Effect of electrode materials in the electroreductive formation of 1,1,2,2-tetramethyl-1,2-diphenyldisilane (
1. Introduction
Polysilanes (
In contrast to the growing interest with the polysilane, the method of preparation hitherto known is highly limited. So far, the almost only practical method is the condensation of organodichlorosilane with alkali metal (Wurtz type condensation). This method, however, requires drastic reaction conditions and hence, is very much limited in the type of substituent that is allowed to be located on the monomer. Although several modified or alternative methods such as sonochemical coupling of dichlorosilane promoted by alkali metal [9-11], transition metal catalyzed reaction of hydrosilane [12, 13], anionic polymerization of masked disilene [14, 15], and ring opening polymerization of cyclic organosilane [16, 17] have been proposed, they are not always extensively effective as preparative methods.
The electroreductive coupling of dichlorosilanes with mercury electrode has been reported by Hengge in 1976 as a method to form disilanes [18], though this method was not effective in the preparation of polysilanes [19, 20].
On the other hand, we have recently found that the electroreduction of organic compounds with Mg electrode promotes a variety of unique reactions which can not be attained without using the Mg electrode. The use of Mg electrodes was highly effective to the formation of Si-Si bond and the synthesis of high molecular weight polysilanes [21, 22].
In this chapter, we describe the details of the electroreductive synthesis of high molecular weight polysilane and some types of functionalized polysilanes and also polygermanes, including the additional information about the effects of electrode material and monomer concentration. We also demonstrate that our electroreduction system is successfully applied for the synthesis of the sequence-ordered oligosilanes and polysilanes.
2. Formation of Si-Si bonds by electroreductive coupling of chlorosilanes [21, 22]
The electroreduction of chlorodimethylphenylsilane (
The material of electrode is one of the most important factors to control the formation of Si-Si bond (
The cathodic coupling of other organochlorosilanes was carried out under the optimized reaction condition, that is, Mg cathode and anode were alternated with the interval of 1 min., supporting electrolyte was LiClO4, solvent was THF, and the electricity passed was 2.0 F/mol (
entry | anode | cathode | alternation | yield of |
1 | Mg | Mg | yes | 92 |
2 | Pt | Pt | yes | 0 |
3 | C | C | yes | 0 |
4 | Zn | Zn | yes | trace |
5 | Pt | Mg | no | 0 |
6 | Mg | Pt | no | 93 |
entry | chlorosilanes | yield of | ||||
R1 | R2 | R3 | ||||
1 | Me | Me | Me | 82 | ||
2 | Me | Ph | Ph | 77 | ||
3 | Ph | Ph | Ph | 85 |
Two types of mechanism may be proposed to this electroreductive Si-Si bond forming reaction. The first prosible mechanism is a radical coupling in which a silyl radical formed by one electron reduction of the starting chlorosilane couples with another silyl radical to give the disilane. In the second mechanism, two-electron reduction of the chlorosilane yields an active species equivalent to silyl anion which reacts with chlorosilane to give a dimer. In order to have an insight into the mechanism, the products obtained in the mixed system of chlorotrimethylsilane (
3. Stepwise synthesis of oligosilanes [22, 23]
This method is also applicable to the synthesis of trisilanes and tetrasilanes. For example, the electroreductive cross coupling of organodichlorosilanes (
The mildness of the reaction conditions of this electroreductive method is favorable for the synthesis of oligosilanes having various functionalities such as Si-H bonds which are known to be reactive under radical or anionic condition. The electroreductive cross-coupling reaction of chlorodimethylsilane (
4. Electroreductive polymerization of dichloromethylphenylsilane [22]
Electroreduction of dichloromethylphenylsilane (
entry | alternation | sonication | yield of | ||
1 | no | no | - | - | - |
2 | yes | no | 4000 | 1.4 | 7 |
3 | no | yes | 3900 | 1.4 | 17 |
