Atomic‐Level Alloying and De‐alloying in Doped Gold Nanoparticles

Atomically precise alloying and de‐alloying processes for the formation of Ag–Au and Cu–Au nanoparticles of 25‐metal‐atom composition (referred to as Ag_xAu_25?x(SR)₁₈ and Cu_xAu_25?x(SR)₁₈, in which R=CH₂CH₂Ph) are reported. The identities of the particles were determined by matrix‐assisted laser desorption ionization mass spectroscopy (MALDI‐MS). Their structures were probed by fragmentation analysis in MALDI‐MS and comparison with the icosahedral structure of the homogold Au₂₅(SR)₁₈ nanoparticles (an icosahedral Au₁₃ core protected by a shell of Au₁₂(SR)₁₈). The Cu and Ag atoms were found to preferentially occupy the 13‐atom icosahedral sites, instead of the exterior shell. The number of Ag atoms in Ag_xAu_25?x(SR)₁₈ (x=0–8) was dependent on the molar ratio of Ag^I/Au^III precursors in the synthesis, whereas the number of Cu atoms in Cu_xAu_25?x(SR)₁₈ (x=0–4) was independent of the molar ratio of Cu^II/Au^III precursors applied. Interestingly, the Cu_xAu_25?x(SR)₁₈ nanoparticles show a spontaneous de‐alloying process over time, and the initially formed Cu_xAu_25?x(SR)₁₈ nanoparticles were converted to pure Au₂₅(SR)₁₈. This de‐alloying process was not observed in the case of alloyed Ag_xAu_25?x(SR)₁₈ nanoparticles. This contrast can be attributed to the stability difference between Cu_xAu_25?x(SR)₁₈ and Ag_xAu_25?x(SR)₁₈ nanoparticles. These alloyed nanoparticles are promising candidates for applications such as catalysis.     

Gold Nanoclusters Confined in a Supercage of Y Zeolite for Aerobic Oxidation of HMF under Mild Conditions

Au nanoclusters with an average size of approximately 1 nm size supported on HY zeolite exhibit a superior catalytic performance for the selective oxidation of 5‐hydroxymethyl‐2‐furfural (HMF) into 2,5‐furandicarboxylic acid (FDCA). It achieved >99 % yield of 2,5‐furandicarboxylic acid in water under mild conditions (60 °C, 0.3 MPa oxygen), which is much higher than that of Au supported on metal oxides/hydroxide (TiO₂, CeO₂, and Mg(OH)₂) and channel‐type zeolites (ZSM‐5 and H‐MOR). Detailed characterizations, such as X‐ray diffraction, transmission electron microscopy, N₂‐physisorption, and H₂‐temperature‐programmed reduction (TPR), revealed that the Au nanoclusters are well encapsulated in the HY zeolite supercage, which is considered to restrict and avoid further growing of the Au nanoclusters into large particles. The acidic hydroxyl groups of the supercage were proven to be responsible for the formation and stabilization of the gold nanoclusters. Moreover, the interaction between the hydroxyl groups in the supercage and the Au nanoclusters leads to electronic modification of the Au nanoparticles, which is supposed to contribute to the high efficiency in the catalytic oxidation of HMF to FDCA.     

Unraveling the Impact of Gold(I)–Thiolate Motifs on the Aggregation‐Induced Emission of Gold Nanoclusters

Aggregation‐induced emission (AIE) provides an efficient strategy to synthesize highly luminescent metal nanoclusters (NCs), however, rational control of emission energy and intensity of metal NCs is still challenging. This communication reveals the impact of surface Au^I‐thiolate motifs on the AIE properties of Au NCs, by employing a series of water‐soluble glutathione (GSH)‐coordinated Au complexes and NCs as a model ([Au₁₀SR₁₀], [Au₁₅SR₁₃], [Au₁₈SR₁₄], and [Au₂₅SR₁₈]^?, SR=thiolate ligand). Spectroscopic investigations show that the emission wavelength of Au NCs is adjustable from visible to the near‐infrared II (NIR‐II) region by controlling the length of the Au^I‐SR motifs on the NC surface. Decreasing the length of Au^I‐SR motifs also changes the origin of cluster luminescence from AIE‐type phosphorescence to Au⁰‐core‐dictated fluorescence. This effect becomes more prominent when the degree of aggregation of Au NCs increases in solution.     

