Mechanistic Features of Boron−Iodine Bond Activation of B−Iodocarboranes

Oxidative addition of the B−I bond of 9-iodo-m-carborane to [(Ph3P)nPd] (n = 3, 4) is
reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence
of [(Ph3P)4Pd] and [Bu4N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to
produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydrido Pd(II) species
generated upon thermal decomposition of [(Ph3P)2Pd(Ph)(O2CH)] and [(Ph3P)2Pd2(Ph)2(μ-O2CH)2] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal
decomposition of [(Ph3P)2Pd2(Ph)2(μ-O2CH)2] in the presence of excess 9-iodo-m-carborane
and PhI (1:1) results in the formation of m-carborane (3%) and [(Ph3P)2Pd2(Ph)2(μ-I)2] (97%),
whose structure was confirmed by single-crystal X-ray diffraction. X-ray analysis of 9-iodo-m-carborane and m-carboran-9-yl(phenyl)iodonium tetrafluoroborate shows that in the
iodonium salt the B−I bond is longer by ca. 0.03 Å than in the iodocarborane. In contrast,
the C−I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) Å) and
in iodobenzene (2.098(4) Å) are only marginally different. The elongation of the B−I bond,
not the C−I bond, likely contributes to (i) the enhanced reactivity of B−carboranyl(phenyl)iodonium cations toward nucleophiles and (ii) the remarkably high selectivity of these SN
reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10-diiodo-m-carborane is reported.