A Pd₄L₂ cage containing Brønsted-base active sites for the one-pot photooxidation/Knoevenagel condensation reaction

Abstract

Brønsted-base active sites on a Pd₄L₂ cage facilitates enhanced catalytic efficiency, wide substrate scope and high turnover number (TON) for the one-pot photooxidation/Knoevenagel condensation reaction under mild conditions.

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Introduction

Both the microenvironment and the active sites are crucial for natural enzymes to realize highly efficient and selective catalytic transformations.^1,2 Developing equivalent artificial catalysts that can mimic enzymes’ structure and function has been a long-sought goal for supramolecular chemists.^3–8 Coordination-assembled metallocages with well-defined three-dimensional (3D) cavities are attractive platforms for supramolecular catalysis.^9–22 The introduction of active sites, such as chiral,^20,23,24 photoactive,^25–27 and Lewis acid–base sites,¹⁰ onto cage skeletons has been shown to be a promising strategy to endow the native host with catalytic functions. Moreover, integration of the active sites and the cage's confinement effect can also lead to enhanced selectivity toward conventional reactions.²⁸

One-pot sequential reactions that can achieve continuous multiple chemical transformations without tedious isolation and purification of reaction intermediates are attractive in organic synthesis.^29–34 Recently, we have reported a series of pyridinium-functionalized organo-palladium cages, which can be utilized for both photooxidation³⁵ and Knoevenagel condensation³⁶ reactions. Herein, we report two Pd₄L₂-type cages (1 and 2) built from the same cis-blocking (tmen)Pd(NO₃)₂ (tmen = tetramethylethylenediamine) metal corners (Pd), and two bis-TPT [TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine] ligands bridged by either a 1,3-dimethylene-pyridine (L¹) or a 1,3-dimethylene-benzene (L²) spacer (Scheme 1). Cage 1, decorated with Brønsted-base pyridine sites, shows enhanced catalytic efficiency toward the one-pot photooxidation/Knoevenagel condensation reaction, with a wide substrate scope, size-selectivity and high reaction turnover number (TON).

Scheme 1 Self-assembly of cages 1 (Pd₄L¹₂) and 2 (Pd₄L²₂), highlighted with cavity modification by introduction of Brønsted-base active sites.

Results and discussion

Ligand design and synthesis

With the aim of introducing Brønsted base functional sites into the cavity of the hosts, ligand L¹ with a 1,3-dimethylene pyridine spacer and ligand L² with 1,3-dimethylene benzene as a control were designed and prepared by similar procedures. Both ligands were synthesized by heating an excess amount of TPT with 1,3-bis(bromomethyl)benzene or 2,6-bis(bromomethyl)pyridine at 115 °C in DMF under a N₂ atmosphere, followed by counterion exchange of excess NaBF₄ (see the Experimental section for details).

Self-assembly of Pd₄L₂-type cages 1 and 2

When L¹ (8.89 µmol) was treated with (tmen)Pd(NO₃)₂ (16 µmol) in 4 mL of D₂O with vigorous stirring at 50 °C for 15 min, the suspension of the ligand turned into a clear light-orange solution. ¹H NMR spectra confirmed the quantitative formation of cage 1 with all the proton signals fully assigned based on a ¹H–¹H COSY experiment (Fig. S5–S10†). Compared to that on free L¹, the downfield-shifting of the pyridyl doublets (9.35 and 9.28 ppm for H_a, 8.76 ppm for H_b) indicated the successful coordination to Pd(II) (Fig. 1A and B). The appearance of only one set of 11 peaks in the ¹H NMR spectrum indicated the high symmetry of cage 1. The diffusion-ordered spectroscopy (DOSY) NMR spectrum also suggested the formation of a single species with a diffusion coefficient of 3.20 × 10⁻¹⁰ m² s⁻¹, corresponding to a diameter of 1.53 nm (Fig. S10†). After counter-ion exchange with NH₄PF₆, ESI-TOF-MS analyses clearly confirmed the formation of the Pd₄L₂-type cages, where a series of peaks with charge states varying from 3+ to 6+ were observed due to the progressive loss of PF₆⁻ counterions. For example, peaks observed at m/z = 536.5657, 672.8510, 877.3541, and 1217.7918 could be assigned to [Pd₄L¹₂(PF₆)₆]⁶⁺, [Pd₄L¹₂(PF₆)₇]⁵⁺, [Pd₄L¹₂(PF₆)₈]⁴⁺, and [Pd₄L¹₂(PF₆)₉]³⁺ charged species, respectively. Cage 2 was also prepared quantitatively by a similar procedure as above by replacing L¹ with L² (Fig. 1C and D).

