English

• 0.   Introduction

  Coordination polymers (CPs) are usually obtained from multi-dentate organic ligands as building templates or assembled from connecting rods with metal ions as nodes^[1-3]. Among them, multi-dentate nitrogen-containing ligands, multi-dentate carboxylic acid ligands, and multi-dentate pyridine carboxylic acid ligands are the preferred ligands to self-assemble organic ligands^[4]. The CPs have potential applications in the fields of catalysis, ion recognition and antioxidation, electrochemistry, etc., and are one of the research hot spots in organometallic chemistry and coordination chemistry^[5-9]. The synthesis of CPs using ligands containing carboxyl groups is more common. In addition to structural stability, due to the multiple coordination modes and bridging modes of carboxyl-containing ligands, the CPs are easy to obtain topological diversity and excellent properties. Ligands containing nitrogen or oxygen groups can be reintroduced during the construction of the CPs to increase the active sites, form more coordination modes, diversify the pore channels, and improve their luminescence performance^[10-13].

  Among them, 4, 4′-oxydibenzoic/4-nitrophthalic acid (H₂oba/4-H₂nph)-like ligands are important ligands for the preparation of functional CPs materials, because having bifunctional groups in the para- position provides the possibility of forming polymer chains of mono- and heterometallic 2D or 3D coordination compounds, which can be used as one of the ligands for the synthesis of bidentate, tridentate or tetradentate chelate CPs based on the characteristics of oba^2-/4-nph^2- ion structure. The presence of empty d orbitals in transition metals allows the use of hybrid orbitals to accept electrons, resulting in stable structures of 16 or 18 electrons, and thus are widely used for the preparation of CPs^[14-17]. Our group chose H₂oba/ 4-H₂nph as the primary ligand and 2, 2′-biimidazole (L) as the secondary ligands and obtained CPs {[Cd(oba)(L)₂]·H₂O}_n (1) and [Cd(4-nph)(L)₂]_n (2). The structures were characterized by IR, elemental analyses, single-crystal X-ray diffraction, and powder X-ray diffraction (PXRD), and their thermal stability and fluorescence properties were also investigated. In addition, the quantum-chemical calculations were accomplished on 'molecular fragments' extracted from the crystal structures of 1 and 2 using the PBE0/LANL2DZ method built in the Gaussian16 program.

  1.   Experimental

  1.1   Material and methods

  All reagents were commercially procured and employed without undergoing any additional purification processes. Elemental analyses (of carbon, hydrogen, and nitrogen) were conducted on a Vario EL Ⅲ Elemental Analyzer. The IR spectrum was documented within a 4 000-400 cm^-1 range on a Nicolet 6700 Spectrophotometer, utilizing a KBr pellet. PXRD patterns were gathered in a 2θ range of 5°-50° at a scan speed of 0.1 (°)·s^-1 on a Bruker D8 Advance instrument, employing Cu Kα radiation (λ=0.154 18 nm, voltage: 40 kV, current: 40 mA) at room temperature. The thermal stability studies were executed on a Perkin-Elmer TGA7 analyzer. The fluorescent studies were implemented on a computer-controlled JY Fluoro-Max-3 spectrometer at room temperature.

  1.2   Synthesis of the complexes

  A mixture of H₂oba (0.20 mmol, 0.051 6 g), Cd(OAc)₂·2H₂O (0.20 mmol, 0.053 0 g), and L (0.20 mmol, 0.026 8 g) was dissolved in deionized water (15 mL). After continuous stirring for 30 min, a sufficient amount of triethylamine was added to the aforesaid solution to adjust the pH value to 8.07. Then the solution was sealed in a Parr Teflon-lined stainless-steel vessel (25 mL) under autogenous pressure at 160 ℃ for 5 d. Subsequently, it was cooled down to 30 ℃ at a rate of 5 ℃·h^-1 for 26 h and then reduced to room temperature. Yellow crystals of complex 1 were obtained, with a yield of 20% (based on Cd). Elemental Anal. Calcd. for C₂₆H₂₂CdN₈O₆(%): C, 47.68; N, 17.11; H, 3.39. Found(%): C, 47.12; N, 16.95; H, 3.01. IR (cm^-1): 3 124 w, 1 596s, 1 547w, 1 498m, 1 384s, 1 305w, 1 259s, 1 162 s, 1 140w, 1 121w, 1 011w, 991w, 882m, 857w, 783m, 771w, 692w, 666w, 625w.

