Robert West University of Wisconsin Madison. Wi 53706 Thomas J. Barton Iowa State University Ames, IA 5001 1
Organosilicon Chemistry Part I
Althoueh com~oundshave been known since .. oraanosilicon .. 1863 1 1 , 2). 1111. lirst real rlou,~riny~,foryantsilinmrhemistry Iwgnn in rhe 1940's 121,t i d h l n y un the cllmmercial success ~,i the silinme polymrrs. At presrnt thr field is undergoing n remarknble rc.n:lisialW~and expnsion, as a result id rrwnt discoveries which have opened entirely new areas: divalent silicon species, catenated polysilanes, biologically active silicon compounds, organosilicon intermediates useful in organic synthesis, and especially, species containing multiple bonds: Si=C, Si=O, Si=Si and Si=N. This resource paper will provide a brief outline of organosilicon chemistry, with suecial emphasis on recent developrnrmts u,hich are n8,t nd(yui~telvtreated in rwirws and tc*xLs (J, I , 51.The mi( le l i intended both for trachvrs who wish tu use tanvr nnrl elastic ~roucrties.T o ohtain the high decree of crosslinking needed-foiresins, substantial amo'nts or organotrichlorosilanes, MeSiCls or PhSiCL, are included in the original hydrolysis reactions. These compounds yield trifunctional RSi03 units which provide abundant cross-links. Some Si-OH end groups are left in the resin, in order to give additional condensation upon curing, which for silicone resin lacquers is carried out above 200'. Silanol functions may also be used for cocondensation with organic polymers, for instance with polyesters containing hydroxyl end groups \ /
-Si-OH
+
HO-C-
/
--r
\
\
-Si-O-C-
/
/ + HO \
Simil;rr condensation reactions nre used to make siliconepd\.ether copdvmrrs which crmtdln Iwth h~drovhohirand hydrophilic portions. They are useful as foaistadilizers, and additives for paints and varnishes. An example of such a polymer structure, hased on dimethyl silicone and ethylene oxide, is C6Hg0 f CHZCHzOi,fSiMe20f , ~ C H S C H ~ O + , C ~ H ~ Because of their "inorganic-organic" structure, silicone polymers have unique properties. Compared to most organic polymers, they are unusually resistant to thermal degradation and oxidation. The viscosity of silicone materials changes remarkably slowly with temperature, and silicone polymers have unusually low glass transition temperatures. These properties make silicones uniquely useful where resistance to extreme temperature changes is desired, for example in aerospace applications. The hoots with which Neil Armstrong made the first human prints on the moon were of silicone ruhber. Because silicones are highly hydrophobic and biologically inert, they are useful as implants in surgery. Drug delivery systems hased on silicone implants have also been developed. 168 / Journal of Chemical Education
,*
0-~
~~
122) Noll, W., "Chemistry and Technology of Siliconps." AcadernicPross, New York, 1968. This erhsurfiw book is the bible of silicone technolagy. (23) Hunter. M.J.."Perspectiuos of Organorikon Chemistry: An Indusvisl Point ofview." Infrs~ScieneeChem. Rept.. 7, 45 (1973).A brief review of historical interest. (241 Watt, J. A. C.,"Silicone Liouid Rubbers? Chem. in Rrrloin. 519 (1974). An oreellent review of the chemistry "f the many rwm-temperature vu1canieing silicones eompositions. (251 Cameron, G. M. and Manend, d. G., "Silane Coupling Agents," Chem. in Brit., 381 (19741. All about surfacetreatment by rilanes.
Catenated Polysilanes A textbook myth holds that only carbon among the group IV elements is capable of extensive catenation. Recent develooments make it clear however that there is no limit on the size and cwnplr~iry~ r catenated f n,mpound; conraining d t im r2ti1. Th(, most sta11k.nnd i~cst srudird n ~ r n v o ~ ~are n d snot exact silicon analogs of hydrocarbons, hut pe&nethylpolysilanes in which chains or rinas of silicon atoms are mainlv "covered" with methyl (27,28). Long chain polysilanes have been made beginning with the six-membered ring (MezSi),j, which is obtained from dimethyldichlorosilane and alkali metals. The cyclic compound can be cleaved with halogens M
Ch
Me~SiCla-t (Me&% +CI(SiMen)&I Monomethylation of the dichloro compound and coupling with alkali metals leads to linear compounds with as many as 24 silicon atoms: (29) CI(SiMezlsC1
CHIMZX
CH:r (SiMezIsCI-
CI(SiMed6C~
Na
Recently, cyclic compounds as large as have been prepared by carefully-controlled condensation of Me2SiCl2 with sodium-potassium alloy ( 3 0 ) : NaIK
MenSiClz +(MenSi), n THF
= 4 through 35
Permethslcvclosilanes obtained in this wav from the lareest h m o l q u cyclic ~ ~ series kntran to chc~niitry.wcepting o n l y the cyclonlknne?. Three-dimcns~mnln m p o u n d i hnte also
I
lr*
aromatic comwunds. form colored charee-transfer complexes with pi-acceptors such as retracyan~gcthylene,and tMelSi~n, like benzene. shows suhstituent directine.- effects on further substitution. The "aromatic" properties of the cyclopolysilanes result because these molecules have high energy, delocalized Si-Si o bonding electrons, and a relatively low energy LUMO* (34). Because the HOMO and LUMO energies lie a t similar energies to those of benzene (Fig. 3), the cyclopolysilanes, like aromatic hydrocarbons, can serve either as electron donors or electron acceptors (26).There is also evidence that longrange transmission of electronic effects takes place in linear permethylsilanes (35). Polysilanes have recently become important as precursors or linear to high-strength silicon carbide (36). When (MezSi)~ oermethvlsilanes are heated ahove 350°, they undergo transiormatim into a "carbosilane" polymer in u,hich metllvlene croups have berome inserted into the polysilane chain. Heating above 1250° converts the carhosilane to &silicon carbide IMeSiL
or IMe.SiI.
