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)₃PNb ₉O₃₄]·22H₂O (1) [triclinic space group P1 (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) ų] 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-O_t-Nb, Na-O_b-Nb₂, and Na-O_t-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.