  The synthesis of complex 2 was the same as that of complex 1, except that H₂oba was replaced with 4-H₂nph (0.20 mmol, 0.042 2 g) and the pH value was adjusted to 6.03 using NaOH. Yellow crystals of 2 were obtained with a yield of 25% (based on Cd). Elemental Anal. Calcd. for C₂₀H₁₅CdN₉O₆(%): C, 40.73; N, 21.37; H, 2.56. Found(%): C, 40.22; N, 20.95; H, 2.11. IR (cm^-1): 3 124w, 2 902w, 1 573m, 1 519m, 1 413w, 1 382 m, 1 346s, 1 325w, 1 247w, 1 158w, 1 121m, 1 065w, 993m, 823w, 750m, 723w, 651m, 667w.

  1.3   Determination of the crystal structures

  Two single crystals with dimensions of 0.16 mm×0.15 mm×0.13 mm (1) and 0.15 mm×0.12 mm×0.10 mm (2) were selected and the data were gathered at 293(2) K on an Oxford Diffraction Gemini R Ultra diffractometer with Mo Kα (λ=0.071 073 nm). The structures were solved through direct methods by employing the SHELXS-97 program of the SHELXL-97 crystallographic software package and refined on F ² through full-matrix least-squares methods. All non- hydrogen atoms were refined anisotropically. All the hydrogen atoms were positioned geometrically and refined using a riding model. A summary of the crystallography data and structure refinements for complexes 1 and 2 is presented in Table 1. The selected bond lengths and angles for 1 and 2 are listed in Table 2.

  Table 1

  

  Table 1.  Crystallography data and structure refinements for complexes 1 and 2

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  Parameter	1	2
  Empirical formula	C26H22CdN8O6	C20H15CdN9O6
  Formula weight	652.90	589.81
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/n	P21/n
  a / nm	0.779 62(4)	0.851 59(8)
  b / nm	1.868 74(9)	1.264 60(13)
  c / nm	1.827 61(9)	2.110 4(2)
  β / (°)	98.864 0(10)	96.100(2)
  Volume / nm3	2.630 9(2)	2.259 8(4)
  Z	4	4
  Dc / (g·cm-3)	1.648	1.734
  GOF	1.041	1.069
  Reflection collected, unique	4 675, 3 126	5 465, 4 763
  Rint	0.038 0	0.031 2
  R [I > 2σ(I)]	0.036 1	0.027 8
  wR	0.083 1	0.060 0

  Table 2

  

