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Palladium-Based Coordination "Clips" with Carboxamide-Pyrazolate Ditopic Ligands: Self-Assembly and Catalytic Properties

Xiao-Peng HU ,  Zhi-Feng WANG ,  Shu-Yan YU

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Citation:  Xiao-Peng HU, Zhi-Feng WANG, Shu-Yan YU. Palladium-Based Coordination "Clips" with Carboxamide-Pyrazolate Ditopic Ligands: Self-Assembly and Catalytic Properties[J]. Chinese Journal of Inorganic Chemistry, 2021, 37(8): 1493-1503. doi: 10.11862/CJIC.2021.159 shu

基于酰胺-吡唑双功能配体的钯配位“夹子”的自组装及其催化性能

  • .

    北京工业大学环境与生命学部, 绿色催化与分离北京市重点实验室, 北京工业大学环境安全与生物效应卓越中心, 自组装化学实验室, 北京 100124

    • 通讯作者: 于澍燕, selfassembly@bjut.edu.cn
  • 基金项目:

    国家自然科学基金 21906002

    北京市自然科学基金 2212002

    北京市科技计划一般项目 KM2020010005010

    北京市高水平创新团队建设项目 IDHT20180504

    北京市卓越青年科学家计划 BJJWZYJH01201910005017

摘要: 利用配位驱动吡唑基配体在溶液中自发去质子自组装的方法，用金属转子[（bpy）2Pd2（NO3）2]（NO3）2、[（dmbpy）2Pd2（NO3）2]（NO3）2（bpy=2，2'-联吡啶，dmbpy=4，4'-二甲基-2，2'-联吡啶）和酰胺-吡唑双功能配体（HL1、HL2）合成了含有Pd（Ⅱ）…Pd（Ⅱ）相互作用的[Pd2L2]2+类型的吡唑基双钯（Ⅱ，Ⅱ）“夹子” C1~C4。利用1H和13C NMR、ESI-MS、红外光谱和X射线单晶衍射等测试手段对配合物C1~C4的结构进行了表征。同时，合成的夹子状双金属配合物对Suzuki-coupling反应均表现出较好的催化活性。

English

  • Coordination - driven self - assembly continues to spark the vibrant field of supramolecular chemistry, owing to its predictability for the construction of elegant functional structures[1-3]. Over the past decades, several metal - directed assembly strategies have been developed to achieve well-defined supramolecular architectures, such as clips, squares, cages, grids, and capsules[4-7]. With the advance of the field, increasing efforts have now been devoted to design architectures with diverse functions and applications, such as catalysis, guests binding, drug delivery, separation, stimuli - responsive materials and cancer theranostics[8-10]. Among these efforts, the selection of a suitable strategy for construction of ensembles with controllable shapes and predictable functions is still drawing particular attentions[11-14].

    Since the recognition of the multiple bonding [Re2Cl8]2- by F. A. Cotton in 1960s, the chemistry of the dimetal - or multimetal - centered compounds have drawn increasing interest due to their structural diversity, rich physical properties and potential applications in catalysis, magnetic coupling and functional materials[15-17]. Recently, the utilization of dimetal units as directional building blocks and the bidentate ligands as linkers has extended such chemistry toward the emerging field of metallo-supramolecular architecture[18-19].

    Meanwhile, the transition metal palladium complexes have been a hot topic in the area of Suzuki-coupling reaction. The mononuclear palladium catalysis had played a crucial role in synthetic organic chemistry for Suzuki - coupling reaction. In the past few years, considerable attention has been paid to functional metal-organic assemblies that show promise in catalysis[20-21]. Especially, palladium was employed in Suzuki-coupling reaction for their high stability and remarkable efficiency. In our search for new coordination motifs as building blocks in supramolecular architecture, we noticed that the versatile ligands 1H-bipyrazoles could be good alternatives for the widely used carboxylates or other bidentate chelating ligands in binding dimetal centers[22-24]. Considering the variety of potential catalysis applications of pyrazolate-bridged multimetal coordination compounds, we studied the self -assembly of metallo-clips with bifunctional ligand.

    In our group, a series of novel metallosupromole- cules have been successfully synthesized through the stepwise ligand substitution reaction of different pyr- azolate ligands with bimetal motifs [(N^N)2Pd2(NO3)2] (NO3)2 (where N^N=2, 2′-bipyridine, bpy; 4, 4′-dimethyl- 2, 2′- bipyridine, dmbpy)[25-26]. In this work, we successfully synthesized a series of functional [M2L2]2+ - type coordination "clips" C1~C4 using a novel kind of carboxamide-pyrazolate ditopic ligands with two dipalladium building units [(N^N)2Pd2(NO3)2] (NO3)2, as shown in Scheme 1. The supramolecular assemblies have been intensively studied by NMR spectroscopy, electrospray ionization mass spectrometry (ESI - MS) in solution and by IR spectroscopy, X-ray crystallograph- ic analysis in the solid state.

