Self-assembly of [PNbxW12−xO40]n− Keggin anions – a simple way to mixed Nb–W polyoxometalates

Abstract

This paper summarizes preparations of mixed phosphoniobotungstates [PNb_xW_12−xO₄₀]^(3+x)− (x = 1–3) in two different ways: (i) incorporation of Nb into pre-existing lacunary phosphotungstates and (ii) self-assembly reactions. A niobium oxalate complex as the source of niobium is used. The reaction products were monitored by ³¹P and ¹⁸³W NMR in solution as well as by ³¹P MAS NMR techniques for precipitated samples. Single-substituted species [PNbW₁₁O₄₀]⁴⁻ forms rapidly and selectively from lacunary [PW₁₁O₃₉]⁷⁻ with one equivalent of [NbO(C₂O₄)₂(H₂O)₂]⁻. With the increasing amount of the niobium oxalate the highest number of Nb atoms was found to be three per Keggin anion, but the disubstituted [PNb₂W₁₀O₄₀]⁵⁻ was always the preferable product whatever the tungsten reagent was. The reaction products with x = 2 and 3 are present as mixtures of all possible isomers, but the ratio between the individual isomers depends on the synthetic pathway.