Keggin POM-based 3D framework tuned by silver polymeric motifs:structural influences of tetrazolate functional groups{

Xiang Wang,ab Jun Peng,*a Kundawlet Alimajea and Zhen-Yu Shia

Received 25th July 2012, Accepted 28th September 2012

DOI: 10.1039/c2ce26193f

By altering the functional groups of tetrazolate ligands in the same Ag–POM reaction system, two

novel inorganic–organic hybrids: [Ag7(L1)4(H2O)(PMo12O40)] (1) and [Ag5(L2)2(PMo12O40)]?H2O (2)

(HL1 = 5-(4-imidazol-1-yl-phenyl)-2H-tetrazole, HL2 = 5-(4-triazol-1-ylmethyl-phenyl)-

2H-tetrazole), have been synthesized under hydrothermal conditions, which are characterized by

single crystal X-ray diffraction, IR spectra and elemental analyses. Compounds 1 and 2 both display

POM-based 3D frameworks modified by different Ag–tetrazolate motifs. In compound 1, a beautiful

POM-inserted 2D network is formed by {Ag6(L1)4}n chains and POM polyoxoanions, which are

linked to each other through covalent bonds into a (6,6,10)-connected 3D framework. The 3D

framework of compound 2 is constructed from chains of circle-connecting-circle and dimeric

polyoxoanions. The different structural features of the two compounds suggest that the functional

groups of tetrazolate ligands should play a key role in the process of assembly. The luminescent

properties for compounds 1 and 2 are investigated.

Introduction

Polyoxometalate (POM)-based organic–inorganic hybrid mate-

rials have received great attention in recent years because of their

intriguing architectures, functional properties and potential

applications in many fields, such as magnetism, catalysis,

photochemistry, gas storage and electrochemistry.1,2 POMs are

usually adopted as basic inorganic building blocks to prepare

organic–inorganic hybrid materials,3,4 among which the Keggin

POMs are extensively studied for constructing fascinating metal–

organic frameworks (MOFs), usually acting as linkers, counter

anions and templates.5,6

In POM-based MOFs, polyazoheteroaromatic molecules, such

as imidazole, triazole, tetrazole, pyrimidine, etc., have been

exploited as types of multidentate ligands,7 especially the

tetrazolate ligands,8,9 which can provide versatile connecting

modes with metal ions.10,11

In our previous research on constructing hybrid organic–

inorganic materials based on the system of POMs, pyridyl-

functionalized tetrazole and Ag, the influences of different

polyoxoanions on the framework structures have been studied.12

In this work, we will fix the Ag–POM hydrothermal systems and

focus on the functionalized tetrazole ligands to investigate the

influences of different functional groups on the architectures

of framework structures. We choose functionalized tetrazole

ligands, 5-(4-imidazol-1-yl-phenyl)-2H-tetrazole (HL1) and 5-(4-

triazol-1-ylmethylphenyl)-2H-tetrazole (HL2), to construct novel

POM-based MOFs in the Ag and H3PMo12O40 (abbreviated to

PMo12) hydrothermal systems. The different configurational

features of the two functional groups (Scheme 1), such as the

numbers of coordination sites, lengths, flexibility, and torsion

angles, may play an key role in inducing the formation of the

final structures. Furthermore, as far as we know, the hybrid

compounds based on HL1 and HL2 ligands in a metal–POM

system have not been reported up to now.

aKey Laboratory of Polyoxometalate Science of Ministry of Education,Faculty of Chemistry, Northeast Normal University, Changchun, Jilin,130024, P. R. China. E-mail: jpeng@nenu.edu.cnbDepartment of Chemistry, Bohai University, Jinzhou, 121000, P. R. China{ Electronic supplementary information (ESI) available: Table ofselected bond lengths and bond angles for compounds 1 and 2;Additional figures, IR, TG and PXRD analysis. CCDC 890707 for 1and 890708 for 2. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c2ce26193f Scheme 1 The L1 and L2 ligand anions.

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Fortunately, two new compounds, [Ag7(L1)4(H2O)(PMo12O40)]

(1) and [Ag5(L2)2(PMo12O40)]?H2O (2), have been obtained.

Compound 1 shows a 3D framework structure, which is extended

by POM-inserted 2D networks through covalent bonds.

Compound 2 also exhibits a 3D network, which is constructed

from chains of circle-connecting-circle and dimeric polyoxoa-

nions, linked to each other to form a 3D network structure. As

expected, the tetrazolate functional groups may play a crucial role

in constructing distinct frameworks. Luminescent properties for

compounds 1 and 2 are also studied.

