Silabenzene

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Silabenzene
Silabenzene.svg
Silabenzene-3D-balls.png
Names
Preferred IUPAC name
Siline [1]
Identifiers
3D model (JSmol)
ChemSpider
PubChem CID
  • InChI=1S/C5H6Si/c1-2-4-6-5-3-1/h1-6H Yes check.svgY
    Key: YJHHPIHPAJYNFT-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/C5H6Si/c1-2-4-6-5-3-1/h1-6H
    Key: YJHHPIHPAJYNFT-UHFFFAOYAP
  • C1=CC=[SiH]C=C1
Properties
C5H6Si
Molar mass 94.188 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Structures of some unstable silabenzenes Silabenzenes.svg
Structures of some unstable silabenzenes

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene (3 theoretical isomers), trisilabenzene (3 isomers), etc.

Contents

Silabenzenes have been the targets of many theoretical and synthetic studies by organic chemists interested in the question of whether analogs of benzene with Group IV elements heavier than carbon, e.g., silabenzene, stannabenzene and germabenzene—so-called "heavy benzenes"—exhibit aromaticity.

Although several heteroaromatic compounds bearing nitrogen, oxygen, and sulfur atoms have been known since the early stages of organic chemistry, silabenzene had been considered to be a transient, un-isolable compound and was detected only in low-temperature matrices or as its Diels-Alder adduct for a long time. In recent years, however, a kinetically stabilized silabenzene and other heavy aromatic compounds with silicon or germanium atoms have been reported.

Synthesis

Stable 2-silanaphthalene and silabenzene Stablesilabenzene.png
Stable 2-silanaphthalene and silabenzene

Several attempts to synthesize stable silabenzenes have been reported from the late 1970s using well-known bulky substituents such as a tert-butyl (1,1-dimethylethyl) or a TMS (trimethylsilyl) group, but such silabenzenes readily react with themselves to give the corresponding dimer even at low temperature (below -100°C) due to the high reactivity of silicon-carbon π bonds. In 1978 Barton and Burns reported that flow pyrolysis of 1-methyl-1-allyl-1-silacyclohexa-2,4-diene through a quartz tube heated to 428 °C using either ethyne or perfluoro-2-butyne as both a reactant and a carrier gas afforded the methyl-1-silylbenzene Diel-Alder adducts, 1-methyl-1-silabicyclo[2.2.2]octatriene or 1-methyl-2,3-bis(trifluoromethyl)-1-silabicyclo[2.2.2]octatriene, respectively, by way of a retro-ene reaction. [2]

A computational investigation in 2013 points out a new route to stable silabenzenes at ambient conditions through Brook rearrangement. [3] The [1,3]-Si → O shift of TMS or triisopropylsilyl (TIPS) substituted precursors with tetrahedral silicon atoms to an adjacent carbonyl oxygen lead to aromatic Brook-type silabenzenes.

Following the synthesis of the naphthalene analog 2-silanaphthalene, [4] [5] the first sila-aromatic compound, by Norihiro Tokitoh and Renji Okazaki in 1997, the same group reported thermally stable silabenzene in 2000 taking advantage of a new steric protective group. [6] A 9-silaanthracene derivative has been reported in 2002, [7] a 1-silanaphthalene also in 2002. [8]

A 1,4-disilabenzene was reported in 2002. [9] In 2007, 1,2-disilabenzene was synthesized via formal [2+2+2] cyclotrimerization of a disilyne (Si-Si triple bonded species) and phenylacetylene. [10]

Some theoretical studies suggest that the symmetric 1,3,5-trisilabenzene may be more stable than 1,2-disilabenzene. [11]

Properties and reactions

Isolated silabenzene reacts with various reagents at 1,2- or 1,4-positions to give diene-type products, so the aromaticity of the silabenzene is destroyed. It is different from benzene, which reacts with electrophiles to give not dienes but substituted benzenes, so benzene sustains its aromaticity. Silicon is a semi-metal element, so the Si-C π bond in the silabenzene is highly polarized and easily broken. The silabenzene is also light-sensitive; Ultraviolet irradiation gives the valence isomer, a silabenzvalene. The theoretical calculations and the NMR chemical shifts of silabenzenes, though, show that silabenzene is an aromatic compound in spite of the different reactivity from benzene and other classical aromatic compounds.

