Organometallics 1995,14,2506-2511
2506
Electrochemical Polymerization of Hydrosilane Compounds Yoshinori Kimata,* Hiroshi Suzuki, Shin Satoh, and Akira Kuriyama" Tsukuba Research Laboratory, Toagosei Co., Ltd., 2 Ohkubo, Tsukuba, Ibaraki 300-33,Japan Received November 16, 1994@ The electrolytic reactions of hydrosilane compounds were studied for the synthesis of polysilanes. Di- and trihydric substituted monosilanes, such as methylphenylsilane, phenylsilane, and n-hexylsilane, were electrolyzed under constant-current conditions with platinum electrodes in an undivided cell containing Bu4NBFdDME as the electrolyte and solvent. In each case, polymerized products were obtained and the formation of silicon catenation was definitely confirmed by W absorption, mass, and Raman spectroscopies. Introduction
Polysilanes are interesting polymers possessing curious properties based on the delocalized o electrons of the silicon-silicon bonds along the backbone. They have been intensely investigated for practical applications such as precursors for silicon carbide ceramics,l photoresists,2 and electroconducting3 and luminescent material^.^ The most general synthetic pathway of polysilanes is the Wurtz coupling reaction of organodichlorosilane compounds with a sodium dispersion in toluene at refluxing t e m p e r a t ~ r e . ~However, -~ there are obvious problems such as the dangerous reaction conditions due to the treatment of moisture-sensitive alkali metals as well as the formation of large amounts of metal chlorides as byproducts. In addition, when monomers having functional groups such as fluoroalkyls were subjected t o the Wurtz type condensation, the corresponding polysilanes were produced in extremely low yield.g Therefore, other synthetic methods have been successively investigated. For example, anionic polymerization of masked disilenes,lOJ1ring-opening polymerization of cyclosilanes,12 the dehydrogenative coupling reaction of hydrosilanes using a transition metal ~ a t a l y s t , ' ~and - ~ electroreductive ~ polymerization of c h l o r ~ s i l a n e s ~ have ~ - ~been ~ reported over the past @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1) Yajima, S.; Omori, M.; Hayashi, J.; Okamura, K.; Matsuzawa, T.; Liaw, C. F. Chem. Lett. 1976, 551. (2) Grifing, B. F.; West, R. Polym. Eng. Sci. 1983, 23, 947. (3)West, R.; David, L. D.; Djurovich, P. I.; Stearley, K. S. V.; Srinivasan, H. Y. J. A m . Chem. SOC.1981,103, 7352. (4) Bianconi, P. A.; Weidman, T. W. J. A m . Chem. SOC.1988, 110, 2342. (5) Trujillo, R. E. J . Organomet. Chem. 1980, 198, C27. (6) Trefonas, P., 111; Damewood, J. R., Jr.; West, R.; Miller, R. D. Organometallics 1986, 4, 1318. (7) West, R. J . Organomet. Chem. 1986, 300, 327. (8) Harrah, L. A.; Zeigler, J. M. Macromolecule 1987,20, 601. (9) Fujino, M.; Hisaki, T.; Fujiki, M.; Matsumoto, N. Macromolecules ~1992.25. _ _ _ 1079. (10) Sakamoto, K.; Obata, K.; Hirata, H.; Nakajima, M.; Sakurai, H. J. Am. Chem. SOC.1989,111, 7641. (11) Sakamoto, K.; Yoshida, M.; Sakurai, H. Macromolecules 1990, 23, 4494. (12) Matyjaszewski, K.; Cypryk, M.; Frey, H.; Hrkach, J.; Kim, H. K.; Moeller, M.; Ruehl, K.; White, M. J . Macromol. Sci.-Chem. 1991, A28 (11 & 121, 1151. (13)Aitken, C.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1986, 269, C11. (14)Aitken, C.; Harrod, J. F.;Samuel, E. J. A m . Chem. SOC.1986, 108,4059. (15)Aitken, C.; Harrod, J. F.; Gill, U. S. Can. J . Chem. 1987, 65, 1804. (16) Harrod, J. F.; Yun, S. S . Organometallics 1987, 6, 1381. , - - 2
several years. The electrochemical process, first studied by Hengge and L i t ~ c h e ris , ~worthy ~ of note since the electrolysis of chlorosilanes can be carried out under very mild conditions a t room temperature. In this method, the practical problem is the anodic oxidation of the electrode itself with chlorine removed from the monomers to form quantitative amounts of metal chloride or corrosive hydrogen chloride.36 Recently, we have found an epoch-making electrochemical pathway for the preparation of Si-Si bonds from methylphenyl~ilane~~ without using any catalysts as investigated by Aitken et al.14 In this paper, we report the novel synthesis of polysilanes from di- and trihydric substituted monosilanes using an electrolytic technique.
