Electrochemical properties of [Mn^III(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 [Mn^III(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 E_1/2 = - 0.170 V versus Ag 10 mM Ag⁺ and one quasi-reversible oxidation wave at E_1/2 = + 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)Mn^II(μ-N₃)₂Mn ^II(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)Mn^IV(μ-O)₂Mn ^IV(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.