
==== Front
Natl Sci Rev
Natl Sci Rev
nsr
National Science Review
2095-5138
2053-714X
Oxford University Press

10.1093/nsr/nwae252
nwae252
Research Article
Chemistry
Nsr/1
AcademicSubjects/MED00010
AcademicSubjects/SCI00010
Regulating interaction with surface ligands on Au25 nanoclusters by multivariate metal–organic framework hosts for boosting catalysis
Wang He Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

Liu Xiaokang National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China

Zhao Yulong Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

Sun Zhihu National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China

Lin Yue Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China

Yao Tao National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China

https://orcid.org/0000-0002-2975-7977
Jiang Hai-Long Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

Corresponding authors. E-mails: yaot@ustc.edu.cn
Corresponding authors. E-mails: jianglab@ustc.edu.cn
Equally contributed to this work.

10 2024
23 7 2024
23 7 2024
11 10 nwae25225 5 2024
26 6 2024
07 7 2024
23 8 2024
© The Author(s) 2024. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
2024
https://creativecommons.org/licenses/by/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

While atomically precise metal nanoclusters (NCs) with unique structures and reactivity are very promising in catalysis, the spatial resistance caused by the surface ligands and structural instability poses significant challenges. In this work, Au25(Cys)18 NCs are encapsulated in multivariate metal–organic frameworks (MOFs) to afford Au25@M-MOF-74 (M = Zn, Ni, Co, Mg). By the MOF confinement, the Au25 NCs showcase highly enhanced activity and stability in the intramolecular cascade reaction of 2-nitrobenzonitrile. Notably, the interaction between the metal nodes in M-MOF-74 and Au25(Cys)18 is able to suppress the free vibration of the surface ligands on the Au25 NCs and thereby improve the accessibility of Au sites; meanwhile, the stronger interactions lead to higher electron density and core expansion within Au25(Cys)18. As a result, the activity exhibits the trend of Au25@Ni-MOF-74 > Au25@Co-MOF-74 > Au25@Zn-MOF-74 > Au25@Mg-MOF-74, highlighting the crucial roles of microenvironment modulation around the Au25 NCs by interaction between the surface ligands and MOF hosts.

Regulating surface ligands on metal nanoclusters by MOF hosts improves single electron transfer and metal site accessibility for enhanced catalysis.

metal nanoclusters
metal–organic frameworks
heterogeneous catalysis
microenvironment modulation
National Key Research and Development Program of China 10.13039/501100012166 2021YFA1500402 National Natural Science Foundation of China 10.13039/501100001809 22331009 U22A20401 Chinese Academy of Sciences 10.13039/501100002367 XDB0450302 XDB0540000 Fundamental Research Funds for the Central Universities 10.13039/501100012226 WK2060000038
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pmcINTRODUCTION

The design and fabrication of structurally precise metal sites are crucial for the development of high-performance heterogeneous catalysts [1–3]. Metal nanoclusters (NCs) with atomic-level structural precision have attracted intense attention due to their small size, uniform active sites and unique electronic structures, giving rise to outstanding activity and selectivity in various catalytic reactions [4–7]. Unfortunately, the surface of metal NCs is always covered with numerous organic ligands that substantially influence catalysis. On the one hand, these surface ligands are detrimental to the accessibility of metal sites [8,9]; on the other, they optimize the electronic structure of metal NCs and improve the activity and selectivity of specific reactions [10–12]. Therefore, the role of surface ligands is akin to a double-edged sword affecting the catalytic properties [13]. In this context, it would be of great importance to preserve the strength yet suppress the drawback of surface ligands on metal NCs, so as to promote the catalysis of metal NCs. Inspired by the crucial roles of the surrounding microenvironment around catalytic centers in bio-enzyme catalysis [14], through the manipulation of the organic surface ligands on metal NCs, it might be possible to improve the substrate accessibility to and electronic structure of catalytic metal sites. To this end, it would be highly desired to develop porous hosts featuring well-defined structures and tunable interaction with the surface ligands. This would allow the regulation of their configuration and movement, thereby influencing the accessibility and activity of central metal sites.

