Silicon bonding catenation

Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the... 

Derivatives with silicon-silicon bonds include cycles, chains and polymers. The relationships between derivatives that contain two or more contiguous silicon centers is shown in Scheme 5. In most cases the origin of the catenated derivatives may be traced back to a dihalosilane. 

Polysilanes are compounds which contain silicon-silicon bonds. Although the disilane Et3SiSiEt3 was reported as early as 18691, and the cyclic perphenyl compounds (Ph2Si) (n = 4-6) were prepared over 70 years ago by Kipping and Sands2, the belief that silicon had limited capability for catenation persisted until quite recently. The isolation within recent years of cyclic dialkylpolysilanes containing up to 40 silicon atoms in a ring, and of linear polymers with more than 40,000 silicon atoms in the chain, have however effectively dispelled this myth. 

For many years, the limited similarity between silicon and carbon excited the scientific community. Carbon and silicon share the same outer shell electronic structure, s, which permits sp hybridization and dominant tetrahedral coordination, as well as dominance of the tetravalent oxidation state. Nevertheless, silicon chemistry is markedly poorer compared to that of carbon. Double silicon bonds and silicon catenation are scarce, and crystalline silicon, which is so widely used in the electronics industry, is never encountered in nature. Instead, sUicon-oxygen bonds dominate natural silicon chanistry, and solid silica and silicates have no common physicochemical features with carbon dioxide and carbonates. The silicon atom is larger than carbon, it is less electronegative, has lower nuclear electric charge shielding and, perhaps most importantly, it has vacant d-orbitals in its outer shell all these dictate the reactivity of silicon. Several consequences of these differences are especially significant, and they are also relevant to sol-gel electrochemistry. 

Catenation is defined as the self-linking of an element to form chains and rings. Carbon, then, given the above discussion, is the all-time champion catenator, much better than silicon (or sulfur, boron, phosphorus, germanium, and tin, the other elements that show this ability). Why should this be so A comparison of the relevant carbon and silicon bond energies as shown below is helpful ... 

Polysilanes are polymers in which there is catenation of silicon, that is, where silicon atoms are bonded to each other in a continuous manner. Synthesis of polysilanes involves the Wurtz coupling of diorganodichlorosilanes with sodium metal (Eq. 2-239) [Baldus and Jansen, 1997 Corriu, 2000 Manners, 1996 Miller and Michl, 1989 West, 1986 West and Maxka,... 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

Further complications with silanes arise from lack of convenient syntheses and difficulties in separation. Nevertheless, compounds from n = I to n = 8 have been isolated, including both straight-chain and branched-chain compounds We should nol judge silicon s tendency to catenate by looking at these hydrides, however, since a much dilfereni result is obtained when substituents other than hydrogen are present.7 Flutters other than inherent Si—Si bond strength must be involved because it is possible to isolate a large number of polysilane polymers 8... 

The term catenation is used to describe the tendency for covalent bond formation between atoms of a given element to form chains, cycles, layers, or 3D frameworks. Catenation is common in carbon compounds, but it only occurs to a limited extent in silicon chemistry. The reason can be deduced from the data listed in Table 14.4.1. 

Germanium forms a wide range of compounds with hydrogen, silicon, the heavier group 14 elements, some main group metals, and transition metals. Catenated hydrides, Ge H2 +2, have already been discussed. The majority have carbon bonded to Ge and are included in the next article (see Germanium Organometallic Chemistry). 

The formation of the Si-Si bond occupies a special position in organosilicon chemistry. Silicon is one of the few elements, which can form stable catenated compounds. Besides high-molecular weight polysilanes, oligomeric polysilanes are a very important class of compounds that are related to silylenes,... 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

---