Unconventional, mild reductants in solid state chemistry: Synthesis of Group 5 and 6 hexanuclear and trinuclear cluster halides

The unconventional, mild, solid-state reductants bismuth, gallium, and Ga₂Cl₄ (i.e., Ga+[GaCl₄] −) have been used to synthesize hexanuclear cluster halides of Group 5 and 6 and trinuclear clusters of tantalum.; The reduction of MOCl₅ with Bi at 350°C in a sealed borosilicate ampule afforded the chloromolybdic acid, (H₃O)₂Mo ₆Cl₁₄(OH₂)₆, in 80% yield after sublimation of BiCl₃ and recrystallization of the solid-state reaction product from hydrochloric acid. Mo₆Cl₁₂ was obtained in virtually quantitative yield by thermolysis of the chloromolybdic acid.; The reduction of TaX₅ (X = Cl, Br) with gallium metal in the presence of an alkali halide (NaCl or KBr, respectively) in a sealed borosilicate ampule at 500°C (for TaCl₅) or 400°C (for TaBr₅), followed by aqueous Soxhlet extraction and subsequent treatment of the extract solution with hydrohalic acid, yielded Ta₆X₁₄·8H ₂O in 92% yield (X = Cl) or 80–84% yield (X = Br). Ga₂Cl ₄, a more convenient reductant than gallium because it is more readily dispersed throughout the solid-state reactant mixture, afforded Ta₆Cl ₁₄·8H₂O in 96% yield after solid-state reduction of TaCl₅ in the presence of NaCl at 500°C followed by Soxhlet extraction of the solid-state reaction product, and treatment of the aqueous extract with hydrochloric acid. The hexatantalum hydrate, Ta₆Cl ₁₄·8H₂O, was converted to the tetraalkylammonium salt, [N(CH₂Ph)(n-Bu)₃]₄Ta ₆Cl₁₈, in 88% yield via ligand exchange. [N(CH₂Ph)( n-Bu)₃]₄Ta₆Cl₁₈, the first tetraalkylammonium salt of Ta₆Cl₁₈4− , was structurally characterized by X-ray diffractometry and found in two crystalline morphologies, one solvated and the other solvent-free.; Ga₂Cl₄ was found to reduce TaCl₅ at relatively low temperatures in solid-state reductions to afford metastable products. The reduction of TaCl₅ with Ga₂Cl₄ in the presence of NaCl at 175°C yielded solid state products from which the trinuclear cluster [Ta₃Cl₇(CH₃CN) ₆][GaCl₄]₂ was extracted into organic solvent, crystallized, and structurally characterized by X-ray diffractometry.