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

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  1
  Cd1—N1	0.224 8(3)	Cd1—N1A	0.224 8(3)	Cd1—N2	0.237 5(3)
  Cd1—N2A	0.237 5(3)	Cd1—O1	0.245 0(3)	Cd1—O1A	0.245 0(3)
  Cd2—N5	0.227 6(3)	Cd2—N5B	0.227 6(3)	Cd2—N6	0.233 0(3)
  Cd2—N6B	0.233 0(3)	Cd2—O4	0.246 3(3)	Cd2—O4B	0.246 3(3)
  N1A—Cd1—N1	180.0	N1—Cd1—N2A	104.94(11)	N1A—Cd1—N2	104.94(11)
  N1—Cd1—N2	75.06(11)	N2A—Cd1—N2	180.0	N1—Cd1—O1A	90.19(10)
  N2—Cd1—O1A	88.45(11)	N1A—Cd1—O1	90.19(10)	N1—Cd1—O1	89.81(10)
  N2A—Cd1—O1	88.45(11)	N2—Cd1—O1	91.55(11)	O1A—Cd1—O1	180.00(7)
  N5—Cd2—N5B	180.00(10)	N5—Cd2—N6	74.97(10)	N5B—Cd2—N6	105.03(10)
  N5—Cd2—N6B	105.03(10)	N6—Cd2—N6B	180.00(11)	N5—Cd2—O4B	90.22(10)
  N6—Cd2—O4B	83.70(10)	N5—Cd2—O4	89.78(10)	N5B—Cd2—O4	90.22(10)
  N6—Cd2—O4	96.30(10)	N6B—Cd2—O4	83.70(10)	O4B—Cd2—O4	180.00(14)
  2
  Cd1—O1	0.230 03(16)	Cd1—O4	0.238 26(16)	Cd1—N1	0.233 4(2)
  Cd1—N2	0.233 43(18)	Cd1—N5	0.232 98(19)	Cd1—N6	0.232 1(2)
  O1—Cd1—N6	161.80(7)	O1—Cd1—N5	101.59(6)	N6—Cd1—N5	74.37(7)
  O1—Cd1—N1	100.26(6)	N6—Cd1—N1	87.76(7)	N5—Cd1—N1	155.95(7)
  O1—Cd1—N2	86.62(6)	N6—Cd1—N2	111.40(7)	N5—Cd1—N2	97.65(7)
  N1—Cd1—N2	73.68(6)	O1—Cd1—O4	76.81(6)	N6—Cd1—O4	85.91(7)
  N5—Cd1—O4	96.26(6)	N1—Cd1—O4	98.42(6)	N2—Cd1—O4	160.20(6)
  Symmetry codes: A: 3-x, -y, -1-z; B: 2-x, -y, -z for 1.

  2.   Results and discussion

  2.1   Description of crystal structure

  The asymmetric unit of complex 1 (Fig.S1, Supporting information) comprises two crystallographically unique Cd cations (its occupancy is 0.5), one oba^2- ligand, two L ligands, and one lattice water molecule. As depicted in Fig.1, the Cd(Ⅱ) ion is situated in a distorted octahedral coordination geometry, being equatorially coordinated by four imidazole N atoms (N1, N1A, N2, and N2A) from two L ligands. At the axial position are carboxylate atoms (O1 and O1A) from two oba^2- ligands. The maximum and minimum bond angles for the Cd²⁺ ion are 180.00(10)° and 74.97(10)°, respectively, which slightly deviate from the angle of 180°/90° in a perfect octahedron. The bond lengths range from 0.224 8(3) to 0.246 3(3) nm, and more detailed selected bond distances and bond angles are presented in Table 2. As shown in Fig.2, four N atoms of two L ligands adopt a chelating μ₁∶η¹∶η¹ mode to connect Cd atoms, and then two [Cd(L)₂] moieties are linked by the oba^2- ligand to form a 1D zigzag chain. Pairs of L ligands are located on the sides of the 1D chain. It is intriguing to note that the carboxylate oxygen atoms of the oba^2- ligand are all deprotonated and take monodentate bridging coordination patterns. Further analysis of the crystal packing revealed hydrogen-bond interactions in carboxy-oxygen atoms and imidazole N atoms (Table 3), as well as π-π stacking interactions between the benzene ring of the oba^2- ligand and the imidazole ring of the L ligand. The shortest centroid distance is 0.366 2(2) nm for the N1C-C18C-N4C-C19C-C20C and N2C-C15C-C16C-N3C-C17C rings (Symmetry code: C: 2-x, -y, -1-z), with the vertical distance being -0.326 18(16) nm and the dihedral angle being 2.6(2)°. Undoubtedly, these stabilize the structure of 1 and form a 2D supramolecular architecture (Fig.3).

  Figure 1

  

  Figure 1.  Coordination environment (at 30% probability level) of the Cd(Ⅱ) center in complex 1

  Symmetry codes: A: 3-x, -y, -1-z; B: 2-x, -y, -z.

