Transition-Metal-Substituted Oligo- and Polysilanes - Advances in

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20 Transition-Metal-Substituted Oligo -and Polysilanes Keith H. Pannell, James M. Rozell, Jr., and Steven Vincenti Department of Chemistry, University of Texas, El Paso, TX 79968-0513

New oligo- and polysilanes containing transition metals have been synthesized and studied. Oligosilanes readily deoligomerize upon photochemical irradiation via metal-silyl(silylene) intermediates. The mechanism was studied in detail via the use of variously substituted disilyl-metal complexes. New high-molecular-weight polysilanes have been synthesized as copolymers from the monomers (C H )(CH )SiCl2 and LM(CH )SiCl , in which LM (ligand-metal) may be [(n -C H )Fe(n -C H )] (Fc) or [(n -C H )Fe(CO) ] (Fp). The new polymers indicate that the metal substituents Fc and Fp cause a photochemical stabilization of the polymers with respect to depolymerization in a manner proportional to the metal content. 6

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T ' RANSITION METAL SUBSTITUENTS in organosilicon compounds create complexes in which the chemical and physical properties of the organosilyl radical are changed significantly (1-3). For example, the presence of the transition metal center allows skeletal rearrangements (4-8), migrations (9-12), and deoligomerizations (12). We investigated the capacity of transition metals to alter the chemical, photochemical, and physical properties of oligo- and polysilane materials.

Monosilane Complexes The synthesis of transition metal complexes containing direct metal-silicon bonds involves three main reaction processes: salt elimination (equation 1) 0065-2393/90/0224-0329$06.00/0 © 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, John M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

(13), oxidative addition (equation 2) (14), and elimination of small molecules (equation 3) (15). ^ 5 - C 5 H 5 ) F e ( C O ) 2 - N a + + (CH 3 ) 3 SiCl-^ ^ 5 -C 5 H 5 )Fe(CO) 2 Si(CH 3 ) 3 + NaCl Fe(CO)5 + R 3 SiH - ^ - » ds-R3SiFe(CO)4H + C O Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 6, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch020

ds-(R 3 P) 2 PtCl 2 + (C 2 H 5 ) 3 SiH

trans-(R3P)2PtCl[Si(C2H5)3] + H C l

(1) (2) (3)

The chemistry of the metal-silicon bond is distinguished by greater thermal and oxidative stability compared with the related metal-carbon analogues. Thus, for example, whereas the ierf-butyliron complex [ ( η 5 C 5 H 5 )Fe(C0 2 )C(CH 3 ) 3 ] (Fp-CMe 3 ) readily undergoes the well-established alkyl migration reaction to form acyl complexes in the presence of ligands (equation 4) (16), the related trimethylsilyl complex does not exhibit this type of chemistry. Only under the influence of UV irradiation will a chemical reaction occur, and this reaction leads to C O substitution (equation 5) (17). (T, 5 -C 5 H 5 )Fe(CO) 2 -C(CH 3 ) 3 + P(C 6 H 5 ) 3 ( O 5 -C 5 H 5 )Fe(CO)[P(C e H 5 )3]-CO-C(CH 3 )3

(4)

(T, -CsH5)Fe(CO)2-Si(CH3)3 + P ( C 6 H 5 ) 3 - ^ 5

^ 5 -C 5 H 5 )Fe(CO)[P(C 6 H 5 ) 3 ]-Si(CH 3 ) 3

(5)

Early transition metal complexes containing a silicon-metal bond, for example, [^ 5 -C 5 H 5 ) 2 Zr[Si(CH 3 ) 3 ]Cl], exhibit both C O and 0 2 insertion re actions (18, 19).

