Stabilizing Pd Nanoparticles via N,O-Chelation in a MOF for General Reductive Amination

Stabilizing Pd Nanoparticles via N,O-Chelation in a MOF for General Reductive Amination | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Stabilizing Pd Nanoparticles via N,O-Chelation in a MOF for General Reductive Amination Mei He, Duoduo Wei, Xiaogang Yin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8088035/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract Herein, we developed a Pd/UiO-66-NO catalyst for the reductive amination of benzaldehyde with aniline to synthesize N -benzylaniline. Through post-synthetic modification, salicylaldehyde was incorporated into the UiO-66-NH 2 framework to construct a support (UiO-66-NO) featuring N,O-bidentate chelating sites, which enabled the stabilization of highly dispersed Pd nanoparticles. Characterization results confirmed the preserved framework integrity and demonstrated efficient secondary amine synthesis under mild conditions (60 °C, 3 h). The catalyst exhibited broad substrate scope with excellent functional group compatibility. Mechanistic studies revealed a two-step pathway involving amine-aldehyde condensation followed by hydrogenation of the imine intermediate. 1 H NMR and GC kinetic analyses evidenced rapid imine formation and subsequent efficient reduction to the secondary amine. This work offers a new strategy for designing high-performance heterogeneous catalysts and paves a promising avenue for sustainable synthesis of secondary amines. Pd/UiO-66-NO salicylaldehyde secondary amines post-synthetic modification MOFs Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1 Introduction Secondary amines and their derivatives represent an important class of nitrogen-containing compounds with broad applications in agrochemicals, pharmaceuticals, natural products, and small-molecule biological probes[ 17 ]. Conventional synthetic approaches to secondary amines include the reductive amination of carbonyl compounds with amines[ 7 ], direct alkylation of amines with alkyl halides, Buchwald-Hartwig[ 28 ] and Ullmann-type C-N cross-coupling reactions[ 10 ], as well as direct N -alkylation of amines with alcohols[ 14 ], Among these methods, the amine-carbonyl reductive amination pathway offers distinct advantages such as readily available substrates, high atom economy, and relatively mild environmental impact. However, this transformation typically requires stoichiometric borohydrides or formic acid as reducing agents to facilitate the reduction of pre-formed or in situ-generated imine intermediates[ 26 ]. In contrast, molecular hydrogen emerges as a more attractive and cleaner alternative. The development of hydrogen-mediated reductive amination processes for secondary amine synthesis holds significant promise, demonstrating considerable value across both academic research and industrial applications. In recent years, one-pot reactions have garnered significant attention in organic synthesis due to their remarkable advantages, including reduced synthetic steps, simplified purification, high efficiency, superior atom economy, minimized waste generation, and shortened reaction times[ 11 , 29 ]. The efficient synthesis of secondary amines can be achieved through the reaction of aldehydes with amines via a one-pot procedure involving sequential imine condensation and hydrogenation. This methodology proceeds through the initial formation of an imine intermediate, followed by its direct in situ hydrogenation to the target product. By circumventing intermediate isolation and effectively suppressing side reactions, this strategy demonstrates considerable potential for synthesizing key pharmaceutical intermediates. Numerous metals, including Fe[ 18 ], Ru[ 4 ], Cu[ 3 ], and Pt[ 9 ], have been employed for the synthesis of secondary amines via reductive amination of aldehydes with amines. Among them, Pd has consistently been a research focus in this domain due to its high catalytic activity[ 27 ]. However, Pd-based catalysts suffer from significant drawbacks during reaction processes, such as pronounced leaching of active species and nanoparticle aggregation, which not only severely compromise catalytic performance but also substantially impede catalyst recovery and reuse[ 23 ]. Consequently, the selection of an appropriate catalyst support is critical for enhancing stability. Metal-organic frameworks (MOFs) present promising support candidates owing to their tunable pore sizes, well-defined metal nodes, and tailorable chemical compositions[ 22 ]. The incorporation of Pd nanoparticles into MOFs offers distinct advantages: the high dispersion of Pd NPs prevents their agglomeration and reduces metal loading, while the adjustable pore sizes facilitate the catalysis of reactants with diverse molecular dimensions[ 8 ]. Conventional MOF supports often fail to effectively stabilize Pd active sites, leading to rapid catalyst deactivation, and metal species may leach from the support into the reaction mixture, adversely affecting catalytic cycling performance. Post-synthetic modification (PSM) provides an effective strategy for engineering MOFs with multiple chelating sites,[ 10 ] where functional groups possessing strong chelating/anchoring capabilities can be readily grafted onto the pore walls of MOFs via a one-step reaction[ 1 ]. In this work, we rationally introduced salicylaldehyde into the UiO-66-NH 2 framework via PSM to construct UiO-66-NO[ 15 ]. The abundant hydroxyl and imine groups on the material surface effectively stabilize the immobilized palladium species, yielding a robust Pd/UiO-66-NO heterogeneous catalyst, which has been successfully applied to the reductive amination of various aldehydes and amines[ 21 ]. 2 Experimentation 2.1 Materials Salicylaldehyde was purchased from Tianjin Guang fu Fine Chemical Research Institute; Zirconium tetrachloride (Shanghai Macklin); 2-Aminoterephthalic acid (Shanghai Macklin); N , N -Dimethylformamide (Tianjin Fuyu Fine Chemical Co., Ltd.); Palladium acetate (Beijing J&K Scientific Ltd.). Alcohol and nitroarene substrates such as 4-fluorobenzaldehyde, 2-methoxynitrobenzene, 4-fluoronitrobenzene, and p-methoxynitro benzene were used as received. All other reagents, including ethyl acetate, petroleum ether, ethanol, etc., were obtained from various commercial sources and could be used without further purification. 2.2 Catalyst Preparation UiO-66-NH 2 was synthesized following the procedure illustrated in Fig. 1 . Typically, zirconium tetrachloride (0.1818 g, 0.78 mmol) was added to a 100 mL beaker containing 20 mL of DMF and sonicated for 10 min to form a homogeneous dispersion. Then, 2-aminoterephthalic acid (0.1406 g, 0.78 mmol) dissolved in 20 mL of DMF was introduced into the mixture, followed by another 10 min of sonication to ensure thorough mixing. The resulting solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120°C for 24 h under static conditions. After cooling naturally to room temperature, the product was collected and sequentially washed three times with DMF and anhydrous methanol to remove unreacted ligands and residual solvents. The filter cake was then immersed in 100 mL of anhydrous methanol in a 500 mL beaker and kept at room temperature for 24 h to facilitate the exchange of DMF trapped in the pores. After soaking, the solid was recovered by filtration, washed three times with anhydrous methanol, and dried at 60°C for 10 h. Finally, the product was activated at 120°C under air for 10 h, affording UiO-66-NH 2 as a light pink powder. UiO-66-NO was synthesized through a Schiff base reaction using salicylaldehyde as the modifying agent. In a typical procedure, 100 mg of UiO-66-NH 2 powder was dispersed in 10 mL of anhydrous dichloromethane in a 25 mL round-bottom flask under ultrasonication for 15 min. To this dispersion, 50 µL of salicylaldehyde was added dropwise, and the reaction was allowed to proceed for 12 h at 30°C under continuous magnetic stirring in an oil bath. After completion, the solid product was collected by filtration and thoroughly washed with dichloromethane until the filtrate became colorless. The obtained material was then dried at 60°C for 10 h to remove residual solvent, followed by activation at 120°C under air for 10 h, yielding UiO-66-NO as a yellow powder. 