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  Figure 2

  

  Figure 2.  View of the 1D zigzag chain of complex 1

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  Table 3

  

  Table 3.  Hydrogen bonds for complexes 1 and 2

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  N3—H3…O1D	0.077(3)	0.222(3)	0.297 3(4)	165(3)
  N4—H4…O2D	0.081(5)	0.179(6)	0.258 9(5)	174(5)
  N7—H7…O5E	0.072(4)	0.199(4)	0.269 8(5)	170(4)
  N8—H8…O4E	0.083(4)	0.197(4)	0.277 5(4)	167(4)
  2
  N3—H3…O2C	0.087(3)	0.188(3)	0.273 7(3)	171(2)
  N4—H4…O1C	0.076(3)	0.193(3)	0.269 0(3)	177(4)
  N7—H7…O3A	0.080(3)	0.196(3)	0.275 9(3)	171(3)
  N8—H8…O4A	0.081(3)	0.193(3)	0.273 2(3)	170(3)
  Symmetry codes: D: -1+x, y, z; E: 1+x, y, z for 1; A: 2-x, 1-y, 1-z; C: 3/2-x, -1/2+y, 1/2-z for 2.

  Figure 3

  

  Figure 3.  View of the 2D supramolecular architecture of complex 1 formed by N—H…O hydrogen bonding and π-π interactions

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  Complex 2 crystallizes in the monoclinic P2₁/n space group, with one Cd(Ⅱ) ion, one 4-nph^2- anion, and two L ligands in its asymmetric unit (Fig.4). The six- coordinated Cd(Ⅱ) center is surrounded by two oxygen atoms (O1, O4) from one 4-nph^2- anion and four nitrogen atoms (N1, N2, N5, and N6) from two different L ligands, adopting a distorted octahedral {CdO₂N₄} geometry. The Cd—O bond lengths are 0.230 03(16) and 0.238 26(16) nm, respectively, and the Cd—N bond distances are 0.233 4(2), 0.233 43(18), 0.232 98(19), and 0.232 1(2) nm, respectively. The O(N)—Cd—N(O) bond angles are in a range of 73.68(6)°-161.80(7)°, all of which are within the reasonable range of those reported for other six-coordinated Cd(Ⅱ) complexes^[18-19]. In the polymeric structure of 2 (Fig.5), the 4-nph^2- anion adopts the bridging-μ₂ mode to connect the cadmium ion, whereas the L ligand adopts a chelting-μ₂ mode to connect the metal ion. Similar to that of complex 1, there are persistent N—H…O hydrogen bonding interactions between the carboxylate oxygen atoms of the 4-nph^2- ligand and the nitrogen atom of the L ligands (Table 3), which are usually significant in the synthesis of supramolecular architecture. Moreover, there are π-π interactions in 2 between the imidazole rings, and between the imidazole ring and benzene ring of 4-nph^2- molecules. The centroid-to-centroid distances between adjacent rings are 0.371 17(14) nm for N5A-C15A-C16A-N7A-C17A and N6A-C18A-N8A-C19A-C20A (Symmetry code: A: 2-x, 1-y, 1-z) imidazole rings and 0.391 36(14) nm for the N6B-C18B-N8B-C19B-C20B imidazole ring and the C2B-C3B-C4B-C5B-C6B-C7B benzene ring (Symmetry code: B: 1+x, y, z). The dihedral angles are 6.49(14)°and 16.88(12)°, respectively. Therefore, through hydrogen bonds and π-π interactions, 2 is further expanded into a 2D supramolecular network framework (Fig.5).

  Figure 4

  

  Figure 4.  Coordination environment of complex 2 with displacement ellipsoids at the 30% probability level

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  Figure 5

  

  Figure 5.  View of the 2D supramolecular architecture of complex 2 formed by N—H…O hydrogen bonding and π-π interactions

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  2.2   IR spectrum analysis

  The solid-state IR spectrum of complexes 1 and 2 was illustrated in a region of 4 000-400 cm^-1 (Fig.S2 and S3). A strong and wide peak at 3 124 cm^-1 was observed, which is characteristic of the stretching vibrations of the N—H bond of the L ligand. The strong peaks at 1 384, 1 596 cm^-1 (1) and 1 346, 1 573 cm^-1 (2) could be ascribed to the asymmetric and symmetric vibrations of the carboxylate groups of the oba^2- anions^[20-21]. The C—N stretching vibrations of the L ligand were observed at 1 162 cm^-1 (1) and 1 211 (2) cm^{-1 [22]}.

