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

摘要

The formation, electronic structure, and reactivity of the recently-reported radical tricationic tetraborane [B-4(hpp)(4)](center dot 3+) (hpp = 1,3,4,6,7,8-hexahydro-2H-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)BPCy3](+). 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)](center dot 3+). Further analysis shows that [B-4(hpp)(4)](center dot 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.