ISSN 1144-0546

www.rsc.org/njc Volume 36 | Number 4 | April 2012 | Pages 845–1122

COVER ARTICLESong, Zhang et al.A self-penetrating network derived from two-fold interpenetrating nets via ligand-unsupported Ag–Ag bonds

New Journal of Chemistry A journal for new directions in chemistry

1144-0546(2012)36:4;1-G

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 877–882 877

Cite this: New J. Chem., 2012, 36, 877–882

An unusual three-dimensional self-penetrating network derived from

cross-linking of two-fold interpenetrating nets via ligand-unsupported

Ag–Ag bonds: synthesis, structure, luminescence, and theoretical studyw

Xue-Zhi Song,ab Chao Qin,a Wei Guan,a Shu-Yan Song*a and Hong-Jie Zhang*a

Received (in Victoria, Australia) 16th October 2011, Accepted 14th December 2011

DOI: 10.1039/c2nj20888a

A novel coordination polymer Ag4L2(4,40-bpy)(H2O)2�6H2O (1) (H2L = bicyclo[2.2.2]oct-7-ene-

2,3,5,6-tetracarboxydiimide, 4,40-bpy = 4,40-bipyridine) has been synthesized and characterized by

IR spectroscopy, elemental analysis, X-ray powder diffraction and single-crystal X-ray diffraction.

Compound 1 exhibits a two-fold interpenetrated array. Remarkably, adjacent sets of 2D

interpenetrating sheets are ultimately cross-linked into an unusual three-dimensional self-

penetrating network when ligand-unsupported Ag–Ag bonds are taken into account. Theoretical

computational methods were applied to further identify argentophilic interactions in this

structure. Additionally, thermal stability analysis and luminescent property of this compound are

also studied in this paper.

Introduction

The interest in entangled systems is rapidly increasing not only

for their potential applications as functional materials1 but

also for their aesthetic and often complicated architectures and

topologies.2 It is well-known that catenanes, rotaxanes, and

molecular knots occupy important positions in the area

of molecular entanglement.3 Interpenetrating networks can be

described as polymeric equivalents of catenanes and rotaxanes.

They, once rarities, are now becoming increasingly common, aided

by the rapid growth of network-based crystal engineering,4 and

many beautiful structures have been constructed and well-discussed

in comprehensive reviews.5More recently, a complete analysis of all

3D interpenetrated metal–organic framework structures contained

in the CSD database has been proposed with a rationalization

and classification of the topology of interpenetration.6 However,

the study of self-penetrating (self-entangled or polyknotting)

networks, in contrast to the extensively studied interpenetrating

nets, is still in its infancy as evidenced in a recent review by Ciani

and co-workers.7 From a rigorous point of view, these structures

are single nets having the peculiarity that the smallest topological

rings are catenated by other rings belonging to the same net.

On a rather intuitive basis, they can be considered as derived

from n-fold interpenetrating networks that are further linked

through additional bridges. Whichever standard is adopted,

this feature is not very common within coordination polymers

and only a limited number of self-penetrated nets have been

reported to date,8 hence there is still a long way to go with this

class. Furthermore self-penetrating nets impart excellent thermal

stability and stability to solvents, vapours and gases that make

them good candidates as porous materials.9

An organic molecule, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetra-

carboxylic dianhydride, can hydrolyze into bicyclo[2.2.2]oct-7-

ene-2,3,5,6-tetracarboxylic acid (H4L0, Scheme 1, top), which

has proved to be a good ligand to assemble metal ions into

extended metal–organic frameworks.10 Likewise, an analogue

bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxydiimide, prepared by

the ammonolysis of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic

dianhydride, which contains two kinds of possible coordination

donors (N and O) to ligate the metal atoms (H2L, Scheme 1,

bottom), should be able to be used as a ligand, however, a search

in the Cambridge Structural Database reveals that none of

metal–organic frameworks with this ligand have been reported

up to now.

