Diboryldiborenes: pi-conjugated B4 chains isoelectronic to the butadiene dication

sp2–sp3 diborane species based on bis(catecholato)diboron and N-heterocyclic carbenes (NHCs) are subjected to catechol/bromide exchange selectively at the sp3 boron atom. The reduction of the resulting 1,1-dibromodiborane adducts led to reductive coupling and isolation of doubly NHC-stabilized 1,2-diboryldiborenes. These compounds are the first examples of molecules exhibiting pi-electron delocalization over an all-boron chain.