Credit: © 2010 ACS

Although silicon is placed just below carbon in group 14 of the periodic table, the differences between hydrocarbons and their silicon analogues can be profound. The hydrocarbons called propellanes — three fused rings with a shared carbon–carbon bond — have unusual bonding that has been much studied in the past. Germanium and tin analogues of these polycyclic compounds exist, and now Wim Klopper, Frank Breher and colleagues have synthesized1 the silicon analogue, a pentasilapropellane.

The team, from the Karlsruhe Institute of Technology in Germany, made the mesityl-stabilized compound in relatively low yield and found that it is extremely sensitive to air and moisture. Similarities to the carbon and hydrogen NMR spectra of the germanium analogue, and the presence of only two peaks in the silicon spectrum, provided evidence for the compound's identity. X-ray diffraction experiments found that the distance between the two bridgehead silicon atoms was 13% longer than for a typical silicon–silicon single bond, but comparable with a known sterically crowded disilane bond.

Although the symmetry of the Raman bands meant the strength of this bond could not be accurately determined by experiment, Klopper, Breher and colleagues used density functional theory to calculate that it is just over half as strong as in normal disilanes. The silicon propellane is unexpectedly more susceptible to nucleophilic attack than the germanium and tin analogues, and reacted with even traces of water in a solution — unlike the heavier analogues that are stable in degassed water.