2.3 Catalyst Characterization The morphology and EDX elemental mapping of the samples were characterized using a Zeiss SUPRA 55 Sapphire field emission scanning electron microscope (FESEM, Germany) at an accelerating voltage of 5 kV. Nitrogen adsorption measurements were performed on a Micromeritics AsAp2020 analyzer (USA), with Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size analyses carried out automatically. The BJH analysis was based on the desorption branch of the isotherm. X-ray diffraction (XRD) patterns were collected on a Rigaku Miniflex600 X-ray powder diffractometer (Japan) using Cu Kα radiation (λ = 1.5406 Å). Infrared spectra were acquired using a PerkinElmer Model 1600 Fourier-transform infrared spectrometer (FT-IR, USA). X-ray photoelectron spectroscopy (XPS) data were obtained on a VG Multi Lab 2000 system (USA) equipped with a monochromatic Al Kα X-ray source. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 electron microscope (Japan) at an accelerating voltage of 200 kV. The Pd loading was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Prodigy 7 instrument from Leeman Labs Inc. (USA). Gas chromatography (GC) yields were measured on a Varian GC 3900 gas chromatograph (USA) equipped with an FID detector. 1 H NMR spectra were recorded at 300.1 MHz and 400.1 MHz on Bruker AVANCE 300 and 400 spectrometers, respectively, using CDCl 3 (7.26 ppm) as the internal standard. 13 C NMR spectra were obtained at 75 MHz or 100 MHz, referenced to the solvent signal (central peak of CDCl 3 at 77.16 ppm). Chemical shifts and coupling constants (J) are reported in ppm and Hz, respectively. Peak multiplicities are denoted as follows: s (singlet), d (doublet), t (triplet), m (multiplet). 2.4 General Procedure for Catalytic Reductive Amination The reductive amination was performed in a miniature high-pressure reactor. In a typical procedure, benzaldehyde (0.25 mmol), aniline (0.35 mmol), Pd/UiO-66-NO catalyst (0.24 mol% Pd), and toluene (2 mL) were successively added to a 5 mL glass vial. The vial was then sealed inside a stainless-steel autoclave. The reactor was purged and pressurized with H 2 to 3.0 MPa via a three-cycle gas exchange process (each cycle: charging H 2 to 0.5 MPa followed by slow release) to ensure an oxygen-free atmosphere. The reaction proceeded at 60°C for 3 hours with constant magnetic stirring at 800 rpm in an oil bath. After completion, the crude product was purified by silica gel column chromatography (eluent: petroleum ether/ethyl acetate = 50:1) to afford the target compound N -benzylaniline. The product was analyzed by gas chromatography (GC) and 1 H NMR spectroscopy to determine the yield and confirm its molecular structure. 3 Results and Discussion 3.1 Sample Characterization The fabrication process is illustrated in Figure. 2a. Starting with stable and readily modifiable UiO-66-NH 2 , salicylaldehyde was successfully anchored into the framework via an amine-aldehyde condensation reaction. This postsynthetic modification yielded a novel support material, UiO-66-NO, which features a well-defined N,O-bidentate chelation mechanism. The obtained UiO-66-NO was first dispersed in anhydrous ethanol under ultrasonication. Subsequently, an ethanol solution of Pd(OAc) 2 precursor was gradually introduced into the mixture, leading to the formation of a stable palladium-based catalyst, Pd/UiO-66-NO, through an alcohol-reduction approach. The microstructural evolution of the catalyst during the synthesis process was investigated by scanning electron microscopy (SEM). As clearly depicted in Figure. 2b-d, all samples, namely UiO-66-NH 2 , UiO-66-NO, and Pd/UiO-66-NO, exhibit well-defined octahedral structures. Importantly, the original morphology remained unchanged after the salicylaldehyde functionalization of UiO-66-NH 2 and the subsequent loading of Pd nanoparticles, indicating the robust stability of the UiO-66-NH 2 precursor. Furthermore, the EDS mapping images Figure. 2(e 1 -e 5 ) confirm the presence and homogeneous distribution of C, N, O, Zr, and Pd elements within the Pd/UiO-66-NO catalyst. The crystal structures of the synthesized materials were characterized by X-ray diffraction (XRD). The results indicate that UiO-66-NH 2 , UiO-66-NO, and Pd/UiO-66-NO all exhibit diffraction patterns highly consistent with the standard UiO-66 structure, confirming successful synthesis and good crystallinity (Figure. 3a)[ 13 ]. It is noteworthy that after Pd loading, the diffraction peak positions remain unchanged, indicating the preservation of the metal–organic framework (MOF) structure. However, the reduction in peak intensity may be attributed to partial framework collapse or strong metal–support interactions[ 19 ]. Fourier transform infrared (FT-IR) spectroscopy further revealed changes in the chemical structure (Figure. 3b): the -NH 2 vibration peaks (3458, 3353 cm − 1 ) in UiO-66-NO and Pd/UiO-66-NO were significantly weakened, while the characteristic C = N peaks at 1570 cm − 1 and 1259 cm − 1 intensified, confirming the successful grafting of salicylaldehyde onto the MOF via a Schiff base reaction[ 24 ]. This chemical modification was accompanied by a visible color change of UiO-66-NH 2 from light pink to bright yellow (Figure. S1), providing evidence of the reaction[ 31 ]. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of characteristic elements such as O, N, C, and Zr in all materials[ 30 ]. The appearance of Pd 3d signals, predominantly in the metallic Pd 0 state, in Pd/UiO-66-NO verified the successful loading of Pd (Figure. 3c-d)[ 12 ]. N 2 adsorption–desorption measurements revealed type-IV isotherms and a bimodal pore size distribution, with micropores of approximately 1 nm and mesopores of 3–5 nm. As the modification with salicylaldehyde and Pd loading proceeded, the specific surface area gradually decreased from 629 m 2 /g for UiO-66-NH 2 to 518 m 2 /g for Pd/UiO-66-NO, accompanied by a synchronous reduction in pore volume. This trend aligns with the mechanism of pore occupation by organic groups and metal nanoparticles (Figure. S2)[ 16 ]. Thermogravimetric analysis (TGA) showed a similar two-stage thermal decomposition behavior for UiO-66-NH 2 and UiO-66-NO, indicating that the modification process did not compromise the thermal stability of the framework (Figure S3)[ 6 ]. To clarify the extent of salicylaldehyde modification on UiO-66-NH 2 , UiO-66-NH 2 and UiO-66-NO powders were dissolved in a mixture of NaOD and D 2 O, respectively. The 1 H NMR spectra of these two samples (Figure. 3e-f) exhibited signals corresponding to 2-aminoterephthalic acid and iminoterephthalic acid. The results indicate that approximately 26% of salicylaldehyde was successfully modified into the framework in UiO-66-NO[ 5 ]. Further characterization of Pd/UiO-66-NO by TEM (Figure. 4) reveals that the Pd nanoparticles (NPs) are predominantly and uniformly dispersed on the UiO-66-NO support, confirming successful loading. The NPs exhibit a narrow size distribution with a mean diameter of 4.10 nm, indicating that most Pd is deposited on the external surface of the carrier rather than embedded within the crystal lattice. However, a certain degree of NP aggregation is observed, likely attributable to the relatively high loading amount (2.2 wt%)[ 25 ]. 3.2 Catalytic Performance Evaluation Secondary amines, serving as pivotal intermediates in the synthesis of pharmaceuticals, cosmetics, and related products, are extensively utilized in industrial processes. The one-pot reductive amination using molecular hydrogen represents a key strategy for efficiently synthesizing secondary amines. To evaluate the catalytic performance of the synthesized Pd/UiO-66-NO material, the reductive amination of benzaldehyde with aniline was selected as a model reaction, employing molecular hydrogen as a green hydrogen source. Reaction conditions for Pd/UiO-66-NO were systematically optimized. The influence of solvent on catalytic activity was first investigated by screening a series of common polar and non-polar solvents. As summarized in Table S1 , toluene afforded the highest yield of 95% (Table S1 , Entry 1). Methanol and ethyl acetate also provided relatively high yields of 81% and 83%, respectively (Table S1 , Entries 3 and 6), while acetonitrile and water resulted in moderate yields of 72% and 68% (Table S1 , Entries 4–5). In contrast, low-boiling-point solvents such as dichloromethane and n-hexane led to significantly lower yields of 35% and 55% (Table S1 , Entries 7 and 9). When high-boiling-point solvents with broad solubility, including tetrahydrofuran, DMF, and dimethyl sulfoxide, were used, the yields further dropped to 0%, 44%, and 