Niobium-substituted arsenotungstates: Controllable transformation between monomers and tetramer

Three niobium-containing arsenotungstates have been synthesized for the first time: peroxoniobium-containing Cs5K[AsW9(NbO2)3O37]·7H2O (1), peroxo-free Cs5.5Na0.5[AsW9Nb3O40]·17H2O (2), and tetramer Cs10K3H3[As4W36Nb16O166]·28H2O (3). Both monomers 1 and 2 can assemble into tetramer 3 in acidic conditions, and 3 can transform to 1 or 2 by adding H2O2 or adjusting the pH value. Comparing the Nb-containing POM systems of A-α-XW9 (X = Si, Ge, As), although they are different only in heteroatoms, there are remarkable differences in reaction conditions and reactivity. The 183W NMR spectra of 1, the UV–Vis spectra of 1–2 along with the IR spectra and thermal stability of 1–3 have been studied and compared with their Ge or Si analogs. The electrochemical properties of 1–3 have also been investigated and the cyclic voltammograms of 3 indicate good electrocatalytic activity towards the reduction of nitrite.