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)).

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…18 Calculations suggest that C2 bonding and C4 bonding proceed via distinctly different reaction pathways involving either C2-H heterolytic bond cleavage or C4-H oxidative addition, implicating an iridium(V) species (see Scheme 2). 19 Such mechanistic proposals were further supported by experimental data, which demonstrate that product distributionsand, thus, the site of metallationsis strongly anion-dependent. Large anions such as BF 4 -typically are only weak partners for hydrogen bonding and effect small changes in charge distribution.…”

Mechanism of the Dehydrogenative Silylation of Alcohols Catalyzed by Cationic Gold Complexes: An Experimental and Theoretical Study

“…A mechanism was postulated and calculated, and the results are in full accord with experimental observations. A few DFT studies pertaining to catalytic processes involving cationic metal complexes explicitly incorporate the counterion, [17] and in two cases, the assistance of the counterion in proton transfer was observed.…”

Stabile cyclische Carbene und verwandte Spezies jenseits der Diaminocarbene

“…Es ist bekannt, dass kleine Anionen in der Lage sind, heterolytische C-H-Spaltungen durch Bildung von Wasserstoffbrücken zu beschleunigen, und interessanterweise machte man sich diesen Effekt bei C2,C5-unsubstituierten Imidazoliumsalzen zunutze, um die Metallierung an die C2-anstelle der C5-Position (die das acidere Proton trägt) zu lenken. [231] Mehrere PEPPSI-Palladiumkomplexe 61 (Schema 60) erwiesen sich als wirksame Präkatalysatoren für die Sonogashira-Kupplung von Aryliodiden und -bromiden mit endständigen Acetylenen. [234] Diese Reaktionen wurden an der Luft in gemischt-wässrigem Medium unter kupferfreien und aminfreien Bedingungen durchgeführt.…”