Fig. 1 (A) ¹H NMR spectra (400 MHz, 298 K) of L¹ in DMSO-d₆, (B) cage 1 in D₂O, (C) L² in DMSO-d₆ and (D) cage 2 in D₂O. ESI-TOF-MS spectra for Pd₄L₂ cages (E) 1 and (F) 2 with the inset showing the observed and simulated isotopic patterns of the 4+ peaks.

Structures of both L¹ and cage 2 have been confirmed by single-crystal X-ray diffraction studies. In the solid state, L¹ adopts a clip conformation with clear intramolecular π–π interaction (3.33 Å) between two TPT panels (Fig. 2A). In the crystal structure of cage 2, two cationic pyridinium ligands are connected by four (tmen)Pd capping units, which is consistent with the Pd₄L₂ crystal structure that we have reported.³⁷ Four Pd(II) ions sit in the middle with the diagonal Pd–Pd distances being 19.2389(6) Å and 17.3411(6) Å (Fig. 2B). The cavity volume of cage 2 was calculated to be 834 ų using the 3 V Web server,³⁸ which is slightly larger than the cavity of our previous Pd₄L₂ cage (Fig. S20†).³⁷

Fig. 2 X-ray structures of ligand L¹ (A) and cage 2 (B) (counterions and solvent molecules are omitted for clarity).

Cyclic voltammetry experiments (CVs) for L¹ and cage 1 were performed to investigate their redox properties (Fig. 3). Three reversible reduction waves for L¹ were observed at around −0.402, −0.526 and −0.975 V vs. Ag/AgCl. The first two reduction peaks are attributed to the reduction processes on pyridinium units, to form a neutral state. The last one is assigned to two-electron reduction processes on the triazine unit. Cage 1 showed four reversible redox waves at around −0.424, −0.556, −1.022 and −1.267 V. Compared with L¹, the reduction potential of cage 1 appeared at more negative positions, indicating that the cage skeleton maintains the redox properties of pyridine derivatives but becomes more difficult to be reduced due to coordination with Pd(II). Moreover, the electronic communication between ligands takes place through coordination bonds.

Fig. 3 Cyclic voltammograms of (A) ligand L¹ (1.0 mM) and (B) cage 1 (0.5 mM) in DMSO at a 0.1 V s⁻¹ scan rate. Bu₄NPF₆ (0.1 M) was used as a supporting electrolyte.

Photocatalytic performance of cage 1 toward a one-pot sequential reaction

The redox-active nature of cage 1 and the installation of Brønsted base active sites motivated us to further study its potential in the one-pot photooxidation/Knoevenagel condensation reaction (Fig. 4). A suspension of 4-bromophenyl methanol (3a, 10 equiv.) was added to a D₂O solution of 1 at room temperature. After the addition of an excess amount of malononitrile (ca. 20 equiv.) as the condensation reactant, the resulting solution was irradiated under purple LEDs at room temperature for 5 h. After reaction, the signals of 3a disappeared completely and a large amount of white solid precipitated. After CH₂Cl₂ extraction, ¹H NMR showed that the substrate was fully consumed and a new set of signals attributable to benzylidene malononitrile (4a) appeared (99% yield and 100% chemoselectivity) (Table 1, entry 1, Fig. S26†). Control experiments (Table 1, entries 3, 6 and Fig. S42, S43†) showed that the photooxidation/Knoevenagel condensation of 3a did not proceed without air or under darkness. Under the same reaction conditions, cage components, L¹ and (tmen)Pd(NO₃)₂, also did not catalyze the reaction, while cage 2, without the pyridine active sites, gave a much lower yield (66%) (Table 1, entries 4, 5, 7 and Fig. S40, S41, S47†).

