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

Oxidative addition of the B−I bond of 9-iodo- m -carborane to [(Ph 3 P) n Pd] ( n = 3, 4) is reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence of [(Ph 3 P) 4 Pd] and [Bu 4 N]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 [(Ph 3 P) 2 Pd(Ph)(O 2 CH)] and [(Ph 3 P) 2 Pd 2 (Ph) 2 (μ-O 2 CH) 2 ] reduce 9-iodo- m -carborane to m -carborane with 100% selectivity. The thermal decomposition of [(Ph 3 P) 2 Pd 2 (Ph) 2 (μ-O 2 CH) 2 ] in the presence of excess 9-iodo- m -carborane and PhI (1:1) results in the formation of m -carborane (3%) and [(Ph 3 P) 2 Pd 2 (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 S N reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10-diiodo- m -carborane is reported.