Proton-Assisted Activation of Dihydrogen: Mechanistic Aspects of Proton-Catalyzed Addition of H₂ to Ru and Ir Amido Complexes

This study examines the acid-catalyzed hydrogenation of ketones by amido-amine chelates of Ru and Ir, focusing on the hydrogen activation step. Addition of H2 to the catalyst Cp*Ir(TsDPEN-H) (1, TsDPEN = racemic H2NCHPhCHPhNTs−) is more favorable than for corresponding (cymene)Ru derivatives. Depending on the acid, the rate of the proton-catalyzed addition of H2 to 1 varies over 3 orders of magnitude even for strong acids. Acids protonate the NH center in the five-coordinate diamides to give the amido-amine, e.g., [Cp*Ir(TsDPEN)]+ ([1H]+). The rate of proton-catalyzed hydrogenation of 1 was found to be first order in both H2 and in [1H]+ for X− = BF4−, OTf−, ClO4−, NO3−. For X− = ClO4− and BArF4− (BArF4− = B(C6H3-3,5-(CF3)2)4−), the rate showed an additional dependence on [1]. The hydrogenation of 1 is proposed to occur via the dihydrogen complex ([1H(H2)]+) followed by proton transfer to 1, either directly (third-order pathway) or via anion-assisted proton transfer (second-order pathway). The pKa (H−H bond) of [1H(H2)]+ is predicted to be 13.88 ± 0.37 (MeCN solution) whereas the pKa (N−H bond) of [1H]+ is about 21.6. The rate of hydrogenation of 1 was fastest for acids about 3 orders of magnitude (pKa ≈ 10) more acidic than [1H(H2)]+, but slower for stronger acids. Although the affinity of H2 for [Cp*Ir(TsDPEN)]+ is orders of magnitude lower than for 1 (298 K), the cationic complex adds H2 far faster. Similar trends are seen for (cymene)Ru(TsDPEN-H) (2) and its derivatives. The affinity of H2 for 2 was found to be 3× less than for 1.