Oligosilane Complexes Several examples of transition-metal-substituted oligosilanes have been re ported (20-23). These materials have been prepared usually via salt elimi nation (equation 6). L M ~ N a + + R 3 Si(SiR 2 ) n Cl-^ LM-(SiR 2 ) n -SiR 3

(6)

In the previous reaction, L M (ligand-metal) is [M(CO)5] (in which M can be Re or Mn) or [^ 5 -C 5 H 5 )M(CO)J (in which M is Fe or Ru when m = 2 and M is Mo when m = 3), and η = 1-6. Like the monosilane complexes, the oligosilanes form thermally and oxidatively stable complexes with the Re, Mn, Ru, and Fe systems, whereas the Mo complexes are significantly less stable. As yet such complexes have not been shown to exhibit any insertion-type reactions. The iron complexes


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Oligo- and Polysilanes

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of the type ^ 5 -C 5 H 5 )Fe(CO) 2 -[Si(CH 3 ) 2 ] n -Si(CH 3 )3 (Fp-Sin) are interest ing, because they are labile with respect to photochemical irradiation. Under such reaction conditions they readily deoligomerize to form the monosilane complex and siloxane polymers (equation 7) (12). FpSi(CH3)2[Si(CH3)2]nSi(CH3)3

FpSi(CH 3 ) 3 + siloxane polymers

(7)


for η — 1 or 2.

Expérimentai Procedures In this section, representative experimental details of the synthesis and photochem ical study of the [(n 5 -C 5 H 5 )Fe(CO) 2 ] (Fp) disilanes and the Fp- and [(η 5 -0 5 Η 5 )Ρβ(η 5 C 5 H 4 )] (Fc)-substituted polysilanes are presented. All manipulations were performed under dry nitrogen or argon atmosphere in oven-dried glassware with dry solvents. Synthesis of [^5-C5H5)Fe(CO)2-Si(CH3)(C6H5)-Si(CH3)3]. A 250-mL, roundbottom flask equipped with a side arm was charged with 0.12 g (5.2 mmol) of Na, which was amalgamated with 15 g of Hg. To this mixture, 1.5 g (4.2 mmol) of [(η 5 C 5 H 5 )Fe(CO)2]2 (Strem Chemicals) in 60 mL of tetrahydrofuran (THF) was added. This mixture was stirred until formation of the Fp salt (24) was complete (1 h). Excess Hg was removed through the side arm, and the solution was cooled to 0 °C. Then, 0.95 g (4.2 mmol) of (CH 3 ) 3 SiSi(CH 3 )(C 6 H 5 )Cl (25) was added. The solution was permitted to warm to room temperature and stirred for 1.5 h. At this time, the solvent was removed under reduced pressure to yield a viscous oil, which was extracted into hexane, filtered, concentrated, and applied to a silica gel column (2.5 by 25 cm). Elution with hexane yielded a bright yellow band, which upon removal of the solvent under reduced pressure yielded 1.1 g (70%) of the required complex as an orange oil. Analysis (Galbraith Laboratories, Inc.) of the complex gave the following results. 1. Elemental analysis [calculated (found), in percent]: C, 55.13 (55.60); H, 5.99 (6.18) 2. Ή NMR spectrum: δ 0.15 [(CH 3 ) 3 Si]; δ 0.65 (CH 3 Si); δ 4.60 (C 5 H 5 ); δ 7.26, 7.51 (C 6 H 5 ) 3.

13

C NMR spectrum: δ -0.32, 1.18 (CH 3 ); δ 83.9 (C 5 H 5 ); δ 127.4, 133.7, 145.8 (C 6 H 5 ); δ 215.3 (CO)

4.

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Si NMR spectrum: δ -11.8 (Fe-Si-St); δ 12.6 (Fe-St)

5. IR spectrum (in hexane): £003, 1998, 1946 c m 1 (CO absorption band) Photochemical Treatment of FpSi(CH3)(C6H5)Si(CH3)3. Photolysis was per formed at room temperature under nitrogen at 88.6 kPa with a Hanovia 450-W medium-pressure mercury lamp. Degassed cyclohexane solutions of the complex in Pyrex 9820 test tubes were illuminated for 1.5 h. Analysis of the product distribution was made with an internal standard (toluene) on a high-pressure liquid chromato graphic system (Beckman Instruments, model 332) with UV detection at 270 nm. A Cis reverse-phase column was used with C H 3 C N - H 2 0 (65:35, vol/vol) as solvent.