    Scheme 1

    Scheme 1.  Schematic illustration of self-assembly of dipalladium "clips" C1·2NO3-~C4·2NO3- and C1·2PF6-~C4·2PF6-

    (i) H2O, acetone, reflux; (ii) KPF6

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    1.   Experimental

    1.1   Reagents and physical instruments

    All chemicals and solvents were of AR grade and used without further purification. Carbon, hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer. 1H and 13C NMR spectra were recorded on Bruker AV 400 MHz spectrometers. The ESI-MS were recorded on Octant TOF LC - plus 4G mass spectrometer. The FT - IR spectra were recorded as KBr pellets on a Shimadzu IR Prestige-21 spectrometer. Single crystal X-ray diffraction analysis were recorded on Bruker apex Ⅱ instrument.

    1.2   Synthesis and characterization of C1·2NO3-~C4·2NO3- and C1·2PF6-~C4·2PF6-

    1.2.1   General pyrazole ligand preparation

    The pyrazole ligand HL1 and HL2 were prepared by using similar methods as described in those works in our group[19, 26] (Scheme 2). N2, N6-dibenzyl-4-(1H-pyrazol-4-yl)pyridine-2, 6-dicarboxamide (HL1): 1H NMR (400 MHz, DMSO - d6, 298 K): δ 4.62 (d, 2H), 7.26 (d, 5H), 8.43 (s, 1H), 8.71(s, 1H), 9.89 (m, 1H), 13.33 (s, 1H). 13C NMR (400MHz, DMSO-d6, 298 K): δ 164.06, 149.90, 144.28, 139.82, 137.83, 128.84, 127.40, 120.23, 118.72, 42.65; N2, N6-dibenzyl-4-(3, 5-dimethyl-1H-pyrazol-4-yl) pyridine-2, 6-dicarboxamide (HL2): 1H NMR (400 MHz, DMSO - d6, 298 K): δ 2.33 (s, 3H), 4.61 (d, 2H), 7.26 (m, 5H), 8.15(s, 1H), 9.96 (m, 2H), 12.24 (s, 1H). 13C NMR (400 MHz, DMSO-d6, 298 K): δ 163.94, 149.51, 145.50, 139.87, 128.81, 127.49, 127.28, 123.03, 114.19, 42.66, 12.41.

    Scheme 2

    Scheme 2.  Synthesis of HL1 and HL2

    (i) POCl3, phenylmethanamine, reflux; (ii) Dmtty, K2CO3, Pd(PPh3)4, reflux; (iii) tty, K2CO3, Pd(PPh3)4, reflux; (iv/v) HCl, methanol, reflux

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    1.2.2   Synthesis and characterization of [M2L2]2+ - type coordination "clips"

    The self-assembly of metallo-clip complexes C1· 2NO3- and C1·2PF6- are shown in Scheme 1. HL1 (41.5 mg, 0.1 mmol) was added to a suspension solution of [(bpy)2Pd2(NO3)2](NO3)2 (38.7 mg, 0.05 mmol) in H2O/acetone (1∶4, V/V). The mixture was stirred for 2 h at room temperature, then was heated at 80 ℃ for 6 h to give C1·2NO3-. C1·2NO3- : 1H NMR (400 MHz, DMSO-d6, 298 K, ppm): δ 8.73 (t, 1H, bpy-1), 8.48 (d, 1H, bpy-2), 7.78 (d, 1H, bpy-3), 8.33 (t, 1H, bpy-4), 9.88 (t, 1H, L1-a), 8.95 (s, 1H, L1-b), 4.61 (s, 2H, L1-c), 7.31 (t, 2H, L1-d), 7.32 (d, 2H, L1-e), 7.26 (m, 1H, L1-f), 8.48 (s, 1H, L1-g). 13C NMR (400 MHz, DMSO-d6, 298 K): δ 163.87, 156.30, 151.71, 150.27, 149.94, 142.82, 140.98, 128.85, 127.42, 124.74, 122.74, 122.49, 120.16, 119.53, 42.64. Elemental analysis Calcd. for C68H56N16O10Pd2(%): C, 55.56; H, 3.84; N, 15.24. Found (%): C, 55.53; H, 3.85; N, 15.24. The PF6- salts of C1· 2NO3- was obtained by adding ten-fold excess of KPF6 to the solution of C1·2NO3-. The mixture was stirred continuously for 6 h, then the precipitation was fil- tered, washed with minimum amount of cold water and dried in vacuum as yellow solid. C1·2PF6- was obtained as yellow needle crystal in quantitative yield (96%, 78.41 mg). C1·2PF6- : 1H NMR (400 MHz, DMSO-d6, 298 K): δ 8.77 (t, 1H, bpy-1), 8.49 (d, 1H, bpy-2), 8.35 (d, 1H, bpy-3), 7.85 (t, 1H, bpy-4), 9.90 (t, 1 H, L1-a), 8.97 (s, 1H, L1-b), 4.60 (s, 2H, L1-c), 7.32 (t, 2H, L1-d), 7.33 (d, 2H, L1-e), 7.24 (m, 1H, L1-f), 8.51 (s, 1H, L1- g). 13C NMR (400 MHz, CD3CN, 298 K): δ 163.88, 156.49, 151.41, 150.27, 142.75, 142.53, 140.04, 139.89, 129.06, 128.18, 127.71, 124.60, 123.38, 123.16, 43.18. FT-IR (KBr, cm-1): 3 336(s), 3 055(s), 1 680(w), 1 606 (m), 1 520(m), 1 459(s), 1 263(s), 1 176(s), 1 042(m), 709.9(m), 636(s). ESI - MS (CH3CN, m/z): Calcd. for [C1]2+ 673.05, Found: 672.58; Calcd. for [C1·PF6-]+ 1 491.07, Found: 1 490.90. Elemental analysis Calcd. for C68H56F12N14O4P2Pd2(%): C, 49.92; H, 3.45; N, 11.99. Found(%): C, 49.92; H, 3.46; N, 11.94.

