Electroreductive Synthesis of Polysilanes with Ordered Sequences

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.


Introduction
Polysilanes (Scheme 1) [1] have attracted considerable attention due to their usefulness as the precursors for thermally stable ceramics [2,3] or a material for microlithography [4,5], and also due to their potentiality in the preparation of new types of material showing semiconducting, photoconducting, or nonlinear optical property [6][7][8].

Scheme 1.
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][10][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.

Formation of Si-Si bonds by electroreductive coupling of chlorosilanes [21, 22]
The electroreduction of chlorodimethylphenylsilane (1a) was studied as the model reaction (Scheme 2) and carried out under a variety of reaction conditions. In the first place, the cathodic reduction was performed in a divided cell since Si-Si bond is electrochemically oxidized at the potential range 0.7-1.6V vs. SCE. The yield of 1,1,2,2-tetramethyl-1,2diphenyldisilane (2a) was, however, unexpectedly low under this reaction conditions. In the next place, the idea of using a sacrificial electrode was studied in order to avoid the undesirable anodic oxidation of Si-Si bond in an undivided cell and it was found that the electroreduction of 1a with Mg electrode was highly effective for the formation of Si-Si bond and 2a was obtained in an excellent yield.

Scheme 2.
The material of electrode is one of the most important factors to control the formation of Si-Si bond ( Table 1). When a solution of 1a in dry THF containing LiClO4 as a supporting electrolyte was electrochemically reduced with Mg cathode and anode with a constant current (current density = 30 mA/cm 2 , supplied electricity = 2.0 F/mol), the coupling product 2a was obtained in 92% isolated yield (entry 1). The results that Pt, carbon, or Zn is not effective electrode in the formation of Si-Si bond (entries 2-4) clearly indicate that Mg plays some important roles in the formation of Si-Si bond. Although details of the role of Mg in the mechanism of formation of Si-Si bond is not always clear at present, the unique reactivity of Mg electrode is undoubtedly shown in this reaction.
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 (Scheme 3). The results summarized in Table 2 show the high potentiality of this method in the synthesis of a variety of disilanes. Moreover, it is remarkable that the extent of contamination with siloxane (Si-O-Si) was less than 2%.  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 (1b) and chlorotriphenylsilane (1d) (1b : 1d = 1 : 1) were studied in detail (Scheme 4). The resulting products were 1 : 1 mixture of the mixed coupling product (2e) and the homocoupling product of 1d (hexaphenyldisilane (2d)), whereas the homocoupling product of 1b, hexamethyldisilane (2b) was not found in the products at all. This result seems to agree with the anionic mechanism. That is, 1d is first reduced to triphenylsilyl anion that reacts with 1b and 1d to afford disilanes 2e and 2d respectively. The electrophilicity of chlorosilanes 1b and 1d are high enough to be attacked equally by the triphenylsilyl anion at the half conversion (supplied electricity = 1 F/mol based on the total amount of 1b and 1d). In the radical mechanism, however, if only 1d is reduced to yield a radical, the formation of 2e is not reasonable, whereas if two types of radical are formed by the reduction of both 1b and 1d, the absence of 2b in the products is unreasonable. Thus, the anionic mechanism is the most reasonable in this coupling reaction. The fact that the reduction potential of 1d is much more positive than that of 1b also supports the above mentioned reaction mechanism. Scheme 4.

Scheme 6.
Electroreductive Synthesis of Polysilanes with Ordered Sequences 343 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 (7) with dichlorodiphenylsilane (3b), in fact, gave the corresponding trisilane 8 (Scheme 7). The Si-H bond was readily transformed to the Si-Cl bond by the treatment with catalytic amount of benzoylperoxide in carbontetrachloride.

