English

• 0.   Introduction

  Metal phosphonates as an important class of metal-organic materials or coordination polymers have received increasing attention in recent years due to not only their fascinating structures but also their promising applications as functional materials in the fields of gas separation/sorption^[1], catalysis^[2], proton conduction^[3], optical applications^[4], magnetism^[5] and so on, all of which are highly structure-sensitive. Phosphonates, [RPO₃]^2-, possess three oxygen atoms that form stronger bonds with metal ions than carboxylate counterparts^[6-8]. Thus one important property of phosphonates is the versatile multisite coordination capability binding from one up to nine metal centers for a single [RPO₃]^2- ligand that would coordinate metals in any degree of protonation^[9]. Furthermore, the adjustable components of phosphonates ranging from mono- to di-, tris-, and tetrakis-phosphonate^{[1, 10-12]} as well as other bifunctional phosphonates like carboxyphosphonates^[13-15] offer an opportunity to create abundant metal phosphonates with fascinating skeletons and characteristics. Cadmium is an excellent candidate for fluorescent sensors due to its d¹⁰ electronic configuration, which is usually used to construct coordination polymers with excellent fluorescence properties. In comparison with other transition metal phosphonates, the employment of cadmium in metal phosphonates is less investigated^[16-24]. As a result, there is still a fairly large amount of work involving cadmium phosphonates that is required to be done.

  Many factors, including the coordination geometry of the central metal ions, the connective modes of the organic ligands, and the synthesis conditions, can affect the final structures. Significant efforts have been undertaken to comprehend these factors by examining various metal centers, functionalized phosphonate ligands, and synthesis conditions. The auxiliary ligand also significantly impacts the final structure, but, until now, a systematic investigation into the effects of auxiliary ligands with varying rigidity on structure formation remains underexplored.

  Very recently we reported metal diphosphonates synthesized from 1,4-naphthalenediphosphonic acid (1,4-ndpaH₄) and two flexible N-donor organic templates, 1,3-di(4-pyridyl)propane (1,3-dpp), bis(imidazol-1-ylmethyl)benzene (1,4-bix)^[25-27]. Among these compounds, flexible N-donor auxiliary ligands act as templates, which is extremely rare among reported phosphonates. We speculate that the rigidity or flexibility of the auxiliary ligands may play an important role in the assembly of diphosphonates. The diphosphonate ligand mainly adopted tetradentate or bidentate coordination modes, providing an outstanding platform to investigate the effect of auxiliary ligands on the structure formation. Therefore, we reacted it with cadmium salts and N-donor auxiliary ligands of different rigidity or flexibility, including 4,4′-bipyridine (4,4′-bpy), 1,4-bis(1-imidazolyl)benzene (1,4-bib), 1,2-di(4-pyridyl)ethylene (1,2-dpe), bis(imidazol-1-ylmethyl)benzene (1,2-bix), and 1,3-dpp (Scheme 1). Fortunately, a series of new metal diphosphonates, namely [Cd_1.5(1,4-ndpaH₂)₂ (4,4′-bpyH)(4,4′-bpy)_0.5(H₂O)₂]₂ (1), [Cd(1,4-ndpaH₂) (1,4-bib)_0.5(H₂O)] (2), [Cd(1,4-ndpaH₃)₂(1,2-dpe)(H₂O)]·(1,2-dpe)·7H₂O (3), (1,2-bixH)[Cd₃(1,4-ndpaH)(1,4-ndpaH₂)₂(H₂O)₂] (4), and [Cd(1,4-ndpaH₂)(H₂O)]·H₂O (5) were obtained successfully. Herein, we report their syntheses, structures, and luminescent properties.

  Scheme 1

  

  Scheme 1.  Molecular structures of 1,4-ndpaH₄ and N-donor auxiliary ligands

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  1.   Experimental

  1.1   Materials and measurements

  1,4-ndpaH₄ was synthesized according to the literature^[28]. All starting materials were of analytical reagent grade and used as received without further purification. Elemental analysis for C, H, and N was performed on a Perkin-Elmer 240C elemental analyzer. Infrared spectra were measured as KBr pellets on a Bruker Tensor 27 spectrometer in a 400-4 000 cm^-1 range. Thermogravimetric analysis (TGA) was performed on a METTLER TOLEDO TGA/DSC-1 from room temperature to 800 ℃ under a nitrogen flow at a heating rate of 10 ℃·min^-1. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ=0.154 06 nm) operating at 45 kV and 40 mA over a 2θ range of 5°-50° at room temperature. The steady fluorescence spectra were attained at Bruker Spectrofluorimeter LS55.

