Design of 4-Coordinate Ti Imido Aryloxide on the Surface of Silica for Catalytic Oxo/Imido Heterometathesis

We report here the synthesis of 4-coordinate Ti imido complexes Ti(=(NBu)-Bu-t)(OArtBu)(2)(py) (2) and Ti(=(NBu)-Bu-t)(OArtBu)(Me(2)Pyr)(py) (3) (Ar-tBu=O-2,6-Bu-t(2)-4-MeC6H2, Me(2)Pyr=2,5-dimethylpyrrolyl) and study their grafting onto the surface of partially dehydroxylated silica with the aim to prepare 4-coordinate Ti imido aryloxide surface species. Study of the resulting materials with IR, solid-state NMR, elemental and mass balance analyses demonstrates that grafting of bis-aryloxide complex 2 leads to a predominant formation of silica-supported Ti amide (equivalent to SiO)Ti((NHBu)-Bu-t)(OArtBu)(2)(py) resulting from the protonation of the imido ligand with the surface silanols and only a smaller fraction of the desired imido species (equivalent to SiO)Ti(=(NBu)-Bu-t)(OArtBu)(py). The use of the tailored complex 3 where one of the aryloxides is replaced with a more reactive pyrrolyl ligand significantly increases the selectivity of grafting leading to the formation of the Ti imido aryloxide (equivalent to SiO)Ti(=(NBu)-Bu-t)(OArtBu)(py) as major surface species. The obtained materials show high activity in catalytic oxo/imido heterometathesis supporting the previously proposed hypothesis that electron-donating environments are favorable for this transformation.