An unstable conjugated homoporphyrin was successfully stabilized by introducing meso-aryl substitutents. It was evident from the moderate diatropic ring current found by NMR analysis that the newly formed 20π conjugated free base and its protonated form exhibited Möbius aromatic character. Furthermore, complexation as a ligand with an Rh ion afforded a unique binding mode and retained the Möbius aromaticity. Overall, these compounds are the smallest Möbius aromatic molecules, as confirmed by spectral and crystal-structure analysis and supported by theoretical studies.
Stable homocarbaporphyrinoids were successfully synthesized by incorporating the m‐o‐m and p‐o‐p terphenyl units into the porphyrin core. The distinct bonding modes of terphenyl in the macrocycle generated two structural isomers with two and four carbon atoms in the macrocyclic environment. The core was utilized to stabilize the RhI ion. The spectral and structural analyses revealed that the restricted (m‐o‐m) and allowed (p‐o‐p) conjugation in the macrocyclic core provide overall non‐aromatic characteristics both to the free bases and their complexes.
Three distinct conformational structures of carbaoctaphyrins were prepared by incorporating bis‐4,4'‐biphenyl units in the macrocyclic core. The free‐base form adopts a figure‐eight conformation, whereas the protonation triggers a conformational change with a pyrrole ring inversion and acquires an open‐framework structure. The insertion of bis‐RhI metal ion in the macrocyclic core affords a singly twisted conformational structure. Furthermore, the local aromaticity in the bis‐4,4'‐biphenyl ring dominates the overall macrocyclic aromaticity in all three forms, and thus adopts nonaromatic characteristics. These results are supported by spectral as well as theoretical studies, and they are unambiguously confirmed by X‐ray crystal analyses.
A new rectangular-shaped carbadecaphyrin was successfully synthesized by introducing a terphenylene unit (m-m-m) in the macrocyclic core. The terphenylene moiety offers an open framework with multiple binding pockets to stabilize two Rh(I) ions in the core. The photophysical and structural studies reveal the non-aromatic character of the ligand and its bis-Rh(I) complex.
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