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

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 D_Mn = −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.