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.