Syntheses Study of Keggin POM Supporting MOFs System - Crystal

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Syntheses Study of Keggin POM Supporting MOFs System Jing-Quan Sha,*,†,‡ Jing-Wen Sun,‡ Cheng Wang,† Guang-Ming Li,† Peng-Fei Yan,*,† and Meng-Ting Li‡ †

School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China School of Pharmacy, Jiamusi University, Jiamusi, 154007, P. R. China

‡

S Supporting Information *

ABSTRACT: To investigate the influence of reactive conditions on the structures of Keggin POM supporting metal−organic frameworks (MOFs), five new compounds, [Ag6(2-pytz)4(H2O)2][HPMo12O40] (1), {[Ag14(2pytz) 12 (H 2 O) 4 ][H 4 PMo 12 O 40 ] 2 }·2H 2 O (2), {[Ag 6 (2-pytz) 4 (H 2 O) 4 ][H2SiW12O40)]}·6H2O (3), {[Ag3(3-pytz)2]2·[AgPMo12O40]}·2H2O (4), [Ag6(4-pytz)4][HPMo12O40] (5) [pytz = 5-(pyridyl)tetrazolate], were hydrothermally synthesized by tuning the reactive species and pH values and characterized by routine method and single-crystal X-ray crystallography. Although compounds 1−5 all show the POM supporting 3D MOFs skeleton, MOFs exhibit diverse structures and POMs play different supporting functions, more specifically, pseudo POM chains supporting MOFs in 1 and 2, isolated POM spheres supporting MOFs in 3, ture POM chains supporting MOFs in 4, and pseudo POM layers supporting MOFs in 5. The roles of the pH value, pytz derivatives, and the Keggin homologues in the formation of the POM supporting MOFs have been discussed. In addition, the photoluminescence properties of compounds 1−3 were investigated in the solid state at room temperature.

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INTRODUCTION The assembly of metal−organic frameworks (MOFs) constructed from metal ions as connectors and ligands as linkers have attracted much attention, owing to their enormous varieties of interesting structural topologies and wide potential applications as functional materials.1,2 Polyoxometalates (POMs), as one kind of important metal oxide nanosize clusters with abundant topologies, have been employed as secondary building units (SBUs) to construct organic− inorganic hybrid materials with great potential applications in catalysis, magnetism and materials science.3−5 Particularly, it is appealing to introduce POMs to different MOFs for constructing the various POM supporting MOFs with desired properties.6 Nevertheless, the rational design and assembly of POM supporting MOFs remain an arduous task for coordination chemists due to the large number of potential coordination sites, the bigger steric hindrance and the relatively weak coordination ability of POMs. The key factor for achieving this aim is to introduce proper POMs into the covalent backbone of MOFs, which can not only direct the formation of the desired architectural, chemical, and physical properties of the resulting hybrid materials but also exhibit a synergic effect with MOFs. To date, remarkable organic− inorganic hybrid compounds based on POMs have been reported.7 However, examples based on POM supporting MOFs are still less.8 And moreover, most of them are connected through noncovalent interactions (van der Waals contacts, hydrogen-bonding, and/or ionic interactions). So it can be seen that the rational design and assembly of POM © 2012 American Chemical Society

supporting MOFs, especially covalent connection, remain a challenge for POM chemists. As is known that pyridyltetrazoles (pytz), as a readily accessible family of polyazaheteroaromatic derivates, have become attractive ligands for the design of MOFs, owning to their abilities of bridging multiple metal sites and considerable variations in tether lengths. Some MOFs constructed by pytz molecules have been synthesized in the past few years.9AlthAlthough these kinds of pytz ligands have proved to be very interesting and useful in building the extended 2D and 3D MOF family, the introduction of pytz ligands in POM supporting MOFs is less explored.10 Additionally, we have also noticed that both the soft d10 metal ions and pytz ligands are good candidates for the construction of high dimensional MOFs. Because the soft d10 metal ions usually adopt versatile coordination geometries under hydrothermal conditions, especially that the Ag+ ions can display two to seven coordination geometries, such as linear, T-type, “seesaw”, “square pyramidal”, “trigonal bipyramidal” geometries, and so on.11 Hence, for considering the synthetic strategy, we choose both Ag ions and pytz ligand for constructing new POM supporting MOFs in this work. Notably, the assembly of POMAg based hybrid compounds is very sensitive about synthetic conditions, such as pH value, the nature of ligands and POMs. Thus, herein we systematically investigated the effect of pH value, the ligands isomers and POM species on assembly of the Received: November 9, 2011 Revised: March 15, 2012 Published: March 26, 2012 2242

