Syntheses, crystal structures and photoluminescent properties of silver(I) and cadmium(II) coordination polymers constructed from flexible hexadentate di-Schiff base ligands

Abstract

A novel flexible S₂N₄ di-Schiff base, 2,2′-((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(N-(pyridin-2-ylmethylene)aniline (pbbpa), was synthesized and used to construct two Ag(I) and Cd(II) coordination polymers, namely [Ag₄(pbbpa)(NO₃)₄]_n (1) and [Cd₂(pbbpa)(NO₃)₄]_n (2), which exhibited complicated 3D frameworks, incorporating Ag–C bonds and a 2D 6³-hcb net, respectively. Their thermal stabilities, UV-vis absorption and photoluminescent properties were also discussed.

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1. Introduction

During the past decades, a great deal of effort has been devoted to the assembly of metal–organic frameworks (MOFs) with multidentate organic ligands, not only due to their novel structures and topologies, but also their appealing properties and potential applications in luminescence, magnetism and catalysis, gas sorption and separation.¹ In contrast to rigid ligands, such as 4,4′-bipyridine and 4,4′-biphenyldicarboxylic acid, that show little or no conformational alteration when they coordinate with metal centers, flexible multidentate ligands can adopt different conformations and have more possible coordination modes according to the different geometric requirements of the metal ion.² Recently, interesting structures constructed from flexible multidentate ligands have shown interesting rotaxane, catenane, and helicate characters.³ Among the multidentate ligands, Schiff base ligands have received much attention due to their multiple coordination sites with different coordination preferences.⁴ As we know, the shape, conformation and coordinating orientation of organic ligands are dominant in affecting the structures of the supramolecular aggregates. So the design and synthesis of new multifunctional ligands are important steps to the assembly of desired coordination compounds. Recently, we have been investigating the use of flexible ligands based on aromatic spacers linked to thioether-carboxylate arms to build coordination polymer arrays.⁵ These organic ligands favor modulable conformations (cis, trans, or any intermediate conformations between them),⁶ and would provide us with an alternative approach for the construction of frameworks with novel patterns that are not easily achievable with simple rigid linear ligands. Moreover, conformational flexibility and different binding modes could increase the possibility of forming supramolecular isomers.⁷

Inspired by the points mentioned above, we designed a novel hexadentate S-shaped di-Schiff base ligand which has two symmetric –CH₂– double arms and two (pyridin-2-ylmethylene)aniline Schiff base arms linked by S donors. The –CH₂– arms and the N and S donors endow this di-Schiff base ligand with flexible conformations and multiple coordination sites with different coordination preferences. Herein, we present the synthesis of this novel di-Schiff base ligand and explore its coordination chemistry towards Ag(I) and Cd(II) ions (Scheme 1).

Scheme 1 Synthetic procedures for Ag(I) and Cd(II) complexes based on the S₂N₄ di-schiff base ligand.

2. Experimental

2.1. Materials and methods

All the reagents and solvents employed were commercially available and used as received without further purification. Infrared spectra were recorded on a Nicolet AVATAT FT-IR330 spectrometer as KBr pellets in the frequency range 4000–400 cm⁻¹. The elemental analyses (C, H, N content) were determined on a CE Instruments EA 1110 analyzer. Photoluminescence measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer with solid powder on a 1 cm round quartz plate. TG curves were measured from 30 to 600 °C on a SDT Q600 instrument at a heating rate 10 °C min⁻¹ under a N₂ atmosphere (100 mL min⁻¹). X-ray powder diffraction was performed on a Panalytical X-Pert Pro diffractometer with Cu-Kα radiation. UV-vis diffuse reflectance spectra were measured from 200 to 800 nm on a Carry 5000 UV-vis-NIR spectrophotometer.

2.2. Syntheses

2.2.1. Synthesis of 2,2′-((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(N-(pyridin-2-ylmethylene)aniline (pbbpa). Synthesis of (I): to a solution of KOH (1.3 g, 19.7 mmol) in methanol (40 mL) was added o-aminothiophenol (2.4 g, 19.2 mmol), followed by p-xylylene dibromide (2.0 g, 7.5 mmol) at room temperature with stirring. After stirring for 2 h, the solid obtained was filtered and washed with methanol (25 mL). Then the solid was washed with water (50 mL) and dried in vacuo to give I as a white solid (2.4 g, yield 90%). ¹H NMR (300 MHz, CDCl₃): 7.21 (q, 2H) , 7.13 (m, 2 H), 6.99 (s, 4 H), 6.69 (m, 4 H), 3.85 (s, 4 H).

