Octahedral {Ta6I12} Clusters

Ta powder reacts with I-2 at 650 degrees C with the formation of Ta6I14, which belongs to the family of {M-6(mu-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)center dot xH(2)o (Ta-Ta 2.9322(6) angstrom, Ta-I 2.8104(7) angstrom, Ta-o 2.3430(5) angstrom). With DMF, [Ta6I12(DMF)(6)]I-2 center dot xDMF was isolated (Ta-Ta 2.9500(2) angstrom, Ta-I 2.8310(4) angstrom, Ta-O 2.2880(7) angstrom). Cyclic voltammetry of [Ta6I12(H2O)(6)](2+) shows two consecutive quasi-reversible one-electron oxidations (E-1/2 0.61 and 0.92 V vs Ag/AgCl). Reaction of Ta6I14 with Bu4NCN yields (Bu4N)(4)[Ta6I12(CN)(6)]center dot xCH(3)CN (Ta-Ta 2.9777(4) angstrom, Ta-I 2.8165(6) angstrom, Ta-C 2.2730(7) A). 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).