Syntheses, structural diversities and characterization of a series of coordination polymers with two isomeric oxadiazol-pyridine ligands

Abstract

In this work two positional-isomeric oxadiazol-pyridine ligands 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁) and 4-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂) have been designed and synthesized. A series of novel coordination polymers, namely [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1), {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2), {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3), {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4), {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5), {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6), [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7) and {[Ag(μ₂-L₂)]·BF₄}_n (8) have been isolated. Both 1 and 2 are 2D Cd^II coordination polymers containing infinite {Cd–NCS–Cd} chains (for 1) or infinite {Cd–dca–Cd} layers (for 2), respectively. 3 is a 2D Cu^II coordination polymer, in which central metal ions are bridged via a bidentate bridging L₁ ligand. While when different Ag^I salts were introduced into the reaction system, 1D Ag^I coordination polymers 4 and 5 with diverse coordination modes can be isolated. Furthermore, when the isomeric oxadiazol-pyridine L₂ is used to replace L₁ in the reaction system, 6–8 can be isolated. 6 is a 2D Cd^II coordination polymer containing {Cd–NCS–Cd} layers. 7 is a 2D neutral Ag^I coordination polymer while 8 is a 2D cationic Ag^I coordination polymer. Variable temperature magnetic susceptibility measurements (2–300 K) reveal anti-ferromagnetic interactions between central copper(II) ions for 3. Solid-state luminescent properties of 1, 2 and 4–8 have been investigated indicating strong fluorescent emissions. Additionally, luminescent measurements illustrate that complex 8 exhibits highly sensitive luminescence sensing for Cr₂O₇²⁻ ions in aqueous solutions with high quenching efficiency K_sv = 2.08 × 10⁴ L mol⁻¹ and low detection limit (0.19 μM (S/N = 3)). 8 also represents the first report of a coordination polymer based on oxadiazol-pyridine derivatives with a luminescence response to Cr₂O₇²⁻ anion pollutants in aqueous solutions.

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Introduction

In the past several decades, metal organic coordination polymers have attracted much interest due to their promising applications for gas storage/separation, catalysis, luminescence, drug delivery, sensors and magnetic materials.^1–9 Among these coordination materials, luminescent metal organic coordination polymers have received great interest because they can be applied in the selective and sensitive detection of environmental pollutants, which can further protect human health and the environment from harm by these pollutants.¹⁰ Their tunable channel size, good structural stability, and large surface area of luminescent coordination polymers make these coordination materials more competitive over other luminescent materials because more analytes can effectively interact with the channel surface areas of the coordination polymers, which can ultimately improve the rate of luminescence response, decreasing the fluorescence detection limit and improving the detection sensitivity.¹¹

In the crystal engineering of functional coordination polymers, the variable selection of metal ions and organic ligands provides endless possible outcomes of the final products. Although these porous coordination polymers with targeted secondary building units (SBUs) and topologies can be synthesized by careful selection of the metal ions and organic ligands, the final structures of the products are still highly influenced by several factors including metal–ligand ratio, pH, solvent, temperature, and soon. In particular, judicious selection of ligands is important for deliberate structural changes and functional properties of coordination polymers.¹² Therefore many new flexible or rigid ligands have been explored and assembled into metal–organic polymers via coordination bonds or supramolecular interactions such as hydrogen bonds, π–π stacking, and host–guest ionic interactions et al.^13–16

1,3,4-Oxidazoles can be employed as one kind of efficient bridging ligands and offer a range of charge-balance requirements, alternative linking modes and different orientations of donor groups.^17,18 Currently oxidazole-pyridine and their derivatives have gained more interest as ligands to bridge different metal ions to form functional coordination polymers because of their intriguing bridging fashions.¹⁹ It is noted that investigation of the substituent position isomerization influence of oxidazole-pyridine ligands on the formation of coordination polymers is still scarcely reported. Especially, to the best of our knowledge, 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁) and 4-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂) are never reported in the coordination chemistry. The development of new ligand systems is continuously an important aspect for the chemistry of metal–organic coordination polymer. Previously we also reported a series of functional coordination polymers using versatile N-containing ligands.²⁰ In this work two position-isomeric oxadiazol-pyridine ligands L₁ and L₂ have been employed, a series of novel coordination polymers, namely [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1), {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2), {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3), {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4), {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5), {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6), [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7) and {[Ag(μ₂-L₂)]·BF₄}_n (8) have been isolated. Variable temperature magnetic susceptibility measurements (2–300 K) reveal anti-ferromagnetic interactions between central copper(II) ions for 3. Solid-state luminescent properties of 1, 2 and 4–8 have been investigated indicating strong fluorescent emissions. Additionally, luminescent measurements illustrate that 8 exhibits highly sensitive luminescence sensing for Cr₂O₇²⁻ in aqueous solutions with high quenching efficiency K_sv = 2.08 × 10⁴ L mol⁻¹ and low detection limit (0.19 μM (S/N = 3)), which make it a promising candidate for sensing Cr₂O₇²⁻ ions in practical. Luminescent measurements illustrate that 8 also represents the first report of coordination polymers based on oxadiazol-pyridine derivatives as luminescent response to Cr₂O₇²⁻ ions in the water solutions. Different coordination modes of L₁ and L₂ are also briefly discussed, which indicates these isomeric oxadiazol-pyridine building blocks have great potential in the construction of these unique metal organic coordination polymers.

Experimental section

General methods

Ligands L₁ and L₂ were prepared by literature methods.²¹ All the other reagents were purchased commercially and used without further purification. Deionized water was used as solvent in this work. C, H, and N microanalyses were carried out with a Perkin-Elmer 240 elemental analyzer. FT-IR spectra were recorded from KBr pellets in the range 4000–400 cm⁻¹ on a Bio-Rad FTS 135 spectrometer. Variable-temperature magnetic susceptibilities were measured using a MPMS-7 SQUID magnetometer. Diamagnetic corrections were made with Pascal's constants for all constituent atoms. Photoluminescence spectra were measured by MPF-4 fluorescence spectrophotometer with a xenon arc lamp as the light source.

Synthesis of 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁). 35 mmol of the nicotinohydrazide were suspended in 95 mmol (2.7 eq.) triethyl orthoacetate. The reaction mixture was heated under reflux for 12 hours. After cooling to ambient temperature the formed solid was filtered off, washed twice with 15 mL of diethylether and dried in air. Yield: 70% of theory. ¹H-NMR (400 MHz, DMSO-d₆): δ = 2.61 ppm (s, 3H, CH₃); 7.88–7.90 (m, 2H, arom. C–H); 8.80–8.82 (m, 2H, arom. C–H). ¹³C-NMR (100 MHz, DMSO-d₆): δ = 10.7 ppm (s, 1C, CH₃); 119.9 (s, 2C, arom. C–H); 130.5 (s, 1C, C_q); 150.9 (s, 2C, arom. C–H); 162.4 (s, 1C, C_q=N); 165.0 (s, 1C, C_q=N). FT-IR (cm⁻¹, KBr): 1565(m), 1549(w), 1465(m), 1430(m), 1349(w), 1247(w), 1131(m), 1086(m), 1044(m), 1014(m), 823(m), 705(m).

