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

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 <i>syn</i> and <i>anti</i> alkylidene isomers, which allowed a study of the interconversion of the two employing ¹H–¹H EXSY methods. The <i>K</i>_eq values ([<i>syn</i>]/[<i>anti</i>]) 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 <i>syn</i> isomer by the sterically demanding NAr* ligand. The rates of interconversion of <i>syn</i> and <i>anti</i> isomers were found to be 1–2 orders of magnitude faster for W MAP complexes than for Mo MAP complexes