Polysilicon hydride
Encyclopedia
Polysilicon hydridesx (where n = 0.2 to 2.5 and x is the number of monomer units) are silicon
backbone polymer
ic solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed
and ignite readily in air.
, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of SinH2n+2 molecules with n greater than about 8 difficult. Greater catenation
of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen through the sigma bonding. Thus the polymeric silicon hydrides are formed along with smaller silicon hydride oligomer
s and hydrogen gas from the spontaneous but slow decomposition, as well as from the accelerated thermolysis, of acyclic and cyclic
liquid silanes
that are higher in molecular weight than monosilane
(SiH4) and disilane
(Si2H6). The high-temperature decomposition products of the latter two gaseous phases are silicon and hydrogen gas.
Other reactions also produce polymeric silicon hydrides. If calcium monosilicide (CaSi), having a zigzag silicon chain, is reacted with hydrochloric acid, polymeric (SiH2)x is formed. This was first reported in 1921 by the German chemists Lothar Woehler (1870–1952) and Friedrich Mueller. In 1923, German chemists Alfred Stock
(1876–1946) and Friedrich Zeidler (1855–1931) found the (SiH)x polymer is formed along with silane gas by the action of sodium amalgam
on dichlorosilane, SiH2Cl2. The reaction first produces the disodiosilane (SiH2Na2) intermediate and NaCl in a Wurtz-like reaction
. The SiH2Na2 dissolves in the mercury to generate the diradical SiH2, which then forms the final products. The (SiH)x polymeric hydride may also be produced by the dehalogenation of polysilicon halides
(for example, debromination of HSiBr3 with Mg in ether: HSiBr3 + 3/2 Mg → 1/x (SiH)x + 3/2 MgBr2).
(has four bonds). If n = 1, quasi two-dimensional (corrugated sheets) or random three-dimensional silicon networks termed polysilynes are obtained in which each silicon atom is bonded to three other silicon skeletal atoms and one hydrogen atom giving, again, a tetravalent Si atom. In the polysilenes and the polysilynes, the backbone is made exclusively of silicon atoms; the pendent
-, or side-groups, are not shared between skeletal atoms of the backbone chain.
Polymeric silane, or silicon tetrahydride, (SiH4)x was recently discovered to form by both the compression of silane gas, and by the compression of the pure elements silicon and hydrogen together, under extremely high pressures (> 90 GPa) for long periods of time (~ 8 months). However, this polymer differs from those shown above in that the silicon atoms are not bonded to one another but, rather, are connected via bridging hydrogen atoms. The silicon atoms are 8-fold coordinated by hydrogen (i.e. each silicon atom is bonded to 8 hydrogen atoms) forming a tetragonal structure
.
and referred to as silico-acetylene.
and aryl groups, polysilanes are obtained. Although organosilicon polymers can be considered structural derivatives of polysilicon hydrides, they are not synthesized directly from them. Traditionally, polysilanes are prepared by the Wurtz-like
sodium or potassium metal-mediated reductive coupling (dehalogenation) of dihaloorganosilanes (R2SiX2) in an inert solvent (e.g. toluene
), very much like Burkhard's original method. Polysilynes have been similarly prepared from trihaloorganosilanes (RSiX3).
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
backbone polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic solids. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that, like silanes, are easily hydrolyzed
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
and ignite readily in air.
Synthesis
The silanes (SinH2n+2) are much less thermally stable than alkanes (CnH2n+2) and they are kinetically labileLability
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...
, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of SinH2n+2 molecules with n greater than about 8 difficult. Greater catenation
Catenation
Catenation is the ability of a chemical element to form a long chain-like structure via a series of covalent bonds. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms. Catenation is the reason for the presence of a large number of organic compounds in nature...
of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen through the sigma bonding. Thus the polymeric silicon hydrides are formed along with smaller silicon hydride oligomer
Oligomer
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...
s and hydrogen gas from the spontaneous but slow decomposition, as well as from the accelerated thermolysis, of acyclic and cyclic
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...
liquid silanes
Silanes
Silanes are chemical compounds of silicon and hydrogen, which are analogues of alkane hydrocarbons. Silanes consist of a chain of silicon atoms covalently bonded to each other and to hydrogen atoms. The general formula of a silane is SinH2n+2...
that are higher in molecular weight than monosilane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
(SiH4) and disilane
Disilane
Disilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and . Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on metal silicides...
(Si2H6). The high-temperature decomposition products of the latter two gaseous phases are silicon and hydrogen gas.
