Contraction and Expansion of the Silicon Scaffold of Stable Si_6R_6 Isomers

摘要

The reactivity of two stable Si_6R_5 clusters (4 and 5, R = 2,4,6-~iPr_3C_6H_2) with unsymmetrical substitution patterns (including Si, SiR, and SiR_2 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 hexasilap-rismane 8a, a saturated-i.e., non-siliconoid-Si_6R_6 isomer is accessible from a suitable Si_3 precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane 6 into the corresponding 1,2,3-derivative 7 prior to the requisite reductive coupling step leading to 8a. On the other hand, a stable expanded Si_(11)-siliconoid 9 was isolated as a minor side product of the thermal isomerization of 4 to 5, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of 9 strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid S with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane 11 in high yield. Conversely, chlorination of the isomeric 4 with BiCl_3 as a mild source of Cl_2 results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of 4. The main product, 1,2-dichlorotricyclo[2.2.0.0~(2,5)]hexasilane 12, undergoes thermal cluster contraction to give tricyclo[2.1.0.0~(2,5)]pentasilane 14 with an exohedral chlorosilyl group.