English

• 0.   Introduction

  As an inexpensive and earth-abundant metal, copper plays a central role in coordination chemistry. This advantage originates from the widespread application of copper complexes in life sciences^[1-5], catalytic reactions^[6-8], magnetism^[9-10], and photoluminescent materials^[11-15]. It is well known that ligand environments influence the properties of copper complexes^[16]. Therefore, much effort has been dedicated to designing ligands for adjusting the properties of the complexes of copper.

  Imidazo[1,5-a]pyridine has recently attracted some attention. It is an important scaffold found in a range of natural and pharmaceutical products^[17-18]. Heterocycles containing imidazo[1,5-a]pyridine moiety have a myriad of applications in organic materials, e.g., OLEDs^[19], organic thin-layer field-effect transistors (FETs)^[20], and organic electronics^[21]. Imidazo[1,5-a]pyridine and its derivatives also act as nitrogen-containing ligands for preparing complexes^[22-30]. In particular, a variety of substituents can be introduced at the 1- or 3-position of imidazo[1,5-a]pyridine, leading to the formation of a series of targeting imidazo[1,5-a]pyridine-based ligands^[31-34]. Among these compounds, 3-(pyridin-2-yl)-imidazo[1,5-a]pyridine (L1) has emerged as an attractive ligand due to its ability to serve as a bidentate nitrogen donor ligand. There are several complexes, including Ni(Ⅱ)^[35], Zn(Ⅱ)^[36], V(Ⅴ)^[37], Mn(Ⅱ)^[38], Re(Ⅰ)^[39-40], and Ir(Ⅲ)^[41], that use L1 as ligands. However, complexes of Cu(Ⅱ) or Cu(Ⅰ) ions that employ L1 and its derivatives as ligands are still very rare^[42-43].

  Considering that the complexes of copper are very important in many fields, in the present work, we employed L1 and its derivative 1-iodo-3-(pyridin-2-yl)imidazo[1,5-a]pyridine (L2) as ligands to obtain differently structured Cu(Ⅱ) and Cu(Ⅰ) complexes. Five complexes, [Cu₂(L1)₂Cl₄]·2CH₂Cl₂ (1·2CH₂Cl₂), [Cu(L1)₂Br][CuBr₂] (2), [Cu(L1)₂]ClO₄ (3), [Cu₂(L2)₂I₂] (4), and [Cu(L2)₂(ClO₄)]ClO₄ (5), were prepared. In particular, in situ cleavage of the C—I bond under solvothermal conditions occurred during the formation of 4. Moreover, by adding different aldehydes to a reaction system of CuCl₂·2H₂O and the L1 ligand, in situ metal-ligand reactions occurred, and complexes [Cu(L3)Cl][CuCl₂] (6), [Cu(L4)Cl][Cu₂(L1)₂Cl₄]_0.5[CuCl₂] (7), and [Cu(L5)Cl]Cl·EtOH (8·EtOH) (L3=1, 1′-(phenylmethylene)bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridine}, L4=1, 1′-{[4-(diethoxymethyl)phenyl] methylene}bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridine}, L5=4-(bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl}methyl)-2-methoxy-phenol) were obtained. The catalytic properties of 1·2CH₂Cl₂, 2-7, and 8·EtOH were investigated. Herein, we present the synthesis, structures, and catalytic activity of these eight complexes.

  1.   Experimental

  1.1   Physical measurements

  All solvents and reagents were purchased from chemical suppliers and used directly without further purification. The ligands L1 and L2 were synthesized following a procedure reported in the literature^[31,43]. The elemental analyses of carbon, hydrogen, and nitrogen were carried out using a Perkin-Elmer 2400 micro analyzer. The infrared data were obtained on a Bruker Tensor 27 Fourier transform infrared spectrometer.

  1.2   Synthesis of complex 1·2CH₂Cl₂

  A long Pyrex tube (10 mL, 20 cm) charged with a solution of the L1 ligand (0.019 6 g, 0.1 mmol) in CH₂Cl₂ (1.0 mL) was layered with a solution of CuCl₂·2H₂O (0.017 0 g, 0.1 mmol) in EtOH (2.0 mL). Then, the Pyrex tube was sealed with Parafilm. Blue crystals formed at the interface of CH₂Cl₂ and EtOH after the tube was allowed to stand still for 3 d. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.022 g (71%). Elemental analysis Calcd. for C₂₄H₁₈Cl₄Cu₂N₆ (1)(%): C, 43.72; H, 2.75; N, 12.79. Found(%): C, 43.56; H, 2.78; N, 12.78. Selected IR data (KBr, cm^-1): 1 598 (w), 1 477 (m), 1 363 (w), 1 302 (w), 1 250 (m), 1 167 (w), 1 144 (w), 1 055 (w), 1 007 (w), 986 (w), 914 (w), 835 (w), 779 (s), 746 (m), 675 (s), 642 (w).

