Synthesis of a Pentacene-Type Silaborin via Double Dehydrogenative Cyclization of 1,4-Diboryl-2,5-disilylbenzene.

A new pentacene-type silaborin, in which three benzene rings are bridged by silicon and boron atoms, has been synthesized and characterized by using NMR spectroscopy and X-ray crystallographic analysis. The precursor, 1,4-bis(dimesitylboryl)-2,5-bis(phenylsilyl)benzene (4), was prepared by stepwise introduction of a silyl group and a boryl group to a benzene ring starting from 1,4-dibromobenzene. Double cyclization of 4 proceeds by a H-Mes exchange and a B-H/C-H dehydrogenative condensation to afford pentacene-type silaborin 5. X-ray crystal structure analysis reveals that 5 adopts a bent structure rather than a planar one. UV/Vis spectra and DFT calculations for 5 reveal a lowering of the LUMO energy level compared with corresponding anthracene-type 3.