English

• 0   Introduction

  Construction and characterization of metal-organic coordination polymers have attracted intense interest due to their potential applications in enantioselective separation^[1-3], nonlinear optical^[4-5], and magnetic properties^[6-8]. Heterometallic compounds formed from lanthanides and transition-metal ions are of interest not only because of their fascinating structural topology, but also for their potential applications as functional materials in luminescence^[9-11], molecular adsorption^[12], bimetallic catalysis^[13-14] and magne-tism^[15-17]. There was also considerable progress to design and fabricate d-f heterometallic coordination polymers, from 1D chains, 2D lattices to 3D frameworks^{[10, 18-20]}. Generally, two routes were used to prepare heterometallic coordination polymers: the first route is based on the use of the presynthesized metal complexes which contain uncoordinated donors to combine with the second type of metal ions^[21]; the second route is based on the use of multidentate organic ligands to bind two types of metal ions in a one-pot reaction^[19-20]. Moreover, it is well known that lanthanide ions have a high affinity for binding to hard donors like the O atom, whereas most transition metal ions prefer to coordinate to soft donors like the N atom^[10]. As a consequence, it should be rational to select the polydentate compartmental ligand containing both O and N donors for the construction of new heterometallic coordination polymers. These ligands such as pyazine carboxylic acid^[22-23], imidazolecarboxylic acid^{[21, 24]} and pyridinecar-boxylic acid^[25] have been widely explored for preparing novel d-transition and f-lanthanide homometallic coordination polymers. Thus, 4-(1H-1, 2, 4-triazol-1-yl) benzoic acid (Htbc), which contains three triazol nitrogen atoms and two carboxylate oxygen atoms, might provide the impetus for the synthesis of novel heterometallic complexes. To the best of our knowledge, there is no report that Htbc ligand is used to construct heterometallic coordination polymers. In the current work, Htbc with both N and O donor atoms was used to fabricate chiral Ln-Ag heterometallic coordination polymers. We herein report the synthesis, structure and characterization of two 2D heterometallic complexes based on Htbc ligands: one is a chiral 2D heterometallic Eu-Ag coordination polymer {[EuAg (tbc)₃(H₂O)]ClO₄·H₂O}_n (1), containing triple-stranded helical (PPM) chains with [Eu-OCO] connectivity; the other is a novel achiral 2D heterometallic Eu-Ag coor-dination polymer {[Eu₂Ag₃(tbc)₆(NO₃)₂(H₂O)₂](ClO₄)₂·2H₂O}_n (2).

  1   Experimental

  1.1   General procedures

  Htbc was obtained from Jinan Henghua science and technology Ltd (Jinan, China). The other chemicals were commercially available reagents of analytical grade and used without further purification. Elemental (C, H, N) analyses were performed on Perkin-Elmer 240 element analyzer. The FT-IR spectra were recorded from KBr pellets in the range of 4000~400 cm^-1 on a Nicolet 5DX spectrometer. Powder X-ray diffraction (PXRD) patterns of the samples were recorded using an X-ray diffractometer (BRUKER D8 ADVANCE) with Cu Kα radiation (λ=0.154 18 nm). Fluorescence spectra was recorded with an F-2500 FL Spectrophotometer analyzer.

  Caution: Although we have experienced no problem with the compounds reported in this work, perchlorate salts of metal complexes with organic ligands are often explosive and should be handled with great caution.

  1.2   Preparation of {[EuAg (tbc₃)(H₂O)] ClO₄·=H₂O}_n (1)

  A mixture of Eu₂O₃ (0.5 mmol), AgNO₃ (0.5 mmol), Htbc (0.5 mmol), HClO₄ (0.2 mL) and distilled water (10 mL) was sealed in a 20 mL Teflon-lined stainless steel reactor and then heated to 150 ℃ for 72 h under autogenous pressure. Then the mixture was slowly cooled to room temperature at a rate of 5 ℃·h^-1, and dark red crystals of 1 suitable for X-ray crystal analysis were obtained. Yield: 50%. Anal. Calcd. for C₂₇H₂₂AgClEuN₉O₁₂(%): C 33.76, H 2.29, N 13.13; Found (%): C 33.92, H 2.11, N 13.35. IR (KBr, cm^-1): 3 456vs, 3 132 m, 2 385 m, 1 606 s, 1 544 s, 1 413 s, 1 282 m, 1 107 m, 1 080 m, 781 m, 501.