  1.3   Synthesis of complex 2

  A mixture of MeOH (1.0 mL), the L1 ligand (0.019 9 g, 0.1 mmol), acetic acid (1.0 mL), and CuBr₂ (0.112 g, 0.05 mmol) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 120 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave dark green lumpy crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.026 g (70%). Elemental analysis Calcd. for C₂₄H₁₈Br₃Cu₂N₆(%): C, 38.07; H, 2.40; N, 11.10. Found(%): C, 38.68; N, 10.68; H, 1.79. Selected IR data (KBr, cm^-1): 1 601 (m), 1 481 (s), 1 411 (w), 1 371 (m), 1 311 (m), 1 249 (m), 1 146 (m), 1 053 (m), 1 009 (m), 984 (w), 916 (w), 858 (w), 824 (w), 781 (s), 737 (s), 677 (s), 638 (m).

  1.4   Synthesis of complex 3

  A mixture of EtOH (2.0 mL), the L1 ligand (0.019 5 g, 0.1 mmol), N, N-dimethylformamide (DMF, 1.0 mL), and Cu(ClO₄)₂·6H₂O (0.017 0 g, 0.1 mmol) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 90 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave red crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.039 g (70%). Elemental analysis Calcd. for C₂₄H₁₈ClCuN₆O₄(%): C, 52.09; H, 3.28; N, 15.19. Found(%): C, 51.95; H, 3.21; N, 15.03. Selected IR data (KBr, cm^-1): 1 597 (s), 1 466 (s), 1 364 (m), 1 302 (w), 1 252 (m), 1 144 (w), 1 054 (w), 1 004 (m), 979 (m), 910 (w), 834 (m), 799 (m), 776 (s), 742 (s), 719 (w), 678 (s), 638 (m).

  1.5   Synthesis of complex 4

  A mixture of MeOH (2.0 mL), the L2 ligand (0.016 g, 0.05 mmol), CuCl₂·2H₂O (0.008 5 g, 0.05 mmol), and DMF (0.1 mL) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 90 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave orange block-like crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.015 g (60%). Elemental analysis Calcd. for C₂₄H₁₆ Cu₂I₄N₆(%): C, 28.17; H, 1.58; N, 8.21. Found(%): C, 28.69; H, 1.58; N, 8.35. Selected IR data (KBr, cm^-1): 1 593 (m), 1 470 (s), 1 359 (m), 1 259 (m), 1 160 (w), 1 150 (w), 1 019 (w), 781 (m), 772 (w), 732 (s), 673 (w).

  1.6   Synthesis of complex 5

  A mixture of EtOH (2.0 mL), the L2 ligand (0.016 g, 0.05 mmol), and Cu(ClO₄)₂·6H₂O (0.018 5 g, 0.05 mmol) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 90 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave orange block-like crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.025 g (55%). Elemental analysis Calcd. for C₂₄H₁₇Cl₂CuI₂N₆O₈(%): C, 31.83; H, 1.89; N, 9.28. Found(%): C, 31.98; H, 1.70; N, 9.21. Selected IR data (KBr, cm^-1): 2 988 (w), 2 902 (w), 1 606 (w), 1 478 (s), 1 362 (w), 1 249 (w), 1 094 (m), 1 080 (s), 1 019 (w), 915 (w), 774 (s), 741 (m), 671 (m), 649 (m), 620 (s).

  1.7   Synthesis of complex 6

  A mixture of MeOH (2.0 mL), the L1 ligand (0.019 5 g, 0.1 mmol), CuCl₂·2H₂O (0.017 0 g, 0.1 mmol), and benzaldehyde (0.5 mL) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 90 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave green flaky crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.016 g (46%). Elemental analysis Calcd. for C₃₁H₂₂Cl₃ Cu₂N₆(%): C, 52.30; H, 3.11; N, 11.80. Found(%): C, 52.50; H, 3.11; N, 11.88. Selected IR data (KBr, cm^-1): 1 596 (m), 1 487 (s), 1 463 (m), 1 427 (w), 1 361 (m), 1 308 (m), 1 272 (m), 1 246 (s), 1 227 (m), 1 186 (w), 1 135 (w), 1 065 (m), 1 004 (m), 963 (w), 832 (m), 773 (s), 748 (s), 708 (s), 685 (s), 640 (w), 622 (m), 607 (w).

  1.8   Synthesis of complex 7

  A mixture of EtOH (2.0 mL), the L1 ligand (0.009 8 g, 0.05 mmol), p-phenylenedicarboxaldehyde (0.006 7 g, 0.05 mmol), and CuCl₂·2H₂O (0.008 5 g, 0.05 mmol) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 110 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave green lumpy crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.004 0 g (21%). Elemental analysis Calcd. for C₄₈H₄₁Cl₅Cu₃N₉O₂(%): C, 50.40; H, 3.61; N, 11.02. Found(%): C, 49.83; H, 3.59; N, 10.88. The selected IR data (KBr, cm^-1): 1 603 (m), 1 486 (s), 1 365 (m), 1 311 (s), 1 246 (m), 1 086 (s), 1 042 (s), 1 007 (m), 969 (m), 823 (m), 770 (s), 729 (s), 694 (s), 668 (s).

