A ketimine–ketoenamine-linked porous organic polymer: synthesis, characterization and applications

Abstract

A novel porous organic polymer (BL-TP-POP) containing β-ketoenamine and ketimine moieties was prepared through Schiff-base condensation, resulting in a framework with high stability and a large surface area. BL-TP-POP emitted fluorescence in the solid state, with a quantum efficiency of 3.56%. The material displayed a high carbon dioxide uptake capacity (up to 121 mg g⁻¹) at 273 K and 1 bar, as well as a high CO₂/N₂ selectivity (53.5). BL-TP-POP also proved effective as a heterogeneous catalyst for Knoevenagel condensation, achieving a reaction yield of 98.6%.

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Introduction

The development of new porous materials has attracted considerable attention in recent decades. Porous organic polymers (POPs), which are composed of light elements (typically C, H, O, N, B), have been extensively explored for applications, such as gas storage and separation,¹ heterogeneous catalysis² and sensing.³ There are several sub-classes of porous organic polymers, including covalent organic frameworks (COFs),⁴ polymers of intrinsic microporosity (PIMs),⁵ hypercross-linked polymers (HCPs),⁶ conjugated microporous polymers (CMPs)⁷ and porous aromatic frameworks (PAFs).⁸ Among these sub-classes, COFs have been extensively studied because of their structural diversity, physicochemical stability and, in particular, their uniform and crystalline microporous architectures. In addition to crystalline COFs, recent years have seen a growing interest in POPs with amorphous structures, such as PIMs, HCPs, CMPs and PAFs, because of their exceptional properties. Compared to other materials, POPs have obvious merits, such as low density, tunable functionalization and high stability.

Climate change has created an urgent need to separate and capture CO₂ from emission sources, such as fuel gases. Several methods, including cryogenic distillation, membranes and adsorbents, have been developed to remove CO₂.⁹ Solid adsorbents such as zeolites,¹⁰ metal oxides,¹¹ activated carbons,¹² metal–organic frameworks (MOFs),¹³ and POPs¹⁴ have been studied for CO₂ capture applications.¹⁵ Currently, the use of POPs with high surface area and high stability plays an important role in the efficient capture and storage of CO₂. EI-Kaderi and coworkers¹⁶ reported a triptycene-derived benzimidazole-linked POP (BILP-3) with a specific surface area of 1306 cm² g⁻¹, and a CO₂ uptake capacity of 225 mg g⁻¹ at 273 K and 1 bar. Yavuz and coworkers¹⁷ successfully developed an ethylenediamine-grafted porous organic polymer (COF-190L-en) with a CO₂ uptake capacity of 99.44 mg g⁻¹ at 298 K. COF-190L-en exhibited a high CO₂/N₂ selectivity of 729 (298 K), and its isosteric heat (Q_st) was as high as 97 kJ mol⁻¹ at zero coverage. It has been known that the presence of polar groups can enhance CO₂ capture efficiency through interactions between polar groups and CO₂via dipole–quadrupole interactions.¹⁸ For example, it has been reported that CO₂-philicity can be enhanced by incorporating N-rich functional groups (such as amine,¹⁹ imine²⁰ and azo²¹) into the backbone of POPs. Another strategy to increase CO₂ uptake capacity involves introducing phenolic moieties into the polymer skeleton. Bhaumik and coworkers²² reported a porous organic polymer (TrzPOP-3) functionalized with phenolic –OH groups in which the CO₂ uptake at 273 K and 1 bar pressure reached up to 375.76 mg g⁻¹. Esteves and coworkers²³ synthesized a series of COFs with different numbers of phenolic groups to study their CO₂ uptake behaviours, which showed, experimentally and theoretically, that the relative number of phenolic groups acted as a threshold for CO₂ uptake capacities of these COFs due to the interaction between CO₂ molecules and the lone electron pair of oxygen atoms from hydroxyl groups. Therefore, the introduction of N-rich functional groups and phenolic groups can be an effective method for the rational design and development of potential CO₂ adsorbents.

