Reactivity of Bis(trimethylsilyl) Sulfate with Respect to the Surface of Silica

12. 13. 14. 15. T. Wentink and V. H. Tiensuu, "Vibrational spectra of BBr 3 and BI3," J. Chem. Phys., 28, No. 5, 826-838 (!958). L. Bellamy, Infrared Spectra of Complex Molecules [Russian translation[, Izd. Inostr. Lit., Moscow (1963). A. A. Chuiko, V. M. Mashchenko, V. A. Tertykh, et al., "Study of the kinetics of chemisorption of n-butyl alcohol vapor by the surface of silica," Kolloidn. Zh., 35, No. I, 110-117 (1973). A. M. Varvarin, L. A. Belyakova, V. A. Tertykh, et al., Teor. Eksp. Khim., 23, No. i, 117-120 (1987). REACTIVITY OF BIS(TRIMETHYLSILYL) SULFATE WITH RESPECT TO THE SURFACE OF SILICA A. M. Varvarin, L. A. Belyakova, A. V. Simurov, V. A. Tertykh, L. A. Lazukina, and V. P. Kukhar' UDC 541.183 It was established by IR spectroscopy and chemical analysis that bis(trimethylsilyl) sulfate reacts chemically with isolated silanol groups of silica even at room temperature, and is quantitatively chemisorbed at 373 K. Sulfidation of the silica does not occur. Chemisorption of [(CH3)3Si]2SO 4 on the surface of silicon(IV) oxide is not complicated by side processes. Bis(trimethylsilyl) sulfate [(CH3)3Si]2S04 is a good silylating [i, 2] and sulfonating [3] reagent. In connection with this, there is unquestionable interest in studying the reaction of bis(trimethylsilyl) sulfate with the surface of finely divided silica that carries isolated silanol groups. There are no data on the chemisorption of bis(trimethylsilyl) sulfate on the surface of silica in the literature. Molded plates of alumina (Ssp = 300 m2/g) with a density of 14-15 mg/cm 2 were used for the tests. The aerosil was first roasted in air at 670 K to remove possible organic impurities and then evacuated at 870 K. The concentration of hydroxyl groups on the surface treated in this way, which we determined from the chemisorption of trimethylcyanosilane, was 2.5 • 0.i ~mole/m 2. Bis(trimethylsilyl) sulfate was obtained by boiling equivalent amounts of trimethylchlorosilane and concentrated sulfuric acid followed by distillation at reduced pressure [4]. Because of the low fugacity of [(CH3)3Si]2SO~, its vapor was frozen in the reaction space of the vacuum cuvette using liquid nitrogen. IR absorption spectra were acquired on an IKS-29 spectrophotometer in the 3900-1200 cm -l frequency interval. After the silica plates are brought into contact with bis(trimethylsilyl) sulfate vapor at room temperature, the absorption band of isolated silanol groups (3750 cm -I) vanishes practically completely from the IR spectrum and new absorption bands appear - 3470, 2970, 2910, 1460, 1410, and 1350 cm -I (Fig. i, curve 2). Similar absorption bands, with the exception of the broad band at 3470 cm -I, are present in the IR spectrum of the individual bis(trimethylsilyl) sulfate (curve 7). Bands with absorption peaks at 2970, 2910, 1460, and 1410 cm -I result respectively from asymmetric and symmetric stretching and bending vibrations of C-n~ bonds in methyl groups, while the absorption band at 1350 cm -I is due to asymmetric stretching vibrations of S -----O bonds in sulfates [5]. Evacuation Of the sample at room temperature does not result in a change of the IR spectrum (curve 3). Only with thermoevacuation is there a significant decrease of the intensity of the broad absorption band at 3470 cm -I and the absorption band at 1350 cm -I (curve 4), which vanish completely with longer evacuation; in this case there is partial restoration of the absorption band of free silanol groups (curve 5). Therefore, the absorption band at 1350 cm -I can be attributed to stretching vibrations of S = O bonds of adsorbed bis(trimethylsilyl) sulfate, and the absorption band at 3470 cm -I can be attributed to stretching vibrations of isolated OH groups Institute of Surface Chemistry, Academy of Sciences of the Ukranian SSR. Institute of Bioorganic Chemistry, Academy of Sciences of the Dkrainian SSR, Kiev. Translated from Teoreticheskaya i ~ksperimental'naya Khimiya, Vol. 25, No. 3, pp. 377-380, May-June, 1989. Original article submitted September 16, 1987. 