Synthesis, Crystal Structures and Catalytic Activities of Two Copper Coordination Compounds Bearing an N,N’-Dibenzylethylenediamine Ligand

Two copper coordination compounds bearing an N,N’-dibenzylethylenediamine ligand, namely [Cu3L(CH3COO)6]n (1) and [(CuCl4)·(C6H5CH2NH2CH2)2] (2) (L = N,N’-dibenzylethylenediamine) were synthesized by the ethanol refluxing method. Powder X-ray diffraction (PXRD), infrared spectra (IR), elemental analyses, and single crystal X-ray diffraction were used to characterize and verify their structures. Structural analyses showed that the asymmetric unit of compound (1), composed of two Cu(II) cations, three acetate anions, and half of the ligand, was bridged by one acetate to obtain an infinite 1D chain structure. The analyses further showed that the asymmetric unit of compound (2), composed of two crystallographically independent [C6H5CH2NH2CH2] units, four chloride anions, and one central Cu(II) cation is connected into an infinite 2D network structure by the hydrogen bonding interactions. The copper compounds were used to catalyze the decomposition of H2O2, and the results showed that both of the compounds exhibited excellent catalytic activities under optimized conditions.


Introduction
Among transition metal coordination compounds, copper coordination compounds occupy a vital position in organometallic chemistry due to their abundant coordination structures and potential applications in various fields [1][2][3][4][5]. In recent years, the catalytic activities of copper coordination compounds with various organic ligands have attracted extensive attention from chemists [6][7][8][9]. To develop novel and efficient catalysts, copper ions coordinated to a suitable ligand with multiple coordination sites and strong coordination abilities have been assembled into different copper coordination compounds [10,11]. Diamine and its derivatives are an important class of organic compounds with highly flexible N,N-donor atoms that can coordinate to different metal ions, thus producing a variety of mononuclear [12][13][14], binuclear [15][16][17], and polynuclear metal coordination compounds with excellent catalytic activities [18][19][20].
Hydrogen peroxide is a kind of oxidant [21], bleaching agent [22], and disinfectant [23] that is widely used in pharmaceutical, textile and chemical industries. [24][25][26]. As a result of the excessive use of H 2 O 2 , local soil and water resources have become severely polluted [27]. Therefore, it is particularly important to develop a practical catalytic system for the decomposition of H 2 O 2 . Presently, metal oxides, metal salts, and some coordination compounds have been used to catalyze the decomposition reaction of H 2 O 2 , and considerable results have been obtained [28][29][30]. In order to better catalyze the decomposition of H 2 O 2 , practical and efficient catalytic systems have become a research hotspot for chemical workers [31].
Therefore, the challenge we faced was to determine how to obtain coordination compounds with strong catalytic activities and low environmental pollution potential. With this purpose, we designed 2.2. Synthesis of [Cu 3 L(CH 3 COO) 6 ] n (1) An ethanol (30 mL) solution of Cu(OAc) 2 ·H 2 O (1.27 g, 2.12 mmol) was added to a 100 mL round-bottom flask with an ethanol (10 mL) solution of N,N'-dibenzylethylenediamine (0.50 mL, 2.12 mmol). The reaction solution was stirred and heated at 78 • C for 24 h and the solution color gradually changed from light blue to deep blue. The reaction solvent was evaporated by vacuum distillation and the residue was carefully washed with n-hexane (2 × 5 mL). The resulting solid was extracted with a co-solvent of dichloromethane and ethanol (8 mL, V dichloromethane /V ethanol = 1/1). The blue crystalline solid of compound (1)  An ethanol (30 mL) solution of CuCl 2 ·2H 2 O (0.72 g, 4.24 mmol) was added to a 100 mL round-bottom flask with an ethanol (10 mL) solution of N,N'-dibenzylethylenediamine (1.00 mL, 4.24 mmol). After being added to two drops of diluted hydrochloric acid under stirring, the reaction solution was stirred and heated at 78 • C for 24 h; as a result, the solution color gradually changed from light blue to deep blue. The reaction solvent was evaporated by vacuum distillation and the residue was carefully washed with n-hexane (2 × 5 mL). The resulting solid was extracted with a co-solvent of n-hexane and methanol (12 mL, V n-hexane /V methanol = 1/2). The blue crystalline solid of compound (2)

