Formation of a Radical Tricationic Tetraborane(4) by Hydride Abstraction from sp 3 –sp 3 ‐Hybridized Diboranes

The formation, electronic structure, and reactivity of the recently‐reported radical tricationic tetraborane [B 4 (hpp) 4 ] · 3+ (hpp = 1,3,4,6,7,8‐hexahydro‐2 H ‐pyrimido[1,2‐ a ]pyrimidinate), which features a planar rhomboid B 4 unit and bridging bicyclic guanidinate (hpp) substituents, is reported. New experiments show that the tetraborane could be synthesized by hydride abstraction either in a very fast reaction from the electron‐rich sp 3 –sp 3 ‐hybridized diborane [HB(hpp)] 2 , or in a very slow reaction from the cationic, less electron‐rich sp 3 –sp 3 ‐hybridized diborane [HB(hpp) 2 BPCy 3 ] + . The results argue for an electron‐transfer in the rate‐determining step of a complex reaction sequence. Quantum chemical calculations highlight the role of the hpp substituents for the stabilization of the radical tricationic tetraborane [B 4 (hpp) 4 ] · 3+ . Further analysis shows that [B 4 (hpp) 4 ] · 3+ is highly reactive, and especially a very strong electrophile. This reactivity makes the compound very interesting because boron atoms in such a low oxidation state (<2) are expected to be strong nucleophiles.