Porous organic polymers are also attractive candidates for catalysis due to their excellent properties.²⁴ POPs function as solid catalysts or catalyst supports for several organic transformations, such as Knoevenagel reaction,²⁵ aldol condensation,²⁶ oxidation,²⁷ hydrogenation,²⁸ transesterification reaction,²⁹ Friedel–Crafts alkylation,³⁰ and epoxide ring-opening reaction.³¹ Knoevenagel reaction, the condensation between aldehydes or ketones and active methylene compounds, is an important reaction in organic chemistry that synthesizes heterocycles, intermediates and fine chemicals by forming new C–C bonds. Homogeneous catalysts for Knoevenagel condensation are highly active but difficult to separate. POPs have shown promising applications as heterogeneous catalysts for the Knoevenagel reaction because of their high stability, high surface area and structural tunability.

In this paper, we successfully synthesized a novel POP (BL-TP-POP) using 3,6-bis((E)-((4-aminophenyl)imino)(phenyl)methyl)-benzene-1,2-diol (BL) and 2,4,6-triformylphloroglucinol (TP) as building blocks (Scheme 1). BL-TP-POP was synthesized through Schiff-base condensation, followed by irreversible enol-to-keto tautomerization to form a β-ketoenamine structure. The building block BL was synthesized through ketimine condensation between 3,6-dibenzoylcatechol and p-phenylenediamine, which introduced phenolic and ketimine groups into the polymer. Therefore, both ketimine and ketoenamine moieties embedded in the backbone of BL-TP-POP. BL-TP-POP was characterized by ¹³C CP-MAS NMR, FT-IR, DRS, BET, SEM and HR-TEM. The material exhibited a high BET surface area of 611 m² g⁻¹, and the CO₂ adsorption reached 121 mg g⁻¹.

Scheme 1 Synthesis of BL-TP-POP.

Results and discussion

Characterization of BL-TP-POP

Fourier transform infrared spectroscopy (FT-IR) and ¹³C cross-polarization with magic-angle spinning (CP-MAS) NMR measurements were employed to verify the formation of imine linkages. As depicted in Fig. 1a, the FT-IR spectrum of BL-TP-POP showed the absence of the characteristic primary amine peaks at 3476 and 3356 cm⁻¹ and the H–C=O stretch band (2870 cm⁻¹), suggesting the occurrence of the condensation reaction. Peaks assigned to carbonyl and imine groups appeared at 1586 and 1613 cm⁻¹ for BL-TP-POP, respectively, indicating the presence of both keto-enamine and the ketoimine moieties in the structure of the polymer. The presence of a stretching vibration peak of the C=C at 1518 cm⁻¹ further proved the formation of the β-ketoenamine structure. The ¹³C solid-state CP-MAS NMR (Fig. 1b) spectrum showed a resonance at 181 ppm for the carbon of the C=N bond, and a shoulder peak at 184 ppm corresponding to the carbon signal of the carbonyl carbon in the keto-enamine moiety, further demonstrating the presence of both keto-enamine and ketoimine groups in the backbone of the BL-TP-POP.

Fig. 1 (a) FTIR spectra of BL, TP and BL-TP-POP. (b) ¹³C solid-state nuclear magnetic resonance spectrum of BL-TP-POP.

