COMPARATIVE-ANALYSIS OF THE CONFORMATIONS OF SYMMETRICALLY AND ASYMMETRICALLY DECASUBSTITUTED AND UNDECASUBSTITUTED PORPHYRINS BEARING MESO-ALKYL OR MESO-ARYL GROUPS

Conformational analysis of highly substituted porphyrins has potential implications for modeling the behavior of macrocycles in tetrapyrrole -containing protein complexes and during catalytic reactions. In order to study the influence of different substituent patterns on the confo rmation of the porphyrin macrocycle, a series of metal free and nickel (II) decasubstituted porphyrins bearing aryl or ethyl groups at opposi te meso positions and alkyl groups at the pyrrole positions have been synthesized and characterized by X-ray crystallography. Crystal struct ures of the flee-base porphyrins with 5,15-diaryl substituents showed negligible out-of-plane distortion but a large amount of in-plane dist ortion along the 5,15-axis accompanied by large bond angle changes sim ilar to those previously seen for related porphyrins with 5,15-dialkyl substituents. Nickel(II) complexes of the 5,15-diaryl-substituted por phyrins show planar or modestly nonplanar conformations, suggesting th at these complexes are not intrinsically nonplanar, whereas a complex with 5,15-diethyl substituents has a very ruffled conformation similar to those observed for related complexes with other metals. The nickel (II) complexes are also elongated along the 5,15-axis in a qualitative ly similar but less dramatic fashion than are the free-base porphyrins . Spectrosopic studies (H-1 NMR, optical, and resonance Raman spectros copy) suggest that conformations similar to those determined by X-ray crystallography are present in solution for the 5,15-dialkyl- and 5,15 -diaryl-substituted porphyrins. Several asymmetric nickel(II) and meta l-free deca- and undecasubstituted porphyrins containing both aryl and alkyl mesosubstituents were also investigated. Metal-free 5,15-disubs tituted porphyrins with one aryl and one alkyl group showed considerab ly elongated porphyrin cores, whereas nickel(II) complexes of porphyri ns with 5,10- or 5,10,15-substitution patterns showed very nonplanar s tructures consisting mainly of ruffle and saddle type distortions.