Magic number metal nanoclusters are atomically precise nanomaterials that have enabled unprecedented insight into structure-property relationships in nanoscience. Thiolates are the most common ligand, binding to the cluster via a staple motif in which only central gold atoms are in the metallic state. The lack of other strongly-bound ligands for nanoclusters with different bonding modes has been a significant limitation in the field. Herein, we report a previously unknown ligand for gold (0) nanoclusters: N-heterocyclic carbenes (NHCs), which feature a robust metal-carbon single bond, and impart high stability to the corresponding gold cluster. The addition of a single NHC to gold nanoclusters results in significantly improved stability and catalytic properties in the electrocatalytic reduction of CO2. By varying the conditions, nature and number of equivalents of the NHC, predominantly or exclusively monosubstituted NHC-functionalized clusters result. Clusters can also be obtained with up to five NHCs, as a mixture of species.

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Copper–hydrides are known catalysts for several technologically important reactions such as hydrogenation of CO, hydroamination of alkenes and alkynes, and chemoselective hydrogenation of unsaturated ketones to unsaturated alcohols. Stabilizing copper-based particles by ligand chemistry to nanometer scale is an appealing route to make active catalysts with optimized material economy; however, it has been long believed that the ligand–metal interface, particularly if sulfur-containing thiols are used as stabilizing agent, may poison the catalyst. We report here a discovery of an ambient-stable thiolate-protected copper–hydride nanocluster [Cu25H10(SPhCl2)18]3– that readily catalyzes hydrogenation of ketones to alcohols in mild conditions. A full experimental and theoretical characterization of its atomic and electronic structure shows that the 10 hydrides are instrumental for the stability of the nanocluster and are in an active role being continuously consumed and replenished in the hydrogenation reaction. Density functional theory computations suggest, backed up by the experimental evidence, that the hydrogenation takes place only around a single site of the 10 hydride locations, rendering the [Cu25H10(SPhCl2)18]3– one of the first nanocatalysts whose structure and catalytic functions are characterized fully to atomic precision. Understanding of a working catalyst at the atomistic level helps to optimize its properties and provides fundamental insights into the controversial issue of how a stable, ligand-passivated, metal-containing nanocluster can be at the same time an active catalyst.

Engineering the surface ligands of metal nanoparticles is critical in designing unique arrangements of metal atoms. Here, we report the synthesis and total structure determination of a large box-shaped Ag67 nanocluster (NC) protected by a mixed shell of thiolate (2,4-dimethylbenzenethiolate, SPhMe2) and phosphine (triphenylphosphine, PPh3) ligands. Single crystal X-ray diffraction (SCXRD) and electrospray ionization mass spectrometry (ESI-MS) revealed the cluster formula to be [Ag67(SPhMe2)32(PPh3)8]3+. The crystal structure shows an Ag23 metal core covered by a layer of Ag44S32P8 arranged in the shape of a box. The Ag23 core was formed through an unprecedented centered cuboctahedron, i.e., Ag13, unlike the common centered Ag13 icosahedron geometry. Two types of ligand motifs, eight AgS3P and eight bridging thiols, were found to stabilize the whole cluster. The optical spectrum of this NC displayed highly structured multiple absorption peaks. The electronic structure and optical spectrum of Ag67 were computed using time-dependent density functional theory (TDDFT) for both the full cluster [Ag67(SPhMe2)32(PPh3)8]3+ and a reduced model [Ag67(SH)32(PH3)8]3+. The lowest metal-to-metal transitions in the range 500–800 nm could be explained by considering the reduced model that shows almost identical electronic states to 32 free electrons in a jellium box. The successful synthesis of the large box-shaped Ag67 NC facilitated by the combined use of phosphine and thiol paves the way for synthesizing other metal clusters with unprecedented shapes by judicious choice of thiols and phosphines.

Surface organic ligands playac ritical role in stabilizing atomically precise metal nanoclusters in solutions. However,itisstill challenging to prepare highly robust ligated metal nanoclusters that are surface-active for liquid-phase catalysis without any pre-treatment. Now,a nN -heterocyclic carbene-stabilized Au 25 nanocluster with high thermal and air stabilities is presented as ah omogenous catalyst for cycloisomerization of alkynyl amines to indoles.T he nanocluster, characterized as [Au 25 ( i Pr 2 -bimy) 10 Br 7 ] 2+ ( i Pr 2 -bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) (1), was synthesized by direct reduction of AuSMe 2 Cl and i Pr 2 -bimyAuBr with NaBH 4 in one pot. X-rayc rystallization analysis revealed that the cluster comprises two centered Au 13 icosahedra sharing av ertex. Cluster 1 is highly stable and can survive in solution at 80 8 8Cf or 12 h, which is superior to Au 25 nanoclusters passivated with phosphines or thiols.DFT computations reveal the origins of both electronic and thermal stability of 1 and point to the probable catalytic sites.T his work provides new insights into the bonding capability of N-heterocyclic carbene to Au in ac luster,a nd offers an opportunity to probe the catalytic mechanism at the atomic level.