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.
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
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-
carbonπ 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
stericprotective group.[6] A 9-sila
anthracene 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-metalelement, 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
NMRchemical 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.
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]
^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.
^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.
^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.
^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.
^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.
^Takeda, N.; Shinohara, A.; Tokitoh, N. (2002). "The First Stable 9-Silaanthracene". Organometallics. 21 (2): 256–258.
doi:
10.1021/om0108301.
^Takeda, N.; Shinohara, A.; Tokitoh, N. (2002). "Synthesis and Properties of the First 1-Silanaphthalene". Organometallics. 21 (20): 4024–4026.
doi:
10.1021/om0205041.
^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.
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10.1021/ja9930061.
^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.
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^Baldridge, K. K.; Uzan, O.; Martin, J. M. L. (2000). "The Silabenzenes: Structure, Properties, and Aromaticity". Organometallics. 19 (8): 1477–1487.
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^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.
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^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.
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^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.
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^Takahasi, M; Kawazoe, Y (2005). "Theoretical Study on Planar Anionic Polysilicon Chains and Cyclic Si6H Anions with D6h Symmetry". Organometallics. 24 (10): 2433–2440.
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^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.
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