No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Kautsky‐Siloxene Analogous Monomers and Oligomers
Abstract
Because of its indirect band structure, crystalline silicon does not show visible light emission at room temperature. Low-dimensional silicon polymers such as polysilanes, or silicon polymers with sheet-like structures, however, have a direct band structure. They attract considerable attention in solid-state physics mainly because of their outstanding luminescence properties. Especially the two-dimensional sheet polymers such as siloxene (Si6O3H6)n or polysilyne (SiH)n are promising candidates for technological application because of their higher mechanical stability and higher conductivity. Since (SiH)n, accessible from (SiBr)n and LiAlH4 or, more recently, from CaSi2 and HCl, is highly unstable and shows luminescence only in the UV part of the spectrum, siloxene with its strong room temperature photoluminescence in the green or yellow has been most thoroughly investigated. Research on siloxene has been further intensified by the recent suggestion that the efficient visible luminescence observed in porous silicon, a phenomenon of considerable current interest, is mainly due to the presence of siloxene species on the surface of the porous silicon particles.
... Although as in CaSi 2 the 6R stacking sequence predominates [3,24,25]. Three different interpretations of the siloxene structure are reported in the literature; [3,4,26] corrugated silicon layers with three SieSi bonds and alternating eH or eOH substituents as in Wöhler siloxene (a), two-dimensional Si sheets made of Si 6 rings linked by oxygen bridges, as in Kautsky Siloxene (b) and one-dimensional SieSi chains interconnected by SieO bonds (c) (Fig. S3). The structure (a) has been reported to be metastable, with a gradual decomposition into Kautsky siloxene (b) through a hydrolysis process of SieH to SieOH and condensation to SieOeSi, enhanced by light and UV irradiation [4,27]. ...
The alloying reaction between 14th group elements and alkali can be detrimental for Li, Na or K-ion batteries due to the large associated volume expansion that leads to a rapid capacity fading. In order to overcome this issue, their associated 2D phases are promising anodes that enable alkali intercalation without alloying reaction and volume expansion. In this study, the lamellar siloxene obtained from topotactic deintercalation of Ca from CaSi 2 delivered reversible capacities of 2300, 311 and 203 mAh/g for Li, Na and K, respectively, with good capacity retention and coulombic efficiency for Li and Na. The intercalation mechanism taking place upon cycling is highlighted on the basis of ex situ Raman characterization combined with IR spectroscopy, SEM and TEM. To the best of our knowledge, it is the first time that a lamellar Silicon based material shows such high stable capacity without volume expansion, representing a real breakthrough for the batteries field and particularly for NIB.
This chapter is dedicated to cyclic and acyclic compounds containing at least one Si-Si bond but no Si-C bonds. The further exclusion of Zintl-type compounds, polymers, and multiply bonded silyl derivatives nevertheless leaves a substantial number of cyclic, acyclic, and cage-type inorganic silanes, which have been discussed systematically by the position of their substituents in the periodic table.
In continuation of our previous contributions concerning the fluorescence behavior of polysiloxanes containing oligosilanyl substructures, the photoluminescence of hydrolysis and condensation products derived from di- and trihnctional cyclooligosilanes has been investigated. To our present knowledge there are two conditions to be met for our siloxene-like polymers in order to exhibit fluorescence: the presence of cyclosilanyl subunits and a two- or three-dimensional polymeric structure.
The cyclic permethylpolysilanes (Me2Si)n and other cyclic alkylpolysilanes have some properties which resemble those of aromatic hydrocarbons: they have strong ultraviolet absorption bands, form electron delocalized anion- and cation-adicals, give charge-transfer complexes with pi acceptors, and show substituent effects on disubstitution. The nature of chemical bonding responsible for these effects in polysilanes is discussed.
Equilibria among the cyclic compounds (Me2Si)n where n = 5, 6 and 7 have been studied between 30–58°C. Thermodynamic values for the redistribution reactions between pairs of compounds are, for n = 5 → 6, ΔH = −18 kcal/mole, ΔS = −20 cal/deg. mole; for n = 7 → 6, ΔH −3, ΔS +33; for n = 7 → 5, ΔH +18, ΔS + 51. The enthalpies indicate that the stabilities of the rings increase in the order (Me2Si)5 < (Me2Si)7 < (Me2Si)6. The differences are smaller than corresponding differences among the cycloalkanes, probably because the silicon compounds are less affected by steric repulsions and angle strain.
