Contraction and Expansion of the Silicon Scaffold of Stable Si₆R₆ Isomers

Abstract

The reactivity of two stable Si(6)R(6) clusters (4 and 5, R = 2,4,6-(i)Pr(3)C(6)H(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 hexasilaprismane 8a, a saturated-i.e., non-siliconoid-Si(6)R(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 5 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.

A heavy analogue of the smallest bridgehead alkene stabilized by a base

“…The facile isomerization between compound 1 and its isomer Si 4 BBY 1 0 was confirmed both experimentally and theoretically. Synthesis and reactions of Si 4 BBE 1 in this study can be regarded also as an example of contraction and expansion of unsaturated molecular silicon clusters [37][38][39][40][41] (termed siliconoids by Scheschkewitz and colleagues 37 ), which have received much attention as possible intermediates of vapour deposition of elemental silicon 42,43 . Transformation via elimination of silylene and/or skeletal isomerization may be a fascinating growth route of well-defined silicon clusters showing unique functions.…”

Effect of Ring Sizes of Cyclooligosilanes on Construction of Organosilicon Clusters

“…Similar examples have been reported in the formation of trisilaallene by the reduction of 1,1,1,2 tetrachlorodisilane 25 and that of the Si11 cluster by the dimerization of an isomer of hexasilabenzene. 32 It is stable below 238 in the solid state under an argon atmosphere in a sealed tube. It is also stable in solution at room temperature under an argon atmosphere.…”

“…There are a few reports on the unusual 29 Si chemical shifts found in silicon clusters. 27,32,44,47,48 In the UV visible spectrum of 8 (Figure 16), an absorption tailing around 650 nm was observed. The lowest energy absorption band at 585 nm (ε 1140) is assignable to the HOMO LUMO transition based on the TD DFT calculations (λ calcd 571.4 nm, f 0.0129), where the HOMO and the LUMO exhibit the dominant contribution of the Si Si π and π orbitals together with some contribution of the σ and σ orbitals of bicyclo[1.1.0]tetrasilane, respectively (Figure 17).…”

Predicted Siliconoids by Bridging Si9 Clusters through sp3-Si Linkers

“…So-called siliconoids [5] are stable unsaturated neutral silicon clusters that show the characteristic structural features of silicon nanoparticles and surfaces in the molecular regime generally realized through the occurrence of one or more unsubstituted Si atoms [5][6][7][8].…”