syntheses, structures, photoluminescence and sensing

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Dual ligands strategy for constructing a series of d10 coordination polymers: syntheses, structures, photoluminescence and sensing properties Di Zhang, Zhen-zhen Xue, Jie Pan, Jin-Hua Li, and Guo-Ming Wang Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Crystal Growth & Design

Dual ligands strategy for constructing a series of d10 coordination polymers: syntheses, structures, photoluminescence and sensing properties Di Zhang, † Zhen-Zhen Xue,† Jie Pan,† Jin-Hua Li,† and Guo-Ming Wang*,† †

College of Chemistry and Chemical Engineering, Qingdao University, Shandong 266071, China

Supporting Information ABSTRACT: Constructed from a rigid pyrazine-tetrazole ligand, 5-(pyrazinyl)tetrazole (Hptz), as well as various polycarboxylate molecules, a series of novel Zn(II)/Cd(II) coordination polymers, [Cd(Hptz)(Hbtc)(H2O)2·H2O]n (1), [Cd2(ptz)(btc)(H2O)2·2H2O]n (2), [Cd2(ptz)2(p-bdc)]n (3), [Cd2(ptz)2(m-bdc)(H2O)]n (4), [Zn2(ptz)(btc)(H2O)2·H2O]n (5) and [Cd2(ptz)(btc)(DMA)(H2O)·4.5H2O]n (6) (H3btc = 1, 3, 5-benzenetricarboxylic acid, p-H2bdc = p-phthalic acid, m-H2bdc = m-phthalic acid, DMA = N,Ndimethylacetamide), have been obtained under hydrothermal or solvothermal conditions. The structural diversity of 1-6 mainly stems from the different and various coordination modes of the N-donor ligand, as different chelating or bridging/chelating fashions have generated various structures with one-, two-, or three-dimensions. Both π-π stacking interactions could be observed in adjacent chains and layers for 1 and 2, respectively, through which leads to 3D supramolecular frameworks. Differently, the 3D complicated architectures are constructed in 3-6 through the linkage of metal centers and organic ligands. All of these interesting structures have been determined and characterized by single-crystal X-ray diffraction analyses. Photoluminescence properties of compounds 1-6 have been investigated. Moreover, photoluminescence sensing experiment indicates that compound 1 and 6 can be acted as promising selective sensors towards Cu2+.

Introduction Metal organic coordination polymers (CPs), presenting one, two or three dimensional structures which consist of metal ions or metal clusters joined by organic linkers through coordination bonds, have attracted great interest in the past two decades.1-6 Apart from their intriguing structures and diverse topologies, the various properties of CPs impart great potential applications in many areas, such as gas storage and separation, optics, host–guest chemistry, catalysis, electronic devices.7-15 It is commonly acknowledged that many factors could influence the structure of CPs, including metal centers, organic linkers, temperature, solvents, pH value, reactant ratio and so on, all of which make the structures diverse and even unexpected.16-19 Notably, the selection and utilization of organic ligands with different rigidities, types, lengths and geometries play a very important role in the fabrication of fascinating architectures with promising properties.20-23 By employing the semirigid organic linkers consisting of different functional groups (-H, -CH3, -Cl), Sheng and co-workers reported a series of isostructural microporous MOF with different H2 uptake capacities.24 Based on a well known MOF74 with original link of ioxidoterephthalate, the long molecular struts with two, three or more phenylene rings gave rise to a series of isostructural MOF-74 structures (IRMOF74-I to XI) with pore apertures being in the range of 14-98 Å.25 Generally speaking, the quest for design of various organic linkers which can act as versatile building blocks to obtain diverse MOFs is still popular. Among the different kinds of organic linkers, N-containing ligands exemplified by imidazole and their derivatives are

