Recent Advances in Ligand Engineering for Gold Nanocluster Catalysis: Ligand Library, Ligand Effects and Strategies

Advances in new ligands in the last decade facilitated in-depth studies on the property-relationship of gold nanoclusters and promoted the rational synthesis and related applications of such materials. Currently, more and more new ligands are being explored; thus, the ligand library of AuNCs is being expanded fast, which also enables investigation of ligand effects of AuNCs via direct comparison of different ligating shell with the identical gold core. It is now widely accepted that ligands influence the properties of AuNCs enormously including stability, catalysis, photoluminescence among others. These studies inspired ligand engineering of AuNCs. One of the goals for ligand engineering is to develop ligated AuNC catalysts in which the ligands are able to exert big-enough influence on electronic and steric control over catalysis as in a transition-metal or an enzyme system. Although increasing attention is paid to the further expansion of ligand library, the investigation of design principles and strategies regarding ligands are still in their infant stage. This review summarizes the ligands for AuNC synthesis, the ligand effects on stability and catalysis, and recently developed strategies in promoting AuNC catalytic performance via ligand manipulation.

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂ CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.

Structural Engineering toward Gold Nanocluster Catalysis

Atomically precise gold nanoclusters provide great opportunities to explore the relationship between the structure and properties of nanogold catalysts. A nanocluster consists of a metal core and a surface ligand shell, and both the core and shell have significant effects on the catalytic properties. Thanks to their precise structures, the active metal site of the clusters can be readily identified and the effects of ligands on catalysis can be disclosed. In this Minireview, we summarize recent advances in catalytic research of gold nanoclusters, emphasizing four strategies for constructing open metal sites, including by post-treatment, the bulky ligands strategy, the surface geometric mismatch method, and heteroatom doping procedures. We also discuss the effects of ligands on the catalytic activity, selectivity, and stability of gold cluster catalysts. Finally, we present future challenges relating to gold cluster catalysis.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au₂₈ (C₂ B₁₀ H₁₁ S)₁₂ (tht)₄ Cl₄ (Au₂₈ -S), was isolated by a facile self-reducing synthesis. Au₂₈ -S adopts an identical Au₂₈ metal framework to that of the reported alkynyl-protected Au₂₈ -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au₂₈ -S shows better catalytic activity for the electrochemical reduction of CO₂ to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

The conversion of CO2 into multicarbon (C2+ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Ligand Engineering toward the Trade-Off between Stability and Activity in Cluster Catalysis

We report the structures, stability and catalysis properties of two Ag₂₁ nanoclusters, namely [Ag₂₁ (H₂ BTCA)₃ (O₂ PPh₂ )₆ ]SbF₆ (1) and [Ag₂₁ (C≡CC₆ H₃ -3,5-R₂ )₆ (O₂ PPh₂ )₁₀ ]SbF₆ (2) (H₄ BTCA=p-tert-butylthiacalix[4]arene, R=OMe). Both Ag₂₁ structures possess an identical icosahedral kernel that is surrounded by eight peripheral Ag atoms. Single-crystal structural analysis and ESI-MS revealed that 1 is an 8-electron cluster and 2 has four free electrons. Theoretical results show that the P-symmetry orbitals are found as HOMO-1 and HOMO states in 1, and the frontier unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) show D-character, indicating 1 is a superatomic cluster with an electronically closed shell 1S² 1P⁶ , while 2 has an incomplete shell configuration 1S² 1P² . These two Ag₂₁ clusters show superior stability under ambient conditions, and 1 is robust even at 90 °C in toluene and under oxidative conditions (30 % H₂ O₂ ). Significantly, 2 exhibits much higher activity than 1 as catalyst in the reduction of 4-nitrophenol. This work demonstrates that ligands can influence the electronic structures of silver clusters, and further affect their stability and catalytic performance.