English

• 0.   Introduction

  Hydrogen (H₂) gas acquires a high combustion calorific value and only produces water during combus- tion, so it is considered as an alternative, sustainable and clean energy^[1]. Artificial photosynthetic H₂ produc-tion is regarded as an effective way to convert and utilize solar energy. Building an efficient, low-cost, nontoxic and chemically stable photocatalytic system is a goal that scientists have always pursued. The current research mainly focuses on the three aspects of photo- catalytic hydrogen production semi - reaction^[2], visible light-driven hydrogen decomposition^[3-5] and hydrogen production coupled with visible light catalysis organic conversion^[6-9]. At present, scientists have developed artificial photocatalytic systems for the generation of hydrogen from water based on molecular photosensitizers (such as eosin, fluorescein and metal complexes) ^[10-18] and semiconductor light-capturing materials (such as TiO₂, C₃N₄ and CdS)^[19-21]. The efficiency and stability of photocatalytic hydrogen production have been continu- ously improved.

  The metal- organic supramolecular complexes with nanoscopic size and well- defined hydrophobic cavities are regarded as mimics of natural photocatalytic system by catching organic dyes in their pockets^[22-27]. Compared to other relevant systems, these supramolec- ular architectures are superior in light-driven hydrogen production by enhancing the proximity between the photosensitizer and the catalytic center and increasing the effective local concentration of the substrates within the container^[24].

  However, host-guest supramolecular system requires a large excess of photosensitizers for hydrogen evolution^[28]. It is mainly due to the poor stability of the photosensitizer and the weak and dynamic host - guest interaction. So, we developed a post-assembly modifica- tion strategy by connecting quantitative photosensitizer fluorescein isothiocyanate (FITC) to metal-organic complex in order to improve the stability of photosensi- tizer. Compared to traditional three - component hydro- gen production system, it was a two-component system consisting of catalyst and electron sacrificial agent. We firstly synthesized the Schiff base ligand L1 and corre- sponding metal-organic triangles Co-L1. Therefore, the functionalized metal- organic triangles Co -L3 were con- structed by complex Co - L2 with amino groups which could be easily bonded to photoactive FITC units. The photocatalytic activity of catalyst Co -L3 in two- component system was higher than that of Co - L1 in three-component system under the same reaction condi- tions. This post-assembly modification strategy pro- vides a novel idea for metal-organic confined supramo- lecular.

  Figure 1

  

  Figure 1.  Structures of ligands L1, L2 and L3

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  1.   Experimental

  1.1   Materials and instruments

  All reactions were carried out under N₂ atmo- sphere with standard Schlenk techniques. Solvents were dried and distilled prior to use according to the standard methods. All chemicals were of reagent grade obtained from commercial sources and used without further purification. ¹H NMR spectra were run on a Bruker-400 spectrometer with tetramethylsilane (¹ H) as an internal standard. The elemental analyses of C, H and N were performed on a Vario EL Ⅲ elemental analyzer. ¹H NMR spectra were measured on a Varian INOVA 400M spectrometer. Electrospray ionization mass spectra (ESI - MS) were obtained on a HPLC - Q - Tof MS spectrometer. The fluorescent spectra were measured on a JASCO FP-6500.

  Electrochemical measurements: Electrochemical measurements were carried out under nitrogen at room temperature on ZAHNER ENNIUM electro-chemical workstation with a conventional three-electrode system with a homemade Ag/AgCl electrode as the reference electrode, a platinum silk with 0.5 mm diameter as the counter electrode, and a glassy carbon electrode as the working electrode. Cyclic voltammograms were obtained with the solution concentrations of 1 mmol·L^-1 for the cobalt-based complexes and 0.1 mol·L^-1 for the supporting electrolyte. The addition of 0.1 mmol·L^-1 NEt₃HCl in dimethyl Formamide (DMF) was carried out with syringe.

