Fused core-modified planar antiaromatic 32π heptaphyrins: unusual synthesis and structural diversity

Abstract

The condensation reaction of fused dithienothiophene (DTT) diol with a core-modified tripyrrane led to the formation of acid and its concentration dependent products, sapphyrin and heptaphyrin. The single crystal X-ray analysis of sulphur and selenium heptaphyrin exhibits planar conformation owing to fusion. The experimental and theoretical calculations revealed the antiaromatic electronic structure of Hückel 4nπ.

[32]π Fused Core‐Modified Heptaphyrin with Möbius Aromaticity

“…Similar trend was observed for 6 at 213–183 K in CD 2 Cl 2 (Figures S7 and S8 in the Supporting Information). The results were further compared with 1 , 2 and 4 : 1) the NH and one of meso ‐mesityl methyl protons are upfield shifted in 1 ; however, the methyl protons were not observed in the negative region, as it was observed in 5 ; 2) the figure‐eight conformation in 2 a adopts nonaromatic character; 3) the non‐symmetrical pattern was observed in 2 b at 183 K; and 4) the DTT and pyrrole units in 4 are inverted and experiencing the paratropic ring current . Overall, these results obtained for 5 are comparable with 1 and 2 b , further confirm the Möbius aromatic character.…”

“…In the case of 6 , the respective angle is 83.3°, and the nonbonding distances are 3.90 and 3.88 Å (Figure S18 in the Supporting Information). On the other hand, the crystal analysis of 1 and 2 a in protonated form and free base 2 b adopt Möbius conformation, while 4 retains the Hückel antiaromatic character in the solid state . An intriguing fact also noticed from crystal structure of 5 is that the molecule exists self‐assembled dimer, one‐ and two‐dimensional arrays through intermolecular hydrogen‐bonding interactions in the solid state (Figures S15–S17 in the Supporting Information).…”

“…The spectral pattern is similar to other Möbius aromatic expanded porphyrins. The results are further compared with 3 and 4 : 1) the redshift of both the Soret and Q‐bands in 5 , compared to 3 , reflects the extension in the π‐electron conjugation; and 2) one fold higher in the molar absorption coefficient of intense band in 5 , compared to 4 , indicates the aromatic character. The absorption spectra of 5 remain unaltered upon changing the polarity of the solvent (Figure S21 in the Supporting Information), as observed in 2 b , proves that the conformation remain intact; however, 1 and 2 a in polar solvent adopt Möbius aromaticity .…”

“…It can be concluded that the Möbius aromaticity with effective π‐electron delocalization, even after protonation, is retained. A comparison of these results with protonated derivatives of 2 and 4 revealed that 2 adopts figure‐eight conformation with Möbius aromatic character, while 4 maintains the Hückel antiaromatic character as such in solution …”

“…This includes Rh I ‐ incorporated 24π N‐fused pentaphyrin, Pd II ‐coordinated N‐fused 28π hexaphyrin, 28π hexaphyrin bearing thienyl‐fused unit, benzopyrane‐fused ring, and diethylamine unit . Recently, we have demonstrated the synthesis of monofused core‐modified 32π‐heptaphyrins 4 , with six meso bridges, which maintains planarity both in freebase, as well as protonated state and follows [4 n ]π Hückel antiaromatic character; however, for the core‐modified expanded porphyrins in general and fused heptaphyrins in particular, the Möbius aromatic character is hitherto unknown in the literature. Herein, we report the synthesis of monofused core‐modified 32π heptaphyrins ( 5 and 6 ) with six meso ‐positions, which exhibits Möbius aromatic character in solution, as well as solid state, and retains the Möbius aromaticity in the protonated state.…”