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• The pursuit of new complexes containing Schiff base remains at the forefront of molecular science because of their potential applications in various fields such as pharmaceutical, catalysis, analytical chemistry, corrosion, and photochromic. Specifically, (1) Schiff bases have antibacterial, bactericidal, antitumor, antiviral biological activities^[1-3]; (2) metalcomplexes containing Schiff bases have been used as catalysts^[4-7]; (3) Schiff bases could be exploited as suitable ligands to identify metal ions and then quantitatively analyze metal ionization^[8-11]; (4) aromatic Schiff bases have been applied as corrosion inhibitors for steel^[12-14]; (5) several kinds of Schiff bases have shown their advantages in the photochromic field^[15-17]. Needless to say, photochromic materials have been applied in many fields such as chemo- and biosensing applications^[18-21], photoelectric devices^[22-23], information storag^[24-25], molecular logic switches^[26-29], ion recognition^[30], molecular self-assembly^[31-32], drug release^[33-37], and super-resolution imaging^[38-39].

  Progress has been made in the past using Schiff base unit to obtain functional materials but usually, those synthetic methods are limited to the multistep reaction, i.e., the condensation between aldehydes and amines should be executed firstly, and the target molecule could be achieved subsequently. Often overlooked though is the impressive ability of in situ ligand reaction to assemble the final product in a feasible way^[40-45].

  In this work, through solvothermal process, 2-(3′, 3′-dimethyl-6-nitrospiro(chromene-2, 2′-indolin)-1′-yl) ethanol reacted with ethylenediamine or 1, 3-propanediamine in situ to form two Schiff base ligands: bis(2 - (methyliminomethyl)-4-nitro-phenol) dianion (L₁)/2-((3- amino-propylimino)-methyl)-4-nitro-phenol monoanion (L₂) (Scheme 1), and two complexes [CuL₁] (1) and [Cu(L₂)(1, 3-DAP)]NO₃ (2) have been obtained.

  Scheme 1

  

  Scheme 1.  Structures of Schiff base ligands

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  1.   Experimental

  1.1   Materials and measurement

  All the chemicals were purchased and used without further purification. Powder X-ray diffraction (PXRD) pattern was collected on a DX-2600 X-ray diffractometer with Mo Kα radiation (λ=0.071 073 nm) at 293 K (U =40 kV, I=30 mA, 2θ =0° - 60°). The IR spectra were obtained on a Varian 640 FT/IR spectrometer with KBr pellet in 4 000-400 cm^-1 region. The UV-Vis absorption spectra were obtained on a TU-1901 spectrometer.

  1.2   Synthesis of [Cu(L₁)] (1)

  Through a one-pot strategy, the product has been prepared in a convenient way (Scheme 2). 2 - (3′, 3′ - dimethyl-6-nitrospiro[chromene-2, 2′-indolin]-1′-yl)ethanol (0.1 mmol), Cu(NO₃)₂·3H₂O (0.3 mmol), n-propanol (NPA, 9 mL), N, N - dimethylformamide (DMF, 3 mL) and ethylenediamine (100 µL) were mixed and stirred for 2 h. The solution in the screw-capped vial was heated under 80 ℃ for 3 d. After cooling to room temperature, the product was washed with water and dried in a vacuum. Yield: 72.0% (based on Cu). Elemental analysis Calcd. for C₁₆H₁₂CuN₄O₆(%): C, 45.77; H, 2.88; N, 13.34. Found(%): C, 44.98; H, 2.79; N, 13.65.

  Scheme 2

  

  Scheme 2.  Synthetic route of complex 1

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  In the spiropyran molecule, the indoline ring and the benzopyran ring are connected by a central spiro carbon atom, and the structure is orthogonal but not conjugated^[46]. Under a mild basic condition in presence of ethylenediamine, in situ ligand reaction is achieved, and complex [Cu(L)] could be formed.

  1.3   Synthesis of [Cu(L₂)(1, 3-DAP)]NO₃ (2)

  As show in Scheme 3, 2-(3′, 3′-dimethyl-6-nitrospiro(chromene-2, 2′-indolin)-1′-yl)ethanol (0.1 mmol), Cu(NO₃)₂·3H₂O (0.3 mmol), NPA (3 mL), DMF (1 mL), and 1, 3 - diaminopropane (1, 3 - DAP, 200 µL) were mixed and stirred for 30 min. The solution in the screwcapped vial was heated under 80 ℃ for 3 d. After cooling to room temperature, green cubic crystals were washed with distilled water and dried in a vacuum. Yield: 68.0% (based on Cu). Elemental analysis Calcd. for C ₁₃H₂₂CuN₆O₆(%): C, 37.01; H, 5.26; N, 19.92. Found(%): C, 36.76; H, 5.15; N, 19.68.