Herein, we report a two-fold interpenetrating coordination

polymer by simultaneous use of the H2L ligand and the rigid

4,40-bpy ligand, whose formula is given as Ag4L2(4,40-

bpy)(H2O)2�6H2O (1). Of particular interest is that adjacent sets

of twofold interpenetrating sheets are further cross-linked by ligand-

unsupported Ag–Ag interactions into a single self-penetrating net

withmbc topology. To the best of our knowledge, such a structural

feature has rarely been observed in reported complexes.Meanwhile,

natural bond orbital (NBO) calculation results also show that

argentophilic interactions exist in this structure.

a State Key Laboratory of Rare Earth Resource Utilization,Changchun Institute of Applied Chemistry, Chinese Academy ofSciences, Changchun, 130022, PR China. E-mail: songsy@ciac.jl.cn,hongjie@ciac.jl.cn; Fax: +86-431-85698041;Tel: +86-431-85262127

bGraduate School of the Chinese Academy of Sciences, Beijing,100039, P. R. China

w Electronic supplementary information (ESI) available: TG curve, IRspectrum, and PXRD pattern for compound 1. CCDC referencenumbers 794548 (1) and 794549 (ligand). For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c2nj20888a

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878 New J. Chem., 2012, 36, 877–882 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

Results and discussion

Crystal structure

Crystals of 1 were obtained in moderate yield, by treatment of

Ag2CO3, H2L, 4,40-bpy, NH3�H2O and H2O under ambient

conditions. The phase purity of bulk products was confirmed

by X-ray powder diffraction (XRPD, Fig. S1, ESIw). Single-crystal X-ray diffraction analysis indicates that the asymmetric

unit contains five crystallographically unique silver atoms, two

L ligands, one 4,40-bpy ligand, two coordinated water molecules

and six lattice water molecules.

As shown in Fig. 1, Ag1 displays a distorted tetrahedral

geometry, defined by two oxygen atoms from two L ligands

and two nitrogen atoms from two 4,40-bpy ligands (Ag–O

2.684(5) and 2.861(5) A, Ag–N 2.142(5) and 2.147(5) A), Ag2

and Ag5 adopt a linear geometry defined by two nitrogen

atoms from two L ligands (Ag–N 2.074(4) and 2.085(5) A),

whereas Ag3 and Ag4 lie on inversion centers, and are linearly

coordinated to two aqua ligands (Ag–Oaqua 2.100(5) and

2.103(5) A). The L ligand presents two cyclic imide groups

attached to a rigid, bicyclic core containing a cyclohexane ring

subunit in the boat conformation and coordinates two AgI atoms

(Ag1 and Ag2) with N and O donors in one end as well as the

third AgI atom (Ag5) with another N donor in the other end.

The Ag1 and Ag2 atoms are first ligated by two oxygen

atoms and two nitrogen atoms of L ligands to form a binuclear

silver unit with a close interdimer Ag–Ag contact

(Ag1–Ag2 2.993 A). The intramolecular Ag–Ag separation is

shorter than the sum of the van der Waals radii of two silver

atoms (3.44 A),11 which indicates relatively strong intra-

molecular argentophilicity in 1. And then these binuclear silver

units are further linked to Ag5 atoms via L ligands to generate

a distinctive mono- and binuclear mixed one-dimensional

chain along the [1 0 2] direction; the adjacent chains are

ultimately pillared via 4,40-bpy ligands along the [1 0 �2]direction, thus forming a two-dimensional puckered sheet

(Fig. 2a). The corrugations in the sheets which make the

interpenetration possible arise from a combination of a distinctly

nonplanar, tetrahedral geometry at the silver centers and the boat

conformation at the cyclohexane ring of ligand L. If one considers

the baricentres of binuclear units to be four-connected nodes, the

single sheet can then be rationalized as a 2D net with (4,4)

topology. Two such independent and identical (4,4) nets then

interpenetrate in a parallel fashion to give a 2D two-fold inter-

penetrating net (Fig. 2b). Notably, even with this interpenetration,

the framework still possesses free void space that is occupied

by free water molecules. PLATON12 analysis showed that the

effective free volume of 1 is 17.2% of the crystal volume (708.2 A3

out of the 4123.5 A3 unit cell volume) after lattice water molecules

have been hypothetically removed. To clarify the structural

integrity after removal of guest molecules, we carried out PXRD

experiment for the heated sample. As shown in Fig. S1 (ESIw), thePXRD pattern of the dehydrated sample (heated at 90 1C under

vacuum for 5 h) is consistent with that of the as-synthesized

sample, suggesting that crystallinity is retained even without the

lattice water molecules.