    The similar procedure as employed for C1·2NO3- and C1·2PF6- were followed except that [(dmbpy)2Pd2 (NO3)2](NO3)2 (41.4 mg, 0.05 mmol) was used as start- ing material to give C2·2NO3- as a light-yellow precipitate (74.6 mg, Yield: 98%), C2·2PF6- as a light-yellow precipitate (82.0 mg, Yield: 97%). C2·2NO3-: 1H NMR (400 MHz, DMSO - d6, 298 K): δ 8.40 (t, 1H, bpy - 1), 8.16 (d, 1H, bpy-2), 7.63 (d, 1H, bpy-3), 2.56 (t, 3H, bpy-4), 9.97 (t, 1 H, L1-a), 8.92 (s, 1H, L1-b), 8.59 (s, 2H, L1-c), 7.31 (t, 2H, L1-d), 7.30 (d, 2H, L1-e), 7.26 (m, 1H, L1-f), 4.60 (s, 2H, L1-g). 13C NMR (400 MHz, DMSO- d6, 298 K): δ 163.83, 155.80, 155.07, 149.87, 139.75, 128.85, 127.41, 125.13, 122.40, 120.07, 42.62, 21.48. Elemental analysis Calcd. for C72H64N16O10Pd2 (%): C, 56.66; H, 4.23; N, 14.68. Found(%): C, 56.64; H, 4.26; N, 14.64. C2·2PF6- : 1H NMR (400 MHz, DMSO, 298 K): δ 8.40 (t, 1H, dmbpy-1), 8.14 (d, 1H, dmbpy-2), 7.62 (d, 1H, dmbpy-3), 2.56 (s, 1H, dmbpy-4), 9.86 (t, 1 H, L1-a), 8.90 (s, 1H, L1-b), 8.59 (s, 1H, L1-c), 7.26 (m, 2H, L1-d), 7.25 (d, 2H, L1-e), 7.24 (d, 1H, L1-f), 4.60 (s, 1H, L1-g). 13C NMR (400 MHz, DMSO-d6, 298 K): δ 163.84, 155.82, 155.05, 150.76, 149.90, 139.77, 128.84, 127.42, 125.16, 122.76, 122.43, 120.10, 119.55, 42.65, 21.50. FT - IR (KBr, cm-1): 3 364(s), 1 668(w), 1 606(m), 1 532(m), 1 422(m), 1 299(m), 1 237(m), 1 066(m), 992(m), 857(m), 562(s). ESI - MS (CH3CN, m/z): Calcd. for [C2]2+ 701.15, Found: 701.62; Calcd. for [C2·PF6-]+ 1 547.17, Found: 1 547.21. Elemental analysis Calcd. for C72H64N14O4F12P2Pd2(%): C, 51.11; H, 3.81; N, 11.59. Found(%): C, 51.14; H, 3.80; N, 11.59.