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Synthesis of [(CeHsXCH^Si^FciCH^Si]». A mixture of 0.5 g (1.7 mmol) of Fe(CH 3 )SiCl 2 and 8.0 g (42 mmol) of (C 6 H 5 )(CH 3 )SiCl 2 in 50 mL of toluene was heated to reflux temperature in a foil-covered three-necked flask. To this refluxing solution was added dropwise 5.8 g (101 mmol) of a 40% dispersion of Na in light mineral oil (Aldrich Chemicals). The dark-blue solution was refluxed for 2 h and cooled to room temperature. A few drops of methanol were added to remove excess Na, and the resulting solution was treated with saturated NaHCQ 3 solution (40 mL) and toluene (40 mL). After separation and drying with magnesium sulfate, the solvent was removed under vacuum to yield an orange oil. Two precipitations from toluene with hexane and two from methanol produced 384 mg of a cream-colored powder. The Ή NMR spectrum indicated a statistical incorporation of the two monomers. Whether the product is made up of block polymers has yet to be determined. Size exclusion chromatography using a μ-styragel 10,000-Â column (Waters Associates) with polystyrene standards indicated a bimodal molecular weight distribution at 200,000 and 11,000 (modal maxima) at a ratio of 1.4:1 (wavelength of maximum absorption [λmax] 340 nm). Synthesis of [(C 6 H 5 )(CH 3 )SiFp(CH 3 )Si]„. The procedure is identical to that described in the previous section, with Fp(CH 3 )SiCl 2 (26) instead of Fc(CH 3 )SiCl 2 and two minor modifications. After removal of the toluene solvent, the product was treated initially with hexane to remove the low-molecular-weight fractions. The residue was dissolved in T H F and filtered, and the T H F was removed under vacuum. The residue was dissolved in toluene, washed with a saturated N a H C 0 3 solution, separated, and dried. Two precipitations from T H F with methanol produced 4% of material with a monomodal molecular weight of 8000. Spectral analysis gave the following results: IR spectrum (in THF), 1998, 1940 c m 1 (CO frequency); UV spec trum, \ m a x = 332 nm; Ή NMR spectrum (in CDC1 3 ), δ 7.2 bd (broad doublet) (C 6 H 5 ); δ 3.8 bd (C 5 H 5 ); δ 0.6 bd (CH 3 ). Photolysis of [[(C 6 H 5 )(CH 3 )Si] n LM(CH 3 )Si] r o . T H F solutions of the polymers, which were obtained from reactions such as those described earlier, in quantities sufficient to produce an absorbance of 1 at k under nitrogen at 88.6 kPa were irradiated in quartz cuvettes in a merry-go-round apparatus at 300 nm for periods of 1-60 s. A sample of authentic [(C 6 H 5 )(CH 3 )Si] „ of almost identical molecular weight was also irradiated at the same time for a comparative study. Typical results are illustrated in Figures 1 and 2. max

Photochemical Deoligomerization ofFp-Si

2

Complexes

Mechanism. We have investigated in detail the mechanism of the photochemical deoligomerization for the Fp-Si 2 system by using variously substituted disilanes in place of the permethylated species. The product distribution obtained upon irradiation of FpSi(CH3)2Si(C6H5)3 suggests a mechanism involving initial CO loss followed by Si-Si bond cleavage and migration to form metal-silyl(silylene) intermediates (12). These interme diates rapidly establish equilibrium via a series of 1,3-alkyl or -aryl shifts, and subsequent recombination of the CO yields the monosilyl complexes (Scheme I).



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Min: -0.1000 300

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Wavelength (nm) Figure la. Photolysis of THF solutions of [(C H )(CH )Si]„ 400,000) at 270 nm. 6

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(molecular weight

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Min: -0.1000[ 300

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1*60

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Wavelength (nm) Figure lb. Photolysis of THF solutions of'[[(C H )(CH )Si] Fc(CH )Si] lecular weight 390,000) at 270 nm. 6

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(mo-

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Max: 1.1000]

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Wavelength (nm) Figure 2a. Photolysis of THF solutions of [(C H )(CH )Si] 4000) at 270 nm. 6

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(molecular weight

Min: -0.1000] 300

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Wavelength (nm) Figure 2b. Photolysis of THF solutions of [[(C H )(CH )Si] Fp(CH )Si] lecular weight 8000) at 270 nm. 6