{Ag3(H2O)2} and {Ag4(H2O)2} act as isolated segments and

are located in the interspaces of two adjacent interpenetrated

layers. Whilst exploring the acting forces between the isolated

{Ag(H2O)2} fragments and neighboring sets of a two-fold

interpenetrating net, we found that there exist significant

ligand-unsupported Ag–Ag interactions, whose distances are

3.087 A for Ag3–Ag5 and 3.114 A for Ag4–Ag5. The Ag–Ag

distances, comparable to the reported ligand-unsupported

Ag–Ag distances in the coordination polymers,13 are shorter

than the sum of the van der Waals radii of two silver atoms

(3.44 A), suggesting significant Ag–Ag bonding. It is also

noteworthy that strong p–p stacking interactions (face-to-face

distance: 3.55 A, center-to-center distance: 3.74 A) arising

from the imide rings of different layers may be partly responsible

for the inter-network silver–silver interactions. Quite intriguingly,

when these Ag–Ag bonds are taken into account, adjacent sets of

2D interpenetrating sheets are ultimately cross-linked into a

three-dimensional overall network (Fig. 3). For any n-fold

interpenetrated net, it is always true that if the extra bonds

that could connect all the n-fold interpenetrated nets together

are added, a single self-penetrating net will be obtained.

Therefore, the resultant framework of 1 is clearly a self-

penetrating net and can be considered as derived from cross-

linking of two-fold interpenetrating nets via ligand-unsupported

Ag–Ag bonds. A topological analysis of this 3D net was

performed with the TOPOS program,14 which reveals a binodal

four-connected mbc-type topology.15 The vertex symbol is

[6.6.6.6.6(2).8(3)][6.6.6.6.6(2).8], which gives the point symbol

658. Catenated hexagonal chair-like circuits are observed in the

Scheme 1 The tetracarboxylic acid H4L0 (top) and the H2L ligand

used herein (bottom).

Fig. 1 ORTEP diagrams showing the coordination environments of

the five unique silver atoms. Symmetry codes: A = 1/2 + x,1/2

� y,�1/2 + z. B = �1 + x,y,�1 + z.

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 877–882 879

interlayer connections (Fig. 3), so that the requirements for

self-penetration are fulfilled, since the catenated 6-rings are

shortest rings.

Extensive attention has been focused on the attractive

interactions between formally closed-shell (such as d10 or s2)

metal centers,16 in which silver–silver interactions are defined

as argentophilicity and found to be relatively weak.17 Among

the reported examples of argentophilic interactions, most are

ligand-supported,18 while only a handful are revealed to be

ligand-unsupported, although the ligand-unsupported d10–d10

metal–metal attractions proved to be potential elements in the

construction of fascinating structures.19 For example, Yaghi’s

group reported a three-fold interpenetrating net derived from

cross-linking chains via ligand-unsupported Ag–Ag bonds.19a

In 1999, Chen and co-workers reported two four-connected

self-penetrating species with ligand-unsupported Ag–Ag bonds.19d

In 2006, Guo et al. reported a class of 1-D to 3-D architectures

constructed through ligand-unsupported argentophilic inter-

actions.19f What we described here, to the best of our knowledge,

is the second self-penetrating example by virtue of argentophilicity.

Theoretical study

To further interpret two kinds of AgI–AgI closed-shell inter-

actions in the present complex, we used the model complexes

[Ag2(C4NH4O2)2] and [Ag(H2O)2]2[Ag(C4NH4O2)2]2 (Fig. 4) in

all the calculations. A natural bond orbital (NBO) calculation

at the B3LYP level was performed through the use of the

Gaussian 03 suite of programs20 based on the experimental

X-ray data. The basis set used for C, H, O, and N is the

standard Gaussian basis set 6-31G(d), in which one set of

d-polarization functions is included. For the Ag transition

metal, the Stuttgart/Dresden effective core potentials basis set

(SDD)21 was used.

Tables 1 and 2 list selected natural bond orbitals, occupancy,

orbital coefficients and hybrids, and the orbital types of model

complexes [Ag2(C4NH4O2)2] and [Ag(H2O)2]2[Ag(C4NH4O2)2]2.

Taking [Ag2(C4NH4O2)2] for example, NBO analysis reveals that

Fig. 2 A view of a 2D puckered sheet with the (4,4) topology (a) and a space-filling diagram of the two parallel interpenetrating (4,4) nets (b).

Fig. 3 A schematic view of the 3D self-penetrating net (left) in which adjacent sets of parallel interpenetrating (4,4) nets are joined together

through ligand-unsupported Ag–Ag bonds (right).

Fig. 4 Schematic structure of the model complexes [Ag2(C4NH4O2)2]

and [Ag(H2O)2]2[Ag(C4NH4O2)2]2 used in the NBO calculations.

