Dynamic covalent bonding for directed construction of molecular cages toward carbon dioxide reduction†

Jingting He‡ ^a, Man Dong‡ ^b, Yang Zhao ^b, Dongxu Cui ^b, Xiaohui Yao ^b, Fanfei Meng ^a, Wei Li ^b, Shuai Yang ^b, Chunyi Sun *^b and Zhongmin Su *^a
aSchool of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022 Jilin, China. E-mail: zmsu@nenu.edu.cn
^bKey Laboratory of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun, 130024 Jilin, China. E-mail: suncy009@nenu.edu.cn

First published on 20th June 2024

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Introduction

Metal–organic cages (MOCs) are discrete molecular assemblies of organic ligands coordinated to metal cations or metal oxide clusters to form structures with intrinsic porosity.^1,2 The first MOC structure was reported by Saalfrank in 1988, and since then, great efforts have been devoted to the design and synthesis of coordination cages.³ Due to their numerous advantages, such as pore tunability, structural diversity, and functionalities,^4,5 MOCs have garnered significant interest in various fields including adsorption, separation, sensing and biomedicine. In recent years, molecular cages have shown fascinating properties and unlimited potential in catalysis because of their unique high dispersibility in solution.^6–14 This inherent property causes them to hold the merits of both homogeneous catalysts with easy access to active sites and heterogeneous materials with recyclable properties.^15–17 However, the limited stability of MOCs resulting from the relatively weak coordination bonds hinders their development in practical applications.

In 2013, Liu et al. reported an extremely stable zirconium-based MOC (Zr-MOC) for the first time.¹⁸ Later, Nam et al. proposed functionalized cages based on active site availability on ligands (ZrT-1-NH₂).¹⁹ The design and application of Zr-MOCs has developed rapidly. Zr-MOCs are usually synthesized by applying a one-pot method of excess zirconium dichloride with organic ligands through a solvothermal reaction in different solvent systems,^20–22 and precise synthesis of Zr-MOC structures remains challenging. Therefore, the development of suitable strategies for the tailored synthesis of Zr-MOCs is a significant and worthwhile research direction.

Herein, we design a method for the precise synthesis of MOCs by constructing dynamic covalent bonds. Employing this approach, a novel lantern-shaped MOC (Schiff-base ZrOC-1) has been successfully designed and its performance in photocatalytic conversion of CO₂ was explored. Experimental findings reveal that the precursor (Zr-CHO) and the Zr-MOCs without dynamic covalent bonds (ZrT-1) exhibit virtually no catalytic activity, whereas Schiff-base ZrOC-1 displays exceptional catalytic properties with CO₂-to-CO reactivity reaching 2.55 mmol g⁻¹ h⁻¹. Moreover, it exhibits attractive stability, high dispersion and recyclability. Mechanistic studies reveal that the newly introduced dynamic covalent bonds should be the catalytic sites and that *COOH is the key intermediate in the photoreduction of CO₂. Furthermore, photoelectrochemical analysis demonstrates that Schiff-base ZrOC-1 has a quicker photocurrent response and higher charge separation efficiency compared to Zr-CHO, implying that the MOC structure has a positive effect on catalytic CO₂ reduction. This work presents a fresh strategy for the rational design and targeted synthesis of MOCs for CO₂ reduction.

Experimental section

Materials

Bis(cyclopentadienyl)zirconium dichloride, hydrazine hydrate, 1,4-dioxane, 1,4-dicarboxybenzene, trifluoroacetic acid, ethanol, 4-formylbenzoic acid, and N,N-dimethylformamide (DMF) were all purchased and used directly.

Synthesis of Zr-CHO crystals

4-Formylbenzoic acid, (0.20 mmol), bis(cyclopentadienyl)zirconium dichloride (0.14 mmol), N,N-dimethylformamide (DMF, 0.5 ml) and distilled water (4 drops) were added to a 5 ml glass vial. Then, the glass vial was placed in a sonicator for 6 minutes and then maintained at 60 °C for 15 h. . Yellow crystals were obtained by filtration, washed with DMF, and then dried in a vacuum oven at 60 °C. Yield: ∼87% based on bis(cyclopentadienyl)zirconium dichloride.