X-ray Crystallography
Single crystals with dimensions of 0.10 × 0.12 × 0.14 mm for compound (1) and 0.22 × 0.25 × 0.27 mm for compound (2) were selected for single-crystal X-ray diffraction analyses on a Bruker APEX-II CCD diffractometer (Bruker, Karlsruhe, Germany). The diffraction data were collected by graphite monochromated Mo-Ka radiation (λ = 0.71073 Å) at 296(2) K. The empirical absorption corrections were performed using the SADABS program. The crystal structures of compounds (1) and (2) were solved by direct methods using the SHELXT program [32] and were refined by full-matrix least-squares on F 2 with the SHELXL 2018/3 program [33]. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms were placed in the geometrically idealized positions. The crystallographic data for the compounds are given in Table 1. The selected bond lengths (Å) and angles ( • ) of the compounds are summarized in Table S1 in Supplementary Materials.  (1) and (2).

Decomposition Reaction of Hydrogen Peroxide
A synthesized compound (1.0 mmol) was added to a 50 mL round-bottom flask with N,N-dimethy lformamide (DMF) (5 mL). An aqueous solution of H 2 O 2 (10 mL, 30 wt %) was diluted 2 times with distilled water (10 mL) and carefully injected into the round-bottom flask. The decomposition reaction proceeded at room temperature for 24 h and the solution gradually became slightly turbid. The reaction process was monitored by hydrogen peroxide test paper. In order to measure the decomposition percentage of H 2 O 2 in the presence of the catalyst, the residual H 2 O 2 concentration was titrated by using a standard solution of KMnO 4 [34].
The nonbonding Cu1…Cu1(ii) distance is 2.6274(16) Å, which is in agreement with those of some similar copper compounds [35,36]. The Cu2 cation is four-coordinated by two nitrogen atoms (N1 and N1(i)) from the N,N'-dibenzylethylenediamine ligand in an η 2 mode and two oxygen atoms (O4 and O4(i)) from two bridging acetates in η 1 mode. This results in a square-planar CuN2O2 coordination geometry. The average bond length of Cu2-O is 1.976 (5) and that of Cu2-N is 2.007(5) Å, both of which correspond with the reported values [37,38]. The sums of the bond angles N1-Cu2- (2)°, and O4-Cu2-N1 of 94.7(2)° when added up equal 368°, which is unequal to 360°, which confirms that the four-coordinated atoms and Cu2 cation are in an imperfect plane. As shown in Figure 2, the CuN2O2 unit and the Cu2(CO2)4 unit are bridged by one acetate in a μ2−η 1 :η 1 -bridging coordination mode, forming an infinite 1D chain along the a axis. In addition, there are weak intramolecular hydrogen bonds between the ligand and the acetate (Table S2). The nitrogen atom (N1) from the ligand acts as an H-donor to the oxygen atom (O5) from one acetate, forming the intramolecular hydrogen bond N1-H1…O5, which further stabilizes the 1D chain structure.   As shown in Figure 2, the CuN 2 O 2 unit and the Cu 2 (CO 2 ) 4 unit are bridged by one acetate in a µ 2 −η 1 :η 1 -bridging coordination mode, forming an infinite 1D chain along the a axis. In addition, there are weak intramolecular hydrogen bonds between the ligand and the acetate (Table S2). The nitrogen atom (N1) from the ligand acts as an H-donor to the oxygen atom (O5) from one acetate, forming the intramolecular hydrogen bond N1-H1 . . . O5, which further stabilizes the 1D chain structure.
