A pentacoordinate boron-containing π-electron system with Cl-B-Cl three-center four-electron bonds.

Tricoordinate boron-containing π-electron systems are an attractive class of compounds with intense fluorescence and strong electron-accepting properties. However, the impact of pentacoordination of the boron atoms on their properties has not been determined. We now disclose a B,B'-bis(1,8-dichloro-9-anthryl)-substituted 9,10-dihydro-9,10-diboraanthracene as a new pentacoordinate organoboron compound. In this skeleton, with the aid of the orthogonal arrangement of the anthryl substituent, the B and Cl atoms can form a three-center four-electron (3c-4e) Cl-B-Cl bond. The pentacoordination of the boron atom significantly perturbs the electronic structure and thereby the photophysical and electrochemical properties.