Electrochemical reduction of l-(meso-tetraphenylporphyrin)-pyridinium cations

1-(M-meso-tetraphenylporphyrin)-4-R-pyridinium cations MTPP-β-(4-R-Py+) (M = H2, ZnII, NiII, CuII or PdII, R = H, phenyl or pyridine), formed by a pyridinium cation bearing a porphyrin, metalated or not, at the N-position, were synthesised electrochemically and studied by polarography, stationary and cyclic voltammetry, UV-Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium and the porphyrin rings are both electroactive species in the range of potentials investigated. The results allowed us to discriminate the distinct sites of the different charge transfers and to propose a mechanism for the primary electrochemical reduction step which corresponds to a one-electron transfer to the pyridinium cation. The redox behaviour of the pyridinium in these systems depends on the nature of the core of the porphyrin (free-base or metalated). The differences observed between the free-base and the metalated porphyrin systems were explained by differences in the distribution of the electronic density on the porphyrin ligand in the two types of molecules. In the metalated porphyrin-pyridinium systems, the measured reduction potentials of the pyridinium cation were correlated with the electronegativity χMP of the metal in the porphyrin core. The obtained correlation revealed the occurrence of strong coulombic interactions between the metal in the porphyrin and the pyridinium. The chemical reactivity of the electrochemically generated pyridinium radical depends on the substitution at the 4-position of the ring. The generation of a dimeric species following the first electron transfer to the pyridinium cation was detected by cyclic voltammetry.