  1.9   Synthesis of complex 8·EtOH

  A mixture of EtOH (2.0 mL), the L1 ligand (0.019 5 g, 0.1 mmol), CuCl₂·2H₂O (0.008 5 g, 0.05 mmol), and 3-methoxy-4-hydroxy-benzaldehyde (0.007 5 g, 0.05 mmol) was loaded into a Pyrex tube (6.0 mL). The tube was sealed and sonicated for 10 min to obtain a homogenous solution. Then, the tube was heated at 110 ℃ for 3 d under autogenous pressure. Cooling of the tube to room temperature for 24 h gave green lumpy crystals. The crystals were collected, washed with EtOH, and dried in air. The yield was 0.017 g (53%). Elemental analysis Calcd. for C₃₄H₂₉Cl₂CuN₆O₃(%): C, 58.00; H, 4.15; N, 11.94. Found(%): C, 57.59; H, 3.87; N, 11.67. Selected IR data (KBr, cm^-1): 3 084 (w), 3 032 (w), 2 964 (w), 1 596 (m), 1 517 (w), 1 485 (s), 1 461 (m), 1 353 (m), 1 309 (m), 1 277 (m), 1 239 (s), 1 213 (m), 1 146 (m), 1 128 (s), 1 055 (m), 1 029 (m), 1 006 (m), 812 (s), 760 (s), 710 (w), 666 (s), 625 (m).

  1.10   General procedure for the ketalization reaction

  Ketone (0.7 mmol), ethylene glycol (0.7 mmol), toluene (0.5 mL), 4A molecular sieves (2.0 mg), and catalyst (2.0 mg) were added to a 10 mL Schlenk flask. The reaction was refluxed at 110 ℃ for 24 h. The catalyst and the molecular sieves were isolated by filtration. All product yields were determined by ¹H NMR spectroscopy (Fig.S17-S48, Supporting information).

  1.11   Crystal structure determination

  The single-crystal X-ray diffraction data for eight complexes were collected on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with graphite-monochromated Mo Kα (λ=0.071 073 nm) using the φ-ω scan technique. The crystal structures were solved with the Olex2 program with the ShelXL refinement package^[44-45]. The refinement details and selected bond lengths and angles are listed in Table S1-S16.

  2.   Results and discussion

  2.1   Synthesis of complexes 1·2CH₂Cl₂, 2, and 3

  We investigated the synthesis of a complex of 3-(pyridin-2-yl)-imidazo[1,5-a]pyridine (L1) by using inexpensive CuCl₂·2H₂O as a starting material under atmospheric pressure at room temperature (Scheme 1). We first added a solution of L1 (in ethanol) to a Pyrex glass tube and then layered a solution of CuCl₂·2H₂O in CH₂Cl₂ on top of the solution of L1. Blue crystals of [Cu₂(L1)₂Cl₄]·2CH₂Cl₂ (1·2CH₂Cl₂) were obtained after the glass tube was allowed to stand still on the bench for one week.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 1-3

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  After the successful synthesis of 1·2CH₂Cl₂, we next attempted to synthesize a bromide-bridged complex by employing CuBr₂ as a Cu(Ⅱ) source. Our efforts to prepare the complex of CuBr₂ by a method similar to that of preparing 1·2CH₂Cl₂ were in vain. Then, the reaction was conducted under solvothermal conditions, and complex [Cu(L1)₂Br][CuBr₂] (2) was generated.

  The reaction of Cu(ClO₄)₂·6H₂O and the L1 ligand in a solvent mixture of EtOH and DMF under solvothermal conditions was also conducted, and a homoleptic complex [Cu(L1)₂]ClO₄ (3) was prepared.

  2.2   Synthesis of complexes 4 and 5

  A remarkable feature of the synthesis of 1·2CH₂Cl₂, 2, and 3 is that three complexes with completely different structures were formed by using different metal salts. These results inspired us to examine whether we could prepare structurally different complexes by using a derivative of L1, 1-iodo-3-(pyridin-2-yl)imidazo[1,5-a]pyridine (L2), as a ligand and different copper salts as metal sources. As shown in Scheme 2, the reactions of L2 with CuCl₂·2H₂O and Cu(ClO₄)₂·6H₂O gave complexes [Cu₂(L2)₂I₂] (4) and [Cu(L2)₂(ClO₄)]ClO₄ (5), respectively.