Synthesis of 4-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂). 35 mmol of the 4-pyridinecarboxylic hydrazide were suspended in 95 mmol (2.7 eq.) triethyl orthoacetate. The reaction mixture was heated under reflux for 12 hours. After cooling to ambient temperature the formed solid was filtered off, washed twice with 15 mL of diethylether and dried in air. Yield: 55% of theory. ¹H-NMR (400 MHz, DMSO-d₆): δ = 2.60 ppm (s, 3H, CH₃); 7.61–7.64 (m, 1H, arom. C–H); 8.32–8.34 (m, 1H, arom. C–H); 8.78–8.79 (m, 1H, arom. C–H); 9.13 (s, 1H, arom. N=C–H). ¹³C{¹H}-NMR (100 MHz, DMSO-d₆): δ = 10.6 ppm (s, 1C, CH₃); 120.0 (s, 1C, C_q); 124.3 (s, 1C, C_q); 133.9 (s, 1C, arom. C–H); 146.9 (s, 1C, arom. C–H); 152.2 (s, 1C, arom. C–H); 164.4 (s, 1C, arom. C–H); 162.1 (s, 1C, C_q=N). FT-IR (cm⁻¹, KBr): 1585(m), 1538(w), 1413(m), 1430(m), 1354(w), 1271(m), 1142(m), 1033(m), 834(m), 770(m).

Preparation of compounds 1–8

[Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1). A mixture of CdCl₂ (18.33 mg, 0.10 mmol), L₁ (17.71 mg, 0.10 mmol), NH₄NCS (15.23 mg, 0.20 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days suitable colorless single crystals were obtained for X-ray diffraction study, yield 55%. Elemental analysis calcd (%) for C₂₀H₁₄Cd₂N₁₀O₂S₄: C 30.82%, H 1.81%, N 17.97%; found: C 30.97%, H 1.93%, N 18.16%. FT-IR (cm⁻¹, KBr): 2090(s), 1572(m), 1558(w), 1465(m), 1263(w), 1131(m), 1098(m), 1033(m), 1014(m), 814(m), 719(m).

{[Cd(μ₂-L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2). A mixture of CdCl₂ (18.33 mg, 0.10 mmol), L₁ (17.71 mg, 0.10 mmol), Nadca (17.81 mg, 0.20 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days suitable colorless single crystals were obtained for X-ray diffraction study, yield 57%. Elemental analysis calcd (%) for C₂₁H₁₁CdN₉O₃: C 45.88%, H 2.02%, N 22.93%; found: C 46.15%, H 2.16%, N 23.12%. FT-IR (cm⁻¹, KBr): 3500(m), 2170(s), 1570(m), 1550(w), 1352(m), 1260(w), 1129(m), 1080(m), 1035(m), 1015(m), 815(m), 720(m).

{[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3). A mixture of CuCl₂ (17.05 mg, 0.10 mmol), L₁ (17.71 mg, 0.10 mmol), NH₄NCS (15.23 mg, 0.20 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days suitable green single crystals were obtained for X-ray diffraction study, yield 37%. Elemental analysis calcd (%) for C₁₈H_14.5CuN₈O_2.5S₂: C 42.35%, H 2.86%, N 21.95%; found: C 42.46%, H 2.97%, N 22.13%. FT-IR (cm⁻¹, KBr): 3410(m), 2090(s), 1557(m), 1420(m), 1359(w), 1268(m), 1093(m), 816(m), 719(m).

{[Ag₂(μ₂-L₁)₃]·2PF₆}_n (4). A mixture of AgPF₆ (25.29 mg, 0.10 mmol), L₁ (17.71 mg, 0.10 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days colorless suitable single crystals were obtained for X-ray diffraction study, yield 59%. Elemental analysis calcd (%) for C₂₄H₂₁Ag₂F₁₂N₉O₃P₂: C 29.14%, H 2.14%, N 12.74%; found: C 29.34%, H 2.38%, N 12.97%. FT-IR (cm⁻¹, KBr): 1589(m), 1420(m), 1270(m), 1110(m), 838(s), 704(m).

{[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃}_n (5). A mixture of AgCF₃SO₃ (25.69 mg, 0.10 mmol), L₁ (17.71 mg, 0.10 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days suitable single crystals were obtained for X-ray diffraction study, yield 39%. Elemental analysis calcd (%) for C₃₅H₂₈Ag₃F₉N₁₂O₁₃S₃: C 29.70%, H 1.99%, N 11.87%; found: C 29.96%, H 2.06%, N 11.96%. FT-IR (cm⁻¹, KBr): 1589(m), 1410(m), 1275(m), 1168(m), 820(m), 1031(bm), 827(m), 720(m).

{[Cd(L₂)₂(μ₂-NCS)₂]}_n (6). 6 was prepared through the similar synthetic method of 3 only using L₂ (17.71 mg, 0.10 mmol) to replace L₁ (17.71 mg, 0.10 mmol) in the reaction system. After a few days suitable colorless single crystals were obtained for X-ray diffraction study, yield 41%. Elemental analysis calcd (%) for C₁₈H₁₄CdN₈O₂S₂: C 39.24%, H 2.56%, N 20.34%; found: C 39.57%, H 2.79%, N 20.59%. FT-IR (cm⁻¹, KBr): 2100 (s), 1617(m), 1567(m), 1251(m), 728(m).

[Ag(μ₂-L₂)·(μ₂-CF₃SO₃)]_n (7). 7 was prepared through the similar synthetic method of 5 only using L₂ (17.71 mg, 0.10 mmol) to replace L₁ (17.71 mg, 0.10 mmol) in the reaction system. After a few days suitable colorless single crystals were obtained for X-ray diffraction study, yield 36%. Elemental analysis calcd (%) for C₉H₇Ag₂F₃N₃O₄S: C 20.55%, H 1.34%, N 7.99%; found: C 20.76%, H 1.57%, N 8.16%. FT-IR (cm⁻¹, KBr): 1616(m), 1565(m), 1418(m), 1272(m), 1142(m), 1033(m), 834(m), 707(m).

{[Ag(μ₂-L₂)]·BF₄}_n (8). A mixture of AgBF₄ (0.10 mmol), L₂ (0.10 mmol) and water (20 mL) was refluxed and stirred for 12 h, then the resulting solutions were filtered. The filtrate was kept in a CaCl₂ desiccator. After a few days suitable single crystals were obtained for X-ray diffraction study, yield 46%. Elemental analysis calcd (%) for C₁₆H₁₄AgBF₄N₆O₂: C 37.17%, H 2.73%, N 16.26%; found: C 37.34%, H 2.97%, N 16.46%. FT-IR (cm⁻¹, KBr): 1616(m), 1566(m), 1416(m), 1266(m), 1051(bm), 836(m), 725(m).