  1.2   Synthesis

  1.2.1   Synthesis of compound 1

  A mixture of Cd(NO₃)₂·4H₂O (61.6 mg, 0.2 mmol), 1,4-ndpaH₄ (28.8 mg, 0.1 mmol), and 4,4′-bpy (15.6 mg, 0.1 mmol) in 10 mL of water, which pH value was adjusted to 3.5 with 0.5 mol·L^-1 NaOH solution, was sealed in a Teflon-lined autoclave and heated at 120 ℃ for 3 d. After cooling to room temperature, faint yellow lamellar crystals were collected and washed with water by suction filtration. Yield: 13.2 mg. Elemental analysis Calcd. for C₇₀H₆₆Cd₃N₆O₂₈P₈(%): C, 41.53; H, 3.29; N, 4.15. Found(%): C, 42.31; H, 3.21; N, 4.09.

  1.2.2   Synthesis of compound 2

  A mixture of Cd(NO₃)₂·4H₂O (61.8 mg, 0.2 mmol), 1,4-ndpaH₄ (28.2 mg, 0.1 mmol), and 1,4-bib (21.6 mg, 0.1 mmol) in 10 mL of water, which pH value was adjusted to 2.8 with 0.5 mol·L^-1 NaOH solution, was sealed in a Teflon-lined autoclave and heated at 120 ℃ for 2 d. After cooling to room temperature, colorless lamellar crystals were collected and washed with water by suction filtration. Yield: 35.8 mg. Elemental analysis Calcd. for C₁₆H₁₅CdN₂O₇P₂(%): C, 36.84; H, 2.90; N, 5.37. Found(%): C, 35.89; H, 2.89; N, 5.31.

  1.2.3   Synthesis of compound 3

  A mixture of Cd(NO₃)₂·4H₂O (61.6 mg, 0.2 mmol), 1,4-ndpaH₄ (28.4 mg, 0.1 mmol), and 1,2-dpe (18.2 mg, 0.1 mmol) in 10 mL of water, which pH value was adjusted to 3.5 with 0.5 mol·L^-1 NaOH solution, was sealed in a Teflon-lined autoclave and heated at 140 ℃ for 3 d. After cooling to room temperature, green-yellow lamellar crystals were collected and washed with water by suction filtration. Yield: 9.8 mg. Elemental analysis Calcd. for C₄₄H₅₄CdN₄O₂₀P₄(%): C, 44.22; H, 4.55; N, 4.69. Found(%): C, 44.25; H, 4.59; N, 4.61.

  1.2.4   Synthesis of compound 4

  A mixture of Cd(NO₃)₂·4H₂O (61.5 g, 0.2 mmol), 1,4-ndpaH₄ (28.8 mg, 0.1 mmol), and 1,2-bix (23.8 mg, 0.1 mmol) in 10 mL of water, which pH value was adjusted to 3.5 with 0.5 mol·L^-1 NaOH solution, was sealed in a Teflon-lined autoclave and heated at 120 ℃ for 3 d. After cooling to room temperature, colorless lamellar crystals were collected and washed with water by suction filtration. Yield: 51.3 mg. Elemental analysis Calcd. for C₄₄H₄₂Cd₃N₄O₂₀P₆(%): C, 36.00; H, 2.88; N, 3.82. Found(%): C, 36.11; H, 2.71; N, 3.79.

  1.2.5   Synthesis of compound 5

  A mixture of Cd(NO₃)₂·4H₂O (61.3 g, 0.2 mmol), 1,4-ndpaH₄ (28.2 mg, 0.1 mmol), and 1,3-dpp (19.6 mg, 0.1 mmol) in 10 mL of water, which pH value was adjusted to 2.5 with 0.5 mol·L^-1 NaOH solution, was sealed in a Teflon-lined autoclave and heated at 120 ℃ for 4 d. After cooling to room temperature, colorless flake crystals were collected and washed with water by suction filtration. Yield: 53.9 mg. Elemental analysis Calcd. for C₁₀H₁₂CdO₈P₂(%): C, 27.64; H, 2.78. Found(%): C, 27.29; H, 2.75.

  1.3   Crystallographic data collection and refinement

  Single crystals with sizes of 0.16 mm×0.15 mm×0.13 mm for 1, 0.26 mm×0.22 mm×0.20 mm for 2, and 0.26 mm×0.23 mm×0.17 mm for 3, 0.10 mm×0.04 mm×0.04 mm for 4, and 0.20 mm×0.15 mm×0.10 mm for 5, were used for structural determination on a Bruker D8 Venture diffractometer using graphite-monochromated (Mo Kα, λ=0.071 073 nm at 100 K for 1, 3, 4, 5 and Cu Kα, λ=0.154 178 nm at 100 K for 2). A hemisphere of data was collected in a 2θ range of 3.148°-58.868° for 1, 3.905°-70.062° for 2, 3.596 2°-52.744° for 3, 3.944°-76.475° for 4, and 5.232°-52.982° for 5. The numbers of observed and unique reflections are 26 430 and 7 863 (R_int=0.084 9) for 1, 16 837 and 4 117 (R_int=0.029 3) for 2, 41 700 and 9 818 (R_int=0.057 4) for 3, 25 735 and 8 691 (R_int=0.048 4) for 4, 23 715 and 2 731 (R_int=0.076 4) for 5. Using Olex2, the structure was solved with the SHELXT structure solution program using Intrinsic Phasing and refined with the SHELXL refinement package using Least Squares minimization. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. Details of the crystal data and refinements of 1-5 are summarized in Table 1, and selected bond lengths and angles of 1-5 are listed in Table S1-S5 (Supporting information).

