Synthesis and Reactivity of 2-Aminoethanethiolato-Bridged Dinuclear Ru(hmb) Complexes (hmb = η<sup>6</sup>-hexamethylbenzene): Mechanistic Consideration on Transfer Hydrogenation with Bifunctional [RuCl<sub>2</sub>(hmb)]<sub>2</sub>/2-Aminoethanethiol Catalyst System
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A series of dinuclear Ru(hmb) complexes with a bridging 2-aminoethanethiolato ligand (S−NH) have been newly prepared by controlling the stoichiometry of starting materials. Their catalytic performance in the racemization of an optically pure secondary alcohol indicated that the relative proportion of the Ru(hmb) and the S−NH unit in these catalyst precursors in addition to the amount of base as an activating agent is crucially important to generate a catalytically active species. Moreover, chiral versions of dinuclear Ru(S−NH)(hmb) complexes displayed excellent catalytic performance in terms of reactivity and enantioselectivity for the asymmetric transfer hydrogenation of aromatic ketones using HCO2H−N(C2H5)3. In agreement with these results, the crossover experiment using a symmetrically bridged dinuclear complex, [Ru(μ-SCH2CH2NH2)(hmb)]2Cl2, and its Cp*Ir dinuclear congener, [Cp*Ir(μ-SCH2CH2NH2)]2Cl2 (Cp* = η5-C5(CH3)5), suggested that the catalytic performance in the hydrogen transfer is originated from the mononuclear amido complex Ru(SCH2CH2NH)(hmb) bearing a Ru/NH bifunctionality.
