A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B–C–B three-centre–two-electron (3c–2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH3 addition reaction, during which a mono-BH3 adduct was formed from a electrophilic addition at the Cγ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H2 evolution is concerted to the second BH3 addition rather than forming B![[double bond, length as m-dash]](http://www.rsc.org/images/entities/char_e001.gif)
C before the second BH3 attack.
![Graphical abstract: Formation of an air-stable diborane via a stepwise BH3 addition of pyrido[1,2-a]isoindole with H2 evolution](http://hg.y866.cn/compound/lib/scimg/usr/1/D1CC04228A.jpg)