  2.3   PXRD and thermal stability analyses

  To substantiate the phase purity of complexes 1 and 2, their PXRD analysis was performed before their photoluminescent properties were measured. The experimental PXRD patterns were in excellent accordance with the corresponding simulated ones (Fig.6) except for the variation in relative intensity because of the preferred orientations of the crystals.

  Figure 6

  

  Figure 6.  PXRD patterns of complexes 1 and 2

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  Complexes 1 and 2 are stable under environmental conditions, and the thermal stability was explored by thermogravimetric analysis (TGA). As depicted in Fig.S4, the TG curve of 1 showed that the complex was stable up to 240 ℃, and then began to lose weight above 240 ℃. Continuous decomposition, attributed to the release of the water molecule and the L ligand, was observed within a temperature range of 240-375 ℃, and the oba^2- ligand commenced to decompose at 376 ℃. The TG curve of 2 indicated no obvious weight loss from 50 to 300 ℃, then the curve presented a platform and the framework started to decompose at 300 ℃, as shown in Fig.S5.

  2.4   Photoluminescent spectrum

  In this work, solid-state luminescent properties were investigated for complexes 1 and 2 (Fig.7), along with H₂oba, 4-H₂nph, and L ligands. The free H₂oba, 4-H₂nph, and L ligands exhibited emissions at 275, 440, and 435 nm, respectively, upon excitation at 335 nm. These emissions of the free ligands are likely attributable to the π*→n or π*→π transitions^[23-27]. The emission peaks of 1 and 2 occurred at 393 and 396 nm, respectively [λ_ex=298 nm (1), λ_ex=308 nm (2)]. Since the d¹⁰ configuration of Cd(Ⅱ) is difficult to oxidize or reduce, the emissions are neither metal-to- ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT). The emission bands of 1 and 2 could be assigned to the emission of ligand-to-ligand charge transfer^[28-29].

  Figure 7

  

  Figure 7.  Solid-state photoluminescent spectra of complexes 1 and 2 at room temperature

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  2.5   Quantum chemistry calculations

  In this work, all calculations were performed on the Gaussian16 program with the parameters of the molecular structure from the experimental data of complexes 1 and 2. Natural bond orbital (NBO) analysis^[30-32] was carried out by density functional theory with the B3LYP^[33], hybrid functional, and the LANL2DZ basis set^[34].

  Table 4 presents the data for the selected natural atomic charges, natural electron configuration, Wiberg bond orders, and NBO bond orders (in atomic units) of complex 1. It is indicated that the electronic configurations of Cd ion, N, and O atoms are 5s^0.294d^9.985p^0.33, 2s^1.37-1.382p^4.17-4.18, and 2s^1.682p^5.11-5.12, respectively. Based on the above results, it can be concluded that the coordination of the Cd1 ion with N and O atoms primarily occurs through the 4d, 5s, and 5p orbitals. Specifically, N atoms form coordination bonds with the Cd1 ion using their 2s and 2p orbitals. All O atoms also supply electrons of 2s and 2p to Cd ions and form coordination bonds. Additionally, the Cd1 ion obtained some electrons from the N atoms of the L ligand and the O atoms of the H₂oba ligand. According to valence-bond theory, the atomic net charge distribution and the NBO bond orders (Table 4) indicate a clear covalent interaction between the coordinated atoms and the Cd ion. Furthermore, the differences in NBO bond orders for Cd—O and Cd—N bonds lead to variations in their bond lengths^[30-32]. This observation aligns well with the X-ray crystal structural data of 1. In summary, the information suggests that there is a covalent interaction between the coordinated atoms (N and O) and the Cd ion in 1. The NBO bond orders and the resulting bond lengths support this conclusion, demonstrating consistency with the X-ray crystal structural data of 1.

