Synthesis, Characterization And Properties Of Binuclear Complexes With Chiral Pyrrolidine Salen Ligands

Manganese is an indispensable element to build the active sites of many enzymes. These active sites containing manganese play important roles in biological systems, such as redox reactions, electron transfer, etc. A series of topics, involving development of efficient biomimetic model complexes of the manganese metal loenzymes with special structures and functions, are of infinite interests. During the research of Cytochrome P₄₅₀, which is an essential oxidation enzyme in nature, a new type of bis-Schiff base ligands (salen) was discovered. These salen ligands have strong coordination ability with most of transition metals to form tetradentate complexes, among which the manganese complexes are efficient catalysts for asymmetric oxidation of various organic substrates.In this work, we found a convenient method to prepare a chiral pyrrolidine salen ligand (R,R)-1 under mild reaction conditions. On the basis of pyrrolidine salen backbone, two novel chiral salen ligands (R,R)-16a, (R,R)-17a and their corresponding Mn^III complexes are prepared, which have the potential ability to construct binuclear complexes via monodentate or bidendate coordination. The structural characterization and the catalytic performance of the complexes are investigated. The theory of the folded conformation of the salen Mn^v=0 intermediate is used to elucidate the effect of the amido-bridged ligand in the asymmetric epoxidation reaction.Novel Mn-Ru and Mn-Re hetero-binuclear complexes are obtained by covalent linkage of the Mn(salen) complex as the catalytic center and the Ru^II(bpy)₃ or the Re^I(bpy) complex as photosensitizer. The steady and transient absorption spectra indicate that the manganese can quench the excited state of the photosensitizer moiety. As a result, both the excitedstate lifetime and the luminescence quantum yield of the photosensitizer part are decreased. Although the covalently linked manganese salen moiety makes the redox potential of ReI/n lowered by 0. 3 V, it is still possible to form the high valent manganese via intramolecular electron transfer from Mnm to photo-generated Rum or Re" as the redox potentials of RuII/In and ReI/n are higher than that of MnIII/Iv.The hetero-binuclear chiral Mn(salen) complexes covalently linked with photosensitizer are applied to the photo-induced catalytic reaction, but no catalytic activity is found by the present experiment.