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If tetrahedrane were to exist, this would be its structure

Tetrahedrane is a platonic hydrocarbon with chemical formula C4H4 and a tetrahedral structure. Extreme angle strain (carbon bond angles deviate considerably from the tetrahedral bond angle of 109.5°) prevents this molecule from forming outside of man-made production.

In 1978, Günther Maier prepared a stable tetrahedrane with four tert-butyl substituents.[1] These substituents are very bulky, and completely envelop the tetrahedrane core. Maier suggested that bonds in the core are prevented from breaking because this would force the substituents closer together (corset effect) resulting in Van der Waals strain. Tetrahedrane is one of the possible platonic hydrocarbons and has the IUPAC name tricyclo[1.1.0.02,4]butane.

Contents

Tetra-tert-butyltetrahedrane

The tert-butyl substituted compound was first synthesised starting from a cycloaddition of an alkyne with t-Bu substituted maleic anhydride [2], followed by rearrangement with carbon dioxide expulsion to a cyclopentadienone and its bromination, followed by addition of the fourth t-Bu group and a photochemical rearrangement with expulsion of carbon monoxide. Tetra-tert-butyl-tetrahedrane synthesis 1978

Tetra(trimethylsilyl)tetrahedrane

Tetra(trimethylsilyl)tetrahedrane is relatively stable

In tetra(trimethylsilyl)tetrahedrane (I) the tert-butyl groups have been replaced by trimethylsilyl groups.[3] This compound is far more stable than the tert-butyl analogue. The silicon-carbon bond is longer than a carbon-carbon bond, and therefore the corset effect is reduced. On the other hand, the trimethylsilyl group is a sigma donor which explains the increased stabilization of the tetrahedrane. Whereas the tert-butyl tetrahedrane melts at 135 °C, at which temperature decomposition to the cyclobutadiene starts, the trimethyl silyl tetrahedrane melts at a much higher temperature of 202 °C, and is stable up to 300 °C, at which point it reverts to the acetylene starting material.

The tetrahedrane skeleton is made up of banana bonds, and hence the carbon atoms are high in s-orbital character. From NMR, sp hybridization can be deduced, normally reserved for triple bonds. As a consequence the bond lengths are unusually short with 152 picometers. The latest development is the organic synthesis and characterization of the tetrahedrane dimer (II). The connecting bond is even shorter with 143.6 pm. An ordinary carbon carbon bond has a length of 154 pm.

synthesis of Tetra(trimethylsilyl)tetrahedrane and the dimer compound

Tetrasilatetrahedrane

In tetrasilatetrahedrane the carbon atoms in the tetrahedrane cage are replaced by silicon. The standard silicon silicon bond is much longer (235 pm) and the cage is again enveloped by a total of 16 trimethylsilyl groups. This makes the compound thermally stable. The silatetrahedrane can be reduced with potassium graphite to the tetrasilatetrahedranide potassium salt. In this compound one of the silicon atoms of the cage has lost a silyl substituent and carries a negative charge. The potassium cation can be captured by a crown ether and in the resulting complex potassium and the silyl anion are separated by a distance of 885 pm. One of the Si- - Si bonds is now 272 pm and its silicon atom has an inverted tetrahedral geometry. Furthermore the four cage silicon atoms are equivalent on the NMR timescale due to migrations of the silyl substituents over the cage.[4]

Tetrasilatetrahedrane

The dimerization reaction observed for the carbon tetrahedrane compound is also attempted for a tetrasilatetrahedrane.[5] In this tetrahedrane the cage is protected by 4 so-called super silyl groups in which a silicon atom has 3 tert-butyl substituents. The dimer does not materialize but a reaction with iodine in benzene followed by reaction with the tri-tert-butyl sila anion results in the formation of an eight membered silicon cluster compound which can be described as a Si2 dumbbell (length 229 picometer and with inversion of tetrahedral geometry) sandwiched between two almost parallel Si3 rings.

silicon cluster compound

In known eight-membered clusters of in the same carbon group, tin Sn8R6 and germanium Ge8R6 the cluster atoms are located on the corners of a cube.

Tetrahedrane outlook

Unsubstituted tetrahedrane is an as yet elusive molecule but it is predicted to be kinetically stable. One strategy that has been explored (but thus far failed) is reaction of propene with atomic carbon [6] Locking away a tetrahedrane molecule inside a fullerene has only been attempted in silico [7]

References

  1. ^ G. Maier, S. Pfriem, U. Schäfer and R. Matusch (1978). "Tetra-tert-butyltetrahedrane". Angewandte Chemie International Edition in English 17 (7): 520–521. doi:10.1002/anie.197805201.  
  2. ^ Tert.-butyl-substituierte cyclobutadiene und cyclopentadienone Günther Maier and Friedrich Boβlet Tetrahedron Letters Volume 13, Issue 11, 1972, Pages 1025-1030 doi:doi:10.1016/S0040-4039(01)84500-7
  3. ^ M. Tanaka and A. Sekiguchi (2005). "Hexakis(trimethylsilyl)tetrahedranyltetrahedrane". Angewandte Chemie International Edition 44 (36): 5821–5823. doi:10.1002/anie.200501605.  
  4. ^ Masaaki Ichinohe, Masafumi Toyoshima, Rei Kinjo, and Akira Sekiguchi (2003). "Tetrasilatetrahedranide: A Silicon Cage Anion" (communication). J. Am. Chem. Soc. 125 (44): 13328–13329. doi:10.1021/ja0305050.  
  5. ^ G. Fischer, V. Huch, P. Mayer, S. K. Vasisht, M. Veith and N. Wiberg (2005). "Si8(SitBu3)6: A Hitherto Unknown Cluster Structure in Silicon Chemistry". Angewandte Chemie International Edition 44 (48): 7884–7887. doi:10.1002/anie.200501289.  
  6. ^ Tetrahedrane—Dossier of an Unknown Adelina Nemirowski, Hans Peter Reisenauer, and Peter R. Schreiner Chem. Eur. J. 2006, 12, 7411 – 7420 doi:10.1002/chem.200600451
  7. ^ Endohedral complex of fullerene C60 with tetrahedrane, C4H4@C60 Xiao-Yuan Ren a, Cai-Ying Jiang a, Jiang Wanga, Zi-Yang Liu Journal of Molecular Graphics and Modelling 27 (2008) 558–562 doi:10.1016/j.jmgm.2008.09.010
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