  Table 1

  

  Table 1.  Crystallographic data and structure refinement details for compounds 1-5

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  Parameter	1	2	3	4	5
  Formula	C70H66Cd3N6O28P8	C16H15CdN2O7P2	C44H54CdN4O20P4	C44H42Cd3N4O20P6	C10H12CdO8P2
  Formula weight	2 024.28	521.65	1195.19	1467.82	434.54
  Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic	Triclinic
  Space group	P21/c	P21/n	P1	P1	P1
  a/nm	1.025 05(3)	0.691 58(11)	1.227 77(4)	1.219 37(2)	0.572 64(5)
  b/nm	2.099 10(4)	1.495 8(2)	1.370 59(4)	1.331 10(3)	1.032 00(8)
  c/nm	1.645 18(3)	1.734 6(3)	1.623 95(3)	1.700 42(3)	1.128 59(12)
  α/(°)			106.821(2)	76.423(2)	86.527(3)
  β/(°)	93.340(2)	93.263(6)	91.942(2)	85.357(2)	85.056(4)
  γ/(°)			110.994(2)	66.453(2)	85.043(3)
  V/nm3	3.533 89(14)	1.791 4(5)	2.413 06(13)	2.459 19(9)	0.661 04(10)
  Z	2	4	2	2	2
  Dc / (g·cm-3)	1.902	1.934	1.645	1.982	2.183
  μ/mm-1	1.171	11.874	0.670	12.893	1.931
  F(000)	2 032.0	1 036.0	1 228.0	1 452	428.0
  Rint	0.084 9	0.029 3	0.057 4	0.048 4	0.076 4
  GOF on F 2	1.053	1.078	1.115	1.018	1.176
  R1, wR2* [I > 2σ(I)]	0.043 6, 0.107 0	0.033 2, 0.081 2	0.060 3, 0.156 7	0.037 1, 0.104 4	0.040 4, 0.085 2
  R1, wR2 (all data)	0.055 5, 0.115 2	0.038 8, 0.084 1	0.071 5, 0.162 1	0.040 7, 0.107 5	0.047 7, 0.088 9
  (Δρ)max, (Δρ)min / (e·nm-3)	2 000, -800	980, -330	2 190, -870	2 990, -707	760, -760
  ${ }^* R=\sum\left\|F_{\mathrm{o}}\left|-\left|F_{\mathrm{c}} \| / \sum\right| F_{\mathrm{o}}\right|, w R=\left[\sum w\left(F_{\mathrm{o}}^2-F_{\mathrm{c}}^2\right)^2 / \sum w\left(F_{\mathrm{o}}^2\right)^2\right]^{1 / 2} .\right.$

  2.   Results and discussion

  2.1   Synthesis

  Five cadmium naphthalene-diphosphonates 1-5 were obtained by hydrothermal reaction of Cd(NO₃)₂·4H₂O, 1,4-ndpaH₄, and different auxiliary ligands 4,4′-bpy, 1,4-bib, 1,2-dpe, 1,2-bix, and 1,3-dpp, respectively (Scheme 2). Their crystal structures were characterized by single-crystal X-ray diffraction analysis, associated with IR spectra (Fig.S1), PXRD patterns (Fig.S1-S6), TGA, and elemental analysis.

  Scheme 2

  

  Scheme 2.  Synthetic routes of compounds 1-5

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  2.2   Structure description

  2.2.1   Structure description of compounds 1 and 2

  Compound 1 crystallizes in the monoclinic system space group P2₁/c, and the asymmetric unit consists of 1.5 crystallographic independent Cd(Ⅱ) cations (with 1.0 occupancy for Cd1, 0.5 occupancy for Cd2), two 1,4-ndpaH₂^2- anions, one singly protonated 4,4′-bpyH⁺ cation, half a 4,4′-bpy molecule, and two coordinated water molecules (Fig. 1a). Here, Cd1 cation is coordinated in a slightly distorted octahedron by two nitrogen atoms (N1 and N2) from two different bridging 4,4′-bpy molecules, two oxygen atom (O1 and O6A) from two different 1,4-ndpaH₂^2- anions, and two oxygen atom from two coordinated water molecules (O13 and O14). Cd2 center is octahedrally coordinated to six oxygen atoms (O2, O2B, O7, O7B, O12A, and O12C) from six different 1,4-ndpaH₂^2- anions (Fig. 1a) (Symmetry codes: A: x+1, y, z; B: 1-x, 1-y, 1-z; C: -x, 1-y, 1-z). The Cd—O bond lengths and O—Cd—O angles fall in a range of 0.221 7(2)-0.246 2(11) nm and 80.3(3)°-180.0(5)°, respectively. The selected bond lengths and bond angles are reported in Table S1.

