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Microwave-assisted organic functionalization of silica surfaces: Effect of selectively heating silylating agents
Journal of Organometallic Chemistry 696 (2011) 825e828
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Communication
Microwave-assisted organic functionalization of silica surfaces: Effect of selectively heating silylating agents Norihisa Fukaya a, *, Hiroshi Yamashita a, Hisato Haga b, Teruhisa Tsuchimoto b, Syun-ya Onozawa a, Toshiyasu Sakakura a, Hiroyuki Yasuda a, * a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Department of Applied Chemistry, School of Science and Technology, Meiji University, Higashimita, Tama-ku, Kawasaki 214-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2010 Received in revised form 30 September 2010 Accepted 5 October 2010 Available online 17 November 2010
Microwave (MW)-assisted (2.45 GHz) organic functionalization of silica surfaces was investigated using (3-chloropropyl)dimethylsilanes with alkoxy, allyl, or aryl leaving groups in heptane or toluene at 80 C. 29 Si and 13C CP/MAS spectroscopy confirmed the 3-chloropropyldimethylsilyl moiety was covalently grafted onto silica for all the samples. The effect of MW irradiation on the loading amount strongly depended on the leaving group as well as the solvent; using methoxysilane and p-anisylsilane in heptane caused a distinct acceleration. The correlation with the dielectric loss factors of the silylating agents suggested that the MW acceleration effect resulted from selectively heating the strongly MW-absorbing silylating agent. For the grafting reaction in toluene, the MW effect was not observed possibly because toluene masked the selective heating effect of the silylating agent. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Organiceinorganic hybrid materials Silica Immobilization Microwave Arylsilane
1. Introduction Organiceinorganic hybrid materials obtained by modifying a silica surface with organic functional groups are widely used as immobilized catalysts, packing materials for chromatography, adsorbents, chemical sensors, etc. [1]. These materials are generally synthesized by reacting “silane coupling reagents”, which are organosilanes possessing alkoxy, halo, acyloxy, or amino groups on silicon atoms as leaving groups, with silanol groups on silica [2]. Silane coupling reagents are easily hydrolyzed and subsequently condensed into the corresponding siloxane compounds. To overcome the weakness of silane coupling reagents, an improved method to modify silica surfaces with allylsilane derivatives, which are stable against hydrolysis, has been developed [3]. Additionally, organic functionalization of silica surfaces can be realized by employing more stable arylsilane derivatives [4]. However, grafting organosilanes, especially allylsilanes or arylsilanes with low reactivities, onto silica requires energy and time because sufficient loading usually requires a high temperature (>100 C) and a long reaction time (>1 day). Therefore, the need to develop more efficient and practical methods for organic functionalization of silica remains. * Corresponding authors. Tel.: þ81 29 861 9399; fax: þ81 29 861 4580. E-mail addresses: [email protected] (N. Fukaya), [email protected] (H. Yasuda). 0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2010.10.008
Inorganic and organic syntheses employing microwave (MW) irradiation have attracted much attention because MW irradiation is a clean, efficient, and convenient energy source [5]. Polar components of a reaction mixture can directly absorb MW to efficiently supply energy to the reactants. Thus, MW heating dramatically reduces reaction times and increases product yields compared to conventional heating methods. Although MW-assisted preparation of organosilicas via solegel processes has become common, there are only a few reports on the use of MW irradiation in the post synthetic attachment of organic functional groups onto silica surfaces [6]. Procopio et al. have described MW-assisted organic modification of mesoporous silica (MCM-41) [6a,b]. They have shown that MW irradiation enhanced the grafting amount of trialkoxysilanes bearing functional groups such as amine, thiol, and chlorine. More recently, Tiemblo et al. have reported MW-assisted grafting of alkyltrimethoxysilanes onto silica nanoparticles [6c]. They have shown that the enhanced grafting amount by MW irradiation strongly depends on the length of the alkyl side chain. Methyltrimethoxysilane (MTMS) induced an enhanced grafting amount under MW irradiation, whereas the MW effect for alkyltrimethoxysilanes with long alkyl side chains was negligible. However, these previous works did not present a comparative study of silylating agents with different types of leaving groups under MW irradiation conditions. Additionally, both studies noted the simultaneous occurrence of self-condensation of trialkoxysilanes, which
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N. Fukaya et al. / Journal of Organometallic Chemistry 696 (2011) 825e828 Table 2 Dielectric properties at 2.45 GHz of silylating agents and solvents.
