Electrocatalytic N–C–N coupling over a hierarchically ordered open single-atom superstructure toward organonitrogen synthesis

Materials

Styrene (99%) was obtained from Aladdin Co., and further washed by sodium hydroxide (NaOH, 10 wt%) aqueous solution and deionized water several times to remove the p-tert-butylcatechol. Potassium persulfate (K₂S₂O₈, A.R., Sinopharm Chemical Reagent Co. (SCRC)) was recrystallized before use. Polyvinyl pyrrolidone (PVP, M_w ~ 58000, Adamas), zinc nitrate hexahydrate (Zn(NO₃)_2‧6H₂O, A.R., SCRC), 2-methylimidazole (C₄H₆N₂, 98%, Aladdin Co.), methanol (CH₃OH, A.R., SCRC), Pt foil (Pt, Wuhan Gaoshiruilian Science and Technology Co.), carbon paper (CP, Toray Industries, Inc), ethanol (CH₂CH₃OH, A.R., SCRC), ammonium hydroxide (NH₃·H₂O, A.R., SCRC), N,N-dimethylformamide (C₃H₇NO, A.R., Xilong Technology), dicyandiamide (C₂H₄N₄, A.R., SCRC), dimethylamine (C₂H₇N, A.R., Shanghai Adamas Reagent Co., 40 wt.% solution in water), N,N,N′,N′-tetramethyldiaminomethane (C₅H₁₄N₂, RG., Shanghai Adamas Reagent Co.), dichloromethane (CH₂Cl₂, A.R., SCRC), 1,2-dichlorobenzene (C₆H₄Cl₂, A.R., SCRC), trichloromethane (CHCl₃, A.R., SCRC), phenol (C₆H₆O, A.R., SCRC), cuprous chloride (CuCl, A.R., SCRC), 2-naphthol (C₁₀H₈O, AG., Shanghai Adamas Reagent Co.), toluene (C₇H₈, A.R., SCRC), 1,2,4-triazole (C₂H₃N₃, AG., Shanghai Adamas Reagent Co.), palladium diacetylacetonate (C₁₀H₁₆O₄Pd, AG., Shanghai Adamas Reagent Co.), imidazole (C₃H₄N₂, AG., Shanghai Adamas Reagent Co.), succinic anhydride (C₄H₄O₃, AG., Shanghai Adamas Reagent Co.), 10-hydroxycamptothecin (C₂₀H₁₆N₂O₅, AG., Shanghai Adamas Reagent Co.), 1,3-propanesulfonate (C₃H₆O₃S, AG., Shanghai Adamas Reagent Co.), methanol-13C (¹³CH₃OH, A.R., Shanghai Adamas Reagent Co.) were used without further purification.

Preparation of polystyrene (PS) monolith

0.5 g of PVP and 100 mL of deionized water were added in a 250 mL of three-necked flask and heated to 70 °C at 600 rpm under N₂ atmosphere. 13 mL of treated styrene was added and kept at 70 °C for 20 min. Then, 20 mL of 0.015 g mL⁻¹ K₂S₂O₈ solution was added, and the reaction was continued for 24 h. The obtained suspension was centrifuged and dried at 60 °C to yield the PS monolith.

Preparation of OM-ZIF-8

First, the methanol solutions of Zn(NO₃)₂·6H₂O (22.5 mL, 0.36 g mL⁻¹) and 2-methylimidazole (22.5 mL, 0.30 g mL⁻¹) were prepared. After thoroughly mixing the above solutions, 4.0 g of PS template was added, and the mixture was impregnated for 30 minutes. Subsequently, the mixture was further vacuum degassed for 20 min. The PS template was taken out and dried at 50 °C for 6 h, and then transferred to 30 mL of methanol and ammonia (v:v = 1:1). The mixture was further vacuum degassed for 10 min and set aside for 24 h at room temperature to promote the growth of ZIF-8. Then, the soaked PS template was extracted and dried at 50 °C to obtain ZIF-8@PS. Finally, the PS template was removed by soaking in DMF, and the solid left was filtrated, rinsed with methanol, and dried to obtain OM-ZIF-8.

Preparation of Zn₁/h-OPNC, Zn₁/OPNC and Zn₁/h-OPNC₉₀₀

Zn₁/h-OPNC was obtained by calcining the mixture of OM-ZIF-8 (100 mg) and DCDA (500 mg) at 1000 °C for 2 h under an Ar atmosphere, with an elevated temperature rate of 2 °C min⁻¹. Zn₁/h-OPNC₉₀₀ and Zn₁/OPNC were prepared in the same way, except that Zn₁/h-OPNC₉₀₀ was calcined at 900 °C, while Zn₁/OPNC was calcined without adding DCDA.