To achieve the aforementioned goal, metal–organic frameworks (MOFs)—a class of crystalline porous materials with coordinatively unsaturated metal sites (for interacting with surface ligands) and tunable pore sizes (for hosting metal NCs)—would be promising candidates [15–18]. MOFs have been demonstrated to be very suitable for incorporating diverse catalytically active species, including single-atom metal sites [19,20], metal nanoparticles [21,22], organic molecules [23], enzymes [24,25], etc., into pore spaces for enhanced catalysis. There have been also a couple of reports on the integration of metal NCs with MOFs in catalysis [26–31], where the regulation of surface ligands on metal NCs has not yet been investigated, except for ligand removal in the only study [32]. Given that the structure and properties of metal NCs are particularly sensitive to the surrounding microenvironment [33], it would be reasonable to adopt MOFs to modulate the microenvironment around metal NCs by regulating their surface ligands based on their interaction with MOFs for enhanced catalytic performance [34]. It is expected that the interactions between surface ligands of metal NCs and MOFs would effectively suppress the free vibration of surface ligands in solution and reduce their hindrance of substrate access to the metal sites [26]. Moreover, the spatial confinement effect of MOFs would significantly improve the catalytic stability of metal NCs [27–29]. However, to our knowledge, it remains unknown how the interaction with porous hosts (e.g. MOFs) regulates the surface ligands on metal NCs for improved catalysis.

In this work, Au25(Cys)18 NCs are encapsulated into representative multivariate MOFs, M-MOF-74 (M = Zn, Ni, Co, Mg), based on coordinated self-assembly and electrostatic interactions to afford Au25@M-MOF-74 (Fig. 1). Significant differences in the fluorescence properties of Au25(Cys)18 are observed among these materials, which can be attributed to the restricted influence of M-MOF-74 encapsulation on the surface-ligand vibration behavior of Au25(Cys)18. Accordingly, the encapsulation of Au25 NCs in M-MOF-74 significantly improves its catalytic efficiency in the intramolecular cascade reaction of 2-nitrophenyl cyanide. Furthermore, X-ray absorption spectroscopy (XAS) results indicate that the electron density of Au25(Cys)18 and the length of the Au–Au bonds within the core of Au25(Cys)18 can be systematically regulated in addition to the interaction strengths between M-MOF-74 and Au25(Cys)18 NCs. Such interaction modulates the microenvironment of Au25(Cys)18, which improves the Au25 accessibility and facilitates the electron transfer from the Au25 NCs to substrates responsible for promoting the catalysis. As a result, the catalytic activity can be further regulated by doping the metal nodes of MOF-74 in the following sequence: Ni > Co > Zn > Mg. Moreover, Au25@Ni-MOF-74 exhibits far superior catalytic activity and stability to the control of Au25/Ni-MOF-74 that is made by supporting the Au25 NCs on the outer surface of Ni-MOF-74. As far as we know, this is the first report on regulating interaction by MOFs with surface ligands of metal clusters, resulting in enhanced catalysis.

Figure 1. Illustration showing the synthetic route to Au25@M-MOF-74 (M = Zn, Ni, Co, Mg) for enhanced catalysis through microenvironment modulation around Au25(Cys)18 by adjusting the metal nodes on MOF pore walls.

