Nitrogen Doping at the Periphery of a Quadruply-Fused Porphyrin and Its Multi-Step Proton-Coupled Electron Transfer...

Four-electron reduction of aromatic compounds to produce four-electron reduced compounds has been extremely difficult because of the highly negative reduction potentials. Herein, we report for the first time formation of stable four-electron reduced aromatic molecules using nitrogen-doping at the peripheral positions of a Zn^II complex of a quadruply fused porphyrin (1), containing four imino-nitrogens in the "formal" 30π aromatic circuit. Peripheral nitrogen atoms have been demonstrated to form hydrogen bonds, as indicated by X-ray diffraction analysis. Addition of a strong acid such as triflic acid (TfOH) enables diprotonation of 1 and the diprotonated form (H₂1²⁺) of 1 shows a large positive shift of the first reduction potential. Furthermore, in the presence of an excess amount of TfOH, chemical or electrochemical reduction causes further protonation of the imino-nitrogen atoms of 1, allowing further reduction of 1 through a proton-coupled electron transfer (PCET) mechanism. The multielectron PCET processes are supported by DFT calculations to clarify a large change in the pK_a of the outer imine nitrogens before and after the 1e^--reduction. Consequently, 1 undergoes its tetraprotonation and four-electron-reduction at a relatively positive potential to form H₄1⁰, which bears the 34π aromaticity despite the fact that it is unable to draw an aromatic circuit with alternating single and double bonds. It should be noted that this is the first time to demonstrate multistep PCET of a π-expanded aromatic system and to achieve the emergence of unique aromatic conjugation with 34π electrons.