Copper complex of phenylglycine-functionalized UiO-66-NH₂: a chiral MOF catalyst for enantioselective Henry reaction

Abstract

In this study, a Zr based MOF chiral catalyst was synthesized, characterized, and then examined in the enantioselective Henry reactions. The process involved the synthesis of UiO-66-NH₂, followed by its functionalization with 2-chloroacetyl chloride. Subsequently, L-phenylglycine was immobilized on the functionalized MOF to prepare a chiral heterogeneous ligand. Finally, the prepared chiral ligand was complexed with Cu(CH₃CN)₄PF₆ to obtain the chiral heterogeneous catalyst. All of the synthesized materials were characterized using various methods including FT-IR, XRD, SEM, EDX, elemental mapping, and BET/BJH analysis. Evaluation of the catalytic activity of the prepared chiral heterogeneous catalyst under different conditions demonstrated that the best results can be achieved under solvent free green conditions at room temperature, producing excellent yields and low to moderate enantioselectivities. The heterogeneous catalyst was recovered and reused easily, maintaining activity over three consecutive cycles.

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1. Introduction

The Henry reaction, known as the nitroaldol reaction, is an atom economic and powerful synthetic strategy for the stereoselective formation of carbon–carbon bonds. This transformation creates a new stereogenic center at the β-position of a nitro group, forming of β-nitro alcohols.^1,2 The asymmetric version of the Henry reaction is of considerable attention due to forming chiral β-nitro alcohols, which are versatile frameworks in the synthesis of biologically active compounds such as fungicide (S)-spirobrassinin,³ the anti-asthmatic drug (R)-salmeterol,⁴ and antibiotics like L-acosamine.⁵ The resulting β-nitro alcohols can also easily be converted to nitro alkenes by dehydration, β-amino alcohols by reduction, and α-hydroxy carbonyl compounds via the Nef reaction.^6–8

The first enantioselective catalytic Henry reaction was described by Shibasaki in 1992, where a lanthanide(III) 2-naphthoxide complex was utilized as a chiral catalyst.⁹ Subsequently, various research groups have successfully employed copper complexes of oxazoline,^10–13 Schiff bases,¹⁴ and others,^15–21 as chiral catalysts.

Economically and environmentally, it is favorable to replace homogeneous catalytic systems with cost-effective and eco-friendly heterogeneous ones. Heterogeneous catalysis has a great number of benefits over its homogeneous counterparts, including reduced waste generation, catalyst reusability, easy separation, etc.^22–25 However, the low activity and selectivity of heterogeneous catalysts are their common drawbacks.^26–29 Several homogeneous catalysts have been immobilized on traditional supports for use in the enantioselective Henry reaction to date. Despite these efforts, heterogenization of homogeneous catalysts on conventional supports, such as oxide supports, poses challenges due to optimization complications, characterization difficulties, and issues with active site accessibility.^30–34

To tackle such limitations, immobilizing the homogeneous catalysts on metal–organic frameworks (MOFs), which are the most significant categories of porous materials, can be a promising approach. The reusability and easy purification of MOFs, diversity in metal centers and linkers, unique topology, ease of modification of their structure, chemical and thermal stability, etc., make them perfect supports for homogeneous catalysts. Moreover, their high surface area and easy adjustability of the pore size result in a high level of reactivity and selectivity in comparison to other commonly used porous materials such as carbons and porous silicas.^35–39

Thanks to their remarkable features, both chiral MOFs or immobilized chiral ligands on MOFs have been extensively used as chiral heterogeneous catalysts in a diverse range of enantioselective transformations, offering higher efficiency than traditional heterogeneous catalysts.^40–48 Zirconium-based MOFs (Zr-MOFs), especially UiO-66 and its amine derivative UiO-66-NH₂, which contain considerable surface area and a high amount of porosity, and also show substantial hydrothermal and chemical stability, are perfect structures for immobilizing homogeneous catalysts.^49–53

Therefore, based on these outstanding merits of MOFs and also in continuing our ongoing studies on enantioselective transformations in the presence of heterogeneous catalysts,^54–59 we immobilized chiral L-phenylglycine (L-PhG) using a post-synthetic modification (PSM) strategy onto UiO-66-NH₂, as a zirconium-based MOF, and examined its catalytic potential as a reusable and recyclable chiral nanocatalyst in the enantioselective Henry reaction.

