N-H and C-H Bond Activations of an Isoindoline Promoted by Iridium- And Osmium-Polyhydride Complexes: A Noninnocent Bridge Ligand for Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

Article abstract of DOI:10.1021/acs.organomet.0c00316

The elusive C-H bond activation of an organic fragment contained in many biologically active molecules and the use of the resulting noninnocent ligand in bimetallic catalysis applied to the acceptorless and base-free dehydrogenation of secondary alcohols has been performed by using the polyhydrides IrH5(PiPr3)2 (1) and OsH6(PiPr3)2 (2). Complex 1 activates the N-H bond of 1,3-bis(6′-methylpyridyl-2′-imino)isoindoline (HBMePHI) to give the mononuclear complex IrH2{κ2-Npy,Nimine(BMePHI)}(PiPr3)2 (3). Both 1 and 2 activate the C(sp2)-H bond at position 4 of the core isoindoline of the BMePHI ligand of 3. The reactions lead to the homobinuclear complex (PiPr3)2H2Ir{μ-(κ2-Npy,Nimine-BMePI-κ2-Nimine,C4iso)}IrH2(PiPr3)2 (4) and the heterobinuclear compound (PiPr3)2H2Ir{μ-(κ2-Npy,Nimine-BMePI-κ2-Nimine,C4iso)}OsH3(PiPr3)2 (5), respectively. The metalated carbon atom of 4 and 5 has a marked nucleophilic character. Thus, it adds the proton of alcohols to afford the respective cations [(PiPr3)2H2Ir{μ-(κ2-Npy,Nimine-BMePHI-κ2-Npy,Nimine)}IrH2(PiPr3)2]+ (6) and [(PiPr3)2H2Ir{μ-(κ2-Npy,Nimine-BMePHI-κ2-Npy,Nimine)}OsH3(PiPr3)2]+ (7), and the corresponding alkoxide. The mononuclear complex 3 and the binuclear compounds 4 and 5 are efficient catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. The binuclear complexes 4 and 5 are significantly more active than 3. The catalytic synergism is a consequence of the mutual electronic influence of the metals through the bridge. X-ray diffraction analysis data of the structures of 3-5 and the reactivity of 4 and 5 support a noninnocent character of the bridging ligand.