Probing the reactivity of oxomanganese-salen complexes: An electrospray tandem mass spectrometric study of highly reactive intermediates

Article abstract of DOI:10.1002/1521-3765(20010202)7:3<591::AID-CHEM591>3.0.CO;2-I

Electrospray ionization in combination with tandem mass spectrometric techniques has been employed to study the formation of oxomanganese-salen complexes upon oxidation of [Mn^III(salen)]⁺ cations as well as the properties and reactions of the oxidized species in the gas phase. Two species could be characterized as the principal oxidation products: the oxomanganese(V) complex, [Mn=O(salen)]⁺, which is the actual oxygen-transfer agent in epoxidation reactions, and the dinuclear, μ-oxo bridged [L(salen)Mn-Q-Mn-(salen)L]²⁺ with two terminal ligands L; the latter acts as a reservoir species. The effects of various substituents in the 5-and 5′-positions, respectively, of the salen ligand on the reactivity of the epoxidation catalyst were determined quantitatively from CID (collision-induced dissociation) experiments and B3LYP density functional calculations. Accordingly, the effect of axial donor ligands on the reactivity of the epoxidation catalyst was studied. Electron-withdrawing substitutents on the salen ligand and additional axial ligands decrease the stability and thus enhance the reactivity of the Mn=O moiety, while electron-donating salen substituents have a strong stabilizing effect. WILEY-VCH Verlag GmbH, 2001.