"MoCl3(dme)" Revisited: Improved Synthesis, Characterization, and X-ray and Electronic Structures.

"MoCl3(dme)" (dme = 1,2-dimethoxyethane) is an important precursor for midvalent molybdenum chemistry, particularly for triply Mo-Mo bonded compounds of the type Mo2X6 (X = bulky anionic ligand). However, its exact structural identity has been obscure for more than 50 years. In search of a convenient, large-scale synthesis, we have found that trans-MoCl4(Et2O)2 dissolved in dme can be cleanly reduced with dimethylphenylsilane, Me2PhSiH, to provide khaki Mo2Cl6(dme)2 in ∼90% yield. If the reduction is performed on a small scale, single crystals suitable for X-ray crystallography can be obtained. Two different crystal morphologies were identified, each belonging to the P21/n space group, but with slightly different unit cell constants. The refined structure of each form is an edge-shared bioctahedron with overall Ci symmetry and metal-metal separations on the order of 2.8 A. The bulk material is diamagnetic as determined by both the Gouy method and SQUID magnetometry. Density functional theory calculations suggest a σ2π2δ*2 ground state for the dimer with the diamagnetism arising from a singlet diradical "broken symmetry" electronic configuration. In addition to a definitive structural assignment for "MoCl3(dme)", this work highlights the utility of organosilanes as easy to handle, alternative reductants for inorganic synthesis.