The material was amorphous as indicated by the powder X-ray diffraction profile of BL-TP-POP (Fig. S6, ESI†) under this synthesis condition (Table S1, ESI†). The low crystallinity of the layer structure was believed to be caused by the nonplanar twisted geometry of building block BL (Fig. S3, ESI†) and sp³ nitrogen node in the keto-enamine moiety, which resulted in the flexibility and twisting of the layers. Field-emission scanning electron microscopy (FE-SEM) was performed to investigate the morphology of BL-TP-POP. As shown in Fig. 2a, SEM revealed that BL-TP-POP was mainly composed of spherical particles of different sizes from about 0.2 μm to 2 μm in diameter. Thermogravimetric analysis (TGA) indicated that BL-TP-POP possessed high thermal stability with minimal weight loss of up to 400 °C (Fig. 2b). Chemical stability was checked by immersing BL-TP-POP samples in THF, CH₂Cl₂, hexane, water, aqueous HCl (3 M) and NaOH (3 M) at room temperature. After 7 days, the FT-IR spectra of these immersed samples (except for the sample in NaOH) showed little change, indicating the unchanged composition. The stability test in NaOH showed retention of FT-IR peak positions after treatment for 1 day and became unstable afterwards (Fig. S20, ESI†). The exceptional thermal and chemical stability was attributed to the irreversible β-ketoenamine structure and stable ketoimine linkage.³² In addition, introducing –OH functionalities adjacent to the Schiff-base –C=N– centres further enhanced the stability of the structure due to the formation of intramolecular O–H⋯N=C hydrogen bonding, which helped to protect the basic imine nitrogen from hydrolysis in the presence of water and acid.³³

Fig. 2 (a) SEM image of the BL-TP-POP bulk material. (b) TGA curve of BL-TP-POP. (c) Nitrogen sorption isotherm curve measured at 77 K. The insert shows the pore-size distribution for BL-TP-POP.

The permanent porous structure of BL-TP-POP was investigated using its nitrogen sorption isotherm at 77 K. Nitrogen adsorption/desorption measurement revealed that the isotherm of the material fit the typical type I sorption model (Fig. 2c) and gave steep nitrogen uptake in the low-pressure range (P/P₀ = 0–0.01), which suggested that the POP showed microporosity. The Brunauer–Emmett–Teller (BET) surface area and pore volume were calculated to be 611 m² g⁻¹ and 0.53 cm³ g⁻¹, respectively. The pore size calculated using the quenched solid density functional theory (QSDFT) method was 0.93 nm.

UV-vis diffuse reflectance spectroscopy (DRS) was conducted to determine the electronic properties of BL-TP-POP. BL-TP-POP showed broad absorptions in the range of 300–550 nm (Fig. 3a). The corresponding band gap (E_g) was determined to be 2.09 eV by the Tauc plot. The semiconductor nature of the material was further investigated by cyclic voltammetry (CV) in a three-electrode system. As shown in Fig. 3b, the onset of reduction and oxidation potentials were −0.81 and 1.32 V (vs. AgCl/Ag), respectively. Accordingly, the HOMO and LUMO energies of BL-TP-POP were calculated to be −5.68 eV and −3.55 eV, respectively, and the band gap was inferred to be 2.13 eV, which is consistent with the value from DRS.

Fig. 3 (a) UV-vis DRS spectrum of BL-TP-POP (insert showing Tauc plot for band gap calculation). (b) Cyclic voltammetry measurement of BL-TP-POP (counter electrode: Pt; reference electrode: AgCl/Ag; scan rate: 100 mV s⁻¹; 0.1 M NBu₄PF₆ in anhydrous MeCN).

The optical property of the material was analysed using a fluorescence spectrometer. Under 380 nm light excitation, the BL-TP-POP powder exhibited yellow fluorescence at 580 nm (Fig. S17, ESI†) with a quantum efficiency of 3.56%, and the average lifetime was measured to be 2.24 ns (Fig. S18, ESI†). The solid powder of BL-TP-POP exhibited visible fluorescence upon 365 nm irradiation with a UV lamp. The high emission of the material in a solid state can be attributed to its nonplanar and twisted molecular structure, which avoids the stacking interaction of π–π during aggregation.

