Highly Active Gold(I)–Silver(I) Oxo Cluster Activating sp3 C–H Bonds of Methyl Ketones under Mild Conditions

The activation of C­(sp3)–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 [OAu3Ag3(PPhpy2)3]­(BF4)4 (1), which can activate C­(sp3)–H bonds under mild conditions for a broad scope of methyl ketones (RCOCH3, 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 RCOCAu4Ag4(PPhpy2)4(BF4)5 (PPhpy2 = bis­(2-pyridyl)­phenylphosphine). Cluster 1 can be generated in situ via the reaction of [OAu3Ag­(PPhpy2)3]­(BF4)2 with 2 equiv of AgBF4. The oxo ion and the metal centers are found to be essential in the cleavage of sp3 C–H bonds of methyl ketones. Interestingly, cluster 1 selectively activates the C–H bonds in −CH3 rather than the N–H bonds in −NH2 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 sp3 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.