widely utilized to construct intriguing CPs with promising properties owing to their diverse coordination modes together with strong coordination abilities. The porous ZIFs (Zeolitic Imidazolate Frameworks), such as ZIF-8, ZIF-67 and so on, are representative of such CPs.26-28 The bridging angle with 145° in the M-Im-M (M = Zn2+, Co2+, etc and Im = imadazolate) could give the opportunity to make novel ZIFs with topological structures based on those of tetrahedral zeolites wherein the Si-O-Si angle prefers to be 145°. Besides that, other five-membered aromatic N-heterocycles together with imidazole derivatives have afforded a large number of metal azolate frameworks (MAFs) when assembling with various metal centers, which lead to a new metal-ligand system and have attracted much interest during the past few years.29-31 In addition, some CPs constructed from triazolepyridine, triazole-pyrazine, triazine-pyridine, etc ligands which are equipped with strong coordination ability have been widely reported recently.32-35 By contrast, 5(pyrazinyl)tetrazole (Hptz) ligands with more N donors, integrating the features of both tetrazole and pyrazine, has been rarely studied. In Hptz molecules, the multiple coordination sites can give rise to numerous CPs with diverse topologies as well as promising properties when acting as a multidentate ligand. What’s more, it has been demonstrated that the versatile bridging modes of polycarboxylate ligands make them be great co-ligands for the formation of intriguing frameworks.36-38 However, systematic study of the Hptzcarboxylate is incomplete and not well understood. Hence, comprehensive research on this topic is necessary. Taking consideration of the above-mentioned, we successfully obtained six novel Zn(II)/Cd(II) CPs with different dimensional networks based on Hptz and various

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All chemicals and reagents were bought commercially. Infrared spectra were performed in 400-4000 cm−1 on an ABB Bomen MB 102 series FT-IR spectrophotometer. An Elemental Vario EL III instrument was used to conduct the elemental analyses (C, H, N). Thermogravimetric measurements were carried out with temperature ranging from 30 °C to 1000 °C on a NETZSCH STA 449 F5 instrument. Using a Philips X’Pert-MPD diffractometer with Cu-Kα1 radiation (λ = 1.54076 Å), powder X-ray diffraction (PXRD) patterns for the six CPs were recorded. The luminescence excitation/emission spectra and solid-state emission lifetimes were recorded using a FLS920 fluorescence spectrophotometer with 450 W xenon light.

carboxylate ligands, namely [Cd(Hptz)(Hbtc)(H2O)2·H2O]n (1), [Cd2(ptz)(btc)(H2O)2·2H2O]n (2), [Cd2(ptz)2(p-bdc)]n (3), [Cd2(ptz)2(m-bdc)(H2O)]n (4), [Zn2(ptz)(btc)(H2O)2·H2O]n (5) and [Cd2(ptz)(btc)(DMA)(H2O)·4.5H2O]n (6). The different coordination modes of Hptz have resulted in diverse architectures in this work. The solid-state photoluminescence properties of CPs 1-6 were investigated under ambient temperature. Furthermore, photoluminescence sensing experiment for typical compounds was also conducted.

Experimental Section Materials and General Methods.

Table 1. Crystal Data and Structure Refinements for CPs 1-6. Compound

1

2

3

4

5

6

Empirical formula Formula weight

C14H14N6O9Cd 522.72

C14H14N6O10Cd2 651.11

C18H10N12O4Cd2 683.18

C18H12N12O5Cd2 701.20

C14H12N6O9Zn2 539.04

C18H26N7O12.5Cd2 765.26

Crystal system

triclinic

triclinic

monoclinic

orthorhombic

triclinic

monoclinic

Space group

P-1

P-1

P2(1)/n

Pbca

P-1

C2/c

a (Å)

9.1280(5)

8.0952(8)

8.4898(7)

12.0252(13)

7.2475(8)

14.0965(15)

b (Å)

9.2135(8)

10.2619(8)

13.1506(10)

11.5033(9)

9.8779(16)

14.3270(13)

c (Å)

11.6394(9)

12.9283(8)

10.0009(8)

31.145(3)

13.3079(15)

29.274(2)

91.426(6)

69.756(6)

90.00

90.00

70.521(13)

90.00

o

α( ) o

β( )

108.842(6)

80.248(7)

108.033(9)

90.00

78.784(9)

97.834(9)

γ (o)

98.266(6)

67.171(9)

90.00

90.00

81.245(11)

90.00

V (Å )

914.14(12)

927.94(13)

1061.71(15)

4308.3(7)

877.1(2)

5857.0(9)

Z

2

2

2

8

2

8

Dc (g/cm3)

1.899

2.330

2.137

2.162

2.041

1.736

µ (mm-1)

1.260

2.365

2.061

2.037

2.804

1.520

θ range (°)

