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

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 basefree dehydrogenation of secondary alcohols has been performed by using the polyhydrides IrH5((PPr3)-Pr-i)(2) (1) and OsH6((PPr3)-Pr-i)(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{kappa(2)-N-py,N-imine (BMePHI)}((PPr3)-Pr-i)(2) (3). Both 1 and 2 activate the C(sp(2))-H bond at position 4 of the core isoindoline of the BMePHI ligand of 3. The reactions lead to the homobinuclear complex ((PPr3)-Pr-i)(2) H2Ir{mu-(kappa(2) -N-py, N-imine-BMePI-kappa(2) -N-imine,C-iso(4))}IrH2 ((PPr3)-Pr-i)(2) (4) and the heterobinuclear compound ((PPr3)-Pr-i)(2)H2Ir{mu-(kappa(2)- N-py,N-imine -BMePI-kappa(2)-Nimine,Ciso4)}OsH3((PPr3)-Pr-i)(2) (5), respectively. The metalated carbon atom of 4 and 5 has marked nucleophilic character. Thus, it adds the proton of alcohols to afford the respective cations [((PPr3)-Pr-i)(2)H2Ir{mu-(kappa(2) -N-py,N-imine-BMePHI-kappa(2)-N-py,N-imine)}IrH2((PPr3)-Pr-i)(2)](+) (6) and [((PPr3)-Pr-i)(2)H2Ir{mu-(kappa(2) -N-py, N-imine-BMePHI-kappa(2)-N-py,N-imine)}OsH3((PPr3)-Pr-i)(2)](+) (7), and the corresponding alkoxide. The mononuclear complex 3 and the binuclear compounds 4 and 5 are efficient catalysts for the acceptorless and basefree 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.