2%, respectively (Table S1 , Entries 8, 10, and 2). These results suggest that solvent properties such as boiling point, polarity, and solubility exhibit no dominant influence on the reaction outcome. Furthermore, the effect of hydrogen pressure was examined. As shown in Table S1 (Entries 11–12), increasing the H₂ pressure from 1 MPa to 3 MPa led to a progressive enhancement in the yield of the target product[ 20 ]. The influence of temperature was investigated, with the corresponding results presented in Figure S4. The reaction was conducted at temperatures ranging from 20°C to 60°C. At 20°C, the yield of the target product was only 42%. Notably, when the temperature was increased to 60°C, the yield reached 95%. This observation indicates that the Pd/UiO-66-NO catalyst exhibits high catalytic activity even at relatively low temperatures. The effect of catalyst loading on the reaction was also examined, as shown in Figure S5. It was found that the reaction did not proceed in the absence of the catalyst. As the catalyst amount was increased from 2 mg to 3 mg, the yield of N -benzylaniline changed significantly. This phenomenon demonstrates that an adequate number of active catalytic sites (Pd) is crucial for driving the reaction forward. Under the optimized conditions, the catalytic performance of the synthesized Pd/UiO-66-NO was systematically compared with those of commercial Pd/C, commercially available Pd nanoparticles, homogeneous Pd(OAc) 2 , Pd@UiO-66-NH 2 (which lacks N-O bidentate chelation), and the bare supports UiO-66-NH 2 and UiO-66-NO. Kinetic profiles of these catalysts were obtained by monitoring the yield of the target product over time (Figure. 5).When the bare supports were used, even after 3 h of reaction, no conversion of the starting materials to the target product was observed, indicating the absence of active Pd sites necessary for the reaction to proceed. With homogeneous Pd(OAc) 2 and Pd nanoparticles, the target product yields reached only 58% and 38%, respectively. These results underscore the importance of an appropriate support in this reaction; in the absence of a support to disperse the active Pd species, even under optimized conditions, homogeneous catalysts fail to achieve satisfactory performance in this amination transformation. When commercial Pd/C was employed, the product yield remained limited to 72%, lower than the 95% achieved with Pd/UiO-66-NO. This inferior performance may be attributed to the larger particle size of Pd nanoparticles in Pd/C. In the case of Pd@UiO-66-NH 2 , the yield of N -benzylaniline was lower than expected, likely due to insufficient chelation ability toward Pd, leading to leaching of active Pd species during the reaction. These findings collectively demonstrate that a support with N-O bidentate chelation capability is a key factor in achieving high product yields[ 21 ]. 3.3 Evaluation of the Catalyst Substrate Scope To evaluate the substrate adaptability of the Pd/UiO-66-NO catalyst, the reaction time was extended to 6 hours under the optimized conditions, and its compatibility with various aromatic amines and aldehydes was investigated. As shown in Fig. 6 , the starting reactants were converted to the corresponding target products with high conversion. Neither electron-donating nor electron-withdrawing groups exhibited a significant influence on the reaction outcome, indicating that electronic effects play a negligible role in this transformation. The catalytic system demonstrated excellent compatibility with monofunctional substrates. Substituents such as fluoro, methoxy, and methyl groups on benzaldehyde derivatives had no notable impact on the yield (Fig. 6 , 2 – 5 ). The yields for monoamine substrates remained consistently high, ranging from 70% to 94% (Fig. 6 , 3 , 9–13). Notably, even 2,5-dimethylaniline—a substrate bearing two substituents—reacted with benzaldehyde to afford the corresponding secondary amine in 91% yield (Fig. 6 , 4 ), suggesting that significant steric hindrance does not impede the reaction. The presence of substituents on both benzaldehyde and aniline did not disrupt the reaction, and the target products were smoothly obtained. Furthermore, when furfural was reacted with aniline or the sterically hindered 2,5-dimethylaniline, the desired products were obtained in 81% and 83% yields, respectively (Fig. 6 , 30, 33). These results collectively demonstrate that the Pd/UiO-66-NO catalyst exhibits a broad substrate scope, efficiently facilitating the one-pot synthesis of secondary amine derivatives from variously functionalized amines and aldehydes under very mild conditions. 3.4 Catalyst Recyclability Studies To investigate the recyclability and reusability of Pd/UiO-66-NO, recycling experiments were conducted under the same reaction conditions as previously described. Although the catalytic activity of Pd/UiO-66-NO declined after four reaction cycles compared to the first run, it still maintained considerable activity, affording target product yields of 75% and 92% (Figure S6), respectively. The observed decrease in activity may be attributed to the fact that most Pd nanoparticles remain predominantly located on the external surface of UiO-66-NO, leading to partial loss of Pd NPs during the reaction process. FT-IR spectra of the Pd/UiO-66-NO catalyst before and after the cycling tests (Figure S7) demonstrated no significant structural alterations, confirming the robustness of the catalyst framework under the investigated reaction conditions. 3.5 Unraveling the Reaction Mechanism of Pd/UiO-66-NO-Catalyzed Aniline-Aldehyde Coupling To investigate the reaction pathway of aniline and benzaldehyde coupling catalyzed by Pd/UiO-66-NO, the transformation process was elucidated by monitoring the temporal concentration changes of reactants, intermediates, and products in the reaction system using a combination of 1 H NMR and gas chromatography (GC). 1 H NMR analysis (Fig. 7 b) revealed that in the presence of both Pd/UiO-66-NO catalyst and H 2 , the characteristic signal intensity of benzaldehyde diminished over time, while the intermediate imine exhibited a characteristic increase followed by a decrease. Given the hydrogenation capability of the palladium catalyst, this observation indicates the gradual reduction of the imine intermediate to the target product, N -benzylaniline, under a hydrogen atmosphere. Analysis of the reaction mixture at different time intervals by GC (Fig. 7 a) further confirmed the proposed amination pathway: aniline and benzaldehyde initially condense to form an imine intermediate, which is subsequently and efficiently hydrogenated to the secondary amine product. The proposed reaction pathway for the Pd/UiO-66-NO-catalyzed transformation is illustrated in Fig. 7 c. Initially, the Lewis acid sites within Pd/UiO-66-NO activate the carbonyl group of benzaldehyde, generating a carbocation (C + ) intermediate via polarization. The lone pair of electrons on the amino group of aniline then attacks this electrophilic C + , forming a hydroxyamine intermediate. Subsequently, the Lewis acid sites further facilitate the dehydration of this intermediate to yield an imine (C = N). Finally, Pd nanoparticles catalyze the hydrogenation of the imine; H 2 undergoes heterolytic cleavage on the Pd surface to produce active hydrogen species (H − /H + ), which selectively reduce the imine to yield N -benzylaniline, concurrently regenerating the catalyst[ 2 ]. 4 Conclusion In this chapter, a Pd/UiO-66-NO catalyst featuring N-O bidentate chelation sites was successfully constructed. Comprehensive characterization confirmed that the salicylaldehyde modification did not disrupt the support framework, while the catalyst exhibited outstanding performance in the reductive amination of benzaldehyde with aniline. Under optimized conditions (toluene, 60°C, 3 MPa H 2 , 3 h), its catalytic efficiency surpassed that of commercial Pd/C and other reference catalysts, which can be attributed to the effective stabilization of Pd nanoparticles by the N-O chelation mechanism. Substrate scope studies revealed good adaptability toward a range of functional groups and sterically hindered substrates. The catalyst maintained a yield of 75% after four recycling runs, with the structural integrity of the support preserved as evidenced by FT-IR analysis. Mechanistic investigations verified a two-step pathway involving condensation to form an imine intermediate followed by hydrogenation. This work offers a new strategy for designing highly stable heterogeneous catalysts and opens a sustainable route for the synthesis of secondary amines. Abbreviations BET Brunauer-Emmett-Teller FTIR Fourier transform infrared MOFs Metal-organic frameworks SEM Scanning electron microscopy TEM Transmission electron microscopy XRD X-ray powder diffraction XPS X-ray photoelectron spectroscopy NMR Nuclear magnetic resonance EDS Electronic Differential System PSM Post-synthetic modification Declarations Ethics and Consent to Participate Not applicable. This study does not involve human participants, animals, or biological materials. Consent for Publication All authors have read and approved the final manuscript for publication. Competing Interests The authors declare no competing interests. Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Author Contribution Mei He: Conceptualization, Methodology, Writing-Original Draft. Duoduo Wei: Formal analysis, Investigation, Data Curation. Xiaogang Yin: Validation, Writing-Review & Editing, Supervision. 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Dalton Trans 47:1674–1681. https://doi.org/10.1039/c7dt04266c Additional Declarations No competing interests reported. Supplementary Files SupportingInformation.pdf floatimage1.png Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 20 Nov, 2025 Reviews received at journal 19 Nov, 2025 Reviews received at journal 18 Nov, 2025 Reviewers agreed at journal 17 Nov, 2025 Reviewers agreed at journal 16 Nov, 2025 Reviewers agreed at journal 15 Nov, 2025 Reviewers agreed at journal 14 Nov, 2025 Reviewers agreed at journal 13 Nov, 2025 Reviewers invited by journal 13 Nov, 2025 Editor assigned by journal 13 Nov, 2025 Submission checks completed at journal 13 Nov, 2025 First submitted to journal 11 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8088035","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":548064131,"identity":"4efb8184-5f7a-453d-a86f-419dcead276b","order_by":0,"name":"Mei He","email":"","orcid":"","institution":"School of Chemistry and Materials Science, Guizhou Normal University","correspondingAuthor":false,"prefix":"","firstName":"Mei","middleName":"","lastName":"He","suffix":""},{"id":548064132,"identity":"b17bad58-d2f0-4961-a613-5d1b7d888f98","order_by":1,"name":"Duoduo Wei","email":"","orcid":"","institution":"School of Chemistry and Materials 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1","display":"","copyAsset":false,"role":"figure","size":172505,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the preparation process of UiO-66-NH\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/8b084f09e240046e08bcbc5a.png"},{"id":96759069,"identity":"f6cf8e5a-a875-4c09-ae34-b1bd7ec42fb7","added_by":"auto","created_at":"2025-11-25 18:37:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2124426,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Schematic illustration of the fabrication process for Pd/UiO-66-NO; Representative SEM images of (\u003cstrong\u003eb\u003c/strong\u003e) UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, (\u003cstrong\u003ec\u003c/strong\u003e) UiO-66-NO, and (\u003cstrong\u003ed\u003c/strong\u003e) Pd/UiO-66-NO; (\u003cstrong\u003ee\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-e\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003c/sub\u003e) Corresponding EDS elemental mapping images of Pd/UiO-66-NO.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/195c8b658bf1bc09a28d0c83.png"},{"id":96759066,"identity":"968ec571-9b9f-45f6-9cfb-b7273ff74a72","added_by":"auto","created_at":"2025-11-25 18:37:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":393210,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) XRD patterns, (\u003cstrong\u003eb\u003c/strong\u003e) FT-IR spectra, (\u003cstrong\u003ec\u003c/strong\u003e) full-scale XPS survey spectra, (\u003cstrong\u003ed\u003c/strong\u003e) high-resolution Pd 3d XPS spectrum of Pd/UiO-66-NO, (\u003cstrong\u003ee-g\u003c/strong\u003e) N₂ adsorption-desorption isotherms and corresponding pore size distribution curves, and (\u003cstrong\u003eh\u003c/strong\u003e) TGA curves of the as-synthesized samples.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/1892c9106773dca5fa9e165f.png"},{"id":96759067,"identity":"cd8551cd-3425-432e-a727-efb5dc78e816","added_by":"auto","created_at":"2025-11-25 18:37:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":533716,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003ea-c\u003c/strong\u003e) TEM images and (\u003cstrong\u003ed\u003c/strong\u003e) the corresponding particle size distribution of Pd nanoparticles (NPs) for the Pd/UiO-66-NO sample.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/2980f6eccfc26e4990897a64.png"},{"id":96914262,"identity":"4e52666a-67dc-40dd-83d9-ea4a0716643f","added_by":"auto","created_at":"2025-11-27 14:05:39","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":837723,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Kinetic profiles and (\u003cstrong\u003eb\u003c/strong\u003e) corresponding product yields of the catalytic reductive amination reaction over different catalysts.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/dd91c40ea28a723dc387ea7d.png"},{"id":96916128,"identity":"596a8835-ebd8-405b-bfa8-d544b6560bba","added_by":"auto","created_at":"2025-11-27 14:08:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1691893,"visible":true,"origin":"","legend":"\u003cp\u003eEvaluation of the reaction generality for the reductive amination of aniline and benzaldehyde catalyzed by Pd/UiO-66-NO. \u003csup\u003ea \u003c/sup\u003eReaction conditions: aldehyde (0.25 mmol), amine (0.35 mmol), toluene (2 mL), Pd catalyst (0.24 mol% [Pd] with respect to benzaldehyde), 60 °C, 3 MPa H\u003csub\u003e2\u003c/sub\u003e, 6 h. \u003csup\u003eb \u003c/sup\u003eReaction time of 3 h; yields refer to isolated yields.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/ff60c1a9ce1f7cb53ef81515.png"},{"id":96914892,"identity":"7f4ec664-9281-46cf-bd20-8d37d1728f4b","added_by":"auto","created_at":"2025-11-27 14:06:33","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1243615,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) gas chromatography analysis; (\u003cstrong\u003eb\u003c/strong\u003e) \u003csup\u003e1\u003c/sup\u003eH NMR spectra (400 MHz) of the reaction mixture at 1 h, 2 h, and 3 h; (\u003cstrong\u003ec\u003c/strong\u003e) proposed reaction mechanism for the transformation of benzaldehyde with aniline.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/11789e2ba7be4c5ad1e7e9e1.png"},{"id":96922591,"identity":"fc5d11ab-d9ce-458e-a2ce-0b7c4c964176","added_by":"auto","created_at":"2025-11-27 14:19:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7647407,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/620c8426-dca7-40e3-83af-3c8148bff6fa.pdf"},{"id":96759077,"identity":"b1336ce9-6090-4bff-90ed-2bf1ade87427","added_by":"auto","created_at":"2025-11-25 18:37:34","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3960077,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/4f8abf7bb3f724f807b85462.pdf"},{"id":96759073,"identity":"b57cff2f-de63-4327-88ac-a949a3bb1248","added_by":"auto","created_at":"2025-11-25 18:37:34","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":83875,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8088035/v1/3dadf604f43ea767e30d1b09.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Stabilizing Pd Nanoparticles via N,O-Chelation in a MOF for General Reductive Amination","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eSecondary amines and their derivatives represent an important class of nitrogen-containing compounds with broad applications in agrochemicals, pharmaceuticals, natural products, and small-molecule biological probes[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Conventional synthetic approaches to secondary amines include the reductive amination of carbonyl compounds with amines[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], direct alkylation of amines with alkyl halides, Buchwald-Hartwig[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] and Ullmann-type C-N cross-coupling reactions[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], as well as direct \u003cem\u003eN\u003c/em\u003e-alkylation of amines with alcohols[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], Among these methods, the amine-carbonyl reductive amination pathway offers distinct advantages such as readily available substrates, high atom economy, and relatively mild environmental impact. However, this transformation typically requires stoichiometric borohydrides or formic acid as reducing agents to facilitate the reduction of pre-formed or in situ-generated imine intermediates[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. In contrast, molecular hydrogen emerges as a more attractive and cleaner alternative. The development of hydrogen-mediated reductive amination processes for secondary amine synthesis holds significant promise, demonstrating considerable value across both academic research and industrial applications.