Synthesis of Schiff-base ZrOC-1 crystals

Zr-CHO crystals (0.24 g), hydrazine hydrate (50 μl), ethanol (0.3 ml), 1,4-dioxane (0.5 ml) and trifluoroacetic acid (10 μl) were added to a 5 ml glass vial. Then, the glass vial was placed in a sonicator for 6 minutes and then maintained at 60 °C for 15 h.  Transparent crystals were obtained by filtration, washed with ethanol, and then dried in a vacuum oven at 60 °C. Yield: ∼76% based on Zr-CHO.

Synthesis of ZrT-1 crystals

ZrT-1 was synthesized according to literature methods.¹⁸

X-ray crystallography studies

All crystal data were collected by an X-ray single crystal system (Bruker D8-Venture) with a Turbo X-ray source (Cu Kα radiation, λ = 1.5418 Å) using a CMOS detector and the direct drive rotating anode technique. Program APEX3 was employed to collect and process the data frames. The structures were solved by the SHELXT program and refined by the SHELXL-2014 program.

Characterization

Powder X-ray diffraction (PXRD) patterns were obtained using an X-ray diffractometer (JEM-2100Plus). Infrared spectra in the wavelength range of 4000–400 cm⁻¹ were acquired from KBr pellets on a Nicolet 380 FT-IR spectrophotometer. A UV-Vis-NIR spectrophotometer was used for the collection of UV-Vis spectra.

Photocatalytic CO₂ reduction

CO₂ photoreduction was performed in 10 ml reactors containing a mixture of Schiff-base ZrOC-1/Zr-CHO/ZrT-1 (1 mg), [Ru(bpy)₃]Cl₂ (7 mg), trolamine (TEOA, 1 ml), and acetonitrile (MeCN, 5 ml). The reactor was bubbled with Ar or CO₂ for 10 min. The system was irradiated with visible light using a 300 W Xe lamp with a 420 nm cutoff filter at 20 °C. The gaseous products were detected by gas chromatography (GC). Liquid products were detected by ion chromatography (IC). An isotopic experiment was performed under similar conditions and ¹³CO analysis was conducted by GC-MS.

Mott–Schottky curve analysis

An ITO (3 cm × 1 cm) substrate was cleaned by sonication in alcohol. A 1 mg sample was scattered into a 500 μl solution (200 μl of ethanol, 100 μl of 0.5 wt% Nafion solution and 200 μl of distilled water), and then sonicated to form a suspension. 120 μl of the prepared sample was uniformly coated on the ITO substrate, and dried overnight at room temperature. The photoelectrochemical characterization was performed on a CHI 760e electrochemical workstation with a three-electrode configuration in 0.2 M Na₂SO₄ as the electrolyte using the assembled photoelectrodes as the working electrode, Ag/AgCl (in 3 M KCl) as the reference electrode and Pt as the counter electrode. The test was conducted at frequencies of 500 Hz, 800 Hz and 1000 Hz.

Results and discussion

Synthesis and characterization

A new method that utilizes a dynamic covalent bond construction strategy has been developed for the meticulous stepwise synthesis of MOCs (Fig. 1). The Schiff-base ZrOC-1 was synthesized directionally by this method, with detailed synthesis procedures provided in the Synthesis section (Fig. S1†).

Initially, single crystal X-ray diffraction (SC-XRD) was used to determine the crystal structure. Bis(cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂) underwent a gradual hydrolysis process, leading to the formation of secondary building units (SBUs) represented by the trinuclear cation [Cp₃Zr₃μ₃-O(μ₂-OH)₃]. These SBUs then coordinated with 4-formylbenzoic acid (4-FBA) ligands, resulting in the formation of precursors (Zr-CHO, [Cp₃Zr₃μ₃-O(μ₂-OH)₃](4-FBA)₃Cl). The SC-XRD result indicates that Zr-CHO crystallizes in the triclinic space group P (Tables S1 and S2†). The 4-formylbenzoic acid ligands are positioned below the Zr₃ triangular plane within the trinuclear cluster of the SBUs, maintaining a close adherence to a C₃ symmetry. Furthermore, the μ₂-OH group is located within the same plane as the trinuclear cluster (Fig. S2 and S3†).

