Osmium- and Iridium-Promoted C–H Bond Activation of 2,2′-Bipyridines and Related Heterocycles: Kinetic and Thermodynamic Preferences

Abstract

The 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).

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