2. Experimental

2.1. Preparation of UiO-66-NH₂ nanoparticles via a solvothermal method

The UiO-66-NH₂ was prepared according to the literature procedure.⁶⁰ In a 250 mL beaker, 0.5 mmol (0.45 g) of 2-aminoterephthalic acid (NH₂-BDC) and 2.35 mmol (0.55 g) of ZrCl₄ were completely dissolved in 125 mL of DMF. Then, 15 mL of acetic acid was added, and the mixture was transferred into a Teflon-lined hydrothermal autoclave reactor and heated at 120 °C for 24 h. After gradually cooling to room temperature, the resulting mixture was centrifuged and washed with anhydrous MeOH (3 × 15 mL) to remove residual DMF. The resulting pale-yellow solid was dried in an oven at 100 °C for 12 h.

2.2. Functionalization of the UiO-66-NH₂ with 2-chloroacetyl chloride (preparation of UiO@Cl)

Under a nitrogen atmosphere, in a 50 mL round-bottom flask, 0.5 g of as-prepared UiO-66-NH₂ was added to 25 mL anhydrous toluene, and then the flask was placed in an ultrasound bath for 30 minutes. Next, 2 mmol (0.15 mL) of 2-chloroacetyl chloride and 2 mmol (0.28 mL) Et₃N were added to the solution, and the mixture was stirred for 15 h at room temperature. The obtained precipitate was centrifuged and washed with anhydrous toluene (2 × 10 mL) and anhydrous MeOH (3 × 10 mL) and then, dried at room temperature.

2.3. Immobilization of L-phenylglycine on the functionalized UiO-66-NH₂ (preparation of UiO@PhG)

Under a nitrogen atmosphere, in a 50 mL round-bottom flask, 10 mL anhydrous toluene, 0.5 g of UiO@Cl, 2 mmol (0.28 mL) of Et₃N and 2 mmol (0.3 g) of L-phenylglycine ligand were added and, then the mixture was stirred for 24 hours at room temperature. After that, the precipitate was isolated by centrifugation and washed with anhydrous toluene (3 × 15 mL) and anhydrous MeOH (3 × 15 mL).⁵⁵

2.4. Incorporation of Cu(CH₃CN)₄PF₆ into UiO-PhG (preparation of heterogeneous catalyst UiO@PhG-Cu)

0.26 mmol (0.1 g) of Cu(CH₃CN)₄PF₆ and 0.5 g of UiO@PhG were transferred in anhydrous acetonitrile (15 mL) into a 50 mL round bottom flask under a nitrogen atmosphere. The resulting mixture was stirred at room temperature for 24 hours. The chiral heterogeneous catalyst UiO@PhG-Cu was separated by filtration and washed with anhydrous methanol (3 × 5 mL).

2.5. Typical catalytic procedure for the enantioselective Henry reaction using the prepared chiral UiO@PhG-Cu catalyst

Under a nitrogen atmosphere, in a 10 mL Schlenk flask, 1 mmol (54 μL) of nitromethane, 5 mg UiO@PhG-Cu, and 0.05 mmol (7 μL) of Et₃N were added and stirred for 15 minutes. Then, 0.1 mmol (0.015 g) of p-nitrobenzaldehyde was added to the reaction mixture. Once the reaction was completed (monitored by TLC), the reaction mixture was filtrated, and the catalyst was washed with ethyl acetate three times, then extracted with ethyl acetate and 5% ammonia. The organic phase was evaporated under reduced pressure, and the resulting product was purified using flash column chromatography (n-hexane/ethyl acetate: 15–25%) to yield a yellow oil.

3. Results and discussion

3.1. Characterization of the as-synthesized MOFs and catalyst

Initially, the UiO-66-NH₂ was synthesized through a solvothermal method by reacting 2-aminoterephthalic acid (NH₂-BDC) with zirconium chloride and acetic acid in DMF, as described above. Then, the free amino groups were modified with 2-chloroacetyl chloride to produce UiO@Cl. In the following, reacting the modified MOF with L-phenylglycine yielded UiO@PhG. Finally, the heterogeneous catalyst (UiO@PhG-Cu) was obtained by complexation of the UiO@PhG with Cu(CH₃CN)₄PF₆ (Scheme 1).

Scheme 1 Preparation of chiral heterogeneous catalyst UiO@PhG-Cu.