Synthesis of (II): I (1.408 g, 4 mmol) and 2-pyridinecarboxaldehyde (0.94 g, 8.8 mmol) were stirred in an acetonitrile–chloroform (v : v = 9 : 1) solvent mixture at room temperature for 3 h. The solution was stirred but the reaction never went to completion as checked by TLC, which showed the presence of unreacted amine even after 3 h. A second portion of 2-pyridinecarboxaldehyde (0.32 g, 3 mmol) was added and the reactants were stirred for another 3 h. Then, the solvent was evaporated in vacuo, and the crude product was purified by recrystallization from chloroform–methanol (0.954 g, yield 45%.). ¹H NMR (300 MHz, CDCl₃): 8.83 (d, 2H), 8.72 (d, 2H), 7.96 (s, 2H), 7.86–7.89 (m, 2H), 7.46–7.55 (m, 2H), 7.17–7.21 (q, 2H), 7.07–7.13 (m, 2H), 7.02 (s, 4H), 6.60–6.70 (m, 4H), 3.85 (s, 4H).

2.2.2. [Ag₄(pbbpa)(NO₃)₄]_n (1). A buffer layer of chloroform (6 mL) was carefully layered over a chloroform (4 mL) solution of pbbpa (15 mg, 0.028 mmol). Then a solution of AgNO₃ (45 mg, 0.26 mmol) in methanol (4 mL) was layered over the buffer layer. The solution was allowed to diffuse in the dark at room temperature for four days to yield a large amount of yellow block crystals. Yield: ca. 48%. Anal. Calc. (found) for C₁₆H₁₃Ag₂N₄O₆S: C, 31.76 (31.33); H, 2.17 (2.94); N, 9.26 (9.94)%. IR (KBr): v (cm⁻¹) = 1729 (s), 1635 (w), 1595 (w), 1396 (s), 1291 (m), 776 (m), 525 (m).

2.2.3. [Cd₂(pbbpa)(NO₃)₄]_n (2). Pbbpa (2.65 mg, 0.005 mmol) and Cd(NO₃)₂·4H₂O (5.3 mg, 0.017 mmol) were dissolved in acetonitrile. Then the solution was sealed in a glass tube, slowly heated to 90 °C from room temperature in 300 min, kept at 90 °C for 4000 min, and then slowly cooled to 30 °C in 600 min. The yellow block crystals that formed were collected, washed with DMF, and dried in air. Yield: ca. 56%. Anal. Calc. (found) for C₃₂H₂₆Cd₂N₈O₁₂S₂: C, 38.30 (38.67); H, 2.61 (2.66); N, 11.17 (11.67)%. IR (KBr): v (cm⁻¹) = 1636 (s), 1384 (s), 1197 (m), 1133 (m), 788 (s).

3. Results and discussion

3.1. X-ray crystallography

Single crystals of complexes 1–2 with appropriate dimensions were chosen under an optical microscope and mounted on a glass fiber for data collection. Single-crystal X-ray diffraction was performed using a Bruker Apex II CCD diffractometer equipped with a fine-focus sealed-tube X-ray source (Mo-Kα radiation, graphite monochromated). In both cases, the highest possible space group was chosen. Both structures were solved by direct methods using SHELXS-97⁸ and refined on F² by a full-matrix least-squares procedure with SHELXL-97.⁹ Atoms were located from iterative examination of difference F-maps following least squares refinements of the earlier models. Hydrogen atoms were placed in calculated positions and included as riding atoms with isotropic displacement parameters 1.2–1.5 times the U_eq of the attached C atoms. Both structures were examined using the Addsym subroutine of PLATON¹⁰ to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collected in Table 1. Selected bond lengths and angles for 1 and 2 are collected in Table 2.