Schiff-base ZrOC-1 with a lantern-like structure based on Cp₃Zr₃μ₃-O(μ₂-OH)₃ units and 4,4′-(hydrazine-1,2-diyilidenebis(methanylylidene)) dibenzoic acid (4-dbahz) was synthesized through the formation of dynamic covalent bonds between Zr-CHO and hydrazine hydrate (Fig. 2a and b). The SC-XRD analysis shows that it crystallizes in the monoclinic space group C2/c (Tables S1 and S2†). Zr-CHO, acting as a unique C₃-symmetric secondary building unit, orients one face towards the aldehyde-based ligand, with the μ₂-OH group extending in the opposite direction. The angle between the C₂ axis of Zr-CHO and the aldehyde-based ligand is approximately 60°. The formula of Schiff base ZrOC-1 is [Cp₃Zr₃μ₃-O(μ_2-OH)₃]₂(4-dbahz)₃Cl₂ (Fig. S4 and S5†).

The powder X-ray diffraction (PXRD) patterns of as-synthesized Zr-CHO and Schiff-base ZrOC-1 are confirmed by the strong agreement with their simulated counterparts (Fig. 2c and Fig. S6†), which indicates the successful synthesis of Zr-CHO and Schiff-base ZrOC-1 with good phase purity and crystallinity. The successful synthesis of Schiff-base ZrOC-1 is further evidenced by Fourier-transform infrared (FT-IR) spectroscopy. Compared with Zr-CHO, a new peak appeared at 1629 cm⁻¹ corresponding to the CN bonds present in Schiff-base ZrOC-1, verifying its successful synthesis (Fig. 2d).^23–25 Mass spectrometry (MS) and proton nuclear magnetic resonance (¹H-NMR) spectroscopy simultaneously confirm the satisfactory synthesis of Schiff-base ZrOC-1 (Fig. S7 and S8†).

Thermogravimetric analysis (TGA) reveals that the crystals are stable up until 200 °C, whereupon any significant weight loss can be attributed to the release of encapsulated solvent. From 400 °C to 450 °C, the weight loss progresses gradually. Beyond this range, there is a marked decrease in weight, indicating the degradation or breakdown of the crystal structure (Fig. S9†).

Photoelectrochemical properties of crystals

Ultraviolet–visible (UV-vis) diffuse reflectance spectra were initially obtained to examine the photoelectrochemical characteristics of the crystals. Fig. 3a shows that Zr-CHO and Schiff-base ZrOC-1 exhibit absorbance in both the UV and visible spectral regions. Utilizing the Tauc plot, the estimated band gaps (E_g) of Zr-CHO and Schiff-base ZrOC-1 are found to be 2.45 eV and 2.5 eV, respectively, showing their typical semiconductor behavior (Fig. 3b).

The flat-band (FB) potentials of Zr-CHO and Schiff-base ZrOC-1 are measured as −0.46 V and −0.47 V versus the normal hydrogen electrode (NHE), respectively, according to the Mott–Schottky (MS) experiments. Moreover, the positive slopes of the MS curves affirm that both materials are n-type semiconductors.²⁶ Given that the conduction band (CB) of n-type semiconductors lies 0.1 V below the FB potential, the CB potentials of Zr-CHO and Schiff-base ZrOC-1 are −0.56 V and −0.57 V versus NHE, respectively.²⁷ By applying the formula E_CB = E_VB − E_g, the valence band (VB) potentials for Zr-CHO and Schiff-base ZrOC-1 were calculated as 1.88 V and 1.94 V versus NHE, respectively. The photoelectrochemical properties of ZrT-1 were also evaluated and its CB and VB were −0.55 V and 2.65 V versus NHE. Evidently, the CB positions of ZrT-1, Zr-CHO and Schiff-base ZrOC-1 relative to the potential of CO₂ reduction (CO₂/CO = −0.53 V vs. NHE) imply that they could be able to drive this reaction (Fig. 3c and d, Fig. S10–S12†).²⁸

Electrochemical impedance spectroscopy (EIS) was employed to examine the charge generation and transport capabilities of the materials. In Fig. 3e and Fig. S13a,† Schiff-base ZrOC-1 exhibits a notably higher charge separation efficiency when compared to Zr-CHO and ZrT-1. Additionally, photocurrent response measurements were conducted. The transient photocurrent density of Schiff-base ZrOC-1 is observed to be greater than that of Zr-CHO and ZrT-1, indicating an enhanced spatial separation of photogenerated charge carriers within Schiff-base ZrOC-1 under light exposure (Fig. 3f and Fig. S13b†). These findings highlight the appealing charge separation and carrier transport properties of Schiff-base ZrOC-1.

