Manganese(III)-Containing Wells–Dawson Sandwich-Type Polyoxometalates: Comparison with their Manganese(II) Counterparts

We present the synthesis and structural characterization, assessed by various techniques (FTIR, TGA, UV–vis, elemental analysis, single-crystal X-ray diffraction for three compounds, magnetic susceptibility, and electrochemistry) of five manganese-containing Wells–Dawson sandwich-type (WDST) complexes. The dimanganese(II)-containing complex, [Na2(H2O)2MnII2(As2W15O56)2]18– (1), was obtained by reaction of MnCl2 with 1 equiv of [As2W15O56]12– in acetate medium (pH 4.7). Oxidation of 1 by Na2S2O8 in aqueous solution led to the dimanganese(III) complex [Na2(H2O)2MnIII2(As2W15O56)2]16– (2), while its trimanganese(II) homologue, [Na(H2O)2MnII(H2O)MnII2(As2W15O56)2]17– (3), was obtained by addition of ca. 1 equiv of MnCl2 to a solution of 1 in 1 M NaCl. The trimanganese(III) and tetramanganese(III) counterparts, [MnIII(H2O)MnIII2(As2W15O56)2]15– (4) and [MnIII2(H2O)2MnIII2(As2W15O56)2]12– (6), are, respectively, obtained by oxidation of aqueous solutions of 3 and [MnII2(H2O)2MnII2(As2W15O56)2]16– (5) by Na2S2O8. Single-crystal X-ray analyses were carried out on 2, 3, and 4. BVS calculations and XPS confirmed that the oxidation state of Mn centers is +II for complexes 1, 3, and 5 and +III for 2, 4, and 6. A complete comparative electrochemical study was carried out on the six compounds cited above, and it was possible to observe the distinct redox steps MnIV/III and MnIII/II. Magnetization measurements, as a function of temperature, confirm the presence of antiferromagnetic interactions between the Mn ions in these compounds in all cases with the exception of compound 2.