X-ray crystallography

Diffraction intensities for complexes 1–8 were collected on a Bruker SMART 1000 CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) by using the ω–φ scan technique. Lorentz polarization and MULTI-SCAN absorption corrections were applied. The structures were solved by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs.^22,23 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The positions of hydrogen atoms of the water molecules were located from difference maps and refined with isotropic temperature factors. The positions of other hydrogen atoms were generated geometrically. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and details of refinements for compounds 1–8 are summarized in Table 1. Selected bond lengths and angles are listed in Table 2. Corresponding hydrogen bonds lengths and angles for 1–8 are listed in Table S1.†

Table 1 Crystal data and structure refinement information for compounds 1–8

	1	2	3	4	5	6	7	8
a R1 = Σ||Fo| − |Fc||/|Fo|, wR2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2]^1/2.
Empricial formula	C20H14Cd2N10O2S4	C21H11CdN9O3	C18H14.5CuN8O2.5S2	C24H21Ag2F12N9O3P2	C35H28Ag3F9N12O13S3	C18H14CdN8O2S2	C9H7Ag2F3N3O4S	C16H14AgBF4N6O2
Fw	779.45	441.70	510.54	989.18	1415.48	550.89	418.11	517.01
Crystal syst.	Monoclinic	Monoclinic	Monoclinic	Triclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic
Temperature (K)	299(2)	298(2)	300(2)	296(2)	296(2)	298(2)	296(2)	127.3(6)
Space group	C2/c	C2	P2(1)/n	P1̄	P1̄	P2(1)/c	P2(1)/n	I2/a
Space group number	15	12	11	2	2	14	14	15
Flack parameter	—	0	—	—	—	—	—	—
a (Å)	12.402(4)	25.8009(18)	8.9845(5)	9.0183(13)	8.6773(11)	14.0427(13)	8.1723(5)	13.7987(5)
b (Å)	15.8257(5)	8.5402(6)	10.6321(6)	10.8981(17)	15.4665(19)	10.0230(10)	19.7588(12)	11.4490(5)
c (Å)	14.732(5)	7.8195(5)	11.8557(7)	19.524(3)	18.997(2)	7.5702(7)	8.6182(5)	12.1306(6)
α (°)	90	90	90	104.313(3)	77.458(3)	90	90	90
β (°)	112.998(11)	100.454(2)	107.885(2)	91.093(3)	82.510(3)	102.733(3)	107.0410(10)	96.794(4)
γ (°)	90	90	90	112.549(3)	76.521(2)	90	90	90
V (Å^3)	2661.6(15)	1694.4(2)	1077.78(11)	1703.2(4)	2411.6(5)	1039.30(17)	1330.52(14)	1902.95(15)
Z	4	4	2	2	2	2	4	4
F(000)	1520	872	519	968	1392	548	816	1024
ρ (mg m^−3)	1.945	1.731	1.573	1.929	1.949	1.760	2.087	1.805
Abs coeff (mm^−1)	1.952	1.322	1.243	1.353	1.444	1.286	1.727	1.124
Data/restraints/params	2596/0/176	3295/1/227	2104/0/153	1281/30/97	11794/0/676	2025/0/143	2753/0/190	1685/49/160
GOF	1.168	1.011	1.045	1.005	1.016	1.056	1.069	1.094
R1^a (I = 2σ(I))	0.0292	0.0220	0.0340	0.0549	0.0482	0.0297	0.0289	0.0414
wR2^a (all data)	0.0363	0.0231	0.0445	0.0858	0.0956	0.0363	0.0343	0.0439

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Table 2 Selected bond lengths (Å) and angles (°) for 1–8^a

