Exploring the redox reactivity of magnesium porphine. Insight into the origins of electropolymerisation

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 X-ray diffraction studies revealed multiple intermolecular associations in the crystal between neighbouring units of 2, namely hydrogen bonds and CH center dot center dot center dot pi. The electrochemical behaviour of 1 was examined by means of cyclic voltammetry. In oxidation, two well-defined and distinct steps are assigned to macrocycle concerned electron transfers yielding initially the pi-cation radical and pi-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 origin of the electropolymerisation. Electrolyses were performed at the potential of the pi-cation radical generation. Investigation of the electrolysed solution by UV-vis 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-vis spectrum. Finally, the mechanism of oxidative oligomerisation is discussed, from the point of view of the initial steps of the electropolymerisation.