A Two-Dimensional Layered Heteropolyoxoniobate Based on Cubic Sn(IV)-Containing {Sn12Nb56O200} Cages with Good Proton Conductivity Property

The first example of a Sn(IV)-containing heteropolyoxoniobate K₆H₁₈[Cu(en)₂]₈{[Sn(OH)₂]₁₂ (HNb₇O₂₂)₈}·2en·88H₂O (1) is built from nanoscale high-nuclearity cubic {[Sn(OH)₂]₁₂(HNb₇O₂₂)₈} cluster and [Cu(en)₂]²⁺ complexes. The cubic {[Sn(OH)₂]₁₂(HNb₇O₂₂)₈} cage is composed of eight {Nb₇O₂₂} clusters and 12 SnO₆ octahedrons. The eight {Nb₇O₂₂} fragments are situated at the vertices of the cubic cage, while the 12 SnO₆ octahedrons are positioned along the edges of the cubic cage. The [Cu(en)₂]²⁺ complexes link the {[Sn(OH)₂]₁₂(HNb₇O₂₂)₈} clusters into two-dimensional (2D) (4,4) grid-like layers. The N-H···O hydrogen bonds between the [Cu(en)₂]²⁺ complexes and the {[Sn(OH)₂]₁₂(HNb₇O₂₂)₈} clusters link the layers to form a 3D supramolecular structure. Compound 1 exhibits a good proton conductivity of 3.1 × 10^-2 S cm^-1 at 85 °C and 98% relative humidity.