RESULTS AND DISCUSSION

Synthesis and characterization of the Au25@Ni-MOF-74 catalyst

The Au25(Cys)18 NCs were selected due to their relatively stable structure and abundance of carbonyl groups on the surface (Fig. S1a) [35]. It was synthesized by the NaOH-mediated NaBH4 reduction method and the high purity of the synthesized Au25(Cys)18 was confirmed by using UV–vis spectroscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Fig. S2) [35]. To prevent structural damage to Au25(Cys)18 under high-temperature solution conditions, in situ growth of the MOF-74 outer-shell was developed. The MOF linker, 2,5-dihydroxyterephthalic acid (DHTP) and Au25(Cys)18 were pretreated in aqueous NaOH solution, inducing the deprotonation and subsequently promoting the dissolution of DHTP in water. This facilitates the reaction with metal ions, leading to the growth of MOF at room temperature, and promotes the dispersion and encapsulation of Au25(Cys)18 in MOF-74. Subsequently, the metal ions underwent coordination self-assembly with Au25(Cys)18 and DHTP at room temperature, resulting in the formation of Au25@MOF-74 [36]. Taking advantage of the multivariate feature of MOFs, M-MOF-74 (M = Ni, Co, Mg) with Ni2+, Co2+ or Mg2+ in the Zn-oxo chains was obtained (Fig. S1b), ensuring their similar crystallinity and sizes, by mixing these metal acetates together with zinc acetate in the synthesis of Zn-MOF-74. Accordingly, Au25@M-MOF-74 (M = Zn, Ni, Co, Mg) were fabricated following a similar synthetic route to Au25@MOF-74, which would serve as an ideal platform for regulating the interaction with the surface ligands of Au25(Cys)18. In addition, due to the presence of a large number of negatively charged carboxyl groups on the surface, Au25(Cys)18 can be attached to the outer surface of Ni-MOF-74 by electrostatic interaction to yield Au25(Cys)18/Ni-MOF-74 [30,31].

Powder X-ray diffraction (XRD) patterns indicate that the M-MOF-74 has similar crystallinity and Au25(Cys)18 encapsulation or support has no influence on the structural integrity and crystallinity of the MOFs (Fig. S3). Nitrogen sorption results demonstrate that Au25@M-MOF-74 and Au25/Ni-MOF-74 maintain the high porosity and very similar pore size distribution, indicating that the introduction of Au25(Cys)18 does not affect the MOF microporous structure (Fig. S4). The Brunauer-Emmett-Teller surface area of Au25@Ni-MOF-74 is 639 m2/g, similar to that of Au25@Zn-MOF-74 (604 m2/g), Au25@Co-MOF-74 (609 m2/g) and Au25@Mg-MOF-74 (610 m2/g), reflecting that different metal nodes have little influence on the surface area. Due to the pore space occupation of MOF-74 by the Au25 NCs, their surface areas are reasonably lower than Au25/MOF-74 (667 m2/g). For Au25@M-MOF-74, Au25(Cys)18 can be isolated by the removal of MOF-74 using dilute hydrochloric acid. UV–vis spectra for the isolated Au25(Cys)18 NCs, upon removing the MOF, showcase the characteristic bands (Fig. S5), demonstrating that MOF-74 encapsulation does not influence the integrity of the Au25(Cys)18 structure.

Scanning electron microscopy (SEM) images show that Au25@M-MOF-74 and Au25/Ni-MOF-74 have similar particle sizes and morphology (Fig. 2a and Fig. S6). From the high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images, it can be observed that Au25(Cys)18 NCs are uniformly dispersed throughout the Ni-MOF-74 support (Fig. 2b and Fig. S7a). Direct comparison of the HAADF-STEM and secondary electron STEM (SE-STEM) images acquired at the same location provides direct evidence that Au25(Cys)18 NCs are encapsulated inside Ni-MOF-74 in Au25@Ni-MOF-74, as indicated by unobservable Au25 NCs in the SE-STEM image (Fig. 2b and c). By comparison, the Au25 NCs can be clearly observed in both images, showing that the Au25 NCs are supported on the surface of Ni-MOF-74 in Au25/Ni-MOF-74 (Fig. S8). HAADF-STEM images of Au25@Ni-MOF-74 projected at different tilt angles (from +30° to −30°) show that the Au25 NCs remain monodispersed in the Ni-MOF-74 (Fig. S9). The HAADF-STEM images and corresponding energy dispersive X-ray spectroscopy (EDS) mapping analyses reveal that both metals in M-MOF-74 and Au25(Cys)18 are uniformly distributed throughout the MOF particle (Fig. 2d and e, and Figs S9–S12). The Au loading amount in Au25@M-MOF-74 is controlled to be ∼2 wt% and the mixed metal molar ratio in M-MOF-74 is also maintained at ∼1 according to inductively coupled plasma atomic emission spectroscopy (ICP-AES) results (Table S1).

