Structure control and evolution of atomically precise gold clusters as heterogeneous precatalysts.

Metal clusters have distinct features from single atom and nanoparticle (>1 nm) catalysts, making them effective catalysts for various heterogeneous reactions. Nevertheless, the ambiguity and complexity of the catalyst structure preclude in-depth mechanistic studies. The evolution of metal species during synthesis and reaction processes represents another challenge. One effective solution is to precisely control the structure of the metal cluster, thus offering a well-defined pre-catalyst. The well-defined chemical formula and configurations make atomically precise metal nanoclusters optimal choices. To fabricate an atomically precise metal nanocluster-based heterogeneous catalyst with enhanced performance, careful structural design of both the nanocluster and support material, an effective assembling technique, and a pre-treatment method for these hybrids need to be developed. In this review, we summarize recent advances in in the development of heterogeneous catalysts using atomically precise gold and alloy gold nanoclusters as precursors. We will begin with a brief introduction to the structural properties of atomically precise nanoclusters and structure determination of cluster/support hybrids. We will then introduce heterogeneous catalysts prepared from medium size (tens to hundreds of metal atoms) and low nuclearity nanoclusters. We will illustrate how ligand modification, support-cluster interaction, hybrid fabrication, and heteroatom (Pt, Pd Ag, Cu, Cd, Fe) introduction affect the structural properties and pretreatment/reaction-induced structural evolution of gold nanocluster pre-catalysts. Lastly, we will highlight the synthetic method of NCs@MOF hybrids and their effectiveness in circumventing the adverse cluster structural evolution. These findings are expected to shed light on the structure-activity relationship studies and future catalyst design strategies using atomically precise metal nanocluster pre-catalysts.

A low-nuclear Ag4 nanocluster as a customized catalyst for the cyclization of propargylamine with CO2.

The preparation of 2-Oxazolidinones using CO2 offers opportunities for green chemistry, but multi-site activation is difficult for most catalysts. Here, A low-nuclear Ag4 catalytic system is successfully customized, which solves the simultaneous activation of acetylene (-C≡C) and amino (-NH-) and realizes the cyclization of propargylamine with CO2 under mild conditions. As expected, the Turnover Number (TON) and Turnover Frequency (TOF) values of the Ag4 nanocluster (NC) are higher than most of reported catalysts. The Ag4* NC intermediates are isolated and confirmed their structures by Electrospray ionization (ESI) and 1H Nuclear Magnetic Resonance (1H NMR). Additionally, the key role of multiple Ag atoms revealed the feasibility and importance of low-nuclear catalysts at the atomic level, confirming the reaction pathways that are inaccessible to the Ag single-atom catalyst and Ag2 NC. Importantly, the nanocomposite achieves multiple recoveries and gram scale product acquisition. These results provide guidance for the design of more efficient and targeted catalytic materials.

Ligand Assisted Thermal Atomization of Palladium Clusters: An Inspiring Approach for the Rational Design of Atomically Dispersed Metal Catalysts.

The transformation from metal nanocluster catalysts to metal single-atom catalysts is an important procedure in the rational design of atomically dispersed metal catalysts (ADCs). However, the conversion methods often involve high annealing temperature as well as reducing atmosphere. Herein, we reported a continuous and convenient approach to transfer Pd nanocluster into Pd single-atom in a ligand assisted annealing procedure, by which means we reduced its activating temperature low to 400 °C. Using ex-situ microscopy and spectroscopy, we comprehensively monitored the structural evolution of Pd species though the whole atomization process. Theoretical calculation revealed that the structural instability caused by remaining Cl ligands was the main reason for this low-temperature transformation. The present atomization strategy and mechanistic knowledge for the conversion from cluster to atomic dispersion provides guidelines for the rational design of ADCs.

Matching Bidentate Ligand Anchoring: an Accurate Control Strategy for Stable Single-Atom/ZIF Nanocatalysts.

