Ligand Variations in Neutral and Cationic Molybdenum Alkylidyne NHC Catalysts

Several neutral and
cationic molybdenum alkylidyne complexes bearing
mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been
prepared. The NHCs were varied in terms of their steric demand, basicity,
and the nature of the chelating function. The introduction of a variety
of different anionic ligands, such as triflates, halides, and alkoxides,
was accomplished by replacing one, two, or three anionic ligands,
allowing for substantial variations in the structure of the complexes. tert-Butyl, anisole, and mesityl groups were chosen as alkylidyne
substituents. Cationic, tetracoordinate complexes were synthesized,
boasting the structure proposed for the active species of molybdenum
alkylidyne NHC complexes in alkyne metathesis reactions. The formation
of unanticipated structures such as a bis-NHC complex (Mo19) and a complex bearing an alkoxide (Mo16) as a result
of the demethylation of 1,2-dimethoxyethane (DME) was observed. The
crystal structures of selected complexes are presented. Notably, this
work focused exclusively on the syntheses and structures of the complexes;
therefore, the reactivity of the complexes in alkyne metathesis reactions
was not investigated.