Oxidation Chemistry of Manganese Nitrido Complexes

This project is mainly concerned with fundamental studies of the oxidation chemistry of manganese(V) nitrido complexes (MnVoN) and other reactive nitrido species generated from them. Metal-catalyzed oxidation of organic compounds is one of the most important processes in organic synthesis. The use of manganese complexes as oxidation catalysts continues to be of great interest since they are relative inexpensive and less toxic than 4d and 5d complexes. However, most manganese catalysts reported so far are in +2 or +3 oxidation states, which are usually high-spin and are intrinsically not very stable. In this project we propose to investigate the use of MnVoN as oxidation catalysts, these d2 complexes are in general stable since the MnV center is stabilized by the strongly electro-donating nitrido ligand. For example, we have recently discovered that (PPh4)2[MnV(N)(CN)4] is a very robust and efficient catalyst for the epoxidation of alkenes and the oxidation of alcohols by hydrogen peroxide at room temperature. By replacing the four cyanide ligands in [MnV(N)(CN)4]2- with multidentate anionic ligands, it is possible to generate more robust catalysts. Also, by designing water soluble MnVoN catalysts, one can perform green oxidation of organic substrates in water. Apart from hydrogen peroxide, various other oxidants such as PhIO, peracetic acid and m-chloroperbenzoic acid will also be studied. There is evidence that with H2O2, the Mn(V) center acts as a Lewis acid; but with other oxidants a novel MnVII(N)(O) intermediate may be involved.Although the nitrido ligand in MnVoN is in general rather inert, our preliminary investigations show that MnV(N)(salen) can be readily activated by a few equivalents of Lewis acids such as BF3 to undergo hydrogen-atom transfer (HAT) reactions with aromatic hydrocarbons at room temperature. An understanding of the mechanism of this novel reaction is of fundamental interest, and may also lead to the development of a system that can carry out selective C-H bond activation.<.p>We also propose to study the reactivity of manganese(VI) nitrido complexes. Report by Weighardt and preliminary investigation by us show that MnVIoN can be readily generated from MnVoN by electrochemical or chemical oxidation. These nitrido species are expected to possess rich electrophilic reactivity, similar to ruthenium(VI) and osmium(VI) nitrides.Our studies should contribute to fundamental understanding of the reactivity of manganese nitrido complexes. It could also lead to the development of green, highly efficient and selective methods for oxidation of various organic substrates based on manganese nitrido complexes.