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

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 C_py,C_NHC-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).