H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
Scheme 1. Model reaction of hydrosilylation of olefins by POSS.
The appropriate amount of a catalyst (in the ratio of 10− 5 mol per 1 mol Si\H) and the ionic liquid (1% based on total weight of combined substrates) were placed into the reaction vessel and heated to 120 °C for 0.5 h. The system was heated for 0.5 h to dissolve the catalyst in ionic liquid, especially if the ionic liquid was solid at room temperature. Then the reaction system was cooled down and the mixture of octakis(hydridodimethylsiloxy)octasilsesquioxane, 1.2 equiv of olefin (calculated per each Si\H group) and toluene (in the amount indispensable for dissolving silsesquioxane) were added and the reaction vessel was heated again to 120 °C. After 1 h, the reaction vessel was cooled to room temperature and then the reaction mixture was separated from the catalytic system by decantation. The mixture was analysed by FT-IR spectroscopy and then, after the evaporation of toluene, the formation of desired products was verified by NMR analysis. The recovered catalytic system (catalyst in ionic liquid) was reused in the next reaction run. 3. Results and discussion Hydrosilylation was carried out using three olefins: hexadecene, allyl glycidyl ether and 4-allyl-1,2-dimethoxybenzene, as shown in Scheme 1. The above reactions were catalysed by systems based on four platinum complexes PtCl4, K2[PtCl4], K2[PtCl6], and [Pt2{(CH2_CHSiMe2)O}3] (Karstedt's catalyst). The complexes were immobilised in the following ionic liquids (IL), shown in Figs. 1–3. Our earlier studies have shown that the effectiveness of a catalytic system for hydrosilylation processes depends on the kind of ionic liquid. The latter should be a good solvent for a metal complex and firmly immobilise the complex while enabling to maintain its high catalytic activity, should form biphasic systems with parent substances and products and should be stable in the presence of reactive parent substances while not interacting with products. All these properties of ionic liquids are affected by the choice of cation and anion present in an ionic liquid. In our earlier studies we employed a dozen of different IL where anions were, among others, halogen ions (Cl−, Br−) or fluorine derivatives (BF4−, PF6−, SO3CF3−) [18,19]. However, these liquids appeared to be ineffective due to either poor solubility of metal complexes in them, difficulties in separation of catalytic system from reaction products (most of halogen anion-containing liquids) or drastic decrease in catalytic activity in subsequent reaction runs (fluorine derivatives). In the latter case, one of the reasons for catalyst
CH3 H3C N
N CH3 MeSO4
I Fig. 1. 1,2,3-Trimethylimidazolium methylsulphate [TriMIM][MeSO4] (I).
deactivation is the decomposition of ionic liquid (caused by the access of moisture) and the release of free fluoride ions that poison the catalyst [19]. Despite drying parent materials and ionic liquid prior to reaction, traces of moisture can access the system during isolation of product from catalytic system. On the other hand, we have found that IL that contain SO2 group in their anion (MeSO4−, (CF3SO2)2N−, Ace−, Sac−) are characterised by good stability and dissolve metal complexes well [20]. This is why the ionic liquids selected for the present study meet the above criteria. However, it has to be added that all our earlier studies on hydrosilylation in the presence of IL were carried out without solvents, but in this study the necessity appeared of using a solvent, because octakis(hydridodimethylsiloxy)octasilsesquioxane is a solid poorly soluble in olefins involved in the reaction. Therefore we applied toluene which dissolves the starting silsesquioxane, and the solution obtained in such a way was mixed with a particular olefin at an appropriate stoichiometric ratio. The mixture of parent materials was added to the catalytic system that was prepared earlier by dissolving a metal complex in an ionic liquid. As it was mentioned earlier, the ionic liquid made 1% based on total weight of combined substrates. Such an amount of ionic liquid secured complete solubility of metal complex and facilitated isolation of reaction product from catalytic system. Too large amount of ionic liquid caused a great dilution of catalyst (that was used at a low concentration of 10− 5 mol per 1 mol of Si\H), which resulted in slowing down the reaction, whereas too small amount of ionic liquid led to difficulties in accurate separation of reaction products from catalytic system. As already mentioned, the choice of ionic liquids (based on our earlier experience) was performed in such a way that complete solubility of employed metal complexes was secured. To obtain a fully homogeneous system, ionic liquid together with a metal complex was heated at 120 °C for half an hour. After cooling down, the catalytic system formed in such a way was homogeneous and no precipitation of metal complex occurred even after long time of storage. In all cases, the catalytic system did not dissolve in the reaction mixture, therefore the process was performed in a biphasic system and, after the reaction completion, the mixture was decanted and a next portion of reactants was introduced into the catalytic system to continue the process. All reactions were carried out for 1 h followed by FT-IR analysis of the post-reaction mixture (after its separation from the catalytic system) and Si\H conversion was determined on the ground of a decrease in the intensity of the band at about 2100 cm− 1. In this way the progress in the reaction carried out in the presence of the same portion of catalyst in subsequent catalytic runs was determined. The formation of a desirable compound was verified
(CH2)3CH3 H3C(H2C)3 P CH3 (CH2)3CH3
MeSO4
II Fig. 2. Tributyl(methyl)phosphonium methylsulphate [TriBMP][MeSO4] (II).
