Highly Active Gold(Ⅰ)-Silver(Ⅰ) Oxo Cluster Activating sp~3 C-H Bonds of Methyl Ketones under Mild Conditions

摘要

The activation of C(sp~3)-H bonds is challenging, due to their high bond dissociation energy, low proton acidity,, and highly nonpolar character. Herein we report a unique gold(Ⅰ)-silver(Ⅰ) oxo cluster protected by hemilabile phosphine ligands [OAu_3Ag_3(PPhpy_2)_3] (BF_4)_4 (Ⅰ), which can activate C(sp~3)-H bonds under mild conditions for a broad scope of methyl ketones (RCOCH_3, R = methyl, phenyl, 2-methylphenyl, 2-aminophenyl, 2-hydroxylphenyl, 2-pyridyl, 2-thiazolyl, tert-butyl, ethyl, isopropyl). Activation happens via triple deprotonation of the methyl group, leading to formation of heterometallic Au(Ⅰ)-Ag(Ⅰ) clusters with formula RCOCAu_4Ag_4(PPhpy_2)_4(BF_4)_5 (PPhpy_2 = bis(2-pyridyl)phenylphosphine). Cluster 1 can be generated in situ via the reaction of [OAu_3Ag(PPhpy_2)_3](BF_4)2 with 2 equiv of AgBF_4. The oxo ion and the metal centers are found to be essential in the cleavage of sp~3 C-H bonds of methyl ketones. Interestingly, cluster 1 selectively activates the C-H bonds in -CH_3 rather than the N-H bonds in - NH_2 or the O-H bond in -OH which is traditionally thought to be more reactive than C-H bonds. Control experiments with butanone, 3-methylbutanone, and cyclopentanone as substrates show that the auration of the C-H bond of the terminal methyl group is preferred over secondary or tertiary sp~3 C-H bonds; in other words, the C-H bond activation is influenced by steric effect. This work highlights the powerful reactivity of metal clusters toward C-H activation and sheds new light on gold(Ⅰ)-mediated catalysis.