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
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 IrIII to give IrV 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 IrIII, not IrV, 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)).
Journal of the American Chemical Society
Appelhans, Leah N.; Zuccaccia, Daniele; Kovacevic, Anes; Chianese, Anthony R.; Miecznikowski, John R.; Macchioni, Alceo; Clot, Eric; Eisenstein, Odile; and Crabtree, Robert H., "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" (2005). Chemistry & Biochemistry Faculty Publications. 10.
Appelhans, L.N.; Zuccaccia, D.; Kovacevic, A.; Chianese, A.R.; Miecznikowski, J.R.; Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree, R.H. “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.”Journal of the American Chemical Society, 2005, 127 (46), 16299-16311.