Polysilicon halides
Encyclopedia
Polysilicon halides are silicon backbone polymer
ic solids. At room temperature, the polysilicon fluorides are colorless to yellow solids while the chlorides, bromides, and iodides are, respectively, yellow, amber, and red-orange. Polysilicon dihaildes (perhalo-polysilenes
) have the general formula (SiX2)n while the polysilicon monohalides (perhalo-polysilynes
) have the formula (SiX)n, where X is F, Cl, Br, or I and n is the number of monomer units in the polymer.
atoms. In the monomeric silicon dihalide (aka dihalosilylene
and dihalosilene) molecule, which is analogous to carbene
molecules, the silicon atom is divalent
(forms two bonds). By contrast, in both the polysilicon dihalides and the polysilicon monohalides, as well as the polysilicon hydrides, the silicon atom is tetravalent with a local coordination geometry that is tetrahedral, even though the stoichiometry of the monohalides ([SiX]n = SinXn) might erroneously imply a structural analogy between perhalopolysilynes and [linear] polyacetylene
s with the similar formula (C2H2)n. The carbon atoms in the polyacetylene polymer are sp2-hybridized and thus have a local coordination geometry that is trigonal planar. However, this is not observed in the polysilicon halides or hydrides because the Si=Si double bond in disilene
compounds are much more reactive than C=C double bonds. Only when the substituent groups on silicon are very large are disilene compounds kinetically non-labile
.
ic intermediates like Si2F4 (•SiF2-F2Si•) and Si3F6 (•SiF2-SiF2-F2Si•),
The polysilicon dihalides also form from the thermally-induced disproportionation
of perhalosilanes (according to: x SinX2n+2 → x SiX4 + (n-1) (SiX2)x where n ≥ 2). For example, SiCl4 and Si forms SinCl2n cyclic oligomers (with n = 12-16) at 900-1200°C. Under conditions of high vacuum and fast pumping, SiCl2 may be isolated by rapidly quenching the reaction products or, under less stringent vacuum conditions, (SiCl2)n polymer is deposited just beyond the hot zone while the perchlorosilanes SinCl2n+2 are trapped farther downstream. The infrared multiphoton dissociation
of trichlorosilane
(HSiCl3) also yields polysilicon dichloride, (SiCl2)n, along with HCl. SiBr4 and SiI4 react with Si at high temperatures to produce SiBr2 and SiI2, which polymerize on quenching.
, (SiF2)n ignites spontaneously in air, whereas (SiCl2)n inflames in dry air only when heated to 150°C.
The halogen atoms in polysilicon dihalides can be substituted with organic groups. For example, (SiCl2)n undergoes substitution by alcohols to give poly(dialkoxysilylene)s.
The polysilicon monohalides are all stable to 400°C, but are also water and air sensitive. Polysilicon monofluoride reacts more vigorously than the heavier polysilicon monohalides. For example, (SiF)n decomposes [to SiF4 and Si] above 400°C explosively.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic solids. At room temperature, the polysilicon fluorides are colorless to yellow solids while the chlorides, bromides, and iodides are, respectively, yellow, amber, and red-orange. Polysilicon dihaildes (perhalo-polysilenes
Polysilicon hydride
Polysilicon hydrides x are silicon backbone polymeric solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed and ignite readily in air.-Synthesis:The silanes are much less thermally stable than alkanes and they are...
) have the general formula (SiX2)n while the polysilicon monohalides (perhalo-polysilynes
Polysilicon hydride
Polysilicon hydrides x are silicon backbone polymeric solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed and ignite readily in air.-Synthesis:The silanes are much less thermally stable than alkanes and they are...
) have the formula (SiX)n, where X is F, Cl, Br, or I and n is the number of monomer units in the polymer.
