Formation of a Radical Tricationic Tetraborane(4) by Hydride Abstraction from sp3–sp3‐Hybridized Diboranes

Abstract

The formation, electronic structure, and reactivity of the recently‐reported radical tricationic tetraborane [B4(hpp)4]·3+ (hpp = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate), which features a planar rhomboid B4 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 sp3–sp3‐hybridized diborane [HB(hpp)]2, or in a very slow reaction from the cationic, less electron‐rich sp3–sp3‐hybridized diborane [HB(hpp)2BPCy3]+. 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 [B4(hpp)4]·3+. Further analysis shows that [B4(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.