Using Fourier transform infrared (FT-IR) spectroscopy, functional groups of the prepared UiO-66-NH₂ before and after post synthetic modification (PSM) and the catalyst were identified.^{52,53,61–63} According to Fig. 1, it can be observed that the intensity of the peaks decreases gradually with each successive modification step, indicating modification of the initial MOF. Although almost all functional groups were present in all of the materials, their FTIR spectra show some differences. In the FTIR spectrum of UiO-66-NH₂, two strong bands related to the stretching vibrations of N–H bonds of the NH₂ groups appeared at 3456 cm⁻¹ and 3352 cm⁻¹. A peak at 2925 cm⁻¹ corresponds to the stretching vibrations of C–H bonds in the aromatic rings. A sharp peak at 1656 cm⁻¹ refers to the carbonyl group of DMF residual trapped within the pores of the UiO-66-NH₂ MOF.⁶⁴ The presence of the characteristic strong absorption bands at around 1581 and 1436 cm⁻¹ relates to asymmetric and symmetric stretching vibrations of carboxylate groups coordinated to Zr⁴⁺. The peak at 1502 relates to stretching vibration of C=C aromatic bonds. The appearance of peaks at 1384 and 1255 cm⁻¹ corresponds to the stretching vibrations of the C–N bonds. The strong band peaks at 769 and 661 cm⁻¹ display the stretching vibration of the Zr–O bond. The same peaks were observed after reacting with 2-chloroacetyl chloride, but with lower intensity. The bands related to the amino groups become broader, and two peaks merged into a broad peak at around 3380 cm⁻¹, indicating the formation of the N–H amide group. It seems that the stretching vibrations of C–Cl at around 750 cm⁻¹ are eclipsed by Zr–O peaks. In the case of UiO@PhG, the intensity of the peaks became lower. A border peak, compared to previous materials, related to CO₂H and N–H bonds was observed at around 3450 cm⁻¹, clearly demonstrating immobilization of the chiral ligand L-phenylglycine on the UiO@Cl. In the FT-IR spectrum of the catalyst UiO@PhG-Cu, the N–H band in the range of 3300–3450 cm⁻¹ appears broader. The Cu–N and Cu–O vibrations in the range of 400–600 cm⁻¹ are apparently obscured by the Zr–O signals.

Fig. 1 The FT-IR spectra of UiO-66-NH₂, UiO@Cl, UiO@PhG and UiO@PhG-Cu.

The crystalline phases of the as-synthesized MOFs were measured using powder X-ray diffraction (PXRD) analysis.^52,53 As indicated in Fig. 2, the PXRD of UiO-66-NH₂ showed distinctive peaks at 2θ angles of 7.3°, 8.5°, and 25.7°, corresponding to the crystal planes (1 1 1), (0 0 2), and (0 0 6), respectively, confirming the successful synthesis of UiO-66-NH₂.^65,66 In the PXRD of all of the modified materials, a similar pattern was observed; however, the existence of slight changes in the position of the characteristic peaks represents the occurrence of reactions and the formation of new materials. Moreover, these slight significant changes indicate that the overall framework of the initial UiO-66-NH₂ remains intact after functionalization and metal complexation.

Fig. 2 The XRD patterns of UiO-66-NH₂, UiO@Cl, UiO@PhG and UiO@PhG-Cu.

To examine the morphology and particle size of the UiO-66-NH₂ and modified MOFs, SEM analysis was applied. As shown in Fig. 3, the SEM image of UiO-66-NH₂ has uniform spherical shape particles ranging from 68–133 nm. The size and morphology of the modified MOFs remained unchanged throughout the modification process, indicating structural stability of the as-synthesized UiO-66-NH₂.

Fig. 3 SEM micrographs of UiO-66-NH₂, UiO@Cl, UiO@PhG and UiO@PhG-Cu.

The elemental composition of the resulting MOF catalyst was measured by EDX analysis. The EDX spectra exhibited the peaks of carbon (C), chlorine (Cl), fluorine (F), nitrogen (N), copper (Cu), oxygen (O), phosphors (P), and zirconium (Zr), validating the synthesis of the catalyst (Fig. 4). Moreover, elemental mapping analysis indicated the presence and homogeneous distribution of these elements throughout the catalyst matrix, confirming successful synthesis of the catalyst UiO@PhG-Cu complex (Fig. 5).

Fig. 4 EDX spectra of the catalyst (UiO@PhG-Cu).