Scheme 1. Grafting of (3-chloropropyl)dimethylsilanes onto silica.
complicates the mechanism of the MW-induced enhancement of the grafting amount. In fact, Tiemblo et al. have demonstrated that the effects of MW on grafting MTMS were mainly due to the acceleration of autocondensation of MTMS itself rather than the reaction with silanol groups on the silica surface [6c]. In this paper, we report MW-assisted organic functionalization of a silica surface using silylating agents with different types of leaving groups, including alkoxy, allyl, and aryl groups. To elucidate the true MW acceleration effect on the grafting reaction (SieOeSi bond formation between silylating agents and the silica surface), we employed a silylating agent possessing a mono-leaving group, which reacted stoichiometrically with a surface silanol group. Moreover, since silylating agents with allyl and aryl leaving groups are water-insensitive, we can exclude the possibility of autocondensation of the silylating agents. We also measured the dielectric properties of the silylating agents and investigated their correlation with the MW effect.
2. Results and discussion We used silica gel (Fuji Silysia Chemical Ltd., CARiACT Q-3) as a grafting support. The surface area, pore volume, and average pore diameter were 713 m2/g, 0.52 cm3/g, and 2.4 nm, respectively. The amount of the surface silanol group on silica estimated by TG-DTA was 5.8 mmol/g. Additionally, we employed as silylating agents (3-chloropropyl)dimethylmethoxysilane (1), allyl(3-chloropropyl)dimethylsilane (2), and aryl(3-chloropropyl)dimethylsilanes (3ae3c) in amounts approximately equimolar with the silanol group. 1e3 were prepared according to the literature [7]. For each silylating agent, grafting of the silica surface was conducted under both MW irradiation conditions and conventional heating conditions to compare the two heating methods. The MW-assisted (2.45 GHz) grafting of the silica surface was performed at 80 C for 1 h. For the control experiment involving conventional heating, the reactor was immersed into an oil bath preheated to 80 C (Scheme 1). Table 1 Comparison of microwave (MW) heating and oil-bath heating for grafting (3-chloropropyl)dimethylsilanes onto silica. Entry
Silylating agent
Solvent
1 2 3 4 5 6 7 8 9 10
1 2 3a 3b 3c 1 2 3a 3b 3c
Heptane Heptane Heptane Heptane Heptane Toluene Toluene Toluene Toluene Toluene
0.79 0.63 0.56 0.08 0.15 0.37 0.24 0.07 Trace Trace
Dielectric loss factor (300 )
tan da
1.217 0.710 0.983 0.664 0.795 0.0008 0.0119
0.202 0.163 0.250 0.191 0.209 0.0004 0.005
a
tan d ¼ 300 /30 .
Covalent grafting of the 3-chloropropyldimethylsilyl moiety in 1e3 onto the silica surface was confirmed by 29Si and 13C CP/MAS spectroscopy (see Supplementary data). In addition to the silica peaks, the 29Si CP/MAS spectrum of the material obtained by reacting 1 with silica under the MW heating condition exhibited a signal assigned to the silicon attached to silica (d14.4 ppm). The 13 C CP/MAS spectrum contained four signals, which corresponded to the carbons of the propyl moiety (d 47.4, 26.3, and 14.5 ppm) and the methyl groups on the silicon atom (d 1.5 ppm). Regardless of the heating method, the chemical shifts in the 29Si and 13C CP/MAS spectra were essentially the same for all modified silicas obtained using 1e3, indicating that the 3-chloropropyldimethylsilyl group was cleanly grafted onto silica via a siloxane (SieOeSi) bond. The loading amount of the 3-chloropropyldimethylsilyl group was determined by elemental analysis of chlorine. Table 1 summarizes the loading amounts obtained by both heating methods and lists the MW effect, which we defined as the ratio of the loading amount by MW heating to the loading amount by oil-bath heating. The MW effect strongly depended on the silylating agents and solvents. The grafting reactions in a heptane solution using 1 (methoxy) and 3a (p-anisyl) were remarkably accelerated by MW heating. The loading amounts of 1 and 3a by MW heating were about one and a half times larger than those by oil-bath heating at the same temperature for 1 h, and were comparable to the loading amounts after 24 h of oil-bath heating (entries 1 and 3). In contrast, for 2 (allyl), 3b (p-tolyl), and 3c (phenyl), the loading amounts for MWirradiated grafting were almost equal to or less than those for conventionally heated experiments (entries 2, 4, and 5). On the other hand, the results of the grafting reaction drastically differed in a toluene solution. Compared to the reaction in heptane, all the silylating agents had reduced loading amounts. Only trace amounts of 3b and 3c were loaded. Additionally, the loading amounts obtained by MW heating were virtually identical to those by oil-bath heating (entries 6e10) [8]. Interestingly, a distinct MW
MW effectb
Loading amount (mmol/g)a MWc
Substance 1 2 3a 3b 3c Heptane Toluene
Oil bathd 1h
24 h
0.51 0.74 0.37 0.09 0.16 0.42 0.29 0.07 e e
0.74 0.95 0.52 0.54 0.52 0.50 0.74 0.27 Trace Trace
1.6 0.85 1.5 0.89 0.94 0.88 0.83 1.0 e e
a
Determined by elemental analysis of chlorine. Ratio of the loading amount by MW heating to the loading amount by oil-bath heating for 1 h. c MW heating at 80 C for 1 h. d Oil-bath heating at 80 C for 1 h or 24 h. b
Fig. 1. Dependency of the MW effect on the dielectric loss factor of the silylating agent.