Figure 2. (a) SEM image, (b) HAADF-STEM and (c) the corresponding SE-STEM image of Au25@Ni-MOF-74. (d) HAADF-STEM image and (e) corresponding EDS elemental mapping of Au, Ni and Zn, and their overlap for Au25@Ni-MOF-74.

Performance of catalytic reduction of 2-nitrobenzonitrile

Given the unique surface and electronic structure of Au25(Cys)18 NCs, they can serve as electron mediators to initiate catalysis via single electron transfer [37,38]. Therefore, the reduction reaction of 2-nitrobenzonitrile containing electron-transfer processes is adopted, which is an important route for the production of significant pharmaceuticals precursor 2-amniobenzamide [37]. Control experiments indicate that no product is detected in the absence of catalysts or with the use of the four M-MOF-74 catalysts (Table S2). When adopting Au25(Cys)18 NCs, the activity and selectivity are 20.6% and 91.0%, respectively, indicating that the Au25 NCs are able to behave as active species. Unfortunately, they are prone to aggregation, as supported by the UV–vis spectrum and HAADF-STEM observation (Fig. S13). Moreover, no obvious difference in catalytic activity is observed when the same amount of Au25 NCs is physically mixed with the four M-MOF-74, respectively, indicating that M-MOF-74 does not affect the catalytic reaction process (Table S2).

Strikingly, the encapsulation of Au25(Cys)18 NCs in M-MOF-74 significantly improves the activity, in which the conversion and selectivity reach 99.8% and 99.2%, respectively, for Au25@Ni-MOF-74 (Fig. 3a). In comparison, the conversion and selectivity of Au25@Co-MOF-74, Au25@Zn-MOF-74 and Au25@Mg-MOF-74 are decreased, with conversion of 72.7%, 46.2% and 32.1%, and selectivity of 97.1%, 94.3% and 93.5%, respectively (Fig. 3a). The reaction yield gradually increases along with reaction time, showcasing activity in the order of Au25@Ni-MOF-74 > Au25@Co-MOF-74 > Au25@Zn-MOF-74 > Au25@Mg-MOF-74 (Fig. 3b). The results indicate that the metal doping in M-MOF-74 exerts a pivotal influence on the catalytic efficiency of Au25(Cys)18. Powder XRD patterns of Au25@M-MOF-74 do not exhibit any discernible decrease in the MOF crystallinity after the catalytic reaction (Fig. S14); the UV–vis spectra of the detached Au25 NCs upon the MOF removal exhibit no substantial alterations with the as-synthesized Au25 NCs, effectively demonstrating the structural integrity of both components in Au25@M-MOF-74 during the catalytic reaction (Fig. 3c). As a control, the Au nanoparticles (NPs) with surface ligands of Cys were synthesized; the UV–vis spectrum displays a characteristic surface plasmon resonance peak at 520 nm and the HAADF-STEM image indicates that the Au NPs are ∼2 nm in size (Fig. S15). Afterward, they were encapsulated into Ni-MOF-74 to obtain AuNPs@Ni-MOF-74 with retained Au sizes and good MOF crystallinity (Fig. S16) via the same coordination self-assembly as that of Au25@Ni-MOF-74. The conversion and selectivity of AuNPs@Ni-MOF-74 in the reaction are only 15.2% and 89.5%, respectively (Table S2). This is possibly attributed to the fact that the charge-transfer effect on the surface of Au NPs is too weak to efficiently facilitate the migration of electrons from NaBH4 to the substrate [37].

Figure 3. (a) The conversion and selectivity of Au25@M-MOF-74 (∼2 wt% Au loading) in the intramolecular cascade reaction of 2-nitrobenzonitrile at 25°C for 1 h. (b) Time-dependent yield of 2-aminobenzamide with Au25@M-MOF-74. (c) UV–vis spectra of as-prepared Au25(Cys)18 and the Au25(Cys)18 NCs detached from Au25@M-MOF-74 after the catalytic reaction. (d) The recyclability tests of Au25@Ni-MOF-74.