Experimental section

Materials and general methods

All reagents were purchased commercially and used without

further purification. Elemental analyses (C, H and N) were

performed on a Perkin-Elmer 2400 CHN elemental analyzer. P,

Mo and Ag were determined by a Leaman inductively-coupled

plasma (ICP) spectrometer. The IR spectra were carried out on

Alpha Centaurt FT-IR spectrometer. The thermal gravimetric

analyses (TGA) were obtained on a Perkin-Elmer DTA 1700

differential thermal analyzer with a rate of 10.00 uC min21

under nitrogen gas. The powder X-ray diffraction (PXRD)

data was collected on a Rigaku RINT2000 diffractometer at

room temperature. The photoluminescence analysis was carried

out on an Edinburgh Fluorescence spectrometer at room

temperature.

Synthesis of [Ag7(L1)4(H2O)(PMo12O40)] (1)

A mixture of H3PMo12O40?12H2O (0.3 g, 0.15 mmol), AgNO3

(0.17 g, 1 mmol), HL1 (0.063 g, 0.3 mmol) and 10 mL water, was

stirred for 1 h. The resulting solution was transferred to a Teflon-

lined autoclave and kept under autogenous pressure at 170 uCfor 3 days. After slow cooling to room temperature, red block

crystals of 1 were filtered, washed with distilled water and dried

at room temperature (Yield, 55%, based on Ag). Anal. Calcd for

C40H30Ag7N24O41PMo12 (3440.21): C 13.97, H 0.88, N 9.77, P

0.90, Mo 33.47, Ag 21.95. Found: C 14.08, H 0.95, N 9.68, P

0.86, Mo 33.41, Ag 21.89. IR (solid KBr pellet, cm21): 3441 (w),

3129 (s), 1612 (s), 1503 (s), 1450 (s), 1062 (s), 957 (s), 876 (s), 797

(s).

Synthesis of [Ag5(L2)2(PMo12O40)]?H2O (2)

The synthetic method was similar to that of compound 1, except

that HL2 (0.066 g, 0.3 mmol) was used instead of HL1. Red block

crystals of 2 were filtered, washed with distilled water and dried

at room temperature (Yield, 45%, based on Ag). Anal. Calcd for

C20H18Ag5N14O41PMo12 (2832.07): C 8.48, H 0.64, N 6.92, P

1.09, Mo 40.65, Ag 19.04. Found: C 8.54, H 0.71, N 6.87, P 1.02,

Mo 40.59, Ag 18.98. IR (solid KBr pellet, cm21): 3444 (w), 3121

(s), 1620 (s), 1530 (s), 1430 (s), 1064 (s), 957 (s), 866 (s), 797 (s).

X-ray crystallographic study

X-ray diffraction analysis data for compounds 1 and 2 were

collected with an Oxford Diffraction Gemini R Ultra diffract-

ometer with graphite-monochromated Mo Ka (l = 0.71073 A) at

293 K. All structures were solved by direct methods and refined

on F2 by full-matrix least-squares methods using the SHELXTL

package.13 The hydrogen atoms attached to water molecules

were not located, but were included in the structure factor

calculations. A summary of the crystal data and structure

refinements of compounds 1 and 2 are provided in Table 1.

Selected bond lengths and angles are listed in Table S1, ESI.{

Table 1 Crystal data and structure refinements for compounds 1 and 2

Compound 1 2

Chemical formula C40H30Ag7Mo12N24O41P C20H18Ag5Mo12N14O41PFW 3438.21 2830.07Crystal system Triclinic Triclinica/A 12.290(5) 12.453(5)b/A 13.700(5) 13.018(5)c/A 13.798(5) 17.071(5)a/u 108.736(5) 95.864(5)b/u 92.793(5) 91.454(5)c/u 114.369(5) 100.003(5)V/A3 1959.3(13) 2708.5(17)Temperature/K 293(2) 293(2)Space group P1 P1Z 1 2m (mm21) 3.675 4.582Dc (g cm23) 2.914 3.47F(000) 1612 2632Final R1

a,wR2

b [I . 2s(I)]0.0714, 0.1442 0.0565, 0.1549

Final R1a,

wR2b (all data)

0.1081, 0.1646 0.0721, 0.1694

Goodness on F2 1.086 1.072a R1 = gDDFoD 2 DFcDD/gDFoD.

b wR2 = [gw(Fo2 2 Fc

2)2/g[w(Fo2)2]1/2.

