Electrochemical properties of [MnIII(terpy)(N3)3] (terpy=2,2′:6′,2″-terpyridine) in CH3CN: Electrogeneration of dimanganese(II) di-μ-azido and dimanganese(IV) di-μ-oxo complexes

The electrochemical behavior of [MnIII(terpy)(N3)3] (1) (terpy=2,2′:6′,2″-terpyridine), a structural model of the azide complex of the manganese superoxide dismutase (MnSOD), has been investigated in acetonitrile (CH3CN) solution. In CH3CN containing either 0.1 M tetra-n-butylammonium perchlorate (Bu4NClO4) or tetraethylammonium trifluoroacetate (Et4NCF3CO2) as supporting electrolytes, the cyclic voltammogram of 1 exhibits one quasi-reversible reduction wave at E1/2=−0.170 V versus Ag ∣ 10 mM Ag+ and one quasi-reversible oxidation wave at E1/2=+0.675 V. These are both one-electron waves, corresponding to the Mn(III)/Mn(II) and Mn(III)/Mn(IV) redox couples respectively. To evaluate the stability of the oxidized and reduced species of 1, exhaustive electrolyses have been carried out. Controlled-potential reductions at −0.35 V of solutions of 1 in CH3CN containing 0.1 M Bu4NClO4 or 0.1 M Et4NCF3CO2 lead to the quantitative conversion of 1 into the bridging N3− dimanganese(II) complex, [(N3)(terpy)MnII(μ-N3)2MnII(terpy)(N3)] (2). This transformation is chemically reversible by an oxidation process. Controlled-potential oxidation at 0.8 V of a solution of 1 in CH3CN+0.1 M Bu4NClO4 produces a new mononuclear Mn(IV) complex characterized by electron paramagnetic resonance spectroscopy, which is stable only at or below −15°C. If this oxidation is conducted in CH3CN+0.1 M Et4NCF3CO2, the stable dimanganese(IV) di-μ-oxo complex [(CF3CO2)(terpy)MnIV(μ-O)2MnIV(terpy)(CF3CO2)]2+ (3) is formed quantitatively owing to the presence of an excess of the coordinating CF3CO2− anions and residual H2O in the CH3CN solution.