\u003c/p\u003e\u003cp\u003eIn recent years, one-pot reactions have garnered significant attention in organic synthesis due to their remarkable advantages, including reduced synthetic steps, simplified purification, high efficiency, superior atom economy, minimized waste generation, and shortened reaction times[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The efficient synthesis of secondary amines can be achieved through the reaction of aldehydes with amines via a one-pot procedure involving sequential imine condensation and hydrogenation. This methodology proceeds through the initial formation of an imine intermediate, followed by its direct in situ hydrogenation to the target product. By circumventing intermediate isolation and effectively suppressing side reactions, this strategy demonstrates considerable potential for synthesizing key pharmaceutical intermediates.\u003c/p\u003e\u003cp\u003eNumerous metals, including Fe[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], Ru[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], Cu[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], and Pt[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], have been employed for the synthesis of secondary amines via reductive amination of aldehydes with amines. Among them, Pd has consistently been a research focus in this domain due to its high catalytic activity[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. However, Pd-based catalysts suffer from significant drawbacks during reaction processes, such as pronounced leaching of active species and nanoparticle aggregation, which not only severely compromise catalytic performance but also substantially impede catalyst recovery and reuse[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Consequently, the selection of an appropriate catalyst support is critical for enhancing stability. Metal-organic frameworks (MOFs) present promising support candidates owing to their tunable pore sizes, well-defined metal nodes, and tailorable chemical compositions[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The incorporation of Pd nanoparticles into MOFs offers distinct advantages: the high dispersion of Pd NPs prevents their agglomeration and reduces metal loading, while the adjustable pore sizes facilitate the catalysis of reactants with diverse molecular dimensions[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eConventional MOF supports often fail to effectively stabilize Pd active sites, leading to rapid catalyst deactivation, and metal species may leach from the support into the reaction mixture, adversely affecting catalytic cycling performance. Post-synthetic modification (PSM) provides an effective strategy for engineering MOFs with multiple chelating sites,[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] where functional groups possessing strong chelating/anchoring capabilities can be readily grafted onto the pore walls of MOFs via a one-step reaction[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. In this work, we rationally introduced salicylaldehyde into the UiO-66-NH\u003csub\u003e2\u003c/sub\u003e framework via PSM to construct UiO-66-NO[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The abundant hydroxyl and imine groups on the material surface effectively stabilize the immobilized palladium species, yielding a robust Pd/UiO-66-NO heterogeneous catalyst, which has been successfully applied to the reductive amination of various aldehydes and amines[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e"},{"header":"2 Experimentation","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eSalicylaldehyde was purchased from Tianjin Guang fu Fine Chemical Research Institute; Zirconium tetrachloride (Shanghai Macklin); 2-Aminoterephthalic acid (Shanghai Macklin); \u003cem\u003eN\u003c/em\u003e,\u003cem\u003eN\u003c/em\u003e-Dimethylformamide (Tianjin Fuyu Fine Chemical Co., Ltd.); Palladium acetate (Beijing J\u0026amp;K Scientific Ltd.). Alcohol and nitroarene substrates such as 4-fluorobenzaldehyde, 2-methoxynitrobenzene, 4-fluoronitrobenzene, and p-methoxynitro benzene were used as received. All other reagents, including ethyl acetate, petroleum ether, ethanol, etc., were obtained from various commercial sources and could be used without further purification.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Catalyst Preparation\u003c/h2\u003e\u003cp\u003eUiO-66-NH\u003csub\u003e2\u003c/sub\u003e was synthesized following the procedure illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Typically, zirconium tetrachloride (0.1818 g, 0.78 mmol) was added to a 100 mL beaker containing 20 mL of DMF and sonicated for 10 min to form a homogeneous dispersion. Then, 2-aminoterephthalic acid (0.1406 g, 0.78 mmol) dissolved in 20 mL of DMF was introduced into the mixture, followed by another 10 min of sonication to ensure thorough mixing. The resulting solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120\u0026deg;C for 24 h under static conditions. After cooling naturally to room temperature, the product was collected and sequentially washed three times with DMF and anhydrous methanol to remove unreacted ligands and residual solvents. The filter cake was then immersed in 100 mL of anhydrous methanol in a 500 mL beaker and kept at room temperature for 24 h to facilitate the exchange of DMF trapped in the pores. After soaking, the solid was recovered by filtration, washed three times with anhydrous methanol, and dried at 60\u0026deg;C for 10 h. Finally, the product was activated at 120\u0026deg;C under air for 10 h, affording UiO-66-NH\u003csub\u003e2\u003c/sub\u003e as a light pink powder.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eUiO-66-NO was synthesized through a Schiff base reaction using salicylaldehyde as the modifying agent. In a typical procedure, 100 mg of UiO-66-NH\u003csub\u003e2\u003c/sub\u003e powder was dispersed in 10 mL of anhydrous dichloromethane in a 25 mL round-bottom flask under ultrasonication for 15 min. To this dispersion, 50 \u0026micro;L of salicylaldehyde was added dropwise, and the reaction was allowed to proceed for 12 h at 30\u0026deg;C under continuous magnetic stirring in an oil bath. After completion, the solid product was collected by filtration and thoroughly washed with dichloromethane until the filtrate became colorless. The obtained material was then dried at 60\u0026deg;C for 10 h to remove residual solvent, followed by activation at 120\u0026deg;C under air for 10 h, yielding UiO-66-NO as a yellow powder.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Catalyst Characterization\u003c/h2\u003e\u003cp\u003eThe morphology and EDX elemental mapping of the samples were characterized using a Zeiss SUPRA 55 Sapphire field emission scanning electron microscope (FESEM, Germany) at an accelerating voltage of 5 kV. Nitrogen adsorption measurements were performed on a Micromeritics AsAp2020 analyzer (USA), with Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size analyses carried out automatically. The BJH analysis was based on the desorption branch of the isotherm. X-ray diffraction (XRD) patterns were collected on a Rigaku Miniflex600 X-ray powder diffractometer (Japan) using Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;). Infrared spectra were acquired using a PerkinElmer Model 1600 Fourier-transform infrared spectrometer (FT-IR, USA). X-ray photoelectron spectroscopy (XPS) data were obtained on a VG Multi Lab 2000 system (USA) equipped with a monochromatic Al Kα X-ray source. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 electron microscope (Japan) at an accelerating voltage of 200 kV. The Pd loading was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Prodigy 7 instrument from Leeman Labs Inc. (USA). Gas chromatography (GC) yields were measured on a Varian GC 3900 gas chromatograph (USA) equipped with an FID detector. \u003csup\u003e1\u003c/sup\u003eH NMR spectra were recorded at 300.1 MHz and 400.1 MHz on Bruker AVANCE 300 and 400 spectrometers, respectively, using CDCl\u003csub\u003e3\u003c/sub\u003e (7.26 ppm) as the internal standard. \u003csup\u003e13\u003c/sup\u003eC NMR spectra were obtained at 75 MHz or 100 MHz, referenced to the solvent signal (central peak of CDCl\u003csub\u003e3\u003c/sub\u003e at 77.16 ppm). Chemical shifts and coupling constants (J) are reported in ppm and Hz, respectively. Peak multiplicities are denoted as follows: s (singlet), d (doublet), t (triplet), m (multiplet).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 General Procedure for Catalytic Reductive Amination\u003c/h2\u003e\u003cp\u003eThe reductive amination was performed in a miniature high-pressure reactor. In a typical procedure, benzaldehyde (0.25 mmol), aniline (0.35 mmol), Pd/UiO-66-NO catalyst (0.24 mol% Pd), and toluene (2 mL) were successively added to a 5 mL glass vial. The vial was then sealed inside a stainless-steel autoclave. The reactor was purged and pressurized with H\u003csub\u003e2\u003c/sub\u003e to 3.0 MPa via a three-cycle gas exchange process (each cycle: charging H\u003csub\u003e2\u003c/sub\u003e to 0.5 MPa followed by slow release) to ensure an oxygen-free atmosphere. The reaction proceeded at 60\u0026deg;C for 3 hours with constant magnetic stirring at 800 rpm in an oil bath. After completion, the crude product was purified by silica gel column chromatography (eluent: petroleum ether/ethyl acetate\u0026thinsp;=\u0026thinsp;50:1) to afford the target compound \u003cem\u003eN\u003c/em\u003e-benzylaniline. The product was analyzed by gas chromatography (GC) and \u003csup\u003e1\u003c/sup\u003eH NMR spectroscopy to determine the yield and confirm its molecular structure.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Sample Characterization\u003c/h2\u003e\u003cp\u003eThe fabrication process is illustrated in Figure. 2a. Starting with stable and readily modifiable UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, salicylaldehyde was successfully anchored into the framework via an amine-aldehyde condensation reaction. This postsynthetic modification yielded a novel support material, UiO-66-NO, which features a well-defined N,O-bidentate chelation mechanism. The obtained UiO-66-NO was first dispersed in anhydrous ethanol under ultrasonication. Subsequently, an ethanol solution of Pd(OAc)\u003csub\u003e2\u003c/sub\u003e precursor was gradually introduced into the mixture, leading to the formation of a stable palladium-based catalyst, Pd/UiO-66-NO, through an alcohol-reduction approach.\u003c/p\u003e\u003cp\u003eThe microstructural evolution of the catalyst during the synthesis process was investigated by scanning electron microscopy (SEM). As clearly depicted in Figure. 2b-d, all samples, namely UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, UiO-66-NO, and Pd/UiO-66-NO, exhibit well-defined octahedral structures. Importantly, the original morphology remained unchanged after the salicylaldehyde functionalization of UiO-66-NH\u003csub\u003e2\u003c/sub\u003e and the subsequent loading of Pd nanoparticles, indicating the robust stability of the UiO-66-NH\u003csub\u003e2\u003c/sub\u003e precursor. Furthermore, the EDS mapping images Figure. 2(e\u003csub\u003e1\u003c/sub\u003e-e\u003csub\u003e5\u003c/sub\u003e) confirm the presence and homogeneous distribution of C, N, O, Zr, and Pd elements within the Pd/UiO-66-NO catalyst.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe crystal structures of the synthesized materials were characterized by X-ray diffraction (XRD). The results indicate that UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, UiO-66-NO, and Pd/UiO-66-NO all exhibit diffraction patterns highly consistent with the standard UiO-66 structure, confirming successful synthesis and good crystallinity (Figure. 3a)[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. It is noteworthy that after Pd loading, the diffraction peak positions remain unchanged, indicating the preservation of the metal\u0026ndash;organic framework (MOF) structure. However, the reduction in peak intensity may be attributed to partial framework collapse or strong metal\u0026ndash;support interactions[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Fourier transform infrared (FT-IR) spectroscopy further revealed changes in the chemical structure (Figure. 3b): the -NH\u003csub\u003e2\u003c/sub\u003e vibration peaks (3458, 3353 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in UiO-66-NO and Pd/UiO-66-NO were significantly weakened, while the characteristic C\u0026thinsp;=\u0026thinsp;N peaks at 1570 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1259 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e intensified, confirming the successful grafting of salicylaldehyde onto the MOF via a Schiff base reaction[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This chemical modification was accompanied by a visible color change of UiO-66-NH\u003csub\u003e2\u003c/sub\u003e from light pink to bright yellow (Figure. S1), providing evidence of the reaction[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of characteristic elements such as O, N, C, and Zr in all materials[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The appearance of Pd 3d signals, predominantly in the metallic Pd\u003csup\u003e0\u003c/sup\u003e state, in Pd/UiO-66-NO verified the successful loading of Pd (Figure. 3c-d)[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. N\u003csub\u003e2\u003c/sub\u003e adsorption\u0026ndash;desorption measurements revealed type-IV isotherms and a bimodal pore size distribution, with micropores of approximately 1 nm and mesopores of 3\u0026ndash;5 nm. As the modification with salicylaldehyde and Pd loading proceeded, the specific surface area gradually decreased from 629 m\u003csup\u003e2\u003c/sup\u003e/g for UiO-66-NH\u003csub\u003e2\u003c/sub\u003e to 518 m\u003csup\u003e2\u003c/sup\u003e/g for Pd/UiO-66-NO, accompanied by a synchronous reduction in pore volume. This trend aligns with the mechanism of pore occupation by organic groups and metal nanoparticles (Figure. S2)[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Thermogravimetric analysis (TGA) showed a similar two-stage thermal decomposition behavior for UiO-66-NH\u003csub\u003e2\u003c/sub\u003e and UiO-66-NO, indicating that the modification process did not compromise the thermal stability of the framework (Figure S3)[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. To clarify the extent of salicylaldehyde modification on UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, UiO-66-NH\u003csub\u003e2\u003c/sub\u003e and UiO-66-NO powders were dissolved in a mixture of NaOD and D\u003csub\u003e2\u003c/sub\u003eO, respectively. The \u003csup\u003e1\u003c/sup\u003eH NMR spectra of these two samples (Figure. 3e-f) exhibited signals corresponding to 2-aminoterephthalic acid and iminoterephthalic acid. The results indicate that approximately 26% of salicylaldehyde was successfully modified into the framework in UiO-66-NO[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther characterization of Pd/UiO-66-NO by TEM (Figure. 