Heterodimers and -trimers of meso-tetra-(isophthalicacid)-porphyrin octaanions with meso- and β-tetramethylpyridinium-porphyrin tetracations and their manganese complexes in water. Electrochemistry, spectroelectrochemistry and fluorescence quenching

doi:10.1016/S0022-0728(00)00538-6

Journal of Electroanalytical Chemistry

Volume 503, Issues 1–2, 20 April 2001, Pages 1-14

https://doi.org/10.1016/S0022-0728(00)00538-6 Get rights and content

Abstract

Non-covalent cofacial heterodimers and -trimers between meso-tetraphenyl-octacarboxylate-porphyrin and β-tetracationic porphyrins have been prepared in bulk water. They are held together by Coulomb interactions between four or eight β-methylpyridinium and meso-phenylcarboxylate ion pairs. The observation of the UV–vis absorption titration indicated quantitative trimerization at concentrations >10⁻⁶ M. The equilibrium constants in water were 2.3×10⁶ M⁻¹ for the dimerization and 1.7×10⁷ M⁻¹ for the conversion of the dimer to the trimer in water. Fluorescence of free base or zinc porphyrins was strongly diminished on heterodimerization and -trimerization by redox quenching. In the case of Mn(III) heterotrimer, electrochemistry and spectroelectrochemistry showed that all three Mn(III) ions were oxidized simultaneously to Mn(IV) at a potential close to 0.20 V versus Ag ∣ AgCl at pH 12. Electroreduction of the peripheral cationic Mn(III) porphyrins was achieved at −0.20 V versus Ag ∣ AgCl and gave the first multivalent trimers, namely Mn(II)P–Mn(III)P–Mn(II)P. The reduction of the central manganese porphyrin finally occurred at −0.60 V yielding Mn(II)P–Mn(II)P–Mn(II)P trimers. Heterotrimers Mn(III)P–H₂P–Mn(III)P and H₂P–Mn(III)P–H₂P were also studied as reference compounds for electrochemical measurements.

Introduction

Manganese constitutes an essential metal in several biological systems that are involved in electron-transfer reactions. Synthetic oxo–manganese porphyrin [1], [2], [3], [4], [5], [6], [7], [8], [9] and the related Schiff base complexes [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] have been applied in the past aimed at mimicking the natural photosynthetic system II [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Coupled face-to-face porphyrins attracted considerable interest due to their catalytic activity involving the oxidation of water to molecular oxygen. Only a few reports have been devoted to heterodimer formation in water in the case of cationic and anionic porphyrins [35], [36] or optimized phthalocyanine–porphyrin dimers [37]. We have recently reported on water-soluble porphyrin heterodimers between tetracationic and -anionic porphyrins [38], [39], [40]. Stereochemically fitting heterodimers were accomplished by combining meso- and β-substituted porphyrin ions in methanol+water (1:1) or water. Electrostatically linked face-to-face heterotrimers and -dimers between β-tetracationic and meso-tetraanionic porphyrins in eclipsed orientations have also been reported in bulk water at pH 12 [40] or adsorbed to gold and embedded in rigid membrane gaps [41].

In the following, we report on a new non-covalent manganese porphyrinate trimer with a central octaanionic porphyrin and two peripheral β- or meso-tetramethylpyridiniumporphyrins. The most important aspects of this investigation are: (i) formation of stable heterotrimers between the octaanionic and tetracationic porphyrins held together by Coulomb interaction (Scheme 2), (ii) comparison of the electrochemical behavior at different pHs with the modifying nature of axial ligands to central neutral ions, (iii) characterization of multivalent-manganese porphyrinates, e.g. Mn(II)P–Mn(III)P–Mn(II)P by electrochemistry and spectroelectrochemistry.

Section snippets

Experimental

(β-Tetraethyl-β-tetrakis(1-methyl-4-pyridinio)porphinato)manganese(III) pentachloride (1), (meso-tetrakis(1-methyl-4-pyridinio)porphinato)manganese(III) pentachloride (2) and (meso-tetrakis(3,5-dicarboxyphenyl)porphinato)manganese(III) chloride (3) were prepared following the method as reported previously [40], [41].

Results

β-Tetramethyl-β-tetrakis(1-methyl-4-pyridinium)porphyrin (1) was synthesized as a mixture of four regioisomers I–IV [40], [41]. Isomers I and II were removed by chromatography on silica gel. The mixture of isomers III and IV (Scheme 1) was used because of good solubility in water. Isomers I and II precipitated in water at high pH and/or in the presence of a conducting salt like KCl (0.2 M), whereas the mixture of isomers III and IV dissolved up to a concentration of about 1×10⁻² M in the

Discussion

The heterotrimer between two tetracationic and one octaanionic porphyrin was verified by the expected minima at a 2:1 ratio in Job plots (Fig. 1B). In the manganese porphyrin heterotrimers 1(Mn)–3(Mn)–1(Mn) or 2(Mn)–3(Mn)–2(Mn), we found, for example, two slopes in the Job plot, first for the heterodimers 1(Mn)–3(Mn) or 2(Mn)–3(Mn), then, upon addition of the second equivalent of tetracationic porphyrin, the 2:1 heterotrimer. The occurrence of the heterotrimer as an intermediate was also

Conclusion

It has not been possible to produce multivalent porphyrinate trimers (e.g. Mn(III)P–Mn(IV)P–Mn(III)P or Mn(IV)P–Mn(III)P–Mn(IV)P) by oxidation. Reduction, however, gave multivalent-manganese porphyrinates, e.g. Mn(II)P–Mn(III)P–Mn(II)P. The most promising candidates for water oxidation processes are presumably the Mn(IV) trimers, which have been described here for the first time. They are accessible by electrochemical oxidation at the modest potential of +0.35 V and could easily take up four

Acknowledgements

This work was supported by the European TMR research network Artificial Photosynthesis for Energy Production (Mn–Ru chemistry, contract CT 96-0031), by a Humboldt fellowship and the Deutsche Forschungsgemeinschaft (SFB 312 ‘Vectorial Membrane Processes’ and SFB 348 ‘Mesoscopic Systems’).

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Heterodimers and -trimers of meso-tetra-(isophthalicacid)-porphyrin octaanions with meso- and β-tetramethylpyridinium-porphyrin tetracations and their manganese complexes in water. Electrochemistry, spectroelectrochemistry and fluorescence quenching

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusion

Acknowledgements

Tetrahedron

Biochim. Biophys. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Angew. Chem.

Angew. Chem.

Angew. Chem.

Inorg. Chem.

J. Phys. Chem.

J. Chem. Phys.

J. Am. Chem. Soc.

Angew. Chem.

Chem. Lett.

J. Chem. Soc. Faraday Trans. I

Chem. Rev.

Coord. Chem. Rev.

Chem. Rev.

Acc. Chem. Res.

Prog. Inorg. Chem.