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Novel three-dimensional Ln–Ag 4d–4f heteropentametallic helix-basedmicroporous metal–organic framework with unprecedented (3,4,5,6)-connected topology constructed from isonicotinate ligand†

Zhao-Yang Li,a Jing-Wei Dai,a Shan-Tang Yue*a and Ying-Liang Liu*b

Received 6th January 2010, Accepted 1st March 2010

First published as an Advance Article on the web 5th March 2010

DOI: 10.1039/c000058b

An unprecedented 4d–4f heterometallic helix-based MMOF with

a new (3,4,5,6)-connected topology, {[Ln2Ag3(ina)6(ox)0.5(m2-

OH2)(H2O)2.5]$1.5H2O$2NO3}n, is presented, which consists of

coaxial bi-strand helices interconnected by the carboxyl groups of

the ligands. Selective anion-exchange functions of the title

compound containing NO3� counter anions were identified.

Microporous metal–organic frameworks (MMOFs) or microporous

coordination polymers (MPCPs) are of great current interest in view

of their fascinating structural topologies and potential applications in

small molecule gas storage, separation, ion exchange, catalysis, etc.,1

because the pore dimensions of the MMOFs typically fall in the range

of ultramicropores (<7 �A). However, most reports have so far

focused on the assembly of 3d block metals and organic ligands as

linkers,2 while many 3d–4f heterometallic MOFs have also been

reported.3 However, 4d–4f heterometallic MMOFs have received less

attention. The preparation of 4d–4f MMOFs has certain difficulties

because of the high coordination numbers of the 4f block metals,

which frequently leads to interpenetration and consequently results in

a decrease of the pore size which can result in non-porous materials.4

Therefore, the selection of the organic ligands becomes a key point in

the preparation of 4d–4f heterometallic MMOFs.

Isonicotinate (ina), the deprotonated form of isonicotinic acid

(Hina), is a rigid and linear ligand that can afford up to three donor

atoms (two O atoms and one N atom) with variable coordination

modes, and hence has a strong potential to construct highly ordered

dimensional structures and becomes an excellent candidate for the

construction of 4d–4f coordination polymers. In the past few years,

very few 4d–4f examples with porous structure constructed from

Hina ligand have been reported.5 Xue et al. reported the first exam-

ples of Ln–Ag supramolecular MOFs with approximately

nanometre-sized channels, owing to the addition of second ligand

(1,3-bdc).5a Xue et al. also reported another example of Ln–Ag

MOFs with approximate nanochannels by mixed connection of

isonicotinate ligand and nicotinate ligand.5b In our system, we chose

the ‘shorter’ and ‘smart’ organic ligand, oxalate as the second ligand,

which plays an important role in forming microporous metal–organic

aSchool of Chemistry and Environment, South China Normal University,Guangzhou, 510006, P. R. China. E-mail: yuesht@scnu.edu.cn; Fax:+86-20-39310187; Tel: +86-13711179206bDepartment of Chemistry, Jinan University, Guangzhou, P. R. China

† Electronic supplementary information (ESI) available: TGA, PXRD,IR and FL spectrum. CCDC reference number 726056. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c000058b

2014 | CrystEngComm, 2010, 12, 2014–2017

frameworks. Up to now, rare 3d–4f or 4d–4f examples, which are

based on the ina and oxalate ligands have been reported with all of

them being non-porous.6

Notably, a large number of helical coordination polymers have

been known, but very few helix-based MOFs have been synthesized.7

Xu et al. reported the first single-metal helix-based MMOFs con-

structed from a single linker.6a Wang and Su et al. synthesized 3D

MOFs featuring nanosized tubular channel based on single-metal

and auxiliary ligand.7c To our knowledge, helix-based MMOFs

constructed from La(III)–Ag(I) heterometallic system have not been

reported. In addition, a variety of uninodal and binodal net topology

that are based on 3, 4, and 6-connected and boracite, pyrite, rutile,

Pt3O4, topologies have been realized.8 However, it is still a great

challenge to synthesize high-nodal mixed-connectivity 4d–4f hetero-

metallic coordination polymers. Herein, we report the first 4d–4f

heterometallic helix-based MMOF with new twelve-nodal

(3,4,5,6)-connected topology, {[Ln2Ag3(ina)6(ox)0.5(m2-OH2)(H2O)2.5]$

1.5H2O$2NO3}n.

On the other hand, as an important property of coordination

frameworks, anion exchange has attracted increasing attention in

recent years because it makes such frameworks potentially attractive

as anion exchange materials. In the past decade, many networks with

anion exchange based on Ag complexes have been reported.9 For

example, in 1996, Yaghi et al. reported a coordination framework,

[Ag(4,40-bpy)](NO3), which exchanged anions with PF6� in 95% yield

after 6 h. In the cases reported, the exchange generally occurs among

guest anion or coordinated anions but the latter is more difficult

because it involves the rupture of the coordinated bonds and

formation of the new bonds. So we report the first 4d–4f hetero-

pentametallic MMOF with partially selective anion exchanged

property.