CO₂ uptake

BL-TP-POP was used as the solid adsorbent for CO₂ capture by taking advantage of its rich polar groups, including –OH, –C=N–, –NH– and C=O. The CO₂ adsorption isotherms of BL-TP-POP were obtained at different temperatures (273 and 298 K) under 1.0 bar pressure. The results are shown in Fig. 4a. At temperatures of 273 and 298 K, the amount of CO₂ uptake steadily increased with the increasing pressure and reached maximum amounts of 121 mg g⁻¹ and 100 mg g⁻¹, respectively. The isosteric heat of the adsorption (Q_st) was calculated by applying the Clausius–Clapeyron equation to evaluate the strength of the interactions between BL-TP-POP and CO₂ (Fig. 4b). The Q_st value was calculated to be 34.5 kJ mol⁻¹ at zero coverage, which is comparable to some other materials reported in the literature, such as the triazine-based framework³⁴ and azine-linked COF,³⁵ indicating strong interactions between CO₂ molecules and adsorbent. Additionally, the material showed a gradual increase in Q_st as a function of the amount of adsorbed CO₂, which could be attributed to the cooperative interactions between adsorbed CO₂ molecules.³⁶

Fig. 4 (a) CO₂ and N₂ isotherms for BL-TP-POP at 273 K and 298 K. (b) Isosteric heat of adsorption of CO₂ for BL-TP-POP.

To test the recyclability of the adsorbent, recycling experiments were performed and showed that BL-TP-POP could be reused without loss of adsorption capacity of CO₂ after four cycles (Fig. S21, ESI†).

The binding sites of CO₂ on BL-TP-POP (CO₂@BL-TP-POP) were simulated with the Materials Studio Package. From the calculation with the sorption module, it was found that CO₂ molecules preferred to adsorb near ketonic oxygens on β-ketoenamine moieties, and attraction around phenolic hydroxyl groups was less favoured (Fig. S26, ESI†). In addition, we found that CO₂ molecules were reluctant to bind to nitrogen on the backbone. Therefore, only oxygen atoms were available for binding CO₂ molecules. As shown in Fig. 5a, from the simulation, when the number of CO₂ was more than the number of oxygen atoms in a unit cell, all oxygen atoms interacted with CO₂, and excess CO₂ molecules (shown by arrow) were distributed in the periphery of the frame, instead of around nitrogen atoms. To illuminate the selectivity of the binding sites, a Hirshfeld population analysis of the density functional electronic charge distribution of BL-TP-POP was performed with the DMol³ module to map the partial atomic charges. As depicted in Fig. 5b, the charges of ketonic oxygens (around −0.294) were more negative than those of oxygens on phenolic hydroxyl groups (−0.198). Higher electron density of ketonic oxygens facilitated the attraction of CO₂ molecule whose carbon atoms showed a partial positive charge. The absolute values of the partial atomic charges of the nitrogen atoms were smaller than those of the oxygens; in particular, the nitrogen atoms on β-ketoenamine moieties exhibited a charge of −0.042, whose absolute value was only slightly higher than that of carbon on the phenyl rings (around −0.030). Although the nitrogen on ketoimines showed a higher negative charge (−0.106), the hydrogen bond formed with nitrogen (O–H⋯N) and steric hindrance due to the hydrogen atoms of adjacent groups prevented CO₂ from accessing this binding site.

Fig. 5 (a) Simulation of binding sites of CO₂@BL-TP-POP. (The CO₂ molecules in dashed circles were attracted around oxygen atoms; CO₂ molecules are shown by arrows distributed in the periphery of the structure.) (b) The Hirshfeld charge distribution in BL-TP-POP (the values of oxygen, nitrogen and some carbon atoms are given. Hydrogens are omitted for clarity).

The selective uptakes of CO₂ and N₂ were studied based on the corresponding sorption isotherm curves (Fig. 4a). The selectivity of CO₂/N₂ was determined by Henry's law based on the initial slope calculation. The ideal adsorption selectivity of CO₂/N₂ was calculated to be 53.5 at 273 K and 41.4 at 298 K. The high selectivity could be attributed to abundant polar groups on the pore wall that were beneficial to bind CO₂ with the quadrupole moment. The decrease in selectivity with an increase in temperature in the Henry regime could be attributed to the attenuation of the interaction between the host and guest molecules at a higher temperature.