  Scheme 2

  

  Scheme 2.  Synthesis of complexes 4 and 5

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  2.3   Synthesis of complexes 6, 7, and 8·EtOH

  Motivated by our previous work showing that in situ metal-ligand reactions occurred in the reaction of FeCl₃, L1, and picolinaldehyde^[31], we explored the in situ metal-ligand reactions of CuCl₂·2H₂O, L1, and other aldehydes. To this end, the reactions of CuCl₂·2H₂O, L1, and benzaldehyde/terephthalaldehyde/4-hydroxy-3-methoxybenzaldehyde, were conducted (Scheme 3). To our delight, three new complexes, [Cu(L3)Cl][CuCl₂] (6), [Cu(L4)Cl][Cu₂(L1)₂Cl₄]_0.5[CuCl₂] (7), and [Cu(L5)Cl]Cl·EtOH (8·EtOH), were formed. In situ metal-ligand reactions occurred, generating three new ligands, L3, L4 and L5, where L3=1, 1′-(phenylmethylene)bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridine}, L4=1, 1′-{[4-(diethoxymethyl)phenyl] methylene}bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridine}, and L5=4-(bis{3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl}methyl)-2-methoxy-phenol.

  Scheme 3

  

  Scheme 3.  Synthesis of complexes 6, 7, and 8·EtOH

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  2.4   Structural descriptions of complexes 1·2CH₂Cl₂, 2, and 3

  The structure of 1·2CH₂Cl₂ was determined by single-crystal X-ray diffraction analysis. Complex 1·2CH₂Cl₂ crystallizes in the triclinic space group P1. The crystal structure of 1·2CH₂Cl₂ is composed of two Cu(Ⅱ) ions, two L1 ligands, four Cl^- ions, and two CH₂Cl₂ molecules (Fig. 1a). The complex 1·2CH₂Cl₂ has a crystallographically imposed C₂ symmetry; thus, there is one crystallographically independent Cu(Ⅱ) ion. The Cu(Ⅱ) ion is bound to two nitrogen atoms from the L1 ligand, two bridging chloride ions, and one terminal coordinated chloride ion. The two Cu(Ⅱ) ions are doubly bridged by two chloride ions to form a Cu₂Cl₂ rhombic ring. The Cu(Ⅱ)…Cu(Ⅱ) distance is 0.342 7 nm, and the bond lengths of Cu1—N1, Cu1—N2, Cu1—Cl1, Cu1—Cl2, and Cu1—Cl2#1 are 0.198 2(3), 0.206 0(3), 0.224 11(9), 0.230 60(8), and 0.264 33(9) nm, respectively.

  Figure 1

  

  Figure 1.  (a) Molecular structure of complex 1·2CH₂Cl₂ with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 1·2CH₂Cl₂; (c) Packing diagram of 1·2CH₂Cl₂ viewed along the b-axis

  The hydrogen atoms and solvent (CH₂Cl₂) molecules are omitted for clarity; Symmetry code: A: 1-x, -y, 1-z.

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  The CShM method (SHAPE 2.0 software program)^[46] was used to analyze the geometry of the Cu(Ⅱ) ion in 1·2CH₂Cl₂ (Table S17). The minimum CShM values for the different geometries show that the Cu(Ⅱ) ion displays a square pyramid geometry with a minimum CShM value of 1.225.

  A detailed structural analysis of 1·2CH₂Cl₂ reveals that π…π stacking interactions exist among the pyridine ring of one molecule and the pyridyl moiety of the imidazo[1,5-a]pyridine motif in another molecule [Fig. 1b, the Cg1…Cg2 distance: 0.362 1(2) nm]. Fig. 1c shows the packing diagram of 1·2CH₂Cl₂ viewed along the b-axis. This packing structure is stabilized by the π…π stacking interactions.

  Single-crystal X-ray diffraction analysis shows that complex 2 crystallizes in the triclinic C2/c space group. The molecular structure of 2 consists of [Cu(L1)₂Br] and [CuBr₂] units. The valence state balance requirement revealed that the copper ion in the [Cu(L1)₂Br] unit is in the +2 valence state and that the other copper ion in the [CuBr₂] unit is in the +1 valence state. The Cu(Ⅱ) ion is coordinated by four nitrogen atoms of the two L1 ligands and a bromide ion (Fig. 2). Thus, the five-coordinate Cu1 ion exhibits a trigonal bipyramidal geometry with a minimum CShM value of 1.752^[46] (Table S17). The Cu1—Br1, Cu1—N2, and Cu1—N1 bond lengths are 0.245 01(8), 0.195 8(3), and 0.211 5(3) nm, respectively.

  Figure 2

  

  Figure 2.  (a) Molecular structure of complex 2 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 2; (c) Packing diagram of 2 viewed along the a-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: 1-x, y, 3/2-z.

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  It is found that there are π…π stacking interactions among the pyridine ring of one molecule and the pyridyl motif of the imidazo[1,5-a]pyridine scaffold in another molecule [Fig. 2b, Cg1…Cg2 distance: 0.356 52(2) nm]. Fig. 2c presents the packing diagram of 2 viewed along the a-axis. In turn, the packing network is stabilized by the π…π stacking interactions.