The colourless crystals of 1 were obtained in good yield by the one-

pot hydrothermal reactions of Ln(NO3)3$6H2O, AgNO3, Hina,

K2C2O4$H2O.‡ Complex 1 crystallizes in space group P21/c and the

unit cell is a dimer lying about an inversion center with the linking

oxalate ligand.x The phase purity of complex 1 was confirmed by

elemental analysis and powder X-ray diffraction (PXRD) (ESI,

Fig. S1).†

An ORTEP view of 1 is shown in Fig. 1. The asymmetric unit of 1

contains two La(III) ions, three Ag(I) ions, six ina ligands, one-half

oxalate ligand, four coordinated water molecules, two lattice water

molecules and two free nitrate anions. The central La1 atom is nine-

coordinated with a distorted tricapped trigonal prismatic geometry:

one coordinated water molecule, two m2-OH2 oxygen atoms and six

carboxylate oxygen atoms, all of which are from four different ina

ligands [La1–O bond length range, 2.452(3)–2.888(4) �A]. The central

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 ORTEP view of the unit cell of complex 1 (50% thermal ellip-

soids).

Fig. 2 The 2D layer structure of complex 1 with bi-strand helical chain

running in the direction of the a axis.

Fig. 3 The 3D frameworks of complex 1.

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La2 atom adopts eight-coordinated with bicapped trigonal prismatic

coordination geometry: one coordinated water molecule, two oxygen

atoms from the oxalate ligand which lies about an inversion center,

and five carboxylate oxygen atoms which are from four different ina

ligands [La2–O bond distance range, 2.442(3)–2.596(3) �A]. Based on

the search of the Cambridge Structural Database, heterometallic

compounds that the central Ln(III) ions adopting two different modes

of eight and nine-coordinated have not been reported. The two-

coordinated Ag1 and Ag3 exhibit linear coordination geometry, and

the three-coordinated Ag2 is surrounded by two N-donor atoms

from the two ina ligands and one O atom from one coordinated

water molecule giving a T-shaped coordination geometry. The Ag–

Ow bond [2.530(8) �A] is shorter than other Ag–O coordinated bond,

and the N–Ag2–N angle [167.12(19)�] are similar to those found in

other Ag(I) complexes having T-shaped configurations. It should also

be noted that the adjacent Ag1/Ag3 and Ag2/Ag2 separation is

3.272 and 3.307 �A, respectively, which is less than the sum of the van

der Waals radii for two silver atoms (3.44 �A), thus showing one weak

argentophilic Ag/Ag interaction in the packing diagram. Mean-

while, the shortest distance Ag1/O17 (2.772 �A) is also less than the

sum of the van der Waals radii of Ag and O atoms (3.20 �A), indi-

cating the weak interaction between Ag and NO3�.

The oxalate dianion behaves as a m-k4 O bridge, connecting the

adjacent La2 ions into a zig-zag chain (ESI, Fig. S2).† The Hina

ligand exhibits two kinds of coordination modes: m3-(k3 N, O4: O4)-

bridging and m2-(k2 N, O4)-bridging (Scheme 1), forming bi-strand

helical chain through connecting the adjacent La1 ions with a pitch of

8.3694(5) �A (ESI, Fig. S3).† These two kinds of chains further extend

to 2D La(III)-carboxylate layer by connections of bidentate Hina

ligands (Fig. 2). From the view of the c-axis, the 2D layer presents

paddle-wheel structure and the two kinds of chains alternately exist.

(ESI, Fig. S4).† It should be noted that the 2D La(III)–carboxylate

layer is not a smooth plane but essentially a chair-like layer (ESI,

Scheme 1 The coordination modes of Hina ligand.

This journal is ª The Royal Society of Chemistry 2010

Fig. S5).† In the packing arrangement of layers, ‘linear’ N–Ag–N

linkages play an important role in connecting the adjacent layers,

forming 3D pillared-layer coordination polymer with microporous

structure (Fig. 3). The water molecule of solvation also gives O–H/O hydrogen-bonding interactions with oxalate and isonicotinate O

acceptors. The remaining free voids are partially filled with free water

molecules and nitrate anions. Calculations using PLATON10 based

on the crystal structure show that the total solvent-accessible volume

comprises 11.7% of the crystal volume. Topological studies

performed using the software package TOPOS 4.011 reveal that this

topology is a unique twelve-nodal (3,4,5,6)-connected net, as

confirmed by TOPOS 4.0 in conjunction with systematic searches in

the literature. It’s Schl€afli symbol is (3$4$52$6$7)(3$4$63$7)(3$52$

64$72$8)(3$52$65$75$82)(32$42$57$64)(4$5$6)(4$52)(4$62)2(42$54$62$75$

82)(43$68$72$82)(52$6).

