Mechanistic features of boron-iodine bond activation of B-iodocarboranes

Oxidative addition of the B-I bond of 9-iodo-m-carborane to [(Ph3P)(n)Pd] (n = 3, 4) is reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence of [(Ph3P)(4)Pd] and [Bu4N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydride Pd(II) species generated upon thermal decomposition of [(Ph3P)(2)Pa(Ph)(O2CH)] and [(Ph3P)(2)Pd-2(Ph)(2)(mu -O2CH)(2)] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal decomposition of [(Ph3P)(2)Pd-2(Ph)(2)(mu -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)(2)Pd-2(Ph)(2)(mu -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 Angstrom than in the iodocarborane. In contrast, the C-I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) Angstrom) and in iodobenzene (2.098(4) Angstrom) are only marginally different. The elongation of the B-I bond, not the C-I bond, likely contribute s 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.