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– (1) as single product, which was isolated as Na+ and mixed Na+/K+ salts. Crystal structures were determined for Na6[(OH)­TeNb5O18]·15H2O (Na6-1) and K6Na­[Nb5.5­{Te­(OH)}0.5O18.5]·26H2O (K6Na-1). Formation of 1 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 1 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 (Cp*2Rh2-1) with two {Cp*Rh}2+ fragments capping the opposing faces of the Lindqvist anion.