  1.3   X-ray crystallographic study

  All the diffraction data for complexes 1 and 2 were collected on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) using the ω-scan technique. Multi-scan absorption corrections were applied with the SADABS program^[26]. The structures were solved by direct methods using the SHELXS-97 program and all the non-hydrogen atoms were refined anisotropically with the full-matrix least-squares on F² using the SHELXL-97 program^[27]. The hydrogen atoms of water molecules were located in the difference Fourier maps and the other hydrogen atoms were generated geometrically and refined as riding atoms with isotropic thermal factors. Crystallographic data and structure determination summaries for 1 and 2 are given in Table 1. Selected bond lengths and angles are listed in Table 2.

  Table 1.  Crystal data for compounds 1 and 2

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  Compound	1	2
  Chemical formula	C27H22AgClEuN9O12.	C54H45Ag3Cl2Eu2N20O30
  Formula weight	959.82	2 152.53
  Crystal system	Orthorhombic	Triclinic
  Space group	P212121	P1
  a/nm	0.975 0(2)	0.975 11(5)
  b/nm	1.353 9(3)	1.285 31(4)
  c/nm	2.369 2(5)	1.428 35(7)
  α/(°)		115.031(4)
  β/(°)		95.365 (4)
  γ/(°)		91.279(3)
  V/nm3	3.127 50(15)	1.611 13(12)
  Z	4	1
  Dc/(g·cm-3)	2.038	2.219
  μ/mm-1	2.779	3.007
  θ range/(°)	2.57~25.01	2.48~25.01
  F(000)	1 880	1 050
  Reflections collected	10 218	12 286
  Independent reflections (Rint)	5 379	5 658 (0.046 7)
  Reflections oliserved [I>2σ(I)]	4 881	4 711
  GOF on F2	1.015	1.017
  R1[I>2σ(I)]	0.034 4	0.046 5
  wR2[I>2σ(I)]	0.055 5	0.109 6
  Symmetry code: i-0.5-x, 3-y, 0.5+z; ii-1-x, -0.5+y, 2.5-z; iii-x, -0.5+y, 2.5-z; iv 1-x, 2-y, 1-z; v 1-x, 1-y, 1-z; vi 1-x, -y, -z.

  Table 2.  Selected bond distances (nm) and angles (°) for 1 and 2

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  CCDC: 1018336, 1; 1024087, 2.

  2.3   Preparation of {[Eu₂Ag₃(tbc)₆(NO₃)₂(H₂O)₂]·(ClO₄)₂ (H₂O)₂}_n (2)

  A mixture of Eu₂O₃ (0.5 mmol), AgNO₃ (0.5 mmol), Htbc (0.5 mmol), HClO₄ (0.1 mL) and distilled water (10 mL) was sealed in a 20 mL Teflon-lined stainless steel reactor and then heated to 170 ℃ for 72 h under autogenous pressure. Then the mixture was slowly cooled to room temperature at a rate of 5 ℃·h^-1, and colorless crystals of 2 suitable for X-ray crystal analysis were obtained. Yield: 10%. Anal. Calcd. for C₅₄H₄₅Ag₃Cl₂Eu₂N₂₀O₃₀(%): C 30.10, H 2.09, N 13.01; Found (%): C 30.47, H 2.14, N 13.35. IR (KBr, cm^-1): 3 128 vs, 2 916 m, 2 788 m, 2 510 m, 1 913 m, 1 691 s, 1 605 s, 1 524 s, 1 445 m, 1 412 s, 1 261m, 1275 s, 1 221m, 1 156 m, 975 s, 861 m, 774 m.

