Global versus local aromaticity in porphyrinoid macrocycles: Experimental and theoretical study of "imidacene", an imidazole-containing analogue of porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole-based analogue of porphycene are described. The macrocycle, given the trivial name "imidacene", was prepared by reductive coupling of a diformyl-substituted 2,2'-biimidazole using low-valent titanium, followed by treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Imidacene displays a porphyrin-like electronic structure, as judged by its H-1 NMR, C-13 NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 pi-electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high-level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 pi-electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 pi-electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 pi- and 22 pi-electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 pi-electron five-membered rings. Our computational analysis predicts that routine H-1 NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.