Fig. 5 Elemental mapping of the catalyst (UiO@PhG-Cu).

Thermal stability of the as-synthesized materials was evaluated by thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) analysis (Fig. 6). The thermograms of all samples showed several stages of mass loss. A small mass loss around 2–5% occurs at under 150 °C, which is due to the evaporation of physically adsorbed trapped moisture and the residual organic solvents into the MOF pores. The thermograph of UiO-66-NH₂ demonstrated that it is more stable than the functioned MOFs. It showed that UiO-66-NH₂ is highly stable up to 350 °C and only begins to decompose significantly above this temperature, which corresponds to the decomposition of the organic linkers (NH₂-BDC). This trend continues gradually until about 500 °C, at which point the organic linkers have fully broken down, leading to the collapse of the MOF's structure. After 500 °C, the weight loss stabilizes, indicating that the remaining material is largely inorganic. In this stage, the zirconium oxo clusters are dehydroxylated, and subsequently, the framework is decomposed. The TGA and DTG graphs of UiO@Cl demonstrated that this framework is stable up to around 300 °C, but weight loss occurs gradually until below 500 °C. In the case of UiO@PhG and UiO@PhG-Cu, significant weight loss is observed at around 200 °C, which can be related to the degradation of phenyl glycine and the copper complex. At around 500 °C, the framework of UiO@PhG is collapsed in a similar pattern to UiO@Cl. This trend was not observed in the thermogram of UiO@PhG-Cu, as presumably due to the existence of copper and formation of CuO the remarkable diminishing in weight did not occur.^67,68 The TGA and CHN analysis also revealed that the amount of phenylglycine loaded on UiO@PhG is 0.78 mmol g⁻¹.

Fig. 6 The TGA and DTG thermograms of UiO-66-NH₂, UiO@Cl, UiO@PhG and UiO@PhG-Cu.

The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) equations were applied to estimate the surface areas, pore diameters and pore volumes of the prepared UiO-66-NH₂ and modified MOFs.⁶⁹ According to the IUPAC classification,^70–72 the nitrogen adsorption–desorption isotherms for UiO-66-NH₂ and UiO@Cl demonstrated a type IV isotherm, mainly indicating the presence of mesopores. A sharp adsorption observed at low relative pressures (p/p₀ < 0.05) is attributed to micropore filling due to strong adsorbent–adsorptive interactions. The H₁-type hysteresis loop, seen between 0.7–0.99 p/p₀, indicates capillary condensation of nitrogen within the pores, suggesting materials with well-defined cylindrical-like pore structures (Fig. 7).

Fig. 7 (a) Nitrogen adsorption–desorption isotherms of all prepared materials. (b) and (c) Zoomed-in views of the isotherms for UiO@PhG and UiO@PhG-Cu, respectively.

The isotherm shapes of UiO@PhG and UiO@PhG-Cu are completely different. Both isotherms are an intermediate between type II and type IV, with an H₃-type hysteresis loop between relative pressures of nearly 0.60 and 0.98 p/p₀, indicating the existence of slit-shaped mesoporous and macropore structures (Fig. 7).

The pore size distribution graphs of UiO-66-NH₂ and UiO@Cl showed primarily mesopores. However, the pore size in the case of UiO@Cl is smaller due to the immobilization of 2-chloroacetyl chloride on UiO-66-NH₂. A broader pore size distribution is observed in the case of UiO@PhG and UiO@PhG-Cu, indicating the presence of mesopores and some macropores (Fig. 8).

Fig. 8 (a) Pore size distribution of all prepared materials. (b) Zoomed-in views of the pore size distribution graphs for UiO@PhG and UiO@PhG-Cu.

As shown in Table 1, the surface areas decrease progressively as the modification of the MOFs advances from the initial UiO-66-NH₂ to UiO@Cl, and then to UiO@PhG, and finally to UiO@PhG-Cu. However, based on the observed isotherm shapes and hysteresis loops, the pore structure undergoes a transition: it changes from well-defined, cylindrical mesopores in UiO-66-NH₂ and UiO@Cl to meso- and macroporous slit-shaped structures in UiO@PhG and UiO@PhG-Cu. This shift in structure and pore may explain the increased pore volume in UiO@PhG-Cu and the rise in mean pore diameter in both UiO@PhG and UiO@PhG-Cu.