N. Fukaya et al. / Journal of Organometallic Chemistry 696 (2011) 825e828
effect, which was observed for the grafting of 1 and 3a in heptane, was not observed in the reaction in toluene. To elucidate the reason why the effect of MW irradiation on organic grafting differed among silylating agents and solvents, we investigated the correlation between the dielectric properties of the silylating agents as well as solvents and the MW effect. The conversion efficiency of MW energy into thermal energy depends on the dielectric properties of materials. The fundamental relationship is described by
P ¼ ue0 e00 jEj2 where P represents power dissipation (conversion of MW to thermal energy), u is the angular frequency, E is the electric field of the sample, 30 is the permittivity of free space, and 300 is the dielectric loss factor (the imaginary part of the complex permittivity) [5a]. Therefore, the conversion of MW energy into thermal energy should be more effective for materials with larger 300 . Table 2 lists the dielectric properties at 2.45 GHz of the silylating agents and the solvents used in this study, which were determined by the resonant cavity perturbation method. The dielectric loss factors (300 ) of 1e3 ranged from 0.664 to 1.217. 1 and 3a were stronger MW absorbers than the other silylating agents. The dielectric loss factor of heptane was about one-fifteenth that of toluene. Fig. 1 plots the MW effect against the dielectric loss factor of the silylating agent. For the grafting reaction in MW-transparent heptane, the MW effect increased as the dielectric loss factor increased. Therefore, the remarkable MW effect observed for 1 and 3a could be rationalized as a consequence of selectively heating strongly MW-absorbing 1 and 3a. In contrast, the MW effect for the reaction in toluene scarcely depended on the dielectric loss factor. Additionally, the difference in the organic loading between the heating methods was small. Although the dielectric loss factor of toluene was much smaller than those of the silylating agents, the amount of toluene in the reaction mixture (5 cm3) greatly exceeded that of the silylating agent (0.2e0.3 cm3). The total thermal energy generated by MW irradiation is determined not only by the dielectric properties of MWabsorbing materials but also by the quantity of the materials [5a]. Therefore, it is possible that MW absorption by toluene masked the effect derived from selectively heating 1 and 3a. Consequently, the apparent MW effect could not be observed even when using a strongly MW-absorbing silylating agent such as 1 and 3a. 3. Conclusions In the organic functionalization of a silica surface via a reaction of organosilanes and silica, the MW acceleration effect strongly depended on the leaving group of the organosilanes as well as the reaction solvent. A distinctive acceleration was observed using methoxysilane and p-anisylsilane in heptane. Analysis of the dielectric properties of the organosilanes suggests that the observed MW effect is due to selectively heating the silylating agent, which has a strong MW-absorbing nature. Alkoxysilanes are occasionally difficult to synthesize and/or handle because they are easily hydrolyzed. Hence, using stable p-anisylsilanes in combination with MW irradiation should be a promising method to efficiently construct structurally diverse organo-functionalized silicas, including silica-immobilized molecular catalysts. 4. Experimental 4.1. Organic grafting onto a silica surface Organic grafting of the silica surface via MW irradiation (2.45 GHz) was carried out in a CEM Discover instrument equipped