Preparation of 2D-ZIF

0.330 g (1.11 mmol) of Zn(NO₃)₂‧6H₂O and 0.985 g (12.0 mmol) of 2-methylimidazole were each dissolved in 90 mL of deionized water. The above two solutions were mixed and stirred at room temperature for 24 h. After standing, the mixture was centrifuged, and the solid was rinsed with methanol and deionized water, and dried at 60 °C to obtain 2D-ZIF.

Preparation of Zn₁/f-PNC

Zn₁/f-PNC was obtained by calcining the mixture of 2D-ZIF (100 mg) and DCDA (500 mg) at 1000 °C for 2 h under an Ar atmosphere, with an elevated temperature rate of 2 °C min⁻¹.

Characterizations

The scanning electron microscopy (SEM) images were obtained on JSM6700-F. Powder X-ray diffraction (PXRD) patterns were gained on a MiniFlex diffractometer (MiniFlex600, Rigaku). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were collected from FEI Titan Themis 200. Atomic force microscope images were obtained in a Bruker Dimension ICON. The X-ray photoelectron spectroscopy (XPS) data were obtained from the Thermo Fisher ESCALAB 250Xi spectrometer. In situ attenuated total reflection-infrared (ATR-IR) spectra were recorded on the NICOLET 6700 instrument. The nuclear magnetic resonance (NMR) spectra were recorded on the ECZ600R. The liquid products were detected by Shimadzu GCMS-QP2020 gas chromatograph mass spectrometer (GC-MS) and Shimadzu GC-2018 gas chromatography (GC) featuring an FID detector and HP-5 column. The mass spectrometry (MS) spectra were recorded on the Bruker Impact II Q-TOF instrument.

3D TEM tomography

3D TEM tomography was collected from JEOL JEM-F200 at 200 kV. The electron micrographs were acquired using a single-axis high-tilt tomography holder from −73° to 71° with a linear tilt step of 2°. At each tilt angle, the beam was focused on a neighboring area to minimize the damage to the region of interest. The application of tophat filtering served to eliminate the back bottom and thereby mitigate the back bottom effect. The manual segmentation method was employed to divide the projection and extract the effective area. Image alignment was performed using the center of mass method. The aligned tilt series were reconstructed using the Fourier-based reconstruction algorithm with 50 iterations. The reconstructed 3D volume was segmented and refined by threshold segmentation and Gaussian filter in MATLAB. The porosity can be calculated as follows: porosity = V_frame/V_total × 100%. Where V_frame is the reconstructed 3D body for binarised acquisition, and V_total is the total volume of the reconstructed 3D body following the implementation of hole-filling techniques.

Electrochemical measurements

The catalyst ink was prepared by mixing 350 μL of H₂O,100 μL of isopropanol, 50 μL of Nafion solution (5 wt% in isopropanol), and 5 mg of catalyst together, and then ultrasonic treatment for 1 h. Then, based on the zinc content, a certain volume of the ink was evenly loaded on the two sides of a carbon paper (1.0 × 1.0 cm²) to obtain a metal loading of 1 mg cm⁻². All the electrochemical measurements were conducted on the CHI760E electrochemical workstation (Shanghai Chen-Hua Instrument Corporation, China) and tested in a proton exchange membrane (Nafion®117) separated H-type cell by using a typical three-electrode system. Pt mesh and Hg/HgO (1 M KOH) electrodes were used as the counter and reference electrodes, respectively. The anodic compartment was filled with 15 mL of 0.7 M K₂CO₃ and 5 mL of a mixed solution of methanol and ammonia with different volume ratios. The cathodic compartment was filled with 20 mL of 0.7 M K₂CO₃. The linear sweep voltammetry (LSV) polarization curves were measured at a scan rate of 10.0 mV s⁻¹. All curves were gained without IR compensation. All electrochemical measurements were performed at ambient temperature (25 °C) and pressure.

Calculation of Faradaic efficiency and electron efficiency

Assuming that the formation of a DMF molecule demands six electrons and one TMDM requires two electrons, the Faradaic efficiency (FE) can be calculated as follows: FE (%) = 100znF/Q. Where z is the number of electrons required to form a molecule of product, n is the mole of product formed, F is the Faraday constant (96485 C mol⁻¹), and Q is the electric quantity. The electron efficiency (EE) is calculated as follows: EE (%) = FE × Yield.