  Table 4

  

  Table 4.  Natural atomic charges, natural valence electron configurations, Wiberg bond indexes, and NBO bond orders for complexes 1 and 2

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  Atom	Net charge	Electron configuration	Bond	Wiberg bond index	NBO bond order / a.u.
  1
  Cd1	1.419 86	[core]5s0.294d9.985p0.33
  O1	-0.801 98	[core]2s1.682p5.12	Cd1—O1	0.177 0	0.192 7
  O1A	-0.800 58	[core]2s1.682p5.11	Cd1—O1A	0.176 1	0.192 2
  N1	-0.576 77	[core]2s1.382p4.18	Cd1—N1	0.140 8	0.209 9
  N1A	-0.574 77	[core]2s1.382p4.18	Cd1—N1A	0.140 0	0.209 2
  N2	-0.558 44	[core]2s1.372p4.17	Cd1—N2	0.135 4	0.198 6
  N2A	-0.557 44	[core]2s1.372p4.17	Cd1—N2A	0.135 2	0.198 4
  2
  Cd1	1.399 21	[core]5s0.294d9.985p0.32
  O1	-0.838 02	[core]2s1.702p5.13	Cd—O1	0.229 7	0.233 9
  O4	-0.809 32	[core]2s1.692p5.11	Cd—O4	0.196 0	0.209 8
  N1	-0.544 06	[core]2s1.382p4.15	Cd—N1	0.129 4	0.191 9
  N2	-0.548 62	[core]2s1.372p4.16	Cd—N2	0.136 1	0.198 3
  N5	-0.548 41	[core]2s1.382p4.15	Cd—N5	0.140 3	0.201 1
  N6	-0.549 67	[core]2s1.372p4.16	Cd—N6	0.138 1	0.200 5

  As shown in Fig.8, the LUMO and HOMO of complex 1 are primarily composed of the H₂oba ligand. This suggests the possibility of intraligand charge transfer (ILCT) within the complex, as indicated by certain contours in the molecular orbitals.

  Figure 8

  

  Figure 8.  Frontier molecular orbitals of complex 1

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  The selected natural atomic charges, natural electron configuration, Wiberg bond, and NBO bond orders (a.u.) for complex 2 are shown in Table 4. It is indicated that the electronic configurations of Cd ion, N, and O atoms are 5s^0.294d^9.985p^0.32, 2s^1.37-1.382p^4.15-4.16, and 2s^1.69-1.702p^4.15-4.16, respectively. Based on the above results, one can conclude that the Cd ion coordination with N and O atoms is mainly on 4d, 5s, and 5p orbitals. N atoms form coordination bonds with Cd ions using 2s and 2p orbitals. All O atoms supply electrons of 2s and 2p to Cd ion and form the coordination bonds. Therefore, the Cd ion obtained some electrons from N atoms of the L ligand and O atoms of the 4-H₂nph ligand. Thus, according to valence-bond theory, the atomic net charge distribution and the NBO bond orders of 2 (Table 4) show the obvious covalent interaction between the coordinated atoms and Cd ions. The differences in the NBO bond orders for Cd—O and Cd—N bonds make their bond lengths different^[30-32], which is in good agreement with the X-ray crystal structural data of 2.

  As can be seen from Fig.9, the LUMO is mainly composed of the L ligand, whereas the HOMO mainly consists of the 4-H₂nph ligand. So, LLCT may be inferred from some contours of the molecular orbital of complex 2.

  Figure 9

  

