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 H-2 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 H-2 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 I was found to be first order in both H-2 and in [1H](+) for X- = BF4-, OTf-, ClO4-, NO3-. For X- = ClO4- and BAr4F- (BAr4F- = 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 clihydrogen complex ([1H(H-2)](+)) followed by proton transfer to 1, either directly (third-order pathway) or via anion-assisted proton transfer (second-order pathway). The pK(a) (H-H bond) of [1H(H-2)](+) is predicted to be 13.88 +/- 0.37 (MeCN solution) whereas the pK(a) (N-H bond) of [1H](+) is about 21.6. The rate of hydrogenation of 1 was fastest for acids about 3 orders of magnitude (pK(a) approximate to 10) more acidic than [1H(H-2)](+), but slower for stronger acids. Although the affinity of H-2 for [Cp*Ir(TsDPEN)](+) is orders of magnitude lower than for 1 (298 K), the cationic complex adds H-2 far faster. Similar trends are seen for (cymene)Ru(TsDPEN-H) (2) and its derivatives. The affinity of H-2 for 2 was found to be 3x less than for 1.