Results and Discussion Electrolysis of Hydrosilanes. The electrochemical properties of some hydrosilanes were examined by Kunai et al.38 An Si-H bond of dimethylphenylsilane is effectively chlorinated to form dimethylphenylchlorosilane by electro-oxidation in the presence of CuCl using platinum electrodes, and the resulting chlorosilanes were subsequently reduced and 1,2-diphenyltetramethyldisilane was produced in a one-pot reaction, if a copper anode was used instead of platinum.38 Using this method, a sacrificial electrode is ultimately necessary to form an Si-Si bond in the same manner as the usual electroreductive polymerization of chlorosilanes, although hydrosilane was used as the starting material. It is obviously unfavorable for a practical electrolysis system to consume the electrode itself. (17) Harrod, J. F. Inorganic and Organometallic Polymers; ACS Symposium Series 360; Zeldin, M., Wynne, K. J., Allcock, H. R., Eds.; American Chemical Society: Washington, DC, 1988; Chapter 7. (18) Aitken, C.; Barry, J. P.; Gauvin, F.; Harrod, J. F.; Malek, A,; Rousseau, D. Organometallics 1989, 8, 1732. (19) Nakano, T.; Nakamura, H.; Nagai, Y. Chem. Lett. 1989, 83. (20) Corey, J. Y.; Zhu, X. H.; Bedard, T. C.; Lange, L. D. Organometallics 1991, 10, 924. (21) Corey, J. Y.; Zhu, X. H. J. Organomet. Chem. 1992,439, 1. (22) Woo, H. G.; Heyn, R. H.; Tilley, T. D. J. A m . Chem. SOC.1992, 114, 5698. (23) Woo, H. G.;Walzer, J. F.; Tilley, T. D. J . A m . Chem. SOC.1992, 114, 7047. (24) Banovetz, J. P.; Suzuki, H.;Waymouth, R. M. Organometallics 1993,12,4700. (25) Hengge, E.; Litscher, G. K. J . Angew. Chem., Int. Ed. Engl. 1976, 15, 370. (26) Corriu, R. J. P.;Dabosi, G.; Martineau, M. J . Organomet. Chem. 1981,222, 195.
0276-7333/95/2314-2506$09.00/0 0 1995 American Chemical Society
Polymerization of Hydrosilane Compounds
Organometallics, Vol. 14, No. 5, 1995 2507
Table 1. Electrochemical Polymerization of Hydrosilanes electricity monomer (F/mol) MePhSiHz PhSiHs nHexSiHs
2.0 3.0 3.0
product -(MePhSi),-(PhSiH),-("HexSiH),-
UV absb molwt" (%) Mw M d M , (nm) &iSi
yield
-
-&nax
60 477 1.05 24Oe 5300 32 640 1.42 24Oe 5700 70 1240 1.09 252 7700
GPC vs polystyrene. Solvent cyclohexane. Shoulder.
