Synthesis and Properties of Bridgehead-Functionalized Permethylbicyclo[2.2.2]octasilanes

A series of previously unknown bridgehead-functionalized bicyclo[2.2.2]­octasilanes, Me3Si-Si8Me12-X, X-Si8Me12-X, and X-Si8Me12-Y [X, Y = −SiMenPh3–n (n = 1, 2) (2, 3, 10), −SiMe2Fc (Fc = ferrocenyl) (4, 11, 13, 14), −COR (R = Me, tBu) (6, 7, 12), COOMe (8), COOH (9)], have been prepared by the reaction of the silanides Me3Si-Si8Me12–K+ or K+–Si8Me12–K+ with proper electrophiles and fully characterized. The molecular structures of 2, 3, 4, 6, 8, 9, 10, and 13 as determined by single-crystal X-ray diffraction analysis exhibit a slightly twisted structure of the bicyclooctasilane cage. Endocyclic bond lengths, bond angles, and dihedral angles are not influenced considerably by the substituents attached to the bridgehead silicon atoms. Due to σ­(SiSi)/π­(aryl) conjugation, a 20–30 nm bathochromic shift of the longest wavelength UV absorption band relative to Me3Si-Si8Me12-SiMe3 (1) is evident in the UV absorption spectra of the phenyl and ferrocenyl derivatives. Otherwise, UV absorption data do not support the assumption of aryl/aryl or aryl/CO interaction via the σ­(SiSi) bicyclooctasilane framework.