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(mo-

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CO

JL hv


CO^Te-SiMe2-SiPh3 CO

C 0

^Fe=siPhMe

c(T

Jfe—SiMe Ph 9

co^\

\

Ph MeSi

2

Ph Si

2

^Fe—SiMe —SiPh 2

3

^.Fe=SiMe„

co^\

2

Ph Si

2

3

RgSi-^ACO Fe—SiMePh /

CO CO

Fe—SiMe Ph il

0

0

0

co

« r /

2

I

^-Fe—SiPh ^ \ w -Wll IQ I

c oCOV

3

Scheme L Deoligomerization of Fp-containing polysilanes. (Reproduced reference 12. Copyright 1986 American Chemical Society.)

from

In subsequent studies, we extended this initial investigation to a series of isomeric pairs of disilyl complexes: (1) FpSi(C6H5)(Crl3)Si(CH3)3 and FpSi(CH 3 ) 2 Si(C 6 H 5 )(CH 3 ) 2 5 (2) FpSi(C 6 H 5 )(CH 3 )Si(CH 3 ) 2 (C 6 H 5 ) and FpSi(CH 3 ) 2 Si(CH 3 )(C 6 H 5 ) 2 , and (3) FpSi(C 6 H 5 )(CH 3 )Si(CH 3 )(C 6 H 5 ) 2 and FpSi(CH 3 ) 2 Si(C 6 H 5 ) 3 . If the mechanism outlined in Scheme I is correct, then regardless of which isomer of a given pair is irradiated, the rapid equilibrium established by the 1,3 shifts should provide the same ratio of final products. Photochemical treatment of the various isomeric pairs resulted in the product distribution data in Table I. The data are unambiguous. Regardless of which isomer of an isomeric pair is irradiated, the product distributions are identical within experimental error. Such results clearly suggest that the mechanism outlined in Scheme I is an accurate description of the processes occurring during irradiation of the disilane complexes. Irradiation of FpSi(CH 3 ) 2 Si(CH 3 ) 2 Si(CH 3 ) 3 yields not only the ultimate product FpSi(CH 3 ) 3 but also transient yields of FpSi(CH 3 ) 2 Si(CH 3 ) 2 .



8 7 2 1

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FpSi(CH ) 2

92 93 51 46 5 7

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FpSi(CH ) (C H ) 3

47 54 89 86

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FpSi(CH )(C H )

Table I. Product Distribution from Photolysis of Fp-Containing Disilanes

FpSi(CH3)(C6H5)Si(CH3)3 FpSi(CH3)2Si(CH3)2(C6H5) FpSi(CH3)(C6H5)Si(CH3)2(C6H5) FpSi(CH3)2Si(CH3)(C6H5)2 FpSi(CH3)(C6H5)Si(CH3)(C6H5)2 FpSi(CH3)2Si(C6H5)3

Starting Complex


6 7

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FpSi(C H )

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FpSi(CH3)2Si(CH3)2Si(CH3)3


F p S i ( C H 3 ) 2 S i ( C H 3 ) 3 ^ FpSi(CH 3 ) 3

(8)

Longer chain oligosilane complexes undergo photochemical redistribution reactions (34). Although our experiments have not ruled out the possibility of a parallel reaction involving direct formation of FpSi(CH 3 ) 3 from FpSi(CH 3 ) 2 Si(CH 3 ) 2 Si(CH 3 )3 via FeSi(Si = Si) intermediates, the results are consistent with the proposed mechanism. Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH 3 ) 3 ] 3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transition-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [(r| 5 -C 5 H 5 )Ru(CO) 2 -Si J, are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. Finally, for the Fe complexes, a direct Fe-Si bond is required to activate the oligomers. When the silyl group is bonded to the complex via the cyclopentadienyl ring, as in [^ 5 -(CH 3 ) 3 Si(CH 3 ) 2 Si(CH 3 ) 2 Si(C 5 H 4 )Fe(CO) 2 CH 3 ], photolysis does not lead to deoligomerization (12). The mode of formation of the siloxane polymers from the photolysis of the Fp-Si n complexes is unclear at present, and we are currently characterizing these materials. However, certain transition metal carbonylates react with silyl halides to form siloxanes, presumably via metal-silicon-bonded complexes (28). Furthermore, (CH 3 ) 3 SiSi(CH 3 ) 3 is an effective reducing agent for C 1 8 0 in the presence of H 2 and Ni catalysts; the reaction leads ultimately to the formation of C H 4 and (CH 3 ) 3 Si 1 8 OSi(CH 3 ) 3 (29). Such results clearly implicate CO as an important reagent for the transformation Si-Si —» Si-O-Si. The inevitable presence of CO during the photochemical treatment of Fp-Si n complexes thus seems to be responsible for siloxane polymer formation. Attempts to trap SiR2 species by using such standard reagents as (C 2 H 5 ) 3 SiH, (CH 3 ) 2 Si(OCH 3 ) 2 , acetylenes, and C H 3 O H have not yielded conclusive evidence regarding the fate of the lost SiR2 fragments.

