Proton-assisted activation of dihydrogen: mechanistic aspects of proton-catalyzed addition of h 2 to RU and IR amido complexes

Abstract

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 F4-(BAr F4- = 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. © 2009 American Chemical Society.