Hexasilabenzene, aromaticity

Another index of delocalization, devised and widely applied by Schleyer et al.,163 is the nuclear-independent chemical shielding (NICS) value. When calculated above the ring, this value corresponds to the -contribution which is a significantly negative quantity for situations with aromatic delocalization . Scheme 21 shows the A and NICS values computed by Schleyer et al.141 for benzene, hexasilabenzene, and hexazine. The three species are seen to possess equally aromatic electron sextets, as indeed anticipated from the appreciable vertical resonance ener-... 

Because H-Si-SiHs is nearly isoenergetic with H2Si = SiH2, it is not obvious which structure would be favored for hexasilabenzene, SieHe. Whereas proponents of aromaticity may take heart from the theoretical calculations that show the hexasilabenzene structure to be 34 kcal/mol more stable than the tris(silylene) form, they will be reminded of how little aromaticity has to do with the magic of six tt electrons by the revelation that the hexasilaprismane is calculated to be 14 kcal/mol more stable yet. 

In addition to the question of geometry a second point of interest is the degree of aromatic stabilization in hexasilabenzene. Using appropriate bond-separation reactions hexasilabenzene is found to possess approximately half the aromatic stabilization energy... 

More recently, Scheschkewitz and coworkers reported the synthesis of an exotic tricyclic isomer stabilized with six 2,4,6-triisopropylphenyl (named Tip) substituents, which is an attempt toward production of the first hexa-silabenzene (Chart 13.19). In the crystalline form, the tricyclic ring adopted a cyclohexane-like puckered geometry, which is significantly distorted from a planar benzene-like geometry. The isomer showcased a unique aromaticity, called dismutational aromaticity, supported by three Si resonances and high-level density functional theory calculations. This result can lead to the first isolation of an ideal hexasilabenzene with higher molecular symmetry (six- and three-fold) by designing more elaborate substituents and reaction routes. 

Abersfelder K, White AJP, Rzepa HS, Scheschkewitz D (2010) A tricyclic aromatic isomer of hexasilabenzene. Science 327 564... 

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