Improving metal loading and controlling the coordination environment is nontrivial and challenging for single-atom catalysts (SACs), which have the greatest atomic efficiency and largest number of interface sites. In this study, a matching bidentate ligand (MBL) anchoring strategy is designed for the construction of CuN4 SACs with tunable coordination environments (Cu loading range from 0.4 to15.4 wt.%). The obtained Cu SA/ZIF and Cu SA/ZIF* (0.4 wt.%) (ZIF and ZIF* = Zeolitic imidazolate framework with Matching bidentate N-ligands) nanocomposites exhibit superior performance in homo-coupling of phenyl acetylene under light irradiation (TON = 580, selectivity > 99%), which is 22 times higher than that of Cu SA/NC-800 (NC = N-doped porous carbon). Experiments and density functional theory calculations confirmed that the specific Cu five-membered ring formed using the MBL anchoring strategy is the key to the immobilization of isolated Cu atoms. This strategy provides a basis for the construction of M SA/MOF, which has the potential to narrow the gap between experimental and theoretical catalysis, as further confirmed by the successful preparation of Fe SA/ZIF and Ni SA/ZIF.

Exploiting the Fracture in Metal-Organic Frameworks: A General Strategy for Bifunctional Atom-Precise Nanocluster/ZIF-8(300 °C) Composites.

Atom-precise nanoclusters-metal-organic framework (APNC/MOF) composites, as bifunctional material with well-defined structures, have attracted considerable attention in recent years. Despite the progress made to date, there is an urgent need to develop a generic and scalable approach for all APNCs. Herein, the authors present the Exploiting Fracture Strategy (EFS) and successfully construct a super-stable bifunctional APNC/ZIF-8(300 °C) composite overcoming the limitations of previous strategies in selecting APNCs. The EFS utilizes the fracture of ZnN in ZIF-8 after annealing at 300 °C. This method is suitable for all kinds of S/P protected APNCs with different sizes, including uncharged clusters Au1 Ag39 , Ag40 , negatively charged Au12 Ag32 , positively charged Ag46 Au24 , Au4 Cu4 and P-ligand-protected Pd3 Cl. Importantly, the generated APNC/MOF show significantly improved performances, for example, the activities of Au12 Ag32 /ZIF-8(300°C), Au4 Cu4 /ZIF-8(300°C), and Au1 Ag39 /ZIF-8(300°C) in the corresponding reactions are higher than those of Au12 Ag32 , Au4 Cu4 , and Au1 Ag39 , respectively. In particular, Au12 Ag32 /ZIF-8(300 °C) shows higher activity than Au12 Ag32 @ZIF-8. Therefore, this work offers guidance for the design of bifunctional APNC/MOF composites with excellent optimization of properties and opens up new horizons for future related nanomaterial studies and nanocatalyst designs.

Atomically Precise Dinuclear Site Active toward Electrocatalytic CO2 Reduction.

The development of atomically precise dinuclear heterogeneous catalysts is promising to achieve efficient catalytic performance and is also helpful to the atomic-level understanding on the synergy mechanism under reaction conditions. Here, we report a Ni2(dppm)2Cl3 dinuclear-cluster-derived strategy to a uniform atomically precise Ni2 site, consisting of two Ni1-N4 moieties shared with two nitrogen atoms, anchored on a N-doped carbon. By using operando synchrotron X-ray absorption spectroscopy, we identify the dynamically catalytic dinuclear Ni2 structure under electrochemical CO2 reduction reaction, revealing an oxygen-bridge adsorption on the Ni2-N6 site to form an O-Ni2-N6 structure with enhanced Ni-Ni interaction. Theoretical simulations demonstrate that the key O-Ni2-N6 structure can significantly lower the energy barrier for CO2 activation. As a result, the dinuclear Ni2 catalyst exhibits >94% Faradaic efficiency for efficient carbon monoxide production. This work provides bottom-up target synthesis approaches and evidences the identity of dinuclear sites active toward catalytic reactions.

Insight into the Mechanism of the CuAAC Reaction by Capturing the Crucial Au4Cu4-π-Alkyne Intermediate.

