Synthesis, Protonation\nand Aromatic Characteristics\nof a Series of 1,10-Phenanthroline-Fused Porphyrinoids

A series of porphyrin analogues with fused 1,10-phenanthroline\nunits were synthesized. The proton NMR spectra for phenanthroline-fused\nheteroporphyrins showed significantly upfield shifted <i>meso</i>-proton resonances compared to related porphyrinoid systems and the\npeaks corresponding to alkyl substituents directly attached to these\nmacrocycles were also observed further upfield. These results indicate\nthat the presence of the phenanthroline unit leads to reduced diatropicity,\nbut the internal NH resonance was also further upfield, a result that\nis inconsistent with this interpretation. A phenanthrene-fused carbaporphyrin\ngave an unexpectedly upfield singlet for the internal C–H at\nnearly −9 ppm, while the NH protons appeared at −6.8\nppm. These unusual chemical shifts again imply enhanced diatropicity\nbut the reduced downfield shifts for the external protons indicates\nthat the aromatic ring current has been significantly reduced. Similar\nresults were obtained for phenanthroline-fused oxybenzi- and oxypyriporphyrins.\nDetailed analyses of the spectroscopic properties for these systems\nare reported and protonation studies were conducted. The conjugation\npathways and aromatic properties were computationally analyzed using\nnucleus independent chemical shifts (NICS) and anisotropy of induced\ncurrent density plots.