pubs.acs.org/Langmuir © 2009 American Chemical Society
Thermally Induced Radical Hydrosilylation for Synthesis of C18 HPLC § phases from Highly Condensed SiH Terminated Silica Surfaces Nicolas Plumere,† Bernd Speiser,*,† Benjamin Dietrich,† Klaus Albert,† Joseph J. Pesek,*,‡ and Maria T. Matyska‡ †
Institut f€ ur Organische Chemie, Universit€ at T€ ubingen, Auf der Morgenstelle 18, 72076 T€ ubingen, Germany, and ‡ Chemistry Department, San Jose State University, One Washington Square, San Jose, California 95192 Received June 3, 2009. Revised Manuscript Received July 21, 2009
Silicon hydride terminated silica surfaces were prepared at high temperatures by a chlorination-reduction sequence. SiH groups are desired for further surface modification as an alternative to the native silanol groups which are unfavorable for RPLC applications. Only few silanol groups remain in these materials and mostly SiH moieties with the highest degree of cross-linking are obtained. The retention properties of basic analytes on the SiH terminated material confirm that the surfaces is mostly free of silanols and that therefore the remaining SiOH groups are bulk species. A reagentless, radical initiated hydrosilylation reaction is introduced for the functionalization of the hydride terminated surface with 1-octadecene. 13C CP/MAS NMR and DRIFT spectroscopy demonstrate the reaction of the carboncarbon double bond and the SiH group as well as the linkage of C18 groups to the silica surface. These novel C18 materials show promising performance in RPLC separation, especially for the separation of organic bases.
Introduction The use of silica as a support has greatly contributed to the improvement and widespread use of high pressure liquid chromatography (HPLC). Reversed-phase liquid chromatography (RPLC) employs modified nonpolar silica stationary phases. However, residual acidic silanol groups are present in these materials. They induce peak tailing and loss of chromatographic resolution of basic analytes.1-4 Isolated (nonhydrogen bonded) silanols are the most acidic and believed to be responsible for the undesired interactions with organic bases.2 The presence of acidic silanol groups also enhances the sensitivity of the bonded phase toward hydrolysis.1,5,6 One approach to minimize the effect of the isolated silanol moieties consists in the rehydroxylation of the silica surface.5 The aim of this strategy is to obtain a high SiOH surface concentration to ensure that most groups can interact via hydrogen bonds. An opposite and more common method to improve the separation efficiency consists in the end-capping of the remaining silanol groups after organosilanization.2,7 Although this method is not quantitative,8 the tailing is reduced and the chromatographic resolution enhanced because the end-capping groups block the access to the silanols groups.2 Bulky substituents9 on the organic § Dedicated to Professor Dr. Ekkehard Lindner on the occasion of his 75th birthday. *Address correspondence to either author. E-mail: bernd.speiser@ uni-tuebingen.de (B.S.);
[email protected] (J.J.P.).
(1) Stella, C.; Rudaz, S.; Veuthey, J.-L.; Tchapla, A. Chromatographia Suppl. 2001, 53, S113–S131. (2) Nawrocki, J. J. Chromatogr., A 1997, 779, 29–71. (3) Wirth, M. J.; Fairbank, R. W. P.; Fatunmbi, H. O. Science 1997, 275, 44–47. (4) Sudo, Y.; Wada, T. J. Chromatogr., A 1998, 813, 239–246. (5) K€ohler, J.; Kirkland, J. J. J. Chromatogr. 1987, 385, 125–150. (6) K€ohler, J.; Chase, D. B.; Farlee, R. D.; Vega, A. J.; Kirkland, J. J. J. Chromatogr. 1986, 352, 275–305. (7) Sudo, Y. J. Chromatogr. A 1997, 757, 21–28. (8) Pesek, J. J.; Matyska, M. T. J. Sep. Sci. 2005, 28, 1845–1854. (9) Kirkland, J. J.; Glajch, J. L.; Farlee, R. D. Anal. Chem. 1989, 61, 2–11. (10) Atamna, I. Z.; Muschik, G. M.; Issaq, H. J. J. Liq. Chromatogr. Relat. Technol. 1990, 13, 863–873. (11) Buszewski, B.; Kulpa, M. J. Liq. Chromatogr. Relat. Technol. 1993, 16, 75–94. (12) Buszewski, B.; Schmid, J.; Albert, K.; Bayer, E. J. Chromatogr. 1991, 552, 415–427.
