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 chemist ry of manganese- and oxomanganes-salen complexes in the gas phase. Electros pray ionization was used to transfer the ionic complexes from solution to t he gas phase. The formation of five- versus six-coordinate manganese(III) s pecies was subsequently probed by ion-molecule reactions with neutral ligan ds, e.g. acetonitrile, pyridine, alcohols, etc. The reactivity of the so fa r elusive oxomanganese(v)-salen complexes, readily accessible by fragmentat ion of mu-axomanganese(IV) dimers, and their coordination chemistry was stu died in the same way. Hybrid Hartree-Fock/density functional calculations h ave been performed to assess the geometries and energies of the triplet and quintet states of the manganese complexes in question. The effects of axia l ligation on the geometry and reactivity of the oxo complex were found to be quite drastic. Finally, the epoxidation of olefins by oxomanganese(V) sa len was studied intramolecularly by tethering the substrate to the metal ce nter. No indication for precoordination of the substrate as prerequisite fo r oxidation was found. (C) 2000 Elsevier Science B,V.