Other reactions also produce polymeric silicon hydrides. If calcium monosilicide (CaSi), having a zigzag silicon chain, is reacted with hydrochloric acid, polymeric (SiH2)x is formed. This was first reported in 1921 by the German chemists Lothar Woehler (1870–1952) and Friedrich Mueller. In 1923, German chemists Alfred Stock
Alfred Stock
Alfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
(1876–1946) and Friedrich Zeidler (1855–1931) found the (SiH)x polymer is formed along with silane gas by the action of sodium amalgam
Sodium amalgam
Sodium amalgam, commonly denoted Na, is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions involving mercury as a major component. Sodium amalgam is often used in reactions as strong reducing agents with better handling properties compared...
on dichlorosilane, SiH2Cl2. The reaction first produces the disodiosilane (SiH2Na2) intermediate and NaCl in a Wurtz-like reaction
Wurtz reaction
The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...
. The SiH2Na2 dissolves in the mercury to generate the diradical SiH2, which then forms the final products. The (SiH)x polymeric hydride may also be produced by the dehalogenation of polysilicon halides
Polysilicon halides
Polysilicon halides are silicon backbone polymeric 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...
(for example, debromination of HSiBr3 with Mg in ether: HSiBr3 + 3/2 Mg → 1/x (SiH)x + 3/2 MgBr2).
Macromolecular Structure
When n = 2 in (SiHn)x, the polymer is termed polysilene, which has a quasi one-dimensional (zigzag) chain structure in which each silicon atom is bonded to two other silicon skeletal atoms and two hydrogen atoms. Thus the Si atom is tetravalentValence (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...
(has four bonds). If n = 1, quasi two-dimensional (corrugated sheets) or random three-dimensional silicon networks termed polysilynes are obtained in which each silicon atom is bonded to three other silicon skeletal atoms and one hydrogen atom giving, again, a tetravalent Si atom. In the polysilenes and the polysilynes, the backbone is made exclusively of silicon atoms; the pendent
Pendant group
A pendant group or side group is a group of molecules attached to a backbone chain of a long molecule. Usually, this molecule would be a polymer...
-, or side-groups, are not shared between skeletal atoms of the backbone chain.
Polymeric silane, or silicon tetrahydride, (SiH4)x was recently discovered to form by both the compression of silane gas, and by the compression of the pure elements silicon and hydrogen together, under extremely high pressures (> 90 GPa) for long periods of time (~ 8 months). However, this polymer differs from those shown above in that the silicon atoms are not bonded to one another but, rather, are connected via bridging hydrogen atoms. The silicon atoms are 8-fold coordinated by hydrogen (i.e. each silicon atom is bonded to 8 hydrogen atoms) forming a tetragonal structure
Tetragonal crystal system
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
.
Inorganic derivatives - siloxene
In addition to the polysilicon halides, another related compound, in which the hydrogen is partially replaced by an inorganic group, is siloxene. In fact, siloxene was first oberved before the polysilicon hydrides but it was initially thought to be a polymeric silicon hydride itself. Siloxene is structurally derived from layered polysilyne by replacing one-half of the hydrogen atoms with OH groups. It has the chemical formula Si2H2O (or [Si6H3(OH)3]x) and it is prepared by reacting calcium disilicide (CaSi2), which has a puckered layer of silicon atoms, with dilute hydrochloric acid. This yellow solid was actually prepared from that route as early as 1900 by Charles Schenck Bradley (1853–1929), a one-time associate of Thomas Edison. However, its structure, the intra-polymer Si-Si bond order, and true chemical formula were not determined for some time afterwards Bradley had erroneously concluded it was the silicon analog, (Si2H2)x, of the unsaturated (double-bond containing) polymer PolyacetylenePolyacetylene
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...
and referred to as silico-acetylene.
Organic derivatives - polysilanes and polysilynes
When hydrogen atoms in linear polysilene are replaced with organic substitutents, particularly alkylAlkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
and aryl groups, polysilanes are obtained. Although organosilicon polymers can be considered structural derivatives of polysilicon hydrides, they are not synthesized directly from them. Traditionally, polysilanes are prepared by the Wurtz-like
Wurtz reaction
The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new carbon-carbon bond:...
sodium or potassium metal-mediated reductive coupling (dehalogenation) of dihaloorganosilanes (R2SiX2) in an inert solvent (e.g. toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
), very much like Burkhard's original method. Polysilynes have been similarly prepared from trihaloorganosilanes (RSiX3).