Elsevier

Tetrahedron

Volume 61, Issue 14, 4 April 2005, Pages 3489-3495
Tetrahedron

Molecular structure of simple mono- and diphenyl meso-substituted porphyrin diacids: influence of protonation and substitution on the distorsion

https://doi.org/10.1016/j.tet.2005.01.128Get rights and content

Abstract

The crystal and molecular structures of three simple porphyrin diacids have been determined from X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. Di-meso-substituted [DPPH4]2+(CF3CO2)2 and, mono-meso-substituted [MPPH4]2+(CF3CO2)2 and [dedmMPPH4]2+(CF3CO2)2 porphyrin diacids show increasingly saddled core conformation. Some of the spectroscopic properties (NMR and UV–visible) of the porphyrin diacids are also discussed in terms of the observed structures.

Di-meso-substituted [DPPH4]2+(CF3CO2)2 and, mono-meso-substituted [MPPH4]2+(CF3CO2)2 and [dedmMPPH4]2+(CF3CO2)2 porphyrin diacids show increasingly saddled core conformation with a decrease of the peripheral substitution.

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Introduction

The conformational flexibility of the porphyrin macrocycle may play an important role in controlling the structure–function relationship in many heme proteins.1, 2 Several structural studies have explored the effects of non-planarity on the porphyrin macrocycle using highly substituted porphyrins such as dodecaphenyl- and dodecaalkylporphyrins3, 4 but much less work has been reported with simple porphyrins such as unsubstituted5 or mono-meso-substituted metalloporphyrins.6 Since the pioneering work of Stone and Fleicher,7 another approach to study the flexibility of the porphyrin core is based on protonating the porphyrin free bases to form N-diprotonated dications, also known as porphyrin diacids.8, 9, 10 Porphyrin diacids typically have non-planar structures with mainly saddle-type distorsions of the porphyrin core, as revealed by X-ray crystallography.7, 8, 9, 10, 11, 12, 13, 14

In this study, we have used X-ray crystallography to characterize the structure of a 5,15-diarylporphyrin diacid and two mono-arylporphyrin diacids as their trifluoroacetate salts. Although crystallographic structures showing non-planar conformational distorsions in a number of mono- and di-meso-substituted porphyrins have been reported, to our knowledge, there is no structural comparison to evaluate the peripheral substitution pattern in a class of relatively unstrained porphyrin diacids. In contrast with previously reported Nickel (II) meso-tert-butylporphyrins,6 an increase of the peripheral substitution leads to a slight decrease of the saddled distorsion. We have also incorporated 1H NMR and UV–visible results since these related data have been used, in some cases, to discuss the flexibility of the porphyrin core in solution.

Section snippets

Crystal structures of 1, 2 and 3

The porphyrin core of 5,15-(3,5-dimethoxyphenyl)porphyrin [DPPH4]2+(CF3CO2)2 1, 5-(3,5-dimethoxyphenyl)porphyrin [MPPH4]2+(CF3CO2)2 2 and 5-(3,4,5-trimethoxyphenyl)-13,17-diethyl-12,18-dimethylporphyrin [dedmMPPH4]2+(CF3CO2)2 3 is saddled as expected for a porphyrin diacid. ORTEP diagrams of 1, 2 and 3 are presented in Figure 1. The β-carbon atoms of each pyrrole are alternatively displaced above and below the mean porphyrin plane. The magnitudes can be seen in Figure 2, which are formal

Conclusion

The crystal structures of the three simple porphyrin diacids show that the distortion of the porphyrin core depends on the nature of the peripheral substituent. A slightly increased distortion in the saddled mono-phenylporphyrin diacids suggest that this difference is the result of the dissymmetry on the molecular conformation. This conclusion needs to be supported by molecular mechanics calculation.

General procedures

Methylene chloride, methanol and pentane were distilled from calcium hydride, magnesium and sodium benzophenone, respectively. Proton NMR spectra were recorded on a Bruker AV 500 (500 MHz) spectrometer at room temperature. Chemical shifts are reported in ppm and referenced to the protonated residual solvent. The porphyrin concentration used for NMR samples was 13 mM in CDCl3 solution (500 μl) with 1% added of trifluoroacetic acid. Spectra were assigned with the aid of HMBC. Visible spectra

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