dx.doi.org/10.1021/cg201478y | Cryst. Growth Des. 2012, 12, 2242−2250

Crystal Growth & Design

Article

spectrometer (Ag, Mo and W). The IR spectra were obtained on an Alpha Centaurt FT/IR spectrometer with KBr pellet in the 400−4000 cm−1 region. The photoluminescence analysis was carried out on an Efinburgh FLS920 fluoresecence spectrometer. The XRPD patterns were obtained with a Rigaku D/max 2500 V PC diffractometer with Cu−Kα radiation, the scanning rate is 4°/s, 2θ ranging from 4 to 50°. Synthesis of [Ag6(2-pytz)4(H2O)2][HPMo12O40] (1). A mixture of H3PMo12O40·xH2O (100 mg), AgNO3 (50 mg), 2-pytz (10 mg), NH4VO3 (12 mg), and H2O (8 mL) was stirred for 1 h in air. The pH was then adjusted to 2.5 with 1 M HCl, and the mixture was transferred to an 18 mL Teflon-lined reactor. After 3 days’ heating at 160 °C, the reactor was slowly cooled to room temperature over a period of 16 h. Brown-red block crystals of 1 were filtered, washed with water, and dried at room temperature (36% yield based on Mo). C24H21N20Ag6PMo12O42 (3091.04): Calcd. C 9.32, H 0.67, N 9.06, Ag 20.96, Mo 37.27%; Found C 9.29, H 0.84, N 9.02, Ag 20.85, Mo 37.23%. IR (KBr, cm−1): 3442(s), 3106(w), 1616(m), 1448(s), 1295(w), 1158(w), 1057(s), 961(s), 869(s), 786(s), 656(m), 596(w), 502(w). Synthesis of {[Ag14(2-pytz)12(H2O)4][H4PMo12O40]2}·2H2O (2). A mixture of H3PMo12O40·xH2O (100 mg), AgNO3 (50 mg), 2-pytz (10 mg), NH4VO3 (12 mg), and H2O (8 mL) was stirred for 1 h in air. The pH was then adjusted to ca. 1.5 with 1 M HCl, and the mixture was transferred to an 18 mL Teflon-lined reactor. After 3 days’ heating at 160 °C, the reactor was slowly cooled to room temperature over a period of 16 h. Green block crystals of 2 were filtered, washed with water, and dried at room temperature (42% yield based on Mo). C72H68N60Ag14P2Mo24O88 (7056.39): Calcd. C 12.24, H 0.96, N 11.90, Ag 21.43, Mo 32.65%; Found C 12.22, H 1.04, N 1.89, Ag 21.42, Mo 32.62%. IR (KBr, cm−1): 3442(s), 2927(w), 1619(m), 1448(s), 1297(w), 1162(w), 1057(s), 948(s), 869(s), 788(s), 659(m), 596(w), 512(w). Synthesis of {[Ag6(2-pytz)4(H2O)4][H2SiW12O40)]}·6H2O (3). A mixture of H4SiW12O40·xH2O (100 mg), AgNO3 (50 mg), 2-pytz (10 mg), NH4VO3 (12 mg), and H2O (8 mL) was stirred for 1 h in air. The pH was then adjusted to 2.58 with 1 M HCl, and the mixture was transferred to an 18 mL Teflon-lined reactor. After 3 days’ heating at 160 °C, the reactor was slowly cooled to room temperature over a period of 16 h. Green block crystals of 3 were filtered, washed with water, and dried at room temperature (46% yield based on W).

POM supporting MOF systems (Scheme 1). As expected, five new POM supporting MOFs were obtained, namely, [Ag6(2Scheme 1. Schematic Representation of POM Supporting MOF Systems

pytz)4(H2O)2][HPMo12O40] (1), {[Ag14(2-pytz)12(H2O)4][H 4 PMo 1 2 O 40 ] 2 }·2H 2 O (2), {[Ag 6 (2-pytz) 4 (H 2 O) 4 ][H2SiW12O40)]}·6H2O (3), {[Ag3(3pytz) 2 ]2 ·[AgPMo12 O40 ]}·2H 2O (4), and [Ag6 (4-pytz) 4][HPMo12O40] (5). The informative structures of these five compounds contribute to understanding of the effect of the pH value, ligands isomers and POM species on POM supporting MOFs .