3.0–30.4

2.9–29.7

2.8-24.9

3.1-31.5

2.9-24.6

2.8-25.2

R(int)

0.0208

0.0228

0.0295

0.0356

0.0341

0.0430

0.0381, 0.1130

0.0380, 0.1134

0.0315, 0.0821

0.0475, 0.1502

0.0446, 0.1291

0.0543, 0.1683

1.041

1.024

1.020

1.087

1.072

1.077

3

a

b

R1, wR2 (I >2σ( I)) GOF on F a

2

R1 =∑ ||Fo| - |Fc||/∑|Fo| . b wR2={∑[w(Fo2 - Fc2)2]/∑w(Fo2)2}1/2

3402(m), 1611(s), 1550(s), 1435(s), 1369(s), 1153(w), 1038(w), 729(m). Synthesis of [Cd2(ptz)2(p-bdc)]n (3). Cd(NO3)2·4H2O (62 mg, 0.2 mmol), Hptz (15 mg, 0.1 mmol), p-H2bdc (17 mg, 0.1 mmol) were put in a vessel, then H2O (5 mL) and C2H5OH (2 mL) were added. The contents were kept at 120 °C. After 3 days, light yellow bulk crystals were harvested with the yield of 63% based on p-H2bdc. Elemental analysis calcd (%) for C18H10N12O4Cd2: C 31.65, H 1.48, N 24.59; found: C 31.33, H 1.79, N 24.36. IR (KBr, cm−1): 3435(m), 1593(m), 1381(m), 1151(w), 1038(w), 862(w), 752(w). Synthesis of [Cd2(ptz)2(m-bdc)(H2O)]n (4). A mixture of Cd(NO3)2·4H2O (62 mg, 0.2 mmol), Hptz (15 mg, 0.1 mmol), m-H2bdc (17 mg, 0.1 mmol), 5 mL H2O and 2 mL CH3CN was sealed into a Teflon-lined stainless steel autoclave, then heated slowly and kept at 120 °C for 3 days. Pale-yellow bulk crystals were obtained in a 35% yield after cooled to 30 °C. Elemental

Synthesis of [Cd(Hptz)(Hbtc)(H2O)2·H2O]n (1). 62 mg Cd(NO3)2·4H2O (0.2 mmol), 15 mg Hptz (0.1 mmol), 21 mg H3btc (0.1 mmol) and 5 mL H2O were mixed together then sealed into a Teflon-lined stainless steel autoclave, and the mixture was heated at 120 °C for 3 days. After slowly cooled to 30 °C, the colorless bulk crystals were formed in a 61% yield based on Hptz. Elemental analysis calcd (%) for C14H14N6O9Cd: C 32.17, H 2.70, N 16.08; found: C 32.56, H 2.97, N 16.55. IR (KBr, cm−1): 3410(s), 1689(s), 1550(m), 1404(s), 1173(w), 1038(w), 864(w), 748(w). Synthesis of [Cd2(ptz)(btc)(H2O)2·2H2O]n (2). A similar procedure as 1 was employed for the synthesis of 2 except that 6 mL H2O-DMF in a volumetric 2 : 1 ratio (DMF = N,Ndimethylformamide) was used instead of 5 mL H2O. Colorless strip-shaped crystals were got in a 55% yield. Elemental analysis calcd (%) for C14H14N6O10Cd2: C 25.82, H 2.17, N 12.91; found: C 26.00, H 2.06, N 12.59. IR (KBr, cm−1):