Hexasilabenzene

In calculations the all-silicon hexasilabenzene Si6H6 is predicted to have 6-fold symmetry [12] or a chair conformation. [13] It was shown that the deviation from planarity in hexasilabenzene is caused by the pseudo Jahn–Teller effect. [14] A stable hexasilaprismane has been known since 1993 [15] A compound isomeric with hexasilabenzene was first reported in 2010. [16] This compound is reported as stable and with according to X-ray crystallography a chairlike tricyclic silicon frame.

Hexasilabenzene.svg

The searching of a planar Si6 analogue to benzene has been extended to anionic cycles and structures bearing lithium atoms replacing hydrogens. [17] Through Density functional theory calculations, it has been shown that from a series of planar and tridimensional structures with molecular formula Si6Li2-8, the global minimum is a Si6Li6 planar ring. This particular ring has D2h symmetry with 4 lithium cations placed between two adjacent silicon atoms –forming three-center two-electron bonds –and two more Li cations located above and below the center of the ring’s plane. A highly symmetric D6h structure analogue to hexalithiumbenzene [18] was found to be higher in energy by 2.04 eV to respect to the minimum. [19]
Aromaticity was also tested using density functional calculations. DFT can be effectively used to calculate the aromaticity of various molecular systems [20] using the B3LYP hybrid density functional; this method has been proved to be the method of choice for computing delocalized systems. [21] The nucleus-independent chemical shifts (NICS) [22] was selected as the quantitative criterion to evaluate the aromatic character of the structures under study. The global minimum (D2h symmetry ring) and the D6h symmetry ring show values of −3.95 and −5.95, respectively. In NICS calculations, negative values indicate aromaticity.
More recently, using a novel genetic algorithm, a Si6Li6 three dimensional structure has been calculated to be more stable than planar isomers. [23]

See also

Related Research Articles

<span class="mw-page-title-main">Aromaticity</span> Chemical property

In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.

Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.

<span class="mw-page-title-main">Silylene</span> Chemical compound

Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exist in the singlet or triplet state, silylene (and all of its derivatives) are singlets.

<span class="mw-page-title-main">Cyclodecapentaene</span> Chemical compound

Cyclodecapentaene or [10]annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry.

<span class="mw-page-title-main">Prismane</span> Chemical compound

Prismane or 'Ladenburg benzene' is a polycyclic hydrocarbon with the formula C6H6. It is an isomer of benzene, specifically a valence isomer. Prismane is far less stable than benzene. The carbon (and hydrogen) atoms of the prismane molecule are arranged in the shape of a six-atom triangular prism—this compound is the parent and simplest member of the prismanes class of molecules. Albert Ladenburg proposed this structure for the compound now known as benzene. The compound was not synthesized until 1973.

<span class="mw-page-title-main">Stannabenzene</span> Chemical compound

Stannabenzene (C5H6Sn) is the parent representative of a group of organotin compounds that are related to benzene with a carbon atom replaced by a tin atom. Stannabenzene itself has been studied by computational chemistry, but has not been isolated.

<span class="mw-page-title-main">Germabenzene</span> Chemical compound

Germabenzene (C5H6Ge) is the parent representative of a group of chemical compounds containing in their molecular structure a benzene ring with a carbon atom replaced by a germanium atom. Germabenzene itself has been studied theoretically, and synthesized with a bulky 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl or Tbt group. Also, stable naphthalene derivatives do exist in the laboratory such as the 2-germanaphthalene-containing substance represented below. The germanium to carbon bond in this compound is shielded from potential reactants by a Tbt group. This compound is aromatic just as the other carbon group representatives silabenzene and stannabenzene.

<span class="mw-page-title-main">Homoaromaticity</span> Organic molecular structure

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

<span class="mw-page-title-main">Sandwich compound</span> Chemical compound made of two ring ligands bound to a metal

In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives. Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the metallocenes.