The classic Fokin mechanism of the CuAAC reaction of terminal alkynes using a variety of Cu(I) catalysts is well-known to include alkyne deprotonation involving a bimetallic σ,π-alkynyl intermediate. In this study, we have designed a CNT-supported atomically precise nanocluster Au4Cu4 (noted Au4Cu4/CNT) that heterogeneously catalyzes the CuAAC reaction of terminal alkynes without alkyne deprotonation to a σ,π-alkynyl intermediate. Therefore, three nanocluster-π-alkyne intermediates [Au4Cu4(π-CH≡C-p-C6H4R)], R = H, Cl, and CH3, have been captured and characterized by MALDI-MS. This Au4Cu4/CNT system efficiently catalyzed the CuAAC reaction of terminal alkynes, and internal alkynes also undergo this reaction. DFT results further confirmed that HC≡CPh was activated by π-complexation with Au4Cu4, unlike the classic dehydrogenation mechanism involving the bimetallic σ,π-alkynyl intermediate. On the other hand, a Cu11/CNT catalyst was shown to catalyze the reaction of terminal alkynes following the classic deprotonation mechanism, and both Au11/CNT and Cu11/CNT catalysts were inactive for the AAC reaction of internal alkynes under the same conditions, which shows the specificity of Au4Cu4 involving synergy between Cu and Au in this precise nanocluster. This will offer important guidance for subsequent catalyst design.

Design and Remarkable Efficiency of the Robust Sandwich Cluster Composite Nanocatalysts ZIF-8@Au25@ZIF-67.

Heterogeneous catalysts with precise surface and interface structures are of great interest to decipher the structure-property relationships and maintain remarkable stability while achieving high activity. Here, we report the design and fabrication of the new sandwich composites ZIF-8@Au25@ZIF-67[tkn] and ZIF-8@Au25@ZIF-8[tkn] [tkn = thickness of shell] by coordination-assisted self-assembly with well-defined structures and interfaces. The composites ZIF-8@Au25@ZIF-67 efficiently catalyzed both 4-nitrophenol reduction and terminal alkyne carboxylation with CO2 under ambient conditions with remarkably improved activity and stability, compared to the simple components Au25/ZIF-8 and Au25@ZIF-8, highlighting the highly useful function of the ultrathin shell. In addition, the performances of these composite sandwich catalysts are conveniently regulated by the shell thickness. This concept and achievements should open a new avenue to the targeted design of well-defined nanocatalysts with enhanced activities and stabilities for challenging reactions.

Atomically Precise Noble Metal Nanoclusters as Efficient Catalysts: A Bridge between Structure and Properties.

Improving the knowledge of the relationship between structure and properties is fundamental in catalysis. Recently, researchers have developed a variety of well-controlled methods to synthesize atomically precise metal nanoclusters (NCs). NCs have shown high catalytic activity and unique selectivity in many catalytic reactions, which are related to their ultrasmall size, abundant unsaturated active sites, and unique electronic structure different from that of traditional nanoparticles (NPs). More importantly, because of their definite structure and monodispersity, they are used as model catalysts to reveal the correlation between catalyst performance and structure at the atomic scale. Therefore, this review aims to summarize the recent progress on NCs in catalysis and provide potential theoretical guidance for the rational design of high-performance catalysts. First a brief summary of the synthetic strategies and characterization methods of NCs is provided. Then the primary focus of this review-the model catalyst role of NCs in catalysis-is illustrated from theoretical and experimental perspectives, particularly in electrocatalysis, photocatalysis, photoelectric conversion, and catalysis of organic reactions. Finally, the main challenges and opportunities are examined for a deep understanding of the key catalytic steps with the goal of expanding the catalytic application range of NCs.

Crystallization-induced emission enhancement: A novel fluorescent Au-Ag bimetallic nanocluster with precise atomic structure.

We report the first noble metal nanocluster with a formula of Au4Ag13(DPPM)3(SR)9 exhibiting crystallization-induced emission enhancement (CIEE), where DPPM denotes bis(diphenylphosphino)methane and HSR denotes 2,5-dimethylbenzenethiol. The precise atomic structure is determined by x-ray crystallography. The crystalline state of Au4Ag13 shows strong luminescence at 695 nm, in striking contrast to the weak emission of the amorphous state and hardly any emission in solution phase. The structural analysis and the density functional theory calculations imply that the compact C-H⋯π interactions significantly restrict the intramolecular rotations and vibrations and thus considerably enhance the radiative transitions in the crystalline state. Because the noncovalent interactions can be easily modulated via varying the chemical environments, the CIEE phenomenon might represent a general strategy to amplify the fluorescence from weakly (or even non-) emissive nanoclusters.

