Combined HPLC-ICP-AES technique as an informative tool for the study of heteropolyniobates†
The combined HPLC-ICP-AES technique was applied to study formation of tungstoniobates and phosphomolybdoniobates. Reaction of potassium hexaniobate with tungstic acid results in the formation of mixed Nb/W Lindqvist hexametalates. The metal ratio depends on the relative amount of H2WO4 and reaction time. The most favored species are [Nb4W2O19]6− and [Nb3W3O19]5− which equilibrate with hexaniobate. Formation of [Nb2W4O19]4− was also observed; its share gradually increases with increasing amount of tungstic acid and decreasing pH. Reaction of tungstic acid with telluropentaniobate [TeNb5O19]7− results only in substitution of W for Te producing the same mixed Nb/W Lindqvist anions, but with different distribution of the individual species. The reaction of phosphomolybdic acid (H3PMo12O40) with a niobium oxalate complex, [NbO(C2O4)2(H2O)2]−, leads to the formation of mixed P/Nb/Mo Keggin anions, unambiguously detected for the first time. Only [PMo11NbO40]4− and [PMo10Nb2O40]5− were observed over the whole concentration range. No trisubstituted [PMo9Nb3O40]6− was detected in the substitution reaction, but is accessible from the self-assembly reaction of the niobium oxalate complex, phosphoric acid, and molybdate.
- This article is part of the themed collection: Equilibrium Solution Coordination Chemistry