  Fluorescence quenching experiments: A solution (2.0 mL) of fluorescein (Fl) at 10 μmol·L^-1 concentra- tion in a EtOH/H₂O solution (1:1, V/V, pH=12) was prepared in a quartz cuvette fitted with a septum cap, and the solution was degassed under N₂ for 15 min. Aliquots of 20 μL (1 mmol·L^-1) of the catalysts Co -L1 were added, and the intensity of the fluorescence was monitored by steady state fluorescence excited at 460 nm on a Spex - Fluoromax - P fluorimeter with a photo- multiplier tube detector. Both excitation and emission slit widths were 2 nm.

  UV-Vis absorption spectra: the cobalt-based complexes and FITC were prepared at concentration of 10 μmol·L^-1 in DMF solution. Adding 2 mL of DMF in each of the two wide quartz cuvettes in order to elimi- nate background interference, then above solutions were gradually added into one of the two quartz cuvettes by a micro-syringe. Solutions were stirred evenly and stood for 3 min. Test range of UV-Vis absorption spectrum was set to 250~700 nm.

  Photocatalytic proton reduction experiments: photoinduced hydrogen evolution was performed in a 20 mL flask. Varying amounts of the catalysts, Fl and NEt₃ in EtOH/H₂O (1:1, V/V) were added to obtain a total volume of 5.0 mL. The flask was sealed with a septum and degassed by bubbling argon for 15 min. The pH of this solution was adjusted to a specific pH by adding HCl or NaOH and measured with a pH meter. The samples were irradiated by a 500 W Xenon lamp with the light filter (< 420 nm), and the reaction temperature was maintained at 293 K by using a ther- mostat water bath. The generated H₂ was characterized by GC 7890T instrument analysis using a 0.5 nm mo- lecular sieve column (0.6 m×3 mm), thermal conductiv-ity detector, and argon used as carrier gas. The amount of hydrogen generated was determined by the external standard method.

  1.2   Syntheses of L1 and L2

  Ligand L1: the ligand L1 was synthesized by adding dimethyl biphenyl-4, 4-dicarboxylate (10 mmol, 2.7 g) and hydrazine monohydrate (40 mL) into a round bottomed flask. After heating at 70 ℃ for 24 hours, the mixture was cooled to room temperature and then washed thoroughly with methanol to remove excess hydrazine hydrate. The above product (5 mmol, 1.35 g) was mixed with 150 mL ethanol in a 250 mL round bot- tom flask, and then 0.25 mL AcOH was added drop- wise. After refluxing for 12 h, the mixture was filtered while hot and washed with hot methanol. Yield: ~90%. Anal. Calcd. for C₅₂H₄₀N₄O₂P₂(%): C, 76.65; H, 4.95; N, 6.88. Found (%): C, 76.29; H, 5.02; N, 6.88. ¹H NMR (400 MHz, DMSO - d₆): δ 12.06 (s, 2H), 9.22 (s, 2H), 8.17~7.79 (m, 12H), 7.43 (s, 14H), 7.24 (s, 6H), 6.88 (s, 2H).

  Ligand L2: dimethyl phthalate (1.00 g, 3.7 mmol) was added into concentrated H₂SO₄ (10 mL, 98%) and stirred at room temperature for 10 min. Nitric acid (760 μL, 65%) was added to a concentrated H₂SO₄ (2 mL) to obtain a mixed acid, which was slowly added dropwise to the above solution. The mixture was stirred for 4 hours under ice bath, and then poured into ice (300 mL) to give a beige solid. Then beige solid (10.3 g, 0.03 mol) was dissolved in 300 mL of acetic acid and iron powder (18.2 g, 0.32 mol) was added. The mixture was stirred at 50 ℃ for 14 h. The suspension was filtered, and the filtrate was partly dried in vacuum. After washing with saturated aqueous sodium hydrogen carbonate, the solution was extracted with CH₂Cl₂. The combined organic layers were dried under vacuum and recrystallized in CH₂Cl₂ for purification. The above intermediate (1.5 g, 5 mmol), 2- diphenylphosphinoben- zaldehyde (3.2 g, 11 mmol) and ethanol (150 mL) were added into a 250 mL round bottom flask, and the mixture was refluxed for 12 h. After completion of the reaction, the mixture was filtered while hot, washed with hot methanol and then evaporated to dryness. Yield: ~70%. Anal. Calcd. for C₅₂H₄₂N₆O₂P₂(%): C, 73.92; H, 5.01; N, 9.95. Found (%): C, 73.06; H, 5.07; N, 10.10. ¹H NMR (400 MHz, DMSO - d₆): δ 11.94 (s, 2H), 9.19 (s, 2H), 8.09 (s, 2H), 7.43 (s, 18H), 7.32 (s, 2H), 7.23 (s, 10H), 7.08 (s, 2H), 6.84 (s, 2H), 4.94 (s, 4H).