  Single-crystal X-ray diffraction analysis demonstrates that complex 3 crystallizes in the triclinic P1 space group. The structure of 3 consists of a [Cu(L1)₂]⁺ cation and a ClO₄^- anion (Fig. 3). The Cu(Ⅰ) ion of the [Cu(L1)₂]⁺ unit is tetra-coordinated by four nitrogen atoms of the two L1 ligands. The central Cu(Ⅰ) displays a seesaw geometry with a minimum CShM value of 6.636^[46] (Table S18). During the synthesis of 3, the reduction of Cu(Ⅱ) to Cu(Ⅰ) occurred due to the presence of DMF. The Cu—N_pyridine bond lengths are 0.207 58(14) and 0.202 75(14) nm, respectively. The Cu—N_imidazole bond distances are 0.207 02(15) and 0.200 22(14) nm, respectively.

  Figure 3

  

  Figure 3.  (a) Molecular structure of complex 3 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 3; (c) Packing diagram of 3 viewed along the b-axis

  The hydrogen atoms are omitted for clarity.

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  Two types of π…π stacking interactions are found in the packing diagram of 3 [Fig. 3b, Cg1…Cg2 distance: 0.256 59(11) nm, Cg3…Cg4 distance: 0.350 46(12) nm]. Fig. 3c displays the packing diagram of 3 viewed along the b-axis. The packing structure is stabilized by the π…π stacking interactions.

  2.5   Structural descriptions of complexes 4 and 5

  Complex 4 crystallizes in the orthorhombic Pbca space group. 4 consists of two L2 ligands, two iodide ions, and two monovalent copper ions (Fig. 4). 4 has a crystallographically imposed C₂ symmetry; therefore, there is only one crystallographically independent Cu(Ⅰ) site. The Cu(Ⅰ) site is tetra-coordinated by two nitrogen atoms from the L2 ligand and two bridging I^- ligands. The two Cu(Ⅰ) ions are doubly bridged by two I^- ligands to form a rhombic Cu₂I₂ ring. The Cu(Ⅰ) ion shows a tetrahedral geometry with a minimum CShM value of 4.467^[46] (Table S18). The bond lengths of Cu1—I1 and Cu1—I1A are 0.257 77(11) and 0.262 32(12) nm, respectively. The bond distances of Cu1—N2 and Cu1—N4 are 0.210 1(6) and 0.207 0(6) nm, respectively. The Cu(Ⅰ)—N distances of 4 are slightly longer than the Cu(Ⅱ)—N distances in 1·2CH₂Cl₂. The Cu(Ⅰ)…Cu(Ⅰ) distance is 0.250 94(18) nm, which is much shorter than the Cu(Ⅱ)…Cu(Ⅱ) distance (0.342 7 nm) in 1·2CH₂Cl₂.

  Figure 4

  

  Figure 4.  (a) Molecular structure of complex 4 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 4; (c) Packing diagram of 4 viewed along the c-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: 1-x, 1-y, 1-z.

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  The structure of 4 deserves some comments. CuCl₂·2H₂O was utilized as the metal source, but the two Cu(Ⅰ) ions were connected by two I^- ions, revealing that an in situ metal-ligand reaction occurred during the process for the formation of 4. Both the reduction of Cu(Ⅱ) to Cu(Ⅰ) in the presence of DMF and the cleavage of the C—I bond under solvothermal conditions occurred, resulting in the iodide-bridged dinuclear Cu(Ⅰ) complex.

  Further structural investigation of 4 reveals π…π stacking interactions between the pyridine ring of one molecule and the pyridyl moiety of the imidazo[1,5-a]pyridine motif in another molecule [Fig. 4b, Cg1…Cg2 distance: 0.376 4(5) nm]. Fig. 4c shows the 2D packing diagram of 4 viewed along the c-axis. The packing structure is stable due to the existence of the π…π stacking interactions.

  Single-crystal X-ray diffraction analysis reveals that complex 5 crystallizes in the monoclinic space group of P2₁/n. The structure of 5 consists of two L2 ligands, two ClO₄^- units, and one Cu(Ⅱ) ion, with one ClO₄^- unit involved in the coordination of the Cu(Ⅱ) ion (Fig. 5) and another ClO₄^- ion serving as a counterion. The Cu(Ⅱ) ion is penta-coordinated by four nitrogen atoms from two L2 ligands and one oxygen atom from one ClO₄^- ion. Thus, the Cu(Ⅱ) ion exhibits a trigonal bipyramidal geometry with a minimum CShM value of 0.995^[46] (Table S17). The bond lengths of Cu—N are in a range of 0.195 1(5)-0.206 7(5) nm, which is similar to those of Cu(Ⅱ)—N bonds in complexes 1·2CH₂Cl₂ and 2.

  Figure 5

  

  Figure 5.  (a) Molecular structure of complex 5 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 5; (c) Packing diagram of 5 viewed along the c-axis

  The hydrogen atoms are omitted for clarity.

