Cation-Dependent Self-assembly of Vanadium Polyoxoniobates

Reaction of Na7H­[Nb6O19]·15H2O with NaVO3·2H2O at 220 °C in the presence of NaHCO3 gives new bicapped α-Keggin vana­dodo­deca­niobate [VNb12O40­{NbO­(CO3)}2]13–, isolated and structurally characterized as Na9H4­[VNb12O40­{NbO­(CO3)}2]·37H2O (1). According to 51V NMR and ESI-MS data, this anion equilibrates in solution with [VNb12­O40]15– and oligomeric species that result from dissociation of the {NbO­(CO3)}+ fragments. In the presence of potassium, the same reaction gives [VxNb24­O76]n− (x = 4, n = 12 (2a); x = 3, n = 17 (2b)). The anions with x = 3 and 4 cocrystallize together, but exist as separate entities both in solid and in solution according to 51V MAS NMR and ESI-MS data.