4) reveals that the Pd nanoparticles (NPs) are predominantly and uniformly dispersed on the UiO-66-NO support, confirming successful loading. The NPs exhibit a narrow size distribution with a mean diameter of 4.10 nm, indicating that most Pd is deposited on the external surface of the carrier rather than embedded within the crystal lattice. However, a certain degree of NP aggregation is observed, likely attributable to the relatively high loading amount (2.2 wt%)[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Catalytic Performance Evaluation\u003c/h2\u003e\u003cp\u003eSecondary amines, serving as pivotal intermediates in the synthesis of pharmaceuticals, cosmetics, and related products, are extensively utilized in industrial processes. The one-pot reductive amination using molecular hydrogen represents a key strategy for efficiently synthesizing secondary amines. To evaluate the catalytic performance of the synthesized Pd/UiO-66-NO material, the reductive amination of benzaldehyde with aniline was selected as a model reaction, employing molecular hydrogen as a green hydrogen source. Reaction conditions for Pd/UiO-66-NO were systematically optimized. The influence of solvent on catalytic activity was first investigated by screening a series of common polar and non-polar solvents. As summarized in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, toluene afforded the highest yield of 95% (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Entry 1). Methanol and ethyl acetate also provided relatively high yields of 81% and 83%, respectively (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Entries 3 and 6), while acetonitrile and water resulted in moderate yields of 72% and 68% (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Entries 4\u0026ndash;5). In contrast, low-boiling-point solvents such as dichloromethane and n-hexane led to significantly lower yields of 35% and 55% (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Entries 7 and 9). When high-boiling-point solvents with broad solubility, including tetrahydrofuran, DMF, and dimethyl sulfoxide, were used, the yields further dropped to 0%, 44%, and 2%, respectively (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Entries 8, 10, and 2). These results suggest that solvent properties such as boiling point, polarity, and solubility exhibit no dominant influence on the reaction outcome. Furthermore, the effect of hydrogen pressure was examined. As shown in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e (Entries 11\u0026ndash;12), increasing the H₂ pressure from 1 MPa to 3 MPa led to a progressive enhancement in the yield of the target product[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe influence of temperature was investigated, with the corresponding results presented in Figure S4. The reaction was conducted at temperatures ranging from 20\u0026deg;C to 60\u0026deg;C. At 20\u0026deg;C, the yield of the target product was only 42%. Notably, when the temperature was increased to 60\u0026deg;C, the yield reached 95%. This observation indicates that the Pd/UiO-66-NO catalyst exhibits high catalytic activity even at relatively low temperatures. The effect of catalyst loading on the reaction was also examined, as shown in Figure S5. It was found that the reaction did not proceed in the absence of the catalyst. As the catalyst amount was increased from 2 mg to 3 mg, the yield of \u003cem\u003eN\u003c/em\u003e-benzylaniline changed significantly. This phenomenon demonstrates that an adequate number of active catalytic sites (Pd) is crucial for driving the reaction forward.\u003c/p\u003e\u003cp\u003eUnder the optimized conditions, the catalytic performance of the synthesized Pd/UiO-66-NO was systematically compared with those of commercial Pd/C, commercially available Pd nanoparticles, homogeneous Pd(OAc)\u003csub\u003e2\u003c/sub\u003e, Pd@UiO-66-NH\u003csub\u003e2\u003c/sub\u003e (which lacks N-O bidentate chelation), and the bare supports UiO-66-NH\u003csub\u003e2\u003c/sub\u003e and UiO-66-NO. Kinetic profiles of these catalysts were obtained by monitoring the yield of the target product over time (Figure. 5).When the bare supports were used, even after 3 h of reaction, no conversion of the starting materials to the target product was observed, indicating the absence of active Pd sites necessary for the reaction to proceed. With homogeneous Pd(OAc)\u003csub\u003e2\u003c/sub\u003e and Pd nanoparticles, the target product yields reached only 58% and 38%, respectively. These results underscore the importance of an appropriate support in this reaction; in the absence of a support to disperse the active Pd species, even under optimized conditions, homogeneous catalysts fail to achieve satisfactory performance in this amination transformation. When commercial Pd/C was employed, the product yield remained limited to 72%, lower than the 95% achieved with Pd/UiO-66-NO. This inferior performance may be attributed to the larger particle size of Pd nanoparticles in Pd/C. In the case of Pd@UiO-66-NH\u003csub\u003e2\u003c/sub\u003e, the yield of \u003cem\u003eN\u003c/em\u003e-benzylaniline was lower than expected, likely due to insufficient chelation ability toward Pd, leading to leaching of active Pd species during the reaction. These findings collectively demonstrate that a support with N-O bidentate chelation capability is a key factor in achieving high product yields[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Evaluation of the Catalyst Substrate Scope\u003c/h2\u003e\u003cp\u003eTo evaluate the substrate adaptability of the Pd/UiO-66-NO catalyst, the reaction time was extended to 6 hours under the optimized conditions, and its compatibility with various aromatic amines and aldehydes was investigated. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the starting reactants were converted to the corresponding target products with high conversion. Neither electron-donating nor electron-withdrawing groups exhibited a significant influence on the reaction outcome, indicating that electronic effects play a negligible role in this transformation. The catalytic system demonstrated excellent compatibility with monofunctional substrates. Substituents such as fluoro, methoxy, and methyl groups on benzaldehyde derivatives had no notable impact on the yield (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The yields for monoamine substrates remained consistently high, ranging from 70% to 94% (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, 9\u0026ndash;13). Notably, even 2,5-dimethylaniline\u0026mdash;a substrate bearing two substituents\u0026mdash;reacted with benzaldehyde to afford the corresponding secondary amine in 91% yield (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), suggesting that significant steric hindrance does not impede the reaction. The presence of substituents on both benzaldehyde and aniline did not disrupt the reaction, and the target products were smoothly obtained. Furthermore, when furfural was reacted with aniline or the sterically hindered 2,5-dimethylaniline, the desired products were obtained in 81% and 83% yields, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, 30, 33). These results collectively demonstrate that the Pd/UiO-66-NO catalyst exhibits a broad substrate scope, efficiently facilitating the one-pot synthesis of secondary amine derivatives from variously functionalized amines and aldehydes under very mild conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Catalyst Recyclability Studies\u003c/h2\u003e\u003cp\u003eTo investigate the recyclability and reusability of Pd/UiO-66-NO, recycling experiments were conducted under the same reaction conditions as previously described. Although the catalytic activity of Pd/UiO-66-NO declined after four reaction cycles compared to the first run, it still maintained considerable activity, affording target product yields of 75% and 92% (Figure S6), respectively. The observed decrease in activity may be attributed to the fact that most Pd nanoparticles remain predominantly located on the external surface of UiO-66-NO, leading to partial loss of Pd NPs during the reaction process. FT-IR spectra of the Pd/UiO-66-NO catalyst before and after the cycling tests (Figure S7) demonstrated no significant structural alterations, confirming the robustness of the catalyst framework under the investigated reaction conditions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Unraveling the Reaction Mechanism of Pd/UiO-66-NO-Catalyzed Aniline-Aldehyde Coupling\u003c/h2\u003e\u003cp\u003eTo investigate the reaction pathway of aniline and benzaldehyde coupling catalyzed by Pd/UiO-66-NO, the transformation process was elucidated by monitoring the temporal concentration changes of reactants, intermediates, and products in the reaction system using a combination of \u003csup\u003e1\u003c/sup\u003eH NMR and gas chromatography (GC). \u003csup\u003e1\u003c/sup\u003eH NMR analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) revealed that in the presence of both Pd/UiO-66-NO catalyst and H\u003csub\u003e2\u003c/sub\u003e, the characteristic signal intensity of benzaldehyde diminished over time, while the intermediate imine exhibited a characteristic increase followed by a decrease. Given the hydrogenation capability of the palladium catalyst, this observation indicates the gradual reduction of the imine intermediate to the target product, \u003cem\u003eN\u003c/em\u003e-benzylaniline, under a hydrogen atmosphere. Analysis of the reaction mixture at different time intervals by GC (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea) further confirmed the proposed amination pathway: aniline and benzaldehyde initially condense to form an imine intermediate, which is subsequently and efficiently hydrogenated to the secondary amine product.