Self-assembly and photocatalytic property of germanoniobate [H6Ge4Nb16O56]10-: encapsulating four {GeO4} tetrahedra within a {Nb16} cage.

An unprecedented organic-inorganic hybrid germanoniobate compound Na4[Cu(en)2(H2O)2]5[Na6Ge8Nb32O108H8(OH)4]·41H2O (1) was synthesized under the hydrothermal condition. In compound 1, the {Nb16} cage containing four {GeO4} tetrahedra in its internal cavity results in a heteropolyniobate anion [H4Ge4Nb16O54(OH)2](10-) ({Ge4Nb16}), which is connected by a {Na6} cluster into the first germanoniobate-based sandwich-type structure. Further, the sandwich germanoniobates are connected by [Na2Cu(en)2O6H8] groups into a porous network with one dimensional channels along the a-axis. Photocatalytic study reveals that compound 1 exhibits good photocatalytic activity for the degradation of methylene blue.