Table 1 Crystal data for 1 and 2

Empirical formula	C16H13Ag2N4O6S	C32H26Cd2N8O12S2
Formula weight	605.10	1003.53
T (K)	298(2)	298(2)
Crystal system	Monoclinic	Orthorhombic
Space group	P21/n	Fdd2
a (Å)	11.0150(12)	35.5141(11)
b (Å)	9.2725(10)	37.5405(12)
c (Å)	18.251(2)	10.7672(3)
α (°)	90.00	90.00
β (°)	99.2030(10)	90.00
γ (°)	90.00	90.00
Volume (Å^3)	1840.1(3)	14355.0(8)
Z	4	16
ρ calc (mg mm^−3)	2.184	1.857
μ (mm^−1)	2.286	1.377
F(000)	1180.0	7968.0
Crystal size (mm)	0.15 × 0.11 × 0.10	0.12 × 0.10 × 0.08
Index ranges	−14 ≤ h ≤ 14, −12 ≤ k ≤ 12, −23 ≤ l ≤ 23	−34 ≤ h ≤ 39, −41 ≤ k ≤ 41, −9 ≤ l ≤ 11
Reflections collected	15090	15155
Independent reflections	4239 [R(int) = 0.0418]	4519 [R(int) = 0.0216]
Data/parameters	4239/289	4519/505
Goodness-of-fit on F^2	1.034	1.015
Final R indexes [I > = 2σ (I)]	R 1 = 0.0481, wR2 = 0.1219	R 1 = 0.0166, wR2 = 0.0359
Final R indexes [all data]	R 1 = 0.0737, wR2 = 0.1344	R 1 = 0.0179, wR2 = 0.0365
Largest diff. peak/hole/e (Å^−3)	1.26/−0.87	0.34/−0.30
Flack parameter	N/A	−0.006(14)

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Table 2 Selected bond lengths (Å) and angles (°) for 1 and 2

Complex 1
Ag1–N2	2.359 (4)	Ag2–O4	2.384 (7)
Ag1–N1	2.419 (5)	Ag2–O1	2.466 (4)
Ag1–O1^i	2.460 (4)	Ag2–S1	2.4700 (13)
Ag1–C10^ii	2.685 (6)	Ag2–O5	2.500 (7)
Ag1–S1	2.9227 (16)
N2–Ag1–N1	69.79 (17)	O1^i–Ag1–S1	93.51 (10)
N2–Ag1–O1^i	155.28 (16)	C10^ii–Ag1–S1	100.23 (15)
N1–Ag1–O1^i	115.91 (18)	O4–Ag2–O1	96.8 (2)
N2–Ag1–C10^ii	108.51 (17)	O4–Ag2–S1	140.5 (2)
N1–Ag1–C10^ii	115.1 (2)	O1–Ag2–S1	121.27 (11)
O1^i–Ag1–C10^ii	91.12 (17)	O4–Ag2–O5	50.78 (19)
Symmetry codes: (i) −x + 3/2, y + 1/2, −z + 3/2; (ii) −x + 1, −y, −z + 1.
Complex 2
Cd1–N4^i	2.316 (3)	Cd2–N1	2.332 (3)
Cd1–O5	2.363 (3)	Cd2–O3	2.357 (2)
Cd1–N3^i	2.384 (3)	Cd2–N2	2.373 (3)
Cd1–O8	2.494 (4)	Cd2–O6	2.382 (3)
Cd1–O10	2.504 (3)	Cd2–O11^ii	2.447 (3)
Cd1–O11	2.532 (3)	Cd2–O2	2.585 (3)
Cd1–O7	2.535 (3)	Cd2–S1	2.7374 (8)
Cd1–S2^i	2.8669 (10)
N4^i–Cd1–O5	84.14 (10)	O10–Cd1–S2^i	73.83 (6)
N4^i–Cd1–N3^i	70.27 (9)	O11–Cd1–S2^i	117.46 (6)
O5–Cd1–N3^i	129.00 (9)	O7–Cd1–S2^i	157.30 (7)
N4^i–Cd1–O8	124.54 (10)	N1–Cd2–O3	132.91 (9)
O5–Cd1–O8	78.04 (10)	N1–Cd2–N2	70.54 (9)
N3^i–Cd1–O8	152.46 (10)	O3–Cd2–N2	149.94 (9)
N4^i–Cd1–O10	134.36 (10)	N1–Cd2–O6	77.23 (10)
O5–Cd1–O10	139.90 (9)	O3–Cd2–O6	92.51 (10)
N3^i–Cd1–O10	82.88 (9)	N2–Cd2–O6	113.53 (9)
O8–Cd1–O10	70.48 (10)	N1–Cd2–O11^ii	85.53 (9)
N4^i–Cd1–O11	87.34 (9)	O3–Cd2–O11^ii	80.58 (10)
O5–Cd1–O11	145.66 (9)	N2–Cd2–O11^ii	83.72 (10)
N3^i–Cd1–O11	77.97 (9)	O6–Cd2–O11^ii	149.07 (9)
O8–Cd1–O11	79.70 (10)	N1–Cd2–O2	81.87 (9)
O10–Cd1–O11	50.46 (8)	O3–Cd2–O2	51.11 (8)
N4^i–Cd1–O7	75.12 (9)	N2–Cd2–O2	146.35 (10)
O5–Cd1–O7	74.58 (9)	O6–Cd2–O2	77.18 (10)
N3^i–Cd1–O7	134.07 (9)	O11^ii–Cd2–O2	75.08 (10)
O8–Cd1–O7	49.60 (10)	N1–Cd2–S1	131.65 (7)
O10–Cd1–O7	101.49 (9)	O3–Cd2–S1	94.09 (6)
O11–Cd1–O7	71.09 (9)	N2–Cd2–S1	71.05 (7)
N4^i–Cd1–S2^i	124.34 (7)	O6–Cd2–S1	92.43 (7)
Symmetry codes: (i) −x + 3/4, y + 1/4, z − 1/4; (ii) x, y, z + 1.