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EXPERIMENTAL SECTION

Materials and General Methods. All reagents were purchased commercially and used without further purification. Elemental analyses were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer (C, H and N) and a Leaman inductively coupled plasma (ICP)

Table 1. Crystal Data and Structure Refinements for Compounds 1−5 Formula Fw T (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) μ (mm−1) F(000) final R1a, wR2b [I > 2σ(I)] final R1a, wR2b (all data) GOF on F2 a

1

2

3

4

5

C24H21N20Ag6 PMo12O42 3091.04 293(2) C2/m 10.297(3) 23.266(6) 12.776(3) 90 94.729(2) 90 3050.33(13) 2 3.360 4.394 2874 0.0627 0.2033 0.0707 0.2082 1.115

C72H68N60Ag14 P2Mo24O88 7056.39 293(2) P1̅ 16.072(6) 16.344(5) 18.111(4) 72.641(2) 86.882(2) 60.58 3930.8(2) 1 2.973 3.672 3300 0.0708 0.2284 0.0949 0.2442 1.104

C24H38N20Ag6 SiW12O50 4288.16 293(2) P2(1)/n 12.661(5) 22.238(7) 12.953(5) 90 111.829(5) 90 3385.5(2) 2 4.185 22.116 3768 0.0695 0.1767 0.0853 0.1827 1.065

C24H20N20Ag7 PMo12O42 3197.86 293(2) P1̅ 10.216(5) 12.903(5) 13.395(5) 96.572(5) 112.215(5) 97.623(5) 1593.9(12) 2 3.394 4.511 1516 0.0608 0.1732 0.0753 0.1891 1.045

C24H17N20Ag6 PMo12O40 3054.97 293(2) P21/c 10.554(5) 23.046(5) 12.291(5) 90 101.345(5) 90 2931.1(19) 4 2.953 5.218 2373 0.0494 0.1117 0.0663 0.1204 1.053

R1 = ∑||Fo|  |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2Fc2)2]/∑[w(Fo2)2]}1/2. 2243



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Table 2. Coordination Numbers and Modes of POMs, Ligands and Ag Ions in Compounds 1−5

C24H38N20Ag6SiW12O50 (4288.16): Calcd. C 6.72, H 0.88, N 6.53, Ag 15.11, W 51.49%; Found C 6.72, H 0.91, N 6.49, Ag 15.09, W 51.47%. IR (KBr, cm−1): 3453(s), 2908(w), 1621(m), 1446(s), 1247(w), 1064(w), 1031(w), 962(s), 916(s), 800(s), 749(w), 518(m). Synthesis of {[Ag3(3-pytz)2]2·[AgPMo12O40]}·2H2O (4). A mixture of H3PMo12O40·xH2O (100 mg), AgNO3 (52 mg), 3-pytz (11 mg), NH4VO3 (13 mg), and H2O (8 mL) was stirred for 1 h in air. The pH was then adjusted to 2.79 with 1 M HCl, and the mixture was transferred to an 18 mL Teflon-lined reactor. After 3 days’ heating at 160 °C, the reactor was slowly cooled to room temperature over a period of 16 h. Red block crystals of 4 were filtered, washed with water, and dried at room temperature (50% yield based on Mo). C24H20N20Ag7PMo12O42 (3197.86): Calcd. C 9.01, H 0.62, N 8.76, Ag 23.64, Mo 36.02%; Found C 9.00, H 0.65, N 8.74, Ag 23.62, Mo 36.00%. IR (KBr, cm−1): 3424(s), 1612(m), 1455(s), 1258(w), 1178(w), 1048(s), 949(s), 861(w), 783(s), 669(w), 601(w). Synthesis of [Ag6(4-pytz)4][HPMo12 O40] (5). A mixture of H3PMo12O40·xH2O (100 mg), AgNO3 (49 mg), 4-pytz (10 mg), NH4VO3 (12 mg), and H2O (8 mL) was stirred for 1 h in air. The pH was then adjusted to 2.53 with 1 M HCl, and the mixture was transferred to an 18 mL Teflon-lined reactor. After 3 days’ heating at 160 °C, the reactor was slowly cooled to room temperature over a period of 16 h. Yellow block crystals of 5 were filtered, washed with water, and dried at room temperature (43% yield based on Mo). C24H17N20Ag6PMo12O40 (3054.97): Calcd. C 9.43, H 0.55, N 9.17, Ag 21.21, Mo 37.71%; Found C 9.42, H 0.63, N 9.16, Ag 21.20, Mo 37.69%. IR (KBr, cm−1): 3432(s), 2925(w), 1623(m), 1429(m), 1162(w), 1053(s), 954(s), 875(m), 785(s), 675(w), 601(w), 539(w). X-ray Crystallographic Study. Crystal data for compounds 1−5 were collected on a Bruker SMART-CCD diffractometer with Mo−Kα monochromatic radiation (λ = 0.71073 Å) at 293 K. All structures were solved by the directed methods and refined by full matrix leastsquares on F2 using the SHELXTL crystallographic software package.12 All of the non-hydrogen atoms were refined anisotropically.

