Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

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Molybdenum and tungsten bispyrrolide alkylidene complexes that contain a 2,6-dimesitylphenylimido (NAr*) ligand have been prepared, in which the pyrrolide is the parent pyrrolide or 2,5-dimethylpyrrolide. Monoalkoxide pyrrolide (MAP) complexes were prepared through addition of 1 equiv of an alcohol to the bispyrrolide complexes. MAP compounds that contain the parent pyrrolide (NC4H4–) are pyridine adducts, while those that contain 2,5-dimethylpyrrolide are pyridine free. Molybdenum and tungsten MAP 2,5-dimethylpyrrolide complexes that contain O-t-Bu, OCMe(CF3)2, or O-2,6-Me2C6H3 ligands were found to have approximately equal amounts of syn and anti alkylidene isomers, which allowed a study of the interconversion of the two employing 1H–1H EXSY methods. The Keq values ([syn]/[anti]) are all 2–3 orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of interconversion of syn and anti isomers were found to be 1–2 orders of magnitude faster for W MAP complexes than for Mo MAP complexes.