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with a magnetic stirrer and a temperature/power controller using a glass vial (10 cm3). Silica gel (0.25 g) and a silylating agent (1.5 mmol) were placed into a nitrogen-purged vial. The vial was capped with a Teflon-coated septum and either dry heptane or dry toluene (5 cm3) was added. The vial was placed into the MW unit, and heated at 80 C for 1 h. The temperature of the reaction mixture reached 80 C within 100 s for the reactions in both toluene and heptane. The temperature profiles for the reactions of 3a with silica in heptane and toluene under the MW heating were shown in Fig. S7 in Supplementary data. Organic grafting via conventional heating was performed by immersing the vial into an oil bath, which was preheated to 80 C, for 1 h or 24 h. The modified silica was filtered, washed successively with ethyl acetate (10 cm3) and dichloromethane (10 cm3), and then dried at 80 C under a vacuum for 3 h. 4.2. Measurements of complex dielectric constants Complex dielectric constants (30 ej300 ) of silylating agents and solvents were measured by the perturbation method with a Kanto Electronic Application & Development system, which was comprised of an Agilent 8720ES vector network analyzer and a 2.45 GHz cylindrical cavity resonator [9]. The measurements were conducted in TM020 mode using Teflon-type sample tubes. The resonance frequency, quality factor of the empty cavity, and their shifts by the insertion of sample materials into the center of the cavity were measured. The dielectric constants were calculated from the resonance frequency shifts. The loss factors were calculated from the changes in the quality factor. 4.3. Determination of surface silanol group on silica The amount of the surface silanol group on the silica support was determined by the weight loss of a silica powder over a temperature range of 25e1000 C [10] which was monitored by TG-DTA (Bruker AXS, TG-DTA200SA). Acknowledgments This research was financially supported in part by the Development of Microspace and Nanospace Reaction Environment Technology for Functional Materials Project of NEDO, Japan. We thank Ms. Hiroko Kobashi for measuring the dielectric properties. Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.jorganchem.2010.10.008. References [1] (a) Special issue on organiceinorganic nanocomposites, Chem. Mater. 13 (10) (2001); (b) C. Sanchez, P. Gómez-Romero (Eds.), Functional Hybrid Materials, WileyVCH, Weinheim, 2004; (c) G. Kickelbick (Ed.), Hybrid Materials: Synthesis, Characterization, and Applications, Wiley-VCH, Weinheim, 2007. [2] E.P. Plueddemann, Silane Coupling Agents, second ed. Plenum, New York, 1991. [3] (a) T. Shimada, K. Aoki, Y. Shinoda, T. Nakamura, N. Tokunaga, S. Inagaki, T. Hayashi, J. Am. Chem. Soc. 125 (2003) 4688; (b) K. Aoki, T. Shimada, T. Hayashi, Tetrahedron: Asymmetry 15 (2004) 1771; (c) Y.-R. Yeon, Y.J. Park, J.-S. Lee, J.-W. Park, S.-G. Kang, C.-H. Jun, Angew. Chem., Int. Ed. 47 (2008) 109. [4] N. Fukaya, H. Haga, T. Tsuchimoto, S. Onozawa, T. Sakakura, H. Yasuda, J. Organomet. Chem. 695 (2010) 2540. [5] (a) C. Gabriel, S. Gabriel, E.H. Grant, B.S.J. Halstead, M.P. Mingos, Chem. Soc. Rev. 27 (1998) 213; (b) C.O. Kappe, D. Dallinger, Mol. Divers. 13 (2009) 71.
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[6] (a) A. Procopio, G. Das, M. Nardi, M. Oliverio, L. Pasqua, ChemSusChem 1 (2008) 916; (b) A. Procopio, G. De Luca, M. Nardi, M. Oliverio, R. Paonessa, Green Chem. 11 (2009) 770; (c) N. García, E. Benito, J. Guzmán, R. de Francisco, P. Tiemblo, Langmuir 26 (2010) 5499. [7] T. Nakashima, R. Fujiyama, H.J. Kim, M. Fujio, Y. Tsuno, Bull. Chem. Soc. Jpn. 73 (2000) 429.
[8] The reason the MW effects in entries 2 and 4e7 in Table 1 were slightly less than 1.0 might be due to lower stirring efficiency of reaction mixtures in the MW instrument (the actual stirring speed has not been disclosed by the instrument vendor). Such influences of stirring speed on the MW and oil-bath heating reactions have been described, see M. Irfan, M. Fuchs, T.N. Glasnov, C.O. Kappe, Chem.-Eur. J. 15 (2009) 11608. [9] H. Kawabata, H. Tanpo, Y. Kobayashi, IEICE Trans. Electron. E87-C (2004) 694. [10] R. Mueller, H.K. Kammler, K. Wegner, S.E. Pratsinis, Langmuir 19 (2002) 160.