Fig. 4 One-pot photooxidation/Knoevenagel condensation of 3a with malononitrile (5) catalyzed by cage 1. ¹H NMR spectra (400 MHz, D₂O, 298 K) of (A) free cage 1, (B) after addition of 3a and 5 to cage 1, (C) after sequential reaction, (D) crude product 4a obtained by extraction and dissolved in CDCl₃ with 1,3,5-trimethoxybenzene as an internal standard (labelled by ■). The signals of cage 1 were labelled with ●.

Table 1 One-pot sequential photooxidation-Knoevenagel condensation reactions^a

Entry	Substrates 3	Time	Catalyst	Yield^b (%)
a Standard reaction conditions: benzyl alcohols (0.01 mmol, 10 equiv.) and malononitrile (0.020 mmol, 20 equiv.) were added to the solution of catalysts (0.001 mmol, 1 equiv.) in 1 mL of D2O and then irradiated under purple LEDs (λ > 395 nm, 6 W) with magnetic stirring in air for 5 h at r.t. Then the reaction mixture was extracted with CH2Cl2 without further purification. b Yields were obtained from ^1H NMR integration based on 1,3,5-trimethoxybenzene as the internal standard (0.003 mmol L^−1) (Fig. S25–S37†). c Ph4BNa (0.002 mmol, 2 equiv.) was added as an inhibitor. d Under a N2 atmosphere. e Replacing cage 1 with L^1·BF4. f Replacing cage 1 with (tmen)Pd(NO3)2. g Under darkness.
1		5 h	1	99
2^c		5 h	1	27
3^d		5 h	1	40
4^e		5 h	L^1·2BF4	14
5^f		5 h	(tmen)Pd(NO3)2	3
6^g		5 h	1	n.d.
7		5 h	2	66
8		10 h	1	61
9		7 h	1	65
10		5 h	1	96
11		7 h	1	77
12		7 h	1	97
13		4 h	1	99
14		12 h	1	76
15		7 h	1	95
16		5 h	1	99
17		24 h	1	n.d.

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We also monitored the two catalytic reaction processes under other conditions (15 equiv. of 3a). The results (Fig. S48†) showed that the yield of 4a catalyzed by cage 1 rapidly reached 71% at 2 h and finally became 97% after 5 h, while the yield catalyzed by cage 2 was only 22% at 2 h and reached 86% after 10 h in comparison. The reaction rates were found to be nearly linearly dependent on the substrate concentration in our system, as the reaction at a concentration of 30 mM of 3a was ca. 1.9 times faster than that at a concentration of 15 mM (Fig. S50†).

A wide substrate scope was noticed for this reaction, as a series of electron-rich and electron-deficient benzyl alcohols (Table 1 and Fig. S27–S37†) could be efficiently converted to the corresponding α,β-unsaturated dinitriles in moderate to good yields (Table 1, entries 8–14), where electron-rich alcohols often show higher yields than electron-deficient ones. While two naphthalenyl alcohols could be used for this reaction, no reaction of the bulky pyren-1-ylmethanol (3k) was observed (Table 1, entry 17). The ¹H NMR spectrum showed that 3k cannot be encapsulated by cage 1 due to its bulky size (Fig. S37†). Meanwhile, in inhibition experiments with sodium tetraphenylborate (Ph₄B⁻, a strong binding guest of cage 1), the formation of 4a was significantly inhibited, resulting in a much lower yield (27%) (Fig. S38 and S39†). This indicated that hydrophobic encapsulation of the substrates is essential for the reaction. The turnover number (TON) of cage 1 was determined by lowering the loading of cage 1 to 0.67 mol% (150 equiv. of 3a in total). A TON of 134 in 40 h was achieved, offering 89% isolated yield of 4a (Fig. S44 and Table S1†). Clean and unchanged ¹H NMR signals after the reaction suggest the structural robustness of cage 1. A slight decrease of the cage's concentration was noticed, which we infer was due to co-precipitation of the cage with the insoluble products (Fig. S45†).

To elucidate the reactive oxygen species involved during the reaction, 10 equiv. of t-butyl alcohol (˙OH capturing agent), NaN₃ (¹O₂ quencher) or benzoquinone (O₂˙⁻ capturing agent) was added to the reaction mixture. While t-butyl showed a negligible effect on the reaction (Fig. S61†), the presence of NaN₃ and benzoquinone significantly inhibited the photooxidation–Knoevenagel condensation of 3a, with only 8% and 18% yields detected, respectively (Fig. S62 and S63†). These results indicate that ¹O₂ and O₂˙⁻ are the oxidizing species in the photooxidation reaction, and O₂˙⁻ should be dominant in the electron transfer process.

