STRUCTURAL HETEROGENEITY AND COORDINATION CHEMISTRY OF NICKEL(II) OCTAETHYL-MESO-NITROPORPHYRINS

The role of steric crowding of peripheral substituents in causing nonplanar distortions of the porphyrin macrocycle has been investigated in a series of nickel(II) meso-nitrooctaethylporphyrins as models for protein-induced nonplanar distortions of metal-tetrapyrrole cofactors. UV-visible absorption spectra, resonance Raman spectra, nuclear magnetic resonance spectra, and molecular mechanics calculations are given for nickel(II) derivatives of 5-mono-, 5,15-di-, 5,10-di-, 5,10,15-tri-, and 5,10,15,20-meso-nitro-substituted octaethylporphyrins. The frequencies of the structure-sensitive Raman lines are found to shift to lower energies in response to sterically induced nonplanar distortion of the porphyrin macrocycle rather than in response to the addition of a strong electron-withdrawing nitro substituent. The Raman spectra of all except tetranitrooctaethylporphyrin show spectroscopic evidence of heterogeneity resulting from the presence of conformers with differing degrees of nonplanarity. Evidence from UV-visible absorption spectra, nuclear magnetic resonance spectra, and molecular mechanics calculations suggests that nickel tetranitrooctaethylporphyrin in solution may exhibit conformers with different types of nonplanar distortions which however do not differ in the relative degree of distortion. We find that the effect of increasing distortion on the chemical reactivity with axial ligating agents, which normally acts to lower the affinity, is overcome in the case of the nitrooctaethylporphyrin series by the increasing electron depletion of the porphyrin ring as the number of nitro substituents increases.