Tripyrrin-armed isosmaragdyrins: synthesis, heterodinuclear coordination, and protonation-triggered helical inversion.

Oxidative ring closure of linear oligopyrroles is one of the synthetic approaches to novel porphyrinoids with dinuclear coordination sites and helical chirality. The spatial arrangement of the pyrrolic groups of octapyrrole () affected the position of the intramolecular oxidative coupling of the pyrrolic units; tripyrrin-armed isosmaragdyrin analogue () containing a β,β-linked bipyrrole moiety was synthesized regioselectively in a high yield by using FeCl. Ni-coordination at the armed tripyrrin site of allowed the formation of diastereomeric helical twisted complexes ( and ) and succeeding Cu-coordination at the macrocyclic core afforded heterodinuclear Ni/Cu-complexes ( and ). Each of them comprised a pair of separable enantiomers, exhibiting - and -helices, respectively. Notably, diastereomeric interconversion from to was quantitatively achieved as a consequence of helical transformation under acidic conditions.