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Communication
Microwave-assisted organic functionalization of silica surfaces: Effect of selectively heating silylating agents Norihisa Fukaya a, *, Hiroshi Yamashita a, Hisato Haga b, Teruhisa Tsuchimoto b, Syun-ya Onozawa a, Toshiyasu Sakakura a, Hiroyuki Yasuda a, * a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Department of Applied Chemistry, School of Science and Technology, Meiji University, Higashimita, Tama-ku, Kawasaki 214-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2010 Received in revised form 30 September 2010 Accepted 5 October 2010 Available online 17 November 2010
Microwave (MW)-assisted (2.45 GHz) organic functionalization of silica surfaces was investigated using (3-chloropropyl)dimethylsilanes with alkoxy, allyl, or aryl leaving groups in heptane or toluene at 80 C. 29 Si and 13C CP/MAS spectroscopy confirmed the 3-chloropropyldimethylsilyl moiety was covalently grafted onto silica for all the samples. The effect of MW irradiation on the loading amount strongly depended on the leaving group as well as the solvent; using methoxysilane and p-anisylsilane in heptane caused a distinct acceleration. The correlation with the dielectric loss factors of the silylating agents suggested that the MW acceleration effect resulted from selectively heating the strongly MW-absorbing silylating agent. For the grafting reaction in toluene, the MW effect was not observed possibly because toluene masked the selective heating effect of the silylating agent. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Organiceinorganic hybrid materials Silica Immobilization Microwave Arylsilane
1. Introduction Organiceinorganic hybrid materials obtained by modifying a silica surface with organic functional groups are widely used as immobilized catalysts, packing materials for chromatography, adsorbents, chemical sensors, etc. [1]. These materials are generally synthesized by reacting “silane coupling reagents”, which are organosilanes possessing alkoxy, halo, acyloxy, or amino groups on silicon atoms as leaving groups, with silanol groups on silica [2]. Silane coupling reagents are easily hydrolyzed and subsequently condensed into the corresponding siloxane compounds. To overcome the weakness of silane coupling reagents, an improved method to modify silica surfaces with allylsilane derivatives, which are stable against hydrolysis, has been developed [3]. Additionally, organic functionalization of silica surfaces can be realized by employing more stable arylsilane derivatives [4]. However, grafting organosilanes, especially allylsilanes or arylsilanes with low reactivities, onto silica requires energy and time because sufficient loading usually requires a high temperature (>100 C) and a long reaction time (>1 day). Therefore, the need to develop more efficient and practical methods for organic functionalization of silica remains. * Corresponding authors. Tel.: þ81 29 861 9399; fax: þ81 29 861 4580. E-mail addresses: [email protected] (N. Fukaya), [email protected] (H. Yasuda). 0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2010.10.008
Inorganic and organic syntheses employing microwave (MW) irradiation have attracted much attention because MW irradiation is a clean, efficient, and convenient energy source [5]. Polar components of a reaction mixture can directly absorb MW to efficiently supply energy to the reactants. Thus, MW heating dramatically reduces reaction times and increases product yields compared to conventional heating methods. Although MW-assisted preparation of organosilicas via solegel processes has become common, there are only a few reports on the use of MW irradiation in the post synthetic attachment of organic functional groups onto silica surfaces [6]. Procopio et al. have described MW-assisted organic modification of mesoporous silica (MCM-41) [6a,b]. They have shown that MW irradiation enhanced the grafting amount of trialkoxysilanes bearing functional groups such as amine, thiol, and chlorine. More recently, Tiemblo et al. have reported MW-assisted grafting of alkyltrimethoxysilanes onto silica nanoparticles [6c]. They have shown that the enhanced grafting amount by MW irradiation strongly depends on the length of the alkyl side chain. Methyltrimethoxysilane (MTMS) induced an enhanced grafting amount under MW irradiation, whereas the MW effect for alkyltrimethoxysilanes with long alkyl side chains was negligible. However, these previous works did not present a comparative study of silylating agents with different types of leaving groups under MW irradiation conditions. Additionally, both studies noted the simultaneous occurrence of self-condensation of trialkoxysilanes, which
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N. Fukaya et al. / Journal of Organometallic Chemistry 696 (2011) 825e828 Table 2 Dielectric properties at 2.45 GHz of silylating agents and solvents.