    The similar procedure as employed for C1·2NO3- and C1·2PF6- were followed except that [(bpy)2Pd2(NO3)2](NO3)2 (38.6 mg, 0.05 mmol) and HL2 (43.6 mg, 0.1 mmol) were used as starting material to give C3· 2NO3- as a light - yellow precipitate (74.5 mg, Yield: 98%), C3·2PF6- as a light-yellow precipitate (81.6 mg, Yield: 96%). C3·2NO3- : 1H NMR (400 MHz, DMSO-d6, 298 K): δ 8.48 (t, 1H, bpy-1), 8.27 (d, 1H, bpy-2), 8.26 (d, 1H, bpy-3), 7.78 (t, 1H, bpy-4), 9.95 (t, 1H, L1- a), 8.75 (s, 1H, L1-b), 7.34 (s, 2H, L1-c), 4.64 (t, 2H, L1- d), 2.61 (d, 3H, L1-e), 7.33 (m, 2H, L1-f), 7.26 (s, 1H, L1-g). 13C NMR (400 MHz, DMSO - d6, 298 K): δ 163.93, 157.25, 151.03, 149.53, 148.82, 144.56, 142.86, 139.77, 128.86, 128.62, 127.45, 127.35, 124.83, 123.71, 42.69, 23.25, 14.02. Elemental analysis Calcd. for C72H64N16O10Pd2 (%): C, 56.66; H, 4.23; N, 14.68. Found(%): C, 56.65; H, 4.24; N, 14.66. C3·2PF6- : 1H NMR (400 MHz, CD3CN, 298 K): δ 8.63 (t, 1H, bpy-1), 8.36 (d, 1H, bpy- 2), 8.32 (d, 1H, bpy-3), 7.75 (s, 1H, bpy-4), 9.81(s, 1H, L2-a), 8.74 (s, 1H, L2-b), 7.31 (m, 2H, L2-c), 7.27 (d, 2H, L2-d), 2.69 (s, 3H, L2-e), 7.27 (d, 2H, L2-f), 7.19 (s, 1H, L2-g). 13C NMR (400 MHz, DMSO - d6, 298 K): δ 163.92, 157.25, 151.03, 149.54, 148.82, 144.54, 142.86, 139.78, 128.84, 127.45, 124.84, 123.69, 117.63, 42.69, 14.01. FT-IR (KBr, cm-1): 3 287(s), 3 000(s), 1 657(w), 1 603(m), 1 525(m), 1 425(m), 1 248(m), 832(m), 764 (m), 558(m). ESI-MS (CH3CN, m/z): Calcd. for [C3]2+ 701.15, Found: 701.17; Calcd. for [C3·PF6-]+ 1 547.17, Found: 1 547.21. Elemental analysis Calcd. for C72H64N14O4F12P2Pd2(%): C, 51.11; H, 3.81; N, 11.59. Found(%): C, 51.12; H, 3.80; N, 11.58.

    The similar procedure as employed for C1·2NO3- and C1·2PF6- were followed except that [(dmbpy)2Pd2(NO3)2](NO3)2 (41.4 mg, 0.05 mmol) and HL2 (43.6 mg, 0.1 mmol) were used as starting material to give C4· 2NO3- as a light - yellow precipitate (78.2 mg, Yield: 99%), C4·2PF6- as a light-yellow precipitate (82.1 mg, Yield: 94%). C4·2NO3- : 1H NMR (400 MHz, DMSO-d6, 298 K): δ 8.24 (t, 1H, bpy-1), 8.09 (d, 1H, bpy-2), 7.60 (d, 1H, bpy-3), 2.57 (t, 3H, bpy-4), 9.96 (t, 1 H, L1 -a), 8.59 (s, 1H, L1-b), 7.33 (s, 2H, L1-c), 4.63 (t, 2H, L1 -d), 2.57 (d, 3H, L1-e), 7.32 (m, 2H, L1-f), 7.26 (s, 1H, L1-g). 13C NMR (400 MHz, DMSO-d6, 298 K): δ 163.91, 156.63, 155.25, 150.24, 149.51, 148.66, 144.55, 139.77, 128.85, 127.44, 125.37, 123.66, 117.52, 42.67, 21.55, 14.01. Elemental analysis Calcd. for C76H72N16O10Pd2 (%): C, 57.69; H, 4.59; N, 14.16. Found(%): C, 57.67; H, 4.56; N, 14.14. C4·2PF6- : 1H NMR (400 MHz, CD3CN, 298 K): δ 8.21 (t, 1H, bpy-1), 7.97 (d, 1H, bpy-2), 7.46 (d, 1H, bpy-3), 2.55 (s, 1H, bpy-4), 9.02 (s, 1H, L2-a), 8.27 (s, 1H, L2-b), 7.35 (t, 2H, L2-c), 4.61 (d, 2H, L2-d), 2.56 (s, 3H, L2-e), 7.32 (t, 2H, L2-f), 7.25 (d, 1H, L2 - g). 13C NMR (400 MHz, DMSO - d6, 298 K): δ 163.88, 156.49, 151.41, 150.27, 142.75, 142.53, 140.04, 139.89, 129.06, 128.84, 128.18, 127.71, 124.60, 123.38, 123.16, 43.18, 21.05, 14.85. FT-IR (KBr, cm-1): 3 294 (s), 1 656(w), 1 532(m), 1 422(m), 1 249(m), 1 030(s), 821(m), 538(s). ESI-MS (CH3CN, m/z): Calcd. for [C4]2+ 729.15, Found: 729.15; Calcd. for [C4·PF6-]+ 1 603.28, Found: 1 603.29. Elemental analysis Calcd. for C76H72N14F12O4P2Pd2(%): C, 52.21; H, 4.15; N, 11.22. Found(%): C, 52.23; H, 4.11; N, 11.23.

