Niobium-substituted arsenotungstates: Controllable transformation between monomers and tetramer

摘要

Three niobium-containing arsenotungstates have been synthesized for the first time: peroxoniobium-containing Cs_5K[AsW_9(NbO _2)_3O_(37)]·7H_2O (1), peroxo-free Cs_(5.5)Na_(0.5)[AsW_9Nb_3O _(40)]·17H_2O (2), and tetramer Cs_(10)K _3H_3[As_4W_(36)Nb_(16)O _(166)]·28H_2O (3). Both monomers 1 and 2 can assemble into tetramer 3 in acidic conditions, and 3 can transform to 1 or 2 by adding H_2O_2 or adjusting the pH value. Comparing the Nb-containing POM systems of A-α-XW_9 (X = Si, Ge, As), although they are different only in heteroatoms, there are remarkable differences in reaction conditions and reactivity. The ~(183)W 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.