The atomic precision of the coin metal nanoclusters lays the foundation for the elucidation of the structure-property correlations at the atomic/molecular level. Herein, the bi-icosahedral [Au₁₃Ag₁₂(PPh₃)₁₀Cl₈]PF₆ and [Au₁₃Ag₁₂(PPh₂Py)₁₀Cl₈]PF₆ (loaded on activated carbon) have been developed as the catalyst for the epoxide cycloaddition with CO₂. The [Au₁₃Ag₁₂(PPh₂Py)₁₀Cl₈]/AC catalytic system shows better performance than its PPh₃-analogue, with turnover number (TON) reaching 3.03 × 10⁴. Specifically, the catalyst shows high substrate tolerance, and is widely applicable to different epoxide substrates bearing aryl, alkyl, halogen, alkenyl and ether groups (yield: 72%–96%). The mechanistic investigation with ultraviolet–visible absorption spectroscopy (UV–Vis), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance spectroscopy (NMR), cyclic voltammetry (CV) and density functional theory (DFT) calculations indicate that both the waisted coordination flexibility and pyridine N atom are important: the electron transfer from epoxide to the electrophilic, waisted Ag center drives the entire reaction, and the pyridyl N-atoms facilitates the CO₂-migration and the subsequent cycloaddition processes. This study proposes the possibility for utilizing structural flexibility of metal nanoclusters as an efficient strategy to activate the substrates, which could hopefully benefit the development of more catalytic systems.

Owing to advantages in synthesis, separation, structure determination, and low cost (compared to noble metal nanoclusters), Ag/Cu hydride clusters (and their alloys) have received increasing research interest in recent decades and have shown great potential in mediating reduction reactions and H₂ storage applications. The atomic precision of the Ag/Cu hydride clusters with the combination of single-crystal X-ray diffraction, ^1/2H nuclear magnetic resonance, electrospray ionization mass spectrometry, density functional theory, and particularly, single-crystal neutron diffraction, has provided pivotal information regarding its structural characteristics, facilitating a deep understanding of the inherent bonding principles therein. This review summarizes the research progress of atomically precise Ag/Cu hydride clusters (and their alloys) over the past three years (2021–2023), mainly focusing on the synthesis, structure analysis, and catalytic applications of the hydride clusters. We believe that this review can benefit the future design of different types of metal hydride clusters and aid in their application in various redox reactions.

The redox property of the ultrasmall coinage nanoclusters (with several to tens of Au/Ag atoms) has elucidated the electron-transfer capacity of nanoclusters, and has been successfully utilized in a variety of redox conversions (such as from CO₂ to CO). Nevertheless, their biological applications are mainly restricted by the scarcity of atomically precise, water-soluble metal nanoclusters, and the limited application (mainly on the decomposition of H₂O₂ in these days). Herein, mercaptosuccinic acid (MSA) protected ultrasmall alloy AuAg nanoclusters were prepared, and the main product was determined [Au₃Ag₅(MSA)₃]⁻ by electrospray ionization mass spectrometry (ESI-MS). The clusters can not only mediate the decomposition of H₂O₂ to generate hydroxyl radicals, but is also able to mediate the reduction of nicotinamide adenine dinucleotide (NAD) to its reduced form of NADH. This is the first time that the atomically precise metal nanoclusters were used to mediate the coenzyme reduction. The preliminary mechanistic insights imply the reaction to be driven by the hydrogen bonding between the carboxylic groups (on the surface of MSA) and the amino N–H bonds (on NAD). In this context, the presence of the carboxylic groups, the sub-nanometer size regime (~ 1 nm), and the synergistic effect of the Au-Ag clusters are pre-requisite to the NAD reduction.