a Symmetry transformations used to generate equivalent atoms: for 1: (A) −x, y, −z + 1/2; (B) x + 1, y, z. For 2: (A) x, y, z − 1. For 3: (A) −x, −y + 1, −z + 1. For 4: (A) x + 1, y, z. For 5: (A) x + 2, y − 1, z + 1. For 6: (A) −x + 2, −y + 1, −z; (B) x, −y + 1/2, z − 1/2. For 7: (A) x − 1, −y + 3/2, z + 1/2. For 8: (A) 3/2 − x, y, 2 − z; (B) 1 − x, 1/2 + y, 3/2 − z.
1
Cd(1)–N(4A)	2.302(4)	Cd(1)–N(4)	2.302(4)	Cd(2)–N(1)	2.402(3)
Cd(1)–N(3A)	2.431(3)	Cd(1)–N(3)	2.431(3)	Cd(2)–N(1A)	2.402(3)
Cd(1)–S(2A)	2.7140(15)	Cd(1)–S(2)	2.7140(15)	Cd(2)–N(5B)	2.291(4)
Cd(2)–N(5)	2.291(4)	N(4)–Cd(1)–S(2A)	93.70(10)	N(3A)–Cd(1)–S(2A)	81.44(9)
N(4A)–Cd(1)–N(4)	102.64(18)	N(4A)–Cd(1)–N(3A)	170.09(13)	N(4A)–Cd(1)–S(2)	93.69(10)
N(4)–Cd(1)–N(3A)	83.24(12)	N(4A)–Cd(1)–N(3)	83.24(12)	N(3A)–Cd(1)–S(2)	94.27(9)
N(4)–Cd(1)–N(3)	170.09(13)	N(3A)–Cd(1)–N(3)	92.13(16)	S(2A)–Cd(1)–S(2)	173.86(6)
2
Cd(1)–N(4)	2.261(3)	Cd(1)–N(6A)	2.264(4)	Cd(1)–N(1)	2.358(3)
Cd(1)–N(7)	2.326(4)	Cd(1)–N(9A)	2.326(3)	Cd(1)–O(2)	2.363(2)
N(4)–Cd(1)–N(6A)	98.40(13)	N(4)–Cd(1)–N(7)	92.38(14)	N(4)–Cd(1)–N(1)	90.76(11)
N(6A)–Cd(1)–N(7)	91.66(17)	N(4)–Cd(1)–N(9A)	168.22(14)	N(7)–Cd(1)–N(1)	173.45(13)
N(6A)–Cd(1)–N(9A)	93.34(14)	N(7)–Cd(1)–N(9A)	86.39(16)	N(4)–Cd(1)–O(2)	84.10(11)
N(6)–Cd(1)–N(1)	93.57(14)	N(7)–Cd(1)–O(2)	86.59(13)	N(6A)–Cd(1)–O(2)	177.01(13)
N(9A)–Cd(1)–N(1)	89.37(13)	N(1)–Cd(1)–O(2)	88.01(9)	N(9A)–Cd(1)–O(2)	84.13(12)
3
Cu(1)–N(4)	1.974(2)	Cu(1)–N(4A)	1.974(2)	Cu(1)–N(1A)	2.0578(19)
Cu(1)–N(1)	2.0578(19)	N(4A)–Cu(1)–N(1A)	88.99(8)	N(1)–Cu(1)–N(1A)	180.0
N(4)–Cu(1)–N(4A)	180.0	N(4)–Cu(1)–N(1)	88.99(8)	N(4)–Cu(1)–N(1A)	91.01(8)
N(4A)–Cu(1)–N(1)	91.01(8)	N(4)–Cu(1)–N(1A)	91.01(8)
4
Ag(1)–N(2)	2.277(4)	Ag(1)–N(4A)	2.307(4)	Ag(2)–N(7)	2.421(4)
Ag(1)–N(8)	2.309(4)	Ag(1)–N(3)	2.406(4)	Ag(2)–N(1)	2.554(4)
Ag(2)–N(6)	2.225(4)	Ag(2)–N(9A)	2.231(4)	N(4A)–Ag(1)–N(3)	98.30(17)
N(2)–Ag(1)–N(4A)	124.79(16)	N(2)–Ag(1)–N(8)	111.05(14)	N(6)–Ag(2)–N(9A)	152.34(16)
N(4A)–Ag(1)–N(8)	109.87(16)	N(2)–Ag(1)–N(3)	102.49(14)	N(8)–Ag(1)–N(3)	108.07(14)
5
Ag(1)–N(4)	2.209(4)	Ag(1)–N(12A)	2.211(4)	Ag(3)–N(10)	2.154(3)
Ag(1)–N(1)	2.458(4)	Ag(2)–N(7)	2.284(3)	Ag(3)–N(9)	2.152(3)
Ag(2)–N(3)	2.297(4)	Ag(2)–N(6)	2.352(4)	Ag(2)–O(5)	2.591(4)
N(4)–Ag(1)–N(1)	103.68(13)	N(12A)–Ag(1)–N(1)	104.13(14)	N(3)–Ag(2)–O(5)	77.79(13)
N(7)–Ag(2)–N(3)	143.19(14)	N(7)–Ag(2)–N(6)	109.20(13)	N(9)–Ag(3)–N(10)	175.45(15)
N(3)–Ag(2)–N(6)	104.96(13)	N(7)–Ag(2)–O(5)	94.06(14)	N(6)–Ag(2)–O(5)	123.73(15)
6
Cd(1)–N(4)	2.325(3)	Cd(1)–N(4A)	2.325(3)	Cd(1)–S(1A)	2.7253(9)
Cd(1)–N(1)	2.364(2)	Cd(1)–N(1A)	2.364(2)	Cd(1)–S(1B)	2.7253(9)
N(4)–Cd(1)–N(4A)	180.00(15)	N(4)–Cd(1)–N(1)	90.51(9)	N(4)–Cd(1)–S(1A)	91.75(8)
N(4A)#1–Cd(1)–N(1)	89.49(9)	N(1)–Cd(1)–S(1B)	90.37(6)	N(1)–Cd(1)–S(1A)	89.63(6)
7
Ag(1)–N(1)	2.165(2)	Ag(1)–N(3A)	2.184(2)	Ag(1)–O(2A)	2.623(2)
Ag(1)–O(4)	2.710(2)	N(1)–Ag(1)–N(3A)	164.72(9)	N(1)–Ag(1)–O(4)	94.19(1)
N(1)–Ag(1)–O(2A)	99.27(1)	N(3A)–Ag(1)–O(4)	83.54(1)
8
Ag(1)–N(1)	2.282(3)	Ag(1)–N(1)	2.282(3)	Ag(1)–N(3A)#1	2.349(3)
Ag(1)–N(3B)	2.349(3)	N(1)–Ag(1)–N(3A)	125.45(12)	N(3A)–Ag(1)–N(3B)	94.65(17)
N(1)–Ag(1)–N(1)	121.29(18)	N(1)–Ag(1)–N(3)	125.45(12)	N(1)–Ag(1)–N(3)	94.85(12)
N(1)–Ag(1)–N(3)	94.85(12)

---

Results and discussion

The crystal samples of complexes 1–8 can retain their crystalline integrity at room temperature for two weeks. As shown in Schemes 1 and 2, only nitrogen atoms of oxadiazole and pyridine rings can participate in coordination and the steric effect prevents oxadiazole oxygen atoms from metal coordination. There are five different coordination modes in 1–8: for 1, 3 and 5, these L₁ ligands in these coordination polymers contain bridging coordination modes a. For 2, these L₁ ligands contain mono-dentate coordination mode b, while in 4 two different coordination (bidentate bridging mode a and tridenate bridging mode c) for L₁ can be observed. For 6, mono-dentate coordination mode d for L₂ can be observed while for 7–8 bi-dentate bridging coordination mode for L₂ can be observed. These various different coordination modes also draw the self-assembly of coordination polymers to form different coordination polymers.

Scheme 1 Two isomeric oxadiazol-pyridine ligands 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁) and 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂).

Scheme 2 Five different coordination modes of L₁ and L₂ in 1–8 (color code: cyan, metal; blue, N; light grey, C).

Structure of [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1)

As shown in Fig. 1, the asymmetric structural unit of [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1) contains two central Cd^II ions (Cd1 and Cd2), two μ₂-bridging L₁ ligands and two μ₂-bridging SCN⁻ groups. The central Cd^II atom is six-coordinated by two nitrogen atoms from L₁, two SCN⁻ sulfur atoms and two SCN⁻ nitrogen atoms forming CdN₄S₂ donor set. The Cd–N distances vary from 2.302(4) to 2.431(2) Å, Cd–S distances are 2.7140(15) Å, all the N–Cd–N and N–Cd–S angles fall in the range of 83.24(12)–94.27(9)°, such coordination mode is in line with an octahedrally coordinated Cd^II center.

Fig. 1 The fundamental structural unit of [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1).

As shown in Fig. 2, these thiocyanate ligands adopt bidentate bridging coordination mode, which link these neighboring Cd^II centers forming {Cd–NCS–Cd} chains. The shortest Cd⋯Cd distance within the {Cd–NCS–Cd} chains is 5.714(20) Å. On the other hand, as listed in Scheme 2(a), these L₁ ligands in 1 adopt μ₂-bridging coordination mode. These {Cd–NCS–Cd} chains are further interlinked via bi-dentate L₁ ligands forming the 2D coordination polymer. These L₁ carbon atoms (C(4) and C(8)) and SCN⁻ nitrogen and sulfur atoms (N(4) and S(1)) generate these non-classical hydrogen bonds (C(4)–H(4)⋯N(4), 3.448(6) Å; C(8)–H(8B)⋯S(1), 3.652(5) Å), which further stabilize the 2D coordination polymer 1.^24,25

Fig. 2 2D coordination polymer [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1) containing infinite {Cd–NCS–Cd} chains.

