Synthesis of a Pentasilapropellane. Exploring the Nature of a Stretched Silicon−Silicon Bond in a Nonclassical Molecule

We report on the successful synthesis of Si5Mes6 (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal [1.1.1] cluster core featuring two ligand-free, “inverted tetrahedral” bridgehead silicon atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si−Si single bond. A detailed analysis of the electronic characteristics of Si5Mes6 reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon−silicon bond in this “nonclassical” molecule.