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modifiers, bidendate silanes,9 high bonding density10,11, or electrostatic shielding12-14 were also used to restrict accessibility.2 Silanol groups can be fully avoided if the stationary phases are based on alternative materials like organic polymers, graphitic carbon, alumina, titania, and zirconia.1 However, organic polymers exhibit decreased mechanical stability. Graphitic carbon possesses lower sample loading capacities and is less efficient than silica based materials. The surface hydroxyl groups of the alternative metal oxides also strongly interact with basic analytes. An improvement in the chromatographic performance, without the drawbacks of the SiOH groups, was obtained from the use of siliceous/organic hybrid materials.15 The SiOH groups of silica based materials may also be replaced by other functional groups for further surface modification: The so-called “type C” silica phases, involving SiH modified surfaces and hydrolytically stable SiC bond formation by hydrosilylation reactions, were successfully developed to enhance the stability of silica-based bonded phases against acid.8,16 However, the various SiH modification procedures17,18 previously used for HPLC applications also leave unreacted SiOH groups and especially T2H moieties (Scheme 1) on the silica surface, which again limits their performance. As a means of reducing the residual SiOH groups, we have recently published a procedure for the preparation of SiH terminated silica surfaces involving a chlorination-reduction sequence at high temperatures.19 The procedure, performed at about 900 °C, results in a highly condensed surface: the reaction of SiOH groups by reduction or dehydroxylation is almost (13) O’Gara, J. E.; Alden, B. A.; Walter, T. H.; Petersen, J. S.; Niederl€ander, C. L.; Neue, U. D. Anal. Chem. 1995, 67, 3809–3813. (14) O’Gara, J. E.; Walsh, D. P.; Alden, B. A.; Casellini, P.; Walter, T. H. Anal. Chem. 1999, 71, 2992–2997. (15) Wyndham, K. D.; O’Gara, J. E.; Walter, T. H.; Glose, K. H.; Lawrence, N. L.; Alden, B. A.; Izzo, G. S.; Hudalla, C. J.; Iraneta, P. C. Anal. Chem. 2003, 75, 6781–6788. (16) Sandoval, J. E.; Pesek, J. J. Anal. Chem. 1991, 63, 2634–2641. (17) Chu, C.-H.; Jonsson, E.; Auvinen, M.; Pesek, J. J.; Sandoval, J. E. Anal. Chem. 1993, 65, 808–816. (18) Sandoval, J. E.; Pesek, J. J. Anal. Chem. 1989, 61, 2067–2075. (19) Plumere, N.; Speiser, B.; Mayer, H. A.; Joosten, D.; Wesemann, L. Chem.; Eur. J. 2009, 15, 936–946.
Published on Web 08/24/2009
DOI: 10.1021/la901986w
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Plumer e et al. Scheme 1. Nomenclature of Si;H Moieties
Scheme 2. Hydrosilylation Reaction with Surface Si;H Groups
quantitative while highly cross-linked T3H groups (Scheme 1) are the main product (although this is, in fact, a dehydration, the latter term is used in silica chemistry to describe the removal of physically adsorbed water from the surface, whereas dehydroxylation is defined as the removal of silanol groups).20 This type of Si;H groups as well as the absence of silanol groups are desired for the stability, homogeneity and inertness of the matrix. This contrasts with SiH modifications obtained from low temperature methods where bulk silanol groups remain after the reaction,17,18 and a significant proportion of T2H groups are produced.17 Moreover, from the chlorination-reduction sequence a high SiH surface concentration is obtained and the physical properties of the materials, such as shape, pore size and surface area, remain essentially unchanged.19 Thus, the chlorination-reduction sequence represents an ideal strategy to provide the starting material for the synthesis of RPLC stationary phases based on SiH modified silica surfaces. The application of this method for the synthesis of “type C” silica based materials may enhance their HPLC separation quality. Because of its low polarity and high bond strength the siliconcarbon bond is best suited for the immobilization of HPLC selectors. Hydrosilylation is the best documented method for SiC bond formation starting from SiH modified surfaces16,18,21-23 (Scheme 2). Hexachloroplatinic acid successfully catalyzes the hydrosilylation of HPLC separation selectors on porous silica.16 However, such a homogeneous catalyst leads to metal contaminations of the surface.24 These form strong adsorption sites for complexing, and especially for chelating analytes, causing peak asymmetry and poor resolution.25 Therefore, the use of transition metal catalysts is not appropriate for the synthesis of separation phases. As an alternative, the hydrosilylation may be induced by free radical starters.24 In this case, the radical starter may also react directly with the surface.26 In order to fully avoid impurities from catalytic or radical initiator residues, a reagentless radical addition would be advantageous. (20) Zhuravlev, L. T. Colloids Surf. A 2000, 173, 1–38. (21) Pesek, J. J.; Matyska, M. T.; Oliva, M.; Evanchic, M. J. Chromatogr. A 1998, 818, 145–154. (22) Marciniec, B.; Gulinski, J.; Urbaniak, W.; Kornetka, Z. W. Comprehensive Handbook on Hydrosilylation; Pergamon: Oxford, 1992. (23) Wiberg, E.; Amberger, E. In Hydrides of the Elements of Main Groups I-IV; Elsevier: Amsterdam, 1971; pp 532 - 562. (24) Pesek, J. J.; Matyska, M. T.; Williamsen, E. J.; Evanchic, M.; Hazari, V.; Konjuh, K.; Takhar, S.; Tranchina, R. J. Chromatogr., A 1997, 786, 219–228. (25) Engelhardt, H.; Lobert, T. Anal. Chem. 1999, 71, 1885–1892. (26) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155.