The positions of hydrogen atoms on carbon atoms were calculated theoretically. The crystal data and structure refinements of compounds 1−5 are summarized in Table 1. Selected bond lengths and angles for compounds 1−5 are listed in Tables S1−S5 (Supporting Information). CCDC reference numbers 840362 for 1, 840363 for 2, 840485 for 3, 849457 for 4 and 844747 for 5.

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RESULTS AND DISCUSSION In the syntheses of compounds 1−5, it is noted that the use of NH4VO3 species is necessary, although it is not involved in the final structures. It may be explained that NH4VO3 can activate the POM clusters while the intact skeleton of the presynthesized Keggin POMs is maintained. Bond valence sum calculations for compounds 1−5 show that all tungsten and molybdenum atoms are in +VI oxidation states, and silver atoms are in +I oxidation states. The crystal photographs of compounds, the coordination number and mode of POMs, ligands and Ag ions are listed in Table 2. The networks of compounds were analyzed by using the TOPOS40 program.13 Structure Description of Compound 1. Crystal structure analysis reveals that the asymmetric unit of compound 1 (Figure S1, left, Supporting Information) is made up of six Ag ions, four 2-pytz ligands, one [PMo12O40]3‑ polyoxoanion (abbreviated to PMo12) and two water molecules. Each PMo12 cluster acts as a tetra-dentate inorganic ligand coordinating with six silver centers (Figure S1, right, Supporting Information). In 1, there are three unique Ag centers (Table 2). The Ag1 adopts the distorted octahedral geometry, coordinated by four N atoms from two 2-pytz and two O atoms of two water molecules, with Ag−N bond distances of 2.0008(9)−2.0093(9) Å, Ag−O bond distance of 2.7593(5) Å. The Ag2 shows a 2244



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three-coordinated “Y-shaped” geometry, coordinated by two N atoms from two 2-pytz and one O atom from one PMo12. The bond distances around Ag2 ion are 2.1900(9) Å for Ag−N, 2.4387(1) Å for Ag−O. The Ag3 exhibits a distorted tetrahedral geometry, coordinated by two N atoms from two 2-pytz and two O atoms from two PMo12, with Ag−N bond distances of 2.3276(9) Å, Ag−O bond distances of 2.5731(1)−2.7548(1) Å. On the other hand, the 2-pytz as a tridentate linkage provides two N atoms to chelate one Ag1 ion, and two of the three remaining N atoms to bridge two Ag ions (Ag2 and Ag3), inducing to an infinite organic−inorganic hybrid chain (Figure 1a). Furthermore, the OW1 molecules covalently bond with the

Ag2 and Ag3 ions as shown in Figure 2a. Additionally, Ag3 ions link PMo12 anions to form an infinite POM-Ag-POM chain along the a axis (Figure 2b). Because that the Ag3 ions are shared by 1D POM-Ag-POM chains and 2D MOFs, the 1D POM-Ag-POM chains are named as pseudo POM chains. So the 3D framework can be viewed as the pseudo POM chains supporting 2D MOFs (Figure 2c). As a result, the overall structure of 1 is an unprecedented (4,8)-connected 3D framework. The Schläfli symbol is (44·62)(416·612) (Figure 2d). In this simplification, the 4-connected nodes are PMo12 anions, and 8-connected nodes are subunits of MOFs (Figure 2S, Supporting Information). Crystal Structure of Compound 2. Single crystal X-ray study reveals that the structure of 2 is also a pseudo POM chains supporting MOF 3D framework. The asymmetric unit (Figure S3, left, Supporting Information) is made up of 14 Ag ions, 12 2-pytz ligands, 2 PMo12 anions and 6 water molecules. There are 2 crystallographically independent PMo12 anions (APMo12 and B-PMo12). Both PMo12 clusters act as tetra-dentate ligands coordinating with four silver centers (A-type: Ag4, Ag4, Ag5 and Ag5, B-type: Ag5, Ag5, Ag8 and Ag8, as shown in Figure S3, right, Supporting Information). There are 8 crystallographically independent silver atoms with diverse coordination modes (Table 2). The Ag1 adopts the squareplanar geometry coordinated by 4 N atoms from 2 2-pytz ligands. Ag2, Ag4 and Ag8 ions are six-coordinated in distorted octahedral geometry. For example, Ag2 is coordinated by 4 N atoms from 2 2-pytz and 2 O atoms from 2 water molecules. Ag4 is coordinated by 5 N atoms from 3 2-pytz ligands and 1 O atom from 1 water molecule, while Ag8 is coordinated by 5 N atoms from 3 2-pytz ligands and 1 O atom from 1 PMo12 anion. Ag3, Ag6 and Ag7 ions are three-coordinated in a “Y-shaped” geometry, namely, Ag6 and Ag7 are coordinated by 3 N atoms from 3 2-pytz, while Ag3 by 2 N atoms from 2 2-pytz and 1 atom from 1 water molecule. Ag5 ion adopts distorted tetrahedral geometry coordinating by 2 N atoms from 2 2pytz and 2 O atoms from 2 PMo12 anions. These Ag ions are connected together through 2-pytz to form duodenary nuclear cycles with a dimension of ca. 7.06 × 11.78 Å (Figure 3a). These adjacent duodenary nuclear cycles are further linked to construct a chain with grid via sharing the longer borders of cycle (border of Ag3- Ag6- Ag1- Ag6- Ag3 or Ag5- Ag7- Ag2Ag7- Ag5). Finally, the chains parallel with each other and connect together through 2-pytz between Ag4 and Ag7, and Ag6 and Ag8 to generate the ladder-like 2D MOF (Figure 3b).