Mechanistic discussion

We rationalize the mechanism for the enhanced one-pot photooxidation/Knoevenagel condensation reaction by cage 1 based on the following facts:

(i) The substrate was encapsulated by cage 1 through a hydrophobic effect and π–π stacking interactions. The ¹H NMR titration spectra of 1 with representative 3a and 1-naphthalenemethanol (3i) showed that the guest signals were clearly downfield-shifted, and cage 1 exhibits a fast-exchange binding mode for 3i with the apparent binding constant determined to be 672 M⁻¹ (Fig. S55 and S56†) by Hill function fitting. The apparent binding constant for the reaction intermediate 4-bromobenzaldehyde was determined to be 223 M⁻¹, which is lower than K_a (273 M⁻¹) for 3a (Fig. S51–S54†). However, cage 1 showed a stronger binding ability towards 1-naphthaldehyde, with K_a = 881 M⁻¹ (Fig. S57 and S58†).

(ii) Upon light irradiation, the electron-deficient pyridinium ligands on cage 1 can receive an electron from the reductive alcohol to form the free radical state. The UV-vis absorption spectra of cage 1 after addition of 3a showed a new charge-transfer band tailing in the range of 300–400 nm, which was enhanced after photoirradiation (Fig. S21 and S22†). Electron spin resonance spectroscopy (ESR) confirmed the generation of the radicals, with the emergence of ESR signals (g = 2.0065) after irradiation of the solution containing cage 1 and 3i (Fig. S24†). The excited state of the host–guest complexes leads to the generation of reactive oxygen species (¹O₂ and O₂˙⁻) by energy transfer or electron transfer, which facilitate the photocatalytic oxidation of benzyl alcohols. It is worth noting that the photo-oxidation rate of 3a is faster with cage 1 as the catalyst compared to that of cage 2, under the same reaction conditions (Fig. 5A and B).

Fig. 5 Yields and the corresponding first-order kinetic plots for the cage (1 and 2) catalysed photooxidation reaction of 3a (A and B), the Knoevenagel condensation of 4-bromobenzaldehyde 6 with malononitrile (C and D) and the deuteration of malononitrile 5 (E and F) in D₂O.

(iii) Malononitrile became deprotonated to give the carbanion nucleophile due to the cationic nature of the cages and the existence of Brønsted-base sites. The H/D exchange reaction of malononitrile catalyzed by cage 1 in D₂O was completed in 100% yield over a period of 5 h. A much faster deuteration rate is observed for cage 1 (k₁ = 0.48 h⁻¹) in comparison with cage 2 (k₁ = 0.15 h⁻¹), verifying the effective C–H activation of malononitrile by the Brønsted-base centers (Fig. 5E and F).

(iv) The carbanion immediately attacks the aromatic aldehyde substrate to form the β-hydroxy intermediate, which is subsequently rearranged and dehydrated to the final product. Kinetic studies on a model Knoevenagel condensation between 4-bromobenzaldehyde (6) and malononitrile shows that this reaction catalyzed by cage 1 is ca. 6 times faster than that catalyzed by cage 2 (Fig. 5C and D).

(v) NMR titrations by adding 4a into cage 1 confirmed that the condensation product cannot be encapsulated by cage 1 (Fig. S60†). This is consistent with the fact that the product settled down during the reaction, which facilitated the turnover of the catalytic cycle.

Conclusions

In conclusion, we have constructed a new Brønsted-base functionalized molecular cage that exhibits excellent catalytic performance in the one-pot sequential photooxidation/Knoevenagel condensation reaction, featuring high reactivity, high TON and broad substrate scope. Our results demonstrate that new catalytic functions can be activated by introducing active sites that cooperate with the confinement cavities of the host, offering new insight into the construction of functionalized nanoreactors.

