Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

Abstract

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 1 H NMR, 13 C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 p-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 p-electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 p-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 p-and 22 pelectron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 p-electron five-membered rings. Our computational analysis predicts that routine 1 H 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.

Porphycenes: Facts and Prospects in Photodynamic Therapy of Cancer

“…Extension of the porphycene core, as in 22π and 26π acetylene-cumulene porphycenes, also shows a clear red-shift pattern, consistent with the increased size of the conjugation pathway from 18 to 22 and 26 π electrons, respectively [78,179]. Finally, red shifts can also be induced by introduction of heteroatoms in the porphycene core [92,110,[115][116][117][118][119][120].…”

“…Finally, red shifts can also be induced by introduction of heteroatoms in the porphycene core [92,110,[115][116][117][118][119][120]. The batho-and hyperchromic effects brought about by aza substitution in porphyrins encouraged exploration of whether such effects would also occur in porphycenes.…”

“…Reduction of (38) with DIBAL-H (diisobutylaluminium hydride) in tetrahydrofuran (THF) at reflux to diol (39) followed by oxidation with DDQ afforded dialdehyde (40), which was converted to 3,6,13,16-tetraazaporphycene (41) upon oxidation of the corresponding [20]annulene with DDQ [120]. Almost simultaneously, a general methodology for the synthesis of 2,7,12,17-tetraaryl-substituted 3,6,13,16-tetraazaporphycenes (46) (Scheme 4), that avoids the development of a de novo synthesis whenever a new peripheral substituent is desired, was described [110].…”

Syntheses and fluorescence of salicylaldimine schiff base derivatives of 1,1′‐di(aminoethylaminocarbonylalkyl)‐2,2′‐biimidazole

“…Similarly, macromolecules containing 2,2'-biimidazole, including polymers [3][4][5][6][7][8] and macrocycles [9,10] have been described in efforts to explore relationships between structure, selective binding of metal ions, and thermal stability. Acyclic derivatives of 2,2'-biimidazole have been shown to exhibit cardiotonic [11] and antiprotozoal properties [12].…”

Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

“…[14][15][16] Porphycene has also been suggested to be the most stable of all porphyrin-like isomers, because of its rectangular cavity with four inner nitrogens and two inner hydrogens that can form strong intramolecular hydrogen bonds. [17][18][19] The aromatic properties of porphycenes have been studied experimentally by 1 H NMR spectroscopy 20 and computationally by calculating aromatic stabilization energies, 20,21 magnetic shielding functions, [21][22][23] and magnetically induced current densities. [21][22][23][24][25] Optical spectroscopy measurements 16,22 and graph-theoretical methods 26 have also been used for assessing aromatic properties of porphycenes.…”