Solid-state Structures and Solution Behavior of Alkali Salts of the $$[\hbox{Nb}_{6}\hbox{O}_{19}]^{8-}$$ Lindqvist Ion

Abstract

The hexaniobate Lindqvist ion \([\hbox{Nb}_{6}\hbox{O}_{19}]^{8-}\) has long been known as the dominant specie in alkaline niobium oxide solutions. Recent advances in heteropolyniobate chemistry continue to be greatly aided by use of \([\hbox{Nb}_{6}\hbox{O}_{19}]^{8-}\) alkali salts as soluble precursors; in particular, potassium, sodium and lithium hexaniobate salts. We report here the solid-state characterization and solution behavior of Li, K, Rb and Cs Lindqvist \([\hbox{Nb}_{6}\hbox{O}_{19}]^{8-}\) salts. Synthesis and single-crystal X-ray diffraction data is reported for nine new hexaniobate salts. These structures differ in the number of charge-balancing alkali cations, protonation of the clusters, relative arrangement of the clusters and alkali metal cations, amount of lattice water and its mode of interaction with other lattice species. Trends of alkali-cluster bonding are observed as a function of alkali radius. Protonation of the clusters in the solid-state is influenced by the method of crystallization of the \([\hbox{Nb}_{6}\hbox{O}_{19}]^{8-}\) salt. Lability of the cluster oxygens is observed by solution 17O NMR experiments. Rates of isotopic enrichment of the bridging oxygen, terminal oxygen and bridging hydroxyl cluster sites are compared for aqueous solutions of Li, K, Rb and Cs hexaniobate salts. Parameters influencing the oxo-ligand exchange rates of the salts are discussed relative to their use as heteropolyniobate precursors.