Annulated 1,4,2,5-diazadiborinine 3 has been synthesized and fully characterized. Computational studies revealed that compound 3 features a relatively small HOMO–LUMO gap (2.61 eV). When 3 was treated with elemental grey selenium (Se), oxidative addition of Se proceeded regioselectively at the boron centres of 3 to give a bicyclo[2.2.2] molecule 4 involving a B–Se–Se–B unit, which can be deemed a heavier analogue of dibora-peroxide.