Nuclear magnetic resonance of {sup 29}Si is used to study structural properties of porous silicon and siloxene. Evidence for changes in the chemical shift of porous silicon due to quantum confinement could not be observed. A hydride phase exhibiting a chemical shift of {minus}75 ppm versus tetramethyl silane (TMS) is found in both porous silicon and siloxene. In as-prepared W{umlt o}hler siloxene, a chemical shift of {minus}83 ppm versus TMS is assigned to threefold coordinated Si atoms forming a planar polysilane. {copyright} {ital 1997 American Institute of Physics.}
Silicon L2,3 x-ray emission spectra (XES) of siloxene powder samples prepared according to Wöohler and Kautsky (Wöhler and Kautsky siloxene) are presented. The results are compared with the Si L2,3 spectra of the reference compounds a-Si, c-Si, SiO2, and SiOx. A close similarity of the electronic structure of Wöhler siloxene to that of a-SiO0.43: H and of Kautsky siloxene to that of a-SiO0.87: H is found. We determine the number of oxygen atoms per Si atom at ~0.5 in Wöhler siloxene and ~0.8 in Kautsky siloxene. The relative concentrations are in good agreement with the results of infrared absorption measurements on the same samples.
The first cobalt-cyclosilanyl derivatives Si6Me11Co(CO)3PPh3, 1,4-[Co(CO)3PPh3]2Si6Me10, and 1-[Co(CO)3PPh3]-4-[Fe(CO)2Cp]Si6Me10 were synthesized from Na[Co(CO)3PPh3] with cyclohexasilanyl halides. 1-[Co(CO)3PPh3]-4-[Fe(CO)2Cp]Si6Me10 was prepared from 1-H-4-[Fe(CO)2Cp]Si6Me10 and 1-Cl-4-[Fe(CO)2Cp]Si6Me10. These compounds are the first examples of cyclohexasilanes bearing two different substituents, other than methyl. In contrast to the Co(CO)3PPh3 group, which does not show a significant influence on the Si-Si bonds compared to non-metal substituents, NMR studies (especially the measurement of the Si-Si coupling constants) reveal a strong influence between the Fe(CO)2Cp group and the Si-Si bonds within the cyclohexasilane cycle.
The separation of 1,3- and 1,4-dichlorodecamethylcyclohexasilanes and the isolation of 1-chloro-3-(chlorodimethylsily)octamethyl-cyclopentasilane through their hydrolysis products are shown. A crystal structure determined by X-ray diffraction of a single crystal containing decamethyl-7-oxahexasilanorbornane and 1,4-dihydroxydecamethylcyclohexasilane is discussed.
The title compound has been prepared by cyclization of 1,6-dibromo-3,4-diphenyldecamethylhexasilane with lithium. The cleavage of the siliconphenyl bonds of the cyclohexasilane by CF3SO3H gives the 1,2-bis-triflate derivate.
Starting from hexamethylhexaphenylcyclohexasilane, some new hexamethyl-cyclohexasilane derivatives (Cl, Br, OCH3, F) were prepared by exchange of the phenyl groups and characterized by analytical and spectroscopic methods.
1. Für das Siloxen wurde auf Grund seiner Eigenschaften, besonders seines Verhaltens gegen Halogene, folgende Strukturformel aufgestellt: magnified image
Es wird auch die abgekürzte Formel verwendet: magnified image
A nurnbcr of clcctron-deficient small molecule additivcs qucncli tlic fluorcsccncc of substitute silane high polymers in solution in a fashion consistcnt with an electron transfer mechanism. The measured rate constants arc, howcver, oftcn much larger tlian cxpcctcd Tor diTiision control. This erect has bccn attributed to rapid cncrgy transfcr among thc chrornophoric scgrncrits comprising the polymcr chain. 'This hypothcsis is furthcr supported by quenching studies or photocxcitcd 9, 10-dicyanoanthraccne, where the cxcitation is locali7cd, by ground state substituted silanc polyrncrs and oligomcrs.
The reaction of SbCl5 with Si6Me12 yields 1,3- and 1,4-Cl2Si6Me10. In order to isolate these isomers, a mixture of 1,3- and 1,4-[Fe(CO)2Cp]2Si6Me10 was synthesized and separated by fractional extraction with benzene. The structure of 1,3-[Fe(CO)2Cp]2Si6Me10 has been revealed by 29Si-INEPT-INADEQUATE spectroscopy. Starting from 1,3-[Fe(CO)2Cp]2Si6Me10, the compounds 1,3-Cl2Si6Me10, 1,3-Br2Si6Me10 and 1,3-H2Si6Me10 were synthesized and characterized.ZusammenfassungDie Reaktion von SbCl5 Mit Si6Me12 führt zu 1,3- und 1,4-Cl2Si6Me10. Zur Isolierung dieser Isomeren wurde daraus ein Gemisch von 1,3- und 1,4-[Fe(CO)2CP]2Si6Me10 hergestellt, welches mittels fraktionierter Extraktion mit Benzol auftrennbar ist. Die Struktur von 1,3-[Fe(CO)2Cp]2Si6Me10 wurde mittels 29Si-INEPT-INADEQUATE NMR-Spektroskopie bewiesen. Ausgehend von 1,3-[Fe(CO)2Cp]2Si6Me10 wurden 1,3-Br2Si6Me10, 1,3-Cl2Si6Me10 und 1,3-H2Si6Me10 rein isoliert und charakterisiert.