    1.3   X-ray crystallography of complex C1·2NO3-

    Single crystals of C1·2NO3- were obtained by vapor diffusion isopropyl ether into its metanol solution. The intensity data collection (single crystals with dimensions of 0.16 mm×0.14 mm×0.12 mm for the complex) was performed on the Bruker Smart APEX Ⅱ CCD diffractometry equipped with graphite monochromated Mo Kα radiation (λ=0.071 073 nm).

    The structure was solved by direct methods and refined employing full - matrix least - squares on F2 by using SHELXTL program[27] and expanded using Fouri- er techniques. The final residuals along with unit cell, space group, data collection, and refinement parame- ters are presented in Table 1. The final selected bond length and bond angles for C1·2NO3- are listed on Table S1 and S2 (Supporting information).

    Table 1

    Table 1.  Crystal structure data and refinement parameters for complex C1·2NO3-
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    Formula C68H56N16O10Pd2 Z 2
    Formula weight 1 470.08 Dc/(Mg·m-3) 1.215
    Crystal system Triclinic Absorption coefficient/mm-1 0.506
    Space group P1 F(000) 1 496
    Volume/nm3 1.925 6(7) Independent reflection 19 853 (Rint=0.027 4, Rσ=0.037 8)
    a/nm 0.127 53(2) Absorption correction Empirical
    b/nm 0.133 84(2) Data, restraint, parameter 19 853, 0, 865
    c/nm 0.246 68(2) Goodness-of-fit on F2 1.074
    α/(°) 97.372(9) Final R indices [I > 2σ(I)] R1=0.029 3, wR2=0.070 4
    β/(°) 102.645(9) R indices (all data) R1=0.038 9, wR2=0.073 4
    γ/(°) 97.553(8) Largest diff. peak and hole/(e·nm-3) 600 and -600

    1.4   Catalytic activity test

    To explore the catalytic activity of pyrazolate-based dipalladium with weak dinuclear Pd(Ⅱ) …Pd(Ⅱ) intra-molecular bonding interaction between two Pd centers, different reaction conditions have been tried and the feasible solution was obtained.

    In a typical experiment, the iodobenzene (204 mg, 1 mmol), 3, 4-dimethoxy benzeneboronic acid (182 mg, 1 mmol), and K3PO4 (318.4 mg, 1.5 mmol), or C1· 2NO3- (14.2 mg, 10 μmmol), or C2·2NO3- (15.2 mg, 10 μmmol), or C3·2NO3- (15.3 mg, 10 μmmol) and C4·2NO3- (15.8 mg, 10 μmmol), or C1·2PF6- (16.3 mg, 10 μmmol), or C2·2PF6- (16.9 mg, 10 μmmol), or C3·2PF6- (16.8 mg, 10 μmmol) and C4·2PF6- (17.4 mg, 10 μmmol) was added into a 100 mL flask. 30 mL 1, 4-dioxane was added and the suspension was stirred at under 100 ℃ and nitrogen atmosphere. After the reactant disappeared (the consumption of the starting iodobenzene was monitored by TLC), the mixture was cooled to room temperature. The mixture was directly filtered and the product was afforded through column chromatograph eluting with hexane/ethyl acetate.

    2.   Results and discussion

    2.1   Characterization of [M2L2]2+ - type coordina-tion "clips"

    The complexes were characterized by electrospray ionization mass spectrometry, NMR spectroscopy, IR spectroscopy and elemental analysis.

    As shown in 1H NMR spectra of [M2L2]2+-type coordination "clips" (Fig. 1, 2, S5, S7~S9, S12~S15, S18 ~S21, S24, S25), the signals of protons of the complexes splited and shifted in comparison with those of the free ligands. The protons of the methyl of the flexible ligand shifted, showing only a single peak in free ligand before self - assembly. The protons of the NH of amide group in the flexible ligand shifted in comparision with free ligand before self-assembly. This can be explained that the CH2-protons and NH-protons of the flexible ligand in the complexes are located in asym- metric chemical environment, which demonstrate a highly symmetrical structure of the complex[28-30].

