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-CBMe] (X = H, CH, CH, CH, CH, CH, and COOCH) show two one-electron anodic oxidation peaks at the Pt electrode in liquid SO. 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-CBMe], identical with the anionic product of its reaction with [EtSi-H-SiEt] and/or EtSi, allows it to be identified as the iodonium ylide anion [{12-(1-H-CBMe)}I]. Its reversible oxidation to a neutral ylide radical [{12-(1-H-CBMe)}{12-(1-H-CBMe)}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 CB 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-CBMe] is followed by a transfer of an iodine atom from the B-I bond to SO to yield a weakly bound radical ISO which disproportionates into SO and I. The other product is the borenium ylide [12-dehydro-1-X-CBMe], which has a strongly Lewis acidic naked vertex in position 12 that rapidly adds to another [1-X-12-I-CBMe] anion to form the observed stable ylide anion [{12-(1-X-CBMe)}I]. In acetonitrile, where it presumably exists as a solvent adduct, [12-dehydro-1-X-CBMe] has been trapped with HO and, to a small extent, with MeOH, but not with several other potential trapping agents.

“Dumbbell”‐ and “Clackers”‐Shaped Dimeric Derivatives of Monocarba‐closo‐dodecaborate

“…So,w en ext envisioned that 3 could be synthesized by the oxidation reaction of the 12-iodocarborane anion 4.A fter extensive experimentation, we found that oxidation of 4 with mCPBA, followed by the addition of 1 successfully gave the desired product 3 in 82 %y ield as the cesium salt (Equation (3) in Scheme 1). [5] To our knowledge, this is the first example of "iodonium" structure. Figure 2a depicts the "dumbbell"-shaped structure of the dianionic species 2,i nw hich the two carbon vertices of the carborane anion units are directly connected.…”

“…[4] On the basis of those results,w edeveloped the palladium-catalyzed dimerization reaction of Cu I -carboranyl complex in the presence of an appropriate oxidant (TCQ:t etrachloro-pbenzoquinone,orchloranil), affording 2 as the cesium salt in 73 %y ield (Equation (1) in Scheme 1). [5] Thereaction between ICl 3 and 1 afforded 3 in moderate yield as the cesium salt with concomitant undesired chlorination of 1 (Equation (2) in Scheme 1). We next investigated the preparation of the "clackers"shaped dicarboranyliodonium monoanion 3.The introduction of the iodonium link between carborane anion molecules should lead to cooperative stabilization of the net negative charge.M ichl and Ludvíke tal.…”

“…tentatively suggested the generation of as imilar iodonium species during the electrochemical oxidation of [1-X-12-I-CB 11 Me 10 ] À anions,p ossibly via the formation of the corresponding borenium ylide [12-dehydro-1-X-CB 11 Me 10 ]a nd its coordination with the parent [1-X-12-I-CB 11 Me 10 ] À anion. [5] Thereaction between ICl 3 and 1 afforded 3 in moderate yield as the cesium salt with concomitant undesired chlorination of 1 (Equation (2) in Scheme 1). So,w en ext envisioned that 3 could be synthesized by the oxidation reaction of the 12-iodocarborane anion 4.A fter extensive experimentation, we found that oxidation of 4 with mCPBA, followed by the addition of 1 successfully gave the desired product 3 in 82 %y ield as the cesium salt (Equation (3) in Scheme 1).…”

“…This enlargement may be attributed to the zwitterionic onium structure and the steric repulsion of the carborane anions.S uch an unusually large angle of B À I À B structure in as imilar iodonium species was previously predicted by means of DFT calculations. [5] To our knowledge, this is the first example of "iodonium" structure. [8] Further, the absence of intermolecular short contact involving the iodine atom indicated as trong donor-acceptor interaction between the highly electron-deficient I + center and adjacent electron-rich carborane anions (Figure 2c).…”

“Dumbbell”‐ and “Clackers”‐Shaped Dimeric Derivatives of Monocarba‐closo‐dodecaborate

“…Michl and Ludvík et al. tentatively suggested the generation of a similar iodonium species during the electrochemical oxidation of [1‐X‐12‐I‐CB 11 Me 10 ] − anions, possibly via the formation of the corresponding borenium ylide [12‐dehydro‐1‐X‐CB 11 Me 10 ] and its coordination with the parent [1‐X‐12‐I‐CB 11 Me 10 ] − anion . The reaction between ICl 3 and 1 afforded 3 in moderate yield as the cesium salt with concomitant undesired chlorination of 1 (Equation (2) in Scheme ).…”