The nonbonding Cu1…Cu1(ii) distance is 2.6274(16) Å, which is in agreement with those of some similar copper compounds [35,36]. The Cu2 cation is four-coordinated by two nitrogen atoms (N1 and N1(i)) from the N,N'-dibenzylethylenediamine ligand in an η 2 mode and two oxygen atoms (O4 and O4(i)) from two bridging acetates in η 1 mode. This results in a square-planar CuN2O2 coordination geometry. The average bond length of Cu2-O is 1.976 (5) and that of Cu2-N is 2.007(5) Å, both of which correspond with the reported values [37,38].  As shown in Figure 2, the CuN2O2 unit and the Cu2(CO2)4 unit are bridged by one acetate in a μ2−η 1 :η 1 -bridging coordination mode, forming an infinite 1D chain along the a axis. In addition, there are weak intramolecular hydrogen bonds between the ligand and the acetate (Table S2). The nitrogen atom (N1) from the ligand acts as an H-donor to the oxygen atom (O5) from one acetate, forming the intramolecular hydrogen bond N1-H1…O5, which further stabilizes the 1D chain structure.     Moreover, there is an abundance of intermolecular and intramolecular hydrogen bonds in compound (2) ( Table S2) that benefit the stability of the crystal structure. As shown in Figure 4, the nitrogen atoms from the N,N'-dibenzylethylenediamine ligand act as an H-donor to the chlorine atoms from [CuCl4] 2-unit, forming the hydrogen bonds N1-H1A…Cl1(iii) of 2.37 Å, N1-H1B…Cl4(iv) of 2.51 Å, and N2-H2B…Cl3(v) of 2.37 Å. The intermolecular hydrogen bonds play an important role in constructing the stable 1D chain structure of compound (2). Another noticeable characteristic of compound (2) is how the intermolecular hydrogen bonding interactions exist among 1D chains, such as N2-H2A…Cl1(iv) of 2.81 Å and C16-H16B…Cl4(v) of 2.82 Å. Under the weak hydrogen bonding interactions, the 1D chain structure is further connected and extended, resulting in an infinite 2D network structure in Figure 5.  Moreover, there is an abundance of intermolecular and intramolecular hydrogen bonds in compound (2) ( Table S2) that benefit the stability of the crystal structure. As shown in Figure 4  Moreover, there is an abundance of intermolecular and intramolecular hydrogen bonds in compound (2) ( Table S2) that benefit the stability of the crystal structure. As shown in Figure 4, the nitrogen atoms from the N,N'-dibenzylethylenediamine ligand act as an H-donor to the chlorine atoms from [CuCl4] 2-unit, forming the hydrogen bonds N1-H1A…Cl1(iii) of 2.37 Å, N1-H1B…Cl4(iv) of 2.51 Å, and N2-H2B…Cl3(v) of 2.37 Å. The intermolecular hydrogen bonds play an important role in constructing the stable 1D chain structure of compound (2). Another noticeable characteristic of compound (2) is how the intermolecular hydrogen bonding interactions exist among 1D chains, such as N2-H2A…Cl1(iv) of 2.81 Å and C16-H16B…Cl4(v) of 2.82 Å. Under the weak hydrogen bonding interactions, the 1D chain structure is further connected and extended, resulting in an infinite 2D network structure in Figure 5.

PXRD Analysis
To verify the phase purities of the crystal samples, PXRD experiments were further carried out on compounds (1) and (2) at room temperature. The simulated and experimental PXRD patterns for compounds (1) and (2) are presented in Figures S1 and S2, respectively. The peaks in the experimental curves matched with those in the simulated curves generated from the single crystal X-ray data, thus confirming that the phase purities of the synthesized compounds are correct. The reflection intensities of the experimental patterns were not in agreement with the corresponding simulated patterns; this may be attributed to the different orientations of the crystal samples during the test. Additionally, the elemental analyses of compounds (1) and (2)