    Figure 1

    Figure 1.  1H NMR spectrum of C1·2NO3-
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    Figure 2

    Figure 2.  1H NMR spectrum of C1·2PF6-
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    The NMR analysis of C1·2NO3- clearly showed that a 1∶1 (bpy)Pd to L1 complex was formed. As shown in Fig. 1, NH-proton of amide group in the ligand of the product turned out to be one singlet at δ =9.88, which presented one singlet at δ=9.89 for L1-NH, respectively before selfassembly (Fig.S1). In addition, two CH2-protons of the ligand of the product turned out to be two singlets at δ =4.61 and 4.60, which presented two sin- glets at δ=4.62 and 4.64 for L1-CH2, respectively before self-assembly. And the results of 13C NMR spectroscopy (Fig.S7) agreed well with the analysis of 1H NMR spectroscopy. The NMR analysis of C1·2PF6- also clearly showed that a 1∶1 (bpy)Pd to L1 complex was formed. As shown in Fig. 2, NH - proton of amide group in the ligand of the product turned out to be only three sin- glets at δ=9.90~9.86, which presented one singlet at δ= 9.89, respectively before self - assembly (Fig. S1). In addition, two CH2 -protons in the ligand of the product turned out to be two singlets at δ=4.61 and 4.60, which presented two singlets at δ =4.62 and 4.64 for L1 - CH2, respectively before self - assembly. And the results of 13C NMR spectroscopy (Fig. S5) agreed well with the analysis of 1H NMR spectroscopy. In the FT - IR spec- trum of C1·2NO3-, the absorption bands in the region of 3 200~3 400 cm-1 can be attributed to the stretching vibrations of N—H. The bands in the region of 2 895~3 010 cm-1 can be ascribed to C—H stretching vibrations of the benzene ring. The absence of the absorption bands at 1 450~1 600 cm-1 can be ascribed to C—C stretching vibrations of the benzene ring. Moreover, the assignment of product C1·2PF6- as [M2L2]2+ - type dimetallo - clip was further proved by ESI - MS studies where multiply charged molecular ions corresponding to intact dimetallo - clip were observed. As shown in Fig. 3, the multiply charged molecular ions of C1·2PF6- at m/z=672.58, 1 490.90 are ascribed to the cations of [C1]2+, [C1·PF6-]+, respectively.

    Figure 3

    Figure 3.  ESI-MS spectrum of C1·2PF6- in acetonitrile

    Inset: isotopic distribution of species [C1·PF6-]+ and [C1]2+

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    The NMR analysis of C3·2NO3- clearly showed that a 1∶1 (bpy)Pd to L1 complex was formed. As shown in Fig.S18, NH-proton of amide group in the ligand of the product turned out to be one singlet at δ =9.81, which presented one singlet at δ =9.89 for L1 - NH, respectively before self -assembly (Fig.S1). In addition, two CH2-protons in the ligand of the product turned out to be two singlets at δ=4.60 and 4.59, which presented two singlets at δ=4.62 and 4.64 for L1-CH2, respectively before self-assembly. And the results of 13C NMR spectroscopy (Fig. S19) agreed well with the analysis of 1H NMR spectroscopy. The NMR spectrum of C3·2PF6- clearly shows that a 1: 1 (bpy)Pd to L2 complex was formed. As shown in Fig.S14, NH-proton of amide group in the ligand of the product turned out to be only one singlet at δ=9.81, which presented one singlet at δ= 9.96, respectively before self - assembly (Fig. S3). In addition, two CH2-protons in the ligand of the product turned out to be two singlets at δ=4.59 and 4.60, which presented two singlets at δ =4.61 and 4.63 for HL2-CH2 before self-assembly; three CH 3 -protons in the ligand of the product turned out to be one singlet at δ =2.69, which presented one singlet at δ =2.33 for HL2 - CH3 before self-assembly. And the results of 13C NMR spectroscopy agreed well with the analysis of 1H NMR spectroscopy (Fig. S15). In the FT - IR spectrum of C3·2NO3-, the absorption bands in the region of 3 100~ 3 350 cm-1 can be attributed to the stretching vibra- tions of N — H. The bands in the region of 2 870~3 000 cm-1 can be ascribed to C—H stretching vibrations of the benzene ring. The absence of the absorption bands at 1 650~1 450 cm-1 can be ascribed to C—C stretch- ing vibrations of the benzene ring. Moreover, the assignment of product C3·2PF6- as [M2L2]2+-type dimetallo - clip was further proved by ESI - MS studies where multiply charged molecular ions corresponding to intact dimetallo -clip were observed. As shown in Fig. S13, the multiply charged molecular ions of C3·2PF6- at m/z=701.12, 1 547.21 are ascribed to the cations of [C3]2+, [C3·PF6-]+, respectively.

    The other four similar complexes C2·2PF6-, C2·2NO3-, C4·2NO3- and C4·2PF6- are also obtained and characterized by the similar method (Fig. S8~S13, S20~S25). All the characterizations have demonstrated that the preparation of these organometallic clips as mentioned was successful which have similar molecu- lar structures composed of two Pd motifs and two monoanionic ligands. The highly symmetrical struc- tures of the [M2L2]2+ - type of these "molecular clips" have been further confirmed by single-crystal X-ray dif- fraction analysis.

