Theoretical Study of the Photolysis Mechanisms of Methylpentaphenyldimetallanes (Ph3MM′Ph2Me; M, M′ = Si and Ge)

The mechanisms of the photolysis reactions are studied theoretically at the M06-2X/6-311G(d) level of theory, using the four types of group 14 molecules that have the general structure, Ph₃M–M′Ph₂Me (M and M′ = Si and Ge), as model systems. This study provides the first theoretical evidence for the mechanisms of these photorearrangements of compounds that contain a M–M′ single bond. The model investigations indicate that the preferred reaction route for the photolysis reactions is, as follows: reactant → Franck-Condon (FC) region → minimum (triplet) → transition state (triplet) → triplet/singlet intersystem crossing → photoproducts (both di-radicals and singlets). The theoretical findings demonstrate that the formation of radicals results from reactions of the triplet states of these reactants. This could be because both the atomic radius and the chemical properties of silicon and germanium are quite similar to each other and compared to other group 14 elements, their photolytic mechanisms are nearly the same. The results for the photolytic mechanisms that are studied in this work are consistent with the available experimental observations and allow for a number of predictions for other group 14 dimetallane analogues to be made.

Substituent effects on the reactivity of the silicon-carbon double bond

Laser flash photolysis of various organosilicon compounds such as aryl-, vinyl-, and alkynyldisilanes, silacyclobutanes and silacyclobutenes, and alpha-silylketenes and -diazomethanes leads to the formation of reactive silenes which can be detected directly in solution, allowing detailed studies of the kinetics and mechanisms of their reactions with nucleophiles. Over 30 transient silenes have now been studied by these methods, providing the opportunity to systematically assess the effects of substituents at silicon and carbon on the reactivity of the Si=C bond.

Photolysis of tris(trimethylsilyl)silane: trapping of sisyl radicals

The photolysis of tris(trimethylsilyl)silane (TTMSS) was studied in the absence and in the presence of added trapping agents such as alkenes and alcohols. It was found that, unlike the case with pyrolysis, silyl radicals rather than silylenes are produced. They may be efficiently trapped with alkenes, to give the hydrosilylation products, but not with alcohols. The major product from the photolysis of TTMSS in the absence of added trapping agent (or with alcohols as trapping agents) was tetrakis(trimethylsilyl)silane. Possible mechanisms to account for the photoproducts are presented.Key words: hydrosilane, photolysis, silylenes, silyl radical, sisyl hydride.