Photocatalytic CO₂ reduction

The photocatalytic CO₂ reduction activity of Schiff-base ZrOC-1 was assessed in a CO₂-saturated acetonitrile solution containing Ru(bpy)₃Cl₂ as a photosensitizer and TEOA as a sacrificial electron donor subjected to visible-light irradiation for 1 hour. For comparison, the photocatalytic performances of Zr-CHO and ZrT-1 were examined. As shown in Fig. 4a, CO and H₂ are the primary reduction products. Schiff-base ZrOC-1 effectively catalyzes the photocatalytic reduction of CO₂ to CO (yielding 1.5 mmol g⁻¹ h⁻¹of H₂ and 2.55 mmol g⁻¹ h⁻¹ of CO), whereas Zr-CHO and ZrT-1 exhibit almost no catalytic activity, indicating that the newly constructed dynamic covalent bonds within the cage are the active sites for CO₂ reduction.^29,30 Considering the preferable dispersion of Schiff-base ZrOC-1, we speculate the superior catalytic ability may be attributed not only to the presence of introduced catalytic sites for CO₂ reduction, but also to its dispersion in the solvent and better charge separation and carrier transport properties.

In the absence of a catalyst, photosensitizer, or light, no substantial carbon reduction products were formed, confirming that the catalytic process is indeed photosensitive and that the catalytically active sites are present on the catalyst (Fig. S14†). The time-dependent CO₂ conversion by Schiff-base ZrOC-1 is presented in Fig. 4b. Over a 90 min period, CO production rises with increasing reaction time. Once the reaction surpasses 90 min, the CO production rate becomes nearly constant. The observed decrease in catalytic performance after this point may be associated with the gradual inactivation of the photosensitizer (Fig. S15a†).

To confirm that CO was derived from the reduction of CO₂, control experiments (using Ar) and isotopic experiments (employing ¹³CO₂ labeling) were conducted. No carbon reduction products in Ar (Fig. 4a) and the ¹³CO signal (m/z = 29) (Fig. 4c) verify that CO originates from the reduction of CO₂.

The stability and reproducibility of a catalyst are of great importance for practical applications. Schiff-base ZrOC-1 displays consistent high catalytic activity over the course of three consecutive cycles (Fig. 4d). More significantly, the FT-IR spectrum of Schiff-base ZrOC-1 remains largely unchanged before and after the catalytic reaction, demonstrating its structural integrity (Fig. S16a†). In contrast, the characteristic peaks of the FT-IR spectrum of Zr-CHO show significant alteration following the catalytic reaction (Fig. S16b†). The stability of Schiff-base ZrOC-1 in the presence of TEOA was carefully investigated using MS and ¹H-NMR. The results reveal its stabilization in the presence of TEOA (Fig. S17 and S18†).

Catalytic experiments were carried out on three batches of the same catalyst and the yields of the products (CO and H₂) from each batch were obtained by GC. The results show that the yields of CO and H₂ are almost the same, indicating favorable reproducibility of the catalysts (Fig. S15b†). These outcomes imply that Schiff-base ZrOC-1 possesses superior structural stability and reproducibility during the photocatalytic process, making it more suitable for practical catalyst applications.