Structure of {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2)

The asymmetric structural unit of {[Cd(μ₂-L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2) contains one central Cd^II ion (Cd1), one μ₂-bridging L₁ ligand, two μ₂-bridging dca⁻ groups and one coordinated water molecule (O2) and one lattice aqua molecule (O3) (Fig. 3). The central Cd^II atom is six-coordinated by one nitrogen atom (N1) from one L₁, four dca⁻ nitrogen atoms (N4, N7, N6A and N9A) and one terminal aqua molecule (O2) forming CdN₅O donor set. The Cd–N distances are 2.261(3)–2.326(4) Å, Cd–O distances are 2.363(2) Å. All the N–Cd–N and N–Cd–O angles fall in the range of 84.10(11)–98.40(13)°, such coordination mode is in accordance with an octahedrally coordinated Cd^II center.

Fig. 3 The fundamental structural unit of {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2).

As shown in Fig. 4, these bidentate bridging dca⁻ ligands connect these neighboring Cd^II centers forming {Cd–dca–Cd} layers. The shortest Cd⋯Cd distances within the {Cd–dca–Cd} layers are 7.819(1) Å and 8.575(4) Å. On the other hand, as listed in Scheme 2(b), these L₁ ligands in 2 adopt mono-dentate coordination mode. Therefore these {Cd–NCS–Cd} layers are further decorated via these mono-dentate L₁ ligands. These coordinated aqua molecules (O(2)), lattice aqua molecules (O(3)), L₁ nitrogen atoms (N(3)) and dca⁻ nitrogen atoms (N(8)) generate these classical hydrogen bonds (O(2)–H(2A)⋯O(3), 2.740(4) Å; O(2)–H(2B)⋯N(3), 2.854(4) Å; O(3)–H(3A)⋯O(2), 2.965(4) Å; O(3)–H(3B)⋯N(8), 2.970(6) Å), which further stabilize the 2D coordination polymer 2.

Fig. 4 Side view of 2D coordination polymer {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2) containing infinite {Cd–dca–Cd} layers.

Structure of {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3)

As shown in Fig. 5, the asymmetric structural unit of 3 contains one central Cu^II ion (Cu1), one μ₂-bridging L₁ ligand and two terminal bridging SCN⁻ groups. The central Cu^II atom is six-coordinated by four nitrogen atoms (N1, N1A, N3A and N3B) from four L₁, two thiocyanate nitrogen atoms (N4 and N4A) forming the CuN₆ donor set. The Cu–N distances are 1.974(2)–2.0578(19) Å, all the N–Cu–N angles fall in the range of 88.99(8)–91.01(8)°, such coordination mode is in accordance with an octahedrally coordinated Cu^II center.

Fig. 5 The fundamental structural unit of {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3).

As shown in Fig. 6 and Scheme 2(a), these L₁ ligands in 3 also adopt μ₂-bridging coordination mode. These L₁ ligands connect neighboring Cu^II centers forming the 2D coordination polymer 3. The corresponding Cu⋯Cu distance within the 2D coordination polymer is 9.997(5) Å. These L₁ carbon molecules (C(1)) and L₁ nitrogen atoms (N(2)) generate these non-classical hydrogen bonds (C(1)–H(1)⋯N(2), 3.304(3) Å), which also further extend 3 into a 3D supramolecular architecture. Such weak intermolecular interactions are key for the stabilization of the 2D coordination polymer 3.

Fig. 6 2D coordination polymer {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3).

Structure of {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4)

When AgBF₄ are used, 1D Ag(I) coordination polymers, {[Ag₂(μ₃-L₁)₂(μ₂-L₁)]·2PF₆}_n (4), can be isolated. For 4, the coordination polymer crystallizes in triclinic P1̄ space group with one formula unit in the asymmetric unit. The asymmetric unit contains two Ag^I atoms (Ag1 and Ag2), two tri-dentate bridging L₁, one bi-dentate bridging L₁ and two free PF₆⁻ anions (Fig. 7). The central Ag^I atom (Ag1) is four-coordinated by four nitrogen atoms (N2, N4, N3A and N4A) from four L₁ forming AgN₄ donor set. The other central Ag^I atom (Ag2) is also four-coordinated by four nitrogen atoms (N1, N6, N7 and N9A) from four L₁ forming AgN₄ donor set. The Ag–N distances are 2.225(4)–2.554(4) Å, all the N–Ag–N angles are in the range of 98.30(17)–152.34(16)°, such coordination mode is in accord with a tetrahedrally coordinated Ag^I center.

Fig. 7 The fundamental structural unit of {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4).

As shown in Fig. 8, Ag1 and Ag2 are inter-linked via oxadiazole nitrogen atoms forming di-nuclear Ag(I) clusters, neighboring Ag⋯Ag distances within di-nuclear Ag(I) clusters are 3.845(6) Å. These bridging L₁ ligands connect adjacent Ag^I ions forming a cluster-based 1D double chain coordination polymer, which is shown along the crystallographic a-axis. These non-classical hydrogen bonds (C(8)–H(8B)⋯F(12), 3.341(9) Å; C(17)–H(17)⋯F(7), 3.190(10) Å; C(17)–H(17)⋯F(8), 3.347(10) Å; C(21)–H(21)⋯N(5), 3.504(8) Å) also can be observed, which further extend 4 into a 3D supramolecular architecture. Such weak intermolecular interactions are important for the stabilization of the 1D cluster-based Ag^I coordination polymer 4.

Fig. 8 1D Ag^I coordination polymer {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4).

Structure of {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5)

When AgCF₃SO₃ is used, 1D Ag(I) coordination polymer, {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5), can be isolated. For 5, the coordination polymer also crystallizes in triclinic P1̄ space group with one formula unit in the asymmetric unit. The asymmetric unit contains three Ag^I atoms (Ag1, Ag2 and Ag3), four bi-dentate bridging L₁, two terminal CF₃SO₃⁻ anions and one free CF₃SO₃⁻ anion (Fig. 9). The central Ag^I atom (Ag1) is three-coordinated by four nitrogen atoms (N4, N1 and N12A) from three L₁ forming AgN₃ donor set. The central Ag^I atom (Ag2) is four-coordinated by three nitrogen atoms (N3, N6 and N7) from three L₁ and one CF₃SO₃⁻ oxygen atom (O5) forming AgN₃O donor set. The central Ag^I atom (Ag3) is three-coordinated by two nitrogen atoms (N9 and N10) from two L₁ and one CF₃SO₃⁻ oxygen atom (O9) forming AgN₂O donor set. The Ag–N distances vary from 2.209(4) Å to 2.458(4) Å, Ag–O distances are 2.591(4) Å, all the N–Ag–N and N–Ag–O angles fall in the range of 77.79(13)–143.19(14)°, All the bond distances and bond angles all fall into the normal range.

Fig. 9 The fundamental structural unit of {[Ag₃(μ₂-L₁)₄(CF₃SO₃)₂]·CF₃SO₃}(5).

