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

Abstract

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 (NC[subscript 4]H[– over 4]) 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(CF[subscript 3])[subscript 2], or O-2,6-Me[subscript 2]C[subscript 6]H[subscript 3] 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 [superscript 1]H–[superscript 1]H EXSY methods. The K[subscript eq] 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.