Synthesis and Characterization of [(OH)TeNb₅O₁₈]^6– in Water Solution, Comparison with [Nb₆O₁₉]^8–

Reaction of [Nb₆O₁₉]^8– with H₆TeO₆ in water gives telluropentaniobate [(OH)­Te­Nb₅O₁₈]^6– (<b>1</b>) as single product, which was isolated as Na⁺ and mixed Na⁺/K⁺ salts. Crystal structures were determined for Na₆[(OH)­TeNb₅O₁₈]·15H₂O (<b>Na</b>_<b>6</b><b>-1</b>) and K₆Na­[Nb_5.5­{Te­(OH)}_0.5O_18.5]·26H₂O (<b>K</b>_<b>6</b><b>Na-1</b>). Formation of <b>1</b> was monitored with electrospray ionization mass spectrometry (ESI-MS) and ¹²⁵Te NMR techniques. Capillary electrophoresis was used to calculate electrophoretic mobilities and radii of the anionic species in solutions of [(OH)­Te­Nb₅O₁₈]^6– and [Nb₆O₁₉]^8– in borate buffer. No condensation or degradation products were detected. Reactions of <b>1</b> with {Cp*Rh}²⁺ sources gives 1:1 and 2:1 hybrid polyoxometalate, which are present in solution as a mixture of isomers, as detected by ¹²⁵Te NMR. The isomerism is related to various possibilities of coordination of {Cp*Rh}²⁺ to different {M₃O₃} faces, relative to the unique Te atom. According to ESI-MS experiments in water and methanol, rapid redistribution of the organometallic fragments between the 1:1 and 2:1 complexes takes place. The 1:1 complexes are more stable in water, while 2:1 complexes dominate in methanol. X-ray structural analysis of the crystals isolated from 2:1 reaction mixture allowed identification of Na₃[{Cp*Rh}₂­TeNb₅O₁₉]·24H₂O (<b>Cp*</b>_<b>2</b><b>Rh</b>_<b>2</b><b>-1</b>) with two {Cp*Rh}²⁺ fragments capping the opposing faces of the Lindqvist anion