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J Am Chem Soc. 2009 Mar 18;131(10):3593-600. doi: 10.1021/ja807277s.

Proton-assisted activation of dihydrogen: mechanistic aspects of proton-catalyzed addition of H2 to Ru and Ir amido complexes.

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  • 1School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, USA.

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*Ir(TsDPEN-H) (1, TsDPEN = racemic H(2)NCHPhCHPhNTs(-)) 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 1 was found to be first order in both H(2) and in [1H](+) for X(-) = BF(4)(-), OTf(-), ClO(4)(-), NO(3)(-). For X(-) = ClO(4)(-) and BAr(F)(4)(-) (BAr(F)(4)(-) = B(C(6)H(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 ([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) approximately 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.

PMID:
19236021
[PubMed]
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