Scheme 1. Grafting of (3-chloropropyl)dimethylsilanes onto silica.
complicates the mechanism of the MW-induced enhancement of the grafting amount. In fact, Tiemblo et al. have demonstrated that the effects of MW on grafting MTMS were mainly due to the acceleration of autocondensation of MTMS itself rather than the reaction with silanol groups on the silica surface [6c]. In this paper, we report MW-assisted organic functionalization of a silica surface using silylating agents with different types of leaving groups, including alkoxy, allyl, and aryl groups. To elucidate the true MW acceleration effect on the grafting reaction (SieOeSi bond formation between silylating agents and the silica surface), we employed a silylating agent possessing a mono-leaving group, which reacted stoichiometrically with a surface silanol group. Moreover, since silylating agents with allyl and aryl leaving groups are water-insensitive, we can exclude the possibility of autocondensation of the silylating agents. We also measured the dielectric properties of the silylating agents and investigated their correlation with the MW effect.
2. Results and discussion We used silica gel (Fuji Silysia Chemical Ltd., CARiACT Q-3) as a grafting support. The surface area, pore volume, and average pore diameter were 713 m2/g, 0.52 cm3/g, and 2.4 nm, respectively. The amount of the surface silanol group on silica estimated by TG-DTA was 5.8 mmol/g. Additionally, we employed as silylating agents (3-chloropropyl)dimethylmethoxysilane (1), allyl(3-chloropropyl)dimethylsilane (2), and aryl(3-chloropropyl)dimethylsilanes (3ae3c) in amounts approximately equimolar with the silanol group. 1e3 were prepared according to the literature [7]. For each silylating agent, grafting of the silica surface was conducted under both MW irradiation conditions and conventional heating conditions to compare the two heating methods. The MW-assisted (2.45 GHz) grafting of the silica surface was performed at 80 C for 1 h. For the control experiment involving conventional heating, the reactor was immersed into an oil bath preheated to 80 C (Scheme 1). Table 1 Comparison of microwave (MW) heating and oil-bath heating for grafting (3-chloropropyl)dimethylsilanes onto silica. Entry
Silylating agent
Solvent
1 2 3 4 5 6 7 8 9 10
1 2 3a 3b 3c 1 2 3a 3b 3c
Heptane Heptane Heptane Heptane Heptane Toluene Toluene Toluene Toluene Toluene
0.79 0.63 0.56 0.08 0.15 0.37 0.24 0.07 Trace Trace
Dielectric loss factor (300 )
tan da
1.217 0.710 0.983 0.664 0.795 0.0008 0.0119
0.202 0.163 0.250 0.191 0.209 0.0004 0.005
a
tan d ¼ 300 /30 .
Covalent grafting of the 3-chloropropyldimethylsilyl moiety in 1e3 onto the silica surface was confirmed by 29Si and 13C CP/MAS spectroscopy (see Supplementary data). In addition to the silica peaks, the 29Si CP/MAS spectrum of the material obtained by reacting 1 with silica under the MW heating condition exhibited a signal assigned to the silicon attached to silica (d14.4 ppm). The 13 C CP/MAS spectrum contained four signals, which corresponded to the carbons of the propyl moiety (d 47.4, 26.3, and 14.5 ppm) and the methyl groups on the silicon atom (d 1.5 ppm). Regardless of the heating method, the chemical shifts in the 29Si and 13C CP/MAS spectra were essentially the same for all modified silicas obtained using 1e3, indicating that the 3-chloropropyldimethylsilyl group was cleanly grafted onto silica via a siloxane (SieOeSi) bond. The loading amount of the 3-chloropropyldimethylsilyl group was determined by elemental analysis of chlorine. Table 1 summarizes the loading amounts obtained by both heating methods and lists the MW effect, which we defined as the ratio of the loading amount by MW heating to the loading amount by oil-bath heating. The MW effect strongly depended on the silylating agents and solvents. The grafting reactions in a heptane solution using 1 (methoxy) and 3a (p-anisyl) were remarkably accelerated by MW heating. The loading amounts of 1 and 3a by MW heating were about one and a half times larger than those by oil-bath heating at the same temperature for 1 h, and were comparable to the loading amounts after 24 h of oil-bath heating (entries 1 and 3). In contrast, for 2 (allyl), 3b (p-tolyl), and 3c (phenyl), the loading amounts for MWirradiated grafting were almost equal to or less than those for conventionally heated experiments (entries 2, 4, and 5). On the other hand, the results of the grafting reaction drastically differed in a toluene solution. Compared to the reaction in heptane, all the silylating agents had reduced loading amounts. Only trace amounts of 3b and 3c were loaded. Additionally, the loading amounts obtained by MW heating were virtually identical to those by oil-bath heating (entries 6e10) [8]. Interestingly, a distinct MW
MW effectb
Loading amount (mmol/g)a MWc
Substance 1 2 3a 3b 3c Heptane Toluene
Oil bathd 1h
24 h
0.51 0.74 0.37 0.09 0.16 0.42 0.29 0.07 e e
0.74 0.95 0.52 0.54 0.52 0.50 0.74 0.27 Trace Trace
1.6 0.85 1.5 0.89 0.94 0.88 0.83 1.0 e e
a
Determined by elemental analysis of chlorine. Ratio of the loading amount by MW heating to the loading amount by oil-bath heating for 1 h. c MW heating at 80 C for 1 h. d Oil-bath heating at 80 C for 1 h or 24 h. b
Fig. 1. Dependency of the MW effect on the dielectric loss factor of the silylating agent.
