Synthesis, Crystal Structure, Electrochemistry and Electro-Catalytic Properties of the Manganese-Containing Polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6−

Robertha Howell,Pedro De Oliveira,Neus Vilà,Israël Mbomekallé,Anne-Lucie Teillout,Alain Walcarius,Jérôme Marrot

doi:10.3390/inorganics7020015

Abstract

We present the synthesis and structural characterization of the manganese-containing polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6− (1), obtained by reaction of MnCl2 with six equivalents of Na2WO4 in the presence of Zn(CH3COO)2 in acetate medium (pH 4.7). This has been assessed by various techniques (FTIR, TGA, UV-Visible, XPS, elemental analysis, single crystal X-ray and electrochemistry). Single-crystal X-ray analyses showed that, in the solid state, 1 forms a 2-D network in which [H2W12O42]10− fragments are linked in pairs via Mn2+ ions, leading to linear chains of the form [(Mn(H2O)3)2(H2W12O42)]n6n−. The connection between chains occurs also via Mn2+ ions which bind [H2W12O42]10− fragments belonging to two adjacent chains, forming an infinite 2-D network. A complete electrochemical study was done in aqueous solution where 1 is stable in the pH range 1 to 6. This complex undergoes multiple electron-transfer processes that lead to the electro-generation of manganese high oxidation state species that catalyse water electro-oxidation. 1 is also effective in the electro-catalytic reduction of nitrite and dioxygen.

Highlights

Polyoxometalates (POMs) constitute a class of inorganic materials whose properties may be modulated upon inserting chosen chemical elements in their scaffold
This method was already successfully used in the past in the case of the synthesis of the species
The Mn2+ centres are oxidised to the Mn4+ state in a single step which depends on the pH and on the composition of the electrolyte

Summary

Introduction

Polyoxometalates (POMs) constitute a class of inorganic materials whose properties may be modulated upon inserting chosen chemical elements in their scaffold. We have established multi-step synthesis protocols allowing to modulate the chemical composition of the equatorial cluster, which open the way for the targeted synthesis of a large set of new compounds [14,28–31] From this perspective, the compounds of the Keggin family [32], which have a higher diversity both in composition and in lacunary structures, are even more interesting. At least four different types of tungstic Keggin fragments may be cited which are prone to the formation of sandwich-type structures They formally result from the partial decay of the parent compound [XW12 O40 ]n − , leading to the formation of the following lacunary entities: [XW8 O31 ]n − , [XW9 O33 ]n − , [XW9 O34 ]n − or [XW10 O36 ]n −

Objectives

Methods

Results

Conclusion