Direct and improved synthesis of the tri-nickel sandwich-type polyoxoanion [Ni3Na(H2O)2(PW9O34)2]11−

The direct use of preformed B,α-Na9PW9O34 was successful in improving the synthesis of Na11[Ni3Na(H2O)2(PW9O34)2]·14H2O. The complex was characterized by elemental analysis, IR spectroscopy, magnetic properties and cyclic voltammetry. The yield was increased to 30%, which is four times more than that obtained in the initial synthesis. The delay before the appearance of the first crystals is drastically shortened, from months to hours, and these crystals are predominantly those of the yellow compound of interest. Keys to these improvements are the use of freshly prepared B,α-Na9PW9O34 through prolonged heating of A,β-Na9PW9O34 and the increase of the concentrations of the reactants in a close-to-neutral pH medium. Starting the synthesis with several concentrations of preformed A,β-Na9PW9O34 revealed the importance of the concentrations of reactants in speeding up the formation of crystals and suggested strongly that the [A-PW9O34]9−→[B-PW9O34]9− isomerization might be the slow step in the whole process. Cyclic voltammograms as well as X-ray diffraction structure determination confirm that the synthesized compound is indeed Na11[Ni3Na(H2O)2(PW9O34)2]·14H2O. A study of the magnetic properties of Na11[Ni3Na(H2O)2(PW9O34)2]·14H2O indicates that the three nickel centres exhibit ferromagnetic exchange interactions.