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3.1. Synthesis and general characterization

The condensation reaction of 2,2′-((1,4-phenylenebis(methylene))bis(sulfanediyl))dianiline with 2-pyridinecarboxaldehyde gave the pbbpa ligand in good yield. The ratio of CH₃CN : CHCl₃ used in the condensation reaction is very significant for the successful preparation of the pbbpa ligand (Scheme 2). Complexes 1 and 2 were synthesized by diffusion and solvothemal methods, respectively. The compositions of 1 and 2 were further deduced from X-ray single crystal diffraction, elemental analysis, and IR spectroscopy. The solid FT-IR spectra (Fig. S1, ESI†) of complexes 1 and 2 show the characteristic NO₃⁻ anion stretching bands at around 1400 and 1200 cm⁻¹, indicating the NO₃⁻ anion displaying more than one binding mode,¹¹ which is consistent with the results of the X-ray diffraction analysis.

Scheme 2 Synthetic procedures of the S₂N₄ di-Schiff base ligand.

3.2. Structure description of [Ag₄(pbbpa)(NO₃)₄]_n (1)

X-ray single-crystal diffraction analysis reveals that 1 is a complicated 3D framework. It crystallizes in the monoclinic crystal system with a space group of P2₁/n, and an asymmetric unit that contains two Ag(I) ions, half a pbbpa ligand and two NO₃⁻ ions. Ag1, O4, O5 and Ag1A, O4A, O5A are disordered over two sites with occupancies of 0.90 : 0.10. The pbbpa ligand is located on a crystallographic inversion center. As depicted in Fig. 1a, Ag1 and Ag2 are located in distorted square pyramidal (τ₅ = 0.07)^12a and tetrahedral (τ₄ = 0.59)^12b geometries, respectively. The coordination environment of Ag1 is completed by two N atoms from one (pyridin-2-ylmethylene)aniline arm, one S donor, one O atom of an NO₃⁻ ion and one C atom. Ag2 is coordinated by three O atoms from two NO₃⁻ ions and one S atom. The average Ag–O, Ag–N and Ag–S bond lengths are 2.452(5), 2.389(4) and 2.696(2) Å, respectively, comparable to those of related complexes.¹³ Notably, one slightly long Ag–C bond of 2.685(6) Å was observed, and the angle of Ag1–C10–Cg is 99.5(3)° (Cg is the centroid of the ring C7–C12), indicating the Ag–π coordination bond. This Ag–C distance is obviously below the margin of significant Ag–C interaction,¹⁴ and statistical analysis has demonstrated that this contact can be considered as weak bonding.¹⁵ Chen’s group reported a Ag(I) coordination polymer, namely, [Ag₂(hmt)₂(bna)]_n (bna = 2,2′-dihydroxy-1,1′-binaphthalene-3,3′-dicarboxylate), in which the aromatic carboxylates bind to the Ag(I) atoms via a unique η²-coordination mode involving their aromatic rings with Ag–C distances of 2.633(9)–2.710(9) Å, in addition to the normal coordination modes utilizing their carboxyl groups.¹⁶ Although Ag–π interactions have a more recognized history in organometallic chemistry, and were well discussed in one comprehensive review by Munakata,¹⁷ coordinating Schiff base fragments to Ag(I) ions through a C atom as the π-donor has rarely been reported.

Fig. 1 (a) Thermal ellipsoid (50%) plot of 1 showing the coordination environment of the Ag(I) ions. (b) The ball and stick view of the binuclear Ag₂(pbbpa) subunit. (c) The ball and stick view of the 1D chain with the Ag–C bond highlighted in green (symmetry codes: (i) −x + 3/2, y + 1/2, −z + 3/2; (ii) −x + 1, −y, −z + 1.; (iii) −x + 2, −y, −z + 1). Color legend: Ag, purple; S, yellow; C, gray; O, red; N, blue.