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analysis calcd (%) for C18H12N12O5Cd2: C 30.83, H 1.72, N 23.97; found: C 30.54, H 1.94, N 23.68. IR (KBr, cm−1): 3406(m), 1606(s), 1564(m), 1367(s), 1151(w), 1038(m), 862(w), 754(m). Synthesis of [Zn2(ptz)(btc)(H2O)2·H2O]n (5). 59 mg Zn(NO3)2·6H2O (0.2 mmol), 15 mg Hptz (0.1 mmol), 21 mg H3btc (0.1 mmol) were put into a vessel and mixed together, then 5 mL H2O and 2 mL CH3CN were added to the mixture. The contents were heated at 120 °C and kept the temperature for 3 days, then cooled to 30 °C naturally. Colorless X-ray quality crystals were generated with a 48% yield. Elemental analysis calcd (%) for C14H12N6O9Zn2: C 31.19, H 2.24, N 15.59; found: C 31.28, H 2.26, N 15.91. IR (KBr, cm−1): 3369(s), 1616(s), 1554(m), 1361(m), 1176(w), 1092(w), 1047(m), 752(m). Synthesis of [Cd2(ptz)(btc)(DMA)(H2O)·4.5H2O]n (6). A mixture of Cd(NO3)2·4H2O (123 mg, 0.4 mmol), Hptz (15 mg, 0.1 mmol), H3btc (21 mg, 0.1 mmol) was dissolved in 6 mL DMA-H2O with a volumetric 2 : 1 ratio. The clear solution was kept at 85 °C for 4 days, giving colorless crystals. Yield: 41% based on Hptz. Elemental analysis calcd (%) for C18H26N7O12.5Cd2: C 28.25, H 3.42, N 12.81; found: C 28.64, H 3.08, N 12.79. IR (KBr, cm−1): 3425(m), 1610(s), 1550(s), 1431(m), 1367(s), 1039(w), 876(w), 733(m). X-ray Crystallographic Study. Crystal data for CPs 1-6 were collected at room temperature on a Rigaku XtaLAB mini CCD diffractometer. All the structures of 1-6 were solved by direct methods. Full-matrix least-squares fitting on F2 by SHELX-97 was employed to refine these structures. All non-hydrogen atoms in these compounds were refined with anisotropic displacement parameters. Hydrogen atoms that from carbon or nitrogen atoms were generated geometrically, while those that from water molecules couldn’t be determined. The information of CPs 1-6 on crystallographic data as well as structure refinement parameters are listed in Table 1. In addition, Table S1-S6 contains some selected bond lengths and angles for 1-6, respectively. The CCDC numbers are 1587869, 1587870, 1587871, 1587872, 1587873 and 1587874 for 1-6, respectively.

two N atoms as well as five O atoms and sits in the center of

Figure 1. (a) Coordination environment for Cd(II) ion in 1; (b) view of the 1D chain; (c) view of the 3D supramolecular structure; (d) the π-π stacking interaction. Symmetry codes: (A) 1+x, y, z.

pentagonal bipyramid geometry. The equatorial plane is made up of N1, N3 from one pyrazine-tetrazole ligand, O1 from one Hbtc2-, and O3A, O4A from another carboxylate, while the positions of axis are occupied by two water molecules (O1W, O2W). The Cd1-N1 and Cd1-N3 bond lengths are 2.548(4) and 2.307(4) Å, respectively, and Cd-O bond distances are 2.248(3)-2.450(3) Å. The N-donor ligand in compound 1 adopts chelating coordination fashion in mode A as displayed in Scheme 1. For 1, the partial deprotonated Hbtc2- bridges two Cd centers in bidentate coordination mode. As clearly shown in Figure 1b, the linkage of Hptz, Hbtc2and Cd centers gives rise to a 1D chain along a axis in 1, in which the uncoordinated pyrazine and tetrazole N atoms as well as uncoordinated carboxyl groups protruding to the two sides. The terminally coordinated water molecules together with partial deprotonated Hbtc2- prevent its further extension to a high-dimensional network. Notably, π-π stacking interactions exist between the neighbouring chains, wherein the centroid-centroid distances of pyrazine rings of Hptz and benzene rings of Hbtc2- are 3.839 Å, leading to a 3D supramolecular framework (Figure 1c and 1d).

Results and Discussion

Scheme 1. Different coordination modes of the pyrazine-tetrazole ligand.

Crystal Structure of [Cd(Hptz)(Hbtc)(H2O)2·H2O]n (1). Compound 1 crystallizes in triclinic crystal system with space group P-1. The existence of one Cd(II) ion, one Hptz ligand, one partial deprotonated Hbtc2-, two terminally coordinated and one lattice water molecules constitutes the asymmetric unit. As depicted in Figure 1a, Cd1 in CP 1 is surrounded by

Figure 2. (a) Coordination environments for Cd(II) ions in 2; (b) view of the 2D layer; (c) view of the 3D supramolecular structure; (d) the π-π stacking interaction. Symmetry codes: (A) -x, -y, 1-z; (B) -1-x, -y, 2-z; (C) x, -1+y, z.