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Scheme 3. Mechanism for Radical-Based Hydrosilylation on Silicon Surfaces33
Indeed, the addition of chlorosilanes to isolated or conjugated CdC bonds under homogeneous conditions, either at high pressure and temperature23 or under high energy irradiation23,27-29 has been reported. Similar methods, with either photochemical30,31 or thermal initiation26,32 are also employed for the hydrosilylation on hydrogen-terminated Si surfaces.33-35 The reaction is believed to start with the homolytic cleavage of the SiH bond (Scheme 3),23,36,37 although the exact origin of the Si radical is not yet defined unequivocally.33,34 A concerted mechanism has also been postulated.38 The free radical initiated hydrosilylation on the silicon surface is supported by the electron withdrawing effect of the Si bulk matrix,39 through a decrease in SiH bond energy. Molecular oxygen as well as traces of silanol groups may be sources of initiators for the formation of a silicon radical.34 Photochemical conditions provide a reagentless way to induce the radical hydrosilylation reaction40 at low temperatures. On SiH terminated silica surfaces, the reaction of carboncarbon double bonds under UV irradiation was described previously for nonporous materials.41 Since amorphous silica is not transparent to UV light, a photochemical reaction is expected to occur only on the outer surface of porous material. The homolytic cleavage of the SiH groups within pores may be achieved by thermal induction. Indeed, the heterogeneous hydrosilylation on SiH terminated silica surfaces with small olefin molecules in the gaseous state under high pressure and temperature conditions was described.42 In this paper we explore the reaction of SiH modified surfaces of porous silica, prepared from the chlorination-reduction (27) Kulicke, K. J.; Giese, B. Synlett 1990, 1, 91–92. (28) Pietrusza, F. W.; Sommer, L. H.; Whitmore, F. C. J. Am. Chem. Soc. 1948, 70, 484–486. (29) Calas, R.; Duffaut, N. Bull. Mens. Inf. ITERG 1953, 7, 438–440. (30) Effenberger, F.; G€otz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem. 1998, 110, 2651-2654; Angew. Chem. Intl. Ed. 1998, 37, 2462-2464. (31) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688– 5695. (32) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Mass, J. H.; de Jeu, W. H.; Zuilhof, H.; Subh€olter, E. J. R. Langmuir 1998, 14, 1759–1768. (33) Buriak, J. M. Chem. Rev. 2002, 102, 1272–1308. (34) Mischki, T. K.; Lopinski, G. P.; Wayner, D. D. M. Langmuir 2009, 25, 5626–5630. (35) Holm, J.; Roberts, J. T. Langmuir 2009, 25, 7050–7056. (36) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188–194. (37) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305–307. (38) Coletti, C.; Marrone, A.; Giorgio, G.; Sgamellotti, A.; Cerofolini, G.; Re, N. Langmuir 2006, 22, 9949–9956. (39) Kanabus-Kaminska, J. M.; Hawari, J. A.; Griller, D.; Chatgilialoglu, C. J. Am. Chem. Soc. 1987, 109, 5267–5268. (40) Faucheux, A.; Gouget-Laemmel, A. C.; Henry de Villeneuve, C.; Boukherroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J.-N. Langmuir 2006, 22, 153–162. (41) Plumere, N.; Speiser, B. Electrochim. Acta 2007, 53, 1244–1251. (42) Tertykh, V. A.; Belyakova, L. A. Solid-Phase Hydrosilylation Reactions with Participation of Modified Surface. In Adsorption on New and Modified Inorganic Sorbents; Da-brovsky, A., Tertykh, V. A., Eds.; Elsevier: Amsterdam, 1996; Chapter 1.6, pp 147-189.
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sequence, with 1-octadecene under high temperature conditions in a condensed phase. The properties of the resulting alkylated silica as chromatographic sorbent will be investigated with the standard reference material 870 (SRM 870) from the National Institute of Standards and Technology (NIST).43 The same reference material will be used to study the interactions of the SiH modified surfaces with basic analytes as a probe for the presence of remaining surface silanol groups.
