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

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.

Synthesis, structure and electrochemical characterization of an isopolytungstate (W4O16) held by MnII anchors within a superlacunary crown heteropolyanion {P8W48}

“…On the positive side of the potential scale, the CV of Mn 10 W 4 -P 8 W 48 exhibits an oxidation wave attributed to the Mn II centres, which is also the case for the majority of POMs containing Mn centres. 30,[48][49][50][51][52][53] The shape of this oxidation wave, as well as that of its associated reduction wave, clearly indicates the involvement of film adsorption and desorption phenomena taking place on the surface of the working electrode, probably made of manganese oxides.…”

“…This behaviour has been analysed in more detail in a recent study using a quartz crystal microbalance and XPS spectroscopy. 50 The formation of manganese oxides has already been observed and described for POMs having Mn II centres, 48,50,51,53 namely, a species having the same structure as Mn 10 W 4 -P 8 W 48 but containing just 4 Mn II centres. 30 If all the Mn II centres were removed from Mn 10 W 4 -P 8 W 48 , possibly as manganese oxides which would be deposited on the surface of the working electrode, the species remaining in solution would be {P 8 W 48 }, which is stable from pH 0 to 8.…”

“…This observation is in good agreement with other studies which have demonstrated that the Mn II centres could be oxidised beyond the oxidation state +IV and be involved in the electro-catalytic oxidation of water. 50,53 In this respect, the behaviour of the compound Mn 10 W 4 -P 8 W 48 is distinct from that of the homologous species having just four Mn centres, Mn 4 -P 8 W 48 . 54 Indeed, the oxidation peak potentials of the Mn II centre waves of the two compounds are very close, at pH 5: 0.82 V for Mn 10 W 4 -P 8 W 48 and 0.83 V for Mn 4 -P 8 W 48 .…”

“…The total weight loss is ca. 12%, which corresponds to the removal of the crystal water molecules (62) and the coordinated water molecules (50) (Fig. S2 †).…”

Polyoxometalates (POMs): from electroactive clusters to energy materials

“…Photo-induced water oxidation at pH 5 exhibited a TOF per Mn center of 0.71 × 10 −3 s −1 , TON up to 5.2, and a Φ of 1.7%. 222 This complex undergoes multiple electron-transfer processes that lead to the electro-generation of manganese high-oxidation-state species that catalyze water oxidation.…”