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

Abstract

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.