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  There exist π…π stacking interactions among the pyridine ring of one molecule and the pyridyl moiety of the imidazo[1,5-a]pyridine motif in another molecule [Fig. 5b, Cg1…Cg2 distance: 0.346 9(4) nm]. The 2D packing diagram of 5 viewed along the c-axis is presented in Fig. 4c. The packing structure is stabilized via the π…π stacking interactions.

  2.6   Structural descriptions of complexes 6, 7, and 8·EtOH

  Complex 6 crystallizes in the monoclinic P2₁/n space group. 6 contains a [Cu(L3)Cl]⁺ cation and a [CuCl₂]^- anion (Fig. 6). The Cu(Ⅱ) ion in the [Cu(L3)Cl]⁺ unit is penta-coordinated by four nitrogen atoms of the in situ-formed L3 ligand and a chloride ligand, and Cu(Ⅱ) ion shows a square pyramid geometry with a minimum CShM value of 1.867^[46] (Table S17). The Cu1—N_pyridine bond lengths are 0.207 58(14) and 0.202 75(14) nm, and the Cu1—N_imidazole distances are 0.207 02(15) and 0.200 22(14) nm, which are typical for Cu(Ⅱ)—N bond lengths.

  Figure 6

  

  Figure 6.  Molecular structure of complex 6 with thermal ellipsoids drawn at the 50% probability level

  The hydrogen atoms are omitted for clarity.

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  Complex 7 crystallizes in the P1 space group of the triclinic crystal system. The structure of 7 is composed of a cationic [Cu(L4)Cl]⁺ unit, an anionic [CuCl₂]^- ion and a half neutral [Cu₂(L1)₂Cl₄] unit (Fig. 7). In the [Cu(L4)Cl]⁺ ion, the Cu(Ⅱ) ion is penta-coordinated by four nitrogen atoms of the in situ-formed L4 ligand and one chloride ligand. The Cu(Ⅱ) ion shows a square pyramid geometry with a minimum CShM value of 2.000^[46] (Table S17). The structure of the neutral [Cu₂(L1)₂Cl₄] unit is exactly identical to that of complex 1, in which the Cu ion exhibits a +2 valence state. In the anionic [CuCl₂]^- unit, the copper ion displays a +1 valence state.

  Figure 7

  

  Figure 7.  (a) Molecular structure of complex 7 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 7; (c) Packing diagram of 7 viewed along the c-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: -x, 1-y, 1-z.

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  A prominent feature of the synthesis of 7 was the occurrence of two types of in situ metal-ligand reactions. An aldehyde group of terephthalaldehyde reacted with the C-1 position of the L1 ligand, connecting two L1 ligands with a carbon atom of terephthalaldehyde. Moreover, another aldehyde group of terephthalaldehyde underwent a kentalization reaction with two solvent (EtOH) molecules. The combination of the two in situ metal-ligand reactions led to the formation of complex 7 with an aesthetically pleasing structure.

  There are π…π stacking interactions among the pyridine ring of one molecule and the pyridyl motif of the imidazo[1,5-a]pyridine scaffold in another molecule [Fig. 7b, Cg1…Cg2 distance: 0.390 7(4) nm]. Fig. 7c shows the 2D packing diagram of 7 viewed along the c-axis. The packing structure is stabilized through the π…π stacking interactions.

  Complex 8·EtOH crystallizes in the orthorhombic Pbcm space group. The structures of 8·EtOH and 6 are similar. The structure of 8·EtOH consists of a cationic [Cu(L5)Cl]⁺ unit, a chloride counterion, and a solvent (EtOH) molecule (Fig. 8). A ligand L5 was formed by the in situ metal-ligand reaction of L1 and 4-hydroxy-3-methoxybenzaldehyde. The Cu(Ⅱ) ion shows a square pyramid geometry with a minimum CShM value of 2.529^[46] (Table S17).

  Figure 8

  

  Figure 8.  (a) Molecular structure of complex 8·EtOH with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 8·EtOH; (c) Hydrogen bonding interactions in 8·EtOH with thermal ellipsoids drawn at the 50% probability level; (d) Packing diagram of 8·EtOH viewed along the a-axis

  The hydrogen atoms and solvent (EtOH) molecules are omitted for clarity; Symmetry code: A: x, y, 1/2-z.

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  A detailed structural study reveals π…π stacking [Fig. 8b, Cg1…Cg2 distance: 0.353 3(4) nm] and hydrogen bonding [Fig. 8c, C7…Cl10 distance: 0.257 45(10) nm, O2B…Cl1B distance: 0.220 98(16) nm] interactions. Fig. 8d shows the 2D packing diagram of 8·EtOH viewed along the a-axis. The packing structure is stabilized through the hydrogen bonding and the π…π stacking interactions.

  2.7   Catalytic properties of the complexes

  Ketalization is an important method for protecting carbonyl groups in organic synthesis^[47-49]. The ketalization reaction generally requires Brønsted acids such as anhydrous HCl and H₂SO₄ as catalysts. However, these inorganic acids cannot be easily isolated and recycled, leading to serious corrosion of equipment and the production of waste materials. Therefore, the development of more efficient, recyclable, and pollution-free catalysts for ketalization reactions is highly important. Some copper complexes are efficient catalysts for this ketalization transformation.