N. Fukaya et al. / Journal of Organometallic Chemistry 696 (2011) 825e828
effect, which was observed for the grafting of 1 and 3a in heptane, was not observed in the reaction in toluene. To elucidate the reason why the effect of MW irradiation on organic grafting differed among silylating agents and solvents, we investigated the correlation between the dielectric properties of the silylating agents as well as solvents and the MW effect. The conversion efficiency of MW energy into thermal energy depends on the dielectric properties of materials. The fundamental relationship is described by
P ¼ ue0 e00 jEj2 where P represents power dissipation (conversion of MW to thermal energy), u is the angular frequency, E is the electric field of the sample, 30 is the permittivity of free space, and 300 is the dielectric loss factor (the imaginary part of the complex permittivity) [5a]. Therefore, the conversion of MW energy into thermal energy should be more effective for materials with larger 300 . Table 2 lists the dielectric properties at 2.45 GHz of the silylating agents and the solvents used in this study, which were determined by the resonant cavity perturbation method. The dielectric loss factors (300 ) of 1e3 ranged from 0.664 to 1.217. 1 and 3a were stronger MW absorbers than the other silylating agents. The dielectric loss factor of heptane was about one-fifteenth that of toluene. Fig. 1 plots the MW effect against the dielectric loss factor of the silylating agent. For the grafting reaction in MW-transparent heptane, the MW effect increased as the dielectric loss factor increased. Therefore, the remarkable MW effect observed for 1 and 3a could be rationalized as a consequence of selectively heating strongly MW-absorbing 1 and 3a. In contrast, the MW effect for the reaction in toluene scarcely depended on the dielectric loss factor. Additionally, the difference in the organic loading between the heating methods was small. Although the dielectric loss factor of toluene was much smaller than those of the silylating agents, the amount of toluene in the reaction mixture (5 cm3) greatly exceeded that of the silylating agent (0.2e0.3 cm3). The total thermal energy generated by MW irradiation is determined not only by the dielectric properties of MWabsorbing materials but also by the quantity of the materials [5a]. Therefore, it is possible that MW absorption by toluene masked the effect derived from selectively heating 1 and 3a. Consequently, the apparent MW effect could not be observed even when using a strongly MW-absorbing silylating agent such as 1 and 3a. 3. Conclusions In the organic functionalization of a silica surface via a reaction of organosilanes and silica, the MW acceleration effect strongly depended on the leaving group of the organosilanes as well as the reaction solvent. A distinctive acceleration was observed using methoxysilane and p-anisylsilane in heptane. Analysis of the dielectric properties of the organosilanes suggests that the observed MW effect is due to selectively heating the silylating agent, which has a strong MW-absorbing nature. Alkoxysilanes are occasionally difficult to synthesize and/or handle because they are easily hydrolyzed. Hence, using stable p-anisylsilanes in combination with MW irradiation should be a promising method to efficiently construct structurally diverse organo-functionalized silicas, including silica-immobilized molecular catalysts. 4. Experimental 4.1. Organic grafting onto a silica surface Organic grafting of the silica surface via MW irradiation (2.45 GHz) was carried out in a CEM Discover instrument equipped