[Cd2(ptz)2(m-bdc)(H2O)]n (4). Compounds 3 and 4 are belonged to monoclinic space group P21/n, and orthorhombic Pbca, respectively. In CP 3, the asymmetric unit consists of one Cd(II) ion, one ptz ligand and half one deprotonated pbdc2- ligand, while for compound 4, two Cd(II) ions, two ptz, one m-bdc2- together with one terminally coordinated water molecule exist in the asymmetric unit. As clearly shown in Figure 3a, Cd1 in compound 3 adopts a six-coordinated mode and sits in an octahedral geometry defined by four N atoms that from three different ptz ligands and two O atoms that from two distinct p-bdc2- ligands. N1 (Cd1-N1 = 2.415(4) Å) and N3 (Cd1-N3 = 2.347(4) Å) from the same N-donor ligand together with O2 (Cd1-O2 = 2.168(3) Å), N4B (Cd1-N4B = 2.290(4) Å) comprise the equatorial plane of octahedron and the axial sites are made up of O1C (Cd1-O1C = 2.257(3) Å) and N1A (Cd1-N2A = 2.443(4) Å). For compound 4, it can be seen from Figure 3b that the two distinct Cd(II) ions adopt the similar coordination modes, which are both six-coordinated and surrounded by four nitrogen atoms as well as two oxygen atoms in distorted octahedral environments. Differently, the two O atoms (O2, O1B) that coordinate to Cd1 center come from two different m-bdc2- ligands, while O3D and O1W which are from one m-bdc2- ligand and one water molecule, respectively, link to Cd2 center. The Cd-N and Cd-O bond lengths are 2.290(6)-2.422(5) Å and 2.175(5)-2.306(5) Å, respectively, which all match well with the previously corresponding reported values.39-41 Notably, the rigid N-donor ligand in these two compounds presents the same bridging/chelating coordination fashion (mode C, Scheme 1). If the linkages through the carboxylate co-ligands are ignored initially, the connection of ptz ligands with Cd(II) ions affords two almost same Cd-ptz 2D layers for compounds 3 and 4. Taking 3 as an example, N1, N3 and N4 atoms from two different ptz ligands coordinate to two Cd centers simultaneously, generating a binuclear [Cd2(ptz)2] moiety with Cd-Cd distance of 4.376 Å. Meanwhile, the adjacent binuclear units are joined together through Cd-N2 bonds, resulting in a binuclear Cd-based 2D sheet with rectangular windows (Figure 3c). Furthermore, when taking the carboxylate ligands into consideration, two entirely different 3D complicated networks are generated. As depicted in Figure 3d, the network of 3 is constructed from the connection of neighbouring layers with two carboxyl groups of each p-bdc2- ligands, wherein a very small offset between the adjacent layers is observed. By contrast, one carboxyl group of the m-bdc2- ligand joins two neighbouring layers together, and the another links with other Cd center which from another different layers, resulting in an intricate 3D network, in which the 2D layers repeat in a staggered stacking mode (···ABAB···) in compound 4 (Figure 3e). Crystal Structure of [Zn2(ptz)(btc)(H2O)2·H2O]n (5). Compound 5 is in a low-symmetry triclinic space group with P-1 based upon XRD analysis. In its asymmetric unit, two Zn(II) ions, one ptz ligand, one completely deprotonated btc3-, two coordinated and one free water molecules exist. In CP 5, the two distinct Zn(II) ions present different coordination modes. Zn1 is six-coordinated and linked with three O atoms (O1, O5B, O6B) and three N atoms (N1, N3, N2A), in which O5B, O6B come from the same carboxylate ligands and N1, N3 are from the same N-donor ligand, exhibiting a somewhat distorted octahedral geometry, while Zn2 is five-coordinated