  2   Results and discussion

  2.1   Description of the crystal structure of 1

  Single-crystal X-ray diffraction analysis of 1 reveals that the complex crystallizes in the orthorhom-bic chiral space group P2₁2₁2₁ and possesses a 2D heterometallic coordination framework. The asymmetric unit of 1 contains one perchlorate (ClO₄^-), three tbc anion, one Eu (Ⅲ) ion, one Ag (Ⅰ) ion, one coordinated water molecule and one uncoordinated water molecule. As shown in Fig. 1, the Eu (Ⅲ) center is coordinated with seven oxygen atoms, among which six oxygen atoms are derived from six different tbc ions and one water molecule completing the coor-dination sphere of the Eu (Ⅲ) ion with O₇ donor set. The Eu (Ⅲ)-O bond lengths (0.231 4(4)~0.240 3(5) nm) are in the normal range^[8]. The bond angles around the central Eu (Ⅲ) atom vary from 68.76(18)° to 177.2(2)°, similar to those reported for other seven-coordinated Eu (Ⅲ) coordination polymers with oxygen donor ligands^[10]. As far as the Ag (Ⅰ) ion in the coordination framework is concerned, it exhibits a linear configuration, being coordinated by two N atoms from two bridging tbc ligands. The Ag-N bond lengths are in the range of 0.213 5(6) to 0.214 5(7) nm, and the N-Ag-N bond angle is 174.5(3)°. In Complex 1, the Htbc ligands exhibit two types of distinctly different coordination modes: one acts as a bridging ligand to coordinate one Ag (Ⅰ) ion and two Eu (Ⅲ) ions (Scheme 1, Mode Ⅰ) and the other acts a terminal ligand to coordinate two Eu (Ⅲ) ions (Scheme 1, Mode Ⅱ).

  Figure Scheme 1.  Coordination modes of the tbc anions in compounds 1 and 2

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  Figure 1.  Coordination environment of the Eu³⁺ and Ag⁺ ions in Complex 1 with 30% thermal ellipsoids

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  In the structure of 1, the chiral unit can be considered as a right-handed helix. A pair of Eu (Ⅲ) ions are interconnected through three carboxylate groups of three ligands tbc-with modes Ⅰ and Ⅱ to give a [Eu (μ₃-OCO)₂(μ₂-OCO)] unit with Eu1…Eu1 distance of 0.499 1 nm, which may be viewed as a secondary building unit (SBU) to further extend into 1D triple strand right-right-left-handed helical (PPM) chain along the a axis (Fig. 2). Although all the components appearing in the system are achiral, 1 crystallizes in the chiral space group P2₁2₁2₁. A direct vision of the topology of the 1D chain structure is illustrated in Fig. 2. As you can see from Fig. 2, the net handedness of the chain is right-handed.

  Figure 2.  1D three-strand helical chains [Eu (μ₃-OCO)₂(μ₂-OCO)] (PPM) in compound 1 along the a axis

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  These parallel right-right-left (PPM)-handed triple stranded helical chains are further linked by the [AgN₂] unit constructed from two triazole nitrogen atoms of the μ₃-(κ³ N, O, O′)-tbc-ligands, leading to the formation of a wave-like 2D 4d-4f heterometallic coordination polymer in the ac plane (Fig. 3). Furthermore, the adjacent 2D layers are linked through hydrogen bonds (Table 3) involving carboxylate oxygen atoms, coordinated water, uncoordinated water molecules, ClO₄^- and triazol nitrogen atoms, thereby producing a 3D supramole-cular network (Fig. 4).