    2.2   Crystal structure of dimetallo complex C1·2NO3-

    The structure of complex C1·2NO3- was successfully determined by X - ray diffraction as depicted in Fig. 4, and labeled ORTEP plots of the cation of the complex are shown in Fig.S1. C1·2NO3- crystallizes in the space group P1. The structure reveals a Pd2 dimetallic clip-shaped structure with two functional ligands doubly bridged [Pd(bpy)]2 dimetal "clip". The [Pd2L2]-type "clip" is supported by two pyrazolate ligands and one [Pd2(bpy)2] motifs (Fig. 4a and 4b). The central six- membered ring consists of two Pd ions and the four pyrazolyl N atoms has a boat-shaped conformation with Pd-Npz distances between 0.19 and 0.20 nm. The Pd1… Pd2 separation is 0.343 nm and the dihedral angle between the planes of two bpy ligands in the "clip" is 118.0°. The deprotonated ligands (L1) are nearly perpendicular to each other, and the dihedral angle is 105. 6° between the planes of pyrazolate groups. And this arrangement creates an "open book" disposition for the square-planar environment of two Pd ions. As shown in Fig. 4c and 4d, dimetallic coordination "clip" can stack into a three-dimensional structure via strong multiple hydrogen bonding and weak π … π stacking interactions. Three intermolecular hydrogen bonds are formed between oxygen acceptors O6, O7 from nitrate anions and amide N donors (N9, N11, N12 and N14). The distances of O…H are 0.211 nm (O7…H12— N12) and 0.224 nm (O7…H14—N14), respectively. In this hydrogen bonding moieties, intramolecular N… H— N bonds are also formed with an average separa- tion of 0.230 nm. The two benzene rings in the complex are parallel, and the distances of the two rings range from 0.513 to 0.520 nm.

    Figure 4

    Figure 4.  Crystal structure of C1·2NO3-: top view (a), side view (b), stacking mode (c); Multiple hydrogen bond interaction modes between nitrate and ligands (d)

    All the solvent molecules are omitted for clarity; Purple: Pd; Green and gray: C; Light grey and pink: H; Blue: N; Red: O

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    2.3   Catalytic activity

    It is well accepted that catalysts containing multiple metal centers in close proximity to each other can lead to better reactivity than the equivalent mixtures of monometallic complexes. In addition, dimetallic catalysts afford a higher nonlocal concentration of the activesites, and this may also lead to better catalytic performance than those of the analogous monometallic catalysts[31-32].

    As shown in the Table 2, the free ligands HL1 and HL2 showed no catalytic activity to Suzuki-coupling reaction of iodobenzene and 3, 4-dimethoxy benzenebo- ronic acid. Meanwhile, the reaction provide product in good yields for C1·2NO3-~C4·2NO3- and C1·2PF6-~C4·2PF6-. This marked difference in catalytic activity may be attributed to the tunable impact and Pd(Ⅱ) …Pd(Ⅱ) intramolecular bonding interaction between the two Pd(Ⅱ) centers. In order to expand the scopes of reaction substrates, six different iodine-substituted and boronic acid-substituted aromatic compounds were chosen to react under the similar condition (Table 3 and S3). In these cases, the desirable products were obtained in high yields, signifying the excellent catalytic activities of C1·2NO3-~C4·2NO3- and C1·2PF6-~C4·2PF6-.

    Table 2

    Table 2.  Catalytic activity of as-prepared catalysts for Suzuki-coupling reaction of iodobenzene and 3, 4-dimethoxy benzeneboronic acid
    下载: 导出CSV
    Entry* Catalyst Yield/%
    1 [(dmbpy)2Pd2(NO3)2](NO3)2 65.4
    2 [(bpy)2Pd2(NO3)2](NO3)2 66.7
    3 HL1 No reaction
    4 HL2 No reaction
    5 C1·2NO3- 89.1
    6 C2·2NO3- 87.1
    7 C3·2NO3- 87.3
    8 C4·2NO3- 86.5
    9 C1·2PF6- 82.7
    10 C2·2PF6- 82.4
    11 C3·2PF6- 82.5
    12 C4·2PF6- 83.1
      * Reaction conditions: co-catalyst: K3PO4, solvent: 1, 4-dioxane, temperature: 100 ℃, time: 24 h.

    Table 3

    Table 3.  Scope of dimetal complex C1·2NO3- catalyst for Suzuki-coupling reactions
    下载: 导出CSV
    Entry* Catalyst R1 R2 Yield/%
    1 C1·2NO3- 81.6
    2 C1·2NO3- 99.0
    3 C1·2NO3- 80.1
    4 C1·2NO3- 77.8
    5 C1·2NO3- 81.5
    6 C1·2NO3- 92.5
    * Reaction conditions: co-catalyst: K3PO4, solvent: 1, 4-dioxane, temperature: 100 ℃, time: 24 h.

