Synthesis and Properties of Benzofuran-Fused Silole and Germole Derivatives: Reversible Dimerization and Crystal Structures of Monomers and Dimers

Benzofuran-fused silole and germole derivatives were synthesized by the reactions of dilithiobi­(benzofuran) with R2SiCl2 and R2GeCl2, respectively. Dilithiobi­(benzofuran) was prepared by treating diiodobi­(benzofuran) with n-BuLi. Optical and electrochemical measurements of the resulting bis­(benzofurano)­metalloles and their DFT calculations indicated that the introduction of a metallole unit enhanced the conjugation of bi­(benzofuran) mainly by raising the HOMO energy level, similarly to the case of bis­(benzothieno)­silole prepared previously. Interestingly, [2+2] dimers were obtained by recrystallization of the bis­(benzofurano)­metalloles, and the dimers readily reverted to the respective monomers upon dissolving in solvent. The crystal structures of the dimers and the monomers were determined by X-ray diffraction studies.