  Table 3.  Hydrogen bonding parameters for Complex 1 and 2

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  Figure 3.  2D chiral heterometallic layer in 1 fabricated by the coordination link of ligand tbc^- in mode Ⅰ between two adjacent right-right-left (PPM)-handed helical chains in the ac plane, in which the ligand tbc^- has coordination mode Ⅱ

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  Figure 4.  Packing structure constructed through hydrogen bonds (dot line) with an-ABAB-sequence along the b axis

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  2.2   Description of the crystal structures of 2

  Single crystal X-ray diffraction analyses reveal that complex 2 crystallizes in the triclinic space group P1 and feature interesting 2D 4d-4f heterometallic coordination polymers. As depicted in Fig. 5, the symmetric unit of 2 consists of one crystallographi-cally independent Eu (Ⅲ) cation, two Ag (Ⅰ) cation (The crystallographically occupancy of the two Ag (Ⅰ) cations is 1 and 0.5 respectively), three unique tbc-anions, one coordinated nitrate ion, one perchlorate (ClO₄^-), one coordinated water molecule, as well as one lattice water molecule. The coordination polyhedron around the central Eu (Ⅲ) atom can be visualized as slightly distorted bicapped trigonal prismatic arrangement with a [EuO₈] coordination sphere: five carboxylate oxygen atoms come from five individual tbc-anions; one oxygen comes from one coordinated water molecule; other two oxygen atoms come from one nitrate anion. The Eu-O bond lengths are 0.230 4(5) and 0.258 6(6) nm. The bond angles around the central Eu (Ⅲ) atom vary from 24.75(17)° to 165.22(18)°, similar to those reported for other eight-coordinated Eu (Ⅲ) coordination polymers with oxygen donor ligands^[26]. In complex 2, the Htbc ligands exhibit two bridging modes (Scheme 1, Mode Ⅰ and Ⅲ): one acts as a μ₂ bridge between one Eu (Ⅲ) ion and one Ag (Ⅰ) ion; the other one acts as a μ₃ bridge between two Eu (Ⅲ) ions and one Ag (Ⅰ) ion. Concerning two Ag (Ⅰ) ions, both Ag1 and Ag₂ ions are two coordinated with two nitrogen atoms belonging to two different tbc^- anions, forming the linear configuration. The Ag-N bond lengths are in the range of 0.211 3(6) to 0.214 3(6) nm, and the N-Ag-N bond angles are 180.0(6)° and 174.1(2)°. The main difference is that Ag1 ion is coordinated by two N atoms from two triazole rings with μ₃-(κ³ N, O, O′) coordination mode, while Ag₂ is coordinated by two N atoms from two triazole groups with μ₃-(κ³ N, O, O′) and μ₂-(κ² N, O) coordination modes.

  Figure 5.  Coordination environment of the Eu³⁺ and Ag⁺ ions in complex 2 with 30% thermal ellipsoids

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  Two of the tbc^- ligands in complex 2 link to two Eu1 atoms in a μ₃-(κ³ N, O, O′) mode (Scheme 1a). As illustrated in Fig. 6, every two Eu1 ions connect to four μ₃-tbc^- ligands and every two μ₃-tbc^- ligands link two Eu1 ions, giving rise to an infinite 1D mesomeric chain consisting of a right-left-handed helical (PM) chains along the a axis (Fig. 6a), where the adjacent two nonbonding Eu (Ⅲ)…Eu (Ⅲ) distances are equal to 0.4935 3(6) nm and 0.527 72(6) nm, respectively. These 1D chains are further linked by the [N-Ag1-N] unit from two triazole nitrogen atoms of the μ₃-tbc^- ligands and the [N-Ag₂-N] unit from two triazole nitrogen atoms of the μ₃-(κ³ N, O, O′)-tbc^- and μ₂-(κ² N, O)-tbc^- ligands, leading to the formation of a n interesting 2D 4d-4f heterometallic coordination polymer in the ab plane (Fig. 7). Furthermore, the 2D networks are stacked via interdigitation along the c axis (Fig. 8). The alternate 2D layers are linked by van der Waals interaction and hydrogen bonds (Table 3) involving oxygen atoms of the ligands, perchlorate oxygen atoms and uncoordinated water molecules, thereby producing a 3D supramolecular network (Fig. 8).