  Figure 9.  Frontier molecular orbitals of complex 2

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  3.   Conclusions

  In summary, we have developed a synthetic strategy for 2D coordination polymers {[Cd(oba)(L)₂]·H₂O}_n (1) and [Cd(4-nph)(L)₂]_n (2) by employing a flexible/rigid dicarboxylate ligand, an L ligand, and a transition metal Cd(Ⅱ) ion. In complex 1, the four N atoms of the L ligand adopt a chelating μ₁∶η¹∶η¹ mode to link adjacent Cd atoms, and the oba^2- ligand links [Cd(L)₂] moieties to form a 1D zigzag chain. In complex 2, the six-coordinated Cd(Ⅱ) center is surrounded by two oxygen atoms (O1, O4) from one 4-nph^2- anion and four nitrogen atoms (N1, N2, N5, and N6) from two different L ligands, adopting a distorted octahedral {CdO₂N₄} geometry. Moreover, the 2D supramolecular architectures are self-assembled through N—H…O hydrogen bonds and π-π stacking interactions. Additionally, complexes 1 and 2 exhibit thermal stabilities and photoluminescent properties in the solid state. Meanwhile, the quantum-chemical calculations of 1 and 2 signify a significant covalent interaction between the coordination atoms and the Cd(Ⅱ) ions.

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

  
  
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  Figure 1  Coordination environment (at 30% probability level) of the Cd(Ⅱ) center in complex 1

  Symmetry codes: A: 3-x, -y, -1-z; B: 2-x, -y, -z.

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  Figure 2  View of the 1D zigzag chain of complex 1

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  Figure 3  View of the 2D supramolecular architecture of complex 1 formed by N—H…O hydrogen bonding and π-π interactions

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  Figure 4  Coordination environment of complex 2 with displacement ellipsoids at the 30% probability level

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  Figure 5  View of the 2D supramolecular architecture of complex 2 formed by N—H…O hydrogen bonding and π-π interactions

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  Figure 6  PXRD patterns of complexes 1 and 2

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  Figure 7  Solid-state photoluminescent spectra of complexes 1 and 2 at room temperature

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  Figure 8  Frontier molecular orbitals of complex 1

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  Figure 9  Frontier molecular orbitals of complex 2

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  Table 1.  Crystallography data and structure refinements for complexes 1 and 2

  Parameter	1	2
  Empirical formula	C26H22CdN8O6	C20H15CdN9O6
  Formula weight	652.90	589.81
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/n	P21/n
  a / nm	0.779 62(4)	0.851 59(8)
  b / nm	1.868 74(9)	1.264 60(13)
  c / nm	1.827 61(9)	2.110 4(2)
  β / (°)	98.864 0(10)	96.100(2)
  Volume / nm3	2.630 9(2)	2.259 8(4)
  Z	4	4
  Dc / (g·cm-3)	1.648	1.734
  GOF	1.041	1.069
  Reflection collected, unique	4 675, 3 126	5 465, 4 763
  Rint	0.038 0	0.031 2
  R [I > 2σ(I)]	0.036 1	0.027 8
  wR	0.083 1	0.060 0

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

  1
  Cd1—N1	0.224 8(3)	Cd1—N1A	0.224 8(3)	Cd1—N2	0.237 5(3)
  Cd1—N2A	0.237 5(3)	Cd1—O1	0.245 0(3)	Cd1—O1A	0.245 0(3)
  Cd2—N5	0.227 6(3)	Cd2—N5B	0.227 6(3)	Cd2—N6	0.233 0(3)
  Cd2—N6B	0.233 0(3)	Cd2—O4	0.246 3(3)	Cd2—O4B	0.246 3(3)
  N1A—Cd1—N1	180.0	N1—Cd1—N2A	104.94(11)	N1A—Cd1—N2	104.94(11)
  N1—Cd1—N2	75.06(11)	N2A—Cd1—N2	180.0	N1—Cd1—O1A	90.19(10)
  N2—Cd1—O1A	88.45(11)	N1A—Cd1—O1	90.19(10)	N1—Cd1—O1	89.81(10)
  N2A—Cd1—O1	88.45(11)	N2—Cd1—O1	91.55(11)	O1A—Cd1—O1	180.00(7)
  N5—Cd2—N5B	180.00(10)	N5—Cd2—N6	74.97(10)	N5B—Cd2—N6	105.03(10)
  N5—Cd2—N6B	105.03(10)	N6—Cd2—N6B	180.00(11)	N5—Cd2—O4B	90.22(10)
  N6—Cd2—O4B	83.70(10)	N5—Cd2—O4	89.78(10)	N5B—Cd2—O4	90.22(10)
  N6—Cd2—O4	96.30(10)	N6B—Cd2—O4	83.70(10)	O4B—Cd2—O4	180.00(14)
  2
  Cd1—O1	0.230 03(16)	Cd1—O4	0.238 26(16)	Cd1—N1	0.233 4(2)
  Cd1—N2	0.233 43(18)	Cd1—N5	0.232 98(19)	Cd1—N6	0.232 1(2)
  O1—Cd1—N6	161.80(7)	O1—Cd1—N5	101.59(6)	N6—Cd1—N5	74.37(7)
  O1—Cd1—N1	100.26(6)	N6—Cd1—N1	87.76(7)	N5—Cd1—N1	155.95(7)
  O1—Cd1—N2	86.62(6)	N6—Cd1—N2	111.40(7)	N5—Cd1—N2	97.65(7)
  N1—Cd1—N2	73.68(6)	O1—Cd1—O4	76.81(6)	N6—Cd1—O4	85.91(7)
  N5—Cd1—O4	96.26(6)	N1—Cd1—O4	98.42(6)	N2—Cd1—O4	160.20(6)
  Symmetry codes: A: 3-x, -y, -1-z; B: 2-x, -y, -z for 1.

