Proton-assisted activation of dihydrogen: mechanistic aspects of proton-catalyzed addition of h ₂ 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*lr(TsDPEN-H) (1, TsDPEN = racemic H ₂NCHPhCHPhNTs-) 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*lr(TsDPEN)] ⁺ ([1H] ⁺). The rate of proton-catalyzed hydrogenation of 1 was found to be first order in both H ₂ and in [1 H] ⁺ for X ^- = BF ₄ ^-, OTf ^-, CI0 ₄ ^-,NO ₃ ^-. For X ^- = CIO ₄ ^- and BAr ^F ₄ ^-(BAr ^F ₄ ^- = B(C ₆H ₃-3,5-(CF ₃) ₂) ₄ ^-, the rate showed an additional dependence on [1 ]. The hydrogenation of 1 is proposed to occur via the dihydrogen complex ([1 H(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 (pf _a≈ 10) more acidic than [1H(H ₂)] ⁺, but slower for stronger acids. Although the affinity of H ₂ for [Cp*lr(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 3× less than for 1.