Experimental section

General

Unless otherwise stated, all chemicals and solvents were purchased from commercial companies and used without further purification. Deuterated solvents were purchased from Admas, Sigma-Aldrich and J&K Scientific. 1D and 2D NMR spectra were measured on a Bruker Biospin Avance III (400 MHz) or a JEOL ECZ600S (600 MHz) spectrometer. The NMR chemical shifts (δ) are reported in ppm (parts per million) with respect to the solvent signal that originated due to incomplete deuteration (δ = 4.79 for D₂O, δ = 2.50 for DMSO-d₆, δ = 7.26 for CDCl₃ in ¹H NMR). UV-Vis absorption spectra were recorded on a UV-2700 UV-Visible spectrophotometer from SHIMADZU Corporation. ESI-TOF-MS spectra were recorded on an Impact II UHR-TOF mass spectrometer from Bruker or LC-QTOF-MS (G6520B). Data analyses were conducted using the Bruker Data Analysis software (Version 4.3) and simulations were performed using the Bruker Isotope Pattern software or Thermo Xcalibur Qual Browser software (Thermo Foundation 2.0). Electrochemical analysis was performed on a CHI 604E analyzer. All cyclic voltammetry (CV) experiments were carried out in argon-purged solutions of DMSO at room temperature. The purple LED light sources used for all the photoreactions in this research were from Wuhan Geo Instruments Science & Technology Co., Ltd (China) (λ > 395 nm; 1 W for every lamp bead; every Schlenk tube was irradiated using 6 lamp beads from the side).

Synthesis of ligands L¹ and L²

2,4,6-Tris(4-pyridyl)-1,3,5-triazine (420 mg, 1.346 mmol) was added to dry dimethylformamide (120 mL) under a nitrogen atmosphere and the suspension was stirred at 120 °C until it became clear. The solution was treated drop-wise with 2,6-bis(bromomethyl)pyridine (143 mg, 0.540 mmol) dissolved in a small amount of DMF and then maintained at 115 °C for 5 h, affording a reddish-brown clear solution. The solution was cooled to room temperature and placed in a refrigerator below 0 °C overnight, affording a reddish-brown turbid liquid. The turbid liquid was filtered to remove excess TPT, giving a reddish-brown clear solution. The solvent was removed by reduced pressure distillation to give a yellowish-green precipitate. Then, the precipitate was collected by filtration and the filter cake was dissolved in hot water to remove excess TPT, followed by the addition of an excess of sodium tetrafluoroborate (NaBF₄). A purple solid of pure L¹ (BF₄)₂ (M = 903.41 g mol⁻¹, 228 mg, yield 46.7%) was collected by filtration. ¹H NMR (600 MHz, DMSO-d₆) δ 9.12 (d, J = 6.8 Hz, 4H), 9.10 (d, J = 7.0 Hz, 4H), 8.69 (d, J = 6.0 Hz, 8H), 8.25 (d, J = 6.0 Hz, 8H), 8.18 (t, J = 7.8 Hz, 1H), 7.76 (d, J = 7.8 Hz, 2H), 6.20 (s, 4H). ¹³C NMR (101 MHz, DMSO) δ 171.3 (s), 168.0 (s), 152.9 (s), 151.2 (s), 149.5 (s), 148.0 (s), 140.9 (s), 126.5 (s), 122.5 (s), 122.2 (s), 65.4 (s), 63.6 (s). ESI-TOF-MS: m/z Calcd for [L(BF₄)]¹⁺ 816.2856 found 816.2843.

L² was synthesized by a similar procedure, starting from 1,3-bis(bromomethyl)benzene (102 mg, 0.385 mmol). Pure L² (BF₄)₂ was obtained in a yield of 10.4% (M = 902.42 g mol⁻¹, 36 mg). ¹H NMR (600 MHz, DMSO-d₆) δ 9.45 (d, J = 6.2 Hz, 4H), 9.39 (d, J = 6.2 Hz, 4H), 8.97 (d, J = 6.0 Hz, 8H), 8.67 (d, J = 6.0 Hz, 8H), 7.63–7.61 (m, 4H), 6.07 (s, 4H). ¹³C NMR (101 MHz, DMSO-d₆) δ 171.7 (s), 165.7 (s), 151.7 (s), 147.0 (s), 142.6 (s), 142.1 (s), 134.8 (s), 130.1 (s), 127.7 (s), 122.8 (s), 75.8 (s), 66.8 (s), 63.7 (s). ESI-TOF-MS: m/z Calcd for [L(BF₄)]¹⁺ 815.2904 found 815.2895.