352 0040-5760/89/2503-0352512.50 9 1989 Plenum Publishing Corporation < I $600 ) JO00 I .9400 ] I800 cm -I Fig. i. IR spectra of silica evacuated for 2 h at 870 K (i), treated with [(CH3)3Si]=SO4vapor for i h at 298 K (2), evacuated for 1 h at 298 K (3), for i h at 573 K (4), for another i h at 573 K (5), treated with [(CH3)~Si]2SO, vapor for i h at 373 K (6); 7) IR spectrum of individual silane. perturbed by adsorption interaction with [(CHa)~Si]2SO ~ molecules. After evacuation at 570 K there remain in the IR spectrum of the sample absorption bands at 2970, 2910, 1460, and 1410 cm -l, which are evidence of chemisorption of bis(trimethylsilyl) sulfate on the silica surface. Thus, when the surface of silica is treated with vapor of bis(trimethylsilyl) sulfate even at room temperature there is a significant decrease of the intensity of the absorption band at 3750 cm -I in the IR spectrum and as a result of treating a silica plate with [(CHs) 3" Si]2SO . vapor at 370 K this band practically disappears (curve 6), and new absorption bands corresponding to grafted trimethylsilyl groups are seen. The absence of the absoprtion band of isolated hydroxyl groups is evidence that they have participated in the chemisorption process by the mechanism of electrophilic substitution of a proton in the free silanol groups ~ - Si - - OH + (CHa)3 Si - - O - - SOs - - O - - Si (CHa)3-~ - - Si - - O - - Si (CHs)~+ + H O - - SOs - - O - - Si (CH3h. It is appropriate to note that bis(trimethylsilyl) e v e n by a t m o s p h e r i c moisture [6]. The r e s u l t i n g the silica surface by t h e s a m e m e c h a n i s m sulfate is trimethylsilyl easily hydrolyzed by w a t e r sulfate evidently reacts and with ~_ Si - - OH + (CHa)a Si - - O - - S02 - - O H - + - - Si - - O - - Si (CH~) 8 + H2SO 4. However, a study of sulfate is not possible, the reaction of the surface s i n c e (CH3)aSiOSO~H h a s n o t of silicon(IV) been isolated oxide with trimethylsilyl in individual state. Mean- while, the possibility of its existence has been pointed out in a number of studies. For example, it was established in [7] that in the hydrolysis of trimethylsilyl chlorosulfonate in an ether solution there forms an unstable trimethylsilyl sulfuric acid (trimethylsilyl sulfate ), the presence of which was demonstrated by methylation of it with diazomethane. It was shown by different methods in [8, 9] that when an excess amount of sulfuric acid reacts with hexamethyldisiloxane the formation of trimethylsilyl sulfate is also possible. The 353 difficulties of recovering trimethylsilyl sulfate from the reaction mixture of products is due to rapidly occurring exchange processes [9]. In our view, the possibility of reaction of bis(trimethylsilyl) sulfate with two isolated OH groups of the silica surface also should not be precluded. Trimethylsilyl groups form in all of the analyzed cases on the silica surface. Sulfonation of silicon(IV) oxide by bis(trimethylsilyl) sulfate does not occur. Otherwise, the IR spectrum of the modified silica would have an absorption band of the S = O bond (1350 cm-1), as, for example, when silica is brought into contact with sulfur trioxide [i0]: --Si--OH § SOa--~Si--O--SOaH. It was shown in [ii] that, along with the disappearance of the 