The flexible pbbpa ligand is folded in an S-shape, such that the (pyridin-2-ylmethylene)aniline arms are positioned on different sides with respect to the central benzene ring, giving an overall anti-conformation (torsional angle across the benzene ring S1–C13⋯C13ⁱⁱⁱ–S1ⁱⁱⁱ = 180°, symmetry code: (iii) −x + 2, −y, −z + 1). In the anti-conformation of pbbpa, the pyridine and benzene rings of (pyridin-2-ylmethylene)aniline are twisted by 5.3(9)° and 36.8(5)° relative to the central benzene ring, respectively. The pyridine and benzene rings of (pyridin-2-ylmethylene)aniline are non-coplanar, with a dihedral angle of 35.8(3)°. The pbbpa ligand adopts a μ₆-κ¹N:κ¹N:κ¹C:κ²S:κ¹N:κ¹N:κ¹C:κ²S binding mode, and binds two Ag(I) ions to form a binuclear subunit (Fig. 1b). The Ag–C bond plays an important role in extending the binuclear Ag(I) subunit into a 1D chain along the a axis (Fig. 1c). The two NO₃⁻ ions show monodentate bridging μ₂-κ¹O, κ¹O and bidentate chelating μ₁-κ¹O:κ¹O′ modes, respectively. The former type of NO₃⁻ ion bridges the 1D chain into a 2D sheet (Fig. 2a), which is further stacked into the complicated 3D framework by bridging S donors (Fig. 2b). The non-classic C–H⋯O hydrogen bonds, ranging from 3.066(7) to 3.335(9) Å, reinforce the 3D framework, in which the distances between aromatic rings are too long to take part in π⋯π interactions, with the shortest distance between centroids being 4.455(4) Å.

Fig. 2 (a) The ball and stick view of the 2D sheet of 1. (b) The 3D framework of 1. Color legend: Ag, purple; S, yellow; C, gray; O, red; N, blue.

3.3. Structure description of [Cd₂(pbbpa)(NO₃)₄]_n (2)

When Ag(I) was replaced by the Cd(II) ion, we obtained complex 2 as a 2D 6³-hcb net. X-ray single-crystal diffraction analysis revealed that 2 crystallizes in the orthorhombic crystal system with a space group of Fdd2, and its asymmetric unit contains two Cd(II) ions, one pbbpa ligand and four NO₃⁻ ions. As shown in Fig. 3a, Cd1 adopts an eight-coordinate square antiprismatic geometry which is completed by five O atoms from three different NO₃⁻ ions, two N atoms and one S atom from the same pbbpa ligand. Cd2 is seven-coordinate, with four O atoms from three different NO₃⁻ ions, two N atoms and one S atom from the same pbbpa ligand, giving a distorted pentagonal bipyramidal coordination geometry. The Cd–O, Cd–N and Cd–S bond lengths are in the range of 2.363(3)–2.585(3), 2.316(3)–2.384(3) and 2.7374(8)–2.8669(10) Å, respectively, comparable to similar complexes.¹⁸ The pbbpa ligand in 2 adopted a similar anti-conformation (torsion angle across the benzene ring S1–C13⋯C20–S2 = 165.88(19)°). In the anti-pbbpa ligand, the two pyridine rings of (pyridin-2-ylmethylene)aniline are twisted by 14.9(2)° and 2.9(2)° relative to the central benzene ring, respectively, while the twist angles between benzene rings of (pyridin-2-ylmethylene)aniline and the central benzene ring are 27.5(2)° and 35.3(2)° . The dihedral angles between pyridine and benzene rings of (pyridin-2-ylmethylene)aniline are 25.3(2)° and 32.4(2)°, respectively. Conversely, the pbbpa ligand only uses the N and S donors to coordinate a pair of Cd(II) ions to form a Cd₂(pbbpa) subunit. In 2, four independent NO₃⁻ ions exhibit three different coordination modes; these are μ₂-κ¹O:κ¹O:κ¹O′ (monodentate bridging + bidentate chelating), μ₁-κ¹O:κ¹O′ (bidentate chelating), and μ₂-κ¹O:κ¹O′ (bidentate bridging), which play different roles in the construction of the resulting 2D network (Scheme S1, ESI†). The μ₂-κ¹O:κ¹O:κ¹O′ (monodentate bridging + bidentate chelating) NO₃⁻ ions link binuclear Cd(II) subunits to form the 1D chain (Fig. 3b). The bidentate chelating NO₃⁻ ions occupy the remaining coordination sites and do not participate in the extension of the 1D chain. Furthermore, the μ₂-κ¹O:κ¹O′ (bidentate bridging) NO₃⁻ ions extend the 1D chain into a 2D sheet (Fig. 3c). To better understand the structure of 2, the topological analysis approach was employed. If all the nodes in one net have identical connectivity, then according to Wells it is a platonic uniform net, and can be represented by the symbol (n,p).¹⁹ In the sheet of 2, all the Cd(II) ions are 3-connecting, and the shortest circuit is a six-membered ring. So this 2D sheet can be simplified to a 6³-hcb net. The packing of the 2D sheets into a 3D framework is due to intersheet C–H⋯O hydrogen bonds ranging from 2.922(4) to 3.455(4) Å.