Crystal Structure of [Cd2(ptz)(btc)(H2O)2·2H2O]n (2). Based upon single crystal data analysis, compound 2 crystallizes in triclinic space group P-1, presenting a 2D bilayer structure. In the asymmetric unit, two Cd(II) ions, one ptz ligand, one fully deprotonated btc3-, four water molecules (two coordinated and two free) exist. Though two kinds of Cd center both present slightly distorted octahedral geometries, they show different coordination environments, which is displayed in Figure 2a. For Cd1, N1, N3 and N4A which are from two distinct deprotonated ptz ligand together with O3B which comes from one btc3- occupy the equatorial plane, while the positions of axis are made up of O1 and O1W, one of which comes from one btc3- ligand, and the other from a water molecule. Unlike Cd1, in the equatorial plane of the octahedron for Cd2, there are three O atoms (O2, O5C, O6C) and one N atom (N5A). O4B from btc3- and O2W from water constitute the axial position. In 2, the bond length of Cd-N/O are in the range of 2.269(5)-2.453(5) Å and 2.275(4)-2.413(4) Å. The N-donor ligand in CP 2 adopts mode B when coordinating to the Cd centers as depicted in Scheme 1. The linkage of ptz, btc3- as well as metal centers results in a 2D bilayer structure, in which both the two ligands are high connected (Figure 2b). The relatively low-dimensional network may originate from the uncoordinated pyrazine and terminally coordinated water molecules. As seen in Figure 2c, adjacent layers are further stacked into a complicate 3D supramolecular architecture. In compound 2, π-π stacking intermolecular interactions can be observed with centroid to centroid distances of 3.675 Å between pyrazine rings of ptz for neighbouring structures (Figure 2d).

Figure 3. (a) and (b) Coordination environments for Cd(II) ions in 3 and 4; (c) the binuclear Cd-based 2D sheet; (d) and (e) view of the different 3D frameworks for 3 and 4. Symmetry codes for 3: (A) 1/2+x, 1/2-y, 1/2+z; (B) 3-x, 1-y, -z; (C) 2-x, 1-y, -z. Symmetry codes for 4: (A) 1/2-x, -1/2+y, z; (B) -x, 1-y, 1-z; (C) 1/2-x, 1/2+y, z; (D) x, 1/2-y, -1/2+z.

Crystal

Structures

of

[Cd2(ptz)(p-bdc)]n

(3)

and


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and surrounded by two carboxyl-O atoms (O3, O4) from the

N5B) from distinct ptz ligands and four oxygen atoms (O1, O2,

Figure 4. (a) and (b) Coordination environments for Zn(II) ions in 5; (b) view of the SBL for 5; (c) view of the 3D frameworks; (d) the two-fold interpenetrated network. Symmetry codes: (A) -1+x, y, z; (B) x, 1+y, z; (C) x, y, -1+z.

Figure 5. (a) Coordination environments for Cd(II) ions in 6; (b) the 1D chain generated from the linkage of ptz and Cd centers; (c) view of the 2D bi-layer; (d) and (e) the 3D complicated network for 6. Symmetry codes: (A) -1/2+x, 1/2+y, z; (B) 3-x, -3-y, 1-z; (C) -1/2+x, -1/2+y, z; (D) 1/2+x, -1/2+y, z; (E) 7/2-x, -1/2+y, 3/2z.

same btc3- ligand, one N atom from a ptz ligand, and two water-O atoms (O1W, O2W) (Figure 4a). The Zn-N/O distances are 2.010(4)-2.319(4) Å and 1.968(3)-2.249(4) Å, respectively, conforming with the values in previous reports.42,43 Each N-donor ligand links three Cd(II) ions by using its tetrazole and pyrazine groups, and similarly to compounds 3 and 4, it also adopts a bridging/chelating coordination fashion (mode D). If the linkage between N6 of pyrazine-tetrazole ligands and Zn2 atoms is ignored for simplification on our purpose, the connection of Zn(II) ions and two different types of organic ligand gives rise to a layer (supermolecular building layer, SBL) in the ab-plane, which is rendered with Zn2 centers protruding to one side of the layer (Figure 4b). The terminally coordinated water molecules prevent the further extension of this 2D structure. Furthermore, neighbouring SBLs join with each other through coordination bonds of Zn2-N6, generating a 3D porous architecture, in which 1D channels along b axis exist as displayed in Figure 4c. It is of note that in this compound, the large pores within the total structure permits interpenetration of an additional independent equivalent framework to make the whole structure stable. Therefore, a two-fold interpenetrated framework is constructed in the structure of compound 5 (Figure 4d). Crystal Structure of [Cd2(ptz)(btc)(DMA)(H2O)·4.5H2O]n (6). Compound 6 crystallizes in monoclinic crystal system and is belonged to space group C2/c. There are two Cd(II) ions, one pyrazine-tetrazole ligand, one btc3- ligand, one coordinated DMA molecule, one terminally coordinated water molecule, four and a half guest water molecules in the asymmetric unit. Though both Cd1 and Cd2 in CP 6 are sevencoordinated in pentagonal bipyramidal geometries, their coordination environments are totally different. As shown in Figure 5a, Cd1 is linked by three nitrogen atoms (N2, N4A,

