Synthesis of Quinoidal Fused Oligosiloles by Rhodium-Catalyzed Stitching Reaction and Theoretical Investigation of Their Properties

New quinoidal fused oligosiloles
containing an even number of silole
units have been synthesized by a rhodium-catalyzed stitching reaction.
Employing [RhCl­(tfb)]2 as the catalyst significantly improved
the stitching efficiency, and up to six siloles could be fused in
quinoidal form. A systematic comparison of the physical properties
of Si1–Si6′ confirmed the
unique trend in their LUMO levels, which become higher with longer
π conjugation. To understand the origin of this unusual trend,
theoretical calculations were also carried out using various model
compounds, and the results indicated that the terminal indenylidene
(cyclopentadienylidene) moieties in Si1–Si6 (Si1a–Si6a) are primarily
responsible for this phenomenon through their frontier orbital correlations
with the HOMO of the central polyene unit, which becomes higher in
energy with longer π conjugation.