Electrochemical properties of [Mn III(terpy)(N 3) 3] (terpy=2,2′:6′,2″-terpyridine) in CH 3CN : Electrogeneration of dimanganese(II) di-μ-azido and dimanganese(IV) di-μ-oxo complexes

Abstract

The electrochemical behavior of [Mn III(terpy)(N 3) 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 (CH 3CN) solution. In CH 3CN containing either 0.1 M tetra- n-butylammonium perchlorate (Bu 4NClO 4) or tetraethylammonium trifluoroacetate (Et 4NCF 3CO 2) 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 CH 3CN containing 0.1 M Bu 4NClO 4 or 0.1 M Et 4NCF 3CO 2 lead to the quantitative conversion of 1 into the bridging N 3 − dimanganese(II) complex, [(N 3)(terpy)Mn II(μ-N 3) 2Mn II(terpy)(N 3)] ( 2). This transformation is chemically reversible by an oxidation process. Controlled-potential oxidation at 0.8 V of a solution of 1 in CH 3CN+0.1 M Bu 4NClO 4 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 CH 3CN+0.1 M Et 4NCF 3CO 2, the stable dimanganese(IV) di-μ-oxo complex [(CF 3CO 2)(terpy)Mn IV(μ-O) 2Mn IV(terpy)(CF 3CO 2)] 2+ ( 3) is formed quantitatively owing to the presence of an excess of the coordinating CF 3CO 2 − anions and residual H 2O in the CH 3CN solution.