  Two 1,4-ndpaH₂^2- ligands are crystallographically distinguished with different coordination modes. One acts as a tridentate bridging ligand, binding to three Cd ions via three phosphonate oxygen (O1, O2 from P1, and O6 from P2) atoms. The remaining three phosphonate oxygen atoms are either protonated (for O3 and O5) or pendent (for O4) (Scheme 3 mode A). In another one, two phosphono groups (PO₃) bind two Cd ions in a bridging monodentate fashion (Scheme 3 mode B), respectively. As a consequence, the two Cd1 and one Cd2 are bridged by one O—P—O linker forming a trimeric unit of Cd₃(O—P—O)₂ (Fig. 1b). The equivalent trimers are fused by 1,4-ndpaH₂^2-, forming a ribbon with pendent monodentate 4,4′-bpyH ligands along the a-axis (Fig. 1b). The ribbons are cross-linked by 4,4′-bpy ligands, forming a 2D layer in the ab plane (Fig. 1b). These layers are stacked alternatively in ABAB mode with the organic groups of the 1,4-ndpaH₂^2- ligands protruding out to the inter-layer space. The shortest interchain Cd…Cd distances between the layers are 0.825 74 nm. The van der Waals interactions are dominant between the layers, leading to a 3D supramolecular structure (Fig. 1c).

  Compound 2 crystallizes in the monoclinic system space group P2₁/n and shows a 3D framework structure. The asymmetric unit contains one Cd(Ⅱ) cation, one 1,4-ndpaH₂^2- anion, half a 1,4-bib molecule as well as one coordinated water molecule (Fig. 1d). Compared with compound 1, Cd1 is five-coordinated with a distorted trigonal-bipyramidal geometry (Fig. 1d). The equatorial positions are occupied by one oxygen atom of phosphonate (O2), one oxygen atom of coordinated water molecule (O1), and one nitrogen atom of 1,4-bib (N1) [Cd1—O: 0.193 7(2)-0.198 7(2) nm, Cd1—N: 0.194 9(2)-0.201 7(2) nm], and while the axial positions are occupied by two phosphonate oxygen atoms [O6A and O7B, Symmetry codes: A: 1/2+x, 3/2-y, 1/2+z; B: -1/2+x, 3/2-y, 1/2+z; Cd—O(N): 0.218 7(2)-0.224 8(3) nm, O(N)—Cd—O(N): 81.28(8)°-155.86(8)°]. The axial O6A—Cd1—O7B angles are 155.8°. In the 1,4-ndpaH₂^2- ligand, one phosphono group binds to a Cd(Ⅱ) cation in a monodentate binding mode, and the second phosphono group binds two Cd(Ⅱ) cations in a bridging bidentate fashion (Scheme 3 mode C), resulting in the formation of a 1D chain. In the crystal packing, these 1D chains are connected by 1,4-ndpaH₂^2- and/or 1,4-bib to form 3D supramolecular arrangements with the 1D channels along the a-axis (Fig. 1f).

  Figure 1

  

  Figure 1.  Building unit of compounds 1 (a) and 2 (d) with atomic labeling scheme; (b) Ribbons cross-linked by 4,4′-bpy in 1; (c) ABAB packing diagram of 1 viewed along the a-axis; (e) 1D chains of 2 connected by 1,4-ndpaH₂^2- and 1,4-bib; (f) Packing diagram of 2 viewed along the a-axis

  All H atoms except those attached to water molecules and phosphonate oxygen atoms are omitted for clarity; Symmetry codes: A: x+1, y, z; B: 1-x, 1-y, 1-z; C: -x, 1-y, 1-z for 1; A: 1/2+x, 3/2-y, 1/2+z; B: -1/2+x, 3/2-y, 1/2+z for 2.

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  Scheme 3

  

  Scheme 3.  Schematic presentation of the coordination modes of the diphosphonate ligands in compounds 1-5

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  2.2.2   Structure description of compound 3

  Compound 3 crystallizes in the triclinic system space group P1 and shows a 1D chain structure. The asymmetric unit contains one Cd(Ⅱ) cation, two 1,4-ndpaH₃^- anions, one monodentate pendant 1,2-dpe ligand, one free coordinated 1,2-dpe ligand, one coordinated water molecule, and seven lattice water molecules (Fig. 2a).

  Figure 2

  

  Figure 2.  (a) Building unit of compound 3 with atomic labeling scheme; (b) 1D double chain along the a-axis with pendentmonodentate 1,2-dpe ligands along the crystallographic b-axis; (c) Packing diagram of 3 viewed along the b-axis; (d) View of well-defined hydrogen-bonding interaction (sky blue dotted lines) between lattice water and phosphonate —OH group

  Symmetry codes: A: 1+x, y, z; B: 2-x, 1-y, -z.

