Electrochemical formation of bi- versus tetranuclear μ-oxo terpyridine manganese complexes in CH3CN. Influence of the terpyridine substituents

To complete the elucidation of the electrochemical properties of MnII-bis(terpyridine) complexes in CH3CN and evaluate the influence of the bulkiness of the terpy substituents, the oxidation processes of [MnII(L)2]2+ (L = terpy for 2,2′:6′,2″-terpyridine, pTol-terpy for 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine and tBu3-terpy for 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) have been investigated in aqueous (1 M) CH3CN solution. In this medium, exhaustive oxidations at 1.10–1.20 V versus Ag/Ag+ release two electrons per molecule of initial complex and lead to clean dimerization processes with the quantitative formation of the oxo-bridged binuclear [Mn2IVO2(L)2(H2O)2]4+ complex for L = tBu3-terpy and of the tetranuclear [Mn4IVO5(L)4(H2O)2]6+ complexes for L = terpy and pTol-terpy. The formation of the tetranuclear complex with the tBu3-terpy derivative is prevented by the steric hindrance induced by the bulkiness of the tert-butyl groups, as confirmed by molecular mechanics calculations, as well as by their strong electron-donating properties. All these electrogenerated multinuclear complexes have been fully characterized in solution by UV–vis and electron paramagnetic resonance (EPR) spectroscopy. A markedly improved chemical synthesis of [Mn4IVO5(terpy)4(H2O)2]6+ is also reported.