Attempted Synthesis of a Homocyclic Bis(silyl)silylene Leads to the Formation of a Tricyclo[3,1,1,1^2,4]octasilane

The reduction of 1,1-dibromo-cyclopentasilane with a mildly reducing magnesium­(I) dimer [{(MesNacnac)­Mg−}₂] was examined, which gave rise to the formation of endocyclic disilene <b>1</b>. The formation of <b>1</b> was further confirmed by a trapping experiment with MeOH. Additionally, <b>1</b> was found to dimerize slowly to <b>1-dimer</b>. Furthermore, the formation of the homocyclic silylene was demonstrated by performing the reduction in the presence of Et₃SiH as a trapping agent. To prevent the 1,2-trimethylsilyl shift, which causes a rapid degradation of the silylene, a second synthetic strategy was established. Therefore, two different tetrasilanes <b>4</b> and <b>5</b> were synthesized. Compounds <b>4</b> and <b>5</b> were subsequently reacted with 2.1 equiv of trifluoromethanesulfonic acid to corresponding bis­(trifluoromethanesulfonates) <b>6</b> and <b>7</b>. 1,4-Dihalo-tetrasilanes <b>8a</b>,<b>b</b> and <b>9</b> as well as hexasilanes <b>10</b> and <b>11</b> were obtained by a subsequent nucleophilic substitution of the triflate substituent with X^– (X = Cl^–, Br^–, or R₃Si^–). Dianionic species <b>12</b> and <b>13</b> were synthesized by the reaction of corresponding hexasilanes <b>10</b> and <b>11</b> with 2.1 equiv of KO<i>t</i>-Bu. Surprisingly, the salt metathesis reaction of dianionic compound <b>13</b> does not lead to the formation of the expected 1,1-halocyclopentasilanes. Instead the formation of cyclobutasilane <b>14</b> was observed. The reaction of <b>9</b> with lithium led again to the formation of <b>14</b> alongside with the formation of cyclohexasilane <b>15</b>. Interestingly, <b>14</b> underwent a ring-opening formation by the reaction with gaseous HCl in the presence of AlCl₃ and compound <b>16</b> was formed. The reduction of <b>16</b> allowed straightforward access to <b>17</b> as a structurally complex hitherto unknown tricyclic polysilane.