  1.3   Syntheses of complexes Co-L1, Co-L2 and Co -L3

  Co-L1: the ligand L1 (0.1 mmol, 0.082 g) and Co (ClO₄)₂ ·6H₂O (0.12 mmol, 0.031 g) were dissolved in 20 mL methanol. After refluxing for 6 h, the red mix- ture was filtered. The red block crystals were obtained by slow evaporation of the solvent in two weeks under room temperature. Yield: ~55%. Anal. Calcd. for C₁₅₆H₁₂₀N₁₂O₆P₆Co₃(%): C, 69.26; H, 4.47; N, 9.32. Found(%): C, 69.28; H, 4.49; N, 9.12. ESI - MS m/z: [3Co+3L1-6H]³⁺, 871.513 5; [3Co+3L1-6H+ClO₄]²⁺, 1 357.244 5.

  Co-L2: the synthesis of complex Co-L2 was simi- lar to Co-L1 with the ligand L2 (0.1 mmol, 0.085 g) and Co(ClO₄)₂·6H₂O (0.12 mmol, 0.031 g). After several days, red block crystals were obtained. Yield: ~35%. Anal. Calcd. for C₁₅₆H₁₂₆N₁₈O₆P₆Co₃(%): C, 69.10; H, 4.68; N, 9.30. Found(%): C, 69.20; H, 4.65; N, 9.31. ESI - MS m/z: [3Co+3L2-6H]³⁺, 901.552 7; [3Co+3L2-6H+ClO₄]²⁺, 1 402.302 1.

  Co-L3: the FITC (0.7 mmol, 29 mg) was dissolved in 10 mL DMF, and then added dropwise to the DMF solution of Co- L2 (0.1 mmol, 28 mg). The mixture was stirred at room temperature for 24 h. The solution was evaporated in vacuo and excess FITC was removed with ethanol. The red block crystals were obtained in two weeks under room temperature. Yield: ~40%. ESI- MS m/z: [3Co+3L2-6H+3FITC]³⁺, 1 290.903 1; [3Co+ 3L2-6H+4FITC]³⁺, 1 420.913; [3Co+3L2-6H+5FITC]³⁺, 1 550.953 1; [3Co+3L2-6H+6FITC]³⁺, 1 680.271 2.

  2.   Results and discussion

  2.1   ESI-MS spectra of Co-complexes

  The ESI - MS spectrum of Co -L1 in DMF solution demonstrated the stoichiometry of the formation of metal- organic triangles (Fig. 2a). Isotopically resolved peaks corresponding to the intact and discrete [3+3] assembly complex Co-L1 was observed at m/z=871.513 5 and 1 357.244 5 which are assigned to the [3Co+3L1-6H]³⁺ and [3Co+3L1-6H+ClO₄]²⁺, further providing convincing evidence for the existence of the metallacy- cle as the only supramolecular species and its stability in DMF solution. Moreover, six hydrogen atoms lost in the entire supramolecular species were derived from the active hydrogen atoms in the hydrazide, which also illustrated the coordination mode of metal ions and li- gands.

  Figure 2

  

  Figure 2.  ESI⁃MS spectra of metallacycle (a) Co⁃L1, (b) Co⁃L2 and (c) Co⁃L3

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  The cobalt centers in complex Co - L2 possessed the same coordination environments as those in Co-L1 (Fig. 2b). Besides, free amino groups were not involved in coordination, which provided possibilities for the post-assembly modification of FITC.