Figure 1. (a) Ball/stick representation of organic−inorganic hybrid chain constructed by Ag1, Ag2, Ag3 and 2-pytz ligands; (b) inorganic chain constructed by Ag1 and OW1; (c) 2D MOFs.

Ag1 ions to form another wavelike inorganic chain (Figure 1b). As a result, these two kinds of hybrid chains are fused together via Ag1 ions, leading to a 2D MOF (Figure 1c). In the 2D MOFs, the organic−inorganic hybrid chains are linked in a fashion of peak-to-foot generating much larger voids with the dimensions of 6.97 × 12.72 Å. And the PMo12 anions are located between these voids of the adjacent 2D MOFs via

Figure 2. (a) Combined polyhedral/ball/stick representation of the supporting mode of POMs; (b) pseudo POM chains; (c) 3D POM supporting MOF structure; (d) view of the topology (the red nodes symbolize the PMo12 clusters, and the yellow nodes symbolize subunits of MOFs). 2245



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anion. The Ag3 ion is six-coordinated in a distorted octahedral geometry, coordinated by 4 N atoms from 2 2-pytz, 2 O atoms from the SiW12 anion and water molecule. These three kinds of Ag ions are linked together via 2-pytz forming octa-nuclear cycles with the dimension of ca.14.82 × 10.46 Å (Figure 5a). The crown-like octa-nuclear cycles are further fused by sharing the Ag3 ions to generate a 2D MOF (Figure 5b).

Figure 3. (a) Ball/stick representation of duodenary nuclear cycles; (b) scheme of the 2D MOFs.

The A-PMo12 connects with two duodenary nuclear cycles (Figure 4a), while the B-PMo12 connects with four duodenary nuclear cycles (Figure 4b), resulting in the 3D covalent framework (Figure 4c). Different from compound 1, the POMAg-POM chains are composed by two kinds of POMs, which alternately array to stabilize the whole structure. From the topological view, if A-PMo12 anions are considered as twoconnected nodes, B-PMo12 anions as four-connected nodes, and duodenary Ag nuclear cycles as nine-connected nodes (Figure.S4), the structure of 2 is a novel (2, 4, 9)-connected framework with (6) (32·42·52) (38·411·59·66·72) topology (Figure 4d). Crystal Structure of Compound 3. Crystal structure analysis reveals that the asymmetric unit of compound 3 is made up of 6 Ag ions, 4 2-pytz ligands, 1 [SiW12O40]4‑ anion (abbreviated to SiW12) and 10 water molecules (Figure S5a, Supporting Information). There is 1 crystallographically independent SiW12, which acts as a hexa-dentate inorganic ligand coordinating to 6 silver centers (Figure S5b, Supporting Information). In compound 3, there are 3 kinds of coordination modes about Ag ions. The Ag1 is four-coordinated in a “seesaw” mode, coordinated by 2 N atoms from 2 2-pytz, 2 O atoms from 1 SiW12 anion and 1 water molecule. The Ag2 is four-coordinated in a distorted tetrahedral geometry, coordinated by 3 N atoms from 3 2-pytz, 1 O atom from the SiW12

Figure 5. (a) Ball/stick representation of octa-nuclear cycles; (b) scheme of the 2D MOFs.

