We report herein unique stepwise protonation at inner imino-nitrogen atoms of a freebase derivative of a quadruply fused porphyrin (H2QFP), which has been newly synthesized. H2QFP has been revealed to have the two inner NH protons on the two nonfused pyrroles by X-ray diffraction analysis and 1H NMR spectroscopy. The first protonation at one of the two imino-nitrogen atoms of the fused pyrroles smoothly proceeds with trifluoroacetic acid (TFA) in CH2Cl2 and the equilibrium constant (K1) of the protonation has been determined to be (1.3 ± 0.1) × 105 M-1. In contrast, the second protonation at the other imino-nitrogen atom is hard to occur unless a large excess amount of TFA is used, as reflected on a much smaller equilibrium constant, K2 = 7.3 ± 0.3 M-1. The stepwise protonation is ascribed to the structural rigidity caused by the ring fusion and the resultant steric repulsion among inner NH atoms of the diprotonated form. Electrochemical studies have revealed that protonation at the pyrrole nitrogen atoms caused positive shifts of the reduction potentials of the QFP derivatives. In addition, the ESR spectrum of the electrochemically one-electron-reduced monoprotonated QFP derivative showed well-resolved hyperfine splitting to represent its unsymmetrical electronic structure due to the monoprotonation.
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