om0c00156_si_001.pdf (2.81 MB)
Osmium- and Iridium-Promoted C–H Bond Activation of 2,2′-Bipyridines and Related Heterocycles: Kinetic and Thermodynamic Preferences
journal contribution
posted on 2020-05-22, 22:30 authored by Lara Cancela, Miguel A. Esteruelas, Ana M. López, Montserrat Oliván, Enrique Oñate, Ainhoa San-Torcuato, Andrea VélezThe d2-hexahydride complex OsH6(PiPr3)2 (1)
promotes the activation of C–H bonds of 2,2′-bipyridines
and related heterocycles. The study of the same reactions with the
deuteride counterpart OsD6(PiPr3)2 (1-d) reveals that the activation
of the C–H bonds situated in the sterically less hindered positions
is kinetically preferred. However, the isolated products are the result
of the thermodynamic control of the reactions. Thus, reactions of 1 with 2,2′-bipyridine, 6-phenyl-2,2′-bipyridine,
and 6-methyl-2,2′-bipyridine give the “rollover cyclometalation”
products OsH3{κ2-C,N-[C5(R)H2N-py]}(PiPr3)2 (R = H (2), Ph (3),
Me (4)), whereas 3,5-dimethyl-6-phenyl-2,2′-bipyridine
affords OsH2{κ3-C,N,C-[C5H3N-(Me)2py-C5H4]}(PiPr3)2 (5), containing a dianionic C,N,C-pincer
ligand. The behavior of substrates pyridyl-benzimidazolium and -imidazolium
is similar. Reaction of 1 with 3-methyl-1-(6-phenylpyridin-2-yl)-1H-benzimidazolium tetrafluoroborate leads to OsH3{κ2-C,C-[MeBzim-C5(Ph)H2N]}(PiPr3)2 (6), bearing an anionic Cpy,CNHC-chelate. On the other hand, 3-methyl-1-(6-phenylpyridin-2-yl)-1H-imidazolium tetrafluoroborate yields [OsH2{κ3-C,N,C-(MeIm-py-C6H4)}(PiPr3)2]BF4 (7), containing a monoanionic C,N,C-pincer with
a NHC-unit coordinated in an abnormal fashion. The reactivity pattern
of these substrates is also observed with the d4-iridium-pentahydride IrH5(PiPr3)2 (8), which has generated IrH2{κ2-C,N-[C5(R)H2N-py]}(PiPr3)2 (R = H, (9), Ph (10)) and
IrH{κ3-C,N,C-[C5H3N-(Me2)py-C5H4]}(PiPr3)2 (11). The osmium(IV)–carbon bonds display a higher degree
of covalency than the iridium(III)–carbon bonds. In contrast
to 2, the metalated carbon atom of 9 undergoes
the addition of a proton of methanol to give [IrH2{κ2-N,N-(bipy)}(PiPr3)2]BF4 (12).