Table 1 N₂ adsorption–desorption data from UiO-66-NH₂, UiO@Cl, UiO@PhG, and UiO@PhG-Cu

Sample	BET surface area (m^2 g^−1)	Mean pore diameter (nm)	BET total pore volume (cm^3 g^−1)	BJH pore volume (cm^3 g^−1)
UiO-66-NH2	698	5.6	0.97	0.78
UiO@Cl	474	5.4	0.65	0.52
UiO@PhG	64	17.3	0.28	0.27
UiO@PhG-Cu	37	46.7	0.43	0.43

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3.2. Catalyst activity of the prepared catalyst in the asymmetric Henry reaction

To evaluate the catalytic activity of the prepared chiral heterogeneous catalyst UiO@PhG-Cu, it was employed in the enantioselective Henry reaction. By varying different parameters such as solvent, temperature, and catalyst loading, we aimed to prepare enantioenriched β-nitro alcohols. In a model reaction, initially, nitromethane (1 mmol) and p-nitro benzaldehyde (0.1 mmol) were reacted in the presence of a catalytic amount of UiO@PhG-Cu (5 mg) and a small amount of Et₃N in EtOH as a green solvent at room temperature, under an inert atmosphere (entry 1). These reaction conditions resulted in high yield (80%) and 31% enantiomeric excess (ee) in 3 hours. In the FT-IR spectrum of the β-nitro alcohol, a characteristic broad band appeared at 3474 cm⁻¹, which is related to the newly generated OH group. Vibration peaks of the nitro group and C–O bond were observed at 1551 cm⁻¹ and 1144 cm⁻¹, respectfully. The ¹H NMR showed a doublet of doublets signal at δ = 5.62 ppm, indicating one hydrogen of the newly formed stereogenic center. Two methylene hydrogen groups appeared as a multiplet at δ = 5.62 ppm. The hydroxyl group was seen at δ = 3.70 ppm. The ¹³C NMR displayed six signals, confirming the synthesis of the product.

To study the impact of the solvent, a wide range of solvents, from protic and non-protic polar to non-polar, and even in solvent-free conditions, were examined. Using water instead of EtOH halved the reaction time and increased the yield to 90%, but the ee decreased to one-third (entry 2). The reaction in methanol proceeded more slowly than that in EtOH, and gave lower yield and ee (entry 3). Employing polar aprotic solvents like DMSO, DMF, and CH₃CN did not improve both the yield and ee, and it also increased the reaction time to several hours (entries 4–6). Conducting the reaction using non-polar solvents resulted in very low product yield, or no product was obtained at all, even after extended reaction times (entries 7–13). Notably, the results were improved when no solvent was used. Under solvent-free conditions, a higher yield (95%) and enantioselectivity (37%) were obtained within 0.5 hours (entry 14). In situ preparation of the catalyst by reacting UiO@PhG with Cu(CH₃CN)₄PF₆ resulted in lower yield and ee (entry 15). The decrease in enantiomeric excess may be attributed to progress of the reaction using uncomplexed copper.

The catalyst loading screen indicated that varying the amount of catalyst did not improve the results. As the amount of the catalyst increased from 5 mg, the ee and yield decreased, and the reaction increased (entries 16 and 17). The same trend was observed when less than 5 mg catalyst was employed (entries 18–20).

Lowering the reaction temperature improves the enantioselectivity to 47%, as expected. However, this comes at the cost of a longer reaction time and a lower yield (entries 21 and 22). Moreover, increasing the temperature gave lower results compared to those obtained at room temperature (entries 23 and 24).

Under the optimal reaction conditions (entry 14), other aromatic aldehydes were further tested to prove the catalyst's general applicability (entries 25–27). The results indicated that the nitro group, the most electron withdrawing group, effectively activated the carbonyl group of the aldehydes, increasing the rate of the reaction. Generally, a direct correlation was observed between the electron withdrawing activity of groups attached on the aldehydes and the improvement in results (Table 2).