Fig. 3 (a) Thermal ellipsoid (50%) plot of 2 showing the coordination environment of Cd(II) ions. (b) The ball and stick view of the 1D chain. (c) The 2D 6³-hcb net (symmetry codes: (i) −x + 3/4, y + 1/4, z − 1/4; (ii) x, y, z + 1). Color legend: Cd, purple; S, yellow; C, gray; O, red; N, blue.

3.4. TGA and powder X-ray diffraction (PXRD) of 1 and 2

Thermogravimetric (TG) curves (Fig. S2, ESI†) were measured in a N₂ atmosphere. For 1 and 2, no weight loss was observed before 164 and 247 °C, respectively. Further heating led to the collapse of the frameworks, accompanying the loss of the organic ligand pbbpa. The powder X-ray diffraction (PXRD) patterns of 1 and 2 are coincident with the simulated pattern derived from the X-ray single crystal data, implying that the bulk sample is same as the single crystal (Fig. 4). The dissimilarities in intensity may be due to the preferred orientation of the crystalline powder samples.

Fig. 4 PXRD patterns for (a) simulated 1, (b) as-synthesized 1, (c) simulated 2, (d) as-synthesized 2.

3.5. UV-vis spectra and photoluminescence properties of 1 and 2

The absorption spectra of free ligand, 1 and 2 are shown in Fig. S3;† all complexes show similar high-energy absorption bands in the range of 200–500 nm, which are attributed to the intraligand π→π* transitions of the pbbpa ligand. As shown in Fig. S4,† the free ligand pbbpa displays photoluminescence, with an emission maximum at 414 nm (λ_ex = 320 nm). It can be presumed that this peak originates from the π→π* transition. The emission spectra of 1 and 2 are shown in Fig. 5. The emission of 1 is very similar to that of the free ligand, so the luminescence of 1 can probably be assigned to the intraligand (IL) π→π* transition²⁰ of the pbbpa ligand. Due to the d¹⁰ configuration of Cd(II) ion, the emission of 2 is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature, since the Cd(II) ion is difficult to oxidize or reduce.²¹ Thus, it also should be assigned to the IL transition.

Fig. 5 The photoluminescence spectra of 1 and 2.

4. Conclusions

In this study, a flexible hexadentate S-shaped di-Schiff base was designed and synthesized by the condensation reaction of 2,2′-((1,4-phenylenebis(methylene))bis(sulfanediyl))dianiline and 2-pyridinecarboxaldehyde. Two silver(I) and cadmium(II) coordination polymers were successfully prepared by the reaction of this new ligand and various metal salts using different methods. This study demonstrates that the flexible double-armed Schiff base organic ligands are versatile building blocks in the construction of polymeric complexes. The thermal stabilities, UV absorption as well as emissive behaviors were also discussed. We are currently extending this result by preparing new flexible symmetric and asymmetric di-Schiff base ligands of this type containing different terminal coordination functional groups.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 21201110), the Independent Innovation Foundation of Shandong University (2011GN030), the Special Fund for Postdoctoral Innovation Program of Shandong Province (201101007 and 201102018) and the China Postdoctoral Science Foundation (2012M511492 and 2011M500725).