Electroreductive polymerization of dichloromethylphenylsilane [22]
Electroreduction of dichloromethylphenylsilane (3a) (Scheme 10) carried out under the above mentioned reaction conditions gave polymethylphenylsilane (13) in low yield ( Table 3, entries 1, 2). The sonication of ultrasound was found to be necessary for the electroreductive polymerization of 3a (entry 3). The low yield of 13 may be explained by the difficulty of keeping the electric current in a suitable level due to the increase of the terminal voltage with progress of the reaction. This difficulty was overcome by the alternation of anode and cathode with a suitable interval (15 sec., Table 3, entry 4) and the material yield of 13 was remarkably improved.  Mg is a remarkably effective material of electrode also for the formation of 13 (Table 4), whereas Al gave low yield (entry 4) and other materials such as Cu and Ni were rather ineffective (entries 2, 3). The electroreduction systems using Al or Cu anode in other electrolytes (Al anode/Bu4NCl/DME [24], Al anode/LiCl/THF-HMPT [25], or Cu/Bu4NClO4/DME [26]) have been also reported, however, the molecular weight of the resulting polysilanes is relatively low. Other than Mg electrodes, the use of Ag anode and Pt cathode in DME containing Bu4NCLO4 is also reported to be effective to obtain high molecular weight polysilane [27,28].  The effect of monomer concentration was investigated in order to obtain high molecular weight polysilane ( Table 5). The molecular weight of 13 becomes higher with the increase in the concentration of 3a. The molecular weight (Mn) of 13 was, for instance, 31,000 when the electroreduction of 3a was carried out under high concentration condition (1.2 mol/L) at 0.5 F/mol of supplied electricity though the material yield of 13 decreased.
The most satisfactory result, in which material yield was 79 % and molecular weight (Mn) was 9900, was obtained when the concentration of 3a was 0.67 mol/L (entry 2). The polysilane 13 obtained here showed relatively sharp monomodal distribution of molecular weight in the elution profile of GPC, whereas the polysilanes prepared by the alkali metal condensation method usually showed broad bimodal distribution.  The mechanism of electroreductive formation of polysilane is not always perfectly clear, though the initial step of reaction is obviously the reduction of 3a to a silyl anion species 14.
Two types of reaction patterns may be proposable to the propagation step. In the first case, the reaction of 14 with 3a gives a dimer that yields trimer, tetramer, and finally polymer upon repeated reaction with 14 (Scheme 11). In another pattern of the propagation, the oligomers such as dimer, trimer, and the like 15 are reduced to give the oligomeric active species, which then react with oligomer 15 or 3a to give finally polymer. (Scheme 12). Although it is not always possible to specify the extent of contribution of each pattern to the propagation step, the former reaction probably proceeds mainly, since the electrochemical reduction of oligomeric silyl chloride 15 may be rather difficult when it is analogized with the electroreduction of long chain alkyl chlorides. Scheme 11. Scheme 12.

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 3a and the dichlorosilanes having protected hydoxyphenyl groups (3d-g) with Mg electrode afforded the corresponding copolymers 16 (16d-g, Scheme 13), and the deprotection of the resulting copolymers gave the polysilanes having hydroxyl groups. The reactivity of 3d-g highly depends on the type of protecting group ( Table 6). Homopolymerization of 3g, for example, gave 16g (entry 10), whereas other monomers did not afford polymers but oligomeric compounds (entries 3, 5, 8).
The modification or the property of polysilane must be achieved by using the hydroxyl group located on the polymer 16d as a key functional group. The deprotection of the methoxymethyl group of 16d ( Table 6, entry 1) with 10%HCl aqueous solution followed by the reaction with hexamethylene diisocyanate resulted in a remarkable increase in the molecular weight with indicating the linkage of the polymer chain (Scheme 14).

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 5 was found to give the corresponding polysilane 18 consisting of disilane units (Scheme 15). The electroreductive polymerization was carried out under a variety of reaction conditions, however, the yield of the resulting polymer was very low ( Table 7). It is probably due to high reactivity of the disilene intermediate formed by the electroreduction of 5. In fact, the addition of naphthalene, which could make a masked disilene intermediate, into the reaction system slightly increased the yield of the polysilane 18 (entry 6).  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.

