SUBSTITUTED DEUTEROPORPHYRINS .V. STRUCTURES STABILITIES AND PROPERTIES OF NICKEL(2) COMPLEXES WITH AXIAL LIGANDS

Divalent nickel porphyrins in the presence of nitrogenous bases form paramagnetic species (II) which exhibit electronic spectra with absorption maxima at wavelengths markedly shifted from those found for the diamagnetic species (I) found in non-coordinating solvents. Absorption spectra in the Soret region in piperidine-chloroform solutions of nickel complexes of protoporphyrin IX and other deuteroporphyrin IX derivatives gave evidence that species II resulted from the addition of two axial piperidine ligands to a planar species I. Differences in porphyrin ring substituents such as found among heme moieties of heme proteins resulted in significant changes in both enthalpy and entropy differences for piperidine binding. Solvent interactions, although difficult to evaluate quantitatively, appear to contribute importantly to these thermodynamic parameters. Reciprocal electronic effects of porphyrin and axial ligands upon the stability of axial ligand binding were noted; the less basic the porphyrin or more basic the axial ligand, the more stable was species II. Steric interference with axial ligand binding exerted by groups either on the periphery of the porphyrin ring or on the axial ligand was indicated. Effects of ligand binding upon the porphyrin π system was observed in electronic spectra and in nuclear magnetic resonance spectra where delocalization of positive and negative spin density led to contact shifts of peripheral ring protons. These studies provide further evidence of the sensitivity of metalloporphyrin reactions and physical properties to changes in porphyrin, ligand, and media of relevance to structure-function relationships among porphyrin proteins and enzymes. © 1968, American Chemical Society. All rights reserved.