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 = −SiMe_<i>n</i>Ph_3–<i>n</i> (<i>n</i> = 1, 2) (<b>2</b>, <b>3</b>, <b>10</b>), −SiMe₂Fc (Fc = ferrocenyl) (<b>4</b>, <b>11</b>, <b>13</b>, <b>14</b>), −COR (R = Me, <i>t</i>Bu) (<b>6</b>, <b>7</b>, <b>12</b>), COOMe (<b>8</b>), COOH (<b>9</b>)], 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 <b>2</b>, <b>3</b>, <b>4</b>, <b>6</b>, <b>8</b>, <b>9</b>, <b>10</b>, and <b>13</b> 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 (<b>1</b>) 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