Polysilane Complexes. The photolability of the Si-Si bonds in the Fp complexes just described suggests that the incorporation of a metal center into high-molecular-weight polysilanes might affect the photodepolymerization of these photoresist materials (30, 31). Incorporation of the metals into the polymers was expected to permit new redox chemistry. Polysilanes



338


are irreversibly oxidized when polysilane-eoated electrodes are used in cyclic voltammetry (32); thus the presence of a reversible low-oxidation-state metal atom in the polymer might introduce a stabilizing effect upon the silane chain and allow switching processes to become available. To investigate these possibilities, we have synthesized two types of metal-substituted polysilane materials incorporating both Fp and Fc groups. Fp was synthesized to investigate, inter alia, the effect on photodepolymerization, whereas Fc seemed to be an excellent model for introducing a reversible low-oxidation-potential metal center. A preliminary communication describing our initial attempts to incorporate Fc has appeared recently (33). The new copolymeric materials were synthesized via a Wurtz-type coupling reaction (equation 9) nLM(CH 3 )SiCl 2 4- m(C6H5)(CH3)SiCl2

Na

' toluene>

[(LMCH 3 Si) n (C 6 H 5 (CH 3 )Si)J. t

(9)

in which L M is Fp or Fc. As determined by NMR spectroscopy, the copolymers formed exhibited a statistical incorporation of monomers in a manner reflecting the initial ratio of starting monomers. For the Fp-containing copolymers, molecular weights were generally low (103-104), whereas for the Fc-containing copolymers, high molecular weights (105-106) were obtained routinely. Yields were low (2-7%). Initial attempts to make homopolymers with Fc-containing monomer were unsuccessful; however, with the Fp-containing monomer, homopolymers with molecular weights of 103-104 were prepared readily. This difference in behavior may be a feature of the particular reaction conditions used, and we are currently studying alternative synthetic procedures. Our initial studies on the new polysilanes have concentrated on their photochemical depolymerization. The results of a study comparing the new polymers with the related homopolymers [(C6H5)(CH3)Si)]n are illustrated in Figures 1 and 2. The photolyses were performed in a merry-go-round apparatus such that both samples were subjected to identical irradiation conditions. The incorporation of both Fc and Fp substituents retarded the depolymerization process. With Fp polymers, a small amount of Si-Fe bond cleavage occurs, as denoted by the appearance of the bridging CO stretching frequency at 1780 cm" 1 , indicating the formation of [(r|-C5H5)Fe(CO)2]2. The degree of photostabilization introduced by the metal substituent is proportional to the amount of metal incorporated. Thus a 5:1 (C 6 H 5 )(CH 3 )Si-Fc(CH 3 )Si copolymer is markedly more photostable than a 10:1 copolymer, which in turn is more stable than the 25:1 copolymer. The homopolymer [ F p S i ( Ç H 3 ) 3 ] n (molecular weight 8000) exhibits depolymerization, but as yet, the needed quantum yields, etc., are not available to describe accurately the effect of metal substitution on the polysilane.


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Acknowledgments


This research has been supported by the Robert A. Welch Foundation, Houston, TX, and a Texas Advanced Technology research award. Financial assistance from the Dow Corning Corporation is also gratefully acknowl edged.

