COMPARATIVE-ANALYSIS OF THE CONFORMATIONS OF SYMMETRICALLY AND ASYMMETRICALLY DECASUBSTITUTED AND UNDECASUBSTITUTED PORPHYRINS BEARING MESO-ALKYL OR MESO-ARYL GROUPS
Mo. Senge et al., COMPARATIVE-ANALYSIS OF THE CONFORMATIONS OF SYMMETRICALLY AND ASYMMETRICALLY DECASUBSTITUTED AND UNDECASUBSTITUTED PORPHYRINS BEARING MESO-ALKYL OR MESO-ARYL GROUPS, Inorganic chemistry, 36(6), 1997, pp. 1149-1163
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.