O5C, O6C) from two btc3- ligands. Four oxygen atoms together with N5B occupy the equatorial plane, while N2 and N4A determine the axial sites. On the other hand, two nitrogen atoms (N1, N3) from a ptz ligand, three oxygen atoms (O2D, O3E, O4E) from two btc3- ligands and two oxygen atoms (O1W, O7) from solvent molecules (H2O and DMF) constitute the vertexes of pentagonal bipyramidal geometry based on Cd2, in which axial positions are defined by two O1W and O7, while the other atoms determine the equatorial plane. The distances of Cd-N are 2.295(6)-2.539(6) Å, and the Cd-O bond lengths are 2.251(8)-2.568(7) Å. Pyrazine-tetrazole ligand in compound 6 chelate/bridge four adjacent Cd centers in mode E as depicted in Scheme 1, and the connection of ptz with two kinds of Cd center generate a 1D chain in advance based on binuclear Cd moiety with CdCd distance of 4.603 Å as displayed in Figure 5b. The coordination interaction between solvent molecules and Cd2 prevents the chains into a higher dimensional Cd-ptz structure. Afterwards, adjacent chains are connected by two carboxyl groups (O1-C-O2 and O5-C-O6) of one same btc3- ligand, expanding into a 2D bi-layer structure with another carboxyl group (O3-C-O4) pointing towards both sides of the layer (Figure 5c). Furthermore, the connection of Cd2, O3, O4 centers joins these layers together into a 3D complicated network with square channels along the [1 0 1] orientation (Figure 5d and 5e).

Thermal Analysis and PXRD Results. To ascertain the thermal stabilities of CPs 1-6, their TG analyses were recorded (Figure 6). The initial weight loss with temperature ranging from 30 to 150°C for compound 1 corresponds to the release of one lattice water molecules (obsd, 3.61%; calcd, 3.44%); from 150 to 210°C, the following step weight loss of 6.47% could correspond to the loss of coordinated water molecules



(calcd, 6.89%). The framework begins to decompose rapidly after 300°C. Compound 2 exhibits a weight loss of 10.73% in 30-210°C, which could be attributed to the removal of both free and coordinated water molecules (calcd, 11.06%), the desolvated framework could be maintained till 400°C, and followed by the rapid decomposition of the residue. For compound 3, no obvious weight loss before 260°C is observed, which is in good accordance with the crystal structure. After that, decomposition of the framework occurred. Compound 4 is stable till 180°C, and then the coordinated water molecule is released (obsd, 2.62%; calcd, 2.57%), immediately followed by the backbone decomposition at around 340°C. For compound 5, the weight loss before 220°C belongs to removal of all the coordinated and free water molecules (obsd, 9.67%; calcd, 10.02%), then the 3D framework starts collapsing after that temperature. There is an initial weight loss at room temperature to 265°C for 6, which arises from the loss of five and a half water molecules and one DMA molecue (obsd, 24.13%; calcd, 24.31%). Then the framework begins to decompose.

organic linkers, which make its conformational rigidity enhanced and nonradiative energy loss decreased. The different photoluminescent behaviors of 1-6 with the same Ndonor linker may originate from the various coordination environments with d10 metal ions as well as the differences that exist in the coordination modes of both Hptz molecules and polycarboxylate co-ligands, all of which could influence the rigidity of the whole network and further affect the energy transfer involved in the luminescence. Furthermore, we have also performed the time-resolved luminescence study. By monitoring the most intense emission of the four compounds that show intense luminescence emission (1, 2, 5 and 6) at room temperature, the emission decay lifetimes are found to be τ = 3.26, 4.46, 4.32 and 4.79 ns, respectively.

Figure 7. The emission spectra of CPs 1-6 in the solid state.

