Electron Spin Resonance and Molecular Oribtal Study of One-Electron-Reduced O,O‘-Diphenylenehalonium Cations:  First Evidence for a Diaryliodine Radical, Ar₂I^• or Simply a New σ*-Radical?

Abstract

One-electron reduction of o,o‘-diphenylenebromonium (DPB) and o,o‘-diphenyleneiodonium (DPI) cations in low-temperature glasses produces free radical intermediates whose halogen hyperfine couplings suggest significant spin densities on bromine (0.13) and iodine (0.30). An adequate theoretical description of these species has been obtained at both semiempirical (PM3) and density functional levels of theory. These calculations show these species are a planar conformation of the 2-halobiphenyl-2‘-yl radicals, stabilized through intramolecular three-electron (or σ*) carbon−halogen bonding. Theory also predicts a nonequivalence of the C−X bonds and unsymmetrical spin density distribution over the two C−X bonding carbons. As compared to DPB, the DPI radical gives evidence for more equivalent bonding of the iodine to both carbons, accompanied by lower potential barriers for intramolecular iodine atom migration (1−2 kcal mol-1) along the σ*-bond. In the case of 3-nitrosubstituted DPI (NDPI) the one-electron-reduced intermediate was observed both as a σ*-radical (in polar glasses) and as a π*-radical (when intercalated into DNA). Calculations suggest that the change from σ* to π* on intercalation into DNA is driven both by electric field of the DNA backbone and by π-stacking of NDPI with DNA bases. One-electron-reduced diphenylenehalonium derivatives were not found to undergo intramolecular free radical addition leading to a cyclohexadienyl-type adduct. This result is supported by theoretical calculations indicating that such a process would be endothermic by 13.9 kcal mol-1 at the ROMP2/6-31G* level.

Breaking Down the Reactivity of λ3-Iodanes: The Impact of Structure and Bonding on Competing Reaction Mechanisms

“…This is similar to the three-center bond breaking process illustrated in Figure . The diaryliodine radical, which “captured” its electron from 3cb nb , will be metastable …”

Cyclic diaryliodonium ions: old mysteries solved and new applications envisaged

“…Although several pieces of indirect evidence for 9-I-2 diaryliodine radicals can be found in the literature, the first direct observation of such a species was reported only very recently. 11 The one-electron reduction of [Ph 2 I] + and 1 via g-irradiation from a 60 Co source was performed in aqueous LiCl or ethylene glycol glasses at 77 K. 11 When the diphenyliodonium cation was reduced, only the EPR signal from a phenyl radical was observed (eqn. (18)).…”

Selective Radiation-Induced Generation of 2-Deoxyribonolactone Lesions in DNA Mediated by Aromatic Iodonium Derivatives

“…By its nature DPI • is a localized phenyl-type radical, partially stabilized through intramolecular C-I * bonding (34). It is a moderate hydrogen abstractor, capable of reacting both with the deoxyribose moiety as well as with radical scavengers in the bulk (reactions 2 and 4).…”

“…1. The source of dibenz[b,d]iodolium chloride 1 and 3-nitrodibenz[b,d]iodolium chloride 2 was the same as in the previous studies (34). Acylation of 3-aminodibenz[b,d]iodolium chloride (35) with bromoacetic anhydride in DMF resulted into the bromoacetyl derivative,…”