Manganese complexes of mixed O, X, O-donor ligands (X = S or Se): synthesis, characterization and catalytic reactivity

Abstract

Complexes of manganese with two tridentate ligands, H₂L^Se and H₂L^S, containing [O, Se, O] or [O, S, O]-donor atoms, respectively, are described. The bisphenolate ligands yield complexes [Mn^III₂L^Se₂(μ-OCH₃)₂(CH₃OH)₂] 1, [Mn^III₂L^S₂(μ-OCH₃)₂(CH₃OH)₂] 2, Bu₄N[Mn^IIIL^Se₂] 3, Bu₄N[Mn^IIIL^S₂] 4, [Mn^IVL^Se₂] 5 and [Mn^IVL^S₂] 6. The ferrocene adduct of 5, i.e. [Mn^IVL^Se₂]·Fe(C₅H₅)₂5a is also described. The crystal structures of 1, 5a and 6 have been determined. The compounds have been thoroughly characterized by various physical methods, including IR, MS, UV-vis, EPR and magnetic susceptibility measurements. Low temperature electrochemical results at −25 °C strongly suggest that the electrochemical oxidations are not only ligand-centered, i.e. formation of phenoxyl radicals from the coordinated phenolates, but also metal-centered generating Mn(IV) species. The manganese(III) monomers 3 and 4 have been found to be oxidation catalysts with dihydrogen peroxide as the oxidant for epoxidation of alkenes, like styrene, stilbene and cyclohexene. The selenium containing ligand exhibits higher turnover numbers than the sulfur containing ligand.