<span class="mw-page-title-main">Hexazine</span> Chemical compound

Hexazine is a hypothetical allotrope of nitrogen composed of 6 nitrogen atoms arranged in a ring-like structure analogous to that of benzene. As a neutrally charged species, it would be the final member of the azabenzene (azine) series, in which all of the methine groups of the benzene molecule have been replaced with nitrogen atoms. The two last members of this series, hexazine and pentazine, have not been observed, although all other members of the azine series have.

<span class="mw-page-title-main">Disilyne</span> Chemical compound

Disilyne is a silicon hydride with the formula Si
2H
2. Several isomers are possible, but none are sufficiently stable to be of practical value. Substituted disilynes contain a formal silicon–silicon triple bond and as such are sometimes written R2Si2 (where R is a substituent group). They are the silicon analogues of alkynes.

<span class="mw-page-title-main">Kekulene</span> Chemical compound

Kekulene is a polycyclic aromatic hydrocarbon which consists of 12 fused benzene rings arranged in a circle. It is therefore classified as a [12]-circulene with the chemical formula C48H24. It was first synthesized in 1978, and was named in honor of August Kekulé, the discoverer of the structure of the benzene molecule.

<span class="mw-page-title-main">Planar hexacoordinate carbon</span>

Planar hexacoordinate carbon in chemistry describes a molecular geometry featuring a planar arrangement of carbon with six surrounding atoms. No actual chemical compounds having this particular hexacoordinate configuration have been reported but quantum mechanical methods have demonstrated that these molecules are a possibility. Examples of molecules investigated with computational methods are the B6C dianion, the CN3Be3+ ion, the CO3Li3+ ion and the CN3Mg3+ ion. A simulated Be2C monolayer is reported to consist of quasi-planar hexacoordinate carbon atoms.

<span class="mw-page-title-main">Butalene</span> Chemical compound

Butalene is a polycyclic hydrocarbon composed of two fused cyclobutadiene rings. A reported possible synthesis of it involves an elimination reaction from a Dewar benzene derivative. The structure itself can be envisioned as benzene with an internal bridge, and calculations indicate it is somewhat less stable than the open 1,4-didehydrobenzene biradical, the valence isomer in which that bridged bond is broken.

<span class="mw-page-title-main">Decamethylsilicocene</span> Chemical Compound

Decamethylsilicocene, (C5Me5)2Si, is a group 14 sandwich compound. It is an example of a main-group cyclopentadienyl complex; these molecules are related to metallocenes but contain p-block elements as the central atom. It is a colorless, air sensitive solid that sublimes under vacuum.

<span class="mw-page-title-main">Trisilaallene</span> Class of silicon chemical compounds

Trisilaallene is a subclass of silene derivatives where a central silicon atom forms double bonds with each of two terminal silicon atoms, with the generic formula R2Si=Si=SiR2. Trisilaallene is a silicon-based analog of an allene, but their chemical properties are markedly different.

<i>N</i>-heterocyclic silylene Chemical compound

An N-Heterocyclic silylene (NHSi) is an uncharged heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.

<span class="mw-page-title-main">Trivalent group 14 radicals</span>

A trivalent group 14 radical (also known as a trivalent tetrel radical) is a molecule that contains a group 14 element (E = C, Si, Ge, Sn, Pb) with three bonds and a free radical, having the general formula of R3E•. Such compounds can be categorized into three different types, depending on the structure (or equivalently the orbital in which the unpaired electron resides) and the energetic barrier to inversion. A molecule that remains rigidly in a pyramidal structure has an electron in a sp3 orbital is denoted as Type A. A structure that is pyramidal, but flexible, is denoted as Type B. And a planar structure with an electron that typically would reside in a pure p orbital is denoted as Type C. The structure of such molecules has been determined by probing the nature of the orbital that the unpaired electron resides in using spectroscopy, as well as directly with X-ray methods. Trivalent tetrel radicals tend to be synthesized from their tetravalent counterparts (i.e. R3EY where Y is a species that will dissociate).