  As shown in Fig. 2c, the Co-L3 generated peaks at m/z of 1 290.903 1, 1 420.913 6, 1 550.593 1 and 1 680.271 2, corresponding to [3Co+3L2-6H+3FITC]³⁺, [3Co+3L2-6H+4FITC]³⁺, [3Co+3L2-6H+5FITC]³⁺ and [3Co+3L2-6H+6FITC]³⁺, respectively. The hexafunc- tionalized metallacycle Co-L3 could be seen as the major product of the reaction when analyzed in conjunction with the ¹H NMR spectra.

  2.2   Structural description for metallacycle Co-L1

  It was a pity that we have not obtained good single crystal diffraction date of Co- L1 because of its poor crystallinity. The composition of Co -L1 was revealed by the ESI-MS spectrum, where signals (m/z) at 871.513 5 and 1 357.244 5 could be classified as [3Co+3L1-6H]³⁺ and [3Co+3L1-6H+ClO₄]²⁺. Each of the isotope patterns is in accord with the calculated ones. The metallacycle of Co-L1 could be seen as a M3L3 triangle consisting of three cobalt ions as verti- ces and three ligands as sides (Fig. 3). The cobalt ions were located on two edges with an average distance of 1.5 nm and the angle between ligand and ligand was 60°. In the structure, the cobalt atoms coordinated to the two N atoms, two O atoms and two P atoms, which is similar to the octahedral configuration.

  Figure 3

  

  Figure 3.  Schematic representation of the metal⁃organic triangle Co⁃L1 showing the coordination geometry of the cobalt ions

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  2.3   ¹H NMR spectra of Co-complexes

  ¹H NMR spectra of the reaction products further supported the structural information about the metalla- cycles. The shift of H5 (-NH₂, δ =4.9) was observed in the spectrum of Co - L2 (Fig. 4), further demonstrating that free NH₂ groups were not involved in coordination.Despite the complexity, features characteristic of Co-L3 could be identified in ¹H NMR spectra. Com- pared to the spectrum of Co-L2, we could infer that amino groups and isothiocyanate groups reacted in a ratio of 1:1 due to the disappearance of shift of H5 (- NH₂, δ =4.9). And integration of the peaks (H1 and H4) in this spectrum also demonstrated that six FITC units were present in each metallacycle of Co-L3 on average. Both ¹H NMR and ESI-MS spectra provided evidence that FITC units were successfully connected to the metal-organic triangles in a covalent post- assembly modification reaction.

  Figure 4

  

  Figure 4.  Partial ¹H NMR spectra of FITC, Co⁃L3 and Co⁃L2

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  2.4   Cyclic voltammetry of Co-L3

  The cyclic voltammetry of Co-L3 showed that the reduction potentials of CoⅡ/CoⅠ and CoⅢ/CoⅡ occurred at -1.14 and -0.31 Ⅴ (Fig. 5a). This reduction potential of CoⅡ/CoⅠ was more negative than the reduction potential of the proton, suggesting that Co-L3 was able to reduce proton hydrogen evolution under electro- chemical conditions. To investigate the role of the Co- L3 as a proton reduction catalyst, cyclic voltammetry was performed in the presence of increasing amounts of NEt₃HCl (Fig. 5b). It was found that a new proton reduc- tion peak was induced near the reduction peak of CoⅡ/ CoⅠ. This result could be regarded as the electrochemical reduction process of protons in solution, indicating that Co-L3 can reduce the proton with a catalysis process.

  Figure 5

  

  Figure 5.  (a) Cyclic voltammetry curve of Co⁃L3 (1.0 mmol·L^-1); (b) Cyclic voltammetry curves of Co⁃L3 (1.0 mmol·L^-1) upon addition of NEt₃HCl with different concentrations

  Scan rate: 100 mV·s^-1

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  2.5   Co-L1 as quencher for the photosensitizer Fl

  As a proton reducing agent, Co - L1 could obtain electrons from the activator in an excited state, and the actinic agent in the excited state reached a steady state usually accompanied by a decrease in the intensity of the photosensitizer emission. As shown in Fig. 6, the addition of an equivalent molar ratio of Co - L1 to an EtOH/H₂O solution containing Fl obviously quenched the emission intensity, which proved possibilities for Fl to activate Co-complexes for the production of H₂. The Stern-Volmer quenching constant K_svl in this system was calculated as 1.51×104. The result showed that the complex Co - L1 could not only realize the process of electrochemical decomposition of water for hydrogen evolution, but also transferred the absorbed light ener- gy to the redox catalyst through the excited state of fluo- rescein after the sensitization of fluorescein.