The incorporation of Au25(Cys)18 into M-MOF-74 has been demonstrated to not only influence the activity as noted above, but also strongly improve the catalytic stability. The catalytic yield of Au25@Ni-MOF-74 is maintained at 99% for 1 h in the three cycles; in contrast, the yield of Au25/Ni-MOF-74 gives 25% in the first cycle and decreases continuously during the next two cycles (Fig. S17), possibly due to the unexpected leaching or aggregation of Au25(Cys)18. HAADF-STEM images show that the size and dispersion of Au25(Cys)18 in Au25@Ni-MOF-74 remain unchanged after three reaction cycles due to the confined protection by the MOF, whereas significant agglomeration of Au25 NCs is observed in Au25/Ni-MOF-74 (Fig. S18). No noticeable change occurs in the Au content in Au25@Ni-MOF-74 after the reaction; however, a decrease of ∼10% can be found for Au25/Ni-MOF-74 (Table S1). These results exemplify the much enhanced stability of Au25(Cys)18 by MOF encapsulation.

The stability and recyclability of Au25@Ni-MOF-74 have been further demonstrated. The activity and selectivity exhibit no significant decrease in the five consecutive cycles with a controlled reaction time of 40 min and ∼80% conversion (Fig. 3d). Powder XRD patterns, HAADF-STEM images and corresponding EDS elemental mapping analyses suggest that the catalyst microstructure can be retained after five consecutive cycles (Figs S19 and S20). In addition, the results of the hot filtration experiment for Au25@Ni-MOF-74 manifest that no leaching occurs in Au25 NCs and the process is truly heterogeneous catalysis (Fig. S21).

Mechanism of the catalytic reaction

Given the significant differences in the activity of Au25@M-MOF-74, relevant investigations have been conducted to understand the intrinsic mechanisms involved. The variations in fluorescence emission intensity and wavelength can reflect the extent of motion associated with the vibration and rotation of the ligands on the surface of the metal NCs [39,40]. Under excitation of 420 nm, the photoluminescence (PL) spectrum of Au25(Cys)18 is in the visible region with a maximum value of ∼720 nm (Fig. 4a) and the PL of Au25@M-MOF-74 is substantially enhanced in intensity and undergoes an obvious blue shift compared with that of Au25(Cys)18. The phenomenon can be ascribed to the coordination interactions between the metal-oxo chain in M-MOF-74 and the free carboxyl groups from the Cys on the surface of Au25(Cys)18, which restricts the vibration and rotation of the Cys ligands and inhibits the non-radiative leaps of Au25(Cys)18, thereby improving the emission efficiency [41]. Moreover, this interaction affects the electronic structure of Au25(Cys)18, giving rise to the blue shift of its fluorescence signal [39]. Therefore, the PL intensity and peak shift of Au25(Cys)18 incorporated in M-MOF-74 can reflect the degree of Cys rigidification (Fig. 4b). The PL intensities and peak shift of Au25(Cys)18 follow the trend of Au25@Ni-MOF-74 > Au25@Co-MOF-74 > Au25@Zn-MOF-74 > Au25@Mg-MOF-74, in agreement with the order of their catalytic activities. This reflects that the difference in the coordination strength between the MOF metal-oxo chains with carboxylate groups on the Au25(Cys)18 surface modulates the microenvironment of Au25(Cys)18, influencing the catalytic activity. The PL intensity and shift of Au25@M-MOF-74 are apparently greater than those of Au25/MOF-74, suggesting that the MOF encapsulation is more favorable for the rigidification of Cys and improves the activity to a larger extent.

Figure 4. (a) Fluorescence spectra of Au25(Cys)18, Au25@M-MOF-74 and Au25@Ni-MOF-74 in aqueous solution. (b) A schematic diagram of encapsulating Au25(Cys)18 by M-MOF-74 restricts Cys ligand vibrations on its surface. (c) The Au 4f XPS spectra of Au25(Cys)18, Au25/Ni-MOF-74 and Au25@M-MOF-74.