PXRD Analysis
To verify the phase purities of the crystal samples, PXRD experiments were further carried out on compounds (1) and (2) at room temperature. The simulated and experimental PXRD patterns for compounds (1) and (2) are presented in Figures S1 and S2, respectively. The peaks in the experimental curves matched with those in the simulated curves generated from the single crystal X-ray data, thus confirming that the phase purities of the synthesized compounds are correct. The reflection intensities of the experimental patterns were not in agreement with the corresponding simulated patterns; this may be attributed to the different orientations of the crystal samples during the test. Additionally, the elemental analyses of compounds (1) and (2) support the results of the PXRD experiments.

Catalytic Activities of the Compounds
Hydrogen peroxide, which has severely polluted local soil and water sources because of its extensive use in production and life, could instead be efficiently decomposed under the catalysis of metal compounds [28][29][30]. To optimize the favorable reaction conditions, such as the pH value and catalyst loading, the decomposition reaction of H2O2 catalyzed by the synthesized compounds was carried out under different conditions. The results are given in Tables 2 and 3.
Firstly, the decomposition reaction of H2O2 with compound (1) as a catalyst with the pH value of 8 was used as a model reaction to evaluate the effect of the catalyst loading on this reaction. When the catalyst loading was 1.0 mmol, the decomposition rate of H2O2 reached 94% (Entry 3). Although the decomposition reaction may efficiently occur under the catalysis of compound (1) with a catalyst loading range between 1.0 and 1.6 mmol, the decomposition rate remained almost unchanged (Entries 3-5), which implied that an excessive amount of catalyst could not effectively improve the decomposition rate. When the catalyst loading was reduced from 1.0 to 0.4 mmol, the decomposition percentage decreased significantly from 94% to 39% (Entries 1-3). The experimental data revealed that H2O2 could not be fully decomposed with the catalyst loading below 1.0 mmol. In comparison with compound (1), compound (2) exhibited stronger catalytic activities in catalyzing the decomposition of H2O2. Surprisingly, when the loading of compound (2) was 1.0 mmol, the decomposition percentage reached nearly 100% (i.e., 99%) (Entry 8). In addition, regardless of the loading of compound (2) was increased or decreased, the decomposition percentage of H2O2 had the same rule as compound (1) (Entries 6 and 7, 9 and 10). Therefore, the most suitable catalyst loading was 1.0 mmol, and thus was be selected for the following studies. The catalytic activities of the N,N'dibenzylethylenediamine ligand were also explored. As we expected, the residual concentration of H2O2 remained almost constant (Entry 11), which confirmed that the ligand had no catalytic activities