Symmetry code: * = 1�x,�y,�z.

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880 New J. Chem., 2012, 36, 877–882 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

the Ag–N single s bond is formed by a Ag (sp1.19d0.16) orbital

and a N (sp2.86) orbital. The [Ag(H2O)2]2[Ag(C4NH4O2)2]2also possesses similar bond character. Furthermore, NBO

results show weak Ag–Ag interactions based on the Wiberg

bond indexes. Ag–Ag bond orders of [Ag2(C4NH4O2)2] and

[Ag(H2O)2]2[Ag(C4NH4O2)2]2 are 0.16 and 0.12, respectively,

which are consistent with their bond-lengths. Ag–Ag distances

in [Ag2(C4NH4O2)2] and [Ag(H2O)2]2[Ag(C4NH4O2)2]2 are

2.99 and 3.11 A, respectively.

Thermal stability analysis

The TG curve of 1 shows three continuous weight loss steps

(Fig. S2, ESIw). The first step (11.28%) in the temperature

range ca. 30–210 1C corresponds to the loss of free and

coordinated water molecules (calcd. 11.80%). The second step

(11.52%) in the temperature range ca. 210–340 1C corresponds to

the loss of the 4,40-bpy ligand (calcd. 12.80%). The remaining

weight of 37.34% indicates that the final product is Ag2O (calcd.

37.98%). It can be observed that the result of the TG analysis

basically agrees with that of the structure determination. An

endothermic peak followed by an exothermic one in the differential

scanning calorimetry (DSC) curve of this compound revealed that

this self-penetrating net was thermodynamic (Fig. S2, ESIw). Thisphenomenon was also verified by the variable XRD patterns of the

as-synthesized samples treated at 120 1C under vacuum for 3 h

(Fig. S1, ESIw).

Luminescent property

The emission spectrum of compound 1 in the solid state at

room temperature is depicted in Fig. 5. It can be observed that

an intense emission occurs at 390 nm upon excitation at 246 nm.

As the luminescent properties of the organic components are

very weak under the same experimental conditions, the intense

luminescence in compound 1 may be attributable to the linear

Ag–Ag interactions. Similar emission bands have also been

observed previously for other silver(I) compounds with Ag–Ag

interactions,22 which indicates that Ag–Ag interactions are

significant for the formation of luminescent species.

Conclusions

In conclusion, we have prepared and characterized a new 3D

self-penetrating coordination net derived from cross-linking of two-

fold interpenetrating nets via ligand-unsupported Ag–Ag bonds.

Our structural investigation combined with NBO calculation

demonstrates that argentophilic interactions do play an essential

role in the formation of the resultant self-penetrating topology.

The successful isolation of this species not only provides an

intriguing example for an entangled system but also provides a

rational route to generating high-dimensional frameworks by

using Ag-containing species. Further research is ongoing to

prepare novel polymeric frameworks using the H2L ligand and

explore their valuable properties.

Experimental

General information

All chemicals purchased were of reagent grade and used

without further purification. Elemental analyses (C, H, N)

were performed on a Perkin-Elmer 2400 CHN elemental

analyzer. The content of Ag was determined by a Leeman

inductively coupled plasma (ICP) spectrometer. Powder

X-Ray diffraction data were collected on a Bruker D8-AD-

VANCE diffractometer equipped with Cu Ka at a scan speed

of 51 min�1. IR spectra were recorded within the 4000–400 cm�1

wavenumber range using a Bruker TENSOR 27 Fourier Trans-

form Infrared Spectrometer with the KBr pellet technique and

operating in the transmittance mode. Thermogravimetric analysis

(TGA) and differential scanning calorimetry (DSC) were

performed on a Perkin-Elmer TGA7 instrument and DSC7

instrument, respectively, in flowing N2 at a heating rate of

10 1C min�1. 1H NMR spectra were recorded on a Bruker

AVANCE spectrometer (600 MHz) with TMS as the internal

standard. The fluorescence excitation and emission spectra

were recorded at room temperature with a Hitachi F-4500

spectrophotometer equipped with a 150W Xenon lamp as an

excitation source.

