Self-Assembly and Photocatalytic Properties of Polyoxoniobates: {Nb24O72}, {Nb32O96}, and {K12Nb96O288} Clusters

Three novel polyoxoniobates, KNa2[Nb24O72H21]·38H2O (1), K2Na2[Nb32O96H28]·80H2O (2), and K12[Nb24O72H21]4·107H2O (3) with molecular triangle, molecular square, and cuboctahedral molecular cage geometries, respectively, have been successfully synthesized by conventional aqueous methods. All the compounds are built from [Nb7O22]9– fundamental building units. Compound 1 is the first isolated {Nb24O72} cluster, featuring three heptaniobate clusters linked in a ring by three additional NbO6 octahedra, while compound 2 is the largest isopolyoxoniobate cluster reported to date, consisting of four heptaniobate clusters linked by four additional NbO6 octahedra. Compound 3 is the largest solid aggregation of polyoxoniobates, assembled by four {KNb24O72} clusters joined by four K ions. To our knowledge, it is the first time these polyoxoniobate clusters have been crystallized with only alkali-metal counterions, thereby giving them the possibility of being redissolved in water. ESI-MS spectra indicate that compounds 1 and 2 remain structural integrity when the pure, crystalline polyanion salts are dissolved in water, while compound 3 is partially assembled into Nb24 fragments. The UV–vis diffuse reflectance spectra of these powder samples indicate that the corresponding well-defined optical absorption associated with Eg can be assessed at 3.35, 3.17, and 3.34 eV, respectively, revealing the presence of an optical band gap and the nature of semiconductivity with a wide band gap. UV-light photocatalytic H2 evolution activities were observed for these compounds with CoIII(dmgH)2pyCl as a cocatalyst and TEA as a sacrificial electron donor.