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  Table 3.  Hydrogen bonds for complexes 1 and 2

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  N3—H3…O1D	0.077(3)	0.222(3)	0.297 3(4)	165(3)
  N4—H4…O2D	0.081(5)	0.179(6)	0.258 9(5)	174(5)
  N7—H7…O5E	0.072(4)	0.199(4)	0.269 8(5)	170(4)
  N8—H8…O4E	0.083(4)	0.197(4)	0.277 5(4)	167(4)
  2
  N3—H3…O2C	0.087(3)	0.188(3)	0.273 7(3)	171(2)
  N4—H4…O1C	0.076(3)	0.193(3)	0.269 0(3)	177(4)
  N7—H7…O3A	0.080(3)	0.196(3)	0.275 9(3)	171(3)
  N8—H8…O4A	0.081(3)	0.193(3)	0.273 2(3)	170(3)
  Symmetry codes: D: -1+x, y, z; E: 1+x, y, z for 1; A: 2-x, 1-y, 1-z; C: 3/2-x, -1/2+y, 1/2-z for 2.

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  Table 4.  Natural atomic charges, natural valence electron configurations, Wiberg bond indexes, and NBO bond orders for complexes 1 and 2

  Atom	Net charge	Electron configuration	Bond	Wiberg bond index	NBO bond order / a.u.
  1
  Cd1	1.419 86	[core]5s0.294d9.985p0.33
  O1	-0.801 98	[core]2s1.682p5.12	Cd1—O1	0.177 0	0.192 7
  O1A	-0.800 58	[core]2s1.682p5.11	Cd1—O1A	0.176 1	0.192 2
  N1	-0.576 77	[core]2s1.382p4.18	Cd1—N1	0.140 8	0.209 9
  N1A	-0.574 77	[core]2s1.382p4.18	Cd1—N1A	0.140 0	0.209 2
  N2	-0.558 44	[core]2s1.372p4.17	Cd1—N2	0.135 4	0.198 6
  N2A	-0.557 44	[core]2s1.372p4.17	Cd1—N2A	0.135 2	0.198 4
  2
  Cd1	1.399 21	[core]5s0.294d9.985p0.32
  O1	-0.838 02	[core]2s1.702p5.13	Cd—O1	0.229 7	0.233 9
  O4	-0.809 32	[core]2s1.692p5.11	Cd—O4	0.196 0	0.209 8
  N1	-0.544 06	[core]2s1.382p4.15	Cd—N1	0.129 4	0.191 9
  N2	-0.548 62	[core]2s1.372p4.16	Cd—N2	0.136 1	0.198 3
  N5	-0.548 41	[core]2s1.382p4.15	Cd—N5	0.140 3	0.201 1
  N6	-0.549 67	[core]2s1.372p4.16	Cd—N6	0.138 1	0.200 5

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