Diboryldiborenes: π-Conjugated B 4 Chains Isoelectronic to the Butadiene Dication.

B(sp2 )-B(sp3 ) diborane species based on bis(catecholato)diboron and N-heterocyclic carbenes (NHCs) underwent catechol/bromide exchange selectively at the sp3 -hybridized 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 π-electron delocalization over an all-boron chain.