Trigonal-Bipyramidal Lewis Base Adducts of Methyltrioxorhenium (VII) and Their Bisperoxo Congeners: Characterization, Application in Catalytic Epoxidation, and Density Functional Mechanistic Study

摘要

Methyltrioxorhenium(VII) (MTO) forms trigonal-bipyramidal adducts with pyridines and related Lewis bases. These complexes have been isolated and fully characterized, and two single-crystal X-ray structures are reported. The complexes react with H_2O_2 to form mono- and bisperoxo complexes which were examined in situ by ~1H and ~(17)O NMR spectroscopy. A clear increase in electron deficiency at the Re center can be observed from the MTO complexes to the bisperoxo complexes in all cases examined. The activity of the bisperoxo complexes in olefin epoxidation depends on the Lewis bases, the redox stability of the ligands, and the excess of Lewis base used. Density functional calculations show that when the ligand is pyridine or pyrazole there are significantly stabilized intermediates and moderate energies of the transition states in olefin epoxidation. This ultimately causes an acceleration of the epoxidation reaction. In contrast, the catalytic performance is reduced when the ligand was a nonaromatic nitrogen base. The frontier orbital interaction between the olefin HOMO #pi# (C-C) and orbitals with #sigma#~* (O-O) character in the LUMO group of the Re-peroxo moiety controls the olefin epoxidation.