Total Structure Determination of Au21(S-Adm)15 and Geometrical/Electronic Structure Evolution of Thiolated Gold Nanoclusters.

The larger size gold nanoparticles typically adopt a face-centered cubic (fcc) atomic packing, while in the ultrasmall nanoclusters the packing styles of Au atoms are diverse, including fcc, hexagonal close packing (hcp), and body-centered cubic (bcc), depending on the ligand protection. The possible conversion between these packing structures is largely unknown. Herein, we report the growth of a new Au21(S-Adm)15 nanocluster (S-Adm = adamantanethiolate) from Au18(SR)14 (SR = cyclohexylthiol), with the total structure determined by X-ray crystallography. It is discovered that the hcp Au9-core in Au18(SR)14 is transformed to a fcc Au10-core in Au21(S-Adm)15. Combining with density functional theory (DFT) calculations, we provide critical information about the growth mechanism (geometrical and electronic structure) and the origin of fcc-structure formation for the thiolate-protected gold nanoclusters.

A New Crystal Structure of Au36 with a Au14 Kernel Cocapped by Thiolate and Chloride.

This study presents a new crystal structure of a gold nanocluster coprotected by thiolate and chloride, with the formula of Au36(SCH2Ph-(t)Bu)8Cl20. This nanocluster is composed of a Au14 core with two Cl atoms, a pair of pentameric Au5(SCl5) staple motifs, and a pair of hexameric Au6(S3Cl4) motifs. It is noteworthy that the "Au-Cl-Au" staple motifs are observed for the first time in thiolate protected gold nanoclusters. More importantly, the formation of the Cl-Au3 motifs is found to be mainly responsible for the configuration of the gold nanocluster. This work will offer a new perspective to understand how the ligands affect the crystal structure of gold nanocluster.

A metal exchange method for thiolate-protected tri-metal M(1)Ag(x)Au(24-x)(SR)(18)(0) (M = Cd/Hg) nanoclusters.

We herein reported the first synthesis of tri-metallic M1AgxAu24-x(SR)18(0) (M = Cd/Hg) nanoclusters by a two-step metal exchange method. Optical spectra suggested that the second and third foreign metals could largely change the electronic structure of homogold Au25(SR)18(-) nanoclusters. This work also provides a novel way to find the doping site of some special metals (such as Cd), which can be done using silver as the isotope of gold.

The structure and optical properties of the [Au18(SR)14] nanocluster.

Decreasing the core size is one of the best ways to study the evolution from Au(I) complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au18(SC6H11)14] nanocluster using the [Au18(SG)14] (SG=L-glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X-ray structure of the cyclohexylthiol-protected [Au18(C6H11S)14] nanocluster. The [Au18(SR)14] cluster has a Au9 bi-octahedral kernel (or inner core). This Au9 inner core is built by two octahedral Au6 cores sharing one triangular face. One transitional gold atom is found in the Au9 core, which can also be considered as part of the Au4(SR)5 staple motif. These findings offer new insight in terms of understanding the evolution from [Au(I)(SR)] complexes into Au nanoclusters.

Heterobimetallic dinuclear lanthanide alkoxide complexes as acid-base difunctional catalysts for transesterification.

A practical lanthanide(III)-catalyzed transesterification of carboxylic esters, weakly reactive carbonates, and much less-reactive ethyl silicate with primary and secondary alcohols was developed. Heterobimetallic dinuclear lanthanide alkoxide complexes [Ln2Na8{(OCH2CH2NMe2)}12(OH)2] (Ln = Nd (I), Sm (II), and Yb (III)) were used as highly active catalysts for this reaction. The mild reaction conditions enabled the transesterification of various substrates to proceed in good to high yield. Efficient activation of transesterification may be endowed by the above complexes as cooperative acid-base difunctional catalysts, which is proposed to be responsible for the higher reactivity in comparison with simple acid/base catalysts.

