The 3+3-type synthesis of a
pyrazole-based expanded
porphyrin 22 H4, a hexaphyrin analogue named
Siamese-twin porphyrin, and its homobimetallic 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
porphyrin-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 spectroscopic evidence as well as structural parameters 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 complexes, each
metal ion is coordinated in a square-planar fashion by a dianionic,
porphyrin-like {N4} binding pocket. The solid-state structures of the dication and both
metal complexes were elucidated by single-crystal diffractometry. 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 permitted resolution of these enantiomeric helimers by HPLC on a chiral phase. The absolute stereostructures of 22 H6(2+), 22 Ni2, and 22 Cu2 were assigned by a combination of experimental 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 macrocycles lays the foundation for the study of the interactions of the
metal centers within their bimetallic complexes.