Effect of Ancillary Ligands and Solvents on H/D Exchange Reactions Catalyzed by Cp*Ir Complexes

A series of complexes of the form Cp*Ir(NHC)(X)n and [Cp*Ir(NHC)(L)2][OTf]2, where NHC = 1,3,4,5-tetramethylimidazol-2-ylidene (n = 2, X = Cl− (1-Cl), NO3− (1-NO3), −OC(O)CF3 (=TFA, 1-TFA); n = 1, X = SO42− (1-SO4); L = H2O (1-H2O), CH3CN (1-CH3CN), OTf = trifluoromethanesulfonato), were prepared. X-ray crystal structures of 1-OH2, 1-SO4, and 1-NO3 and the dimeric complex [(Cp*Ir(NHC)Cl)2][OTf]2 (2) were obtained. In solution, the complex 1-TFA was found to exist in equilibrium with [Cp*Ir(NHC)(OH2)2][OCOCF3]2 (1-aqua-TFA), where the aqua ligands are strongly hydrogen bound to the −OCOCF3 counterion. A van’t Hoff plot from −10 to 30 °C yielded values for the reaction enthalpy and entropy of ΔH° = −7.6 ± 0.7 kcal/mol and ΔS° = −30.6 ± 2.4 eu, respectively. These data are consistent with the observation that at higher temperatures the complex 1-TFA is favored. An X-ray crystal structure of 1-aqua-TFA was obtained. Catalytic H/D exchange reactions between benzene and various deuterium sources (CD3OD, CF3COOD, CD3COCD3, and D2O) were performed and assessed by GC-MS. The best deuterium sources for this reaction were found to be CD3OD or CD3OD/D2O (1:1) mixtures. The highest turnover numbers (TONs) were observed for the H/D exchange reactions catalyzed by Cp*Ir(NHC) complexes with labile ligands. These results suggest that the dissociation of the ancillary ligand to form an unsaturated 16-electron intermediate is an important step prior to C−H activation in the catalytic cycle, which is consistent with the Shilov electrophilic C−H activation mechanism. In contrast, the most effective deuterium source for H/D exchange with the aqua complex [Cp*Ir(OH)3][OTf2] (3) was the acidic solvent CF3COOD. Thus, the σ-donating NHC ligand serves to attenuate the electrophilicity of the metal center so that milder reaction conditions are required for the C−H activation reactions.