Fig. 1 (a) The coordination modes of Ag ions and ligands in compound

1. (b) The coordination environment of PMo12 polyoxoanion. The

hydrogen atoms are omitted for clarity. Symmetry codes: #1 2 2 x, 1 2

y, 2 2 z; #2 1 2 x, 21 2 y, 1 2 z; #3 21 + x, 22 + y, 21 + z; #4 2 2 x,

21 2 y, 1 2 z; #5 21 + x, y, z; #6 1 2 x, 1 2 y, 2 2 z; #7 1 2 x, 2y, 1 2

z.

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Crystallographic data of compounds 1 and 2 have been

deposited in the Cambridge Crystallographic Data Center, and

CCDC numbers are 890707 for 1, 890708 for 2.

Results and discussion

Description of the crystal structures

Crystal structure of compound 1. X-ray crystal structure

analysis reveals that compound 1 consists of one PMo12

polyoxoanion, four L1 ligands, seven silver ions (disordered

Ag4 ion over two positions with half-occupancy), and one

disordered water molecule. In the PMo12 polyoxoanion, the Mo–

O and P–O bond distances are in the normal ranges,14 and the

central four m4-oxygen atoms are disordered over eight positions

with each oxygen site half-occupied, which is a common problem

for POM clusters.15 Bond valence sum calculations show that all

molybdenum atoms are in the +VI oxidation state, and silver

atoms are in the +I oxidation state.16

In the structure of compound 1, there are four crystal-

lographically independent Ag ions with three kinds of coordina-

tion modes (Fig. 1a). Ag1 ion is four-coordinated in a ‘‘seesaw’’

geometry by two O atoms (O10, O13) from two PMo12

polyoxoanions and two N atoms (N10, N11) from two L1

ligands. Ag2 ion is three-coordinated in a ‘‘Y-shape’’ geometry

by three N atoms (N6, N8, N12) from two L1 ligands. Ag3 ion is

coordinated in a distorted tetrahedron geometry by three N

atoms (N1, N2, N7) from three L1 ligands and one oxygen atom

(O18) from the PMo12 polyoxoanion. Ag4 ion is four-coordi-

nated by two N atom (N9, N5) from two L1 ligands, one oxygen

atom (O16) from the PMo12 polyoxoanion and one water

molecule. The PMo12 polyoxoanion acts as an octa-dentate

inorganic ligand to link eight silver ions (Fig. 1b). The bond

lengths and angles are listed in Table S1, ESI.{A structural feature of compound 1 is that the 3D framework

structure is constructed from two moieties: {Ag6(L1)4}n chains

and POM polyoxoanions. In detail, six Ag ions (two Ag2, two

Ag3, two Ag4 ions) are aggregated by four tetrazolyl groups,

forming a hexa-nuclear subunit. The hexa-nuclear subunits are

linked together via Ag3–N2 bonds, forming an infinite

{Ag6(L1)4}n chain. Furthermore, these chains are bridged via

double-bridge Ag1 ions coordinated by N10 and N11 atoms of

the remaining imidazolyl groups from adjacent chains to form a

beautiful 2D network with ca. 15.75 6 12.29 A circles, which are

big enough to accommodate PMo12 polyoxoanions (Fig. 2a).

Accordingly, the PMo12 polyoxoanions are incorporated into

these circles as hexa-dentate inorganic ligands, utilizing four

terminal and two bridging oxygen atoms to link six Ag ions (four

Ag1 and two Ag4 ions) of the circles (Fig. 2b). Considering the

PMo12 polyoxoanions, the double-bridge Ag1 ions, and the

hexa-nuclear subunits as four, six, and eight-connected nodes,

respectively, the 2D network can be considered as a (4,6,8)-

connected network structure (Fig. 2c).

Fig. 2 (a) View of the 2D network in compound 1. (b) View of the POM clusters inserted into the 2D network. The hydrogen atoms are omitted for

clarity. (c) Topology of the 2D network in compound 1. The green, orange and blue nodes represent the PMo12 polyoxoanions, the double-bridge Ag1

ions, and the hexa-nuclear subunits, respectively.

