Contraction and Expansion of the Silicon Scaffold of Stable Si₆R₆ Isomers

Abstract

The reactivity of two stable Si₆R₆ clusters (<b>4</b> and <b>5</b>, R = 2,4,6-^<i>i</i>Pr₃C₆H₂) with unsymmetrical substitution patterns (including Si, SiR, and SiR₂ vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term “siliconoids” for silicon clusters with unsaturated valencies. With the hexasilaprismane <b>8a</b>, a saturatedi.e., non-siliconoidSi₆R₆ isomer is accessible from a suitable Si₃ precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane <b>6</b> into the corresponding 1,2,3-derivative <b>7</b> prior to the requisite reductive coupling step leading to <b>8a</b>. On the other hand, a stable expanded Si₁₁-siliconoid <b>9</b> was isolated as a minor side product of the thermal isomerization of <b>4</b> to <b>5</b>, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of <b>9</b> strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid <b>5</b> with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane <b>11</b> in high yield. Conversely, chlorination of the isomeric <b>4</b> with BiCl₃ as a mild source of Cl₂ results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of <b>4</b>. The main product, 1,2-dichlorotricyclo­[2.2.0.0^2,5]­hexasilane <b>12</b>, undergoes thermal cluster contraction to give tricyclo­[2.1.0.0^2,5]­pentasilane <b>14</b> with an exohedral chlorosilyl group