C-H Activation of Pyridines by (PBP)Ir Complexes: Elucidation of Boryl-Directed C-H Oxidative Addition to Ir, and Discovery of Transition Metal-Assisted Reductive Elimination from Boron

A combination of experimental and theoretical techniques was used to investigate the mechanism of C-H activation of pyr-idines by complexes of Ir supported by a diarylboryl/bis(phosphine) PBP pincer. This pincer ligand in the critical intermedi-ate (PBP)IrCO (4) contains a three-coordinate boron bound to Ir that retains some Lewis acidity in the perpendicular direc-tion. Coordination of the pyridine to this boron center in 4 leads to fast insertion of Ir into the 2-CH bond of pyridine, providing a different topology of direction than the conventional directed C-H activation where both the directing group coordination and C-H activation happen at the same metal center. Beyond this critical sequence, the system possesses sig-nificant complexity in terms of possible isomers and pathways, which have been thoroughly explored via theoretical and experimental means. Kinetic and thermodynamic preferences for the activation of differently substituted pyridines were also investigated. In experimental work, the key intermediate 4 is accessed via elimination of benzene from a phe-nyl/hydride containing precursor (PBPhP)IrHCO (3). DFT investigations of the mechanism of benzene loss from 3 revealed the possibility of a genuinely new type of mechanism, whereby the Ph-H bond is made in a concerted process that is best described as C-H reductive elimination from boron, assisted by the transition metal (TMARE). For Ir, this pathway was pre-dicted to be competitive with the more conventional pathways involving C-H reductive elimination from Ir, but still higher in energy barrier. However, for the Rh analog 3-Rh, TMARE was calculated to be the preferred pathway for benzene loss and this prediction was experimentally corroborated through the study of reaction rates and the kinetic isotope effect