Experimental Section Materials. All chemicals were reagent grade and used as supplied by Aldrich. The solvents for chromatographic evaluation were HPLC grade. SRM 870 was obtained from Merck. The Kromasil particles (M2, 5 μm, 300 A˚ pore diameter) were received from EKA Chemicals (the same nomenclature will be used in the present paper for the SiH modified Kromasil particles (M2SiH) as in ref 19). Analytical Techniques. The 1H and 13C solution NMR spectra were measured on a Bruker Advance 400 spectrometer, which operated at 400.16 MHz for 1H and 100.62 MHz for 13 C nuclei. All high resolution NMR spectra were recorded at 295 K in CDCl3, chemical shifts were calibrated to the residual proton and carbon resonance of the deuterated solvent. 29 Si CP/MAS NMR experiments were performed as described in ref 19. 13C CP/MAS NMR experiments were performed in 4 mm ZrO2 rotors with a Bruker DSX 200 spectrometer operating at a resonance frequency of 200.13 MHz for 1H and 50.33 MHz for 13C. About 200 000 transients were accumulated at 300 K. A relaxation delay of 1 s and a contact time of 3 ms were chosen. The proton 90° pulse length was set to 3.5 μs. DRIFT experiments were performed on a Bruker IFS 25 IR spectrometer with the Praying Mantis DRIFT unit from Harrick. The samples were dried at 100 °C under reduced pressure for several hours and mixed with dry KBr at a ratio of 1:20. The DRIFT spectra were recorded from 4000 to 500 cm-1 versus pure KBr as blank. The specific SiH amount was semiquantitatively obtained from the integration of the stretching vibration bands in the DRIFT spectra. To enable an accurate comparison between different spectra, the integration value from the signal of the Si;O combination band at 1870 cm-1 was used as an internal standard.44 The integration values for νSiH were normalized with respect to this integral. The quantitative determination of the specific SiH amount was obtained by gas chromatographic analysis of the hydrogen evolved after treatment of the SiH material with a proton source (ethanol or water) in presence of a base catalyst (KOH) as described earlier.19 Synthesis of Hydride Modified Silica (M2SiH). Hydride modified silica M2SiH was prepared according to ref 19. IR (DRIFT): ν∼ (cm-1), 3746 (w, νOH, isolated SiOH), 37353450 (w, νOH, hydrogen bonded SiOH), 2284 (s, νSiH), 1981, 1874, and 1636 (m, combination of vibrations of the SiO2 network45), 1260-1000 (s, νSiOSi, asymmetric). Synthesis of 1-Octadecene Modified Silica (M2C18). Hydride modified silica M2SiH (2 g) was dried under vacuum at 100 °C overnight and suspended in neat 1-octadecene (50 mL, 90%). The suspension was purged with argon for 30 min and heated to 180 °C for 3 days. During both steps the suspension was stirred. After the reaction, the silica was separated from the solvent by centrifugation and washed with hexane in a Soxhlet apparatus overnight. Finally, the silica was dried overnight under vacuum at 110 °C. 13C CP/MAS NMR (50.32 MHz): δ (ppm) 12.42 (br, SiCH2, ;CH3), 22-35 (;CH2;). IR (DRIFT): ν∼ (cm-1), 3760-3550 (w, νOH, SiOH), 2960 (m, νCH, CH3), (43) Sander, L. C.; Wise, S. A. J. Sep. Sci. 2003, 26, 283–294. (44) Murthy, R. S. S.; Leyden, D. E. Anal. Chem. 1986, 58, 1228–1233. (45) Lenza, R. F. S.; Vasconcelos, W. L. Mater. Res. 2001, 4, 189–194.
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Figure 1. DRIFT spectrum of M2SiH; inset: Gaussian deconvolution (top, gray lines) with envelope (bottom, dashed line) of the expended νSiH signal (bottom). 2927, 2857 (s, νCH, CH2), 2279 (s, νSiH), 1981, 1870, and 1636 (m, combination of vibrations of the SiO2 network45), 1466 (w, δCH), 1200-1000 (νSiOSi). Elemental analysis: C=3.36%. Recovered 1-octadecene after the reaction: 1H NMR (CDCl3, 400.16 MHz): δ (ppm) 0.91 (t, 3H, CH3), 1.29 (m, 26H, ;CH2;), 1.41 (m, 2H, ;CH2;), 2.07 (m, 2H, CH2dCH;CH2;), 4.945.04 (m, 2H, CH2=CH;), 5.79-5.89 (m, 1H, CH2dCH;). 13 C{1H};NMR (CDCl3, 100.62 MHz): δ (ppm), 14.13 (;CH3), 22.73 (;CH2;CH3), 28.99, 29.21, 29.42, 29.57, 29.68, 29.75 (;CH2;), 31.98 (;CH2;CH2;CH3), 33.87 (CH2dCH; CH2;), 114.07 (CH2dCH;), 139.23 (CH2dCH;). HPLC Column Packing. The chromatographic sorbents M2SiH and M2C18 were slurry-packed into 125 4.6 mm stainless steel columns from Bischoff at 35 MPa employing a Knauer pneumatic HPLC pump. Thus, 1.6 g of the modified silica were dispersed in 25 mL of 2-propanol by sonication in an ultrasonic bath for about 10 s. This suspension was poured into the reservoir of the packing system and the system was topped off. The column was downward packed with 2-propanol as solvent. The excess of stationary phase on the top of the column was carefully removed, the inlet frit and end-fitting were installed and the ends plugged. The columns were conditioned for 10 h with a methanol/water mobile phase at a flow rate of 0.1 mL min-1. HPLC Tests. Chromatographic tests were performed at 23 °C using a series 1100 HPLC instrument (Agilent) with UV-detection at 210, 254, and 480 nm. All solvents were filtered and degassed before use. The test mixture (Standard Reference Material 870; SRM 870)43 was a methanolic solution of uracil, toluene, ethylbenzene, quinizarin, and amitriptyline. Under isocratic elution conditions, the mobile phase was 80% methanol/ 20% buffer (v/v). When a gradient elution was used, 60% methanol/ 40% buffer (v/v) as mobile phase was held 5 min and then a linear gradient to 80% methanol/ 20% buffer (v/v) was performed within 1 min. The buffer, 20 mmol L-1 aqueous potassium phosphate, was adjusted to pH 7.0 ( 0.1 by mixing solutions of the dibasic and monobasic forms of the buffer. The mobile phase had a flow rate of 2.0 mL min-1.
