Energetics and Structural Consequences of Axial Ligand Coordination in Nonplanar Nickel Porphyrins

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins
have been investigated with UV−visible absorption spectroscopy, resonance Raman spectroscopy, X-ray
crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition
analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding
affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of
the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered
affinity, resulting from the large energies required to expand the core and flatten the porphyrin to
accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices
and the function of heme proteins and methyl-coenzyme M reductase.