4 | yes | yes | 5200 | 1.5 | 43 |
Mg is a remarkably effective material of electrode also for the formation of
entry | electrode materials | yield of | ||
1 | Mg | 5200 | 1.5 | 43 |
2 | Cu | 700 | 1.1 | - |
3 | Ni | 640 | 1.1 | - |
4 | Al | 4700 | 1.5 | 15 |
The effect of monomer concentration was investigated in order to obtain high molecular weight polysilane (
The most satisfactory result, in which material yield was 79 % and molecular weight (
entry | monomer | supplied electricity, F/mol | yield of | ||
1 | 0.33 | 4.0 | 5200 | 1.5 | 43 |
2 | 0.67 | 4.0 | 9900 | 2.1 | 79 |
3 | 2.5 | 2.2 | 18000 | 2.1 | 43 |
4 | 6.3 | 0.8 | 19000 | 2.8 | 15 |
5 | 12 | 0.5 | 31000 | 1.8 | 8 |
The mechanism of electroreductive formation of polysilane is not always perfectly clear, though the initial step of reaction is obviously the reduction of
5. Electroreductive synthesis of functionalized polysilanes [29]
The mildness of the polymerization conditions of the electroreductive method is favorable for the synthesis of the polysilanes having a variety of hydroxyl-related functional groups. The electroreduction of a mixture of
The modification or the property of polysilane must be achieved by using the hydroxyl group located on the polymer
entry | charged mol% of | yield of | Mne | Mw/Mne |
1 | 7 ( | 79 (7) | 9900 | 1.9 |
2 | 10 ( | 57 (12) | 6900 | 1.7 |
3 | 100 ( | 28 (100) | 1100 | 1.2 |
4 | 10 ( | 36 (11) | 6100 | 1.5 |
5 | 100 ( | -f (100) | 1100 | 1.2 |
6 | 10 ( | 50 (6) | 4500 | 1.3 |
7 | 50 ( | 22 (46) | 4600 | 1.3 |
8 | 100 ( | -f (100) | 1700 | 1.3 |
9 | 10 ( | 56 (17) | 4600 | 1.3 |
10 | 100 ( | 57 (100) | 4000 | 1.1 |
6. Electroreductive Polymerization of Dichlorooligosilanes [23]
The electroreductive polymerization of the dichlorooligosilanes is highly promising for the synthesis of sequence-ordered polysilanes. The electroreduction of dicholodisilane
entry | dichlorosilane | supplied electricity, F/mol | polysilane | ||
yield, % | |||||
1 | 0.11 | 4 | 2900 | 2.7 | 2.7 |
2 | 0.33 | 4 | 3600 | 2.3 | 2.7 |
3 | 0.67 | 4 | 2100 | 2.9 | 1.0 |
4 | 0.33 | 2 | 2800 | 2.7 | 3.9 |
5 | 0.33 | 6 | 3000 | 3.0 | 1.0 |
6 | 0.33 | 4 | 2500 | 1.8 | 13.0 |
Dichlorooligosilanes, such as dichlorotrisilane 9 was found to be good monomers for the electroreductive synthesis of the polysilanes having longer sequence units (Schemes 16). The temperature control is found to be very important in the electroreductive polymerization of 9 (Table 8). The reaction at higher temperature, the backbiting reaction of the propagating polymer proceeded forming cyclohexasilane as a by-product (entry 1). This side reaction was successfully suppressed when the reaction was carried out below 0°C, and polysilanes 19 having relatively high molecular weight were obtained (entries 3, 4). In the optimized reaction conditions, the electroreduction of dichlorotetrasilane 12 gave the corresponding polysilane 20, units of which were ordered in four sequences in satisfactory yield (Scheme 17). The polymerizability of dichlorooligosilanes under the electroreduction conditions seems to be mainly affected by the substituents on the chlorinated terminal silicon atom, and this fact provides a wide possibility to design the oligosilane sequences of the inner silicon atoms.
entry | polymerization temperature, °C | yield of | ||
1 | 18 | 3800 | 1.44 | (42) |
2 | 0 | 4700 | 1.87 | 50 |
3 | -10 | 5500 | 1.54 | 35 |
4 | -15 | 4400 | 1.42 | 16 |
7. Electroreductive polymerization of dichlorosilanes in the presence of disilane additives [30]
The disilane additives, which are generated
The mechanism for the triphenylsilyl substituted disilane to control the electroreductive polymerization suggested is as follows (
entry | disilane additives | yield of | ||
1 | - | 3200 | 1.65 | 38 |
2 | Me3SiSiMe3 | 3700 | 1.39 | 54 |
3 | Me3SiSiPh3 | 3000 | 1.10 | 56 |
4 | Ph3SiSiPh3 | 2600 | 1.08 | 59 |
triphenylsilyl group terminated polysilane. The resulting polysilane is isolable but does not lose its polymerizability completely since the triphenylsilyl group at the terminal position acts as an activator, that is, it probably reacts as a macroinitiator.
