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

Electrospray ionization in combination with tandem mass spectro metric tech niques has been employed to study the formation of oxomanganese-salen compl exes 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) c omplex, [Mn=O(salen)](+), which is the actual oxygen-transfer agent in epox idation reactions, and the dinuclear, mu -oxo bridged [L(salen)Mn-O-Mn-(sal en)L](2+) with two terminal ligands L; the latter acts as a reservoir speci es. The effects of various substituents in the 5- and 5'-positions, respect ively of the salen ligand on the reactivity of the epoxidation catalyst wer e determined quantitatively from CID (collision-induced dissociation) exper iments and B3LYP density functional calculations. Accordingly, the effect o f axial donor ligands on the reactivity of the epoxidation catalyst was stu died. Electron-withdrawing substitutents on the salen ligand and additional axial ligands decrease the stability and thus enhance the reactivity of th e Mn=O moiety, while electron-donating salen substituents have a strong sta bilizing effect.