Table 2. Characterization of the Oligomers -[MePhSil,- Fractionated by HPLC Wabsb degree of fraction no. GPCn MS A,, (nm) polymerization yieldc (%)
1 2 3 4
219 304 3 78 427
242 362
230 243 251 257
2 3 4
5
10 14 16 6
Polystyrene standards, peak top. Solvent acetonitrile. Based
on the weight of isolated fraction by HPLC.
electrolysis product of methylphenylsilane in the present work is poly(methylphenylsi1ane) as shown by eq 1. CH3 I H-Si-H
electrolysis 0.2M "Bu~NBF@ME Pt anode, cathode passed 2 F/mol
io3 300 200 mol wt. by standard polystyrenes
3000
Figure 1. GPC profiles of electrolyzed (a) methylphenylsilane, (b) phenylsilane, and (c) n-hexylsilane.
According to these results, the Si-Si bond can be electrochemically formed by condensation of monosilanes having Si-H bonds. In order to confirm the generality of this polymerization method, we examined the electrolysis of other hydrosilanes in a similar manner to methylphenylsilane. The mean degree of polymerization of the obtained product from phenylsilane was estimated to be about 6 based on the molecular weight by GPC. The di-, triand tetramers, which corresponded to the components from n = 2 to 4 in Figure lb, were purified and eluted by HPLC. Each of them exhibited obvious W absorption maximums which red-shifted with increasing degree of polymerization (Figure 31, while the oligomer mixture showed a Am, at 240 nm as a shoulder shape (Table 1). This is distinct evidence for the formation of silicon catenation. It is necessary to determine not only if the products have Si-Si bonds but also if the backbone structure is linear, cyclic, or branched, because phenylsilane has three reactive Si-H bonds. The FTIR spectra, shown in Figure 4, indicate that although the intensity ratio of (I((GsiH)/I(YsiH)) in monomers was 1.2, it certainly decreased to 0.60 after electrolysis. This result suggests the formation of the -(PhSiH)- structure from phenylsilane by the polymerization process.15 The 29Si-NMRspectrum was measured by a single pulse without decoupling in order to obtain information about the number of hydrogens directly bonded on the silicon
We have found that di- and trihydric substituted monosilanes are suitable for electrochemical polymerization without using sacrificial electrodes. The electrolysis of methylphenylsilane was at first carried out in THF containing lithium perchlorate as the supporting electrolyte. However, reductive formation of metallic lithium occurred on the cathode and no compound having an Si-Si bond was obtained. When tetrabutylammonium tetrafluoroborate (TBAF) was used instead of lithium perchlorate as the electrolyte, the solvent decomposed to form poly(THF1 during electrolysis. Therefore, we examined the electrolysis in 1,2-dimethoxyethane (DME)A'BAF as the solvent/electrolyte system. In this case, the electrochemical polymerization of methylphenylsilane occurred satisfactorily (Table 1). The product was obtained as a mixture of oligomers containing more than five components (Figure la). The FT-IR spectrum indicates that the polymerized meth(27)Hengge, E.;Firgo, H. J. Organomet. Chem. 1981,212,155. (28)Shono, T.; Kashimura, S.; Ishifune, S.; Nishida, R. J. Chem. ylphenylsilane has Si-CH3 (1250 cm-l), Si-Ph (1428 SOC.,Chem. Commun. 1990,1160. cm-l), and Si-H (2106 cm-l) groups. Since the strength (29)Umezawa, M.;Takeda, M.; Ichikawa, H.; Ishikawa, T.; Koizumi, T.; Fuchigami, T.;Nonaka, T. Electrochim. Acta 1990,35, 1867. of the absorption band between 1000 and 1100 cm-l was (30)Bordeau, M.; Biran, C.; Lambert, M. P. L.; Dunogues, J. J. very weak, few siloxane bonds were present in the Chem. Soc., Chem. Commun. 