Each SiW12 cluster is situated between the 2D MOFs via six Ag ions from four octa-nuclear Ag cycles (Figure 6a), which results in forming isolated POM supporting MOFs 3D structure (Figure 6b). As a result, the overall structure of 3 is an unprecedented (4,8)-connected 3D framework with (32·53·6)(34·44·512·67·7) topology (Figure 6c). In this simplification, the 4-connected nodes are SiW12 clusters and 8connected nodes are octa-nuclear Ag cycles (Figure S6, Supporting Information). Crystal Structure of Compound 4. Single-crystal X-ray diffraction analysis reveals that 4 shows a complicated POM supporting MOF 3D structure, which is constructed by {AgPMo12O40}n anion chains and [Ag3(3-pytz)2]n MOFs (Figure S7, left, Supporting Information). There is one crystallographically independent PMo12, which acts as an octa-dentate inorganic ligand coordinating with eight silver centers (Figure S7, right, Supporting Information). Note that

Figure 4. (a) Combined polyhedral/ball/stick representation of the supporting mode of A-POMs; (b) B-POMs; (c) 3D POM supporting MOFs structure; (d) view of the topology (the green nodes symbolize the A-PMo12, the blue nodes symbolize the B-PMo12, and the purple nodes symbolize duodenary Ag nuclear cycles). 2246



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Figure 8. (a) Ball/stick representation of the supporting mode of POMs; (b) polyhedral representation of the POM-Ag-POM column; (c) 3D POM supporting MOFs structure; (d) view of the topology (the green nodes symbolize the PMo12 clusters, the orange nodes symbolize Ag1, the purple nodes symbolize Ag2, the blue nodes symbolize Ag3 (2-in-1) and Ag4 (2-in-1).

Figure 6. (a) Combined polyhedral/ball/stick representation of the supporting mode of POMs; (b) POM supporting MOFs 3D structure; (c) view of the topology (the blue nodes symbolize the SiW12 clusters and the yellow nodes symbolize octa-nuclear Ag cycles).

the coordination number in compound 4 represents the highest coordination number of Keggin POMs up to date. There are four kinds of crystallographically unique Ag ions. The Ag1 shows a distorted “T shaped” geometry by two N atoms from two different 3-pytz ligands and one terminal oxygen atom from the PMo12 anion. Ag2 is square-planar coordination geometry surrounded by four bridging oxygen atoms from two PMo12 anions. And Ag3 and Ag4 centers show similar tetrahedral geometry coordination, which are coordinated by three N atoms from three 3-pytz ligands and one terminal oxygen atom from the PMo12 anion, respectively. Additionally, there are two types of 3-pytz (A- and B- type), which link the Ag1, Ag3 and Ag4 forming 2D MOFs (Figure 7). Note that there is an infinite double nuclear Ag-chain formed by Ag3, Ag4 ions and A-type ligands.

eight-connected nodes, Ag1 as five-connected nodes, Ag2 as two-connected nodes, the structure of 4 is a novel (2,5,8,8,8)connected framework with (5)(33·43·53·6)(34·42·56·610·75·9)(36·48·58·66)(38·412·57·6) (Figure 8d). Crystal Structure of Compound 5. Single-crystal X-ray diffraction analysis reveals that 5 shows a complicated pseudo POM layers supporting MOF 3D structure, which is constructed by one PMo12 anion, four 4-pytz ligands and six Ag ions (Figure S8, left, Supporting Information). There is one crystallographically independent PMo12, which acts as an octadentate inorganic ligand coordinating to eight silver centers (Figure S8, right, Supporting Information). Note that the coordination number in 5 also represents the highest coordination number of Keggin POMs up to date. There are three crystallographically unique Ag centers (Table 2). The Ag1 and Ag3 ions are three-coordinated in “Y-type” geometry, namely, Ag1 is coordinated by three N atoms from three 4-pytz ligands, while Ag3 is coordinated by two N atoms from two 4pytz ligands and one terminal O atom from PMo12 anion. The Ag2 ion is five-coordinated in a “square-pyramidal” geometry, coordinated by two N atoms from two 4-pytz ligands, three O atoms from three PMo12 anions. Further, 4-pytz (A-type and Btype) link Ag anions to form the 2D MOFs (Figure 9a). Unlike compounds 1−4, there is a 2D POM-Ag-POM layer in 5, more specifically, PMo12 clusters link each other by sharing two Ag2 centers. Each PMo12 cluster is surrounded by six adjacent PMo12 clusters leading to the 2D POM-Ag-POM layer (Figure 9b). Finally, the POM-Ag-POM layer perpendicularly supports the 2D MOFs through sharing of the same Ag2 and Ag3 ions to generate a pseudo POM layer supporting MOF 3D structure (Figure 10). As a result, the overall structure is an unprecedented (4,5,6)-connected 3D framework. The Schläfli symbol is (43·56·6)(46·66·83)(54·6·8). In this simplification, the 4-connected nodes are Ag1 ions, 5-connected nodes are 2-in-1 (Ag2 and Ag3), and the 6-connected nodes are PMo12 anions. Influence of Reaction Conditions on the Structures of Compounds. Although some POM supporting MOF compounds with diverse structural architectures have been synthesized by using hydrothermal methods, in the crystal engineering point of view, the control of self-assembly processes is still a very challenging work to realize the target