Catalysing Knoevenagel reaction

From the structure of BL-TP-POP, the new material contained a Schiff-base (–C=N–) linkage and a secondary amine (CH–NH) moiety in the framework. Because of rich Lewis basic active sites and nanopore structure, BL-TP-POP was further employed as a heterogeneous catalyst for Knoevenagel condensation, which is a classical base-catalysed reaction widely used in the synthesis of fine chemicals and pharmaceuticals.³⁷ The reaction between malononitrile and benzaldehyde in the presence of BL-TP-POP proceeded effectively to yield a product, benzylidenemalononitrile, in benzene, while the blank experiment without catalyst afforded a negligible amount of product (Table 1).

Table 1 Effect of catalyst on the Knoevenagel condensation reaction^a

Entry	Catalyst	Mass (mg)	Solvent	Time (h)	T (°C)	Yield^b (%)
a Reaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), benzene (2 ml). b The yield of the product was determined by GC–MS using benzylidenemalononitrile as the external standard. The selectivity of the condensed product was greater than 99% in all cases.
1	—	—	Benzene	8	80	<1
2	BL	25	Benzene	8	80	41.4
3	TP	25	Benzene	8	80	<1
4	BL-TP-POP	15	Benzene	8	80	7.5
5	BL-TP-POP	20	Benzene	8	80	82.5
6	BL-TP-POP	25	Benzene	8	80	98.6
7	BL-TP-POP	25	THF	8	66	84.4
8	BL-TP-POP	25	EtOH	8	78	76.7
9	BL-TP-POP	25	Benzene	6	80	78.9
10	BL-TP-POP	25	Benzene	8	60	43.6

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With the increase in the loading amount of BL-TP-POP, the productivity of benzylidenemalononitrile was enhanced. It was found that the productive rate could reach up to 98.6% when 25 mg of BL-TP-POP was used to catalyse 1 mmol of benzaldehyde and 1 mmol of malononitrile at 80 °C for 8 h in benzene. Condensations in different solvents, such as THF and EtOH, were investigated, and experimental results showed that yields of benzylidenemalononitrile in these solvents were lower than those in benzene, which could be attributed to the solvation effect of the active sites by the polar solvent.³⁸ The catalytic performances of the monomers were also evaluated. It was observed that TP exhibited poor catalytic activity and BL showed moderate yield, which further demonstrated that base groups played a key role in the catalysis. We noticed that the catalytic efficiency of BL was much lower than expected because there were more base groups in BL than in BL-TP-POP with the same weight. The low catalytic efficiency of BL could be attributed to its low solubility in benzene and the small surface area of the solid compound (Fig. S23, ESI†).

The investigation of the reusability of BL-TP-POP showed that the catalytic activity did not decrease considerably after repeating four times (Fig. 6a). The FT–IR spectra showed no significant difference between the prepared sample and the 4th cycle sample, indicating that BL-TP-POP was stable after 4 cycles (Fig. 6b).

Fig. 6 (a) Recyclability study of the catalytic activities of BL-TP-POP. (b) IR spectra of as-synthesized BL-TP-POP after 4 cycles.

Conclusions

A novel porous organic polymer (BL-TP-POP) was synthesized using Schiff-base condensations. The framework contained both ketimine and keto-enamine moieties in the backbone. The material showed high stability and a large BET surface area. The solid polymer emitted yellow fluorescence with a high quantum yield. At 273 K and 1 bar, the material gave a high CO₂ uptake capability of 121 mg g⁻¹ and the adsorption selectivity of CO₂/N₂ was as high as 53.5. The catalytic efficiency of the porous organic polymer was investigated for Knoevenagel condensation. The yield of Knoevenagel condensation catalysed by BL-TP-POP was up to 98.7%, and the catalyst maintained good catalytic performance after four cycles.