<span class="mw-page-title-main">Gregory H. Robinson</span> American inorganic chemist

Gregory H. RobinsonFRSC is an American synthetic inorganic chemist and a Foundation Distinguished Professor of Chemistry at the University of Georgia. Robinson's research focuses on unusual bonding motifs and low oxidation state chemistry of molecules containing main group elements such as boron, gallium, germanium, phosphorus, magnesium, and silicon. He has published over 150 research articles, and was elected to the National Academy of Sciences in 2021.

<span class="mw-page-title-main">Hexaphosphabenzene</span> Chemical compound

Hexaphosphabenzene is a valence isoelectronic analogue of benzene and is expected to have a similar planar structure due to resonance stabilization. Although several other allotropes of phosphorus are stable, no evidence for the existence of P6 has been reported. Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1, and would therefore not be observed in the uncomplexed state under normal experimental conditions. The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6. The moderate barrier suggests that hexaphosphabenzene could be synthesized from a [2+2+2] cycloaddition of three P2 molecules. Currently, this is a synthetic endeavour which remains to be conquered.

References

  1. ↑ Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 392. doi:10.1039/9781849733069-FP001. ISBN   978-0-85404-182-4.
  2. ↑ Barton, T. J.; Burns, G. T. (1978). "Unambiguous generation and trapping of a silabenzene". Journal of the American Chemical Society. 100 (16): 5246. doi:10.1021/ja00484a075.
  3. ↑ Rouf, Alvi Muhammad; Jahn, Burkhard O.; Ottosson, Henrik (14 January 2013). "Computational Investigation of Brook-Type Silabenzenes and Their Possible Formation through [1,3]-Si→O Silyl Shifts". Organometallics. 32 (1): 16–28. doi:10.1021/om300023s.
  4. ↑ Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; von Ragué Schleyer, P.; Jiao, H. (1997). "A Stable Neutral Silaaromatic Compound, 2-{2,4,6-Tris[bis(trimethylsilyl)methyl]phenyl}- 2-Silanaphthalene". Journal of the American Chemical Society. 119 (29): 6951–6952. doi:10.1021/ja9710924.
  5. ↑ Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S.; von Ragué Schleyer, P.; Jiao, H. (1999). "Synthesis of Stable 2-Silanaphthalenes and Their Aromaticity". Journal of the American Chemical Society. 121 (49): 11336–11344. doi:10.1021/ja992024f.
  6. ↑ Wakita, K.; Tokitoh, N.; Okazaki, R.; Takagi, N.; Nagase, S. (2000). "Crystal Structure of a Stable Silabenzene and Its Photochemical Valence Isomerization into the Corresponding Silabenzvalene". Journal of the American Chemical Society. 122 (23): 5648–5649. doi:10.1021/ja000309i.
  7. ↑ Takeda, N.; Shinohara, A.; Tokitoh, N. (2002). "The First Stable 9-Silaanthracene". Organometallics. 21 (2): 256–258. doi:10.1021/om0108301.
  8. ↑ Takeda, N.; Shinohara, A.; Tokitoh, N. (2002). "Synthesis and Properties of the First 1-Silanaphthalene". Organometallics. 21 (20): 4024–4026. doi:10.1021/om0205041.
  9. ↑ Kabe, Y.; Ohkubo, K.; Ishikawa, H.; Ando, W. (2000). "1,4-Disila(Dewar-benzene) and 1,4-Disilabenzene: Valence Isomerization of Bis(alkylsilacyclopropenyl)s". Journal of the American Chemical Society. 122 (15): 3775–3776. doi:10.1021/ja9930061.