Results and Discussion Base Materials. The materials M2 studied are based on 5 μm porous silica particles (Kromasil). Synthesis and characterization of a SiH terminated Kromasil material were described recently.19 The presence of silicon monohydride groups on the silica surface was proven by DRIFT and 29Si CP/MAS NMR spectroscopy.19 Further information is obtained from the DRIFT spectrum after optimized Gaussian deconvolution of the νSiH band DOI: 10.1021/la901986w
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Figure 2.
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C CP/MAS NMR (left) and DRIFT (right) spectra of M2C18.
(Figure 1), resulting in two peaks at 2250 cm-1 and 2283 cm-1. The envelope generated by summation of the individual peaks excellently fits the measured spectrum. According to 29Si CP/ MAS NMR spectroscopic results, the main groups in M2SiH are T3H groups, i.e., SiH groups cross-linked to the silica matrix via 3 siloxane bonds (Scheme 1). Therefore the main component of the νSiH band at 2283 cm-1 may be assigned to these T3H groups. Those are the desired groups for further modification because of their high degree of cross-linking with the silica matrix. The T2H groups observed at 2250 cm-1 are only a minor component: The integration values of the T3H and T2H components represent 82 and 18% of the total νSiH band, respectively. If we assume equal extinction coefficients for the SiH groups in both T3H and T2H, their respective surface concentrations (Γ T3H and Γ T2H) are 2.62 μmol m-2 and 0.58 μmol m-2 (total ΓSiH =3.2 μmol m-2).19 The SiOH surface concentration (ΓSiOH) may also be estimated: In previous work,19 we demonstrated (DRIFT spectroscopy) that about 95% of the SiOH groups of M2 have reacted after chlorination and reduction. Since the ΓSiOH of native silica is 8 μmol m-2,2,3,20 the remaining ΓSiOH of M2SiH is about 0.40 μmol m-2, if we assume equal extent of reaction of the SiOH groups on the surface and in the bulk of the silica that is accessible to DRIFT spectroscopy. In comparison, for monomeric C18 phases, ΓSiOH ranges from 3 to 5 μmol m-2.46 The correlation between the estimates of ΓSiOH and Γ T2H in M2SiH suggests that the SiOH groups present in M2SiH are mostly due to the presence of the T2H groups. The accessibility or activity of the SiOH groups on M2SiH is assessed in the HPLC part of this paper. Immobilization of 1-Octadecene by Thermal Hydrosilylation. Thermal hydrosilylation requires temperatures above 160 °C. The high boiling point of 1-octadecene makes it possible to perform the high temperature modification of SiH terminated silica in neat olefin under atmospheric pressure. The porous M2SiH material was used as a hydride modified silica matrix to test the hydrosilylation under thermal initiation (Scheme 2, R= ;(CH2)14;CH3). The immobilization of 1-octadecene on the hydride modified silica M2SiH was observed at temperatures above 160 °C. We will denote the alkyl modifier with 18 carbon atoms as C18 in the following and the material obtained from this reaction is referred to as M2C18. In the thermal reaction, traces of O2 as well as remaining silanol groups might induce the radical mechanism as recently discussed for Si.34 (46) Scholten, A. B.; de Haan, J. W.; Claessens, H. A.; van de Ven, L. J. M.; Cramers, C. A. Langmuir 1996, 12, 4741–4747.
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Table 1. Characterization of Materials M2 before and after C18 Modification nSiHa/μmol g-1
relative νSiH integral b
M2SiH 337.3 1.41 230.0 0.91 M2C18 difference /% -31.7 -35.7 a Specific SiH amount from GC-TCD analysis. b From DRIFT measurements.
