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)­(η2-alkene), MoTp­(NO)­(DMAP)­(η2-ketone), and MoTp­(NO)­(DMAP)­(η2-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-η2) and monocoordinated (κ-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)­(η2-naphthalene) and MoTp­(NO)­(DMAP)­(α-pinene).