Coordination chemistry of manganese-salen complexes studied by electrospray tandem mass spectrometry: the significance of axial ligands

Tandem mass spectrometric techniques in combination with high-level quantum chemical calculations have been employed to study the coordination chemistry of manganese- and oxomanganes-salen complexes in the gas phase. Electrospray ionization was used to transfer the ionic complexes from solution to the gas phase. The formation of five- versus six-coordinate manganese(III) species was subsequently probed by ion-molecule reactions with neutral ligands, e.g. acetonitrile, pyridine, alcohols, etc. The reactivity of the so far elusive oxomanganese(v)-salen complexes, readily accessible by fragmentation of mu-axomanganese(IV) dimers, and their coordination chemistry was studied in the same way. Hybrid Hartree-Fock/density functional calculations have been performed to assess the geometries and energies of the triplet and quintet states of the manganese complexes in question. The effects of axial ligation on the geometry and reactivity of the oxo complex were found to be quite drastic. Finally, the epoxidation of olefins by oxomanganese(V) salen was studied intramolecularly by tethering the substrate to the metal center. No indication for precoordination of the substrate as prerequisite for oxidation was found. (C) 2000 Elsevier Science B,V.