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Crystalmaker structure showing bonds
Crystalmaker structure showing bonds













Consequently, while several stable germaaromatic derivatives have been synthesized, including germabenzene 6, 35 2-germanaphthalene 7, 36,37 9-germaanthra- cene 8 38 and 9-germaphenanthrene 9 38 (Scheme 2), only one stable stannaaromatic compound has been reported, namely, 2-stannanaphthalene 10 39 (Scheme 2). The problem of kinetic stabilization of heavy aromatics becomes increasingly important descending group 14 because of the general tendency of reluctance of the heavier elements (particularly, Sn and Pb) for s,p-hybridization and formation of multiple bonds. Utilizing the same synthetic procedure and taking advantage of the protective power of Tbt and Bbt (2,6-bis-4-phenyl) groups, the same authors prepared the most fundamental silabenzene 2, 29,30 as well as 1-silanaphthalene 3 31,32 and 9-silaanthracene 4 33 and 9-silaphenanthrene 5 34 (Scheme 2).

crystalmaker structure showing bonds

in 1997 by the dehydrohalogenative reduction of the bromosilane pre- cursor with t BuLi in hexane 9,27,28 (Scheme 1). 25,26 The first silaaromatic compound stable at room temperature, 2-silanaphthalene 1, was prepared by Tokitoh et al. 21 Accordingly, the synthesis of hexasilaprismane (Ar 6 Si 6, Ar = 2,6-diisopropylphenyl) and its low-temperature photochemical isomerization to hexasila-Dewar(benzene) were reported. 23 Similar to the case of Si 6 H 6, the prismane was found to be the most stable E 6 H 6 system, which was explained by the reluctance of the heavy group 14 elements to form double bonds, as well as their ability to release the strain of polycyclic compounds. 21 Sub- sequent calculations showed that the D 3d structure of Ge 6 H 6 is 14.5 kcal mol À 1 more stable than its planar D 6h isomer, and the aromaticity of hexagermabenzene in its D 6h form is comparable to that of hexasilabenzene: NICS (Ge 6 H 6 ) = À 14.4 vs. In the same direction, the planar D 6h structure of E 6 H 6 is progressively destabilized compared with the puckered D 3d structure: 9.1 (Ge 6 H 6 ), 23.1 (Sn 6 H 6 ) and 63.3 (Pb 6 H 6 ) kcal mol À 1.

crystalmaker structure showing bonds

22 The departure of the heavy arenes E 6 H 6 (E = Ge, Sn, Pb) from planarity becomes more prominent going down group 14: the puckering angles for Ge 6 H 6, Sn 6 H 6 and Pb 6 H 6 are 38.0, 50.8 and 58.0 1, respectively. 24 A study on the influence of substituents on the structure and aromaticity of R 6 Si 6 isomers showed that the strongly s -accepting groups (F) impose the largest deviation from planarity and stretching of the Si–Si bonds, whereas s -donating (SiH 3 ) and p -accepting (BH 2 ) substituents exhibited completely opposite effect, resulting in overall aromatic stabilization and flattening of the R 6 Si 6 molecule. 24 This situation is completely opposite to that of organic C 6 H 6 isomers, of which benzene is by far the most stable isomer followed by prismane (+117.5), Dewar(benzene) (+81.0) and benzvalene (+74.8 kcal mol À 1 ). 23 The most stable Si 6 H 6 structural isomer is hexasilaprismane, which is more favorable than the planar D 6h hexasilabenzene, puckered D 3d hexasilabenzene, hexasila(Dewar)benzene and hexasilabenzvalene by 11.9, 7.7, 10.5 and 5.9 kcal mol À 1, respectively. 14,17,18,23 This agreed well with the magnetic considerations: NICS (Si 6 H 6 ) = À 14.1 vs. 50–80% that of benzene (depending on the theoretical method used). The value of the aromatic stabilization energy for the planar D 6h hexasilabenzene was reported to be ca.

crystalmaker structure showing bonds

15,20–23 However, at all computational levels, the Si–Si bond distances in both the D 6h and D 3d structures were found to be intermediate between the typical Si Q Si (2.15 A ̊) and Si–Si (2.34 A ̊ ) bond lengths, implying the presence of the cyclic electron delocalization.

crystalmaker structure showing bonds

17–19 However, subsequent higher-level computations found a chair-like puckered D 3d geometry for Si 6 H 6 as the true minimum structure. The early calculations at the HF/3-21G level described Si 6 H 6 as a D 6h structure with identical Si–Si bond lengths of ca. comprehensively studied heavy arene representative.















Crystalmaker structure showing bonds