X-ray photoelectron spectroscopy (XPS) analysis demonstrates a significant increase in the Au electron density of Au25(Cys)18 upon integration with M-MOF-74 (Fig. 4c). Interestingly, the electron density of Au in Au25@M-MOF-74 is in a sequence that is consistent with their activity order, disclosing the fact that the charge-transfer interaction between Au25(Cys)18 and M-MOF-74 benefits the activity. In addition, the change in the Au electron density in Au25/Ni-MOF-74 is relatively small, further suggesting that Au25(Cys)18 supported on MOF cannot create a strong interaction with Au25(Cys)18.

UV–vis spectroscopy and electron spin resonance (ESR) spectroscopy are further adopted to investigate the electron-transfer process in the catalytic reactions. The Au25 NCs with different charges have unique UV–vis absorption spectra, which can clearly distinguish the charge states of Au25 NCs [42]. When 2-nitrobenzonitrile is added as the substrate to the Au25(Cys)18 with a negative charge (abbreviated as Au25−) in the aqueous solution, its UV–vis spectrum displays a distinct change from the characteristic spectrum of uncharged Au25(Cys)18 (abbreviated as Au250). After NaBH4 is added, the characteristic absorption peak of Au25− reappears (Fig. S22), indicating the recovery of Au25−. Furthermore, ESR experiments indicate that Au25@Ni-MOF-74 and the mixture of Ni-MOF-74 with 2-nitrobenzonitrile do not result in any signal, whereas the mixture of Au25(Cys)18 or Au25@Ni-MOF-74 with 2-nitrobenzonitrile results in a triple peak corresponding to the N radicals (Fig. S23). These results confirm that Au25(Cys)18 is an electronic mediator that continuously transfers electrons from NaBH4 to the substrate. In addition, a range of catalytic experiments with substrate analogs have been performed (Table S3). The functional group of the substrates can be efficiently and completely reduced when the unsaturated group is placed at the ortho position only, suggesting that an intermolecular cascade reaction has occurred. To further investigate the catalytic mechanism, deuterium-labeling experiments are also conducted. When H2O is replaced by D2O and NaBH4 is replaced by NaBD4 in the reaction system, the molecular weight of the products detected by using mass spectrometry increases (Fig. S24), implying that H2O and NaBH4 are involved in the reaction. As a result, reaction paths can be proposed (Fig. S25).

The geometric structure and electronic properties of Au25(Cys)18

Given that the X-ray absorption fine structure (XAFS) is sensitive to the structure of metal NCs [43,44], the Au L3-edge XAFS is adopted to investigate the effect of MOF encapsulation on the geometric structure and electronic properties of Au25(Cys)18. X-ray absorption near edge structure (XANES) spectra display obvious differences between the peak profiles of Au25(Cys)18 and fcc-structured Au foil (Fig. 5a). Moreover, the peak profile of Au25(Cys)18 almost remains after its integration with M-MOF-74, suggesting that the structure of Au25(Cys)18 is preserved in Au25@M-MOF-74, which is in agreement with the above results. The white line peak corresponds to the electronic transition from the core level to the unoccupied 5d valence level and the variation in the intensity of the white line peak can drive the absorption edge to different directions of energy, thus explaining different electronic states [45]. In comparison with that of the Au foil, the spectrum of Au25(Cys)18 exhibits a more intense white line peak at ∼11 924 eV, which is due to the electron-withdrawing property of the thiol ligand and the reduced 5d electron density of surface Au atoms. On the other hand, the white line intensity of Au25@M-MOF-74 is lower than that of Au25(Cys)18, suggesting the greater occupation of the 5d electronic state in Au25@M-MOF-74. This is likely due to the MOF’s constraining thiol ligand vibrations on the Au25(Cys)18 surface, thereby weakening the electron-withdrawing effect of the thiol ligand toward the Au atoms. In addition, the intensity order of the white line peaks in Au25@M-MOF-74 is further demonstrated by means of differential spectroscopy of XAFS, which is consistent with the results of XPS analysis (Fig. S26), confirming that the microenvironment modulation caused by the interaction between M-MOF-74 and the thiol ligand regulates the Au25 electronic structure.

