title: Addition of C-C and C-H bonds by pincer-iridium complexes: a combined experimental and computational study.
abstract: We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C-C bond of biphenylene. The sterically crowded species ((tBu)PCP)Ir ((R)PCP = κ(3)-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)-H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C-C addition product, ((tBu)PCP)Ir(2,2'-biphenyl), is observed. The much less crowded ((iPr)PCP)Ir undergoes relatively rapid C-C addition at room temperature. The large difference in the apparent barriers to C-C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the (R)PCP ligands. Based on DFT calculations the large difference in the barriers to C-C addition can be explained in terms of a "tilted" transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°-60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the (tBu)PCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C-C bond of biphenyl by ((R)PCP)Ir.
authors.affiliation: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, USA. alan.goldman@rutgers.edu kroghjes@rutgers.edu.
authors.forename: David A
authors.initials: DA
authors.lastname: Laviska
authors.forename: Changjian
authors.initials: C
authors.lastname: Guan
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Emge
authors.forename: Miles
authors.initials: M
authors.lastname: Wilklow-Marnell
authors.forename: William W
authors.initials: WW
authors.lastname: Brennessel
authors.forename: William D
authors.initials: WD
authors.lastname: Jones
authors.forename: Karsten
authors.initials: K
authors.lastname: Krogh-Jespersen
authors.forename: Alan S
authors.initials: AS
authors.lastname: Goldman
mesh_terms: D001713:Biphenyl Compounds / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry*; D007495:Iridium / Q000737:chemistry*; D008024:Ligands; D008968:Molecular Conformation; D013816:Thermodynamics
chemical_list: D001713:Biphenyl Compounds; D002244:Carbon; D056831:Coordination Complexes; D006859:Hydrogen; D007495:Iridium; D008024:Ligands; C010574:biphenyl