Phenylthio substituted oligosilanes are obtained from the reaction of oligomeric silicon halides with alkali metal thiophenolates MSPh (M = Li, Na, K). Si—Si bonds are formed when phenylthiooli-gosilanes are reacted with alkali metal silicon compounds. Alkali metal thiophenolates, the second reaction products can be separated easily. Quite similarly, Fe—Si-bonds are obtained from phenyl-thiosilanes and Na[Fe(CO)2Cp]. In contrast to the related alkali metal halide elimination reactions of organohalooligosilanes, transmetallations are never observed in the thiophenolate reactions.
Powder-x-ray-diffraction measurements on siloxene and calculations of the diffraction patterns for the three commonly proposed structures of siloxene (Si6H6O3) clearly show that only a structure with silicon layers is ever observed. The formation of siloxene from CaSi2 is topotactic, with the Si layers remaining intact and with no oxygen insertion into these layers. Siloxene prepared at 0 °C has very little oxygen incorporated into the interlayer gaps, and can be described as hydrogen terminated silicon layers (Si6H6), which we call layered polysilane. Layered polysilane can be purified by reaction with aqueous HF and is pyrophoric on contact with air. In some sense, layered polysilane can be considered the silicon equivalent of graphite.
The two-dimensional silicon backbone structure of planar polysilane and Wöhler siloxene is responsible for their exciting luminescing properties. We have prepared single crystals of siloxene by a topotactic reaction from crystalline CaSi2. The chemical composition was determined as [Si6H3(OH)3]n. The x-ray crystal structure analysis identifies the so-called Wöhler siloxene as 2D-poly[1,3,5-trihydroxocyclohexasilane]. Polysilane exhibits the same structural properties but with a chemical composition [Si6H6]n. The optical properties (infrared transmission, photoluminescence, excitation spectroscopy) of these well-defined materials are presented. A heat treatment above 350°C in vacuum of Wöhler siloxene results in a destruction of the planar ∞2 [Si-] structure by internal rearrangements, which is evidenced by the x-ray-diffraction pattern and characteristic changes in the optical spectra. The involvement of Wöhler siloxene in the optical properties of porous Si is critically reviewed.
Optically active organosilicon compounds having reactive functional groups bonded to asymmetric silicon have recently been synthesized and their study has revealed that many organosilicon reactions are stereospecific.
The present paper deals with the problem of correlation of configuration for six key compounds containing the α-naphthylphenylmethylsityl group. The solution of this problem by the use of chemical and physical methods is described. In addition the stereochemistry of more than twenty stereospecific reactions of silicon is considered. The probable fundamental relationships between organosilicon stereochemistry and the reaction mechanisms of monofunctional R3SiY compounds are discussed in a preliminary way.
The optical and structural properties of anodically oxidized porous silicon and of siloxene (Si6O3H6) are discussed. Both materials exhibit a strong visible luminescence at room temperature. In siloxene, the luminescence can
be traced to six-fold silicon rings which act as efficient radiative recombination centers. In the case of porous silicon,
on the other hand, spatial confinement of charge carriers in quantum wires has been proposed as the origin of the luminescence.
Based on the similarities between the optical and structural properties observed for certain siloxene derivates and for porous
silicon, it is argued that the luminescence in porous silicon actually stems from siloxene derivates formed during the etching
process.
Using Raman and infrared transmission spectroscopy, the vibrational properties of siloxene (Si6O3H6) and its derivatives are investigated and interpreted in terms of various structural modifications of siloxene which have been proposed in the past. On the basis of experimental and theoretical investigations siloxene is found to be a mixture of Si6 rings and/or linear Si chains interconnected by oxygen, and Si planes terminated by H and OH. The influence of thermal annealing, chemical treatment, and laser irradiation on the structure of siloxene is discussed in terms of the corresponding changes of the vibrational spectra and the x-ray-diffraction patterns. The vibrational properties of siloxene are very similar to those of electrochemically anodized porous Si. Raman, infrared transmission, and photoluminescence measurements of the two classes of materials are compared and a possible mechanism for the efficient luminescence in porous Si is discussed in light of the similarities between siloxene and porous Si.
Siloxene (Si6O3H6) and its derivatives show strong visible luminescence. By applying semiempirical quantum chemical calculations to the siloxene structure, we demonstrate that the presence of oxygen in a planar array of silicon atoms results in molecular orbitals confined to Si rings or chains which are responsible for the luminescence properties. Our results on the optical and vibrational properties of siloxene support earlier experimental results identifying siloxenelike structures as the luminescent agent in porous silicon.
Tailor-Made Silicon-Oxygen Compounds
- M. S. Brandt
- T. Puchert
- M. Stutzmann
The Silicon-Heteroatom Bond
- D. A. Armitage
Dissertation Technische Universität Graz
- K Renger