Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds

Abstract

This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). (1)H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8-xM6O19·nH2O (A = alkali ion; M = Nb, Ta). (93)Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), (93)Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The (93)Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D (23)Na MAS and 2D (23)Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The (23)Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR ((1)H, (23)Na, and (93)Nb).

High‐field95Mo and183W static and MAS NMR study of polyoxometalates

“…Keplerate‐type POMs for instance had never been succeeded to be characterized by NMR in solution despite the numerous trials even at high‐magnetic field . With the recent development of NMR instrumentation, challenging NMR nuclei like 93 Nb can be recorded in solid state, and resolved enough spectra of some polyoxoniobates were obtained combining high‐field and high‐rotation spinning speed …”

Thermochemical Measurements of Alkali Cation Association to Hexatantalate

“…A high-resolution single-crystal structure of Li-Ta 6 has never been possible, due to poor crystal quality and perhaps disorder of the hydrated lithium in the lattice. We surmise that Li-Ta 6 is not isostructural with the Li-Nb 6 analogue (Li 8 Nb 6 O 19 • 22H 2 O) due to the lack of a reported single-crystal structure for lithium hexatantalate [30,31]. However, Li is generally bonded to water in hydrated salt lattices, and this is reflected in its less exothermic formation enthalpy, compared to the salts of the heavier alkalis.…”

Extraction of Nb(v) by quaternary ammonium-based solvents: toward organic hexaniobate systems

“…This can also be put in perspective with the numerous hexaniobate salts that can be isolated in the solid-state. 9 The reactions mentioned above underline the complexity of the extraction of polyoxoniobate ions, even if only some extracted species may dominate the mass transfer process. Some features of the above phenomena are discussed in more detail hereafter.…”

“…Niobium, which only exhibits the +V oxidation state in aqueous solutions, is also soluble at a basic pH due to the formation of polyoxometalates. While various polyoxoniobates can be isolated in the solid state or at a near-neutral pH, 8,9 the aqueous chemistry of Nb(V) in basic media (at pH ≥ 9) is dominated by the Lindqvist ions, i.e., H 21 Few aqueous hexatantalate and hexaniobate complexes with ruthenium have also been recently reported. 22,23 The unusually high alkalinity required to form and stabilize the hexaniobate and hexatantalate ions also prevent their interaction in aqueous solutions with other polyoxometalates, like those of tungsten and molybdenum, which only exist in acidic media.…”