H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
(CH2)5CH3 N H3C(H2C)5 P (CH2)13CH3 F3CO2S SO2CF 3 (CH2)5CH3
Fig. 3. Trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide [TriHTP] [(CF3SO2)2N] (III).
by means of NMR analysis which was performed for the compound obtained after solvent evaporation. In all runs one product was obtained. In most cases, experiments were discontinued after 8 catalytic runs carried out in the presence of the same portion of catalyst, in spite of the fact that catalytic activity was still maintained. All catalytic systems showed high activity in the first run, whereas some activity differences between them appeared in next runs carried out with the use of the same portion of the catalyst. We have listed TON (turnover number) values which is a suitable tool for comparing catalytic activity for all catalytic systems used. Table 1 shows TON values obtained for hydrosilylation of olefins in the presence of platinum complexes immobilised in the ionic liquid I ([TriMIM][MeSO4]). On the ground of the obtained results, it can be concluded that the kind of olefin significantly influences TON values, and for this reason also affects the possibility of multiple use of the same portion of catalyst. In most cases, TON values decrease beginning from non-polar (hydrophobic) hexadecene to more polar and hydrophilic allyl glycidyl ether. The above conjecture was confirmed by determining Pt content in the post-reaction mixtures. Measurements performed by using ICP technique have shown that the post-reaction mixture after 8 runs contained from 5 to 30% of the initial platinum amount (for PtCl4, depending on the kind of olefin). Such a tendency is maintained, except for K2[PtCl4] and [Pt2{(CH2_CHSiMe2)O}3]. A similar situation was observed for hydrosilylation carried out in the presence of the ionic liquid II ([TriBMP][MeSO4]), as it results from data shown in Table 2. Also in this case the best results were obtained for hydrosilylation of hexadecene. It is worth of mentioning that in the presence of ionic liquid I and ionic liquid II, the highest catalytic activity was observed for PtCl4. Nevertheless, it should be emphasised that results obtained in the ionic liquid I are better than those in ionic liquid II, which suggests a better immobilisation of platinum complexes in the ionic liquid I, while maintaining high catalytic activity. These differences were even more clear when olefin hydrosilylation was performed in the presence of the ionic liquid III ([TriHTP][(CF3SO2)2N]) (Table 3). While comparing results discussed above one can notice that the effectiveness of the ionic liquid III in immobilising platinum complex and maintaining its catalytic activity is definitely the lowest one as well as that PtCl4 shows the highest catalytic activity also in the latter liquid. It is worth to add that Karstedt's catalyst, although it was not the most active one, showed comparable activity in all cases studied, i.e. it was not influenced by the kind of olefin or ionic liquid. The effect of the type of ionic liquid on the extent of leaching is particularly clear in the case of metal complexes at higher oxidation states, whereas in Table 1 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid I. Catalyst
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
Table 2 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid II. Catalyst
III
TON
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
87 600 35 200 67 000 32 400
62 500 55 900 22 000 34 300
45 900 45 500 3900 41 700
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
TON Hexadecene
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
74 700 32 100 66 400 32 500
49 800 48 900 44 800 17 800
37 000 29 200 39 300 31 800
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
that of Pt(0) complex the leaching is similar for all systems studied and equals to 5–10%. Results of our study bring into conclusion that the ionic liquid III is the least effective one and that the catalytic activity in the presence of this liquid decreases with each subsequent reaction run. One of reasons for such a situation can be too weak bonding between metal complexes and the liquid III, which can result in a gradual leaching of metal complex by reaction products (particularly by those of more hydrophilic nature) and by toluene solvent. ICP analyses have shown that contents of the platinum in the post-reaction mixture increased after each subsequent reaction run carried out in the presence of the ionic liquid III to reach after 8 runs the value of 45–65% of initial platinum amount (in the case of PtCl4) and even up to 90% (in that of K2 [PtCl6]). It is also possible that the low effectiveness of the ionic liquid III can be a result of catalyst deactivation caused by poisoning with fluoride ions formed during decomposition of ionic liquid anion, as already mentioned. The PtCl4 catalyst showed the highest activity for all reactions of olefin hydrosilylation studied in this work by using octakis(hydridodimethylsiloxy)octasilsesquioxane. At the same time, its immobilisation in the ionic liquids employed in this study was the best one, which points to its stronger interaction with ionic liquids. In most cases, platinum compounds at higher oxidation state (PtCl4 and K2[PtCl6]) showed higher catalytic activity, however, at the same time they were more susceptible to the influence of the kind of olefin as well as the kind of cation and anion present in ionic liquid. This fact suggests the occurrence of ionic interactions that are the stronger the higher platinum oxidation state. It is worth to add that catalytic activities for reactions catalysed by Pt(0) complex (Karstedt's catalyst) are comparable irrespective of the kind of olefin and ionic liquid, which suggests that in this case the effect of ionic interactions is considerably smaller. In order to determine what is the actual catalyst for hydrosilylation reactions, a study is underway on stoichiometric reactions of platinum complexes with ionic liquids and results of this study will be the subject of a separate paper. 4. Conclusions The developed catalytic systems enable highly efficient synthesis of organofunctional silsesquioxanes, their easy isolation from postTable 3 TON values after 8 catalytic runs for reactions of olefin hydrosilylation catalysed by Pt complexes immobilised in the ionic liquid III. Catalyst
Hexadecene
3
PtCl4 K2[PtCl4] K2[PtCl6] [Pt2{(CH2_CHSiMe2)O}3]
TON Hexadecene
4-Allyl-1, 2-dimethoxybenzene
Allyl glycidyl ether
48 700 300 4400 30 100
40 700 800 3900 32 800
37 900 4900 3700 25 000
[HSi`]:[CH2_CH\]:[cat] = 1:1,2:10− 5; IL content was 1% based on total weight of combined substrates; T = 120 °C; t = 1 h.
4
H. Maciejewski et al. / Catalysis Communications 24 (2012) 1–4
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