Cyclotrisilenylium ion: the persilaaromatic compound.

The highly crowded 3,3-bis(di-tert-butylmethylsilyl)-1,2-bis(tri-tert-butylsilyl)cyclotrisilene (3) was newly designed as a precursor of the cyclotrisilenylium ion and prepared by the reaction of 2 equiv of dilithiosilane (tBu2MeSi)2SiLi2 (1) with 2,2,3,3-tetrabromo-1,1,1,4,4,4-hexa-tert-butyltetrasilane. The reaction of 3 with triphenylmethylium tetraarylborate in toluene produced (di-tert-butylmethylsilyl)bis(tri-tert-butylsilyl)cyclotrisilenylium ion (4+), which was isolated in the form of the tetraarylborate salt as extremely air- and moisture-sensitive yellow crystals, representing the first isolable silicon congener of the cyclopropenylium ion. The molecular structure of 4+.TSFPB- (TSFPB- = tetrakis[4-(tert-butyldimethylsilyl)-2,3,5,6-tetrafluorophenyl]borate) was established by X-ray crystallographic analysis, showing that the three-membered ring constitutes an almost equilateral triangle with average Si-Si bond lengths of 2.216(3) A. The X-ray crystal structure and spectral data show that 4+ is not only a free silyl cation but also a 2pi electron aromatic species with delocalization of the positive charge over the three-membered skeleton.