As shown in Fig. 10, L₁ adopt bi-dentate bridging coordination mode, which connect neighboring Ag1, Ag2 and Ag3, forming 1D double chain coordination polymer 5. The shortest neighboring Ag⋯Ag distances within 1D double chain are 4.478(2) and 4.858(8) Å. These non-classical hydrogen bonds (C(1)–H(1A)⋯O(13), 3.348(7) Å; C(4)–H(4)⋯N(2), 3.259(6) Å; C(12)–H(12)⋯N(5), 3.241(6) Å; C(15)–H(15)⋯O(11), 3.393(7) Å) also can be observed, which further extend 5 into a 3D supramolecular architecture. Such weak intermolecular interactions are key for the stabilization of 1D Ag^I double chain coordination polymer 5.

Fig. 10 1D Ag^I coordination polymer {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃}(5).

Structure of {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6)

As shown in Fig. 11, the asymmetric structural unit of {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6) contains one central Cd^II ions (Cd1), two terminal coordinated L₁ ligands and two μ₂-bridging NCS⁻ groups. The central Cd^II atom is six-coordinated by two nitrogen atoms (N1 and N1A) from two L₂, two nitrogen atoms (N4 and N4A) and two sulfur atoms (S1A and S1B) form four NCS⁻ groups forming CdN₄S₂ donor set. The Cd–N distances vary from 2.325(3) to 2.364(2) Å, Cd–S distances are 2.7253(9) Å. All the N–Cd–N and N–Cd–O angles are in the range of 89.63(6)–91.75(8)°, such coordination mode is in accordance with an octahedrally coordinated Cd^II center.

Fig. 11 The fundamental structural unit of {[Cd(μ₂-L₂)₂(μ₂-NCS)₂]}_n (6).

As shown in Fig. 12, these bidentate bridging NCS⁻ ligands connect neighboring Cd^II centers forming the {Cd–NCS–Cd} layers. The shortest distances of Cd⋯Cd within the {Cd–NCS–Cd} layers are 6.280(1) and 7.570(1) Å. On the other hand, as listed in Scheme 2(b), these L₂ ligands in 6 adopt mono-dentate coordination mode. Therefore these {Cd–NCS–Cd} layers are further decorated via these mono-dentate L₂ ligands. These L₂ carbon atoms (C(1)) and nitrogen atoms (N(3)) generate non-classical hydrogen bonds (C(1)–H(1)⋯N(3), 3.528(4) Å), which further stabilize the 2D coordination polymer 6.

Fig. 12 2D coordination polymer {[Cd(μ₂-L₂)₂(μ₂-NCS)₂]}_n (6) containing infinite {Cd–NCS–Cd} layers.

Structure of [Ag(μ₂-L₂)·(μ₂-CF₃SO₃)]_n (7)

As shown in Fig. 13, the asymmetric structural unit of [Ag(μ₂-L₂)·(μ₂-CF₃SO₃)]_n (7) contains one central Ag^I ions (Ag1), one μ₂-bridging L₂ ligand, one μ₂-bridging CF₃SO₃⁻ group. For 7, the coordination polymer crystallizes in monoclinic P2₁/n space group with one formula unit in the asymmetric unit. The central Ag^I atom (Ag1) is four-coordinated by two nitrogen atoms (N1 and N3A) from two L₁ and two CF₃SO₃⁻ oxygen atoms (O2A and O4) forming AgN₂O₂ donor set. Ag–N distances are 2.165(2)–2.184(2) Å, Ag–O distances are 2.623(2)–2.710(2) Å, all the N–Ag–N and N–Ag–O angles are in the range of 83.54(1)–94.19(1)°, such coordination mode is in accord with a tetrahedrally coordinated Ag^I center.

Fig. 13 The fundamental structural unit of [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7).

As shown in Fig. 14, these CF₃SO₃⁻ anions adopt bi-dentate bridging coordination modes via its two oxygen atoms (O2 and O4). On the other hand, these L₂ ligands also adopt bridging coordination mode as listed in Scheme 2(e). These bridging CF₃SO₃⁻ anions and L₂ ligands link these neighboring Ag(I) centers, which ultimately forming the 2D coordination polymer 7. These L₂ carbon atoms (C(2) and C(6)) and CF₃SO₃⁻ oxygen and fluoride atoms (O(4) and F(1)) generate these non-classical hydrogen bonds (C(2)–H(2C)⋯F(1), 3.338(5) Å; C(6)–H(6)⋯O(4), 3.308 Å), which further extend 7 into a 3D supramolecular architecture. Such weak intermolecular interactions are important for the stabilization of the 1D cluster-based Ag^I coordination polymer 7.

Fig. 14 2D coordination polymer [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7).

Structure of {[Ag(μ₂-L₂)]·BF₄}_n (8)

As shown in Fig. 15, the fundamental structural unit of {[Ag(μ₂-L₂)]·BF₄}_n (9) contains one central Ag^I ions (Ag1), one μ₂-bridging L₂ ligand, one free BF₄⁻ group. For 8, the coordination polymer crystallizes in monoclinic I₂/a space group with one formula unit in the asymmetric unit. The central Ag^I atom (Ag1) is four-coordinated by four nitrogen atoms (N1, N3A, N1A and N3B) from four L₂ forming AgN₄ donor set. The Ag–N distances are 2.282(3)–2.349(3) Å, all the N–Ag–N angles are in the range of 94.65(17)–125.45(12)°, such coordination mode is in accord with a tetrahedrally coordinated Ag^I center.

Fig. 15 The fundamental structural unit of {[Ag(μ₂-L₂)]·BF₄}_n (8).

As shown in Fig. 16, these L₂ ligands also adopt bridging coordination mode as listed in Scheme 2(e). These bridging L₂ ligands link these neighboring Ag(I) centers, which ultimately forming the 2D grid-like cationic coordination polymer. These non-classical hydrogen bonds (C(4)–H(4)⋯F(2), 3.085(9) Å) also can be observed, which further extend 8 into a 3D supramolecular architecture. Such weak intermolecular interactions are important for the stabilization of the 2D Ag^I coordination polymer.

Fig. 16 2D cationic Ag^I coordination polymer {[Ag(μ₂-L₂)]·BF₄}_n (8).

Magnetic properties of 3

Variable temperature dc magnetic susceptibility data were collected on a microcrystalline powder sample of 3 in a 1000 Oe field in the temperature range 1.8–300 K. The temperature-dependent magnetic susceptibility data of complexes 3 have been measured in the temperature range of 2–300 K. The diamagnetic correction was evaluated by using Pascal's constants. As shown in Fig. 17, The χ_mT values are ca. 0.416 cm³ kmol⁻¹ (for 3) at room temperature, which are expected spin-only values for an isolated Cu(II) ion. When the temperature is lowered, χ_MT values remain constants and then smoothly decrease and drop rapidly below ca. 20 K reaching the minimum values at 2 K. Fitting the magnetic susceptibility data with the Curie–Weiss law χ_M = C/(T − θ), where C and θ represent the Curie constant and the Weiss temperature, it can give θ values of −1.05 K. These θ values reveal anti-ferromagnetic interactions between central Cu^II ions. The result of weak anti-ferromagnetic interactions can be attributed to can be attributed to very weak antiferromagnetic interactions as indicated by large metal–metal distances or the presence of zero-field splitting or both in 3.²⁶

Fig. 17 Variable-temperature magnetic susceptibility data for 3 in the 2–300 K temperature range.