Tea‐Bag‐Like Polymer Nanoreactors Filled with Gold Nanoparticles

Gold‐containing polymer nanotubes, which showed both catalytic activity and resistance to leaching, were prepared by the “tubes by fiber templates” (TUFT) process. For this purpose, electrospun polymer nonwovens with incorporated poly(L ‐lactide)‐stabilized gold nanoparticles were coated with poly(p‐xylylene) by the chemical vapor deposition process, and then the inner fiber templates were removed. The resulting polymer tubes carried encapsulated gold nanoparticles which were shown to be immobilized and featured pronounced catalytic activity towards the hydrolytic oxidation of dimethylphenylsilane and the alcoholysis of dimethylphenylsilane with n‐butanol. The macroscopic nonwovens could be used as tea‐bag‐like catalyst systems and showed excellent reusability.     

Controlling the Atomic Structure of Au30 Nanoclusters by a Ligand‐Based Strategy

We report the X‐ray structure of a gold nanocluster with 30 gold atoms protected by 18 1‐adamantanethiolate ligands (formulated as Au₃₀(S‐Adm)₁₈). This nanocluster exhibits a threefold rotationally symmetrical, hexagonal‐close‐packed (HCP) Au₁₈ kernel protected by six dimeric Au₂(SR)₃ staple motifs. This new structure is distinctly different from the previously reported Au₃₀S(S‐^tBu)₁₈ nanocluster protected by 18 tert‐butylthiolate ligands and one sulfido ligand with a face‐centered cubic (FCC) Au₂₂ kernel. The Au₃₀(S‐Adm)₁₈ nanocluster has an anomalous solubility (it is only soluble in benzene but not in other common solvents). This work demonstrates a ligand‐based strategy for controlling nanocluster structure and also provides a method for the discovery of possibly overlooked clusters because of their anomalous solubility.     

Blue Luminescence of Dendrimer‐Encapsulated Gold Nanoclusters

Direct evidence for the blue luminescence of gold nanoclusters encapsulated inside hydroxyl‐terminated polyamidoamine (PAMAM) dendrimers was provided by spectroscopic studies as well as by theoretical calculations. Steady‐state and time‐resolved spectroscopic studies showed that the luminescence of the gold nanoclusters consisted largely of two electronic transitions. Theoretical calculations indicate that the two transitions are attributed to the different sizes of the gold nanoclusters (Au₈ and Au₁₃). The luminescence of the gold nanoclusters was clearly distinguished from that of the dendrimers.     

Giant Emission Enhancement of Solid‐State Gold Nanoclusters by Surface Engineering

Ligand‐induced surface restructuring with heteroatomic doping is used to precisely modify the surface of a prototypical [Au₂₅(SR¹)₁₈]^? cluster ( 1 ) while maintaining its icosahedral Au₁₃ core for the synthesis of a new bimetallic [Au₁₉Cd₃(SR²)₁₈]^? cluster ( 2 ). Single‐crystal X‐ray diffraction studies reveal that six bidentate Au₂(SR¹)₃ motifs (L2) attached to the Au₁₃ core of 1 were replaced by three quadridentate Au₂Cd(SR²)₆ motifs (L4) to create a bimetallic cluster 2 . Experimental and theoretical results demonstrate a stronger electronic interaction between the surface motifs (Au₂Cd(SR²)₆) and the Au₁₃ core, attributed to a more compact cluster structure and a larger energy gap of 2 compared to that of 1 . These factors dramatically enhance the photoluminescence quantum efficiency and lifetime of crystal of the cluster 2 . This work provides a new route for the design of a wide range of bimetallic/alloy metal nanoclusters with superior optoelectronic properties and functionality.     