Macromolecular Structure
The polysilicon halides can be considered structural derivatives of the polysilicon hydrides, in which the side-group hydrogen atoms are substituted with halogenHalogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
atoms. In the monomeric silicon dihalide (aka dihalosilylene
Silylene
Silylenes are chemical compounds containing a divalent silicon atom without any electrical charge. Both dicoordinate and tricoordinate silylenes are reported in the literature. They are considered to be heavier analogues of carbene. In earlier times, they were called silene, but this is a mistake,...
and dihalosilene) molecule, which is analogous to carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
molecules, the silicon atom is divalent
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
(forms two bonds). By contrast, in both the polysilicon dihalides and the polysilicon monohalides, as well as the polysilicon hydrides, the silicon atom is tetravalent with a local coordination geometry that is tetrahedral, even though the stoichiometry of the monohalides ([SiX]n = SinXn) might erroneously imply a structural analogy between perhalopolysilynes and [linear] polyacetylene
Polyacetylene
Polyacetylene is an organic polymer with the repeat unit n. The high electrical conductivity discovered for these polymers beginning in the 1960's accelerated interest in the use of organic compounds in microelectronics...
s with the similar formula (C2H2)n. The carbon atoms in the polyacetylene polymer are sp2-hybridized and thus have a local coordination geometry that is trigonal planar. However, this is not observed in the polysilicon halides or hydrides because the Si=Si double bond in disilene
Disilene
Disilenes are compounds containing a silicon–silicon double bond and are considered to be heavier analogues of alkenes. They are sometimes also called disilaalkenes.-History:...
compounds are much more reactive than C=C double bonds. Only when the substituent groups on silicon are very large are disilene compounds kinetically non-labile
Lability
Lability refers to something that is constantly undergoing change or something that is likely to undergo change.-Chemistry:The term is used to describe a relatively unstable and transient chemical species...
.
Synthesis
The first indication that the reaction of SiX4 and Si yields a higher halide SinX2n+2 (n > 1) was in 1871 for the comproportionation reaction of SiCl4 vapor and Si at white heat to give Si2Cl6. This was discovered by the French chemists Louis Joseph Troost (1825 - 191) and Paul Hautefeuille (1836–1902). Since that time, it has been shown that gaseous silicon dihalide molecules (SiX2) are formed as intermediates in the Si/SiX4 reactions. The silicon dihalide gas molecules can be condensed at low temperatures. For example, if the gaseous SiF2 (difluorosilylene) produced from SiF4 (g) and Si (s) at 1100-1400°C is condensed at temperatures below -80°C and subsequently allowed to warm to room temperature, (SiF2)n is obtained. That reaction was first observed by Donald C. Pease, a DuPont scientist in 1958. The polymerization is believed to occur via paramagnetic di-radical oligomerOligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...
ic intermediates like Si2F4 (•SiF2-F2Si•) and Si3F6 (•SiF2-SiF2-F2Si•),
The polysilicon dihalides also form from the thermally-induced disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
of perhalosilanes (according to: x SinX2n+2 → x SiX4 + (n-1) (SiX2)x where n ≥ 2). For example, SiCl4 and Si forms SinCl2n cyclic oligomers (with n = 12-16) at 900-1200°C. Under conditions of high vacuum and fast pumping, SiCl2 may be isolated by rapidly quenching the reaction products or, under less stringent vacuum conditions, (SiCl2)n polymer is deposited just beyond the hot zone while the perchlorosilanes SinCl2n+2 are trapped farther downstream. The infrared multiphoton dissociation
Infrared multiphoton dissociation
Infrared multiphoton dissociation is a technique used in mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original molecule. -How it works:...
of trichlorosilane
Trichlorosilane
Trichlorosilane is a chemical compound containing silicon, hydrogen, and chlorine. At high temperatures, it decomposes to produce silicon, and therefore purified trichlorosilane is the principal source of ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to produce...
(HSiCl3) also yields polysilicon dichloride, (SiCl2)n, along with HCl. SiBr4 and SiI4 react with Si at high temperatures to produce SiBr2 and SiI2, which polymerize on quenching.
Reactivity
The polysilicon dihalides are generally stable under vacuum up to about 150-200°C, after which they decompose to perhalosilanes, SinX2n+2 (where n = 1 to 14), and to polysilicon monohalides. However, they are air and moisture sensitive. Polysilicon difluoride is more reactive than the heavier polysilicon dihalides. In stark contrast to its carbon analog, polytetrafluoroethylenePolytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
, (SiF2)n ignites spontaneously in air, whereas (SiCl2)n inflames in dry air only when heated to 150°C.
The halogen atoms in polysilicon dihalides can be substituted with organic groups. For example, (SiCl2)n undergoes substitution by alcohols to give poly(dialkoxysilylene)s.
The polysilicon monohalides are all stable to 400°C, but are also water and air sensitive. Polysilicon monofluoride reacts more vigorously than the heavier polysilicon monohalides. For example, (SiF)n decomposes [to SiF4 and Si] above 400°C explosively.