Reactivity of 9-anthracenecarboxylate in the presence of Mn(II) and Mn(III) ions: Biomimetic aerobic oxidative decarboxylation catalysed by a manganese(III) 2,2′-bipyridine complex

Abstract

The stability of the 9-anthracenecarboxylate (AnthCO2–) in the presence of manganese ions and 2,2′-bipyridine (bpy) was explored. Two Mn(II) species were isolated: a one-dimensional system [Mn(bpy)(AnthCO2)2]n (1) and a dinuclear [{Mn(bpy)(AnthCO2)}2(μ-OH2)(μ-AnthCO2)2] compound (2), which was characterized by X-ray diffraction. Compound 2 can be obtained by hydrolysis of 1. These compounds present a specific electron paramagnetic resonance (EPR) signature and weak antiferromagnetic coupling, which is slightly stronger for the dinuclear complex (−4.8 cm−1) than for the chain (−1.3 cm−1). In the presence of Mn(III) ions AnthCO2– is not stable for long time. From the Mn(III) solution obtained by a comproportionation reaction between Mn(II) and Mn(VII) salts, first, a tetranuclear [Mn4(μ-AnthCO2)6(μ-O)2(bpy)2(ClO4)2] compound was isolated, showing the antiferromagnetic properties typical of a butterfly [Mn4O2]8+ core (J1 = −41.0 cm−1, J2 = −8.4 cm−1, J3 = −10.6 cm−1 and DMn = −3.7 cm−1). After that, the solution became yellow due to the reduction of the Mn(III) complex to a Mn(II) complex, and the decarboxylation and oxidation of the AnthCO2– ligand. This oxidation can lead to different compounds, depending on the reaction conditions (temperature and amount of O2): an ester, derived from the AnthCO2– and the intermediate 10-hydroxyanthrone, and the 9,10-anthraquinone.