Figure 5. (a) Au L3-edge XANES spectra of Au foil, Au25(Cys)18 and Au25@M-MOF-74. (b) Au L3-edge FT-EXAFS spectra of Au25(Cys)18 and Au25@M-MOF-74. (c) Attribution of the EXAFS-analysed Au25(Cys)18 structure to three distinct bond domains (the Au–S, Au–Au-1 and Au–Au-2 highlighted with ellipses). (d) The bond lengths of Au–Au-1 and Au–Au-2 are extracted from the refinement of the FT-EXAFS spectra of Au25(Cys)18 and Au25@M-MOF-74.

To quantify the local atomic structure of Au25(Cys)18 in MOF-74, the extended X-ray absorption fine structure (EXAFS) region has been analysed. All Au25@M-MOF-74 samples exhibit oscillation patterns that are similar to those of Au25(Cys)18, whereas the oscillation intensity increases, suggesting that the surrounding microenvironment modulation driven by the MOF encapsulation can affect the degree of Au25(Cys)18 disorder (Fig. S27). Fourier transform (FT) is performed in the R-space of the EXAFS spectra for Au25(Cys)18 and Au25@M-MOF-74 (Fig. 5b). For Au25(Cys)18, three prominent features are shown at 1.9, 2.4 and 2.7 Å; thus, the fittings are performed using three coordination paths. As previously reported [46], the first peak represents the S atom directly coordinated to Au, i.e. Au–S on the motif of Au25(Cys)18, and the other two peaks represent contributions from two Au–Au pathways (Fig. 5c). By comparing the curve-fitting analysis of Au25(Cys)18 and Au25@M-MOF-74 EXAFS results (Table S4), it is observed that the bond length of Au–S is very similar (Fig. S28), while Au–Au-1 and Au–Au-2 increase when Au25(Cys)18 NCs are encapsulated in Ni-MOF-74 and Co-MOF-74, suggesting that the Au25(Cys)18 nucleus undergoes expansion in response to the stronger interaction with M-MOF-74 (Fig. 5d). Notably, the Au–Au bond length sequence in Au25@M-MOF-74 is in good agreement with their catalytic activity, suggesting that the catalytic performance of metal NCs can be optimized by manipulating the metal-oxo chains in M-MOF-74 hosts. Furthermore, the Au L3-edge XANES and EXAFS spectra of Au25@Ni-MOF-74 after the reaction are similar to those before the reaction (Fig. S29), further supporting that the structure of the Au25 NCs in Au25@Ni-MOF-74 barely changes after the catalytic reaction, matching the UV–vis spectra above (Fig. 3c) and demonstrating that MOF encapsulation improves the structural stability of metal NCs.

Based on the above analysis, it can be concluded that the strong coordination interaction between M-MOF-74 and Au25(Cys)18 results in the rigidification of surface ligands and the expansion of the Au nucleus. Given that ligand vibrations on the surface of Au25 NCs in the reaction solution prevent the substrate from accessing the Au sites, rigidifying the surface ligands and expanding the gold nucleus are expected to increase the available spatial domain for substrate accessibility, thereby boosting activity. Additionally, the different interaction between tunable M-MOF-74 and surface ligands of Au25(Cys)18 effectively regulates the electron transfer from the MOF to Au25 NCs. Due to the electrophilicity of the nitro and cyano groups in 2-nitrobenzonitrile, metal surfaces with higher electron densities will give stronger interactions with the substrate [47,48]. Overall, optimization of the accessibility and electron density of the Au25 NCs facilitates electron transfer with the substrate and promotes the conversion [49].