8. Electroreductive block copolymerization using triphenylsilyl group-terminated polysilane [32]
The triphenylsilyl group terminated polysilanes have been synthesized by the electroreductive polymerization of dichloromethylphenylsilane (
Polydibutylsilane-block-polymethylphenylsilane (
entry | preparation of the macroinitiator | polymethylphenylsilane- | |||||||
disilane additives | yield, % | ||||||||
observed | calculated | ||||||||
1 | Me3SiSiMe3 | 3740 | 1.9 | 5530 | 1.4 | 83 : 17 | 70 : 30 | 16 | |
2 | Me3SiSiPh3 | 3000 | 1.3 | 4080 | 1.4 | 66 : 34 | 77 : 23 | 28 | |
3 | Ph3SiSiPh3 | 3350 | 1.4 | 4730 | 1.2 | 75 : 25 | 74 : 26 | 38 |
The UV absorption spectra of the resulting polysilane
9. Conclusion
The formation of Si-Si bonds was achievable by the electroreductive condensation of organochlorosilanes with Mg sacrificial electrode. Disilanes, trisilanes, and tetrasilanes were readily obtained in good to moderate yield. Moreover, this method was also remarkably effective to the synthesis of polysilanes. The molecular weight and yield of the polymers was controlled by the concentration of monomers and the supplied electricity. The mildness of the reaction conditions allowed to use a wide variety of monomers, and enabled the synthesis of the functionalized polysilanes and the structure-controlled polysilanes. The electroreductive polymerization of the dichlorooligosilanes was highly useful for the synthesis of sequence-ordered polysilanes. Moreover, this electroreductive method also provided a new procedure to synthesize well-controlled di-block polysilane copolymers. Since the present electroreductive polymerization requires only a single compartment cell, it is undoubtedly one of the simplest and most powerful tools for synthesis of polysilanes.
References
- 1.
West R. 1986 The polysilane high polymers J. Organomet. Chem. 300(1-2):327 EOF 346 EOF - 2.
Yajima S. Hasegawa Y. Hayashi J. Iimura M. 1978 Synthesis of continuous silicon carbide fiber with high tensile strength and high Young’s modulus Part 1. Synthesis of polycarbosilane as precursor. J. Mater. Sci.13 12 2569 2576 - 3.
Hasegawa Y. Okamura K. 1985 Silicon carbide-carbon composite materials synthesized by pyrolysis of polycarbosilane. J. Mater. Sci. Lett.4 3 356 8 - 4.
Miller R. D. Willson C. G. Wallroff G. M. Clecak N. Sooriyakumaran R. Michl J. Karatsu T. Mc Kinley A. J. Klingensmith K. A. Downing J. 1989 Polysilanes: photochemistry and deep-UV lithography. Polym Eng Sci29 13 882 886 - 5.
Miller R. D. Michl J. 1989 Polysilane high polymers. Chem. Rev.89 6 1359 1410 - 6.
West R. David L. D. Djurovich P. I. Stearley K. L. Srinivasan K. S. V. Yu H. 1981 Phenylmethylpolysilanes: formable silane copolymers with potential semiconducting properties J. Am. Chem. Soc.103 24 7352 7354 - 7.
Kepler RG, Zeigler JM, Harrah LA, Kurtz SR 1987 Photocarrier generation and transport in σ-bonded polysilanes. Phys. Rev. B35 6 2818 2822 - 8.
Baumert J. C. Bjorklund G. C. Jundt D. H. Jurich M. C. Looser H. Miller R. D. Rabolt J. Soorijakumaran R. JD Swalen Twing. R. J. 1988 Temperature dependence of the third-order nonlinear optical susceptibilities in polysilanes and polygermanes. Appl. Phys. Lett.53 13 1147 1149 - 9.
Matyjaszewski K. Greszta D. Hrkach J. S. Kim H. K. 1995 Sonochemical synthesis of polysilylenes by reductive coupling of disubstituted dichlorosilanes with alkali metals Macromolecules28 1 59 72 - 10.
Jones RD, Holder SJ 2006 High-yield controlled syntheses of polysilanes by the Wurtz-type reductive coupling reaction Polym. Int.55 7 711 718 - 11.
Koe J. 2008 Contemporary polysilane synthesis and functionalisation Polym. Int.58 3 255 260 - 12.
Tilley TD 1993 The coordination polymerization of silanes to polysilanes by a " σ-bond metathesis" mechanism. Implications for linear chain growth. Acc. Chem. Res.26 1 22 29 - 13.
Minato M. Matsumoto T. Ichikawa M. Ito T. 2003 Dehydropolymerization of arylsilanes catalyzed by a novel silylmolybdenum complex. Chem. Commun. (24):2968 EOF 9 EOF - 14.
Sanji T. Kawabata K. Sakurai H. 2000 Alkoxide initiation of anionic polymerization of masked disilenes to polysilanes J. Organomet. Chem. 611(1-2):32 EOF - 15.
Sanji T. Isozaki S. Yoshida M. Sakamoto K. Sakurai H. 2003 Functional transformation of poly(dialkylaminotrimethyldisilene) prepared by anionic polymeriztion of the masked disilenes. The preparation of a true polysilastyrene. J. Organomet. Chem. 685 (1-2): 65-69. - 16.
