Electron-Transfer Chain Catalysis of Η2-Arene, Η2-Alkene, and Η2-Ketone Exchange on Molybdenum

An oxidant-initiated, substitution process for dihapto-coordinated ligands is described for the {MoTp(NO)(DMAP)} system. Complexes of the form MoTp(NO)(DMAP)(eta(2)-alkene), MoTp(NO)(DMAP)(eta(2)-ketone), and MoTp(NO)(DMAP)(eta(2)-arene) (where Tp = hydridotris(pyrazolyl)borate and DMAP = 4-(dimethylamino)pyridine) undergo an alkene-to-ketone exchange that is catalyzed by the addition of <0.1 equiv of a metallocene oxidant (ferrocenium, permethylferrocenium, or cobaltocenium). A similar acceleration was observed in the presence of the H-bond donor hexafluoroisopropanol (HFIP). From experimental observations, a radical chain propagation mechanism is proposed that is dependent on the equilibrium between dihapto-coordinated (C, O-eta(2)) and monocoordinated (kappa-O) isomers and the differing redox characteristics of these two isomeric forms. This concept was then applied to the search of sodium-free reduction conditions for the conversion of MoTp(NO)(DMAP)(I) to various molybdenum(0) complexes of unsaturated ligands, including MoTp(NO)(DMAP)(eta(2)-naphthalene) and MoTp(NO)(DMAP)(alpha-pinene).