Luminescence properties

Coordination polymers have been investigated for potential applications as luminescent materials, such as light-emitting diodes (LEDs).²⁷ Owing to the ability of affecting the emission wavelength and strength of organic materials, syntheses of coordination polymers by judicious choice of conjugated organic spacers and transition metal centers can be an efficient method for obtaining new types of luminescent materials, especially for d¹⁰ or d¹⁰–d¹⁰ systems.²⁸ In the present work, we have explored the luminescent properties of L₁, L₂ ligands and organic/inorganic coordination polymers 1, 2 and 4–8 based on the ligands in the solid state.

As shown in Fig. 18, at room temperature, free L₁ and L₂ ligands in the solid state are luminescent, which show the broad emission maximum centered at 360 nm (λ_ex = 280 nm) and 361 nm (λ_ex = 280 nm), respectively. For the free L₁ and L₂ ligands, the chromophores are aromatic pyridine and oxadiazol rings, the observed emission is due to π–π* transition. In comparison with that of free ligand, upon excitation of solid samples of 1, 2, 6 and 8 show emission bands with maximum at ca. 360 nm (for 1), ca. 361 nm (for 2), ca. 363 nm (for 6) and ca. 362 nm (for 8) respectively. The emissions of 1, 2, 6 and 8 should be assigned to intraligand π–π* transitions.²⁹ The slight shifts of the emission bands relative to free L₁ and L₂ ligands can be attributed to the coordination action of ligand to central metal ions.³⁰

Fig. 18 Solid-state fluorescence spectra of 1–2, 4–8 and L₁, L₂ ligand molecules.

On the other hand, for these 1D or 2D cluster-based Ag^I coordination polymers 4, 5 and 7, it is noted that red-shift of luminescent emissions can be observed (emission bands with maximum centered at 395 nm for 4 (λ_ex = 300 nm), 403 nm for 5 (λ_ex = 300 nm) and 372 nm for 7 (λ_ex = 300 nm)). The photoluminescence origin of the emission bands should be attributed to metal-to-ligand charge transfer (MLCT) as discussed in the previous literature.¹ Because 5 and 6 contain similar 1D structural motifs, different band shapes in the luminescent emissions of 5 and 6 can be ascribed to different guest PF₆⁻/CF₃SO₃⁻ anions, which can greatly affect luminescence emission bands.³¹ These cluster-based Ag^I coordination polymers increase the ligand conformational rigidity, thereby reducing the non-radiative decay of the intraligand (π–π*) excited state. The syntheses of new Cd(II) and Ag(I) complexes with these isomeric oxadiazol-pyridine ligands can be an efficient method for obtaining new types of luminescent materials.^32,33

Luminescent sensing properties of Cr₂O₇²⁻ anion pollutants for 8

In general, water pollutions are a global environmental issue, and considerable research attention has been paid to the removal of pollutants from waste water. With the development of modern industry, Cr₂O₇²⁻ anions have been widely used in the field of chromium plating, metallurgy, pigment manufacturing, leather tanning, and wood preservation. These anion pollutants have been a focus of concern because they cause serious damage to human health and the environment. Therefore, the exploration of new materials for the efficient detection of these anion pollutants is highly important.³⁴

In this work, the product of 8 as the luminescent probes was carefully explored for sensing diverse ions, owing to its strong visible blue light when excited by ultraviolet light. The photoluminescence emission bands of 8 in aqueous solutions are centered at around 360 nm. These emission bands should be more likely to be due to intra-ligand transitions because similar photo-luminescent bands can be observed for free L₂ ligands in aqueous solutions. On the other hand, water stability of 8 was also investigated, the solid state samples were suspended in aqueous solutions for 12 h, then water solutions were filtrated and solid state samples were dried by the vacuum oven. Then solid state samples were characterized by PXRD patterns. As shown in Fig. S1,† PXRD patterns of solid state samples suspended in the water solutions and these calculated patterns deduced from single-crystal X-ray data are in good agreement. The result indicates that the coordination polymer structure is unchangeable and can retain stable in aqueous solutions.

In order to study the luminescent responses to different anions, the products of 8 were ground into powder and suspended in the water solution containing different salt solutions of the same concentration (10⁻⁴ M) of SiF₆²⁻, CO₃²⁻, HCO₃²⁻, BF₄⁻, NO₃⁻, SCN⁻, Ac⁻, SO₄²⁻, Br⁻, Cl⁻, F⁻, I⁻, BrO₃⁻, IO₃⁻, CrO₄²⁻, HPO₄²⁻ and PO₄³⁻ and Cr₂O₇²⁻. Aqueous solutions of SiF₆²⁻, CO₃²⁻, HCO₃²⁻, BF₄⁻, NO₃⁻, SCN⁻, Ac⁻, SO₄²⁻, Br⁻, Cl⁻, F⁻, I⁻, BrO₃⁻, IO₃⁻, CrO₄²⁻, HPO₄²⁻ and PO₄³⁻ and Cr₂O₇²⁻ were prepared from these corresponding potassium salts, respectively. As shown in Fig. 19(a), Cr₂O₇²⁻ gave significant fluorescence quenching effect, while there was only a negligible effect on the luminescence intensity for other anions. PXRD pattern of 8-Cr₂O₇²⁻ was measured and remains well consistent with the simulated one of 8, indicating that 8 still remains stable. The result indicates the high selectivity of 8 for the detection and specific recognition of Cr₂O₇²⁻ anions in aqueous solutions. Therefore 8 may be chosen as a candidate for the selective sensing of Cr₂O₇²⁻.

Fig. 19 (a) Comparison of the luminescence intensity of 8 at 360 nm incorporating aqua solutions (10⁻⁴ M) upon the addition of various anions; (b) liquid luminescence spectra of 8 under different concentrations of Cr₂O₇²⁻ aqueous solution; (c) comparison of the luminescence intensity of 8 under different concentrations of Cr₂O₇²⁻ anion pollutants aqueous solution. (d) The linear luminescence intensity vs. Cr₂O₇²⁻ anion pollutants concentration plot.

Generally, waste water contains more than one type of pollutant anions, and therefore, it is essential to investigate the influence of mixed anions on the luminescence of 8. The detailed experiments are as follows: 0.4 mL of Na₂Cr₂O₇ (10⁻⁴ M) and 0.4 mL of other anion solutions (10⁻⁴ M) were slowly dropped into a 3.2 mL suspension of 8, respectively. The luminescence measurements of resultant solutions, containing Cr₂O₇²⁻ and other anions, were carried out at once. It is also noted that there was only a negligible effect on the luminescence intensity for Cr₂O₇²⁻ anions. This selective detection of Cr₂O₇²⁻ anions has no interfering effects from other anions such as SiF₆²⁻, CO₃²⁻, HCO₃²⁻, BF₄⁻, NO₃⁻, SCN⁻, Ac⁻, SO₄²⁻, Br⁻, Cl⁻, F⁻, I⁻, BrO₃⁻, IO₃⁻, CrO₄²⁻, HPO₄²⁻ and PO₄³⁻, suggesting that 8 can selectively detect Cr₂O₇²⁻ anions among the above anions.

