Syntheses, molecular structures, and dynamic solution behavior of nitrido (oxo) molybdenum(VI) siloxides

Abstract

Cocondensation reaction of MoO 3 with hexamethyldisilazane, followed by work-ups produced the nitrido molybdenum(VI) compounds, [MoN(OSiMe 3) 3(NH 3)] 2 ( 1), [MoN(OSiMe 3) 3(py)] ( 2), and a very unusual μ 2-oxo-μ 2-nitrido tetranuclear molybdenum(VI) compound, [Mo 2N 2O(OSiMe 3) 4(py) 2] 2·2py ( 3). When 1 is reacted with excess methanol it results in another oxonitrido Mo(VI) compound, Mo 4N 4O 2(OH) 8(H 2O)(MeOH) 3 ( 4). The compounds were characterized by elemental analyses, FT-IR, 1H and 13C NMR, mass spectrometry, and by single-crystal X-ray diffraction for 1 and 3. Compound 1 is dimeric in the solid state with two bridging siloxo groups; however, in toluene solution and at ambient temperature, the dimer readily dissociates into the monomer. The molecular geometry changes rapidly via a pseudorotation process as deduced from the variable temperature 1H and 13C NMR study. However, as the temperature is lowered, monomer↔dimer equilibrium is attained. The Δ G ≠ for the pseudorotation of 1 is 13.1 kcal mol −1 and Δ G ≠ for the monomer↔dimer association is 9.9 kcal mol −1. Compound 2 is also fluxional, but remains monomeric in toluene solution down to 190 K. The Δ G ≠ for the pseudorotation of 2 is 12.2 kcal mol −1. In contrast, 3 is tetranuclear in the solid state and is not fluxional in solution. Instead, it is slowly disproportionated to 2 and, presumably, an oxonitrido Mo(VI) pyridine adduct.