Synthesis, Protonation and Aromatic Characteristics of a Series of 1,10-Phenanthroline-Fused Porphyrinoids

A series of porphyrin analogues with fused 1,10-phenanthroline units were synthesized. The proton NMR spectra for phenanthroline-fused heteroporphyrins showed significantly upfield shifted meso-proton resonances compared to related porphyrinoid systems and the peaks corresponding to alkyl substituents directly attached to these macrocycles were also observed further upfield. These results indicate that the presence of the phenanthroline unit leads to reduced diatropicity, but the internal NH resonance was also further upfield, a result that is inconsistent with this interpretation. A phenanthrene-fused carbaporphyrin gave an unexpectedly upfield singlet for the internal C-H at nearly -9 ppm, while the NH protons appeared at -6.8 ppm. These unusual chemical shifts again imply enhanced diatropicity but the reduced downfield shifts for the external protons indicates that the aromatic ring current has been significantly reduced. Similar results were obtained for phenanthroline-fused oxybenzi- and oxypyriporphyrins. Detailed analyses of the spectroscopic properties for these systems are reported and protonation studies were conducted. The conjugation pathways and aromatic properties were computationally analyzed using nucleus independent chemical shifts (NICS) and anisotropy of induced current density plots.