Trigonal-bipyramidal Lewis base adducts of methyltrioxorhenium(VII) and their bisperoxo congeners: Characterization, application in catalytic epoxidation, and density functional mechanistic study

Methyltrioxorhenium (VII) (MTO) forms trigonal-bipyramidal adducts with pyr idines and related Lewis bases. These complexes have been isolated and full y characterized, and two single-crystal X-ray structures are reported. The complexes react with H2O2 to form mono- and bisperoxo complexes which were examined in situ by H-1 and O-17 NMR spectroscopy A clear increase in elect ron deficiency at the Re center can be observed from the MTO complexes to t he bisperoxo complexes in all cases examined. The activity of the bisperoxo complexes in olefin epoxidation depends on the Lewis bases, the redox stab ility of the ligands, and the excess of Lewis base used. Density functional calculations show that when the ligand is pyridine or pyrazole there are s ignificantly stabilized intermediates and moderate energies of the transiti on 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 inte raction between the olefin HOMO pi(C-C) and orbitals with sigma*(O-O) chara cter in the LUMO group of the Re-peroxo moiety controls the olefin epoxidat ion.