1991,1476. backbone. The complete separation of the oligomers (31)Kunai, A.; Kawakami, T.; Toyoda, E.; Ishikawa, M. Organometallics 1991,10,893. into compounds ranging from dimers to pentamers was (32)Kunai, A.;Kawakami, T.; Toyoda, E.; Ishikawa, M.Organosuccessfully carried out by HPLC. The isolation yield, metallics 1991,10,2001. (33)Biran, C.; Bordeau, M.; Leger, M. P. Inorg. Organomet. Polym. molecular weight, and maximum wavelength of the UV Special Prop. 1992,206,79. absorption (A), are summarized in Table 2. Obviously, (34)Shono, T.: Kashimura, S.: Murase. H. J . Chem. Soc., Chem. the Am= shows a remarkable red shift with increasing Commun. 1992,12,896. (35)Aeiyach, S.;Lacaze, P. C.; Satge, J. Synth. Met. 1993,58,267. molecular weight. Furthermore, 1,2-dimethyl-l,2-diphe(36)Jammegg, C.;Graschy, S.; Hengge, E. Organometallics 1994, nyldisilane (mlz = 242) and 1,2,3-trimethyl-1,2,3-tri- 13,2397. (37)Kimata, Y.;Suzuki, H.; Satoh, S.; Kuriyama, A. Chem. Lett. phenyltrisilane (mlz = 362) were detected by mass 1994,1163. analysis corresponding to fractions no. 1and 2, respec(38)Kunai, A,; Kawakami, T.; Toyoda, E.; Sakurai, T.; Ishikawa, M. Chem. Lett. 1993,1945. tively (Figure 2). We can now conclude that the
Kimata et al.
2508 Organometallics, Vol. 14, No. 5, 1995
_i
I 4
(a)
Figure 2. Mass spectra of (a) 1,2-dimethyl-1,2-diphenyldisilane and (b) 1,2,3-trimethyl-1,2,3-triphenyltrisilane obtained by the electrolysis of methylphenylsilane.
4000
1 . ~ . . " . ' . ' . . . ' " . 1 220
240
260
280
3000
2000
1000 500
Wavenumberkml 300
WavelenghUnm
Figure 3. U V absorption spectra of phenylsilane oligomers eluted with acetonitrile by HPLC.
atoms. As shown in Figure 5, resonances due to the linear main chain were observed in the range of -60 to -70 ppm, and two doublet peaks assigned to silicon nuclears possessing one hydrogen distinctly appeared with reasonable coupling constants (JSiH). Waymouth synthesized stereoregular poly(phenylsi1ane) by the catalytic dehydrogenative coupling reaction of phenylsilane and reported that the resonance of 29Si-NMRdue t o the SiH2 end groups appeared sharply at -58 (39)Banovetz, J. P.; Stein, K M.; Waymouth, R. M. Organometallics 1991,10, 3430.
Figure 4. FT-IR spectra of (a) phenylsilane oligomer and (b) PhSiHa (KBr method).
We also observed at -58.56 ppm the terminal SiH2 as a triplet signal. No peak due t o branched silicon atoms appeared in the high magnetic field. The lH-NMR spectrum also supported the presence of a linear silicon backbone, since the existing ratio of Si-H to phenyl groups was 1.1which is slightly lower than the theoretical ratio of 1.3 calculated as a hexamer. It is considered that the electrochemically synthesized oligo(phenylsilane) consisted of -PhSiH- in the main chain and HzPhSi- as the end groups. On the other hand, n-hexylsilane, a typical hydrosilane possessing a non-aryl substituent, was also subjected to electrolysis, and a transparent liquid polymer was obtained in 70% yield. In Figure IC,a simple
Polymerization of Hydrosilane Compounds
Organometallics, Vol. 14,No. 5, 1995 2509
.a
.O2ppm
C u)
-s C
200
400
600
800
1000
1200
1400
Raman Shifl /cm-'
Figure 7. Raman spectra of (a) poly(n-hexylsilane)and (b) "HexSiHs obtained with 647.09 nm excitation. -40
-50
-60
- 70
-80 WPPm
Table 3. Reduction and Oxidation Potentials" of Hydrosilane Compounds
Figure 5. 29Si-NMRspectrum of phenylsilane oligomer in benzene-ds.
hydrosilane
Ered
MePh Si H 2 PhSiH3 nHexSiH3 a
(v)
Eo, (VI $0.7