3750 cm -I band, there appears at 1340 cm -I the absorption band of stretching vibrations of S -----0 bonds in sulfo groups. Sulfuric acid does not react with the silica surface [12]. It must be noted that the concentration of the grafted trimethylsilyl groups, as determined by elemental analysis for carbon (2.58 • 0.16 ~mole/m2), corresponds to the concentration of free silanol groups on the surface of the starting silica. Along with the results obtained by IR spectroscopy, this is further evidence that only one type of active site participates in the chemisorption of bis(trimethylsilyl) sulfate-isolated OH groups, which are characterized by the absorption band at 3750 cm -I. It was shown earlier that the effectiveness of the reaction of some organosilicon compounds with isolated hydroxyl groups on the surface of silica is basically determined by the proton-acceptor properties of substituents of the reagent bonded to the silicon atom and the electrophilic character of the silicon atom in the attacking molecule [13]. In particular, it was established in [14] that the activity of organosilazanes, chloro- and alkoxysilanes, and organosiloxanes with the same types of substituents on the silicon in reactions with free OH groups of silica decreases in the order Si-N > Si-O(C) > Si-CI > Si-O(Si) > Si-C. It is suggested that the reactivity of these compounds is determined mainly by the proton-acceptor properties of the atoms bonded to the silicon, which can be characterized by the magnitude of the shift of the absorption band at 3750 cm -l (A~OH) as a result of adsorption interaction with the corresponding compounds. In the case of adsorption of [(CHa)3Si]2SO 4 on silica AVOH = 280 cm -I, while for (CHa)aSiOCH 3 it is 475 cm -I It was logical to propose that trimethylmethoxysilane is more active in the reaction with OH groups of silica than bis(trimethylsilyl) sulfate. However, in fact the reverse is seen. For (CH3)~SiOCH 3 the temperature of intensive interaction is 453 K [14], while for [(CHa)~Si]2SO~ it is 298-373 K. It is probable that the mild conditions of chemisorption of bis(trimethylsilyl) sulfate on silica are due to the electrophilic character of the silicon atom in the trimethylsilyl group. It appears that this is characteristic for Si-O-S and Si-S bonds. For example, it was shown in [2] that bis(trimethylsilyl) sulfate is much more active than trimethylchlorosilanes in the silylation of ketones. The relative reaction rate constant is 2.7.102 (the reactivity of (CH3)aSiCI is taken as unity). There are data in [15] indicating the trimethylsilyltrifluoromethyl sulfonate (CHs)aSiOSO2CF a even at room temperature is chemisorbed significantly on the surface of silica, while in the case of hexamethyldisilthiane [(CHa)3Si]2S under the same conditions there is complete replacement of the structural hydroxyl groups by trimethylsilyl groups. Thus, IR spectroscopy and chemical analysis methods have been used to show that bis(trimethylsilyl) sulfate reacts chemically with isolated silanol groups of silica even at room temperature and is quantitatively chemisorbed at 373 K. Sulfonation of silica does not occur. Chemisorption of [(CH3)aSi]2SO u on the surface of silicon(IV) oxide is not complicated by side processes. LITERATURE CITED i. 2. 3. 4. 354 M. G. Voronkov, V. K. Roman, and E. A. Maletina, "Bis(trimethylsilyl) sulfate as an organosilicon synthon," Synthesis (BRD), No. 4, 277-280 (1982). H. H. Hergott and G. Simchen, Reaktionsfahigkeit von Trimethylsilylierungsreagentien Liebigs Ann. Chem., No. ii, 1718-1721 (1980). P. Bourglois and N. Duffaut, Le sulfate de bis(trimethylsilyle)reactif de sulfonation en chimie organique Bull. Soc. Chim. Fr., Ii, No. 3, 195-199 (1980). M. G. Voronkov, V. K. Roman, and E. A. Maletina, Reaction of Bis(trialkylsilyl) Sulfates with Salts of Mineral and Organic Acids, in: The Chemistry of Organometallic Compounds [in Russian], Nauka, Leningrad (1976), pp. 49-52. 