Single and Double Hydroboration of B–B Triple Bonds and Convergent Routes to a Cationic Tetraborane

A compound with a boron–boron
triple bond is shown to undergo
stepwise hydroboration reactions with catecholborane to yield an unsymmetrical
hydro­(boryl)­diborene and a 2,3-dihydrotetraborane. Abstraction of
H– from the latter compound produces an unusual
cationic, planar tetraborane with a hydrogen atom bridging the central
B2 moiety. Spectroscopic and crystallographic data and
DFT calculations support a “protonated diborene” structure
for this compound, which can also be accessed via direct protonation
of the corresponding diborene.