  Scheme 3

  

  Scheme 3.  Synthetic route of complex 2

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  1.4   X-ray crystallography

  Suitable crystals of complexes 1 and 2 were selected and the crystal data were collected on an Oxford Diffraction Gemini R Ultra diffractometer with graphitemonochromated Cu Kα (1: λ=0.154 184 nm) and Mo Kα (2: λ=0.071 073 nm) at 293(2) K. The structures were solved by direct methods and refined on F² by fullmatrix least - squares methods using the SHELXTL package. A summary of the crystal data of the two complexes is provided in Table 1.

  Table 1

  

  Table 1.  Crystal data and structure refinements for complexes 1 and 2

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  Parameter	1	2
  Empirical formula	C16H12CuN4O6	C13H22CuN6O6
  Formula weight	419.84	421.90
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	0.788 92(4)	0.721 36(5)
  b / nm	0.953 46(6)	1.048 78(7)
  c / nm	1.142 50(9)	1.200 46(7)
  α/(°)	111.563(7)	85.322(5)
  β(°)	91.554(5)	76.661(5)
  γ/(°)	98.858(5)	82.513(6)
  Volume / nm3	0.786 43(10)	0.874 92(10)
  Z	2	2
  Dc / (g·cm-3)	1.773	1.601
  μ/mm-1	2.399	1.293
  F(000)	426	438
  Reflection collected	4 888	7 125
  Independent reflection	3 027	4 014
  Data, restraint, parameter	3 027, 0, 244	4 014, 0, 259
  Goodness-of-fit on F2	1.113	1.040
  Final R indexes [I > 2σ(I)]*	R1=0.033 0, ωR2=0.082 3	R1=0.041 5, ωR2=0.088 1
  Final R indexes (all data)*	R1=0.040 6, ωR2=0.107 9	R1=0.055 6, ωR2=0.095 0
  $*{R_1} = \sum \left\| {{F_{\rm{o}}}\left| - \right|{F_{\rm{c}}}} \right\|/\sum \left| {{F_{\rm{o}}}} \right|, w{R_2} = \sum \left[ {w{{\left( {{F_{\rm{o}}}^2 - {F_{\rm{c}}}^2} \right)}^2}} \right]/\sum {\left[ {w{{\left( {{F_{\rm{o}}}^2} \right)}^2}} \right]^{1/2}}.$

  CCDC: 2053028, 1; 2104827, 2.

  1.5   Experiment of photocatalytic degradation

  Complex 1 (8 mg) was added to an aqueous solution (40 mL) of pararosaniline hydrochloride (PH, c=10 mmol·L^-1) or methylene blue (MB, c=10 mmol·L^-1) and stirred for 30 min in the dark to ensure the adsorptiondesorption equilibrium of the resulting solution. Then the solution was exposed to UV irradiation from a 100 W Hg lamp (λ=365 nm) and it was kept for stirring during the irradiation. At every 30 min interval, 4 mL solution was taken out for the UV-Vis measurement.

  Complex 2 (20 mg) was added to an aqueous solution (40 mL) of methylene blue (MB, c=12 mmol· L^-1) and stirred for 30 min in the dark to ensure the adsorption - desorption equilibrium. Then the solution was exposed to a 100 W Hg lamp (λ =365 nm) and it was kept for stirring during the irradiation. At every 30 min interval, 4 mL solution was taken out for the UV-Vis measurement.

  2.   Results and discussion

  2.1   Structural analysis of 1

  Complex 1 crystallizes in the triclinic crystal system with space group P1. The complex is composed of a Cu(Ⅱ) ion and a Schiff base ligand L₁. As shown in Fig. 1, N1 and N2 atoms, O1 and O2 atoms are coordinated with the central ion, to form a four- coordinated structure, in which the Schiff base unit is achieved through the in situ ligand reaction (Fig. 1a). Aided with those C—H…O hydrogen bondings, a 1D supramolecular chain could be observed (Fig. 1b). Moreover, considering interchain hydrogen bondings, each [Cu(L₁)] molecule is connected with five [Cu(L₁)] molecules (Fig. 1c) to form a 3D network and it could be simplified to 5 - connected bnn topology (Fig. 1d).