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  The Cd ion is coordinated by one nitrogen (N1) from 1,2-dpe, four phosphonate oxygen (O2, O8, O5A, O13) atoms from two equivalent 1,4-ndpaH₃^- ligands, and one water molecule (O1) (Symmetry codes: A: 1+x, y, z; B: 2-x, 1-y, -z). Rarely, compared with rigid 4,4′-bpy in compound 1 or 1,4-bib in compound 2, the 1,2-dpe ligand plays two roles in compound 3, either as a monodentate ligand bridging a cadmium ion or as a guest molecule embedded within the molecular voids. The phosphonic acid group in each diphosphonate segment is singly deprotonated and displays the same monodentate coordination mode (Scheme 3 mode B). The bidentate diphosphonate ligands bridge the Cd(Ⅱ) cations into a 1D double chain in the a-axis with pendent monodentate 1,2-dpe ligands along the crystallographic b-axis (Fig. 2b). In the crystal packing, these 1D chains are connected by hydrogen-bonding interactions between the O atom of a lattice water molecule and the phosphonate —OH group to form 2D layer arrangements (Fig. 2c and 2d).

  2.2.3   Structure description of compound 4

  Compound 4 crystallizes in the triclinic space group P1. Its asymmetric unit consists of three distinct Cd(Ⅱ) centers, two 1,4-ndpaH₂^2-, one 1,4-ndpaH^3-, one singly protonated 1,2-bixH⁺ cation, and two coordinated water molecules (Fig. 3a).

  Figure 3

  

  Figure 3.  (a) Building unit of compound 4 with the atomic labeling scheme; (b) Inorganic chains in 4; (c) 2D polyhedral view of 4; (d) Packing diagram of 4 with 1,2-bixH⁺ ion embedding between the layers

  Symmetry codes: A: x+1, y-1, z; B: -x+1, -y+1, -z; C: x, y-1, z; D: x+1, y, z: E: -x+1, -y+2, -z.

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  One of the crystallographic distinct Cd(Ⅱ) centers, Cd1, has a seven-coordinated environment, surrounded by seven phosphonate oxygen atoms from five diphosphonate ligands to give a {CdO₇} geometry. The Cd—O bond lengths are 0.228 0(3) and 0.233 4(3) nm. The rest of the crystallographic distinct Cd(Ⅱ) centers, Cd2 and Cd3 are each six-coordinated with a slightly distorted octahedral geometry. Cd2 is surrounded by four oxygen atoms from four different diphosphonate ligands and two oxygen atoms from two coordinated water molecules, while Cd3 is coordinated by five oxygen atoms from five different diphosphonate ligands and one oxygen atom from one coordinated water molecule. Interestingly, a water molecule (O100) bridges the two cadmium ions (Cd2 and Cd3), which is not common for cadmium phosphonates. The Cd2…Cd3 distance over the μ₂-OH₂ bridge is 0.417 06 nm. The Cd2—O100—Cd3 angle is 112.8°.

  Three crystallographically distinguished naphthalene-diphosphates are present in the structure. The 1,4-ndpaH₂^2- anion is a zwitterion with a singly protonated in each phosphonate group, whereas the two phosphonate groups in the 1,4-ndpaH^3- anion are fully deprotonated in one and singly protonated in another. The 1,4-ndpaH₂^2- anion acts as a bidentate ligand bridging the Cd ions through two phosphonate oxygen from one phosphono group, and the second phosphonate group as a pendant is free to coordination (Scheme 3 mode D). The 1,4-ndpaH^3- anion serves as a hexadentate ligand, chelating and bridging the Cd ions through five phosphonate oxygens. The remaining phosphonate oxygen atom is pendant (Scheme 3 mode E, F). As a consequence, the Cd(Ⅱ) ions are cross-linked by the O—P—O units and/ —O—, forming a double chain running along the b-axis. The double chains are further fused by ligand 1,4-ndpaH^3-, forming a 2D layer in the ab plane. These layers are stacked with the organic groups of the 1,4-ndpaH₂^2- ligands protruding out to the inter-layer space. The shortest interchain Cd…Cd distances within the layer and between the layers are 1.219 and 1.345 nm, respectively. The singly protonated 1,2-bixH⁺ ions are embedded in inter-layer space.