Synthesis of other N–C–N bonded organonitrogen compounds: N,N′-diethyl-N,N′-dimethylmethanediamine, N,N,N′,N′-tetraethylmethanediamine and dimorpholinomethane

Other N–C–N bonded organonitrogen compounds were synthesized according to the procedures for TMDM, with the use of different amines as the reagents. The cathodic compartment was filled with 20 mL of 0.7 M K₂CO₃, while the anodic compartment was filled with 15 mL of 0.7 M K₂CO₃, 1 mL of methanol, and 4 mL of 40 wt% amine solution. After electrolysis for 3 h at 0.8 V, the electrolytes were extracted with dichloromethane, and the products were quantified by GC.

Synthesis of the organonitrogen compounds with (dimethylamino)methyl substituent

2-[(Dimethylamino)methyl]phenol

Following electrolysis, the electrolyte was extracted with 5 mL of trichloromethane and dried with anhydrous sodium sulfate. The resulting solution was then added to a single-necked flask with 94.1 mg (1 mmol) of phenol and 4.04 mg (0.04 mmol) of CuCl and stirred at 50 °C for 12 h. The product was quantified by GC. Yield: 56 mg (72%). ¹H NMR (600 MHz, DMSO-d₆): δ 7.13-6.98 (m, 2H), 6.72 (dd, J = 9.3, 5.1 Hz, 2H), 3.55 (s, 2H), 2.22 (s, 6H).

1-[(Dimethylamino)methyl]-2-naphthalenol

Following electrolysis, the electrolyte was extracted with 5 mL of trichloromethane and dried with anhydrous sodium sulfate. The resulting solution was then added to a single-necked flask with 169 mg (1.17 mmol) of 2-naphthol, 8 mg of K₂CO₃, and refluxed at 60 °C for 6 h. The resulting solution was extracted with 1 M HCl, and then the pH was adjusted to 8 by the addition of K₂CO₃. The precipitated solid was filtered and dried, resulting in the product. Yield: 204 mg (86%). ¹H NMR (600 MHz, DMSO-d₆): δ 7.93 (d, J = 5.7 Hz, 1H), 7.77 (d, J = 5.4 Hz, 1H), 7.70 (d, J = 5.9 Hz, 1H), 7.42 (t, J = 5.1 Hz, 1H), 7.26 (t, J = 4.9 Hz, 1H), 7.06 (d, J = 5.9 Hz, 1H), 3.98 (s, 2H), 2.29 (s, 6H); HRMS (m/z): [M + H]⁺ calcd. for C₁₃H₁₆NO, 202.1154; found, 202.1235.

N,N-dimethyl-1-1,2,4-triazol-1-methanamine

Following electrolysis, the electrolyte was extracted with 5 mL of toluene and dried with anhydrous sodium sulfate. The resulting solution was then added to a single-necked flask with 69.1 mg (1 mmol) of 1,2,4-triazole, 1 mg of Pd(acac)₂, and refluxed at 90 °C for 4 h. Subsequently, after filtration, the product was obtained by rotary evaporation of the filtrate. Yield: 108 mg (85%). ¹H NMR (600 MHz, DMSO-d₆): δ 8.55 (s, 1H), 7.97 (s, 1H), 4.98 (s, 2H), 2.19 (s, 6H). HRMS (m/z): [M-C₂H₂N₃]⁺ calcd. for C₃H₈N, 58.0675; found, 58.0500.

N,N-dimethyl-1H-imidazole-1-methanamine

Following electrolysis, the electrolyte was extracted with 15 mL of trichloromethane and dried with anhydrous sodium sulfate. The resulting solution was then added to a single-necked flask with 138 mg (2 mmol) of imidazole, 152 mg (1.1 mmol) of K₂CO₃, and 111 mg (1.1 mmol) of succinic anhydride, and the mixture was stirred at room temperature for 1 h. After reaction, 50 mL of trichloromethane was added, and the organic phase was washed with 1 M NaOH and saturated NaCl solution, respectively. The organic phase was then dried with anhydrous sodium sulfate, and the resulting product was obtained by rotary evaporation. Yield: 94 mg (94%). ¹H NMR (600 MHz, DMSO-d₆): δ 7.62 (s, 1H), 7.15 (s, 1H), 6.90 (s, 1H), 4.73 (s, 2H), 2.15 (s, 6H). HRMS (m/z): [M + H]⁺ calcd. for C₅H₁₁N₄, 125.175; found, 126.1013.

