Two non-identical twins in one unit cell: characterization of 34π aromatic core-modified octaphyrins, their structural isomers and anion bound complexes

摘要

Four different core-modified planar 34π octaphyrins ( 10 , 11 , 13 , and 15 ) which exhibit rotational isomerism have been synthesized and characterized both in solution and solid states. Octaphyrins 10 , 11 and 13 show two inseparable isomers A and B which crystallize in the same unit cell. However, 15 forms two identical isomers of A . Structurally, the two isomers in 10 , 11 and 13 ( A and B ) are different only in the ring inversion of one of the thiophene or selenophene rings present in the terthiophene subunit of the macrocycle. In isomer A , the middle thiophene or selenophene rings are inverted, while in isomer B , the terminal thiophene rings are inverted. The ~(1) H NMR spectrum of these macrocycles shows peaks assignable to protons of both the isomers in toluene D _(8) . The single crystal structure analysis of 10 reveals the presence of both isomers 10A and 10B in a single unit cell with the P 2 _(1) / n space group. Both the isomers exhibit aromatic behaviour in the freebase form. Protonation of pyrrole nitrogens leads to exclusive formation of isomer B for 10 and 11 . However, both the isomers are present upon protonation of 13 where the central heterocyclic ring of terthiophene subunits has thiophene and selenophene rings. Octaphyrin 15 crystallizes in the P 2 _(1) / c space group and exclusively isomer A was formed in the reaction. Protonation of pyrrole nitrogens leads to significant increases in aromaticity as revealed by ~(1) H NMR chemical shift data. The NICS values calculated for the individual heterocyclic rings before and after protonation support such a conclusion. The AICD plots exhibit clockwise orientation of current density vectors suggesting the presence of diatropic ring current in the octaphyrins. Energy calculations at the M06L/CC-pVTZ//M06L/6-31G** level qualitatively account for exclusive stabilization of a particular isomer relative to the other upon protonation. To the best of our knowledge 10 represents the first example in expanded porphyrin chemistry where two different structural isomers crystallize in a single unit cell.