Rotational Isomers of <i>N</i>-Alkylpyridylporphyrins and Their Metal Complexes. HPLC Separation, ¹H NMR and X-ray Structural Characterization, Electrochemistry, and Catalysis of O₂^•^- Disproportionation

Abstract

Rotational (atropo-) isomers of Mn(III) meso-tetrakis(N-alkylpyridinium-2-yl)porphyrins and corresponding metal-free porphyrin ligands (where alkyl is methyl, ethyl, n-butyl, n-hexyl) and Zn(II) meso-tetrakis(N-methyl(ethyl,n-hexyl)pyridinium-2-yl)porphyrins were separated and isolated by reverse-phase HPLC. The identity of the rotational isomers of metal-free meso-tetrakis(N-methylpyridinium-2-yl)porphyrin was established by 1H NMR spectra and by the crystal structure of the fastest eluting fraction (Rf = 7.7%, Rw = 9.2%, P21/c, Z = 8, a = 14.2846(15) Å, b = 22.2158(24) Å, c = 29.369(3) Å, β = 95.374(2)°) which, in accordance with 1H NMR interpretation, proved to be the αβαβ isomer. This result, together with elution intensity patterns, was used to identify the fractions of other Mn(III)-porphyrins, Zn(II)-porphyrins, and corresponding metal-free ligands in the series. All of the atropoisomers were inert toward isomerization which was not observable for 30 days at room temperature and reached only 50% in 16 days at 90 °C in the case of the Mn(III)-ethyl analogue. However, a complete freeze-dry removal of the mobile phase from the HPLC fractions caused an almost 100% isomerization. The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, as a mixture of atropoisomers (AEOL-10113), has been shown to offer protection in oxidative stress injury ascribed to its high reactivity toward superoxide (kcat = 5.8 × 107 M-1 s-1) as a consequence of its favorable redox potential (E1/2 = +228 mV vs NHE). In this work, the atropoisomers were found to have similar redox potentials ranging from +240 to +220 mV, to be similarly potent catalysts of O2•- disproportionation (dismutation), with kcat ranging from 5.5 × 107 to 6.8 × 107 M-1 s-1, and not to preferentially bind to biological tissue.