    3.   Conclusions

    In conclusion, two ditopic carboxamide-pyrazolate ligands were synthesized and used to construct novel pyrazolate - bridged dipalladium coordination "clips" C1·2NO3- ~C4·2NO3- and C1·2PF6-~C4·2PF6- with strong Pd(Ⅱ) …Pd(Ⅱ) bonding interaction. The coordina- tion "clips"(C1·2NO3-) can trap two nitrate anions via multiple N—H…O hydrogen bonds in the semi - open cavity composed of two amide groups. The complexes were characterized by elemental analysis, NMR, ESI - Ms and IR spectroscopy. Moreover, these pyrazolate - based dipalladium "clips" containing a dimetal active center exhibited good catalytic activity for Suzuki- coupling reaction.

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


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  • Scheme 1  Schematic illustration of self-assembly of dipalladium "clips" C1·2NO3-~C4·2NO3- and C1·2PF6-~C4·2PF6-

    (i) H2O, acetone, reflux; (ii) KPF6

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    Scheme 2  Synthesis of HL1 and HL2

    (i) POCl3, phenylmethanamine, reflux; (ii) Dmtty, K2CO3, Pd(PPh3)4, reflux; (iii) tty, K2CO3, Pd(PPh3)4, reflux; (iv/v) HCl, methanol, reflux

    下载: 全尺寸图片 幻灯片

    Figure 1  1H NMR spectrum of C1·2NO3-

    下载: 全尺寸图片 幻灯片

    Figure 2  1H NMR spectrum of C1·2PF6-

    下载: 全尺寸图片 幻灯片

    Figure 3  ESI-MS spectrum of C1·2PF6- in acetonitrile

    Inset: isotopic distribution of species [C1·PF6-]+ and [C1]2+

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    Figure 4  Crystal structure of C1·2NO3-: top view (a), side view (b), stacking mode (c); Multiple hydrogen bond interaction modes between nitrate and ligands (d)

    All the solvent molecules are omitted for clarity; Purple: Pd; Green and gray: C; Light grey and pink: H; Blue: N; Red: O

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    Table 1.  Crystal structure data and refinement parameters for complex C1·2NO3-

    Formula C68H56N16O10Pd2 Z 2
    Formula weight 1 470.08 Dc/(Mg·m-3) 1.215
    Crystal system Triclinic Absorption coefficient/mm-1 0.506
    Space group P1 F(000) 1 496
    Volume/nm3 1.925 6(7) Independent reflection 19 853 (Rint=0.027 4, Rσ=0.037 8)
    a/nm 0.127 53(2) Absorption correction Empirical
    b/nm 0.133 84(2) Data, restraint, parameter 19 853, 0, 865
    c/nm 0.246 68(2) Goodness-of-fit on F2 1.074
    α/(°) 97.372(9) Final R indices [I > 2σ(I)] R1=0.029 3, wR2=0.070 4
    β/(°) 102.645(9) R indices (all data) R1=0.038 9, wR2=0.073 4
    γ/(°) 97.553(8) Largest diff. peak and hole/(e·nm-3) 600 and -600
    下载: 导出CSV

    Table 2.  Catalytic activity of as-prepared catalysts for Suzuki-coupling reaction of iodobenzene and 3, 4-dimethoxy benzeneboronic acid

    Entry* Catalyst Yield/%
    1 [(dmbpy)2Pd2(NO3)2](NO3)2 65.4
    2 [(bpy)2Pd2(NO3)2](NO3)2 66.7
    3 HL1 No reaction
    4 HL2 No reaction
    5 C1·2NO3- 89.1
    6 C2·2NO3- 87.1
    7 C3·2NO3- 87.3
    8 C4·2NO3- 86.5
    9 C1·2PF6- 82.7
    10 C2·2PF6- 82.4
    11 C3·2PF6- 82.5
    12 C4·2PF6- 83.1
      * Reaction conditions: co-catalyst: K3PO4, solvent: 1, 4-dioxane, temperature: 100 ℃, time: 24 h.
    下载: 导出CSV

    Table 3.  Scope of dimetal complex C1·2NO3- catalyst for Suzuki-coupling reactions

    Entry* Catalyst R1 R2 Yield/%
    1 C1·2NO3- 81.6
    2 C1·2NO3- 99.0
    3 C1·2NO3- 80.1
    4 C1·2NO3- 77.8
    5 C1·2NO3- 81.5
    6 C1·2NO3- 92.5
    * Reaction conditions: co-catalyst: K3PO4, solvent: 1, 4-dioxane, temperature: 100 ℃, time: 24 h.
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  • 发布日期:  2021-08-10
  • 收稿日期:  2021-03-06
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