  Figure 6

  

  Figure 6.  Emission spectra of Fl solution (10 μmol·L^-1) in an EtOH/H₂O (1:1, V/V) solution upon addition of Co⁃L1 (1 mmol·L^-1) with various concentrations

  Inset: Stern⁃Volmer plot for the photoluminescence quenching of Fl by catalyst Co⁃L1

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  2.6   Photophysical properties of Co-L3

  The factors that provide Co-L3 with photoactivity and stability were investigated based on the light absor- bance and structural stability. Usually, cationic metal- organic macrocycles or cages constructed by Schiff base ligands have non-fluorescent emission due to intramolecular photoinduced electron transfer (PET) between luminophores and electron-deficient units. However, Co-L3 exhibited certain photophysical prop- erties in various solvent systems despite of reduced flu- orescence intensity due to interference from metal ions (Fig. 7a). The light absorbances of Co - L3, Co - L1 and FITC were compared via UV-Vis spectrometry. The same number of FITC units were prepared in each sample, and the corresponding absorptions in the wave- length range from 260 to 700 nm were compared. As shown in Fig. 7b, the complex Co-L3 modified with FITC units showed higher light absorption, indicating that the discrete metal-organic triangles of Co-L3 improved the light absorption in the solution.

  Figure 7

  

  Figure 7.  (a) Fluorescence spectra of FITC and Co-L3; (b) UV-Vis spectra of FITC and Co-L3

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  2.7   Catalytic behavior of Co-L1 and Co-L3 in H₂ evolution

  Proton reduction catalytic activity of Co - L3 was evaluated in an EtOH/H2O (1:1, V/V) solvent mixture irradiated with visible light (λ > 420 nm) containing NEt3 as an electronic sacrificial agent under room temperature^[29-31]. It was a two -component photocatalytic hydrogen production system without extra photosensi- tizer. The volume of H2 was quantified at the end of the photolysis by gas chromatographic analysis. The optimum pH value of the reaction mixture was main-tained at 12.0 (Fig. 8a), decreasing or increasing the pH value both resulted in a lower initial rate and shorter system lifetime for hydrogen evolution. The decrease of the efficiency at higher pH values was likely due to the lower concentration of proton in solution. The optimal concentration of NEt3 is 5%(V/V) when fixing the pH of the system at the optimum value of 12.0 (Fig. 8b). It was mainly because the high concentration of electron sac- rificial agents would accelerate the decomposition of photosensitizers. Under optimal conditions, Co-L3 was highly active for the evolution of H2, the produced hy- drogen volume exhibited a linear relationship with the catalyst concentration in a range of 10.0 to 100.0 μmol·L^-1 (Fig. 8c) and the calculated turnover number (TON) was approximately 80. In addition, hardly any observable amount of hydrogen was detected in the dark or in the absence of Co-catalyst under the measured conditions, which showed that Co - catalyst was essential for an efficient photochemical reaction.

