crystals Article Synthesis, Crystal Structures and Catalytic Activities of Two Copper Coordination Compounds Bearing an N,N’-Dibenzylethylenediamine Ligand Chao Liu *, Weiwei Zhang and Gaigai Cai School of Chemistry and Chemical Engineering, Suzhou University, Suzhou 234000, China; [email protected] (W.W.Z.); [email protected] (G.G.C.) * Correspondence: [email protected]; Tel.: +86-557-2875018 Received: 25 May 2020; Accepted: 17 June 2020; Published: 21 June 2020 Abstract: Two copper coordination compounds bearing an N,N’-dibenzylethylenediamine ligand, namely [Cu L(CH COO) ]n (1) and [(CuCl ) (C H CH NH CH ) ](2) (L = N,N’-dibenzylethylenediamine) 3 3 6 4 · 6 5 2 2 2 2 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. Keywords: N,N’-dibenzylethylenediamine; crystal structure; catalyst; coordination compound 1. 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 [1–5]. In recent years, the catalytic activities of copper coordination compounds with various organic ligands have attracted extensive attention from chemists [6–9]. 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 [12–14], binuclear [15–17], and polynuclear metal coordination compounds with excellent catalytic activities [18–20]. Hydrogen peroxide is a kind of oxidant [21], bleaching agent [22], and disinfectant [23] that is widely used in pharmaceutical, textile and chemical industries. [24–26]. As a result of the excessive use of H2O2, 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 H2O2. Presently, metal oxides, metal salts, and some coordination compounds have been used to catalyze the decomposition reaction of H2O2, and considerable results have been obtained [28–30]. In order to better catalyze the decomposition of H2O2, 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 Crystals 2020, 10, 528; doi:10.3390/cryst10060528 www.mdpi.com/journal/crystals Crystals 2020, 10, 528 2 of 10 and synthesized two novel copper coordination compounds bearing an N,N’-dibenzylethylenediamine ligand. Simultaneously, the catalytic activities of the coordination compounds were investigated for the decomposition reaction of H2O2 under mild conditions. 2. Materials and Methods 2.1. General Considerations (NH ) CuCl 2H O and N,N’-dibenzylethylenediamine were obtained from Shanghai Aladdin 4 2 4· 2 Bio-Chem Technology Co., Ltd. in Shanghai, China. Other reagents, such as KMnO ,H O , Cu(OAc) H O, 4 2 2 2· 2 and CuCl 2H O, were obtained from commercial sources and were used as received. 2· 2 An FTS-40 spectrometer (Bio-Rad, Santa Clara, CA, USA) with a KBr pellet was used to collect the 1 FT-IR spectra of the compounds in the 400–4000 cm− range. A DX-2600 diffractometer (Dandong, Liaoning, China) with Cu-Kα radiation was employed to obtain powder X-ray diffractions at ambient temperature. A Vario Micro Cube elemental analyzer (Elementar, Frankfurt, Germany) was used to test the elemental analyses of compounds (1) and (2). 2.2. Synthesis of [Cu3L(CH3COO)6]n (1) An ethanol (30 mL) solution of Cu(OAc) H O (1.27 g, 2.12 mmol) was added to a 100 mL 2· 2 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, Vdichloromethane/Vethanol = 1/1). The blue crystalline solid of compound (1) was obtained upon standing the resulting solution for 48 h at room temperature in a 64% yield. Melting point: 215.0–216.5 ◦C. Anal. Calcd. for C28H38Cu3N2O12 1 (Formula weight = 785.22): C, 42.83; H, 4.88; N, 3.57%; Found: C, 42.64; H, 5.11; N, 3.79%. IR (KBr, cm− ): 3426(s), 3066(w), 2978(m), 2924(s), 2765(s), 2689(m), 2568(m), 2404(m), 1564(s), 1400(s), 1301(w), 1023(m), 908(s), 842(w), 799(m), 744(s), 695(m), 646(s), 504 (w). 2.3. Synthesis of [(CuCl ) (C H CH NH CH ) ] (2) 4 · 6 5 2 2 2 2 An ethanol (30 mL) solution of CuCl 2H O (0.72 g, 4.24 mmol) was added to a 100 mL round-bottom 2· 2 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, Vn-hexane/Vmethanol = 1/2). The blue crystalline solid of compound (2) was obtained upon standing the resulting solution for 24 h at room temperature in a 79% yield. Melting point: 189.5–190.5 ◦C. Anal. Calcd. for C16H22Cl4CuN2 (Formula weight = 447.69): C, 42.92; H, 4.95; N, 1 6.26%; Found: C, 43.21; H, 5.03; N, 6.06%. IR (KBr, cm− ): 3443(s), 3055(w), 2902(m), 2847(w), 2771(m), 2361(s), 2334(m), 1640(s), 1591(s), 1553(s), 1487(m), 1411(s), 1148(m), 1066 (m), 1023(m), 974(w), 842(w), 739(m), 701(s), 684(m), 631(w). 2.4. 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). Thediffractiondatawerecollectedbygraphitemonochromated 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 F2 with Crystals 2020, 10, 528 3 of 10 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 Table1. The selected bond lengths (Å) and angles ( ◦) of the compounds are summarized in Table S1 in Supplementary Materials. Table 1. Crystallographic data for compounds (1) and (2). Compound (1) (2) Formula C28H38Cu3N2O12 C16H22Cl4CuN2 Formula weight 785.22 447.69 Temperature 296(2) 296(2) Crystal system Monoclinic Triclinic Space group C2/c P¯ı a (Å) 15.903(13) 6.5935(9) b (Å) 16.403(15) 11.9953(18) c (Å) 15.679(12) 12.4736(17) α 90.00 84.199(4) β 96.06(2) 88.408(4) γ 90.00 77.972(4) Volume (Å3) 4067(6) 959.9(2) Z 4 2 3 Density (calculated)/(g cm− ) 1.283 1.549 1 · µ (mm− ) 1.605 1.694 F (000) 1612 458 Reflections collected 13,134 6888 Unique reflections (Rint) 4648 (0.064) 3346 (0.040) Gof 1.026 1.057 Final R indices [I > 2σ(I)] R1 = 0.0931, wR2 = 0.2708 R1 = 0.0300, wR2 = 0.0770 R indices (all data) R1 = 0.1098, wR2 = 0.2899 R1 = 0.0322, wR2 = 0.0786 Largest diff. peak and hole (e Å 3) 0.96 and 1.13 0.30 and 0.49 − − − 2.5. 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 H2O2 (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.