Solution equilibria and redox reactivities of a dioxo-bridged manganese complex

Abstract

The dinuclear complex [(bipy)₂Mn^IIIO₂Mn^IV(bipy)₂]³⁺(bipy = 2,2′-bipyridyl)1 was stabilised in bipy-Hbipy⁺ buffer where it coexists with the diaqua derivative [(H₂O)₂Mn^IIIO₂Mn^IV(bipy)]³⁺2(equilibrium constant K^H). Reduction of 1 by NO₂^– has been found to be kinetically insignificant, but that of 2 is rapid, quantitative and follows the sequence Mn^IIIMn^IV[graphic omitted](Mn^III)₂ [graphic omitted]Mn^IIMn^III [graphic omitted] 2Mn^II. In the presence of an excess of NO₂^–, first-order kinetics was observed at 830 nm, but a biphasic profile at 525 nm. At 30.0 °C and I= 1.0 mol dm^–3, k₁K¹_H and k₂K²_H are 34.9 ± 0.8 and 3.3 ± 0.2 dm³ mol^–1 s^–1 respectively. Added bipy retards, while Mn²⁺ accelerates, the reaction. The solution equilibria are much more complex for 1 in HNO₃ and H₃PO₄, but the known solution chemistry of 1 in such different media may be correlated by a unified reaction scheme.