A Stable Open-Shelled Au Nanocluster with Remarkable Performance in Selective Oxidation of Benzyl Alcohol.

Open metal sites are crucial in catalysis. We have used a "loose coordination strategy" (LCS) to preorganize open metal sites in gold cluster catalysts. A gold nanocluster with composition of [Au(3,4-Me-Ph-form)(Pr-imy)(MeS)](BF) (Pr-imy=1,3-Diisopropylimidazolium tetrafluoroborate, 3,4-Me-Ph-form=N,N'-Di(3,4-dimethyl-phenyl)formamidine) (Au) has been obtained by one pot synthesis, i.e. the direct reduction of MeSAuCl in the presence of N-heterocyclic carbenes and amidinate ligands. ESI-TOF-MS reveals that the MeS ligand is detached from the cluster to form open sites. The accessibility of the exposed Au atoms has been confirmed quantitatively by luminescent titration with 2-naphthalenethiol. Surprisingly, Au has 15 valence electrons, and the presence of an unpaired electron is confirmed by superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR). This open-shelled Au not only shows unexpected high stability but also exhibits excellent catalytic performance toward the selective oxidation of benzyl alcohol to benzaldehyde, achieving a remarkable turnover number up to 100670.