  2.2.4   Structure description of compound 5

  The flexible 1,2-bix was further changed to flexible 1,3-dpp and this resulted in a new compound 5. Single-crystal X-ray diffraction analysis reveals that compound 5 crystallizes in the triclinic space group P1 and displays a 2D layer structure. The asymmetric unit consists of one crystallographically independent Cd(Ⅱ) ion, one 1,4-ndpaH₂^2- ion, one coordinated water molecule, and one disordered lattice water molecule (Fig. 4a), corresponding to a formula of [Cd(1,4-ndpaH₂)(H₂O)]·H₂O. Cd1 has a distorted octahedral environment with the six sites occupied by five phosphonate oxygen (O2, O3B, O4C, O4D, and O6A), and one coordination water oxygen (O7, Symmetry codes: A: 1+x, y, z; B: 1+x, 1+y, z; C: 2-x, 1-y, 2-z; D: x, 1+y, z). The Cd1—O (1,4-ndpaH₂^2-) and Cd1—O (water) bond lengths are 0.189 1(6)-0.204 4(8) nm, and O—Cd—O angles are 94.4(3)°-122.0(3)°.

  1,4-ndpaH₂^2- ligand behaves as a pentadentate ligand, and adopts mode G (Scheme 3) to coordinate with five Cd ions via its five phosphonate oxygen atoms from two phosphonate groups. Two neighboring edge-shared {CdO₆} octahedrons are corner-shared with eight {PO₃C} tetrahedrons and vice-versa, forming an infinite inorganic chain along the a-axis (Fig. 3b). The inorganic chains are further cross-linked by 1,4-ndpaH₂^2- forming a 2D layer in the ab plane. The layers are stacked along the c-axis with hydrogen bonds O7—H7A…O8B (H7A…O8B: 0.180 8 nm) and O8B—H8B#1…O5 (H8B#1…O5: 0.202 2 nm) interactions between naphthalene groups and lattice water molecules, thereby forming a 3D supramolecular structures (Fig. 4c and 4d). The shortest Cd…Cd distances are 0.653 2 nm within the supramolecular layer and 1.687 7 nm between the layers.

  Figure 4

  

  Figure 4.  (a) Building unit of 5 with the atomic labeling scheme; (b) Single layer structure of 5 where the inorganic chains are cross-linked by naphthalene groups; (c) Packing diagram of 5 viewed along the a-axis and hydrogen-bonding interaction (green dotted lines) between lattice water uncoordinated acidic OH of the phosphonate group

  Symmetry codes: A: 1+x, +y, +z; B: 1+x, 1+y, +z; C: 2-x, 1-y, 2-z; D: +x, 1+y, +z.

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  2.2.5   Effect of the auxiliary ligands on the structure formation

  In this work, a series of N-donor pyridine/imidazole-containing auxiliary ligands with different sizes, shapes, and rigidity were employed as second ligands to investigate their effect on the structure formation in metal naphthalene-diphosphonates. From the results of the structural analysis, it is clear that the rigidity of the auxiliary ligands posed a remarkable influence on the structures of the reaction products. As representatives of rigid ligands, 4,4′-bpy and 1,4-bib both participated in coordination with metals during the self-assembly of the compounds, yielding 2D layered (compound 1) and 3D network structures (compound 2), respectively. When the rigidity of the auxiliary ligand is reduced, as in the case of 1,2-dpe, the coordination mode of the auxiliary ligand is significantly changed, as in the case of compound 3, where there are two crystallographically independent 1,2-dpe molecules in the crystal structure, one of which is coordinated to the metal in a pendent monodentate mode, and the other one exists as a guest molecule. When the rigidity of the auxiliary ligand continues to decrease to flexible 1,2-bix, as in compound 4, 1,2-bix exists exclusively as a guest molecule and does not participate in coordination with the metal. A 2D layer structure is found in compound 4, where the guest molecules 1,2-bix are embedded in inter-layer space. Upon changing the flexible ligand from 1,2-bix to 1,3-dpp, in compound 5, we were surprised to find that more flexible 1,3-dpp was neither involved in coordination nor present as a guest molecule. Experiments have shown that without 1,3-dpp, the same reaction would yield only an unknown powder. As far as we are aware, there are many examples of the addition of auxiliary ligands to modulate the crystal structure of metal phosphonates, but none of them discuss the rigidity of the ligand leading to changes in its coordination ability. We speculate that the rigid ligand can participate in the coordination assembly with the naphthalene diphosphonate ligand in a self-adaptive manner due to its distortion difficulty, while the flexible ligand can automatically adjust its configuration according to the space size to keep the structure of the complex in a stable state due to the uncertainty of structural distortion.