\u003c/p\u003e\u003cp\u003eThe proposed reaction pathway for the Pd/UiO-66-NO-catalyzed transformation is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. Initially, the Lewis acid sites within Pd/UiO-66-NO activate the carbonyl group of benzaldehyde, generating a carbocation (C\u003csup\u003e+\u003c/sup\u003e) intermediate via polarization. The lone pair of electrons on the amino group of aniline then attacks this electrophilic C\u003csup\u003e+\u003c/sup\u003e, forming a hydroxyamine intermediate. Subsequently, the Lewis acid sites further facilitate the dehydration of this intermediate to yield an imine (C\u0026thinsp;=\u0026thinsp;N). Finally, Pd nanoparticles catalyze the hydrogenation of the imine; H\u003csub\u003e2\u003c/sub\u003e undergoes heterolytic cleavage on the Pd surface to produce active hydrogen species (H\u003csup\u003e\u0026minus;\u003c/sup\u003e/H\u003csup\u003e+\u003c/sup\u003e), which selectively reduce the imine to yield \u003cem\u003eN\u003c/em\u003e-benzylaniline, concurrently regenerating the catalyst[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eIn this chapter, a Pd/UiO-66-NO catalyst featuring N-O bidentate chelation sites was successfully constructed. Comprehensive characterization confirmed that the salicylaldehyde modification did not disrupt the support framework, while the catalyst exhibited outstanding performance in the reductive amination of benzaldehyde with aniline. Under optimized conditions (toluene, 60\u0026deg;C, 3 MPa H\u003csub\u003e2\u003c/sub\u003e, 3 h), its catalytic efficiency surpassed that of commercial Pd/C and other reference catalysts, which can be attributed to the effective stabilization of Pd nanoparticles by the N-O chelation mechanism. Substrate scope studies revealed good adaptability toward a range of functional groups and sterically hindered substrates. The catalyst maintained a yield of 75% after four recycling runs, with the structural integrity of the support preserved as evidenced by FT-IR analysis. Mechanistic investigations verified a two-step pathway involving condensation to form an imine intermediate followed by hydrogenation. This work offers a new strategy for designing highly stable heterogeneous catalysts and opens a sustainable route for the synthesis of secondary amines.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eBET\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eBrunauer-Emmett-Teller\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eFTIR\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eFourier transform infrared\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eMOFs\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eMetal-organic frameworks\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSEM\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eScanning electron microscopy\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eTEM\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eTransmission electron microscopy\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eXRD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eX-ray powder diffraction\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eXPS\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eX-ray photoelectron spectroscopy\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eNMR\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eNuclear magnetic resonance\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eEDS\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eElectronic Differential System\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003ePSM\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003ePost-synthetic modification\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cb\u003eEthics and Consent to Participate\u003c/b\u003e\u003c/p\u003e\u003cp\u003eNot applicable. This study does not involve human participants, animals, or biological materials.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003cp\u003eAll authors have read and approved the final manuscript for publication.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThe authors declare that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eMei He: Conceptualization, Methodology, Writing-Original Draft. Duoduo Wei: Formal analysis, Investigation, Data Curation. Xiaogang Yin: Validation, Writing-Review \u0026amp; Editing, Supervision.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors thank the School of Chemistry and Materials Science, Guizhou Normal University for providing laboratory facilities and technical support.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data and materials that support the findings of this study are available on request from the corresponding authors.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAn B, Zeng L, Jia M et al (2017) Molecular Iridium Complexes in Metal\u0026ndash;Organic Frameworks Catalyze CO\u003csub\u003e2\u003c/sub\u003e Hydrogenation via Concerted Proton and Hydride Transfer. 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Dalton Trans 47:1674\u0026ndash;1681. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c7dt04266c\u003c/span\u003e\u003cspan address=\"10.1039/c7dt04266c\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Pd/UiO-66-NO, salicylaldehyde, secondary amines, post-synthetic modification, MOFs","lastPublishedDoi":"10.21203/rs.3.rs-8088035/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8088035/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHerein, we developed a Pd/UiO-66-NO catalyst for the reductive amination of benzaldehyde with aniline to synthesize \u003cem\u003eN\u003c/em\u003e-benzylaniline. Through post-synthetic modification, salicylaldehyde was incorporated into the UiO-66-NH\u003csub\u003e2\u003c/sub\u003e framework to construct a support (UiO-66-NO) featuring N,O-bidentate chelating sites, which enabled the stabilization of highly dispersed Pd nanoparticles. Characterization results confirmed the preserved framework integrity and demonstrated efficient secondary amine synthesis under mild conditions (60 °C, 3 h). The catalyst exhibited broad substrate scope with excellent functional group compatibility. Mechanistic studies revealed a two-step pathway involving amine-aldehyde condensation followed by hydrogenation of the imine intermediate. \u003csup\u003e1\u003c/sup\u003eH NMR and GC kinetic analyses evidenced rapid imine formation and subsequent efficient reduction to the secondary amine. This work offers a new strategy for designing high-performance heterogeneous catalysts and paves a promising avenue for sustainable synthesis of secondary amines.\u003c/p\u003e","manuscriptTitle":"Stabilizing Pd Nanoparticles via N,O-Chelation in a MOF for General Reductive Amination","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 18:37:29","doi":"10.21203/rs.3.rs-8088035/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-20T10:24:04+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-19T21:57:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-18T11:33:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"172898658554357957794801000401984157488","date":"2025-11-18T01:02:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"94563049270661418708126669340346774498","date":"2025-11-16T09:25:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"102348405277308835570146032519336708898","date":"2025-11-15T10:47:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"312643142325810136302779018243066773940","date":"2025-11-14T11:28:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"318951680488696632160150939114522745766","date":"2025-11-14T00:21:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-13T10:28:03+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-13T06:30:12+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-13T06:29:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Catalysis Letters","date":"2025-11-11T14:37:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2783e5a1-edfb-4e65-ba0a-9e511cef6a2d","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-12-06T12:53:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-25 18:37:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8088035","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8088035","identity":"rs-8088035","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}