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

Abstract

Three novel polyoxoniobates, KNa(2)[Nb(24)O(72)H(21)]·38H(2)O (1), K(2)Na(2)[Nb(32)O(96)H(28)]·80H(2)O (2), and K(12)[Nb(24)O(72)H(21)](4)·107H(2)O (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 [Nb(7)O(22)](9-) fundamental building units. Compound 1 is the first isolated {Nb(24)O(72)} cluster, featuring three heptaniobate clusters linked in a ring by three additional NbO(6) octahedra, while compound 2 is the largest isopolyoxoniobate cluster reported to date, consisting of four heptaniobate clusters linked by four additional NbO(6) octahedra. Compound 3 is the largest solid aggregation of polyoxoniobates, assembled by four {KNb(24)O(72)} 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 Nb(24) fragments. The UV-vis diffuse reflectance spectra of these powder samples indicate that the corresponding well-defined optical absorption associated with E(g) 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 H(2) evolution activities were observed for these compounds with Co(III)(dmgH)(2)pyCl as a cocatalyst and TEA as a sacrificial electron donor.