References 1. Aylett, B. J. Adv. Inorg. Chem. Radiochem. 1982, 25, 1. 2. Cundy, C. S.; Kingston, B. M.; Lappert, M . F. Adv. Organomet. Chem. 1973, 11, 253. 3. Pannell, Κ. H . In Silicon Compounds: Registry and Review; Anderson, R.; Arkles, B. C.; Larson, G. L . , Eds.; Petrarch Systems: Bristol, PA, 1987; p 32. 4. Pannell, Κ. H . J. Organomet. Chem. 1970, 21, C17. 5. Pannell, Κ. H . Transition Met. Chem. (Weinheim, Ger.) 1975/6, 1, 32. 6. Lewis, C.; Wrighton, M . S. J. Am. Chem. Soc. 1983, 101, 279. 7. Windus, C.; Giering, W. P. J. Organomet. Chem. 1975, 101, 279. 8. Pannell, Κ. H . ; Rice, J. R. J. Organomet. Chem. 1974, 78, C35. 9. Thum, G.; Ries, W.; Greissinger, D.; Malisch, W. J. Organomet. Chem. 1983, 252, C67. 10. Berryhill, S. R.; Clevenger, G. L . ; Burdurli, Yu, P. Organometallics 1984, 4, 1509. 11. Pannell, K. H.; Vincenti, S. P.; Scott, R. C., III Organometallics 1987, 6, 1593. 12. Pannell, Κ. H . ; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti, S. Organ ometallics 1986, 5, 1056. 13. Piper, T. S.; Lemal, D.; Wilkinson, G. Naturwissenschaften 1956, 43, 129. 14. Smid, G.; Balk, H . J. Chem. Rer. 1970, 103, 2240. 15. Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. 16. Pannell, K. H . ; Kapoor, R. N.; Giassoli, A. J. Organomet. Chem. 1983, 316, 127. 17. King, R. B.; Pannell, Κ. H . Inorg. Chem. 1968, 7, 1510. 18. Tilley, T. D. J. Am. Chem. Soc. 1985, 107, 4084. 19. Arnold, J.; Tilley, T. D. J. Am. Chem. Soc. 1985, 107, 6409. 20. King, R. B.; Pannell, K. H . ; Ishaq, M . ; Bennett, C. J. J Organomet. Chem. 1969, 19, 327. 21. Nicholson, Β. K.; Simpson, J. J. Organomet. Chem. 1974, 72, 211. 22. Malisch, W. J. Organomet. Chem. 1972, 39, C28. 23. Pannell, K. H.; Rozell, J. M . ; Tsai, W. M . Organometallics 1987, 6, 2085. 24. Pannell, K. H . ; Jackson, D. J. Am. Chem. Soc. 1976, 98, 4443. 25. Tamao, K.; Kumada, M . J. Organomet. Chem. 1971, 30, 329. 26. Malisch, W.; Kuhn, M . Chem. Ber. 1974, 107, 979. 27. Pannell, Κ. H.; Rozell, J. M . ; Wang, L. J. Organometallics 1989, 8, 550. 28. Curtis, M . D. Inorg. Nucl. Chem. Lett. 1970, 6, 859. 29. Vollhardt, K. P. C.; Yang, Ζ. Y. Angew. Chem. Int. Ed. Engl. 1984, 23, 460. 30. Miller, R. D.; Hofer, D. C.; McKean, D. R.; Willson, G. C.; West, R.; Trefonas, P. T. In Materials for Microlithography; Thompson, L. F.; Willson, G. C.; Frechet, J. M . J., Eds.; ACS Symposium Series 266; American Chemical Society: Washington, D C , 1984.


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31. Zeigler, J. M . ; Harrah, L. Α . ; Johnson, A . W. Proc. SPIE Conf., Adv. Resist Technol. Process. 1985, 539, 166.

32. Diaz, Α.; Miller, R. D. J. Electrochem. 1985, 132, 834.

33. Pannell, Κ. H . ; Rozell, J. M . ; Zeigler, J. M . Macromolecules 1988, 21, 276. 34. Pannell, K. H . ; Wang, L. J.; Rozell, J. M . Organometallics 1989, 8, 550.


RECEIVED for review May 27, 1988. ACCEPTED revised manuscript April 4, 1989.


Transition-Metal-Substituted Oligo- and Polysilanes - Advances in

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