The practical application of MMOF is dominated by thermal

stabilities, so TG and PXRD analysis were performed to examine the

permanent porosity of the MMOF 1. The TGA curve for 1 indicates

that the loss of the lattice water molecules occurred in the temperature

range 50–150 �C (found, 2.02%; calculated, 1.69%, ESI, Fig. S6).†

The complex begins to decompose at 300 �C. Variable-temperature

PXRD patterns reveals that the framework integrity of 1 is main-

tained after the removal of the water molecules and can stable up to

about 280–300 �C, which is in agreement with the TG analysis (ESI,

Fig. S7).†

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The photoluminescent property of 1 was investigated in the solid

state at room temperature. But we didn’t observe the distinctly

luminescence property of 1, the probable reasons are from two sides:

the 4f shell of La(III) ion is empty and the intermolecular and intra-

molecular charge transfer between the ligands and Ag(I) ion is

inefficient (ESI, Fig. S8).†

As revealed by the crystal structure of 1, the anions NO3� were

located within the open structure. Since the complex 1 is not fully

soluble in common solvents, this cationic layered complex is expected

to display anion-exchange property. Excess Na2SO4 was added to

a suspension of well-ground complex 1 in water at room temperature.

The mixture was stirred for 24 h to allow anion exchange, then it was

filtered, and washed with water several times. The IR spectra of the

exchanged solid and the original 1 are shown in the ESI, Fig. S9.†

Intense bands from 1100 to 1160 cm�1, which originate from the

SO42� ion, appeared, while the intense band of the NO3

� ion is still

exists in the spectra of the exchanged solid. The anion-exchange

reactions were also monitored by X-ray powder diffraction tech-

niques (ESI, Fig. S10).† When complex 1 was exchanged with

Na2SO4, the characteristic peaks were different from those of original

1. The slight shift and extension of some peaks may be attributed to

incomplete recovery of the symmetry of the structure.12 The results

confirm that the anion exchange occurs without the destruction of the

frameworks. The anion exchanged products are also evidenced by

elemental analysis (see the ESI).† We also try to prove that such

anion exchange occur by means of a solid-state or a solvent-mediated

process, but convincing results were not obtained by NMR analysis.

Moreover, the anion exchange of 1 was found to be highly selective in

water phase, only the exchange of NO3� by SO4

2� was achieved,

whereas anion exchanges of NO3� with ClO4

�, SO32�, PF6

�, and

WO4� proved unsuccessful and the frameworks decomposed which

were evidenced by the IR spectra.

In summary, we report a novel 3D 4d–4f heterometallic micro-

porous coordination polymer with pillared helical-layer and unique

mixed connected topology structure. The luminescent property is

investigated, together with the interesting selective anion-exchanged

properties. The results presented herein indicate that the method of

one-pot synthesis provides an applicable way of constructing heter-

ometallic MMOF, and mixed organic ligands can be used as struc-

ture-directing agents to form heteropolymetallic complexes.

Acknowledgements

This work was financially supported by Guangdong Provincial

Science and Technology Bureau (grant 2008B010600009), and NSFC

(grant no. 20971047 and U0734005).

Notes and references

‡ Synthesis of complex 1: a mixture of isonicotinic acid (0.1037 g, 0.5mmol), K2C2O4$H2O (0.0552 g, 0.3 mmol), La(NO3)3$6H2O (0.135 g, 0.3mmol), AgNO3 (0.051 g, 0.3 mmol) and H2O (10 mL) was heated to 160�C for 72 h in a 23 mL Teflon-lined stainless-steel autoclave (pH¼ 5) andthen cooled to room temperature in a rate of 5 �C h�1. Colourless pris-matic crystals were collected and dried in air. Yield: 65%, based on La.

x Crystal data for 1: C74H68Ag6La4N16O50, monoclinic, P21/c spacegroup, a ¼ 17.1160(8) �A, b ¼ 33.9351(16) �A, c ¼ 8.3694(4) �A, b ¼95.9240(10)�, Mr ¼ 3184.31, V ¼ 4835.3(4) �A3, T ¼ 298 K, Z ¼ 2, Dc ¼2.187 g cm�3, F(000) ¼ 3068.0, reflections collected 26 645, uniquereflections 9481, R1 [I > 2s(I)] ¼ 0.0366, wR2 [I > 2s(I)] ¼ 0.0717, Rint ¼0.0437, GOF ¼ 1.042.

2016 | CrystEngComm, 2010, 12, 2014–2017

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