Nuclearity versus oxidation state in the catalytic efficiency of MnII/III azo Schiff base complexes: computational study on supramolecular interactions and phenoxazinone synthase-like activity

Abstract

A novel mononuclear Mn(III) complex, [Mn^III(L¹)Cl(H₂O)]·H₂O (1), and a tetranuclear Zn(II)–Mn(II) complex, [{Zn^II₂(L²)₂Cl₂}Mn^II₂(μ_1,1-N₃)₂(H₂O)₂]·2H₂O (2), have been synthesized involving azo Schiff base ligands, viz. H₂L¹ = (E)-6,6′-((1E,1′E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol) and H₂L² = (E)-6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol), respectively. The solid-state structures were determined by single crystal X-ray crystallography. In complex 1, the Mn(III) centre adopts slightly distorted octahedral geometry, while in complex 2 the Mn(II) centre residing in the outer core of the ligand adopts a distorted pentagonal bipyramidal geometry. In complex 1, the chloride ion simply acts as a terminal coligand, while in complex 2, azide ions bind the metal centres in an end-on bridging fashion to produce a tetranuclear complex. The phenoxazinone synthase-like activity of both complexes has been examined and a detailed investigation of the structure–property correlation has been performed. Whereas the mononuclear complex 1 exhibits significant phenoxazinone activity, complex 2 is almost inactive, although in both complexes labile sites are available at manganese centres for substrate binding. The present work therefore highlights the importance of higher oxidation states of manganese over nuclearity for the development of better in vitro catalysts. In addition, extensive efforts have been made to visualize and quantify all supramolecular interactions present in 1 and 2.