CH,
5
I HI
CSi-CH%+
BSiC
+
CH.
+
H,
If the carbosilane polymer is drawn into fibers before the final heat treatment, the silicon carbide is formed as fibers which compare favorably with the strongest fibers known. Tensile streneths well ahove 300.000 osi have been reported for such fiber; which seem likely to become important materials in manufacture of high-strength composites. Figure 3. Qualitative MO energy ievei diagram lor (Me2Si)~. (Me2Si)Band benzene.
been made. Condensation of MeSiCl3 with MezSiClz produces the 2.2.2 and 3.3.1 hicyclopolysilanes, along with other po!ycyclics whose structures are still unknown (31) Electron delocalization is responsible for the special properties of polysilanes which make them unusually interesting. Linear polysilanes absorb ultraviolet light, the wavelength and extinction increasing with increasing chain length, thus resemhling conjugated polyenes. 1)isilanes undergo some reartions similar to those of olefins, for example 2 + 2 addition with acetylenes (32) LZMe
I
Ph
0 SiM%
&M*
Ph
m
".-y..-.
C
I
CO,Me Ph
(26) Wost, R. and Carbeny, E.. "Pcrmethylwlysiknos: S i l i m n h d o g . of Hyd-bons: Science, 183.179 11975). Covem synthosia ofcyelicpermethyiwlysilanesanddeais emnaively with electron-delocaliration and "aromaticity" in thwe moleeulw. (27) Henggc. E.. "Prapertios and Preparations of Si-Si Linkages: Topie* in Current Chmlsfry, No. 51, Springer-Verk &din, 1374.Acomplete treatment of plysilane chemistry, emphasizing synthesis and readom. See all*, Hengge, E., "Prapenies and Formation Reactions of Cyclic Palysilanes? in (Editor: Rheingold, A. L.) "Homoatomic Rings, Chains and Msaomoleculos of Main Group Element.." Elsevier. Amterdsm, 1917. p. 235. (28) Kumadq M., "Some Recent Studies of Skeletal k a f o r m s f i o n of Organop~lysih~." J Olgowmetol. Chrm.. 1W.127 11975).Describes rearcangcment reactions cbrsneri& of wlysilsnes, suite diffwent from thoae of hydraearbons. 129) Bobemki, W. G, end Ailred, A. L.."Preperation ofPermethylatadecssilanp and Per. methyl Tetracosssilane". J. O~ganomeloi.Cham.. 71. C27 11974). (30) Brough, L. F., Matrumurn. K. and West, R.,"The Homalogous Series ofCyelicPalyailanas. 1MeSi)s to IMepSi)z: Angem. Chem.. in press. 1311 Indriksons, A. and West, R., 'Cyclic Polysilsnes VI. Bicyelie and Cage Polyailanes? J. Amw Chem. Soc., 94,6110 11972). 1321 Sakurai, H.. Ksmiyama, Y. and Nskadaira, Y., "Novel (o+ r)Reaetiona of Hexaorgandidlane with Amtylenes Cafalyled by Palladium Complexes? J Amen Chem. Soc., 97.931 (1975). (33) Boek,H.. Kaim, W., Kirs,M.and Wost,R.,"NowlOrganoeilieonRsdicalCations: The One-electron Oridsfion of Permethylcyelopolysila~~~:J. Amor Chem. Soc.. in
Ph
Continuing this analow, the cyclic oolysilanes resemble aromatic hydr&arhons in.hany of fhei; pkperties. For instance, they can be reduced to anion-radicals (Me2SiL7or oxidized to cation-radicals, (MezSi),t (33). Esr spectra for both the cation and anion radicals indicate that the unpaired electron spin is fully delocalized over the ring. Cyclopolysilanes, like
(34) Pitt. C. G., "The Conjugative Propertie8 of Polydsnos and Catenated d3yatems,l. in Rheingold, p. 203. Thir veryreadsble review summarize theoretical treatment. of chemical bonding in polysilanes using ths linear combinstian of bond orbitals (LCBO) MO method and the "Sandorfy C" method. 135) Allred. A. L.. Emst, C. A, and Rstner, M.A.."Co"formatiansand Long-Range Interactions in Permethylpolysilan-,"in Rheingold, (see Ref. 127)) p. 307. (36) Yajima, S.. Oksmura, K., Hawahi, J. end Omori. M., "Synthesis of Continuaua SIC Fibemv6th Hkh TennileStrength." J Amer. Ceramic Soc., 59.324 (1976).and other papem by the Yajimsgroup.
'The LUMO may be r ( 3 d ) or o* type; the point is controver-
sial.
Volume 57, Number 3. March 1980 1 169
