Comparative Analysis of the Conformations of Symmetrically and Asymmetrically Decaand Undecasubstituted Porphyrins Bearing Meso-Alkyl or -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 conformation of the porphyrin macrocycle, a series of metal free and nickel(II) decasubstituted porphyrins bearing aryl or ethyl groups at opposite meso positions and alkyl groups at the pyrrole positions have been synthesized and characterized by X-ray crystallography. Crystal structures of the free-base porphyrins with 5,15-diaryl substituents showed negligible out-of-plane distortion but a large amount of in-plane distortion along the 5,15-axis accompanied by large bond angle changes similar to those previously seen for related porphyrins with 5,15-dialkyl substituents. Nickel(II) complexes of the 5,15-diaryl-substituted porphyrins show planar or modestly nonplanar conformations, suggesting that 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 qualitatively similar but less dramatic fashion than are the free-base porphyrins. Spectrosopic studies (1H NMR, optical, and resonance Raman spectroscopy) 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 metal-free deca- and undecasubstituted porphyrins containing both aryl and alkyl mesosubstituents were also investigated. Metal-free 5,15-disubstituted porphyrins with one aryl and one alkyl group showed considerably elongated porphyrin cores, whereas nickel (D) complexes of porphyrins with 5,10- or 5,10,15-substitution patterns showed very nonplanar structures consisting mainly of ruffle and saddle type distortions.
Publication Source (Journal or Book title)
Senge, M., Medforth, C., Forsyth, T., Lee, D., Olmstead, M., Jentzen, W., Pandey, R., Shelnutt, J., & Smith, K. (1997). Comparative Analysis of the Conformations of Symmetrically and Asymmetrically Decaand Undecasubstituted Porphyrins Bearing Meso-Alkyl or -Aryl Groups. Inorganic Chemistry, 36 (6), 1149-1163. https://doi.org/10.1021/ic961156w