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)

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((OPr)-Pr-i)(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 ((PrO)-Pr-i)(3)M(mu-(OPr)-Pr-i)(3)MoO(mu-O)(2)MoO(mu-(OPr)-Pr-i)(2)MoO(mu-O)(2)MoO(mu-(OPr)-Pr-i)(3)M((OPr)-Pr-i)(3) non-linear chain molecules with 2 Mo-Mo bonds (2.5836(8) Angstrom) and short but non-bonding Mo-M distances (3.1791(8) Angstrom for 1 and 3.1746(8) Angstrom 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((OPr)-Pr-i)(10) 3. Compound 3 can be isolated in a pure form from the reaction of MoO((OPr)-Pr-i)(4) with Ta((OPr)-Pr-i)(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)(2)C(OMe)(2) ketal as organic byproduct. The molecules of 3 are 5-member rings with a MoO(mu-O)(2)MoO fragment in the basis (Mo-Mo 2.5730(13) Angstrom), coupled to two (mu-(OPr)-Pr-i)(2)Ta((OPr)-Pr-i)(3) fragments that are joined together by an oxomolybdate ligand (mu-O)(2)MoO2. According to NMR-spectroscopic data the aggregate is preserved and rigid in solution. Mo4Ta4O16((OPr)-Pr-i)(12) 4 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((OPr)-Pr-i)(4) and WO((OPr)-Pr-i)(4) in toluene yields (Mo4O8)-O-V(Mo,W)O-VI(2)2((OPr)-Pr-i)(12) 5 with a molecular structure very close to its homometallic analog Mo6O10((OPr)-Pr-i)(12). The complete X-ray single crystal study was carried out for the sample of 5 with (Mo4O8)-O-V(Mo0.45W0.55)O-VI(2)2((OPr)-Pr-i)(12) composition.