Experimental section

General considerations

Unless stated otherwise, all reagents were purchased from commercial sources and used without purification. Compounds including phloroglucinol, trifluoroacetic acid, catechol, benzoin, diboron trioxide, chromium trioxide, anhydrous 1,4-dioxane, mesitylene, o-dichlorobenzene, benzaldehyde, malononitrile, benzylidenemalononitrile, and p-phenylenediamine were purchased from InnoChem. Ethanol, n-butanol, acetone, tetrahydrofuran, dichloromethane, acetic acid, hexamethylenetetramine, sulfuric acid and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. 2,4,6-Triformylphloroglucinol (TP)³⁹ was synthesized according to a method in the literature. Benzaldehyde was distilled under reduced pressure, and p-phenylenediamine was sublimated before use.

X-ray powder diffraction (PXRD) patterns were recorded using a Rigaku Miniflex 600 diffractometer with Cu Kα radiation (λ = 1.54056 Å) over the 2θ range of 2.5–30° and using a SAXSess mc2 diffractometer with Cu Kα radiation (λ = 1.54056 Å) over the 2θ range of 1–5°. FT-IR spectra were measured using a PerkinElmer Frontier infrared spectrometer FTIR-650 ranging from 4000 to 400 cm⁻¹. Thermogravimetric analysis was performed using a Netzsch TG 209 F1 Libra thermogravimetric analyzer with a ramp rate of 10 K min⁻¹ under an atmosphere of nitrogen. A Shimadzu UV-2600 UV-vis spectrophotometer, equipped with integration sphere ISR-2200, was used to measure the UV-vis diffuse reflectance spectrum of solid powders at room temperature. SEM images were obtained using a JSM-6480LV at 5.0 kV. An FEI (Jeol FEG 2100F) high-resolution transmission electron microscope (HRTEM) equipped with a field emission source operating at 300 kV was used to record the TEM images. The nitrogen adsorption and desorption isotherms were measured at 77 K using an Autosorb-iQ (Quantachrome) surface area size analyzer. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. CO₂ adsorption measurements were performed at 273 and 298 K, and 1 bar. Before measurement, the samples were degassed in a vacuum at 120 °C for 6 h. ¹H NMR spectra were recorded using a Bruker Avance NEO 400 MHz NMR spectrometer, where chemical shifts (δ in ppm) were determined with a residual proton of solvent as standard. Solid state cross polarization magic angle spinning (CP/MAS) NMR was performed using an AVIII 500 MHz solid-state NMR spectrometer. Electrical measurement was performed using a CHI 900C electrochemical workstation produced by Chenhua Instrument Co., Ltd. The MALDI-TOF mass spectrum was obtained using the Shimadzu MALDI-8020. Fluorescence spectra were measured using a Hitachi F-7000 Photoluminescence Spectrometer. The fluorescence lifetime and quantum yield were measured using EI FLS980 Fluorescence Spectrometers produced by Edinburgh Instruments. GC-MS measurement was performed using Agilent 7890A-5975C GC/MS equipped with an HP-5MS column with helium as the carry gas. The melting point was measured using an RY-1G melting point instrument.

Synthesis of tetraphenyl-o-benzodifuran (1)

The synthesis followed a literature report⁴⁰ with modification. 20.00 g (94.3 mmol) of benzoin, 7.00 g (63.4 mmol) of catechol and 3.00 g of B₂O₃ (43.1 mmol) were mixed in a crucible and heated to around 280 °C until the solid melted to form bubbles and then continued for 10 min. After cooling to room temperature, the solid was smashed and ground. The obtained powder was dissolved with dichloromethane and then washed with water 3 times. The organic layer was separated, and the solvent was removed by rotovap. The solid obtained was washed with water and boiling ethanol. After filtration, 7.85 g of 1 as a white solid was obtained (17.0 mmol, 36.1%).

Data for 1: ¹H NMR (400 MHz, DMSO-d₆): δ 7.66 (br, 4H), 7.55 (br, 10H), 7.43 (br, 6H), 7.33 (br, 2H).

Synthesis of 2,3-bis(benzoyloxy)-1,4-dibenzoylbenzene (2)

The synthesis followed a literature report⁴⁰ with modification. 1.50 g (3.25 mmol) of compound 1 was added to 15 ml of boiling glacial acetic acid. 1.00 g (10.0 mmol) of CrO₃ was added in portions to the mixture for 1 h. After the addition, the reaction continued refluxing for 15 minutes. The crude mixture was then cooled to room temperature and kept overnight. After filtration, the solid was recrystallized from ethanol to give 0.98 g of 2 as a white solid (1.86 mmol, 57.3%).

