title: An anion-dependent switch in selectivity results from a change of C-H activation mechanism in the reaction of an imidazolium salt with IrH5(PPh3)2.
abstract: Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).
authors.affiliation: Department of Chemistry, 225 Prospect Street, Yale University, New Haven, Connecticut 06520-8107, USA.
authors.forename: Leah N
authors.initials: LN
authors.lastname: Appelhans
authors.forename: Daniele
authors.initials: D
authors.lastname: Zuccaccia
authors.forename: Anes
authors.initials: A
authors.lastname: Kovacevic
authors.forename: Anthony R
authors.initials: AR
authors.lastname: Chianese
authors.forename: John R
authors.initials: JR
authors.lastname: Miecznikowski
authors.forename: Alceo
authors.initials: A
authors.lastname: Macchioni
authors.forename: Eric
authors.initials: E
authors.lastname: Clot
authors.forename: Odile
authors.initials: O
authors.lastname: Eisenstein
authors.forename: Robert H
authors.initials: RH
authors.lastname: Crabtree