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The 3D framework structure of compound 1 is constructed

from the 2D networks through covalently connected PMo12

polyoxoanions. Each PMo12 polyoxoanion from a network

offers two terminal oxygen atoms (O18) to coordinate to Ag3

ions from two neighboring networks (Fig. 3a). Thus, the PMo12

polyoxoanion has reached its highest coordination number of

eight in the Keggin POM-based MOFs up to now.9a Considering

the PMo12 polyoxoanions and the double-bridge Ag1 ions as six-

connected nodes respectively, the hexa-nuclear subunits as ten-

connected nodes, as a result, the overall structure of compound 1

is a (6,6,10)-connected 3D framework with (36?46?53)(34?47?54)

(38?412?520?65) topology (Fig. 3b).

Crystal structure of compound 2. Crystal structure analysis

reveals that compound 2 consists of five silver ions, two L2

ligands, one PMo12 polyoxoanion, and one crystallization water

molecule (Fig. 4).

In compound 2, there are five crystallographically independent

Ag ions. Ag1 ion is linearly coordinated by two N atoms (N2,

N10) from two L2 ligands. Ag2, Ag3, Ag4, and Ag5 ions are

four-coordinated in a ‘‘distorted seesaw’’ geometry, among

which Ag2, Ag3 and Ag4 ions are coordinated by two O atoms

and two N atoms from different PMo12 polyoxoanions and L2

ligands, respectively: O32, O39, N13 and N14 for Ag2 ion; O5,

O17, N1 and N9 for Ag3 ion; O21, O31, N4 and N7 for Ag4 ion.

Ag5 ion is coordinated by three O atoms (O9, O16, O20) from

different PMo12 polyoxoanions and one N atom (N8) from a L2

ligand. The corresponding bond lengths and angles are given in

Table S1, ESI.{ The basic motifs of the 3D structure in compound 2 are a

dimeric polyoxoanion linkage and an infinite chain of circle-

connecting-circles. The POM dimer is formed from two PMo12

polyxoxanions bridged by two Ag5 ions, each of which is

coordinated by three O atoms of polyoxoanions (Fig. 5a). The

chain consists of two kinds of {Ag2(L2)2} square circles: A- and

B-circles. The A-circle is formed by two Ag4 (or Ag2) ions and

two L2 ligands which are bridged in a head-to-tail mode. The

B-circle is generated from the connection of two A-circles via two

Ag ions (Ag1 and Ag3). Then the two kinds of circles are

extended to a channel-like chain (Fig. 5b). Furthermore, the

chains are connected by the dimeric polyoxoanions to generate a

2D layer (Fig. 5c), in which each dimeric polyoxoanion provides

Ag5 ions and three oxygen atoms to link two N atoms (N8) and

three Ag ions (Ag2, Ag3 and Ag4) to fuse neighboring chains,

respectively.

Finally, each PMo12 polyoxoanion in a layer offers three

terminal oxygen atoms to connect three Ag ions (Ag2, Ag3 and

Ag4) from two neighboring layers, establishing its eight-

coordination number, to extend the layers into a 3D framework

structure (Fig. 6).

Influences of tetrazolate functional groups on the structures. It is

worth noting that compounds 1 and 2 are formed in the same Ag–

POM system; the only difference is that the tetrazolate ligands

have different functional groups. The functional group (4-

imidazol-1-ylphenyl) of the HL1 ligand exhibits a rigid feature,

which results in the formation of a 2D network composed of

circles, and the dimension (ca. 15.75 6 12.29 A) of the circles are

large enough to accommodate the PMo12 polyoxoanions. While

the functional group (4-triazol-1-ylmethylphenyl) of HL2 makes

Fig. 3 (a) View of the 3D structure of compound 1. The hydrogen

atoms are omitted for clarity. (b) Schematic view of the 3D framework

structure of compound 1. The green, orange and blue nodes represent the

PMo12 polyoxoanions, the double-bridge Ag1 ions, and the hexa-nuclear

subunits, respectively.

Fig. 4 Ball/stick/polyhedral view of the asymmetric unit of compound

2. The hydrogen atoms and crystallization water molecule are omitted for

clarity. Symmetric codes: #1 1 2 x, 1 2 y, 21 2 z; #2 1 + x, 1 + y, 21 +

z; #3 1 2 x, 2 2 y, 2z; #4 1 2 x, 2 2 y, 21 2 z; #5 2 2 x, 3 2 y, 21 2 z.