References

1. (a) T. T. Luo, H. C. Wu, Y. C. Jao, S. M. Huang, T. W. Tseng, Y. S. Wen, G. H. Lee, S. M. Peng and K. L. Lu, Angew. Chem., Int. Ed., 2009, 48, 9461; (b) G. Férey, Chem. Soc. Rev., 2008, 37, 191; (c) R. A. Fisher and C. Wöll, Angew. Chem., Int. Ed., 2008, 47, 8164; (d) S. Ma, D. Yuan, X. S. Wang and H. C. Zhou, Inorg. Chem., 2009, 48, 2072; (e) P. K. Thallapally, J. Tian, M. R. Kishan, C. A. Fernandez, S. J. Dalgarno, P. B. McGrail, J. E. Warren and J. L. Atwood, J. Am. Chem. Soc., 2008, 130, 16842; (f) L. Hou, Y. Y. Lin and X. M. Chen, Inorg. Chem., 2008, 47, 1346; (g) B. T. N. Pham, L. M. Lund and D. Song, Inorg. Chem., 2008, 47, 6329; (h) A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong and J. Li, Angew. Chem., Int. Ed., 2009, 48, 2334; (i) K. L. Hu, M. Kuromo, Z. Wang and S. Gao, Chem.–Eur. J., 2009, 15, 12050; (j) J. Sun, F. N. Dai, W. B. Yuan, W. H. Bi, X. L. Zhao, W. M. Sun and D. F. Sun, Angew. Chem., Int. Ed., 2011, 50, 7061; (k) F. N. Dai, H. Y. He and D. F. Sun, J. Am. Chem. Soc., 2008, 130, 14064; (l) C. Zou, Z. J. Zhang, X. Xu, Q. H. Gong, J. Li and C. D. Wu, J. Am. Chem. Soc., 2012, 134, 87; (m) C. Yan, K. Li, S. C. Wei, H. P. Wang, L. Fu, M. Pan and C. Y. Su, J. Mater. Chem., 2012, 22, 9846; (n) J. Zhang, S. M. Chen, R. A. Nieto, T. Wu, P. Y. Feng and X. H. Bu, Angew. Chem., Int. Ed., 2010, 49, 1267; (o) D. Q. Yuan, D. Zhao, D. J. Timmons and H. C. Zhou, Chem. Sci., 2011, 2, 103.
2. (a) P. L. Caradoc-Davies and L. R. Hanton, Chem. Commun., 2001, 1098; (b) P. L. Caradoc-Davies, L. R. Hanton, J. M. Hodgkiss and M. D. Spicer, J. Chem. Soc., Dalton Trans., 2002, 1581; (c) L. R. Hanton and K. Lee, J. Chem. Soc., Dalton Trans., 2000, 1161.
3. (a) A. Harada, Acc. Chem. Res., 2001, 34, 456; (b) C. Marchal, Y. Filinchuk, X. Y. Chen, D. Imbert and M. Mazzanti, Chem.–Eur. J., 2009, 15, 5273; (c) A. K. Bar, R. Chakrabarty and S. Munherjee, Inorg. Chem., 2009, 48, 10880; (d) J. Wang, Z. J. Lin, Y. C. Ou, Y. Shen, R. Herchel and M. L. Tong, Chem.–Eur. J., 2008, 14, 7218.
4. (a) J. Doemer, J. C. Slootweg, F. Hupka, K. Lammertsma and F. E. Hahn, Angew. Chem., Int. Ed., 2010, 49, 6430; (b) W. Meng, B. Breiner, K. Rissanen, J. D. Thoburn, J. K. Clegg and J. R. Nitschke, Angew. Chem., Int. Ed., 2011, 50, 3479; (c) H. Mitsunuma and S. Matsunaga, Chem. Commun., 2011, 47, 469; (d) C. Camp, V. Mougel, P. Horeglad, J. Pecaut and M. Mazzanti, J. Am. Chem. Soc., 2010, 132, 17374; (e) P. D. Frischmann, S. Guieu, R. Tabeshi and M. J. MacLachlan, J. Am. Chem. Soc., 2010, 132, 7668.
5. (a) F. Dai, D. Sun and D. Sun, Cryst. Growth Des., 2011, 11, 5670; (b) F. Dai, S. Gong, P. Cui, G. Zhang, X. Qiu, F. Ye, D. Sun, Z. Pang, L. Zhang, G. Dong and C. Zhang, New J. Chem., 2010, 34, 2496; (c) P. P. Cui, J. M. Dou, D. Sun, F. N. Dai, S. N. Wang, D. F. Sun and Q. Y. Wu, CrystEngComm, 2011, 13, 6968.
6. X.-L. Zhao, D. Sun, T.-P. Hu, S. Yuan, L.-C. Gu, H.-J. Cong, H.-Y. He and D.-F. Sun, Dalton Trans., 2012, 41, 4320.
7. (a) B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629; (b) B. Moulton and M. J. Zaworotko, Adv. Supramol. Chem., 2000, 7, 235; (c) L. Carlucci, G. Ciani, D. M. Proserpio and L. Spadacini, CrystEngComm, 2004, 6, 96; (d) A. J. Blake, N. R. Brooks, N. R. Champness, M. Crew, A. Deveson, D. Fenske, D. H. Gregory, L. R. Hanton, P. Hubberstey and M. Schroder, Chem. Commun., 2001, 1432.