  Figure 1

  

  Figure 1.  Crystal structure of 1: (a) coordination environment of Cu(Ⅱ) ion; (b) 1D supramolecular chain formed by hydrogen bonding; (c) each [Cu(L₁)] connecting with five [Cu(L₁)] molecules; (d) diagram of bnn topology

  Symmetry codes: ⁱ-1+x, -1+y, -1+z; ⁱⁱ -2+x, -2+y, -2+z

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  2.2   Structural analysis of 2

  Complex 2 crystallizes in the triclinic crystal system with space group P1. This complex is composed of a Cu(Ⅱ) ion, one 1, 3-DAP, and a Schiff base ligand L ₂. As shown in Fig. 2a, four N atoms and one O atom are coordinated with the Cu²⁺ to form a five-coordinated tetragonal pyramid configuration [CuN₄O], and the NO₃^- acts as the counter anion. Noticeably, the bond length of Cu1—N1, Cu1—N2, Cu1—N4, and Cu1—O1 are 0.200 1(2), 0.204 7(2), 0.202 1(3), and 0.197 77(18) nm, respectively, whereas the bond length of Cu1—N3 is relatively long (0.219 7(3) nm), indicating the John- Teller effect of Cu²⁺ ion. Through hydrogen bonding interactions (Fig. 2b), each [Cu(L₂)(1, 3-DAP)]NO₃ molecule is interacted with four neighboring molecules (Fig. 2c) to achieve 3D supramolecular network, and it could be simplified to 4-connected bnn topology (Fig. 2d).

  Figure 2

  

  Figure 2.  Crystal structure of 2: (a) asymmetric unit; (b) hydrogen bonding interactions; (c) each [CuL₂(1, 3-DAP)]NO₃ interacting with four neighbouring molecules; (d) diagram of simplified bnn topology

  Symmetry codes: ⁱ-x, 2-y, 2-z; ⁱⁱ 1-x, 1-y, 1-z; ⁱⁱⁱ-x, 1-y, 2-z; ^iv-x, 1-y, 1-z

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  2.3   IR spectra of complexes 1 and 2

  As for 1 (Fig. 3a), the weak absorption peak at 2 923 cm^-1 is assigned to the C—H stretching vibration. A strong band at 1 643 cm^-1 can be attributed to the stretching vibration of the —CH=N of Schiff base unit. Those peaks at 1 596 and 1 303 cm^-1 can be assigned to the —NO₂ group. In addition, the peak at 902 cm^-1 is the characteristic absorption peak of =CH— of the phenyl ring, while a peak at 694 cm^-1 arises from the para-substituted characteristic peak of the phenyl ring.

  Figure 3

  

  Figure 3.  IR spectra of complexes 1 (a) and 2 (b)

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  As for 2 (Fig. 3b), the weak absorption peak at 2 935 cm^-1 is assigned to the C—H stretching vibration. A strong band at 1 634 cm^-1 can be attributed to the stretching vibration of the NH₂ group. Those peaks at 1 547 and 1 310 cm^-1 can be assigned to the —NO₂ group. In addition, the peak at 902 cm^-1 is the characteristic absorption peak of =CH— of the phenyl ring, while a peak at 699 cm^-1 arises from the para-substituted characteristic peak of the phenyl ring.

  2.4   PXRD of complexes 1 and 2

  As shown in Fig. 4, the simulated PXRD patterns and the experimental results of 1 and 2 are consistent in the main locations, demonstrating the single- phase purity of those bulk samples.

  Figure 4

  

  Figure 4.  PXRD patterns of complexes 1 (a) and 2 (b)

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  2.5   Photocatalytic degradation analysis

  Fig. 5a and 5b show the changes in UV-Vis spectra of PH and MB solution photocatalytically degraded by complex 1, respectively. The degradation rates of PH and MB were calculated to be 35.9% and 34.6%, respectively. There was no detectable photocatalytic degradation activity of 2 to PH. Nevertheless, complex 2 could degrade MB in a certain way (Fig. 5c). The degradation rate of MB was 24.3%.

