Solid-State Structural Characterization of a Rigid Framework of Lacunary Heteropolyniobates

In our ongoing investigations of heteropolyniobate chemistry, a phase featuring decorated, A-type trivacant α-Keggin
ions linked by their charge-balancing sodium cations has been isolated and structurally characterized. This is the
first heteropolyniobate reported that has a true lacunary structure type. Na15[(PO2)3PNb9O34]·22H2O (1) [triclinic
space group P1̄ (No. 2); a = 12.242 (2) Å, b = 12.291 (3) Å, c = 22.056 (4) Å; α = 93.12 (3)°, β = 99.78 (3)°,
γ = 119.84 (3)°; Z = 4, V = 2799.2 (10) Å3] is composed of bilayers of the heteropolyanions alternating with
layers of hydrated Na+ cations. Sodium cations also bridge the clusters within their layers through Na−Ot−Nb,
Na−Ob−Nb2, and Na−Ot−P bonds (t = terminal and b = bridging). This phase is poorly soluble in water, suggesting
that it is more characteristic of a framework of linked heteropolyanions rather than a water-soluble heteropolyanion
salt. Two-dimensional solid-state 23Na multiple-quantum magic angle spinning (MAS) NMR of 1 reveals five distinctive
chemical and structural environments for sodium, which agrees with the crystallographic data. The 23Na and 1H
MAS NMR studies further illustrate the rigid and immobile nature of this framework of cations and anions.