Moreover, to explore the detection limit of 8 as a luminescent probe for detecting Cr₂O₇²⁻, as shown in Fig. 19(b), a series suspension of 8-Cr₂O₇²⁻ (5 μM to 100 μM) were prepared by dropping different concentrations of Cr₂O₇²⁻ solutions into the suspension of 8. The luminescence intensity of 8 gradually decreases with increasing the concentration of Cr₂O₇²⁻. As shown in Fig. 19(c) and (d), the luminescence intensity linearly decreases with the concentration of Cr₂O₇²⁻ ranging from 0.5 μM to 70 μM. The detection limit of 8 as a luminescent probe for detecting Cr₂O₇²⁻ is supported by the calculated results based on the equation: detection limit = 3σ/k (σ is the standard deviation of blank measurement; k is the slope between the luminescence intensity vs. log[Cr₂O₇²⁻]). To further explore the relationship between the quenching effect and Cr₂O₇²⁻ concentration, the linear luminescence intensity vs. Cr₂O₇²⁻ concentration plot was made, which can be fitted into I₀/I = 1 + K_sv[Cr₂O₇²⁻] (I₀ and I represent the luminescence intensity of 8 before and after adding Cr₂O₇²⁻, respectively; [Cr₂O₇²⁻] represents the concentration of Cr₂O₇²⁻, and K_sv represents the quenching rate constant). The K_sv value is calculated to be 2.08 × 10⁴ L mol⁻¹, indicating the high quenching efficiency of Cr₂O₇²⁻ in the emission of 8.^35–37 The high quenching efficiency and low detection limit 0.19 μM (S/N = 3) also reveal that 8 can act as luminescent probes for discrimination and detection of Cr₂O₇²⁻.

8 is a cationic coordination polymer, in which free BF₄⁻ anions are located between these 2D cationic layers, while in other Ag^I coordination polymers these anions are tightly linked to main frameworks through coordination bonds. As illustrated in the previous literature, these cationic coordination polymers can effectively capture Cr₂O₇²⁻ anions through anion exchange process,³⁸ which may effectively lead to the fluorescent quenching of 8. On the other hand, in comparison with other anions, only Cr₂O₇²⁻ exhibits two wide absorption bands from 230 to 413 nm.³⁹ The bands almost cover the whole ranges of absorption bands that arise from 8. Thus, the luminescence quenching mechanism also further causes by the competition of excitation energy between Cr₂O₇²⁻ anions and 8. Upon illumination, the adsorption of Cr₂O₇²⁻ ions for excitation energy hinders the UV/vis absorption of the target coordination polymer, thus resulting in a decrease, or even full quenching, of the luminescence intensities. This luminescent sensing mechanism for Cr₂O₇²⁻ is consistent with those previously proposed by previous literature.^40,41 To the best of our knowledge, work on these metal–organic polymers as luminescent probes for discrimination and detection of Cr₂O₇²⁻ is important and still limited.⁴² 8 also represents the first report of coordination polymers based on oxadiazol-pyridine derivatives as luminescent response to Cr₂O₇²⁻ in the water solutions.

Conclusions and perspectives

In conclusion, in this work two position-isomeric oxadiazol-pyridine ligands 3-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₁) and 4-(5-methyl-1,3,4-oxadiazol-2-yl)pyridine (L₂) have been designed and synthesized. A series of novel coordination polymers, namely [Cd₂(μ₂-L₁)₂(μ₂-NCS)₄]_n (1), {[Cd(L₁)(μ₂-dca)₂(H₂O)]·H₂O}_n (2), {[Cu(μ₂-L₁)₂(NCS)₂]·0.5H₂O}_n (3), {[Ag₂(μ₂-L₁)(μ₃-L₁)₂]·2PF₆}_n (4), {[Ag₃(μ₂-L₁)₄(μ₂-CF₃SO₃)(CF₃SO₃)]·CF₃SO₃} (5), {[Cd(L₂)₂(μ₂-NCS)₂]}_n (6), [Ag(μ₂-L₂)(μ₂-CF₃SO₃)]_n (7) and {[Ag(μ₂-L₂)]·BF₄}_n (8) have been isolated. Both 1 and 2 are 2D Cd^II coordination polymers containing infinite {Cd–NCS–Cd} chains (for 1) or infinite {Cd–dca–Cd} layers (for 2). 3 is a 2D Cu^II coordination polymer, in which central metal ions are bridged via bidentate bridging L₁ ligand. While when different Ag^I salts were introduced into the reaction system, 1D cluster-based Ag^I coordination polymers 4 and 5 with diverse coordination modes can be isolated. Further when the isomeric oxadiazol-pyridine L₂ is used to replace L₁ in the reaction system, 6–8 can be isolated. 6 is a 2D Cd^II coordination polymer containing 2D {Cd–NCS–Cd} layers. 7 is a 2D neutral Ag^I coordination polymer while 8 is a 2D cationic Ag^I coordination polymer. Variable temperature magnetic susceptibility measurements (2–300 K) reveal anti-ferromagnetic interactions between central copper(II) ions for 3. Solid-state luminescent properties of 1, 2 and 4–8 have been investigated indicating strong fluorescent emissions. Additionally, luminescent measurements illustrate that complex 8 also exhibits highly sensitive luminescence sensing for Cr₂O₇²⁻ anion pollutants in aqueous solutions with high quenching efficiency K_sv = 2.08 × 10⁴ L mol⁻¹ and low detection limit (0.19 μM (S/N = 3)). 8 also represents the first report of coordination polymers based on oxadiazol-pyridine derivatives as luminescent response to Cr₂O₇²⁻ anion pollutants in the water solutions. On the basis of this work, further syntheses, structures and properties studies of these coordination polymers using these isomeric oxadiazol-pyridine derivatives as basic building blocks are also under way in our laboratory.

Acknowledgements

This work was supported financially by the Open Project of Key Lab Adv Energy Mat Chem (Nankai Univ), Tianjin Educational Committee (20120508), Natural Science Foundation of Tianjin (Grant no. 14JCQNJC05900), Young Scientist Fund (Grant no. 21301128), National Natural Science Foundation of China (Grant No. 21375095) and the Program for Innovative Research Team in University of Tianjin (TD12-5038).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1486502 (1), 1518712 (2), 1486501 (3), 1486507 (4), 1486509 (5), 1486505 (6), 1486508 (7), and 1487692 (8). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra28153b

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