Figure 7. Scheme of the 2D MOFs. (Inset) Plot of the stick representation of double nuclear Ag-chain formed by Ag3 and Ag4 ions (left) and the coordination mode of A-, B-type ligands (right).

Each PMo12 ions supports the 2D MOFs by sharing the Ag1, Ag3 and Ag4 ions (Figure 8a), which results in forming a POM supporting MOF 3D structure (Figure 8c). It is interesting that PMo12 ions are linked together via Ag2 forming the POM-AgPOM chain along the a axis (Figure 8b), which further enhance the stability of the whole framework. To the best of our knowledge, the POM-Ag-POM chains have not been observed in other POM supporting MOF system. From the topological view, if PMo12, Ag3 (2-in-1) and Ag4 (2-in-1) are considered as 2247



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Figure 9. (a) Scheme of 2D MOFs; (b) polyhedral/ball representation 2D POM-Ag-POM layer.

Figure 10. (a) Combined polyhedral/stick representation of the supporting mode of POMs; (b) 3D POM supporting MOFs structure; (c) view of the topology (the bule nodes symbolize the PMo12 clusters, the green nodes symbolize the Ag1 and the pink nodes symbolize 2-in-1 (Ag2 and Ag3).

Figure 11. Coordination representation of (a) 3-pytz, (b) 4-pytz and (c) 2-pytz.

specifically, the 3- and 4-pytz molecules acting as radiated linkers afford five bridging sites from different directions, which generate the compacted 2D nets without voids in 4 and 5. However, in the compounds 1−3, two 2-pytz molecules tether one Ag anion forming a rigid metal-complex linker, which can control the direction of the spatial expansion of the ligand to some extent, so that the 2D net with voids is obtained (Figure 11). pH Value. Compounds 1 and 2 were synthesized under the same reaction conditions, except for the alternation of one pH unit. It is clear that the acid−base chemistry of the molecular building units can explain their differences. First, the free Ag+ ions possess good fitting coordination environment under different pH values and readily transform to Ag or Ag2O under

syntheses of this series. Many factors can significantly affect the structures. To explore this aspect, in this work, our attention is focused on studying the effect of reaction conditions, such as pH value, the POM species and the ligand isomers. Ligand Isomers. Compounds 1−3 were synthesized by using 2-pytz, while 4 and 5 using 3-pytz or 4-pytz, respectively. It is clear that the relationship between the structure of the 2D network and the species of the ligand is the key role in assembly of 2D porous MOFs. During the investigation on 2D network structures, we find that compounds 1−3 possess voids in the 2D network due to the 2-pytz molecule acting as both bridging linker and chelator. Contrastively, 3-pytz and 4-pytz molecules only act as bridging linkers inducing to the disappearing of voids in the 2D networks of compounds 4 and 5. More 2248



