A Stable Open-Shelled Au26 Nanocluster with Remarkable Performance in Selective Oxidation of Benzyl Alcohol.

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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 [Au26(3,4-Me2-Ph-form)9(iPr2-imy)3(Me2S)](BF4)2 (iPr2-imy=1,3-Diisopropylimidazolium tetrafluoroborate, 3,4-Me2-Ph-form=N,N'-Di(3,4-dimethyl-phenyl)formamidine) (Au26) has been obtained by one pot synthesis, i.e. the direct reduction of Me2SAuCl in the presence of N-heterocyclic carbenes and amidinate ligands. ESI-TOF-MS reveals that the Me2S 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, Au26 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 Au26 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.