Crystal structure of selenolate-protected Au24(SeR)20 nanocluster.

We report the X-ray structure of a selenolate-capped Au24(SeR)20 nanocluster (R = C6H5). It exhibits a prolate Au8 kernel, which can be viewed as two tetrahedral Au4 units cross-joined together without sharing any Au atoms. The kernel is protected by two trimeric Au3(SeR)4 staple-like motifs as well as two pentameric Au5(SeR)6 staple motifs. Compared to the reported gold-thiolate nanocluster structures, the features of the Au8 kernel and pentameric Au5(SeR)6 staple motif are unprecedented and provide a structural basis for understanding the gold-selenolate nanoclusters.

A rhodamine-based "turn-on" fluorescent probe for Fe3+ in aqueous solution.

A water-soluble "turn-on" fluorescent probe (RD1) for Fe(3+) based on rhodamine B was designed and synthesized. The fluorescent probe showed "turn-on" fluorescent and colorimetric responses to Fe(3+) with a high selectivity in water containing less than 1% organic cosolvent. Furthermore, bioimaging investigations indicated that the new probe was cell permeable and suitable for monitoring intracellular Fe(3+) in living cells by confocal microscopy with low cytotoxicity.

Remarkable effect of alkali metal on polymerization of cyclic esters catalyzed by samarium-alkali metal multinuclear alkoxide clusters.

The remarkable effect of alkali metal on catalytic reactivity of samarium-alkali metal multinuclear alkoxide clusters is systematically studied. Three samarium-alkali metal multinuclear alkoxide clusters are synthesized in high yield by the reaction of anhydrous SmCl(3) with different molar ratios of alkali metal alkoxide and MOH (M = Na or K) in tetrahydrofuran (THF). These clusters were fully characterized by elemental analysis, IR, (1)H NMR and single-crystal structural analysis. These clusters exhibited good catalytic activity for the ring-opening polymerization of ε-caprolactone (ε-CL), L-lactide (L-LA) and trimethylene carbonate (TMC). It is interesting to note that the catalytic activity is much influenced by the alkali metals of the clusters. For the polymerization of these cyclic esters, the catalytic activities all increase with the increase of the molar ratio of alkali metal to samarium metal.

Novel mixed-metal alkoxide clusters of lanthanide and sodium: synthesis and extremely active catalysts for the polymerization of epsilon-caprolactone and trimethylene carbonate.

Novel mixed-metal alkoxide clusters of lanthanide and sodium [Ln2Na8(OCH2CH2NMe2)12(OH)2], which were synthesized in reproducible high yields and structurally characterized, were found to be extremely active catalysts for the ring-opening polymerization of epsilon-caprolactone and trimethylene carbonate.

Ytterbium(II) complex bearing a diaminobis(phenolate) ligand: synthesis, structure, and one-electron-transfer and epsilon-caprolactone polymerization reactions.

The first divalent ytterbium complex supported by a diaminobis(phenolate) ligand, YbL(THF)2.0.5C7H8 (1; THF = tetrahydrofuran), was synthesized in good yield by the amine elimination reaction of Yb[N(SiMe3)2]2(THF)2 with H2L (L = [Me2NCH2CH2N(CH2-2-OC6H2-3,5-But2)2]) in a 1:1 molar ratio. X-ray structural determination shows complex 1 to be a THF-solvated monomer, which adopts a distorted octahedral coordination geometry around the Yb atom. Complex 1 can react with PhNCO and PhCCH, as a single electron-transfer reagent, to give the corresponding reduction coupling product [(YbLOCNPh)(THF)]2.4THF (2) and the alkynide complex YbLCCPh(DME) (3; DME = 1,2-dimethoxyethane). Complexes 2 and 3 have been characterized by X-ray crystal structural analysis. In complex 2, the dianionic oxamide ligand resulting from the reductive coupling of two phenyl isocyanate molecules coordinates to two Yb atoms in a mu,eta4 fashion. Complex 3 has a monomeric structure with a Yb-C(terminal phenylacetynide) bond length of 2.374(3) A. Complex 1 is also a highly efficient catalyst for ring-opening polymerization of epsilon-caprolactone.