Table 2 Evaluation of the asymmetric Henry reaction using heterogeneous catalyst UiO@PhG-Cu

Entry	X	Solvent	Catalyst (mg)	T (°C)	Time (h)	Yield^b (%)	ee (%)
a The complex was prepared in situ by reacting UiO@PhG with Cu(CH3CN)4PF6. b Isolated yield based on aldehyde.
1	p-NO2	EtOH	5	r.t.	3	80	31
2	p-NO2	H2O	5	r.t.	1.5	90	10
3	p-NO2	MeOH	5	r.t.	6	53	16
4	p-NO2	DMSO	5	r.t.	16	26	20
5	p-NO2	DMF	5	r.t.	3	83	18
6	p-NO2	CH3CN	5	r.t.	20	45	20
7	p-NO2	EtOAc	5	r.t.	48	0	—
8	p-NO2	THF	5	r.t.	19	20	13
9	p-NO2	CH2Cl2	5	r.t.	48	0	—
10	p-NO2	Ether	5	r.t.	48	0	—
11	p-NO2	CHCl3	5	r.t.	19	30	13
12	p-NO2	Dioxane	5	r.t.	48	0	—
13	p-NO2	Toluene	5	r.t.	22	30	8
14	p-NO2	S.F.	5	r.t.	0.5	95	37
15^a	p-NO2	S.F.	5	r.t.	0.5	90	28
16	p-NO2	S.F.	6.25	r.t.	1	89	33
17	p-NO2	S.F.	7.5	r.t.	2.5	84	28
18	p-NO2	S.F.	3.75	r.t.	1.5	90	30
19	p-NO2	S.F.	2.5	r.t.	2	84	22
20	p-NO2	S.F.	1.25	r.t.	2	78	9
21	p-NO2	S.F.	5	0	6	52	40
22	p-NO2	S.F.	5	−10	10	43	47
23	p-NO2	S.F.	5	50	0.5	85	22
24	p-NO2	S.F.	5	100	0.45	77	10
25	H	S.F.	5	r.t.	24	45	12
26	p-Cl	S.F.	5	r.t.	18	69	19
27	m-NO2	S.F.	5	r.t.	0.5	78	25

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3.3. Reusability of the chiral heterogeneous MOF catalyst

To study the reusability of the catalyst, after the reaction was completed under optimal conditions, the reaction mixture was filtered and washed with ethyl acetate three times. The catalyst was then dried in an oven and then reused in the optimal reaction conditions. This process was repeated five times. According to the results shown in Scheme 2, there was no significant decrease in the catalytic activity after three times. After the third run, the reduction in the catalyst activity, commonly observed in MOF catalytic systems, may result from the blockage of active sites in the MOF.⁷³ Trapped molecules, such as reactants, products, or solvents, can occupy and block the MOF pores, reducing the catalytic activity. It should be noted that, based on ICP analysis, the copper content was 3.14 mol% before use in the reaction and 2.67 mol% after three runs, indicating relatively low copper leaching.

Scheme 2 Reusability of the UiO@PhG-Cu catalyst.

3.4. Reaction mechanism

According to the literature,^1,11,74–76 a plausible mechanism for enantioselective Henry reaction using the UiO@PhG-Cu catalyst is depicted in Scheme 3. The catalytic cycle starts with the elimination of the α-hydrogen of nitromethane by base (Et₃N), resulting in a nitronate anion. The resulting nitronate anion then coordinates with the copper, which is located in the middle of the chiral complex. Consequently, the oxygen atom of the aldehyde coordinates with the copper. In the most stable transition state (TS1), the nitronate anion attacks from the re face of the aldehyde, resulting in the S-product. Conversely, the less favorable transition state (TS2), due to unfavorable interactions, leads to the R-product.

Scheme 3 Proposed mechanism of the enantioselective Henry reaction catalyzed by UiO@PhG-Cu.

4. Conclusions

In summary, we have introduced a new chiral heterogeneous catalyst by immobilizing L-phenylglycine on functionalized UiO-66-NH₂, and subsequent complexation with Cu(CH₃CN)₄PF₆. All of the synthesized processes were characterized and then the catalyst's activity was studied in the enantioselective Henry reaction. This chiral Zr-based MOF catalyst results in excellent yield and moderate enantioselectivity under solvent free conditions. Moreover, the catalyst demonstrates good reusability, as it can be used effectively for three successive cycles. Further optimizations are being undertaken in our laboratory to enhance the enantioselectivity.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to the University of Kurdistan Research Councils and the Iran National Science Foundation (Project No: 4003026) for providing financial support for this research. The authors also gratefully thank Dr Rezgar Ahmadi for his helpful discussion.

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Footnote

† Electronic supplementary information (ESI) available: The ESI provides materials and characterization methods as well as spectroscopic characterization and spectra of the prepared chiral organic products 2a–c. See DOI: https://doi.org/10.1039/d4nj03149k

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