Diboryldiborenes: π‐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 π‐electron delocalization over an all‐boron chain.