Topotecan hydrochloride

Following electrolysis, the electrolyte was extracted with 5 mL of trichloromethane and dried with anhydrous sodium sulfate. The extracts from six electrolysis batches were combined and added to a single-necked flask, together with 1.00 g (2.69 mmol) of 10-hydroxycamptothecin. The mixture was then stirred at 45 °C for 24 hours. After the reaction, the solid was obtained by rotary evaporation. The solid was dissolved in 20 mL of 0.4 M HCl and stirred at room temperature for 16 hours. The resulting solution was filtered and freeze-dried to obtain the product. Yield: 1.16 g (95%). ¹H NMR (600 MHz, DMSO-d₆): δ 9.82 (s, 1H), 8.98 (s, 1H), 8.17 (d, J = 6.2 Hz, 1H), 7.72 (d, J = 6.1 Hz, 1H), 7.28 (s, 1H), 6.53 (s, 1H), 5.42 (s, 2H), 5.26 (s, 2H), 4.72 (s, 2H), 2.83 (s, 6H), 1.87 (m, J = 14.2, 9.4, 4.8 Hz, 2H), 0.88 (t, J = 4.9 Hz, 3H); UV/Vis: λmax 225 nm; HRMS (m/z): [M–Cl]⁺ calcd. for C₂₃H₂₄N₃O₅, 422.1710; found, 422.1706.

3-(((4-ethyl-4,9-dihydroxy-3,14-dioxo-3,4,12,14-tetrahydro-1H pyrano[3′,4′:6,7]indolizino[1,2-b]quinolin-10 yl)methyl)dimethylammonio)propane-1-sulfonate (Sulfonate functionalized topotecan)

Topotecan hydrochloride (0.2 mmol, 92 mg) was combined with 5 mL of trichloromethane and 24.4 mg (0.2 mmol) of 1,3-propanesulfonate, and refluxed at 80 °C for 36 h. After the reaction, the solid was obtained by rotary evaporation at 45 °C. The solid was continuously rinsed with trichloromethane, dispersed in trichloromethane, ultrasonicated to dissolve a small number of impurities, filtered, and dried to obtain the product. Yield: 80 mg (74%). ¹H NMR (600 MHz, DMSO-d₆): δ 9.39 (s, 1H), 8.93 (s, 1H), 8.18 (d, J = 6.1 Hz, 1H), 7.64 (d, J = 6.1 Hz, 1H), 7.29 (s, 1H), 6.53 (s, OH), 5.42 (s, 2H), 5.27 (s, 2H), 4.73 (s, 2H), 3.71 (t, J = 4.5 Hz, 2H), 2.85 (s, 6H), 2.53 (t, 2H), 2.0 (m, 2H), 1.87 (m, J = 14.2, 9.5, 4.7 Hz, 2H), 0.88 (t, J = 4.8 Hz, 3H); ¹³C NMR (150 MHz, DMSO-d₆): δ 172.64, 156.98, 156.94, 150.18, 149.66, 145.59, 143.47, 133.22, 130.68, 129.09, 126.55, 122.18, 118.67, 108.50, 96.28, 72.51, 65.33, 50.62, 50.35, 48.57, 44.85, 42.65, 30.32, 28.97, 7.89; HRMS (m/z): [M–C₃H₇O₃S]⁺ calcd. for C₂₃H₂₄N₃O₅, 422.1710; found, 422.1709.

Product quantification and identification

After electrolysis, the electrolyte was divided into two parts and extracted by dichloromethane and 1,2-dichlorobenzene, respectively. Dichloromethane extract was used for the detection of DMF, and 1,2-dichlorobenzene extract was used for the detection of TMDM. The final samples were further identified by GC-MS (Shimadzu GCMS-QP2020) and quantified by GC (Shimadzu GC-2018). Formate was quantified by ¹H nuclear magnetic resonance (¹H NMR) (ECZ600R, 600 MHz) using H₃BTC as the internal standard. In detail, the mixture of 500 μL of electrolyte and 100 μL of D₂O containing 10 mM H₃BTC solution was used as the test sample.

Computational details for calculations

First-principles calculations based on density functional theory (DFT) were conducted using the Vienna ab initio Simulation Package (VASP). The projector augmented wave (PAW) method was employed to solve the Kohn-Sham equations, while the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) described exchange-correlation interactions. The van der Waals interactions were incorporated into the standard DFT framework via the DFT-D3 dispersion correction. A 15 Å vacuum layer was introduced to eliminate the interaction between the periodically repeated structures. In this work, key computational parameters included: a 400 eV plane-wave cutoff energy, a 2 × 2 × 1 Monkhorst-Pack k-point grid, and convergence criteria of 10⁻⁵ eV (energy) and 0.05 eV Å^–1 (residual forces). The density of states (DOS) was computed with Gaussian smearing (0.1 eV width). Free energy diagrams were generated via the computational hydrogen electrode (CHE) method. The Gibbs free energies were calculated at 298 K and 1 atm according to G = E + E_ZPE − TS, where E is the total energy, E_ZPE is the zero-point vibrational energy, T is the absolute temperature, and S is the entropy.