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

The role of steric crowding of peripheral substituents in causing nonp lanar distortions of the porphyrin macrocycle has been investigated in a series of nickel(II) meso-nitrooctaethylporphyrins as models for pr otein-induced nonplanar distortions of metal-tetrapyrrole cofactors. U V-visible absorption spectra, resonance Raman spectra, nuclear magneti c resonance spectra, and molecular mechanics calculations are given fo r 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 freque ncies of the structure-sensitive Raman lines are found to shift to low er 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 al l except tetranitrooctaethylporphyrin show spectroscopic evidence of h eterogeneity resulting from the presence of conformers with differing degrees of nonplanarity. Evidence from UV-visible absorption spectra, nuclear magnetic resonance spectra, and molecular mechanics calculatio ns suggests that nickel tetranitrooctaethylporphyrin in solution may e xhibit conformers with different types of nonplanar distortions which however do not differ in the relative degree of distortion. We find th at 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 incr easing electron depletion of the porphyrin ring as the number of nitro substituents increases.