Siamese‐Twin Porphyrin: A Pyrazole‐Based Expanded Porphyrin of Persistent Helical Conformation

The 3 + 3-type synthesis of a pyrazole-based expanded porphyrin 22 H4, a hexaphyrin analogue named Siamese-twin porphyrin, and its homo- bimetallic diamagnetic nickel(II) and paramagnetic copper(II) complexes, 22 Ni2 and 22 Cu2, are described. The structure of the macrocycle composed of four pyrroles and two pyrazoles all linked by single carbon atoms, can be interpreted as two conjoined porphyr- in-like subunits, with the two opposing pyrazoles acting as the fusion points. Variable-temperature 1D and 2D NMR spectroscopic analyses suggested a conformationally flexible structure for 22 H4. NMR and UV/Vis spectro- scopic evidence as well as structural pa- rameters proved the macrocycle to be non-aromatic, though each half of the molecule is fully conjugated. UV/Vis and NMR spectroscopic titrations of the free base macrocycle with acid showed it to be dibasic. In the com- plexes, each metal ion is coordinated in a square-planar fashion by a dianionic, porphyrin-like {N4} binding pocket. The solid-state structures of the dica- tion and both metal complexes were elucidated by single-crystal diffractom- etry. The conformations of the three structures are all similar to each other and strongly twisted, rendering the molecules chiral. The persistent helical twist in the protonated form of the free base and in both metal complexes per- mitted resolution of these enantiomeric helimers by HPLC on a chiral phase. The absolute stereostructures of 22 H6 2 + , 22 Ni2, and 22 Cu2 were as- signed by a combination of experimen- tal electronic circular dichroism (ECD) investigations and quantum-chemical ECD calculations. The synthesis of the first member of this long-sought class of expanded porphyrin-like macrocy- cles lays the foundation for the study of the interactions of the metal centers within their bimetallic complexes.