Soft,Oxidative Stripping of Alkyl Thiolate Ligands from Hydroxyapatite‐Supported Gold Nanoclusters for Oxidation Reactions

A strategy for the mild deprotection of alkyl‐thiolated (6‐mercaptohexanoic acid, MHA, and 3‐mercaptopropanoic acid, MPA) gold nanoclusters (Au NCs) supported on hydroxyapatite (HAP) has been developed by employing a peroxide (tert‐butyl hydroperoxide, TBHP, or hydrogen peroxide, H₂O₂) as an oxidant. The thiol ligands on the supported Au NCs were removed after oxidation, and the size and integrity of the supported clusters were well‐preserved. The bare gold clusters on HAP after removal of the ligands were catalytically effective for the epoxidation of styrene and the aerobic oxidation of benzyl alcohol. These two reactions were also investigated on calcined Au NCs that were supported on HAP for comparison, and the resulting Au NCs that were prepared by using this new strategy showed superior catalytic activity.     

Gold(I) Catalysis at Extreme Concentrations Inside Self‐Assembled Nanospheres

Homogeneous transition‐metal catalysis is a crucial technology for the sustainable preparation of valuable chemicals. The catalyst concentration is usually kept as low as possible, typically at mM or μM levels, and the effect of high catalyst concentration is hardly exploited because of solubility issues and the inherent unfavorable catalyst/substrate ratio. Herein, a self‐assembly strategy is reported which leads to local catalyst concentrations ranging from 0.05 M to 1.1 M , inside well‐defined nanospheres, whilst the overall catalyst concentration in solution remains at the conventional mM levels. We disclose that only at this high concentration, the gold(I) chloride is reactive and shows high selectivity in intramolecular C? O and C? C bond‐forming cyclization reactions.     

One‐Pot Synthesis of Phenylmethanethiolate‐Protected Au20(SR)16 and Au24(SR)20 Nanoclusters and Insight into the Kinetic Control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (‐SCH₂Ph)‐protected Au₂₀(SR)₁₆ and Au₂₄(SR)₂₄ nanoclusters in one‐pot by kinetic control. Unlike the previously reported methods for thiolate‐protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH₄ and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold–thiolate cluster synthesis. We demonstrate that the synthesis of ‐SCH₂Ph‐protected Au₂₀ and Au₂₄ nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH₄ under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH₄ under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au₂₀(SCH₂Ph)₁₆ and Au₂₄(SCH₂Ph)₂₀ clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH₄ solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH₂S‐protected Au₂₀ and Au₂₄ nanoclusters with the PhC₂H₄S‐protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH₂S‐capped nanoclusters when compared to the PhC₂H₄S‐capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.     

Taming Gold(I)–Counterion Interplay in the De‐aromatization of Indoles with Allenamides

A careful interplay between the π electrophilicity of a cationic Au^I center and the basicity of the corresponding counterion allowed for the chemo‐ and regioselective inter‐ as well as intramolecular de‐aromatization of 2,3‐disubstituted indoles with allenamides. The silver‐free bifunctional Lewis acid/Brønsted base complex [{2,4‐(tBu)₂C₆H₃O}₃PAuTFA] assisted the formation of a range of densely functionalized indolenines under mild conditions.     

Gold(I)‐Catalyzed Cycloisomerizations and Alkoxycyclizations of ortho‐(Alkynyl)styrenes

Indenes and related polycyclic structures have been efficiently synthesized by gold(I)‐catalyzed cycloisomerizations of appropriate ortho‐(alkynyl)styrenes. Disubstitution at the terminal position of the olefin was demonstrated to be essential to obtain products originating from a formal 5‐endo‐dig cyclization. Interestingly, a complete switch in the selectivity of the cyclization of o‐(alkynyl)‐α‐methylstyrenes from 6‐endo to 5‐endo was observed by adding an alcohol to the reaction media. This allowed the synthesis of interesting indenes bearing an all‐carbon quaternary center at C1. Moreover, dihydrobenzo[a]fluorenes can be obtained from substrates bearing a secondary alkyl group at the β‐position of the styrene moiety by a tandem cycloisomerization/1,2‐hydride migration process. In addition, diverse polycyclic compounds were obtained by an intramolecular gold‐catalyzed alkoxycyclization of o‐(alkynyl)styrenes bearing a nucleophile in their structure. Finally, the use of a chiral gold complex allowed access to elusive chiral 1H‐indenes in good enantioselectivities.     

An Original L‐shape,Tunable N‐Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis

The synthesis and characterization of original NHC ligands based on an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold‐catalyzed C?N, C?O, and C?C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5^Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic Au^I active species by ligand 1^Me .