Synthesis of a Linear Tetraboron Cation via Deprotonation of a Nonsupported Dimeric Hydroborylborenium Ion

The nonsupported B-H-B moiety of a dimeric hydroborylborenium ion can be readily deprotonated by 4-(dimethylamino)pyridine, affording a H-bridged linear tetraboron cation. The resulting tetraboron cation shows diverse reactivity with small molecules. Treatment of this tetraboron cation with H₂ leads to the first example of complete hydrogenolysis of main-group homocatenated chains. On the other hand, in the presence of tBuNC, only the internal B-B bond of the tetraboron cation is cleaved, resulting in the formation of an azadiborabutadiene cation. Additionally, during the preparation of the hydroborylborenium, an unprecedented formal boryl cage-walking was observed. Experimental and theoretical studies suggest this process involves consecutive activation of carboranyl B(9)-H bonds.