Comparative Analysis of the Conformations of Symmetrically and Asymmetrically Deca- and Undecasubstituted Porphyrins Bearing Meso-Alkyl or -Aryl Groups Mathias O. Senge,* ,1a,b Craig J. Medforth, 1a Timothy P. Forsyth, 1a David A. Lee, 1a Marilyn M. Olmstead, 1a Walter Jentzen, 1c Ravindra K. Pandey, 1a John A. Shelnutt, 1c and Kevin M. Smith* ,1a Department of Chemistry, University of California, Davis, California 95616, Institut fu ¨r Organische Chemie (WE02), Freie Universita ¨t Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Fuel Science Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-0710 ReceiVed September 18, 1996 X 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 ( 1 H 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 meso- substituents were also investigated. Metal-free 5,15-disubstituted porphyrins with one aryl and one alkyl group showed considerably elongated porphyrin cores, whereas nickel(II) complexes of porphyrins with 5,10- or 5,10,15- substitution patterns showed very nonplanar structures consisting mainly of ruffle and saddle type distortions. Introduction The conformations of porphyrins are currently under active scrutiny with regard to the connection between macrocycle distortion and physicochemical properties. 2,3 The flexibility of the tetrapyrrole system was demonstrated in the early porphyrin crystal structures of Hoard, 4 and since then, a considerable body of information has been accumulated about how the conforma- tions of simple porphyrin compounds such as octaethylporphyrin (OEP, 1) and tetraphenylporphyrin (TPP, 2) can be influenced by metal, packing, and axial ligand effects. 5 The concept of conformationally flexible tetrapyrrole macrocycles has wide implications for the study of chromophore-protein interactions in ViVo. 2,3,6 Some years ago, it was realized that the confor- mational flexibility might modulate the biological properties of tetrapyrroles in ViVo, 2 and nonplanar tetrapyrrole conformations have been observed in the bacterial photosynthetic reaction center, 7 a photosynthetic antenna complex, 8 heme proteins, 9 methyl reductase, 10 and vitamin B 12 -dependent enzymes. 11 Physical studies have shown a direct correlation between macrocycle nonplanarity and properties such as spin delocal- ization, redox potential, and the positions of optical absorption X Abstract published in AdVance ACS Abstracts, February 15, 1997. 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