Supramolecular chemistry of an antiaromatic tetrapyrrole: zinc(II) norcorrole

Zinc norcorrole was prepared by metalation of freebase norcorrole in the presence of pyridine. Zinc norcorrole is isolated as its pyridine complex, which is square-pyramidal at zinc. The pi-conjugated norcorrole fragment is appreciably curved, as demonstrated by both X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy, the latter of which shows characteristic ring current deshielding effects. Ligand exchange of pyridine with the bidentate ligand DABCO results in the (ZnNc)2•DABCO sandwich complex, which is also characterised by crystallography and NMR spectroscopy. The NMR resonances of the axial ligands in all of the complexes demonstrate that the ring current in zinc norcorrole is approximately 40 nA/T, which is comparable in magnitude to that in porphyrin. Analysis of the ligand-exchange processes on addition of DABCO to ZnNc•pyridine show that zinc norcorrole coordinates to axial nitrogen-containing ligands with approximately 10^3-fold higher binding constants than analogous porphyrins.