Data for 2: ¹H NMR (400 MHz, DMSO-d₆): δ 7.82 (d, 4H), 7.77 (s, 2H), 7.66 (m, 6H), 7.56 (m, 6H), 7.36 (t, 4H).

Synthesis of 3,6-dibenzoylcatechol (3)

The synthesis followed a literature report⁴⁰ with modification. 1.00 g (1.90 mmol) of compound 2 was mixed with 10 ml of concentrated sulfuric acid and stirred at room temperature for 15 min. Then, the mixture was slowly poured into 100 ml of water in a beaker cooled in an ice water bath. The mixture was stirred for 30 min. After filtration, the solid was recrystallized with ethanol to give 0.49 g of 3 as a yellow flaky crystal (1.54 mmol, 81.1%).

Data for 3: ¹H NMR (400 MHz, DMSO-d₆): δ 10.35 (s, 2H), 7.79 (d, 4H), 7.68 (t, 2H), 7.57 (t, 4H), 6.95 (s, 2H).

Synthesis of 3,6-bis((E)-((4-aminophenyl)imino)(phenyl)meth-yl)benzene-1,2-diol (BL)

A mixture of 0.80 g (2.5 mmol) of 3,6-dibenzoylcatechol (3) and 0.70 g (6.5 mmol) of p-phenylenediamine was added to a Schlenk tube. The mixture was heated by a heat gun at around 180 °C under nitrogen for 10 min to obtain a dark red liquid. The crude product was cooled to room temperature, and a red solid was obtained. 5 ml of methanol was added into the Schlenk tube, and the tube was sonicated until all the solids dispersed in methanol. After filtration, the product was dried under vacuum at 120 °C for 3 h to obtain BL as a red solid with a yield of 92.4%.

Data for BL: ¹H NMR (400 MHz, CDCl₃): δ (ppm) 15.53 (s, 2H), 7.33 (m, 6H), 7.16 (m, 4H), 6.63 (d, 4H), 6.47 (d, 4H), 6.33 (s, 2H), 3.58 (s, 4H). IR (KBr): υ = 3476, 1613, 1586, 1518, 1468, 1431, 1255, 1155, 1081, 880, 834, 711, 537 cm⁻¹. MALDI-TOF MS (dithranol matrix) m/z = 499.2 (M + H)⁺. Anal. calcd for C₃₂H₂₆N₄O₂: C (77.09%), H (5.26%), N (11.24%); found: C (77.28%), H (5.22%), N (11.15%). MP = dec. at 285 °C (¹³C NMR is not available for BL due to its low solubility).

Synthesis of BL-TP-POP

To a 20 ml Teflon-lined reactor autoclave, BL (64.0 mg, 0.13 mmol), TP (17.8 mg, 0.085 mmol), 1,2-dichlorobenzene (2.0 ml), n-BuOH (2.0 ml), and 6 M aqueous acetic acid (0.80 ml) were added. The mixture was sonicated for 20 min and then degassed by bubbling with nitrogen for 20 min. The reactor was sealed off and heated at 120 °C for 72 h. The resulting red solid was separated and washed three times with THF and acetone sequentially. Finally, the solid was vacuum dried at 120 °C for 24 h to obtain BL-TP-POP with a yield of 63%.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Cultivating project for youths of Mudanjiang Normal University (1453QN017), Cultivating project for youths of Mudanjiang Normal University (1453QN020), “Open Competition to Select the Best Candidates” project of Mudanjiang Normal University (kjcx2023-055mdjnu), the Natural Science Foundation of Heilongjiang Province (SS2023B003), Collaborative quality improvement project for teaching and research (XTTZ001), Teaching and researching improvement project of Mudanjiang Normal University (22-XJ22011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04667f

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