Octahedral {Ta6I12} Clusters.

Ta powder reacts with I2 at 650 °C with the formation of Ta6I14, which belongs to the family of {M6(μ-X)12} clusters. It undergoes aquation with the formation of the intensely colored [Ta6I12(H2O)6]2+. The crystal structure was determined for [Ta6I12(H2O)6](BPh4)2·xH2O (Ta-Ta 2.9322(6) Å, Ta-I 2.8104(7) Å, Ta-O 2.3430(5) Å). With DMF, [Ta6I12(DMF)6]I2·xDMF was isolated (Ta-Ta 2.9500(2) Å, Ta-I 2.8310(4) Å, Ta-O 2.2880(7) Å). Cyclic voltammetry of [Ta6I12(H2O)6]2+ shows two consecutive quasi-reversible one-electron oxidations (E1/2 0.61 and 0.92 V vs Ag/AgCl). Reaction of Ta6I14 with Bu4NCN yields (Bu4N)4[Ta6I12(CN)6]·xCH3CN (Ta-Ta 2.9777(4) Å, Ta-I 2.8165(6) Å, Ta-C 2.2730(7) Å). Quantum chemical calculations reproduce well the experimental geometry of the aqua complex and show the essentially Ta-centered nature of both the HOMO and LUMO. The long-term stability of [Ta6I12(H2O)6]2+ solutions can be greatly enhanced in the presence of polystyrenesulfonate (PSS), which forms nanoparticle associates with the aqua complex in water (ca. 1 cluster per 3 PSS monomeric units).