Luminescent Sensing. Compounds 1 and 6 were selected to study the luminescence responses to various metal ions owing to their good stabilities as well as relatively high luminescence performance. 1 or 6 samples (10 mg) were grinded into powder and dispersed in aqueous solution (10 mL) containing 1 mM of M(NO3)x (M = Li+, Na+, K+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Cr3+).The suspensions were sonicated for 40 min and then the photoluminescence experiment was conducted. Interestingly, the luminescence intensity could be dependent on the species of metal ions. As shown in Fig. 8a and S7, Cu2+ exhibits an excellent quenching effect towards both 1 and 6 with sharply decreased luminescence intensities, indicating that they can be acted as promising selective sensors. In addition, the sensing sensitivities of the samples on Cu2+ were further examined. With the increasing concentration of Cu(NO3)2, the luminescence intensities were gradually decreased for the suspensions. Taking 1 as an example, the luminescence intensity is highly sensitive to the Cu2+ concentration in the range of 0-400 µM. As seen in Fig. 8b, more than 60% of emission intensity was decreased when the Cu2+ concentration increasing to 50 µM and nearly quenched to 400 µM. The reason for luminescence quenching effect may possibly be attributed to the donor-acceptor electron transfer. The unsaturated electronic state (3d9) makes the empty orbital of Cu2+ become an ideal acceptor for excited electrons from the organic conjugate molecules through the interactions between Cu2+ and nitrogen or oxygen atoms from the ligands, leading

Figure 6. TG curves for CPs 1-6.

The purity of CPs 1-6 is demonstrated by powder X-ray diffraction (PXRD) analyses. It can be observed in the experimental spectra of 1-6 that all the main peaks are almost in accordance with its simulated spectrum, which indicates the phase purity of these compounds (Fig. S1-S6).

Luminescent Properties. The d10 metal compounds are well known for their multipurpose photoluminesecent properties. Therefore, we have investigated the solid-state luminescence properties of CPs 1-6 at room temperature. As illustrated in Figure 7, upon photoexcitation at 330 nm, weak and broad emissions for 3 and 4 are observed with maximum peaks of 390 and 395 nm, respectively. For CPs 1, 2, 5 and 6, they display the maximum emission peaks at 435 nm (λex = 350 nm), 416 nm (λex = 330 nm), 430 nm (λex = 350 nm), and 416 nm (λex = 330 nm), respectively. Compared with the emission of Hptz at 452 nm,44,45 the emission bands of these compounds show somewhat blue shifts, which can be due to the ligand-centered nature of the emission.46-49 Besides that, when comparing with the free pyrazine-tetrazole ligand, the enhancement of luminescence intensities for compounds 1, 2, 5 and 6 could be mainly attributed to the coordination interactions between the Zn(II)/Cd(II) metal ions and the rigid


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to the luminescence quenching phenomenon.50-52

via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21571111, 21601100), and the Natural Science Foundation of Shandong Province (ZR2016BP02).

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Figure 8. (a) The luminescence intensity of 1 with various metal ions; (b) The emission spectra of 1 with different Cu2+ concentration.

Conclusions In this work, assembly of 5-(pyrazinyl)tetrazole and various polycarboxylates with Zn(II)/Cd(II) ions has afforded a series of fascinating CPs with different dimensions, such as 1D for 1, 2D for 2, and 3D for 3-6. Systematic investigation of these diverse structures confirms that the pyrazine-tetrazole ligand can adopt various coordination modes and act as versatile building units when bridging to metal centers. Additionally, the carboxylate co-ligands also play a critical role in modulating the generation of the final structures. The blue luminescent emissions at room temperature for CPs 1-6, indicate that they may be potentially applicable in photoactive materials. Moreover, Cu2+ exhibits a quenching effect towards 1 and 6, suggesting that the two compounds could be considered as luminescent probes for Cu2+.

ASSOCIATED CONTENT Supporting Information. Selected bond lengths and angles, PXRD curves for compounds 1-6 and luminescence intensity of 6 with various metal ions. This material is available free of charge



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For Table of Contents Use Only Dual ligands strategy for constructing a series of d10 coordination polymers: syntheses, structures, photoluminescence and sensing properties Di Zhang, Zhen-Zhen Xue, Jie Pan, Jin-Hua Li and Guo-Ming Wang*

A series of Zn(II)/Cd(II) coordination polymers were obtained by using pyrazine-tetrazole and polycarboxylate ligands, in which different coordination modes of the ligands result in distinct structures.


syntheses, structures, photoluminescence and sensing

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