Synthesis of H2L

Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (0.248 g,

1 mmol) was dispersed in 10 mL of water. Then, 1 mL 28%

NH3�H2O was quickly added to the above solution under

vigorous stirring. The transparent reaction mixture appeared

immediately and then was left to slowly evaporate at room

Table 1 The selected natural bond orbitals, occupancy, orbitalcoefficients and hybrids, and orbital type of [Ag2(C4NH4O2)2]

NBOs Occupancy Orbital coefficients and hybrids Orbital type

Ag1–N1 1.88 0.30 (sp1.19d0.16)Ag + 0.95 (sp2.86)N sAg1–N2 1.88 0.31 (sp1.14d0.15)Ag + 0.95 (sp2.87)N s

Table 2 The selected natural bond orbitals, occupancy, orbitalcoefficients and hybrids, and orbital type of[Ag(H2O)2]2[Ag(C4NH4O2)2]2

NBOs Occupancy Orbital coefficients and hybrids Orbital type

Ag1–N1 1.88 0.31 (sp1.13d0.14)Ag + 0.95 (sp2.78)N sAg1–N2 1.88 0.31 (sp1.16d0.15)Ag + 0.95 (sp2.59)N s

Fig. 5 Solid-state excitation and emission spectra for 1 at room

temperature.

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 877–882 881

temperature. Colorless crystals of H2L were obtained after two

weeks. 1H NMR (600 MHz, d6-DMSO): d 11.14 (s, 2H, NH),

6.13 (m, 2H, CHQCH), 3.27 (s, 2H, CHCHQCHCH), 3.05

(s, 4H, CHCQO); anal. calcd (%) for C12H10N2O4: C, 58.54;

H, 4.09; N, 11.38%. Found: C, 58.62; H, 4.15; N, 11.30%.

Synthesis of Ag4L2(4,40-bpy)(H2O)2�6H2O (1)

A mixture of Ag2CO3 (0.055 g, 0.20 mmol), H2L (0.049 g,

0.20 mmol) and 4,40-bpy�2H2O (0.019 g, 0.10 mmol) was

placed in 20 mL water. About 0.5 mL NH3�H2O was added

dropwise into the mixture under vigorous stirring. Twenty

minutes later, the solution was filtered and left undisturbed at

room temperature to allow for slow evaporation in the dark.

Colorless prismatic crystals of 1 were obtained after two weeks

as a pure phase. Crystals of 1 were collected and washed with

distilled water and ethanol, then dried under ambient conditions.

The quantity of compound 1 was 0.0697 g with a yield of 56.7%

based on Ag2CO3. A great deal of parallel experiments indicated

that the ratios of the starting material have no effect on the

isolation of the aimed product, except that the yields are some-

what different. The ratio given herein is the optimal condition.

Anal. calcd (%) for C34H40N6O16Ag4: C, 33.47; H, 3.30; N, 6.89;

Ag, 35.36%. Found: C, 33.35; H, 3.43; N, 6.75; Ag, 35.45%.

X-Ray crystallography

Single crystals of compound 1 and ligand H2L were glued on a

glass fiber. Data collection was performed on a Bruker Smart

Apex II CCD diffractometer with graphite-monochromated

Mo Ka radiation (l = 0.71073 A) at low temperature.

Empirical absorption correction was applied. The structures

were solved by the direct method and refined by the full-matrix

least-squares method on F2 using the SHELXTL crystallo-

graphic software package.23 Anisotropic displacement parameters

were applied to all non-hydrogen atoms. The organic hydrogen

atoms were generated geometrically; the aqua hydrogen atoms

were located from difference Fourier maps. The crystal data and

structure refinement of compound 1 and H2L are summarized in

Table 3 and Table S1 (ESIw).

Acknowledgements

The authors are grateful to the financial aid from the National

Natural Science Foundation of China (Grant No. 21071140

and 21001101), National Natural Science Foundation of

China Major Project (Grant No. 91122030), and National

Natural Science Foundation for Creative Research Group

(Grant No. 20921002).

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Table 3 Crystallographic data for 1

1

Empirical formula C34H40N6O16Ag4M 1220.20T/K 185(2)l/A 0.71073Crystal system MonoclinicSpace group P21/na/A 12.3937(7)b/A 19.3300(9)c/A 17.2992(7)a/1 90b/1 95.7500(10)g/1 90V/A3 4123.5(3)Z 4m/mm�1 1.949R1

a [I 4 2s(I)] 0.0424wR2

b [I 4 2s(I)] 0.0889GOF on F2 1.019

a R1 = S||F0|–|Fc||/S|F0|.b wR2 = S[w(F2

0–F2c)2]/S[w(F2

0)2]1/2.

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882 New J. Chem., 2012, 36, 877–882 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

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