8. G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure Determination, University of Göttingen, Germany, 1997.
9. G. M. Sheldrick, SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Germany, 1997.
10. A. L. Spek, Implemented as the PLATON Procedure, a Multipurpose Crystallographic Tool, Utrecht University, Utrecht, the Netherlands, 1998.
11. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, 5th ed., Wiley-Interscience, New York, 1997, pp. 87–89.
12. (a) A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349; (b) L. Yang, D. R. Powell and R. P. Houser, Dalton Trans., 2007, 955.
13. (a) D. Sun, L. Zhang, Z. Yan and D. Sun, Chem.–Asian J., 2012, 7, 1558; (b) D. Sun, G.-G. Luo, N. Zhang, R.-B. Huang and L.-S. Zheng, Chem. Commun., 2011, 47, 1461; (c) D. J. Hutchinson, S. A. Cameron, L. R. Hanton and S. C. Moratti, Inorg. Chem., 2012, 51, 5070; (d) V. Domasevitch, P. V. Solntsev, I. Y. A. Gural'skiy, H. Krautscheid, E. B. Rusanov, A. N. Chernega and J. A. K. Howard, Dalton Trans., 2007, 3893; (e) I. Y. A. Gural'skiy, D. Escudero, A. Frontera, P. V. Solntsev, E. B. Rusanov, A. N. Chernega, H. Krautscheid and K. V. Domasevitch, Dalton Trans., 2009, 2856; (f) Y. F. Hsu, H. L. Hu, C. J. Wu, C. W. Yeh, D. M. Proserpio and J. D. Chen, CrystEngComm, 2009, 11, 168; (g) C. J. Wu, M. J. Sie, H. L. Hsiao and J. D. Chen, CrystEngComm, 2011, 13, 4121.
14. Q. M. Wang and T. C. W. Mak, Chem.–Eur. J., 2003, 9, 44.
15. M. Mascal, J. L. Kerdelhue, A. J. Blake and P. A. Cooke, Angew. Chem., Int. Ed., 1999, 38, 1968.
16. S. L. Zheng, M. L. Tong, S. D. Tan, Y. Wang, J. X. Shi, Y. X. Tong, H. K. Lee and X. M. Chen, Organometallics, 2001, 20, 5319.
17. M. Munakata, L. P. Wu and G. L. Ning, Coord. Chem. Rev., 2000, 198, 171.
18. (a) D. B. Cordes, A. S. Bailey, P. L. Caradoc-Davies, D. H. Gregory, L. R. Hanton, K. Lee and M. D. Spicer, Inorg. Chem., 2005, 44, 2544; (b) Y. F. Hsu, H. L. Hu, C. J. Wu, C. W. Yeh, D. M. Proserpio and J. D. Chen, CrystEngComm, 2009, 11, 168; (c) J. J. Cheng, Y. T. Chang, C. J. Wu, Y. F. Hsu, C. H. Lin, D. M. Proserpio and J. D. Chen, CrystEngComm, 2012, 14, 537; (d) Z. K. Chan, T. R. Chen, J. D. Chen, J. C. Wang and W. Liu, Dalton Trans., 2007, 3450.
19. A. F. Wells, Three-dimensional nets and polyhedra, Wiley-Interscience, New York, 1977.
20. (a) S. L. Zheng and X. M. Chen, Aust. J. Chem., 2004, 57, 703; (b) C. L. Chen, B. S. Kang and C. Y. Su, Aust. J. Chem., 2006, 59, 3; (c) X. Zhao, L. Zhang, H. Ma, D. Sun, D. Wang, S. Feng and D. Sun, RSC Adv., 2012, 2, 5543; (d) J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126.
21. (a) A. Barbieri, G. Accorsi and N. Armaroli, Chem. Commun., 2008, 2185; (b) V. W.-W. Yam, K. K.-W. Lo, W. K.-M. Fung and C.-R. Wang, Coord. Chem. Rev., 1998, 171, 17.

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Footnote

† Electronic Supplementary Information (ESI) available: Synthetic and crystallographic details, additional measurements, figures and tables. CCDC numbers 888434 and 888435. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra21533k

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