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hydrothermal conditions. At the lower pH values, the Ag+ ions, without hydrolysis and with higher reaction activity, are sufficient and lively to the coordination with others (eight unique silver ions for 2), which results in the ladder-like 2D MOFs in 2. With the pH value continuous increasing, the number and activity of free Ag+ ions may decrease (three unique Ag ions for 1), which results in the wave-like 2D MOFs in 1. Additionally, due to the protonation, the extent of coordination of POMs becomes a little weaker as the decrease of pH value. So the PMo12 anions act as hexa-dentate inorganic ligands in 1, while as tetra-dentate in 2. POM Species. Compounds 1 and 3 were synthesized under the same reaction conditions, except for the alternation types of polyoxoanions (PMo12 for 1 and SiW12 for 3). In 1 and 3, they all possess one crystallographically independent POM and three independent Ag ions. Furthermore, POMs also all act as hexa-dentate inorganic ligands, which are consistent with the above discussion results. Due to the changes of POMs, the nature of the whole reaction system may be altered correspondingly, such as the coordination modes of Ag ions, ligands and POMs. In compound 1, 2-pytz ligands act as tridentate chelate and bridging ligands to coordinate with the Ag centers to generate wave-like 2D MOFs with larger voids (12.72 × 6.97 Å3). Owing to the bigger dimensions of the PMo12 cluster (ca.10.36 × 10.36 Å), the PMo12 clusters act as pendent supports to connect the adjacent 2D MOFs. However, in compound 3, 2-pytz ligands act as tri- and tetra-dentate chelate and bridging ligands to coordinate with the Ag centers generating 2D MOFs with the larger voids (14.82 × 10.46 Å3). Because the spherical diameter of the SiW12 cluster (ca. 10.45 Å) is smaller than the voids, the SiW12 clusters as supports occupy the voids. FT-IR, XRPD Characterization. The IR spectra of compounds 1−5 are shown in Figure S9 (Supporting Information). Characteristic bands at 1057, 961, 869, and 786 cm−1 for 1, 1057, 948, 869, and 788 cm−1 for 2, 1048, 949, 861, and 783 cm−1 for 4 and 1053, 954, 875, and 785 cm−1 for 5 are attributed to ν (P−O), ν (MoO), and ν (Mo−O−Mo) vibrations, respectively. Characteristic bands at 962, 916, and 787 cm−1 for 3 are attributed to ν (WO), ν (Si−O), and ν (W−O−W) vibrations, respectively. Band in the regions of 1616−1158 cm−1 for 1, 1162−1619 cm−1 for 2, 1161−1621 cm−1 for 3 are attributed to the 2-pytz, 1178−1612 cm−1 for 4 are attributed to the 3-pytz and 1162−1623 cm−1 for 5 are attributed to the 4-pytz. The XRPD patterns for 1−5 are presented in the Figure S10 (Supporting Information). The diffraction peaks of both simulated and experimental patterns match well, thus indicating that the phase purities of the compounds 1−5 are good. The difference in reflection intensities between the simulated and the experimental patterns is due to the different orientation of the crystals in the powder samples. Luminescent Properties. Inorganic−organic hybrid compounds have been investigated for fluorescence properties and for potential applications as luminescent materials.14 Owing to the higher thermal stability of inorganic−organic hybrid compounds and the ability of affecting the emission wavelength of organic materials, syntheses of inorganic−organic hybrid compounds by the judicious choice of conjugated organic spacers and other centers can be an efficient method for obtaining new types of electroluminescent materials, especially comprising the d10 metal center and aromatic system. In the present work, photoluminescence properties of compounds 1−

3 were investigated in the solid state at room temperature. In order to understand the nature of the luminescence, the emission spectra of free ligand (2-pytz) were also investigated under identical experimental conditions (Figure 12). As shown

Figure 12. Solid-state emission spectra of 2-pytz and 1−3 at room temperature.

in Figure 12, a blue-fluorescent emission maximum at 470 nm was observed for free ligands upon excitation at 390 nm, and the emission bands are probably attributable to the π*→π transitions.15 Compounds 1−3 display two emission bands at ca. 440 nm and ca.470 nm when excitation occurs at 390 nm, which indicates that the emission of the free ligands were significantly affected by their incorporation into the Agcontaining hybrid compounds. The origin of the emission (ca. 440 nm) can be tentatively attributable to ligand-to-metal charge transfer (LMCT), and the enhanced emission (ca. 470 nm) in the same region as that of ligand can be tentatively assigned to the intraligand transition of the ligand, since a similar emission was observed for the free ligands. Since the compound is insoluble in water and common solvents such as ethanol, acetone, acetonitrile and benzene, it may be a good candidate for solvent-resistant green fluorescent material.

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CONCLUSIONS

Five new Keggin POM supporting MOF compounds with three kinds of smart pyridyltetrazoles ligands have been synthesized under hydrothermal conditions. These compounds display intriguing and versatile coordination features with isolated POM, pseudo POM chain, ture POM chain and pseudo POM layer supporting MOFs. The influences of the pH value, POM species and ligand isomers on the structures of the hybrid compounds have been investigated. The results indicate that the assembly of new POM supporting MOF hybrids with Ag ions depends on a synergic effect of polyoxoanion, ligands and pH value. The photoluminescent emissions show that these compounds may be good candidates for optical materials. This study will give impetus to the investigation of the POM supporting MOF system. It is believed that more POM supporting MOF compounds with interesting structures as well as properties will be synthesized in the future. 2249



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ASSOCIATED CONTENT

S Supporting Information *

Supplementary tables and figures. Crystallographic data and CCDC can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation (Grant Nos. 20901031 and 21072049), the Natural Science Foundation (No. B200916), and the Education Ministry key Teachers Foundation (1155G53).

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REFERENCES

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Syntheses Study of Keggin POM Supporting MOFs System - Crystal

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