The presence of the alkyl groups on the silica surface after reaction, thorough Soxhlet washing and drying steps is unambiguously shown by the signals between 20 and 40 ppm in the 13 C CP/MAS NMR spectrum and between 2800 and 3000 cm-1 in the DRIFT spectrum (Figure 2). A quantitative determination of the C18 surface concentration (ΓC18) is obtained from elemental analysis and the specific BET surface area.15,47 Carbon could not be detected in the starting M2SiH materials. On the other hand, after C18 modification, the carbon content is 3.4%. By using the surface area of M2SiH (103.6 m2 g-1; determined from nitrogen adsorption/desorption isotherms),19 ΓC18 =1.5 μmol m-2 is calculated. The absence of signals between 110 and 140 ppm in the 13C CP/ MAS NMR spectrum and between 3000 and 3100 cm-1 in the DRIFT spectrum shows that the carbon-carbon double bond of 1-octadecene has fully reacted. Moreover, the intensity decrease of the SiH stretching vibration in the DRIFT spectrum upon hydrosilylation also shows that silicon hydride units have reacted. The relative amount of the reacted SiH groups was semiquantitatively determined from the DRIFT spectra. For this purpose, the materials were diluted in KBr, so that the absorbances of the signals between 1500 and 4000 cm-1 are low enough to allow their quantitative analysis.44 In addition, the absolute value of the specific SiH amount nSiH remaining in M2C18 was determined by the GC/TCD method described earlier.19 The comparison of the SiH IR-signal integration on one hand, and the nSiH obtained from GC/TCD measurements of M2SiH and M2C18 on the other hand (Table 1) show that about 1/3 of the initial silicon hydride groups have reacted during the hydrosilylation reaction. The absence of any CdC bond related signal and the decrease of the νSiH intensity in M2C18 provide indirect evidence for the formation of a SiC bond in a hydrosilylation reaction. In order to provide more direct evidence for this linkage, M2C18 was investigated with 29Si CP/MAS NMR spectroscopy. However, the TH (47) Berendsen, G. E.; de Galan, L. J. Liq. Chromatogr. Relat. Technol. 1978, 1, 561–586.
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Scheme 4. Possible Mechanism for the Oligomerization of 1-Octadecene on M2SiH Surface at High Temperature
signals due to the remaining SiH groups are broad and overlap with the region where T3 groups are expected in the spectrum (-65 ppm).48 In the 13C CP/MAS NMR spectrum of M2C18, the signal at 12.4 ppm may be assigned to the carbon atom of a SiC bond. However, it could also be attributed to the CH3 group of the C18 chain. The unsymmetrical shape of this signal indicates a possible overlapping contribution from both the CH3 and SiC resonances. Consequently, no final conclusion about the formation of a SiC bond can be drawn from the 13C NMR spectra. The absence of any unexpected signal in both DRIFT and solid state NMR spectra of M2C18 provides good evidence for the absence of side reactions occurring during the reaction. Also, the high resolution 1H and 13C NMR spectra of the 1-octadecene recovered from the reaction mixture after separation of the particles show only the well-known signals for 1-octadecene and for the impurities originally present in this hydrocarbon (≈10% of octadecane and branched octadecene). Therefore, the reaction of 1-octadecene on the particle surface occurs without polymerization in the solution. On the other hand, oligomerization (possibly telomerization) on the silica surface may occur.23,42 Indeed, the reaction between a silicon radical and the CdC bond yields, in a first step, a free radical center on the β-carbon in the addition product. This radical is expected to react with the resulting hydrogen radical or with an H atom from a neighboring SiH bond (Scheme 3). However, it may also react with another alkene (Scheme 4). This yields a new C;C σ bond, which, after subsequent repetition of this step, would result in short polymers of the C18 alkene covalently linked to the silica surface. The DRIFT and solid state NMR spectra do not make it possible to discriminate between the hydrosilylation product and this type of side reaction products. Indirect information about this side reaction is derived from the surface concentration ΓC18. From the GC-TCD analysis, we know that nSiH of M2C18 is lower than nSiH of M2SiH (Table 1). The data correspond to a decrease in ΓSiH of 1.04 μmol m-2. In comparison, ΓC18 is 1.5 μmol m-2. Since the amount of C18 material present on the surface of M2SiH is about 50% higher than the amount of SiH groups that have reacted, some oligomerization probably occurs. Another possible side reaction may be Markovnikov addition. Neither the solid state NMR spectrum nor the DRIFT spectrum enables to discriminate Markovnikov and anti-Markovnikov products on the silica surface. However, since the reaction is induced by a free radical, the addition of the SiH bond to the CdC bond is expected to yield the anti-Markovnikov product with high selectivity (Farmer’s rule).49 (48) Albert, K. J. Sep. Sci. 2003, 26, 215–224. (49) Marciniec, B.; Gulinski, J.; Urbaniak, W.; Kornetka, Z. W. In Comprehensive Handbook on Hydrosilylation; Pergamon: Oxford, 1992; p 11.
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Figure 3. Separation of SRM 870 with the chromatographic sorbent M2SiH under isocratic elution conditions (MeOH/buffer 80/20) with UV detection at 254 nm.
