Electrochemical Oxidation of [1-X-12-I-CB11Me10-] Anions: Formation of Borenium Ylides [12-Dehydro-1-X-CB11Me10] and Iodonium Ylide Anions [{12-(1-X-CB11Me10-)}2I+]

Abstract

Cyclic voltammograms of 12-iodinated icosahedral carborane anions [1-X-12-I-CB₁₁Me₁₀^-] (X = H, CH₃, C₂H₅, C₃H₇, C₄H₉, C₆H₁₃, and COOCH₃) show two one-electron anodic oxidation peaks at the Pt electrode in liquid SO₂. Oddly, the first is irreversible and the second partially reversible. Mass spectrometry of the principal anionic product of preparative anodic oxidation of [1-H-12-I-CB₁₁Me₁₁^-], identical with the anionic product of its reaction with [Et₃Si-H-SiEt₃]⁺ and/or Et₃Si⁺, allows it to be identified as the iodonium ylide anion [{12-(1-H-CB₁₁Me₁₀^-)}₂I⁺]. Its reversible oxidation to a neutral ylide radical [{12-(1-H-CB₁₁Me₁₀^•)}{12-(1-H-CB₁₁Me₁₀^-)}I⁺] is responsible for the second peak. A DFT geometry optimization suggests that both the ylide anion and the ylide radical are very crowded and have an unusually large C-I-C valence angle of ∼132°; they are the first compounds with two bulky highly methylated CB₁₁ cages attached to the same atom. Molecular iodine is another product of the electrolysis. We propose an electrode mechanism in which initial one-electron oxidation of [1-X-12-I-CB₁₁Me₁₀^-] is followed by a transfer of an iodine atom from the B-I bond to SO₂ to yield a weakly bound radical ISO₂^• which disproportionates into SO₂ and I₂. The other product is the borenium ylide [12-dehydro-1-X-CB₁₁Me₁₀], which has a strongly Lewis acidic naked vertex in position 12 that rapidly adds to another [1-X-12-I-CB₁₁Me₁₀^-] anion to form the observed stable ylide anion [{12-(1-X-CB₁₁Me₁₀^-)}₂I⁺]. In acetonitrile, where it presumably exists as a solvent adduct, [12-dehydro-1-X-CB₁₁Me₁₀] has been trapped with H₂O and, to a small extent, with MeOH, but not with several other potential trapping agents.