“N-Fused Porphyrin”: A New Tetrapyrrolic Porphyrinoid with a Fused Tri-pentacyclic Ring

Abstract

The syntheses and X-ray structures of novel porphyrinoids, “N-fused porphyrins (NFPs)”, and their reactivity were described. NFP was spontaneously produced from the bromo-substituted N-confused tetraarylporphyrin in a pyridine solution at room temperature. X-ray diffraction analyses revealed that the porphyrinoid core containing a fused tri-pentacyclic ring is almost planar. The deviation from the mean plane of 4e‘, for example, was within 0.30 Å. The peripheral aryl substituents were tilted 50.4, 53.5, 64.4, and 12.4° relative to the porphyrin mean plane, respectively. The occurrence of a three-centered hydrogen bonding in the NFP core was inferred by the downfield shift of the inner NH signal (e.g., 8.48 ppm for 4e‘) in 1H NMR and the short distances (within 2.4−2.9 Å) among the inner core nitrogens, N2, N3, and N4. The optical absorption spectra of NFPs exhibit Soret-like transitions around 360, 500, and 550 nm and weak Q-like bands around 650, 700, 850, and 940 nm in CH2Cl2. The electrode process of 4e‘ showed the first oxidation at 0.08 V and reduction at −1.37 V (vs Fc/Fc+), which suggested the small energy gap attributed to the unusual long-wavelength absorption was mainly due to the rising of the HOMO energy level. The first-order rate constants (kf) for the transformation from NCPs to NFPs were largely affected by the substituents at the meso position, showing a good correlation with Hammet σ+ parameters. Moreover, the reverse reaction from NFPs to NCPs was observed in the CH2Cl2 solution by treating with a base. Dynamic ring inversion in the tetrapyrrolic porphyrin core is discussed.