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 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.