  2.3   Thermal stability of compounds 1-5

  To study the thermal stability of compounds 1-5, TGA was performed on the prepared powders in a temperature range of between room temperature and 800 ℃ under an N₂ atmosphere at a 10 ℃·min^-1 heating rate. The TGA curve of compound 1 showed an initial weight loss of 1.63% at 167 ℃. This weight loss is consistent with the removal of the two coordinated water molecules (Calcd. 1.78%, Fig. 5). The dehydrated framework of compound 1 was stable at 365 ℃, above which the second and third steps were observed, corresponding to the decomposition of the organic ligands and the collapse of the structure. For compound 2, the first step occurred below 245 ℃ with a weight loss of 3.41%, in agreement with the removal of one coordinated water molecule water molecule (Calcd. 3.48%). This was followed by a long plateau until 353 ℃, above which a quick weight loss was observed corresponding to the release of the coordination water molecules and the decomposition of the organic components. Several steps of weight loss were present on the TGA curve of compound 3. The first one (11.44%), in a range of 32-225 ℃, corresponds to the removal of one coordinated and seven lattice water molecules (12.05%). It started to decompose at about 305 ℃, accompanying the release of 1,2-dpe ligand molecules and the collapse of the structure. The thermal stability of compound 4 was much better. No obvious weight loss can be observed below 160 ℃. The first weight loss (2.34%) in a range of 160-190 ℃, matches well with the value of two coordinated water molecules in each formula (2.45%). It did not decompose until the temperature reached 330 ℃, then it began to decompose. For compound 5, two steps of weight losses (8.14%) were observed in a range of 25-250 ℃, corresponding to the removal of one coordinated water molecule and one lattice water molecule (Calcd. 8.28%). A decomposition process occurred at a temperature approaching 470 ℃.

  Figure 5

  

  Figure 5.  TGA curves of compounds 1-5

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  2.4   Photoluminescence properties

  Cd(Ⅱ)-containing compounds have received much attention for their potential applications as optical materials or chemical sensors. Thus, the photoluminescence properties of compounds 1-5 as well as the free 1,4-ndpaH₄ ligand were investigated in the solid. It was found the free 1,4-ndpaH₄ ligand displayed photoluminescence with emission maxima at about 388 nm. It can be presumed that these emissions originated from the π*→π or π*→n transitions. Upon compoundation with Cd(Ⅱ) ions, intense fluorescence emissions were observed at 404 nm for 1, 401 nm for 2, 413 nm for 3, 406 and 425 (shoulder) nm for 4, and 400 nm for 5. Compared with the emissions of the free ligand, different band shapes and redshifts were observed for compounds 1-5. Since Cd(Ⅱ) ions are hard to oxidize or reduce, these bands can likewise be regarded as intraligand fluorescence emissions which are greatly affected by the different coordination environments around the corresponding metal centers and the different deprotonation degrees of the 1,4-ndpaH₄ ligands at room temperature (Fig. 6). This result indicates that although the auxiliary ligands with different rigidity exert a crucial influence on the structure of cadmium phosphonates, they do not significantly affect the fluorescence properties of this class of compounds.

  Figure 6

  

  Figure 6.  Normalized emission spectra of 1,4-ndpaH₂ and compounds 1-5

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  3.   Conclusions

  By employing 4,4′-bipyridine (4,4′-bpy) and 1,4-bis(1-imidazolyl)benzene (1,4-bib) as rigid ligands and 1,2-di(4-pyridyl)ethylene (1,2-dpe), bis(imidazol-1-ylmethyl)benzene (1,2-bix) and 1,3-di(4-pyridyl)propane (1,3-dpp) as flexible ligands in the self-assembly of metal diphosphates respectively, we succeeded in isolating five compounds with formulae [Cd_1.5(1,4-ndpaH₂)₂(4,4′-bpyH)(4,4′-bpy)_0.5(H₂O)₂]₂ (1), [Cd(1,4-ndpaH₂)(1,4-bib)_0.5(H₂O)] (2), [Cd(1,4-ndpaH₃)₂(1,2-dpe)(H₂O)]·(1,2-dpe)·7H₂O (3), (1,2-bixH)[Cd₃(1,4-ndpaH)(1,4-ndpaH₂)₂(H₂O)₂] (4), and [Cd(1,4-ndpaH₂)(H₂O)]·H₂O (5). Compounds 1 and 2 show 2D layer structure and 3D open-framework structure, respectively. The typical characteristic of these two compounds is that the rigid 4,4′-bpy or 1,4-bib ligands bridge the inorganic metal chain, forming a lamellar or network structure. In compound 3, 1,2-dpe, which is less rigid than 4,4′-bpy, is embedded in the skeleton in the form of monodentate pendant or guest molecules. Compounds 4 and 5 all display 2D layer structures. The flexible ligands 1,2-bix embed as guest molecules in 4, while 1,3-dpp was absent in 5. Of particular interest is the observation that the coordination capacity of the auxiliary ligand is controlled by its rigidity or flexibility, a previously unreported phenomenon. This finding brings a new route to prepare metal phosphonates with controllable structures for potential applications in separation, transport, and catalysis.