The free radical initiated hydrosilylation reactions can successfully be applied to modify SiH terminated silica surfaces. Evidence for a strong attachment of the organic molecules on the silica surface is observed. However, only indirect evidence for the SiC bond formation was established. Moreover, a radical initiated surface oligomerization during the hydrosilylation is suspected. This is, however, not expected to impair the chromatographic performance of the materials. HPLC Separation Experiments. The standard reference material 870 (SRM 870) from the National Institute of Standards and Technology (NIST)43 was used as a test mixture to examine the general characteristics of the SiH and C18 bonded materials M2SiH and M2C18 as chromatographic sorbents. SRM 870 is a methanolic solution of uracil, toluene, ethylbenzene, quinizarin and amitriptyline. Uracil is used as an indicator of the void time of the LC column. The retention of the nonpolar components (ethylbenzene and toluene) provides a measure of the column retentiveness (hydrophobicity). Quinizarin (1,4-dihydroxyanthraquinone) is a strong metal chelating agent. The asymmetry of the quinizarin peak is indicative of the presence of metals in the chromatographic system. Amitriptyline is an organic base. The asymmetry of its peak is an appropriate measure of the silanol activity of the separation phase.43 The HPLC separation is, by convention, performed under isocratic elution conditions. Gradient elution was also used for optimization of the separation in some experiments. HPLC Separation of SRM 870 with M2SiH. M2SiH is the base material for the synthesis of the C18 phase and is not intended to be used as such for HPLC applications. As a support for the selectors, the absence of a contribution of M2SiH to the HPLC separations is preferred. The HPLC separation is performed with M2SiH in order to assess the extent of its interaction with the standard analytes of SRM 870. The synthesis of M2SiH was designed to obtain a silanol free surface. Therefore, the HPLC separation of amitryptilin is of particular interest in this case. For the M2SiH materials, only two distinct signals are observed in the chromatogram (Figure 3). The peaks were identified on the basis of the relative absorption of the compounds at the various detection wavelengths (210, 254, and 480 nm). Based on the signal intensity in the spectrum at 254 nm as well as the absence of absorbance at 480 nm, the signal at t=1.30 min is attributed to DOI: 10.1021/la901986w
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Table 2. Chromatographic Characteristics of M2SiH under Isocratic Elution Conditionsa k0
N/m-1
amitriptyline 0.52 28000 a UV detection at 254 nm, turacil = t0 = 0.93 min.
As 2.02
amitriptyline. The other components coelute without significant retention at t=t0 =0.93 min. The use of a gradient elution (not shown) does not enable a better separation. The coelution and low retention times show that the SiH groups, as well as the siloxane bonds,2 do not interact with the polar (quinizarin) and nonpolar components (toluene, ethylbenzene) of SRM 870 under these standard RPLC conditions. Amitriptylin is the only compound of SRM 870 that shows a significant retention. The separation of amitriptyline from the other components makes it possible to determine the peak asymmetry (As; Table 2) which is an appropriate measure of the silanol activity.43 The peak asymmetry value of about 2 for amitriptyline is low. In comparison, modern C18 phases with end-capping and embedded polar groups typically display As values for amitryptiline ranging from 1.5 to 5.7.43 This indicates a weak activity of the M2SiH materials toward the undesired retention of the organic base by the silanol groups, although no end-cappping was performed and no bulky groups block the access to the surface. This result may seem unexpected with regard to the spectroscopic characterization of these materials: According to 29Si CP/ MAS NMR investigations, Q3 groups (SiOH groups cross-linked via three siloxane bonds) are still present in M2SiH.19 Moreover, with the help of DRIFT spectroscopy, a ΓSiOH of about 0.40 μmol m-2 was estimated (see above). Although the amount of SiOH is small, a large peak asymmetry for basic solutes is expected. Generally, attempts to fully eliminate the silanol groups in order to diminish the silanol activity, by dehydroxylation for example, are counter productive:6 if only few silanol groups remain, those would be of the isolated type which are the most able to undergo ion-exchange interactions with organic bases.2 Indeed, the presence of isolated silanol groups in M2SiH resulting from the chlorination-reduction sequence, is confirmed by the sharp signal at 3746 cm-1 in the DRIFT spectrum (Figure 1). Still, the HPLC separation of amitriptyline with M2SiH does not display significant peak tailing (As =2.02). Consequently, the Q3 groups detected by solid state NMR spectroscopy are not located on the surface and even the isolated silanol groups detected by DRIFT spectroscopy are not accessible to the basic solutes in the mobile phase. The surface silanol groups, which were accessible to chlorination and reduction have reacted to a large extent. On the other hand, the bulk silanol groups only undergo condensation which is known to leave unreacted isolated silanol groups whose amount depends on the applied temperature.20 The HPLC evaluation demonstrates that in these experiments the M2SiH materials have a low silanol activity and therefore the SiOH groups are mainly bulk species. Moreover, the silicon hydride, which is the main functional group present on the surface, does not show significant interaction with the other components of SRM 870. Therefore, the material obtained from the chlorination-reduction sequence displays the desired inertness for a base material in RPLC separation. HPLC Separation of SRM 870 with M2C18. The M2C18 phase obtained from the thermal hydrosilylation reaction of 1-octadecene on the M2SiH material is expected to display an increased retention of the components of SRM 870 as compared 13486 DOI: 10.1021/la901986w
Figure 4. Separation of SRM 870 with the chromatographic sorbent M2C18; Gradient elution: 60% methanol/ 40% buffer (v/v) to 80% methanol/20% buffer (v/v). Table 3. Chromatographic Characteristics of M2C18 with Gradient Elutiona k0
N/m-1
As
toluene 0.30 32 100 ethylbenzene 0.47 31 800 quinizarin 1.70 22 100 1.72 amitriptyline 10.38 39 800 2.96 a UV detection at 254 nm, turacil = t0 = 0.93 min, Rethylbenzene/toluene = 1.57.
