Iridium-Pentahydride Referee for Competition of Activations between C–H and C–F Bonds and between C–H Bonds Located in Different Positions

Abstract

Polyhydride IrH₅(PⁱPr₃)₂ (1) activates an ortho-CH bond of acetophenone and an ortho-CF bond of 2,3,4,5,6-pentafluoroacetophenone to give IrH₂{κ²-C,O-[C₆H₄C(O)CH₃]}(PⁱPr₃)₂ (2) and IrH₂{κ²-C,O-[C₆F₄C(O)CH₃]}(PⁱPr₃)₂ (3). When the phenyl group contains ortho-CH and ortho-CF bonds, ortho-CH bond activation is kinetically favored. Thus, complexes IrH₂{κ²-C,O-[C₆H₃FC(O)CH₃]}(PⁱPr₃)₂ (4) and IrH₂{κ²-C,O-[C₆H₄C(O)C₆H₃F₂]}(PⁱPr₃)₂ (5) are obtained from the reactions of 1 with 2-fluoroacetophenone and 2,6-difluorobenzophenone. Complex 1 also activates an ortho-CH bond of the 4-fluorophenyl group of 2-(4-fluorophenyl)pyridine. The reaction leads to IrH₂{κ²-C,N-[C₆H₃F-py]}(PⁱPr₃)₂ (6). Replacement of the hydrogen atom of one of the ortho-CH bonds with a fluorine accelerates orthometalation, while the ortho-CH and ortho-CF bonds compete for the metal center. Thus, 2-(2,4-difluorophenyl)pyridine produces a 1:9 mixture of 6 and IrH₂{κ²-C,N-[C₆H₂F₂-py]}(PⁱPr₃)₂ (7). Complex 1 activates in a competitive manner ortho-CH and ortho-CF bonds of 2,6-bis(2,4-difluorophenyl)pyridine to give mixtures of IrH{κ³-C,N,C-[C₆H₂F₂-py-C₆H₂F₂]}(PⁱPr₃)₂ (8) and IrH{κ³-C,N,C′-[C₆H₂F₂-py-C₆H₃F]}(PⁱPr₃)₂ (9). H/D Isotopic exchange experiments indicate that these orthometalations are thermodynamically assisted by chelating effect resulting from coordination of carbonyl or pyridyl groups. However, the activation of other C(sp²)–H bonds less sterically hindered is kinetically favored. The distribution of deuterium atoms in the orthometalated phenyl ring also shows kinetic preference for the activation of bonds located ortho to fluorine.