Synthesis of cage 1 (Pd₄L¹₂)

Ligand L¹ (BF₄ salt, 8.03 mg, 8.89 µmol) was treated with (tmen)Pd(NO₃)₂ (5.54 mg, 16.00 µmol) in D₂O (4 mL) at 50 °C for 15 min. The suspension progressively dissolved to generate a homogeneous light-orange solution. ¹H NMR confirmed the quantitative formation of Pd₄L¹₂·(BF₄)₄(NO₃)₈ (cage 1, M = 3193 g mol⁻¹).¹H NMR (400 MHz, D₂O) δ 9.35 (d, J = 5.8 Hz, 8H), 9.28 (d, J = 5.8 Hz, 8H), 9.09 (d, J = 6.6 Hz, 8H), 9.04 (d, J = 6.4 Hz, 8H), 8.76 (t, J = 6.6 Hz, 16H), 8.24 (t, J = 7.8 Hz, 2H), 7.95 (d, J = 7.8 Hz, 4H), 6.10 (s, 8H), 3.16 (s, 16H), 2.74 (d, J = 7.6 Hz, 48H). ¹³C NMR (101 MHz, D₂O) δ 169.6 (s), 168.7 (s), 151.9 (s), 151.2 (s), 150.2 (s), 145.9 (s), 145.3 (s), 126.7 (s), 126.6 (s), 126.0 (s), 64.8 (s), 62.6 (s), 50.3 (s), 50.1 (d).

Pd₄L¹₂·(PF₆)₁₂ was prepared by treating Pd₄L¹₂·(BF₄)₄(NO₃)₈ with excess ammonium hexafluorophosphate in an aqueous medium for electrospray ionization mass spectra (ESI-MS) measurement. ESI-TOF-MS: m/z Calcd for [Pd₄L¹₂·(PF₆)₈]⁴⁺ 877.3555 found 877.3541.

Synthesis of cage 2 (Pd₄L²₂)

Ligand L² (BF₄ salt, 8.02 mg, 8.89 µmol) was treated with (tmen)Pd(NO₃)₂ (5.54 mg, 16.00 µmol) in D₂O (4 mL) at 70 °C for 30 min. The suspension progressively dissolved to generate a homogeneous light-yellow solution.¹H NMR confirmed the quantitative formation of Pd₄L²₂·(BF₄)₄(NO₃)₈ (cage 2, M = 3191 g mol⁻¹). ¹H NMR (600 MHz, D₂O) δ 9.33 (d, J = 6.0 Hz, 8H), 9.26 (d, J = 6.0 Hz, 8H), 9.13 (d, J = 6.8 Hz, 8H), 9.02 (d, J = 6.8 Hz, 8H), 8.76 (d, J = 7.2 Hz, 8H), 8.74 (d, J = 6.0 Hz, 8H), 7.96 (d, J = 9.6 Hz, 2H), 7.85–7.80 (m, 6H), 5.93 (s, 8H), 3.14 (s, 16H), 2.70 (d, J = 15.2 Hz, 48H). ¹³C NMR (101 MHz, D₂O) δ 169.5 (s), 168.6 (s), 151.9 (s), 149.9 (s), 145.3 (s), 133.0 (s), 132.8 (s), 126.9 (s), 126.0 (s), 64.3 (s), 62.8 (s), 62.6 (s), 50.3 (s), 50.1 (s).

Pd₄L²₂·(PF₆)₄ was prepared by treating Pd₄L²₂·(BF₄)₄(NO₃)₈ with excess ammonium hexafluorophosphate in an aqueous medium for electrospray ionization mass spectra (ESI-MS) measurement. ESI-TOF-MS: m/z Calcd for [Pd₄L²₂·(PF₆)₈]⁴⁺ 876.8579 found 876.8575.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grants 2021YFA1500400 and 2022YFA1503300), the National Natural Science Foundation of China (Grants 22171264, 22171262 and 21825107), and the Natural Science Foundation of Fujian Province (Grants 2021J01516 and 2021J02016).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2246529 and 2246530. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt00815k

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