Table 4. Chromatographic Characteristics of M2C18 under Isocratic Elution Conditions [a] k0
N/m-1
quinizarin 0.58 11 000 amitriptyline 5.94 4700 [a] UV detection at 254 nm, turacil = t0 = 0.87 min.
As 3.34 3.59
to M2SiH due to the presence of the C18 selectors. Indeed, the separation of all five compounds of SRM 870 is achieved when a gradient elution is performed (Figure 4). The elution order is in agreement with the elution order obtained with other C18 phases (chromatograms of SRM 870 on various commercial C18 phases are given in ref 43). The separation factor of ethylbenzene and toluene is about 1.57 under the gradient elution conditions. The chromatographic results for the individual components of SRM 870 are given in Table 3. The resolution of toluene and ethylbenzene is not sufficient in order to determine As of these two components. In order to compare the chromatographic characteristics of the M2C18 material with other separation phases, the HPLC test must be performed under isocratic conditions.43 In this case, ethylbenzene and toluene almost coelute with a retention factor of 0.18. Therefore, M2C18 as a HPLC separation phase shows poor methylene selectivity. Optimization of the thermal hydrosilylation reaction, in order to achieve higher surface concentration and lower the possible side reactions, may improve the separation quality. On the other hand, quinizarin and amitriptyline are well separated under the standard test conditions. The chromatographic characteristics for these two components are given in Table 4. The peak for amitriptyline shows an increased asymmetry compared to the separation with M2SiH. This indicates that the Langmuir 2009, 25(23), 13481–13487
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surface concentration of isolated silanol groups has increased during the hydrosilylation reaction. The presence of water traces in the hydrosilylation reaction mixture may explain the production of silanol groups by rehydroxylation of the surface siloxane bonds under the high temperature conditions. However, the peak tailing remains moderate (As=3.59). This value is in the range of those obtained for C18 columns with steric or electrostatic blocking of the silica surface.43 Therefore, the production of isolated silanol groups is only a minor side reaction. This is in accordance with the DRIFT spectrum of M2C18 (Figure 2) which shows only a limited increase in the intensity of the silanol stretching vibration. The asymmetry factor As of quinizarin is typically between 1 and 4 for commercial columns.43 Within this range, the M2C18 phase displays a relatively high peak tailing (As=3.34). This could indicate the presence of some metal ions on the silica surface. Still, according to the As value, the amount of metal impurities is not higher than is observed for commercial type B silica. This is a direct consequence of the use of the free radical induced hydrosilylation reaction for the surface modification with C18. In comparison, when the standard transition metal catalyzed hydrosilylation reaction is used, the silica surface can be contaminated by the metal catalyst.24 The peak tailing of chelating analytes may still be improved by testing different sources of silica as starting materials, as well as using high purity alkenes for the hydrosilylation reaction.
Conclusions The chlorination-reduction sequence for the preparation of silicon hydride modified silica surfaces was designed to obtain materials with low silanol content. The presence of only a few silanol groups was previously demonstrated by IR as well as
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NMR spectroscopy. The HPLC investigations of the SiH modified silica provide additional information on the surface chemistry of these materials. The low retention and peak asymmetry obtained for an organic base demonstrate that the silica surface is almost free of isolated silanol groups and therefore the remaining SiOH moieties are nonaccessible bulk species. This opens a new route for the preparation of RPLC phases where blocking the accessibility to the surfaces is not necessary. For the modification of these surfaces with C18 groups, a thermally initiated free radical hydrosilylation reaction was developed. Strong attachment of the organic molecules on the silica surface is obtained. Indirect evidence, like decrease in the SiH surface concentration, or absence of CdC double bonds in the modified materials are in agreement with the formation of an SiC bond. The thermally induced hydrosilylation is advantageous over the transition metal catalyzed or radical initiator induced reactions because it leaves the silica surface free of impurities. The chlorination-reduction sequence followed by C18 surface modification presents a dramatically different strategy for the synthesis of C18 HPLC phases as compared to the conventional silylation methods. Even at the early stage of their development, these C18 phases already match their silylated equivalent for the separation of organic bases. Optimization of the hydrosilylation reaction may also improve their methylene selectivity as well as the separation of chelating agents. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft (Graduiertenkolleg 441 “Chemie in Interphasen”), the National Science Foundation (Grant 0724218) for support of this work, and the Max-Buchner-Forschungsstiftung for a fellowship to NP. We thank Professor Dr. Lars Wesemann for providing the DRIFT facilities.
DOI: 10.1021/la901986w
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