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 (18.104.22.168.1.0), 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.
Bibliographical noteFunding Information:
Support is acknowledged from the US National Science Foundation (CHE-1057904 to J.L.S.), the Robert A. Welch Foundation (F-1018 to J.L.S.), Grant-in-Aid (No. 20108010 to S.F. and 23750014 to K.O.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, the World Class University Program (R32-2010-000-10217-0 to D.K. and R31-2008-000-10010-0 to S.F.) of the Ministry of Education, Science and Technology, the Basic Science Research Programs, National Research Foundation (2009-0087013 to C-H.L), BK21, Council of Scientific and Industrial Research, India (from a senior research fellowship to T.S.) and the Department of Science and Technology India (SR/S1/IC-20/2007 to P.K.P.).
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)