Catalytic Activities of the Compounds
Hydrogen peroxide, which has severely polluted local soil and water sources because of its extensive use in production and life, could instead be efficiently decomposed under the catalysis of metal compounds [28][29][30]. To optimize the favorable reaction conditions, such as the pH value and catalyst loading, the decomposition reaction of H 2 O 2 catalyzed by the synthesized compounds was carried out under different conditions. The results are given in Tables 2 and 3.
Firstly, the decomposition reaction of H 2 O 2 with compound (1) as a catalyst with the pH value of 8 was used as a model reaction to evaluate the effect of the catalyst loading on this reaction. When the catalyst loading was 1.0 mmol, the decomposition rate of H 2 O 2 reached 94% (Entry 3). Although the decomposition reaction may efficiently occur under the catalysis of compound (1) with a catalyst loading range between 1.0 and 1.6 mmol, the decomposition rate remained almost unchanged (Entries 3-5), which implied that an excessive amount of catalyst could not effectively improve the decomposition rate. When the catalyst loading was reduced from 1.0 to 0.4 mmol, the decomposition percentage decreased significantly from 94 to 39% (Entries 1-3). The experimental data revealed that H 2 O 2 could not be fully decomposed with the catalyst loading below 1.0 mmol. In comparison with compound (1), compound (2) exhibited stronger catalytic activities in catalyzing the decomposition of H 2 O 2. Surprisingly, when the loading of compound (2) was 1.0 mmol, the decomposition percentage reached nearly 100% (i.e., 99%) (Entry 8). In addition, regardless of the loading of compound (2) was increased or decreased, the decomposition percentage of H 2 O 2 had the same rule as compound (1) (Entries 6 and 7, 9 and 10). Therefore, the most suitable catalyst loading was 1.0 mmol, and thus was be selected for the following studies. The catalytic activities of the N,N'-dibenzylethylenediamine ligand were also explored. As we expected, the residual concentration of H 2 O 2 remained almost constant (Entry 11), which confirmed that the ligand had no catalytic activities in the decomposition reaction. According to the related literature [39], CuCl 2 ·2H 2 O and Cu(OAc) 2 ·H 2 O are also excellent catalysts in the decomposition of H 2 O 2 . We found that the decomposition percentages of 84% and 77% were obtained with a catalyst loading of 1.0 mmol, respectively (Entries 12 and 13). The copper metal contents of CuCl 2 ·2H 2 O and Cu(OAc) 2 ·H 2 O were higher than those of the synthesized compounds, and thus may cause more serious environmental damage. The catalytic activities of (NH 4 ) 2 CuCl 4 ·2H 2 O were also surveyed (Entry 14), and from this, we found that the decomposition percentage of 81%, which is lower than the 99% of compound (2), proved that the ligand unit in this compound also plays an important role in the catalytic process. To study the effect of pH levels on the rate of the decomposition of H 2 O 2 , we tested the catalytic activities of the coordination compounds with a catalyst loading of 1.0 mmol under different pH conditions. We found that the pH value of the solution had a considerable influence on the catalytic activities of the coordination compounds. Under acidic conditions, the catalytic activities of the coordination compounds were very poor (Entries 1 and 2, 5 and 6), but the catalytic activities strengthened significantly with the increase in pH value. When the solution was weakly alkaline, such as at pH 8, the decomposition percentages of H 2 O 2 reached 94% and 99%,respectively (Entries 4 and 10). The experimental results showed that when the pH value of the solution was higher than 9, the decomposition rate of H 2 O 2 decreased significantly; this may be due to the side reactions between the compounds and the hydroxyl ion. We concluded that the weak alkaline solution with the pH value of 8 is suitable for the decomposition of H 2 O 2 . Considering the decomposition rate and the concepts of green chemistry, compound (2) rather than compound (1) and the copper salts was found to be a more suitable catalyst with a catalyst loading of 1.0 mmol and a pH value of 8 for the decomposition reaction of H 2 O 2.

Conclusions
In this study, two copper coordination compounds bearing an N,N'-dibenzylethylenediamine ligand were synthesized. The crystal structure of compound (1) revealed that the Cu1 cation, as well as the Cu1(ii) cation, lies in an almost perfect square pyramidal CuO 5 coordination environment and that the Cu2 cation lies in a distorted square planar CuN 2 O 2 coordination geometry. Both the CuN 2 O 2 unit and the Cu 2 (CO 2 ) 4 unit were bridged to generate an infinite 1D chain structure by the bidentate acetate. The crystal structure of compound (2) revealed that the Cu1(II) cation forms a distorted tetrahedral [CuCl 4 ] 2− coordination environment. Two [C 6 H 5 CH 2 NH 2 CH 2 ] + units and the [CuCl 4 ] 2− anion constituted the asymmetric unit of compound (2), which is further connected to an infinite 2D network structure via the hydrogen bonding interactions. In addition, examination on their catalytic activities indicated that compounds (1) and (2) with a catalyst loading of 1.0 mmol and a pH value of 8 could efficiently catalyze the decomposition of H 2 O 2 . Considering the decomposition rate and the concepts of green chemistry, compound (2) is a more suitable catalyst for the decomposition of H 2 O 2 than compound (1) .