  Figure 8

  

  Figure 8.  8 (a) Light⁃driven H₂ evolution of systems containing Co⁃L3 (40 μmol·L^-1) and NEt₃ (5%, V/V) in EtOH/H₂O solutions (1:1, V/V) with different pH values; (b) Light⁃driven H₂ evolution of systems containing Co⁃L3(40 μmol·L^-1) in EtOH/H₂O solution (1:1, V/V) at pH=12.0 with different concentrations of NEt₃; (c) Light⁃driven H₂ evolution of systems containing NEt₃ (5%, V/V) in EtOH/H₂O solutions (1:1, V/V) at pH=12.0 with various concentrations of Co⁃L3; (d) Light⁃driven H₂ evolution in EtOH/H₂O (1:1, at pH=12.0 of three⁃component system containing Co⁃L1 (100 μmol·L^-1), Fl (100 μmol·L^-1) and NEt₃ (5%, V/V), and two⁃component system containing Co⁃L3 (100 μmol·L^-1) and NEt₃ (5%, V/V)

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  Furthermore, a system containing Fl (0.6 mmol·L^-1), Co - L1 (0.1 mmol·L^-1) and NEt3 (5%, V/V) in an EtOH/H2O (1:1, V/V) solution was also used for hydro- gen generation. The two - component system containing Co-L3 and NEt3 showed almost 30 times the hydrogen production efficiency of above three-component system (Fig. 8d). From fluorescence quenching experiments, we could infer that Co-L1 exhibited weak host-guest in- teractions with fluorescein molecule. Besides, hydro- gen evolution was maintained over a period of at least 36 hours in two-component systems containing Co-L3, indicating the introduction of FITC extended the sys- tem lifetime for hydrogen production (Fig. 8c). In a word, this artificial photosynthetic system works well in the absence of the Fl, demonstrating that post - assem- bly modification strategy is an effective and feasible way for the hydrogen generation.

  3.   Conclusions

  In summary, a new post-assembly strategy for constructing artificial photocatalytic systems to generate hydrogen from water by combining an organic photo- sensitizer FITC with a redox active metal- organic macrocycle was reported. New synthesized metal complexes Co-L3 act as both transition metal catalysts and photosensitizers that capture light energy, hence becoming a two -component hydrogen evolution system without extra photosensitizers. This work may provide a new idea for post - assembly strategy of supramolecular systems in hydrogen production.

  
  
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  Figure 1  Structures of ligands L1, L2 and L3

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  Figure 2  ESI⁃MS spectra of metallacycle (a) Co⁃L1, (b) Co⁃L2 and (c) Co⁃L3

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  Figure 3  Schematic representation of the metal⁃organic triangle Co⁃L1 showing the coordination geometry of the cobalt ions

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  Figure 4  Partial ¹H NMR spectra of FITC, Co⁃L3 and Co⁃L2

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  Figure 5  (a) Cyclic voltammetry curve of Co⁃L3 (1.0 mmol·L^-1); (b) Cyclic voltammetry curves of Co⁃L3 (1.0 mmol·L^-1) upon addition of NEt₃HCl with different concentrations

  Scan rate: 100 mV·s^-1

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  Figure 6  Emission spectra of Fl solution (10 μmol·L^-1) in an EtOH/H₂O (1:1, V/V) solution upon addition of Co⁃L1 (1 mmol·L^-1) with various concentrations

  Inset: Stern⁃Volmer plot for the photoluminescence quenching of Fl by catalyst Co⁃L1

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  Figure 7  (a) Fluorescence spectra of FITC and Co-L3; (b) UV-Vis spectra of FITC and Co-L3

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  Figure 8  8 (a) Light⁃driven H₂ evolution of systems containing Co⁃L3 (40 μmol·L^-1) and NEt₃ (5%, V/V) in EtOH/H₂O solutions (1:1, V/V) with different pH values; (b) Light⁃driven H₂ evolution of systems containing Co⁃L3(40 μmol·L^-1) in EtOH/H₂O solution (1:1, V/V) at pH=12.0 with different concentrations of NEt₃; (c) Light⁃driven H₂ evolution of systems containing NEt₃ (5%, V/V) in EtOH/H₂O solutions (1:1, V/V) at pH=12.0 with various concentrations of Co⁃L3; (d) Light⁃driven H₂ evolution in EtOH/H₂O (1:1, at pH=12.0 of three⁃component system containing Co⁃L1 (100 μmol·L^-1), Fl (100 μmol·L^-1) and NEt₃ (5%, V/V), and two⁃component system containing Co⁃L3 (100 μmol·L^-1) and NEt₃ (5%, V/V)

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