(N-Salicylidene)aniline Derived Schiff Base Complexes of Methyltrioxorhenium(VII): Ligand Influence and Catalytic Performance

Met hyltrioxorlienium (VII) (MTO) readily forms 1:1 adducts with several N-(salicylidene)ani line derived Schiff bases. If the aromatic rings of the N-(salicylidene)aniline ligands display non-donating or electron withdrawing substituent groups, the resulting MTO adducts show good activities in olefin epoxidations. However, steric effects seem to play a major role, leading often to instable o- and m-Schiff base-MTO adducts, while p-substituted Schiff bases usually lead to more stable adducts. In catalysis, electron-withdrawing substituents oil the aniline moiety lead to better catalysts than electron donating ones. The gap between good catalysts and instable or non-existing compounds, however, is small. The general tendency, however, that good donors on the Schiff base ligands lead to shorter Re-O(Schiff base) bridges and lower catalytic activity, while the opposite is true with acceptor ligands oil the Schiff bases, seems to be quite clear.