Iridium-Mediated Regioselective B–H/C–H Activation of Carborane Cage: A Facile Synthetic Route to Metallacycles with a Carborane Backbone

One-pot
reactions of carborane carboxylic acids (L), [Cp*IrCl2]2, and silver salt are reported, which lead to
regioselective B–H or C–H bond activation at ambient
temperature in good yields. This process is demonstrated for three
carborane (o-, m-, p-) dicarboxylates, and metal-mediated B–H functionalization
of a p-carborane derivative is accomplished for the
first time. Two metal-induced self-assembly routes to tetra-nuclear
metallacycles 3 and 5 were performed through
B­(4, 7)/H and B­(2, 10)/H activation, respectively, and the two metallacycles
were found to be stable and to exist in solution as discrete complexes.
Different activation modes in the carborane cage were ascribed to
the characteristic structure of the products and the electronic density
differences. The analogous reaction of o-carborane
monocarboxylate with the same metal precursor gave the C–H
activation complex 6, indicating that the B–H
bond is more stable than the C–H bond in this carborane cage.
The selective activation was confirmed by DFT calculation results.
In this study, a facile and efficient synthetic route has been developed
through specific B–H bond activation to construct carborane-based
metallacycles that are unavailable by conventional methods.