  After the successful synthesis of 1·2CH₂Cl₂, 2-7, and 8·EtOH, we examined their catalytic activity toward the ketalization of acetone. First, 1·2CH₂Cl₂ was selected as the catalyst (x=0.2%), ethylene glycol was chosen as the alcohol source, and toluene was used as the reaction solvent. The product 2, 2-dimethyl-1, 3-dioxolane was obtained in 45% yield (Table 1, entry 1) after the reaction mixture was heated at 110 ℃ for 24 h. We then conducted the reactions catalyzed by 2-7 and 8·EtOH under the same conditions as those in which 1·2CH₂Cl₂ was used as the catalyst (Table 1, entries 2-8). Except for complex 4 (Table 1, entry 4), the remaining seven complexes were all catalytically active for this reaction, and the product 2, 2-dimethyl-1, 3-dioxolane was obtained in 87%-91% yields by using complexes 2-3, 5-7, and 8·EtOH, respectively, as catalysts. Complex 4 was catalytically inactive in this reaction. It is reasoned that the larger atomic radius of bridging iodide ions and shorter Cu(Ⅰ)…Cu(Ⅰ) distance in 4 hindered the coordination of the substrates to the metal center, leading to the catalytic inactivity of complex 4.

  Table 1

  

  Table 1.  Ketalization of acetone catalyzed by complexes 1·2CH₂Cl₂, 2-7, and 8·EtOH^a

  下载: 导出CSV

  Entry	Catalyst	xcat / %	T / ℃	t / h	Yieldb / %
  1c	1	0.2	110	24	45
  2	2	0.2	110	24	89
  3	3	0.2	110	24	88
  4	4	0.2	110	24	0
  5	5	0.2	110	24	91
  6	6	0.2	110	24	90
  7	7	0.2	110	24	87
  8c	8	0.2	110	24	91
  9d			25	24	0
  10d	2	0.2	25	24	21
  a Conditions: acetone (0.7 mmol), ethylene glycol (0.7 mmol), and toluene (0.5 mL); b Yield was determined by 1H NMR analysis; c Solvents were not used; d Irradiated by 400-410 nm LEDs.

  To study whether the reaction is photocatalyzed or if complex 2 could serve as a photocatalyst, the ketalization of acetone was conducted by irradiating the reaction with 400-410 nm LEDs without the use of 2 (Table 1, entry 9) as the catalyst at room temperature. No product was detected. In addition, the reaction was performed by employing 2 as the catalyst under irradiation with 400-410 nm LEDs (Table 1, entry 10). A 21% yield of the product was obtained. These results indicate that this reaction is not affected by light and that complex 2 cannot be used as a photocatalyst.

  Next, the scope of the ketalization reaction was expanded. The ketalization of acetone, butanone, acetylacetone, and cyclohexanone by ethylene glycol catalyzed by 1·2CH₂Cl₂, 2-3, 5-7, and 8·EtOH was explored (Table 2). Except for the ketalization of acetylacetone catalyzed by 1·2CH₂Cl₂, which gave 1-(2-methyl-1, 3-dioxolan-2-yl)propan-2-one in a low yield of 49% (Table 2, entry 8), the yields of the other reactions were high, and the yields of the other ketals ranged from 77% to 100%, indicating the high catalytic activity of these complexes.

  Table 2

  

  Table 2.  Ketalization of ketones catalyzed by complexes 1·2CH₂Cl₂, 2-3, 5-7, and 8·EtOH^a

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  Entry	Ketone	Product	Catalyst	xcat / %	T / ℃	Yieldb / %
  1c			1	0.2	110	80
  2			2	0.2	110	82
  3			3	0.2	110	97
  4			5	0.2	110	98
  5			6	0.2	110	93
  6			7	0.2	110	96
  7c			8	0.2	110	92
  8c			1	0.2	110	49
  9			2	0.2	110	89
  10			3	0.2	110	97
  11			5	0.2	110	97
  12			6	0.2	110	91
  13			7	0.2	110	91
  14c			8	0.2	110	87
  15c			1	0.2	110	85
  16			2	0.2	110	100
  17			3	0.2	110	99
  18			5	0.2	110	98
  19			6	0.2	110	98
  20			7	0.2	110	99
  21c			8	0.2	110	97
  a Conditions: acetone (0.7 mmol), ethylene glycol (0.7 mmol), and toluene (0.5 mL); b Yield was determined by 1H NMR analysis; c Solvents were not used.