Electroreductive polymerization of dichlorosilanes in the presence of disilane additives [30]
The disilane additives, which are generated in situ in electroreductive coupling of the corresponding chlorosilanes, were found to be effective to the promotion of the electroreductive polymerization of dichloromethylphenylsilane (3a) and the control of the molecular weight distribution of the resulting polysilanes (Scheme 18, Table 9). The electroreduction of dichlorosilane 3a in the presence of 1,1,1-trimethyl-2,2,2triphenyldisilane gives the corresponding polysilane 13 in 56% yield, and the number average molecular weight and the molecular weight distribution are determined by GPC to be 3000, and 1.10 respectively ( Table 9, entry 3). The reduction of dichloromethylphenylsilane (3a) by Wurtz type condensation using metal lithium in the presence of catalytic amount 1,1,1-trimethyl-2,2,2-triphenyldisilane affords five-and sixmembers ring products [31]. On the other hand, the cyclosilanes are not detected under the electroreductive conditions. The use of 1,1,12,2,2-hexaphenyldisilane affords the polysilane 13 in 59% yield, and the Mw/Mn is 1.08 (entry 4). Thus, the polysilanes prepared in the presence of the disilane additive containing triphenylsilyl group show narrower molecular weight distributions than the polysilane prepared without the disilane additive.
The mechanism for the triphenylsilyl substituted disilane to control the electroreductive polymerization suggested is as follows (Scheme 19). The electroreductively generated reactive silyl anion species at the terminus of the propagating polymer is trapped with the disilane forming a relatively stable triphenylsilyl anion and inhibits the undesirable side reactions such as backbiting reaction. The triphenylsilyl anion attacks as a nucleophile to the chlorinated silicon atom at the terminal of the propagating polymer to give the  Table 9. Electroreductive polymerizartion of dichlomethylphenylsilane (3a) in the presence of disilane additives a 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.

Electroreductive block copolymerization using triphenylsilyl groupterminated polysilane [32]
The triphenylsilyl group terminated polysilanes have been synthesized by the electroreductive polymerization of dichloromethylphenylsilane (3a) in the presence of the electroreductively prepared disilane additives 2. The electroreductive termination with chlorotriphenylsilane (1d) was carried out to ensure the terminus of the resulting polysilane for triphenylsilyl group. The polysilanes were obtained as white powders in 15-32% yields after reprecipitation from benzene-ethanol, and the number average molecular weights were estimated by GPC to be 3000-3740 (Table 10). By using the isolated triphenylsilyl groupterminated poly(methylphenylsilane) 13 as a macroinitiator, the electroreductive polymerization of dibutyldichlorosilane (3h) was carried out (Scheme 20, Table 10). Under these conditions dichlorsilane 3h was first electroreduced to form the corresponding oligomeric silyl anion. Electroreductive copolymerization was found to proceed by the attack of the oligomeric silyl anion to triphenylsilyl group-terminated polysilane 13 and further electroreductive condensation with dichlorosilane 3h affording the corresponding copolymer, polymethylphenylsilane-block-polydibutylsilane (21) Polydibutylsilane-block-polymethylphenylsilane (21') was also obtained by using triphenylsilyl group-terminated polydibutylsilane (22)   The UV absorption spectra of the resulting polysilane 21 (entry 3, Table 10) was compared with those of poly(methylphenylsilane) (Mn = 3350, Mw/Mn = 1.4), poly(dibutylsilane) (Mn = 3950, Mw/Mn = 1.7), and poly(methylphenylsilane-co-dibutylsilane) (random copolymer) ( Figure). Poly(methylphenylsilane) showed a -* band at 271 nm (max = 4831) and a-* band at 326 nm (max = 4766) at 4°C (Figure (a)). Poly(methylphenylsilane) (Mw = 9000) prepared by Wurtz coupling under sonication was reported to show a -* band at 332 nm (max = 4100) [33]. The max value of the electroreductively prepared poly(methyphenylsilane) was a little shorter and it is probably due to relatively lower molecular weight. In fact Wurtz coupling-synthesized poly(methylphenylsilane) having 5000 of Mw showed a -* band at 328 nm [34]. Poly(dibutylsilane) showed only a -* band at 309 nm, and the max value (9611) was two times higher than that of poly(methylphenylsilane) (Figure (b)). The UV absorption spectrum of the helical form of poly(di-n-alkylsilane)s is typically centered near 315 nm, while the more planar conformation is typically centered at 375 nm [35,36]. Poly(dibutylsilane) obtained in this study showed the max value corresponding to helical backbone conformation. The max of the -* band of poly(methylphenylsilane) was observed to decrease with an increase in temperature [5]. The UV spectra of copolymer 5 showed a -* band at 273 nm (max = 2723) and a -* band at 306 nm (max = 2949) (Figure (d)), while the random copolymer showed a -* band at 272 nm (max = 2438) and a -* band at 309 nm (max = 2065) (Figure (c)). The max value of the -* band of copolymer 21 was higher than that of the random copolymer, and less temperature dependence of max was observed in the spectrum of copolymer 21.
These results indicate that copolymer 21 has long sequences of dibutylsilylene units, that is, block structure.

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.

Manabu Ishifune
Department of Applied Chemistry, Faculty of Science and Engeneering, Kinki University, Higashi-Osaka, Osaka, Japan