Tetrahydrofuran-Stabilized Molybdenum III Halides

Submitted by RINALDO POLI, STEVEN T. KRUEGER, and SUNDEEP P. MATTAMANA  

Monomeric, soluble Mo(III) precursors are extremely useful chemical intermediates, their uses ranging from transformation to other Mo(III) coordination compounds,1 reduction to dinitrogen complexes,2 conversion to metal-metal bonded compounds,3 and formation of new materials.4 

A common problem of this synthesis is the decomposition of the product to a black insoluble and pyrophoric powder (presumably metallic Mo), which is possibly caused by the use of an impure starting material, to the adventitious introduction of air, to excessive exposure to light, or to a combination of those factors. If a black powder is formed, this can be removed at the end of the reaction by filtration through a glass frit the subsequent workup is as described above. Such decomposition reduces the yield but does not lead to a less pure product. If larger amounts of Mo(CO)6 are used, the sublimation of larger quantities of Mo(CO)6 to the condenser occurs, which may require interruption of the procedure and manual scraping of the sublimed solid back into the flask with a spatula under a flow of N2. 

A variety of cosolvents can be used as catalysts for this reaction (e.g., diglyme, MeCN, or 1,4-dioxane). For instance, the use of 1,4-dioxane catalyst (0.5 mL, 5.8 mmol) in a synthetic procedure starting from Mo(CO)6 (4.88 g, 18.48 mmol) in 50 mL of toluene has reduced the reflux time to 16h and has led to the formation of 4.00 g of spectroscopically pure product (80% yield). In our hands, the use of MeCN leads to impurities of Mo(CO)3(MeCN)3 in the final product. 

Prolonged standing in a THF solution results in its decomposition as outlined above however, by brief warming in THF followed by slow cooling, single crystals of the compound have been obtained. Its molecular stereochemistry is meridional in the solid state.9 Substitution reactions of the THF ligand by phosphine and halide ligands have been described.9,15 

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