5. 6. 7. 8. 9. I0. II. 12. 13. 14. 15. L. Bellamy, Infrared Spectra of Complex Molecules [Russian translation], Izd-vo Inostr. Lit., Moscow (1965). K. A. Andrianov, Methods of Organoelemental Chemistry [in Russian], Nauka, Moscow (1968). M. Schmidt and H. Fischer, Partielle Hydrolyse yon Chloroschwefelsauresilylestern , Z. Chem., No. 7, 253 (1968). F. P. Price, "Freezing point depression of sulfuric acid by siloxanes," J. Am. Chem. Soc., 70, No. 2, 871-872 (1948). B. V. Molchanov, V. G. Ryzhova, E. A. Chernyshev, et al., Study of the Reaction Products of Hexamethyldiloxane with Sulfuric Acid, Paper deposited at ONIITEKhim, April 17, 1986, No. 590. N. V. Ke'tsev, Sh. O. Minasyan, N. S. Torocheshnikov, et al., "Chemisorption of sulfur trioxide on silica gel," Zh. Fiz. Khim., 44, No. 6, 1592-1594 (1970). V. V. Pavlov, V. A. Tertykh, A. A. Chuiko, and K. P. Kazakov, "Chemical rearrangements in the surface layer of finitely divided silicas," Adsorbts. Adsorb. No. 4, 62-69 (1976). R. K. Iler, The Chemistry of Silica [Russian translation], Vol. i, Mir, Moscow (1982). pp. 258-259. V. A. Tertykh. V. V. Pavlov, K. I. Tkachenko, and A. A. Chuiko, "Basic laws of the reaction of silanol groups of silica with alkylchlorosilanes of the series ClnSi(CH3)~-n(n = 0-4)" Teor. Eksp. Khim., ii, No. 2, 174-181 (1975). V. A. Tertykh and V. V. Pavlov, "Problems of reactivity of molecules attacking a fixed site," Adsorbts. Adsorb., No. 6, 67-75 (1978). J. Chmielowiec and B. A. Morrow, "Alkylation of silica surfaces," J. Colloid Interface Sci., 94, No. 2, 319-327 (1983). 35CI NQR SPECTRA OF ORGANYLTRICHLOROGERMANES CONTAINING I THE CI3Ge--C--N-C(O)X FRAGMENT I I V. P. Feshin, P. A. Nikitin, T. K. Gar, O. A. Dombrova, and N. A. Viktorov UDC 543.425+547.246 3sCl NQR spectra of compounds containing the fragment CI~Ge-C-N-C(O)X, which were acquired at 77 K, indicate that in most of the investigated compounds the germanium atom is pentacoordinated owing to the intramolecular interaction Ge § 0. The ~sCl NQR frequencies in these compounds satisfactorily correlate with the bond length of the corresponding Ge-CI bond. The relationship between the length of the axial Ge-CI bond and the Ge...O spacing is described by an exponential function. The charge on the chlorine atoms in the investigated models was estimated from the experimental NQR frequencies. Under favorable steric conditions in organyltrichlorogermanes that contain the fragment CI3Ge-C-C-C(O)X, closure of the five-membered ring as a result of intermolecular interaction of the carbonyl oxygen with the germanium atom, which becomes pentacoordinated, is possible. One of the chlorine atoms bonded to it occupies axial position, and the other two equatorial positions, of a trigonal bipyramid [1-3]. Such interaction can usually be detected rapidly and reliably by the 35CI NQR method [1-3], since this method is highly sensitive to a change of the electron distribution of chlorine atoms (see, for example, [4]). In 3sCl NQR spectra of organyltrichlorogermanes of this structure the frequency of the axial chlorine atom is much lower than that of the equatorial atoms [1-3]. Institute of Organic Chemistry, Siberian Branch, Academy of Sciences of the USSR, Irkutsk. Translated from Teoreticheskaya i ~ksperimental'naya Khimiya, Vol. 25, No. 3, pp. 381-383, May-June, 1989. Original article submitted August 25, 1986. 0040-5760/89/2503-0355512.50 9 1989 Plenum Publishing Corporation 355