  Figure 6.  (a) 1D meso-chain [Eu (μ₃-OCO)₂] (PM) in compound 2 along the a axis; (b) A fragment of the 1D meso-chain of complex 2

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  Figure 7.  2D heterometallic layer in 2 fabricated by the coordination link of ligand tbc^- in modes Ⅰ and Ⅲ between two adjacent meso-chains in the ab plane

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  Figure 8.  3D supramolecular framework organized by the H-bonding (dot line) in complex 2

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  2.3   PXRD pattern and photoluminescent properties

  The PXRD pattern of 1 is in good agreement with the ones simulated from single-crystal structural data, which confirmed the purity of the bulk sample (Fig. 9).

  Figure 9.  PXRD pattern of 1

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  Because of the excellent luminescent properties of Eu (Ⅲ) ions, the luminescences of 1 and 2 were investigated. The emission spectrum of 1 (Fig. 10a) at room temperature upon excitation at 395 nm displays red luminescence and exhibits the characteristic transition of ⁵D₀→⁷F_J (J=0~4) of Eu (Ⅲ) ions. The emission spectrum is dominated by the characteristic ⁵D₀→⁷F₂ electron dipole transition at 615 nm. In addition, two weak peaks at 651 nm and 705 nm can be attributed to the ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transition, respectively^[28]. Similarly, complex 2 exhibits the characteristic emission of Eu (Ⅲ) ions (Fig. 10b).

  Figure 10.  Solid-state emission spectra of complexes 1 (a) and 2 (b)

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  3   Conclusions

  In conclusion, we have successfully synthesized two novel 2D Eu-Ag heterometallic coordination polymers. Compound 1 is chiral and its 2D lattices are constructed from the parallel right-right-left (PPM)-handed triple stranded helical chains [Eu (OCO)₃]_n which are further linked by Ag (Ⅰ) and the fragments of tbc^- ligands. In complex 2, the 1D chains of [Eu (OCO)₂]_n are linked by the [N-Ag1-N] unit from two triazole nitrogen atoms of the μ₃-tbc^- ligands and the [N-Ag₂-N] unit from two triazole nitrogen atoms of the μ₃-(κ³ N, O, O′)-tbc^- and μ₂-(κ² N, O)-tbc^- ligands, leading to the formation of an 2D d-4f heterometallic coordination polymer. The complexes 1 and 2 exhibit characteristic lanthanide luminescence of rare earth ion Eu³⁺. The result obtained here can enrich the structure chemistry of transition-lanthanides with chiral helix Ln-OCO-Ln cluster cores.

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• 

  Scheme 1  Coordination modes of the tbc anions in compounds 1 and 2

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  Figure 1  Coordination environment of the Eu³⁺ and Ag⁺ ions in Complex 1 with 30% thermal ellipsoids

  All hydrogen atoms are omitted for clarity; Symmetry code: ⁱ 0.5+x, 2.5-y, 3-z; ⁱⁱ 0.5+x, 2.5-y, 2-z; ⁱⁱⁱ x, y, 1+z

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  Figure 2  1D three-strand helical chains [Eu (μ₃-OCO)₂(μ₂-OCO)] (PPM) in compound 1 along the a axis

  Symmetry code: ⁱ 1.5+x, 2.5-y, 3-z; ⁱⁱ 1+x, y, z; ⁱⁱⁱ 0.5+x, 2.5-y, 3-z; ^iv-0.5+x, 2.5-y, 3-z; ^v-1+x, y, z; ^vi-1.5+x, 2.5-y, 3-z; ^vii-2+x, y, z

