Exploring the redox reactivity of magnesium porphine. Insight into the origins of electropolymerisation
Résumé
Magnesium(II) porphine, MgP (1), was synthesised according to the Lindsey procedure allowing to isolate and crystallise 1-formyldipyrromethane (2) as a synthetic intermediate. Unprecedented Xray diffraction studies revealed multiple intermolecular associations in the crystal between neighbouring units of 2, namely hydrogen bond and CH … π. The electrochemical behaviour of 1 was examined by means of cyclic voltammetry. In oxydation, two well-defined and distinct steps are assigned to macrocycle concerned electron transfers yielding initially the π-cation radical and π dication, respectively. The highly reactive dication condenses neutral magnesium porphine to form a diprotonated di-isoporphine species, which is assumed to be a key intermediate at the orig in of the electropolymerisation. Electrolyses were performed at the potential of the π-cation radical generation. Investigation of the electrolysed solution by UV-visible spectroscopy and Maldi-tof spectrometry revealed the presence of several oligoporphyrins, with diporphine as an important product. In all the oligomers series, extensive demetallation is evidenced, due to the increasing acidity of the medium as oligomerisation progresses. This demetallation could be prevented by addition of 2,6-lutidine as a base prior to electrolysis. In the oligomer series, the porphine units seem to be connected through meso-meso bonds, an argument supported by certain features of the UV-visible spectrum. Finally, the mechanism of oxidative oligomerisation is discussed, from the point of view of the initial steps of the electropolymerisation.
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