Laser flash photolysis study of Jacobsen catalyst and related manganese(III) salen complexes. Relevance to catalysis.

The laser flash photolysis and emission properties of a set of five-coordinate manganese(III) Schiff-base complexes have been examined. In contrast to the intramolecular electron transfer between Mn3+ and the equatorial salen ligand reported to occur in the absence of axial coordination, our laser flash photolysis study has shown that the reactivity of the respective excited states is appreciably influenced by the electron donor strength of the apical ligand at the metal center. In fact, homolytic and heterolytic photocleavage of the metal-ligand apical bond can be the most important processes upon laser excitation, their relative contribution being influenced by medium effects and the sigma-charge donation of the axial ligand. On the other hand, the detection of reactive intermediates such as the oxomanganese(V) salen complex (lambda(max) 530 nm) by laser flash photolysis opens the way to apply this fast detection technique to the study of reaction mechanisms in catalysis by metallic complexes. As a matter of fact, quenching of oxomanganese(V) salen by simple alkenes has been observed by laser flash.