A Neutral Silicon(II) Half-Sandwich Compound

Metathesis reaction of a dilithio
borole dianion, a cyclic π-ligand
isoelectronic to ubiquitous cyclopentadienyls, with two equivalents
of “silicocenium” cation [Cp*Si]+ as a source
of low-valent Si­(II), cleanly gives a borole half-sandwich π-complex
of Si­(II) and silicocene. The resulting half-sandwich complex is a
neutral isoelectronic analogue to the iconic silicocenium cation and
features the rare structural motif of an apical silicon­(II) atom with
an energetically high lying lone pair of electrons that is shown to
be accessible for coordination chemistry toward tungsten carbonyl.
Protonation at the Si­(II) atom with [H­(OEt2)2]­[Al­{OC­(CF3)3}4] induces formal
oxidation, and the compound rearranges to incorporate the Si atom
into the carbocyclic base to give an unprecedented cationic 5-sila-6-borabicyclo[2.1.1]­hex-2-ene.
This rearrangement is accompanied by drastic changes in the 11B and 29Si NMR chemical shifts.