Cypryk M. Gupta Y. Matyjaszewski K. 1991 Anionic ring-opening polymerization of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane J. Am. Chem. Soc.113 3 1046 7 - 17.
Suzuki M. Kotani J. Gyobu S. Kaneko T. Saegusa T. 1994 Synthesis of sequence-ordered polysilane by anionic ring-opening polymerization of phenylnonamethylcyclo-pentasilane. Macromolecules27 8 2360 2363 - 18.
Hengge E. Litscher G. 1976 A new electrochemical method for the formation of silicon-silicon bonds. Angew] Chem] 88(12): 414. - 19.
Hengge E. Litscher G. 1978 Electrochemical formation of di-, oligo- and polysilanes. Monatsh. Chem.109 5 1217 1225 - 20.
Hengge E. Firgo H. 1981 An electrochemical method for the synthesis of silicon-silicon bonds. J. Organomet. Chem.212 2 155 161 - 21.
Shono T. Kashimura S. Ishifune M. Nishida R. 1990 Electroreductive formation of polysilanes J. Chem. Soc. Chem. Commun. (17):1160 EOF - 22.
Kashimura S. Ishifune M. Yamashita N. Bu H. B. Takebayashi M. Kitajima S. Yoshihara D. Kataoka Y. Nishida R. Kawasaki S. Murase H. Shono T. 1999 Electroreductive synthesis of polysilanes, polygermanes, and related polymers with magnesium electrodes. J. Org. Chem.64 18 6615 6621 - 23.
Ishifune M. Kashimura S. Kogai Y. Fukuhara Y. Kato T. Bu H. B. Yamashita N. Murai Y. Murase H. Nishida R. 2000 Electroreductive synthesis of oligosilanes and polysilanes with ordered sequences. J. Organomet. Chem. 611(1-2): 26-31. - 24.
Umezawa M. Takeda M. Ichikawa H. Ishikawa T. Koizumi T. Nonaka T. 1991 Electroreductive polymerization of mixtures of chloromonosilanes. Electrochim. Acta 36(3-4): 621-624. - 25.
Biran C. Bordeau M. Pons P. Leger M. P. Dunogues J. 1990 Electrosynthesis, a convenient route to di- and polysilanes. J. Organomet. Chem. 382 (3): C17 C20. - 26.
(Kunai A. Kawakami T. Toyoda E. Ishikawa M. 1991 ) Electrochemistry of organosilicon compounds. 2. Synthesis of polysilane oligomers by a copper electrode system. Organometallics10 6 2001 2003 . - 27.
Okano M. Takeda K. Toriumi T. Hamano H. 1998 Electrochemical synthesis of polygermanes Electrochim. Acta44 4 659 666 - 28.
Yamada K. Okano M. 2006 Electrochemical synthesis of poly(cyclotetramethylene-silylene) Electrochemistry74 8 668 671 - 29.
Kashimura S. Ishifune M. Bu H. B. Takebayashi M. Kitajima S. Yoshihara D. Nishida R. Kawasaki S. Murase H. Shono T. 1997 Electroorganic chemistry. 153. Electroreductive synthesis of some functionalized polysilanes and related polymers. Tetrahedron Lett.38 26 4607 4610 - 30.
Ishifune M. Kogai Y. Uchida K. 2005 Effect of disilane additives on the electroreductive polymerization of organodichlorosilanes J. Macromol. Sci. Part A Pure and Appl. Chem.42 7 921 929 - 31.
Chen S. M. David L. D. Haller K. J. Wadsworth C. L. West R. 1983 Isomers of (PhMeSi)6 and (PhMeSi)5 Organometallics2 3 409 414 - 32.
Ishifune M. Sana C. Ando M. Tsuyama Y. 2011 Electroreductive block copolymerization of dichlorosilanes in the presence of disilane additives Polym. Int.60 8 1208 1214 - 33.
Terunuma D. Nagumo K. Kamata N. Matsuoka K. Kuzuhara H. 2000 Preparation and characterization of water-soluble polysilanes bearing chiral pendant ammonium moieties Polymer Journal32 113 117 - 34.
Herzog U. West R. 1999 Hererosubstituted polysilanes. Macromolecules32 2210 2214 - 35.
Hu Z. Zhang F. Huang H. Zhang M. He T. 2004 Morphology and structure of poly(di-n-butylsilane) single crystals prepared by controlling kinetic process of solvent evaporation. Macromolecules37 3310 3318 - 36.
Chunwachirasiri W. Kanaglekar I. MJ Winokur Koe. J. C. West R. 2001 Structure and chain conformation in poly(methyl-n-alkyl)silanes. Macromolecules34 6719 6726