Manganese catalysed reduction of dioxygen to hydrogen peroxide: structural studies on a manganese(III)–catecholate complex

Abstract

The complex [Na] 5[Mn(3,5-(SO 3) 2Cat) 2]·10H 2O (H 2Cat=1,2-dihydroxybenzene) has been synthesised and characterised by X-ray diffraction (triclinic system, space group P 1 ̄ ). The characterisation of this complex supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short ‘bite distance’ of catecholate oxygen atoms (2.60 Å) which are unable to span the elongated coordination axes of the Jahn–Teller distorted Mn(III) ion. Hydrogen peroxide production is demonstrated with [Na] 5[Mn(3,5-(SO 3) 2Cat) 2]·10H 2O using dioxygen and hydroxylamine as substrates at pH 8.0 in aqueous solution under ambient conditions. In the presence of added Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt monohydrate) turn over frequency (TOF) in H 2O 2 (the number of moles of H 2O 2 per moles of manganese per hour) of ∼10 000 h −1 are obtained. The redox and structural motif characteristics imposed on this system by catecholate ligands together with the precise electronic requirements determined by the substituents on the catechol ring provide the enzyme-like characteristics of this system's unique ability to activate O 2 for reduction to hydrogen peroxide.