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the ligand exhibit an ‘‘L-type’’ configuration due to the existence

of the methylene tether (the torsion angle of C–C–N is ca. 114u)(Fig. S1, ESI{). The ‘‘L-type’’ configuration is favorable for the

formation of closed square circles. The dimensions (ca. 8.3 68.2 A and 7.8 6 6.1 A) of such formed square circles are too small

to envelop the POM clusters, thus the PMo12 polyoxoanions are

arranged in pairs on the two side of the channel-like chains of

circle-connecting-circles (Fig. S2, ESI{). It suggests that the

features of the functional groups of tetrazolate ligands may result

in different silver polymeric motifs and different connection

modes of the polyoxoanions in the assembly processes.

FT-IR spectra, TG analyses and powder X-ray diffraction

(PXRD). The IR spectra of compounds 1 and 2 are shown in Fig.

S3, ESI.{ For compound 1, the bands at 1062, 957, 876 and 798

cm21 could be ascribed to the characteristic peaks of nas(P–Oa),

nas(Mo–Od), nas(Mo–Ob–Mo) and nas(Mo–Oc–Mo).17 The bands

in the region of 3441 to 1306 cm21 could be ascribed to the

characteristic peaks of the L1 ligand. For compound 2, the

character bands at 1064, 957, 866 and 797 cm21 are attributed to

the nas(P–Oa), nas(Mo–Od), nas(Mo–Ob–Mo) and nas(Mo–Oc–

Mo) bands. The bands in the region of 3444 to 1303 cm21 could

be ascribed to the characteristic peaks of the L2 ligand.

The TG measurement of compounds 1 and 2 also supports the

chemical composition. As shown in Fig. S4, ESI,{ both curves

show one-step weight loss processes. The weight loss step in the

range of 300–700 uC is ascribed to the decomposition of water

molecules and organic ligands, 24.8% (calcd 25.1%) for 1; 15.8%

(calcd 16.6%) for 2.

To indicate the phase purities of compounds 1 and 2, PXRD

experiments were carried out. As shown in Fig. S5, ESI,{ the

diffraction peaks of both simulated and experimental patterns

match well in the key positions, which indicates that the phase

purities of the two compounds are good.

Photoluminescent properties. It is well known that the

inorganic–organic hybrid compounds, especially those composed

of a d10 metal center and aromatic ligands, can exhibit strong

photoluminescence properties.1f,18 In this work, the photolumi-

nescence properties of compounds 1 and 2 are investigated, as

well as HL1 and HL2 ligands for comparison, in the solid state at

room temperature. As shown in Fig. 7, it can be observed that

the HL1 and HL2 ligands shows intense emission peaks at ca. 503

nm upon excitation at 251 nm for HL1, 453 nm upon excitation

at 278 nm for HL2, which can be assigned to an intraligand (p–

p*) charge-transfer.19 However, the emission peaks of com-

pounds 1 and 2 are found at ca. 404 and 391 nm upon excitation

at 250 nm for 1, and at 264 nm for 2, which may be attributed to

ligand-to-metal charge transfer (LMCT) as reported for the d10

Fig. 5 (a) View of the dimeric polyoxoanion linkage. (b) Ball/stick view of the channel-like chain of circle-connecting-circles in compound 2. (c) View

of the 2D layer of compound 2. The hydrogen atoms are omitted for clarity.

Fig. 6 View of the 3D structure of compound 2. The hydrogen atoms

are omitted for clarity.

Fig. 7 The emission spectra of HL1, HL2, compound 1 and compound

2 at room temperature.

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metal complexes with organic ligands previously.20 The sig-

nificant blue-shift compared to the free ligands may be due to the

coordination of ligands with Ag ions. These observations suggest

that compounds 1 and 2 could be anticipated as potential

fluorescent materials.

Conclusions

In a word, by changing the functional groups of tetrazolate

ligands, two distinct POM-based MOFs are formed from the

same Ag–POM system. Compound 1 shows a (6,6,10)-connected

3D framework constructed from 2D networks and polyoxoa-

nions through covalent bonds. Compound 2 also displays a 3D

framework generated by the chains of circle-connecting-circles

and dimeric polyoxoanions. The results of structural compar-

isons indicate that the tetrazolate functional groups should play

a key role in inducing MOFs. Furthermore, the emission spectra

of two compounds suggest their potential applications in the

fields of solid luminescent materials.

Acknowledgements

This work is financially supported by the National Natural

Science Foundation of China (21071029).

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8514 | CrystEngComm, 2012, 14, 8509–8514 This journal is � The Royal Society of Chemistry 2012

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