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

  
  
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• 

  Scheme 1  Molecular structures of 1,4-ndpaH₄ and N-donor auxiliary ligands

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  Scheme 2  Synthetic routes of compounds 1-5

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  Figure 1  Building unit of compounds 1 (a) and 2 (d) with atomic labeling scheme; (b) Ribbons cross-linked by 4,4′-bpy in 1; (c) ABAB packing diagram of 1 viewed along the a-axis; (e) 1D chains of 2 connected by 1,4-ndpaH₂^2- and 1,4-bib; (f) Packing diagram of 2 viewed along the a-axis

  All H atoms except those attached to water molecules and phosphonate oxygen atoms are omitted for clarity; Symmetry codes: A: x+1, y, z; B: 1-x, 1-y, 1-z; C: -x, 1-y, 1-z for 1; A: 1/2+x, 3/2-y, 1/2+z; B: -1/2+x, 3/2-y, 1/2+z for 2.

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  Scheme 3  Schematic presentation of the coordination modes of the diphosphonate ligands in compounds 1-5

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  Figure 2  (a) Building unit of compound 3 with atomic labeling scheme; (b) 1D double chain along the a-axis with pendentmonodentate 1,2-dpe ligands along the crystallographic b-axis; (c) Packing diagram of 3 viewed along the b-axis; (d) View of well-defined hydrogen-bonding interaction (sky blue dotted lines) between lattice water and phosphonate —OH group

  Symmetry codes: A: 1+x, y, z; B: 2-x, 1-y, -z.

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  Figure 3  (a) Building unit of compound 4 with the atomic labeling scheme; (b) Inorganic chains in 4; (c) 2D polyhedral view of 4; (d) Packing diagram of 4 with 1,2-bixH⁺ ion embedding between the layers

  Symmetry codes: A: x+1, y-1, z; B: -x+1, -y+1, -z; C: x, y-1, z; D: x+1, y, z: E: -x+1, -y+2, -z.

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  Figure 4  (a) Building unit of 5 with the atomic labeling scheme; (b) Single layer structure of 5 where the inorganic chains are cross-linked by naphthalene groups; (c) Packing diagram of 5 viewed along the a-axis and hydrogen-bonding interaction (green dotted lines) between lattice water uncoordinated acidic OH of the phosphonate group

  Symmetry codes: A: 1+x, +y, +z; B: 1+x, 1+y, +z; C: 2-x, 1-y, 2-z; D: +x, 1+y, +z.

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  Figure 5  TGA curves of compounds 1-5

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  Figure 6  Normalized emission spectra of 1,4-ndpaH₂ and compounds 1-5

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  Table 1.  Crystallographic data and structure refinement details for compounds 1-5

  Parameter	1	2	3	4	5
  Formula	C70H66Cd3N6O28P8	C16H15CdN2O7P2	C44H54CdN4O20P4	C44H42Cd3N4O20P6	C10H12CdO8P2
  Formula weight	2 024.28	521.65	1195.19	1467.82	434.54
  Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic	Triclinic
  Space group	P21/c	P21/n	P1	P1	P1
  a/nm	1.025 05(3)	0.691 58(11)	1.227 77(4)	1.219 37(2)	0.572 64(5)
  b/nm	2.099 10(4)	1.495 8(2)	1.370 59(4)	1.331 10(3)	1.032 00(8)
  c/nm	1.645 18(3)	1.734 6(3)	1.623 95(3)	1.700 42(3)	1.128 59(12)
  α/(°)			106.821(2)	76.423(2)	86.527(3)
  β/(°)	93.340(2)	93.263(6)	91.942(2)	85.357(2)	85.056(4)
  γ/(°)			110.994(2)	66.453(2)	85.043(3)
  V/nm3	3.533 89(14)	1.791 4(5)	2.413 06(13)	2.459 19(9)	0.661 04(10)
  Z	2	4	2	2	2
  Dc / (g·cm-3)	1.902	1.934	1.645	1.982	2.183
  μ/mm-1	1.171	11.874	0.670	12.893	1.931
  F(000)	2 032.0	1 036.0	1 228.0	1 452	428.0
  Rint	0.084 9	0.029 3	0.057 4	0.048 4	0.076 4
  GOF on F 2	1.053	1.078	1.115	1.018	1.176
  R1, wR2* [I > 2σ(I)]	0.043 6, 0.107 0	0.033 2, 0.081 2	0.060 3, 0.156 7	0.037 1, 0.104 4	0.040 4, 0.085 2
  R1, wR2 (all data)	0.055 5, 0.115 2	0.038 8, 0.084 1	0.071 5, 0.162 1	0.040 7, 0.107 5	0.047 7, 0.088 9
  (Δρ)max, (Δρ)min / (e·nm-3)	2 000, -800	980, -330	2 190, -870	2 990, -707	760, -760
  ${ }^* R=\sum\left\|F_{\mathrm{o}}\left|-\left|F_{\mathrm{c}} \| / \sum\right| F_{\mathrm{o}}\right|, w R=\left[\sum w\left(F_{\mathrm{o}}^2-F_{\mathrm{c}}^2\right)^2 / \sum w\left(F_{\mathrm{o}}^2\right)^2\right]^{1 / 2} .\right.$

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