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  Figure 3  2D chiral heterometallic layer in 1 fabricated by the coordination link of ligand tbc^- in mode Ⅰ between two adjacent right-right-left (PPM)-handed helical chains in the ac plane, in which the ligand tbc^- has coordination mode Ⅱ

  Hydrogen atoms, ClO₄^- anions, coordinated water and uncoor-dinated water molecules have been omitted for clarity

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  Figure 4  Packing structure constructed through hydrogen bonds (dot line) with an-ABAB-sequence along the b axis

  Symmetry code: ⁱ 0.5+x, 1.5-y, 2-z; ⁱⁱ 1+x, -1+y, -1+z; ⁱⁱⁱ 0.5+x, 1.5-y, 2-z; ^iv 0.5-x, 1-y, -0.5+z; ^v-x, -1.5+y, 1.5-z

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  Figure 5  Coordination environment of the Eu³⁺ and Ag⁺ ions in complex 2 with 30% thermal ellipsoids

  All hydrogen atoms are omitted for clarity; Symmetry code: ⁱ 1-x, 2-y, 1-z; ⁱⁱ 2-x, 2-y, 1-z; ⁱⁱⁱ 2-x, -y, -z; ^iv 1-x, -y, -z

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  Figure 6  (a) 1D meso-chain [Eu (μ₃-OCO)₂] (PM) in compound 2 along the a axis; (b) A fragment of the 1D meso-chain of complex 2

  H atoms of Carbons have been omitted for clarity; Symmetry code: ⁱ 1+x, y, z; ⁱⁱ 1-x, 2-y, 1-z; ⁱⁱⁱ-x, 2-y, 1-z; ^iv-1+x, y, z; ^v-1-x, 2-y, 1-z; ^vi-2+x, y, z

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  Figure 7  2D heterometallic layer in 2 fabricated by the coordination link of ligand tbc^- in modes Ⅰ and Ⅲ between two adjacent meso-chains in the ab plane

  Hydrogen atoms, ClO₄^- anions, coordinated NO₃^- anions, coor-dinated water and uncoordinated water molecules have been omitted for clarity

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  Figure 8  3D supramolecular framework organized by the H-bonding (dot line) in complex 2

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  Figure 9  PXRD pattern of 1

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  Figure 10  Solid-state emission spectra of complexes 1 (a) and 2 (b)

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  Table 1.  Crystal data for compounds 1 and 2

  Compound	1	2
  Chemical formula	C27H22AgClEuN9O12.	C54H45Ag3Cl2Eu2N20O30
  Formula weight	959.82	2 152.53
  Crystal system	Orthorhombic	Triclinic
  Space group	P212121	P1
  a/nm	0.975 0(2)	0.975 11(5)
  b/nm	1.353 9(3)	1.285 31(4)
  c/nm	2.369 2(5)	1.428 35(7)
  α/(°)		115.031(4)
  β/(°)		95.365 (4)
  γ/(°)		91.279(3)
  V/nm3	3.127 50(15)	1.611 13(12)
  Z	4	1
  Dc/(g·cm-3)	2.038	2.219
  μ/mm-1	2.779	3.007
  θ range/(°)	2.57~25.01	2.48~25.01
  F(000)	1 880	1 050
  Reflections collected	10 218	12 286
  Independent reflections (Rint)	5 379	5 658 (0.046 7)
  Reflections oliserved [I>2σ(I)]	4 881	4 711
  GOF on F2	1.015	1.017
  R1[I>2σ(I)]	0.034 4	0.046 5
  wR2[I>2σ(I)]	0.055 5	0.109 6
  Symmetry code: i-0.5-x, 3-y, 0.5+z; ii-1-x, -0.5+y, 2.5-z; iii-x, -0.5+y, 2.5-z; iv 1-x, 2-y, 1-z; v 1-x, 1-y, 1-z; vi 1-x, -y, -z.

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  Table 2.  Selected bond distances (nm) and angles (°) for 1 and 2

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  Table 3.  Hydrogen bonding parameters for Complex 1 and 2

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