Catalytic mechanism study

In situ Fourier-transform infrared (FT-IR) spectroscopy was performed to characterize the intermediates formed during the CO₂RR. In the catalytic system saturated with CO₂, a significant enhancement of several peaks was detected within the range of 1000 to 2000 cm⁻¹ after 30 minutes of continuous illumination, suggesting the accumulation of intermediates. The peaks at 1360 cm⁻¹ and 1510 cm⁻¹ are assigned to *COOH, which represents a key intermediate in the transformation of CO₂ to CO (Fig. 5a).²⁴

To delve into the photocatalytic mechanism, the electron transfer process during the CO₂RR was first investigated. Various quantities of TEOA or catalyst were introduced into an acetonitrile solution containing Ru(bpy)₃Cl₂. The quenching mechanism was elucidated by monitoring changes in the emission spectrum of Ru(bpy)₃Cl₂. The CH₃CN solution of [Ru(bpy)₃]Cl₂ displays an emission band centered at 623 nm at an excitation wavelength of 382 nm. Upon increasing the amount of TEOA, the fluorescence spectrum of Ru(bpy)₃Cl₂ remains practically unchanged (Fig. 5b). Conversely, the Ru(bpy)₃Cl₂ spectrum exhibits a gradual quenching effect when the quantity of the catalyst increases (Fig. 5c). These findings confirm that the excited electrons from Ru(bpy)₃Cl₂ are transferred directly to the catalyst, rather than to the sacrificial reagent TEOA, indicating an oxidative quenching mechanism between the catalyst and Ru(bpy)₃Cl₂.³¹

Based on the aforementioned experimental data, a plausible catalytic mechanism has been formulated. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of Ru(bpy)₃Cl₂ are estimated to be 1.24 V and −1.25 V, respectively.³² It is noteworthy that the conduction band (CB) potential of Schiff-base ZrOC-1 is lower than the LUMO energy level of Ru(bpy)₃Cl₂, suggesting that the excited electrons from Ru(bpy)₃Cl₂ can be effectively transferred to the catalyst to stimulate the CO₂RR (Fig. S19†).

In detail, the photocatalytic mechanism proceeds as follows: upon light irradiation, Ru(bpy)₃Cl₂ gets excited to form Ru(bpy)₃²⁺. Subsequently, electrons are transferred from Ru(bpy)₃²⁺ to the catalyst, leading to the reduction of CO₂via the intermediacy of *COOH, eventually forming CO. Concurrently, Ru(bpy)₃²⁺ accepts electrons from the sacrificial agent TEOA, transforming into Ru(bpy)₃³⁺. The TEOA donates electrons to regenerate Ru(bpy)₃²⁺ (Fig. 5d and Fig. S20†).³¹

Conclusions

In this work, a method for the step-by-step precise synthesis of metal–organic cages via a constructing dynamic covalent bonds strategy was developed, and a novel lantern-like metal–organic cage (Schiff-base ZrOC-1) was synthesized using this method specifically for photocatalytic CO₂ reduction. Compared to Zr-CHO and ZrT-1 with almost no catalytic activity, Schiff-base ZrOC-1 exhibits commendable photocatalytic performance, efficiently reducing CO₂ to CO with a yield of 2.55 mmol g⁻¹ h⁻¹, wherein the dynamic covalent bonds act as the catalytic centers. Photoelectrochemical analysis shows that Schiff-base ZrOC-1 exhibits a faster photocurrent response and higher charge separation efficiency, indicating that the MOC structure positively influences its catalytic performance. This research offers new insights into the rational design of targeted MOCs for CO₂ photoreduction.

Author contributions

J. T. H., M. D. and C. Y. S. conceived the ideas. J. T. H., M. D. and Y. Z. designed the experiments and wrote the paper. X. H. Y. and F. F. M. revised the paper. D. X. C., W. L. and S. Y. were responsible for crystal analysis. Z. M. S. supervised the project and analyzed the data.

Data availability

Crystallographic data for Zr-CHO and Schiff-base ZrOC-1 has been deposited at the CCDC under 2270600 and 2270602.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the NSFC of China (No. 22371033 and 22101067), Jilin Provincial Department of Science and Technology (No. 20230508108RC, 20230508183RC and 20210101131JC), the Fundamental Research Funds for the Central Universities Excellent Youth Team Program (2412023YQ001) and the Postdoctoral Fellowship Program of CPSF (GZC20230939).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2270600 and 2270602. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01043d

‡ These authors contributed equally to the experiments and writing of the article.

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