Cage and Cluster Compounds of Silicon, Germanium, and Tin

Akira Sekiguchi; Hideki Sakurai    Department of Chemistry and Organosilicon Research Laboratory, Faculty of Science, Tohoku University, Sendai 980-77, Japan

I INTRODUCTION

Highly symmetrical polyhedranes such as tetrahedrane, prismane, and cubane have long fascinated chemists (1,2). Synthesis of polyhedranes comprising Si, Ge, and Sn is one of the greatest synthetic challenges in the chemistry of higher row group 14 elements. Until relatively recently, highly strained polyhedranes of Si, Ge, and Sn were thought to be synthetically inaccessible. However, after the discovery of octasilacubane (3) and hexagermaprismane (4), the chemistry of the field developed rapidly. At present, tetrasilatetrahedrane, hexasilaprismane, and octasilacubane along with germanium and tin analogs are available (except for the tetrahedranes of Ge and Sn and the prismane of Sn). Unlike polyhedranes of carbon, those of Si, Ge, and Sn exhibit quite unique physical and chemical properties owing to the highly rigid framework made up of σ bonds with low ionization potentials. Since reviewing the development of this field (5), we ourselves reported syntheses and X-ray crystallographic structures of octasilacubane, octagermacubane, hexasilaprismane, and hexagermaprismane (6,7). Three reports on the crystal structures of octasilacubanes including ours appeared almost simultaneously (6,8,9). More recently, the synthesis and crystal structure of tetrasilatetrahedrane has been reported (10). Thus, the chemistry of polyhedranes of Si, Ge, and Sn has advanced by rapid strides, providing many novel and unusual structures which are the subject of this article.

II THEORETICAL STUDIES

For the Si4H4 potential energy surface, a local minimum has been predicted for tetrasilatetrahedrane (11,12). In contrast, Nagase and Nakano reported that tetrasilatetrahedrane is unlikely to be a minimum on the potential energy surface (13). Breaking of the two Si–Si bonds is predicted to occur without a significant energy barrier to form an isomer with one four-membered ring. However, it is also indicated that silyl substituents can stabilize the tetrasilatetrahedrane framework (14a,b). Therefore, the derivative (H3SiSi)4 is an interesting synthetic target. For Si6H6 (15a–d) and Si8H8 (16a,b), hexasilaprismane and octasilacubane are highly feasible molecules according to the theoretical calculations.

Geometries are compared by theoretical calculations of tetrahedrane, prismane, and cubane and silicon analogs. Figure 1 shows calculated geometries of the both the carbon and silicon compounds. The Si–Si bond lengths increase in the order Si4H4 (2.314 Å) < Si6H6 (2.359 and 2.375 Å) < Si8H8 (2.396 Å) (16a). The Si–Si bond lengths in the three-membered rings are shorter than the typical Si–Si single bond, whereas those in the four-membered rings are longer.

Table I summarizes the calculated strain energies of compound of the type EnHn, where n is 4, 6, and 8 and E is C, Si, Ge, and Pb, derived from homodesmic reactions (16b). The strain energies of silicon and carbon tetrahedranes are very similar (140 kcal/mol). However, replacement of the three-membered rings by four-membered rings results in a significant decrease in strain energy: 140.3 (Si4H4) > 118.2 (Si6H6) > 99.1 (Si8H8) kcal/mol, whereas those in carbon compounds remain roughly unchanged. Nagase proposed that this was caused by an increasing tendency to maintain the (ns)2(np)2 valence electron configuration in compounds with the heavier atoms (17). As a result, a four-membered ring with 90° bond angles is made favorable. The substituent effect is very important for the stabilization of polyhedranes. It is calculated that the SiH3 substituent leads to a remarkable relief of the strain: 114.5 kcal/mol for (SiSiH3)4 (tetrasilatetrahedrane), 95.7 kcal/mol for (SiSiH3)6 (hexasilaprismane), and 77.9 kcal/mol for (SiSiH3)8 (octasilacubane) (14).

Among polyhedranes, prismane is of special interest since hexasilabenzene, hexagermabenzene, and other heavier atom analogs of benzene are still elusive. Benzene is situated at the minimum of the C6H6 potential energy surface. Strained isomers such as prismane, Dewar benzene, and benzvalene lie at higher energy (1,2). However, the relative energies for the heavier elements are sharply different from those for carbon. Table II gives the calculated relative energies of E6H6 valence isomers, indicating the saturated prismanes to be the most stable valence isomers among E6H6 compounds (E = Si, Ge, Sn, Pb) (15a). This is caused by the fact that the higher row group 14 elements are rather reluctant to form double bonds (18a–e). Thus, it is quite reasonable to assume that the tetrahedrane, prismane, and cubane analogs of the heavier atoms are reasonably accessible and can be synthetic targets, provided that appropriate bulky groups can stabilize the polyhedranes.

Ill SUBSTITUENT, PRECURSOR, AND REDUCING REAGENT

A Substituent

The selection of substituent is critical for the successful isolation of polyhedranes composed of higher row group 14 elements. Figure 2 shows the first ionization potentials of Me3E–EMe3 (Ip, E = Si, Ge, and Sn), which are appreciably lower than those of the carbon analogs (e.g., 8.69 eV for Me3Si–SiMe3, 12.1 eV for H3C–CH3) (19). In fact, the E–E bonds of the small ring compounds comprising Si, Ge, and Sn are readily oxidized because of the existence of high-lying orbitals and the inherent strain. Therefore, full protection of the framework by the bulky substituent is required to suppress the attack by external reagents.

B Precursor

The proper choice of starting compounds with suitable bulky substituents is of crucial importance. The most logical precursors of polyhedranes are three-membered and four-membered compounds bearing halogens (Scheme 1). These ring compounds may give cubanes and prismanes made up of the heavier atoms through the dimerization reaction of the reactive E = E double bond. Tetrahalogenocyclobutanes consisting of the heavier elements will be candidate precursors of tetrahedranes since the higher row group 14 elements prefer saturated structures to unsaturated ones. Compounds of the type REX3 and REX2–EX2R, with judicious selection of R, also can serve as precursors of polyhedranes through multistep reactions. The steric bulkiness of the substituent R determines the ring size and the shape of polyhedranes.

C Reducing Reagent

The role of metals as reducing reagents is...