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with a magnetic stirrer and a temperature/power controller using a glass vial (10 cm3). Silica gel (0.25 g) and a silylating agent (1.5 mmol) were placed into a nitrogen-purged vial. The vial was capped with a Teflon-coated septum and either dry heptane or dry toluene (5 cm3) was added. The vial was placed into the MW unit, and heated at 80 C for 1 h. The temperature of the reaction mixture reached 80 C within 100 s for the reactions in both toluene and heptane. The temperature profiles for the reactions of 3a with silica in heptane and toluene under the MW heating were shown in Fig. S7 in Supplementary data. Organic grafting via conventional heating was performed by immersing the vial into an oil bath, which was preheated to 80 C, for 1 h or 24 h. The modified silica was filtered, washed successively with ethyl acetate (10 cm3) and dichloromethane (10 cm3), and then dried at 80 C under a vacuum for 3 h. 4.2. Measurements of complex dielectric constants Complex dielectric constants (30 ej300 ) of silylating agents and solvents were measured by the perturbation method with a Kanto Electronic Application & Development system, which was comprised of an Agilent 8720ES vector network analyzer and a 2.45 GHz cylindrical cavity resonator [9]. The measurements were conducted in TM020 mode using Teflon-type sample tubes. The resonance frequency, quality factor of the empty cavity, and their shifts by the insertion of sample materials into the center of the cavity were measured. The dielectric constants were calculated from the resonance frequency shifts. The loss factors were calculated from the changes in the quality factor. 4.3. Determination of surface silanol group on silica The amount of the surface silanol group on the silica support was determined by the weight loss of a silica powder over a temperature range of 25e1000 C [10] which was monitored by TG-DTA (Bruker AXS, TG-DTA200SA). Acknowledgments This research was financially supported in part by the Development of Microspace and Nanospace Reaction Environment Technology for Functional Materials Project of NEDO, Japan. We thank Ms. Hiroko Kobashi for measuring the dielectric properties. Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.jorganchem.2010.10.008. References [1] (a) Special issue on organiceinorganic nanocomposites, Chem. Mater. 13 (10) (2001); (b) C. Sanchez, P. Gómez-Romero (Eds.), Functional Hybrid Materials, WileyVCH, Weinheim, 2004; (c) G. Kickelbick (Ed.), Hybrid Materials: Synthesis, Characterization, and Applications, Wiley-VCH, Weinheim, 2007. [2] E.P. Plueddemann, Silane Coupling Agents, second ed. Plenum, New York, 1991. [3] (a) T. Shimada, K. Aoki, Y. Shinoda, T. Nakamura, N. Tokunaga, S. Inagaki, T. Hayashi, J. Am. Chem. Soc. 125 (2003) 4688; (b) K. Aoki, T. Shimada, T. Hayashi, Tetrahedron: Asymmetry 15 (2004) 1771; (c) Y.-R. Yeon, Y.J. Park, J.-S. Lee, J.-W. Park, S.-G. Kang, C.-H. Jun, Angew. Chem., Int. Ed. 47 (2008) 109. [4] N. Fukaya, H. Haga, T. Tsuchimoto, S. Onozawa, T. Sakakura, H. Yasuda, J. Organomet. Chem. 695 (2010) 2540. [5] (a) C. Gabriel, S. Gabriel, E.H. Grant, B.S.J. Halstead, M.P. Mingos, Chem. Soc. Rev. 27 (1998) 213; (b) C.O. Kappe, D. Dallinger, Mol. Divers. 13 (2009) 71.
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[6] (a) A. Procopio, G. Das, M. Nardi, M. Oliverio, L. Pasqua, ChemSusChem 1 (2008) 916; (b) A. Procopio, G. De Luca, M. Nardi, M. Oliverio, R. Paonessa, Green Chem. 11 (2009) 770; (c) N. García, E. Benito, J. Guzmán, R. de Francisco, P. Tiemblo, Langmuir 26 (2010) 5499. [7] T. Nakashima, R. Fujiyama, H.J. Kim, M. Fujio, Y. Tsuno, Bull. Chem. Soc. Jpn. 73 (2000) 429.
[8] The reason the MW effects in entries 2 and 4e7 in Table 1 were slightly less than 1.0 might be due to lower stirring efficiency of reaction mixtures in the MW instrument (the actual stirring speed has not been disclosed by the instrument vendor). Such influences of stirring speed on the MW and oil-bath heating reactions have been described, see M. Irfan, M. Fuchs, T.N. Glasnov, C.O. Kappe, Chem.-Eur. J. 15 (2009) 11608. [9] H. Kawabata, H. Tanpo, Y. Kobayashi, IEICE Trans. Electron. E87-C (2004) 694. [10] R. Mueller, H.K. Kammler, K. Wegner, S.E. Pratsinis, Langmuir 19 (2002) 160.