  10. ↑ Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto, M.; Nagase, S. (2007). "Reactivity of a Disilyne RSi≡SiR (R=SijPr(CH(SiMe3)2)2) Toward π-Bonds: Stereospecific Addition and a New Route to an Isolable 1,2-Disilabenzene". Journal of the American Chemical Society. 129 (25): 7766–7767. doi:10.1021/ja072759h. PMID   17542592.
  11. ↑ Baldridge, K. K.; Uzan, O.; Martin, J. M. L. (2000). "The Silabenzenes: Structure, Properties, and Aromaticity". Organometallics. 19 (8): 1477–1487. doi:10.1021/om9903745.
  12. ↑ Dewar, M. J. S.; Lo, D. H.; Ramsden, C. A. (1975). "Ground States of Molecules. XXIX. MINDO/3 Calculations of Compounds Containing Third Row Elements". Journal of the American Chemical Society. 97 (6): 1311–1318. doi:10.1021/ja00839a005.
  13. ↑ Nagase, S.; Teramae, H.; Kudo, T. (1987). "Hexasilabenzene (Si6H6). Is the Benzene-Like D6h Structure Stable?". The Journal of Chemical Physics. 86 (8): 4513–4517. Bibcode:1987JChPh..86.4513N. doi:10.1063/1.452726.
  14. ↑ Ivanov, A.; Boldyrev. A (2012). "Si6−nCnH6 (n = 0-6) Series: When Do Silabenzenes Become Planar and Global Minima?". J. Phys. Chem. A. 116 (38): 9591–9598. Bibcode:2012JPCA..116.9591I. doi:10.1021/jp307722q. PMID   22946702.
  15. ↑ Sekiguchi, A.; Yatabe, T.; Kabuto, C.; Sakurai, H. (1993). "Chemistry of Organosilicon Compounds. 303. The "Missing" Hexasilaprismane: Synthesis, X-Ray Analysis and Photochemical Reactions". Journal of the American Chemical Society. 115 (13): 5853–5854. doi:10.1021/ja00066a075.
  16. ↑ Abersfelder, K.; White, A.; Rzepa, H.; Scheschkewitz, D. (2010). "A Tricyclic Aromatic Isomer of Hexasilabenzene". Science. 327 (5965): 564–566. Bibcode:2010Sci...327..564A. doi:10.1126/science.1181771. PMID   20110501. S2CID   206523406.
  17. ↑ Takahasi, M; Kawazoe, Y (2005). "Theoretical Study on Planar Anionic Polysilicon Chains and Cyclic Si6H Anions with D6h Symmetry". Organometallics. 24 (10): 2433–2440. doi:10.1021/om050025c.
  18. ↑ Xie, Y; Schaefer, H (1991). "Hexalithiobenzene: a D6h Equilibrium Geometry with Six Lithium Atoms in Bridging Positions". Chemical Physics Letters. 179 (5, 6): 563–567. Bibcode:1991CPL...179..563X. doi:10.1016/0009-2614(91)87104-J.
  19. ↑ Zdetsis, A (2007). "Stabilization of Flat Aromatic Si 6 Rings Analogous to Benzene: Ab initio Theoretical Prediction". The Journal of Chemical Physics. 127 (21): 214306. Bibcode:2007JChPh.127u4306Z. doi:10.1063/1.2805366. PMID   18067356.
  20. ↑ De Proft, F; Geerlings, P (2001). "Conceptual and Computational DFT in the Study of Aromaticity". Chemical Reviews. 101 (5): 1451–1464. doi:10.1021/cr9903205. PMID   11710228.
  21. ↑ Nedel, M; Houk, K; Tolbert, L; Vogel, E; Jiao, H; von Rague Schleyer, P (1998). "Bond Alternation and Aromatic Character in Cyclic Polyenes: Assessment of Theoretical Methods for Computing the Structures and Energies of Bismethano[14]annulenes". The Journal of Physical Chemistry A. 102 (36): 7191–7198. Bibcode:1998JPCA..102.7191N. doi:10.1021/jp9820976.
  22. ↑ von Rague Schleyer, P; Maerker, C; Dransfeld, A; Jiao, H; van Eikema Hommes, N (1996). "Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe". Journal of the American Chemical Society. 118 (26): 6317–6318. doi:10.1021/ja960582d. PMID   28872872. S2CID   207152799.
  23. ↑ Santos, J; Contreras, M; Merino, G (2010). "Structure and Stability of Si6Li6: Aromaticity vs Polarizability". Chemical Physics Letters. 496 (1–3): 172–174. Bibcode:2010CPL...496..172S. doi:10.1016/j.cplett.2010.07.026. hdl: 10533/144740 .
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