  3.   Conclusions

  Eight copper complexes were prepared and characterized. Complexes 1-3 were synthesized by the reactions of different copper salts with 3-pyridinylimidazo[1,5-a]pyridine (L1). Complex 4 was obtained by the use of an iodide-containing L2 ligand to react with CuCl₂·2H₂O, leading to the formation of iodide-bridged dinuclear Cu(Ⅰ) complex 4 via in situ cleavage of the C—I bond of the L2 ligand. Three complexes, 6-7 and 8·EtOH, were prepared by intuitively inducing an in situ metal-ligand reaction via the addition of aldehydes to the reaction system of CuCl₂·2H₂O and the L1 ligand. The catalytic activity of these eight complexes toward the ketalization of a series of ketones by ethylene glycol was explored. Complex 4 was inactive in this reaction. This may be due to the short Cu(Ⅰ)…Cu(Ⅰ) distance and large radius of the iodide ion in 4, rendering the coordination of the substrates to the Cu(Ⅰ) ion more difficult. The remaining seven complexes all showed excellent catalytic behavior toward this ketalization reaction. This work provides a perspective for the synthesis of economically friendly and environmentally friendly Cu(Ⅱ) and Cu(Ⅰ) complexes with catalytic activity.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Scheme 1  Synthesis of complexes 1-3

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  Scheme 2  Synthesis of complexes 4 and 5

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  Scheme 3  Synthesis of complexes 6, 7, and 8·EtOH

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  Figure 1  (a) Molecular structure of complex 1·2CH₂Cl₂ with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 1·2CH₂Cl₂; (c) Packing diagram of 1·2CH₂Cl₂ viewed along the b-axis

  The hydrogen atoms and solvent (CH₂Cl₂) molecules are omitted for clarity; Symmetry code: A: 1-x, -y, 1-z.

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  Figure 2  (a) Molecular structure of complex 2 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 2; (c) Packing diagram of 2 viewed along the a-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: 1-x, y, 3/2-z.

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  Figure 3  (a) Molecular structure of complex 3 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 3; (c) Packing diagram of 3 viewed along the b-axis

  The hydrogen atoms are omitted for clarity.

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  Figure 4  (a) Molecular structure of complex 4 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 4; (c) Packing diagram of 4 viewed along the c-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: 1-x, 1-y, 1-z.

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  Figure 5  (a) Molecular structure of complex 5 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 5; (c) Packing diagram of 5 viewed along the c-axis

  The hydrogen atoms are omitted for clarity.

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  Figure 6  Molecular structure of complex 6 with thermal ellipsoids drawn at the 50% probability level

  The hydrogen atoms are omitted for clarity.

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  Figure 7  (a) Molecular structure of complex 7 with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 7; (c) Packing diagram of 7 viewed along the c-axis

  The hydrogen atoms are omitted for clarity; Symmetry code: A: -x, 1-y, 1-z.

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  Figure 8  (a) Molecular structure of complex 8·EtOH with thermal ellipsoids drawn at the 50% probability level; (b) π…π stacking interactions in 8·EtOH; (c) Hydrogen bonding interactions in 8·EtOH with thermal ellipsoids drawn at the 50% probability level; (d) Packing diagram of 8·EtOH viewed along the a-axis

  The hydrogen atoms and solvent (EtOH) molecules are omitted for clarity; Symmetry code: A: x, y, 1/2-z.

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  Table 1.  Ketalization of acetone catalyzed by complexes 1·2CH₂Cl₂, 2-7, and 8·EtOH^a

  Entry	Catalyst	xcat / %	T / ℃	t / h	Yieldb / %
  1c	1	0.2	110	24	45
  2	2	0.2	110	24	89
  3	3	0.2	110	24	88
  4	4	0.2	110	24	0
  5	5	0.2	110	24	91
  6	6	0.2	110	24	90
  7	7	0.2	110	24	87
  8c	8	0.2	110	24	91
  9d			25	24	0
  10d	2	0.2	25	24	21
  a Conditions: acetone (0.7 mmol), ethylene glycol (0.7 mmol), and toluene (0.5 mL); b Yield was determined by 1H NMR analysis; c Solvents were not used; d Irradiated by 400-410 nm LEDs.

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  Table 2.  Ketalization of ketones catalyzed by complexes 1·2CH₂Cl₂, 2-3, 5-7, and 8·EtOH^a

  Entry	Ketone	Product	Catalyst	xcat / %	T / ℃	Yieldb / %
  1c			1	0.2	110	80
  2			2	0.2	110	82
  3			3	0.2	110	97
  4			5	0.2	110	98
  5			6	0.2	110	93
  6			7	0.2	110	96
  7c			8	0.2	110	92
  8c			1	0.2	110	49
  9			2	0.2	110	89
  10			3	0.2	110	97
  11			5	0.2	110	97
  12			6	0.2	110	91
  13			7	0.2	110	91
  14c			8	0.2	110	87
  15c			1	0.2	110	85
  16			2	0.2	110	100
  17			3	0.2	110	99
  18			5	0.2	110	98
  19			6	0.2	110	98
  20			7	0.2	110	99
  21c			8	0.2	110	97
  a Conditions: acetone (0.7 mmol), ethylene glycol (0.7 mmol), and toluene (0.5 mL); b Yield was determined by 1H NMR analysis; c Solvents were not used.

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