Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phen

Abstract

Proton-coupled electron transfer (PCET) processes are among the most important phenomena that control a variety of chemical and biological transformations. Although extensively studied in a variety of natural systems and discrete metal complexes, PCET mechanisms are less well codified in the case of purely organic compounds. Here we report that a planar β,β′-phenylene-bridged hexaphyrin (1.0.1.0.1.0), a 24 π-electron antiaromatic species termed rosarin, displays unique redox reactivity on protonation. Specifically, treatment with acid (for example, HI) yields a 26 π-electron aromatic triprotonated monocationic species that is produced spontaneously via an intermediate—but stable—25 π-electron non-aromatic triprotonated monoradical dication. This latter species is also produced on treatment of the original 24 π-electron antiaromatic starting material with HCl or HBr. The stepwise nature of the proton-coupled reduction observed in the planar rosarin stands in marked contrast to that seen for non-annulated rosarins and other ostensibly antiaromatic expanded porphyrinoids. A 24 π-electron antiaromatic hexaphyrin derivative (rosarin) with a near-planar geometry enforced by bridging phenylene groups has been shown to undergo a proton-coupled electron transfer reduction when treated with certain protic acids. The reduction proceeds in a stepwise fashion to give first a 25 π-electron non-aromatic system and then a 26 π-electron aromatic ring.

Cite this article

Ishida, M., Kim, SJ., Preihs, C. et al. Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phenylene-bridged annulated rosarins. Nature Chem 5, 15–20 (2013). https://doi.org/10.1038/nchem.1501

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