CONCLUSION

In summary, atomically precise Au25(Cys)18 NCs have been successfully encapsulated in M-MOF-74 with different metal nodes through coordination self-assembly, yielding Au25@M-MOF-74 composites for the intramolecular cascade reaction of 2-nitrobenzonitrile. Strikingly, the activity and stability of Au25(Cys)18 are significantly enhanced upon being incorporated into MOF-74, surpassing those of Au25/MOF-74. Remarkably, doping different metal species into the metal-oxo chains in MOF-74 showcases significant activity difference in the order of Au25@Ni-MOF-74 > Au25@Co-MOF-74 > Au25@Zn-MOF-74 > Au25@Mg-MOF-74. Both fluorescence and XAFS analyses demonstrate that the engineering of metal-oxo nodes in MOFs gives rise to the rigidification of surface ligands on Au25(Cys)18 and induces expansion at the Au nucleus level, improving the accessibility of the Au sites. Moreover, the stronger coordination interaction between M-MOF-74 and Au25(Cys)18 further increases the electron density of Au25 NCs, which is favorable to the substrate activation, leading to enhanced activity. This work describes improved single electron transfer and metal site accessibility of metal NCs by regulating the interaction between their surface ligands and multivariate MOF hosts, which opens a new avenue for boosting the catalysis of metal NCs by surface microenvironment modulation.

MERHODS

Synthesis of Au25@Zn-MOF-74

Typically, 0.2 mL of aqueous solution of Au25(Cys)18 (15 mg/mL) was added to 1 mL of 0.73 M NaOH aqueous solution of DHTP (0.183 mmol) at 25°C. Then, 1 mL of aqueous solution of Zn(CH3COO)2·2H2O (0.732 mmol) was mixed with the solution, sonicated for 1 min and stirred for 6 h at 25°C. The precipitate was collected by centrifugation and washed with H2O and MeOH three times. Finally, the precipitate was soaked in MeOH for 48 h and dried under a vacuum at 60°C overnight.

Synthesis of Au25@M-MOF-74 (M = Ni, Co, Mg)

Typically, 0.2 mL of Au25(Cys)18 aqueous solution (15 mg/mL) was added into 1 mL of 0.73 M NaOH aqueous solution of DHTP (0.183 mmol) at 25°C. Then, 1 mL of aqueous solution of M(CH3COO)2·4H2O (0.183 mmol) (M = Ni, Co, Mg) and Zn(CH3COO)2·2H2O (0.183 mmol) was mixed with the solution, sonicated for 1 min and stirred for 6 h at 25°C. The precipitate was collected by centrifugation and washed with H2O and MeOH three times. Finally, the precipitate was soaked in MeOH for 48 h and dried under a vacuum at 60°C overnight.

X-ray absorption spectra

The Au L3-edge XANES and EXAFS experiments were conducted at the 1W1B beamline station in the Beijing Synchrotron Radiation Facility (BSRF) and the BL14W1 beamline station in the Shanghai Synchrotron Radiation Facility (SSRF). All the data were collected in transmission mode. The incident beam was monochromatized using a Si (111) double-crystal monochromator and a harmonic rejection mirror (S4) was used to eliminate harmonics at high X-ray energy levels. Data reduction, analysis and EXAFS fitting were performed using the Athena and Artemis software packages. The energy calibration of the catalysts was conducted using a standard metal foil as a reference, which was measured simultaneously.

Supplementary Material

nwae252_Supplemental_File

ACKNOWLEDGEMENTS

We thank the 1W1B station at BSRF and BL14W1 station at SSRF for XAS measurements. This work was partially carried out at the Instruments Center for Physical Science, University of Science and Technology of China.

FUNDING

This work was supported by the National Key Research and Development Program of China (2021YFA1500402), the National Natural Science Foundation of China (22331009 and U22A20401), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0450302 and XDB0540000) and the Fundamental Research Funds for the Central Universities (WK2060000038).

AUTHOR CONTRIBUTIONS

H.W. and H.-L.J. conceived the idea. H.W. designed and performed the experiments. H.-L.J. and T.Y. supervised and directed the project; Y.-L.Z. repeated the synthesis and catalytic experiments; H.W. and Y.L. performed TEM experiments. X.-K.L., Z.-H.S. and T.Y. performed XAFS experiments and analysed the data. H.W. and H.-L.J. co-wrote the manuscript. All authors discussed the results and edited the paper.

Conflict of interest statement. None declared.
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