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 (E_1/2 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).