title: Investigations of iridium-mediated reversible C-H bond cleavage: characterization of a 16-electron iridium(III) methyl hydride complex.
abstract: New iridium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP), have been prepared and used in the study of hydrocarbon C-H bond activation. Intermolecular oxidative addition of a benzene C-H bond was directly observed with [(PONOP)Ir(I)(cyclooctene)][PF(6)] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir(I) methyl complex yielded the corresponding iridium(III) methyl hydride cation, a rare five-coordinate, 16-valence electron transition metal alkyl hydride species which was characterized by X-ray diffraction. Kinetic studies of C-H bond coupling and reductive elimination reactions from the five-coordinate complexes have been carried out. Exchange NMR spectroscopy measurements established a barrier of 17.8(4) kcal/mol (22 degrees C) for H-C(aryl) bond coupling in the iridium(III) phenyl hydride cation and of 9.3(4) kcal/mol (-105 degrees C) for the analogous H-C(alkyl) coupling in the iridium(III) methyl hydride cation. The origin of the higher barrier of H-C(aryl) relative to H-C(alkyl) bond coupling is proposed to be influenced by a hindered rotation about the Ir-C(aryl) bond, a result of the sterically demanding PONOP ligand.
authors.affiliation: Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA.
authors.forename: Wesley H
authors.initials: WH
authors.lastname: Bernskoetter
authors.forename: Susan Kloek
authors.initials: SK
authors.lastname: Hanson
authors.forename: Sara K
authors.initials: SK
authors.lastname: Buzak
authors.forename: Zoe
authors.initials: Z
authors.lastname: Davis
authors.forename: Peter S
authors.initials: PS
authors.lastname: White
authors.forename: Rodney
authors.initials: R
authors.lastname: Swartz
authors.forename: Karen I
authors.initials: KI
authors.lastname: Goldberg
authors.forename: Maurice
authors.initials: M
authors.lastname: Brookhart
mesh_terms: D034242:Cyclooctanes / Q000737:chemistry; D004583:Electrons; D007495:Iridium / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D009942:Organometallic Compounds / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D034242:Cyclooctanes; D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D007495:Iridium