The solution thermolysis approach to molybdenum(V) alkoxides: synthesis, solid state and solution structures of the bimetallic alkoxides of molybdenum(V) and niobium(V), tantalum(V) and tungsten(VI)

Abstract

No complex formation can be observed between molybdenum(VI) oxoalkoxides and the alkoxides of niobium(V) or tantalum(V) at room temperature. The bimetallic derivatives of molybdenum(V), Mo₄M₂O₈(OⁱPr)₁₄, where M = Nb 1 and Ta 2, were instead isolated on cooling from the solutions of the isopropoxides in toluene subjected to a short-time reflux. The X-ray single crystal study showed both 1 and 2 to be built of (ⁱPrO)₃M(μ-OⁱPr)₃MoO(μ-O)₂MoO(μ-OⁱPr)₂MoO(μ-O)₂MoO(μ-OⁱPr)₃M(OⁱPr)₃ non-linear chain molecules with 2 Mo–Mo bonds (2.5836(8) Å) and short but non-bonding Mo–M distances (3.1791(8) Å for 1 and 3.1746(8) Å for 2). According to NMR and EXAFS data this structure becomes very fluxional or might even be partially broken into homometallic components in hydrocarbon solutions. The oxidation of 2 with traces of oxygen leads to the formation of Mo₃Ta₂O₈(OⁱPr)₁₀3. Compound 3 can be isolated in a pure form from the reaction of MoO(OⁱPr)₄ with Ta(OⁱPr)₄(OMe) 6: the presence of methoxide ligands leads to the formation of additional oxoligands via non-reductive thermolysis leading to the formation of a (CH₃)₂C(OMe)₂ ketal as organic byproduct. The molecules of 3 are 5-member rings with a MoO(μ-O)₂MoO fragment in the basis (Mo–Mo 2.5730(13) Å), coupled to two (μ-OⁱPr)₂Ta(OⁱPr)₃ fragments that are joined together by an oxomolybdate ligand (μ-O)₂MoO₂. According to NMR-spectroscopic data the aggregate is preserved and rigid in solution. Mo₄Ta₄O₁₆(OⁱPr)₁₂4 was found to be one of the products of complete oxidation of 2 (and 3) on prolonged contact with dry oxygen. The thermal treatment of the solutions of MoO(OⁱPr)₄ and WO(OⁱPr)₄ in toluene yields Mo^V₄O₈(Mo,W)^VI₂O₂(OⁱPr)₁₂5 with a molecular structure very close to its homometallic analog Mo₆O₁₀(OⁱPr)₁₂. The complete X-ray single crystal study was carried out for the sample of 5 with Mo^V₄O₈(Mo_0.45W_0.55)^VI₂O₂(OⁱPr)₁₂ composition.