  Figure 5

  

  Figure 5.  Changes in UV-Vis spectra of PH (a) and MB (b) solution photocatalytically degraded by complex 1; Changes in UV-Vis spectra of MB (c) solution photocatalytically degraded by complex 2

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  3.   Conclusions

  In summary, two complexes containing Schiffbase ligand have been reported. Photocatalytic experiments with pararosaniline hydrochloride and methylene blue have shown those two complexes had certain activity for dye degradation. Notably, the key step in the assembly process is the in situ ligand reaction. Through it, new complexes have been observed and it is anticipated that various Schiff base ligands could be achieved. With elevated temperatures and pressures under hydrothermal/solvothermal experimental conditions, in situ ligand reaction could act as the bridge between coordination chemistry and organic synthetic chemistry because it could harvest new molecules that are inaccessible under mild experimental conditions. To fully utilize the power of in situ ligand reaction and expand the scope of multifunctional complexes, the system of spiropyran and different metal ions in presence of various diamine molecules (including chiral molecules) are actively pursued in the laboratory.

  
  
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      Baldrighi M, Locatelli G, Desper J, Aakeroy C B, Giordani S. Probing Metal Ion Complexation of Ligands with Multiple Metal Binding Sites: The Case of Spiropyrans[J]. Chem. Eur. J., 2016, 22(39):  13976-13984.

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  Scheme 1  Structures of Schiff base ligands

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  Scheme 2  Synthetic route of complex 1

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  Scheme 3  Synthetic route of complex 2

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  Figure 1  Crystal structure of 1: (a) coordination environment of Cu(Ⅱ) ion; (b) 1D supramolecular chain formed by hydrogen bonding; (c) each [Cu(L₁)] connecting with five [Cu(L₁)] molecules; (d) diagram of bnn topology

  Symmetry codes: ⁱ-1+x, -1+y, -1+z; ⁱⁱ -2+x, -2+y, -2+z

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  Figure 2  Crystal structure of 2: (a) asymmetric unit; (b) hydrogen bonding interactions; (c) each [CuL₂(1, 3-DAP)]NO₃ interacting with four neighbouring molecules; (d) diagram of simplified bnn topology

  Symmetry codes: ⁱ-x, 2-y, 2-z; ⁱⁱ 1-x, 1-y, 1-z; ⁱⁱⁱ-x, 1-y, 2-z; ^iv-x, 1-y, 1-z

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  Figure 3  IR spectra of complexes 1 (a) and 2 (b)

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  Figure 4  PXRD patterns of complexes 1 (a) and 2 (b)

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  Figure 5  Changes in UV-Vis spectra of PH (a) and MB (b) solution photocatalytically degraded by complex 1; Changes in UV-Vis spectra of MB (c) solution photocatalytically degraded by complex 2

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  Table 1.  Crystal data and structure refinements for complexes 1 and 2

  Parameter	1	2
  Empirical formula	C16H12CuN4O6	C13H22CuN6O6
  Formula weight	419.84	421.90
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	0.788 92(4)	0.721 36(5)
  b / nm	0.953 46(6)	1.048 78(7)
  c / nm	1.142 50(9)	1.200 46(7)
  α/(°)	111.563(7)	85.322(5)
  β(°)	91.554(5)	76.661(5)
  γ/(°)	98.858(5)	82.513(6)
  Volume / nm3	0.786 43(10)	0.874 92(10)
  Z	2	2
  Dc / (g·cm-3)	1.773	1.601
  μ/mm-1	2.399	1.293
  F(000)	426	438
  Reflection collected	4 888	7 125
  Independent reflection	3 027	4 014
  Data, restraint, parameter	3 027, 0, 244	4 014, 0, 259
  Goodness-of-fit on F2	1.113	1.040
  Final R indexes [I > 2σ(I)]*	R1=0.033 0, ωR2=0.082 3	R1=0.041 5, ωR2=0.088 1
  Final R indexes (all data)*	R1=0.040 6, ωR2=0.107 9	R1=0.055 6, ωR2=0.095 0
  $*{R_1} = \sum \left\| {{F_{\rm{o}}}\left| - \right|{F_{\rm{c}}}} \right\|/\sum \left| {{F_{\rm{o}}}} \right|, w{R_2} = \sum \left[ {w{{\left( {{F_{\rm{o}}}^2 - {F_{\rm{c}}}^2} \right)}^2}} \right]/\sum {\left[ {w{{\left( {{F_{\rm{o}}}^2} \right)}^2}} \right]^{1/2}}.$

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