Carboxylate-assisted acylamide metal–organic frameworks: synthesis,structure, thermostability and luminescence studies{

Gong-ming Sun, Hai-xiao Huang, Xiao-zhao Tian, Yu-mei Song, Yan Zhu, Zi-jun Yuan, Wen-yuan Xu,

Ming-biao Luo, Shu-juan Liu, Xue-feng Feng and Feng Luo*

Received 19th April 2012, Accepted 18th June 2012

DOI: 10.1039/c2ce25602a

We present a series of carboxylate-assisted ethylamide metal–organic frameworks, namely,

Zn2(L)2(oba)2?2H2O(1, L = N1,N3-di(pyridin-3-yl)isophthalamide, H2oba = 4,49-oxybis(benzoic

acid)), Zn2(L)(nap)2?(DMF)2.5H2O(2, H2nap = 1,4-naphthalic acid, DMF = dimethylformamide),

Cd(H2O)(L) (ip)?3H2O (3, H2ip = 1,3-benzenedicarboxylic acid), Cd(L)(ip)?2H2O(4). Polymer 1

crystallizes in the acentric Pca21 space group and presents an unprecedented topology prototype of

noz-4-Pcca that allows three-fold interpenetration. Polymer 2 is a 3D supramolecular net with a

considerable solvent-accessible void that is occupied by DMF and water molecules. As for polymers 3

and 4, the different formation of them is somewhat controlled by the concentration of initial materials

such as 0.1 mmol Cd(NO3)2, L, ip in 8 mL water used for 3, 0.2 mmol Cd(NO3)2, L, ip in 8 mL water

used for 4. In 3, a highly rare 1D porous ladder-type structure is observed, and further through

hydrogen bonds a 2D supramolecular net is built. By contrast, a 3D a-Po net with two-fold

interpenetration, based on a dinuclear Cd2(CO2)4 building block, is observed in 4. Moreover, their

thermostability and luminescence properties are explored.

Introduction

The growing interesting in the crystal engineering of metal–

organic frameworks (MOFs) is driven by not only their extensive

applications in many fields (i.e. gas storage, ion exchange,

fluorescence sensors),1–3 but also their intriguing topology

matrixes.4,5

To construct MOFs for functional claims or topology goals,

the choice of suitable organic ligands will get twice the result with

half the effort. Aiming at porous MOFs, the organic multi-

carboxylate ligands with rigid, big skeleton are favorable,

whereas from the viewpoint of magnetic MOFs the effort should

be focused on the polytopic, small organic ligands.6 As for

topology goal, in light of the current state of the art, it is still in

an uncontrolled phase. However, following the quick increase in

this field a large number of MOFs covering various topological

prototypes are obtained and discussed in comprehensive reviews

or text book by O’Keeffe et al.4,7 Thanks to the increasing

knowledge in this field, sometimes we can imitate some well-

known topological net via the choice of proper metal ions and

organic ligands.7

On the other hand, acylamide functional ligands with the

unique potential to provide two types of hydrogen bonding sites,

the –NH moiety acting as an electron acceptor and the –CLO

group acting as an electron donor, is now receiving extensive

attention and a mass of supramolecular frameworks or MOFs

are prepared.8–13 In recent years, we also devoted our efforts to

this field and launched the low-temperature solvothermal

synthesis of carboxylate-assisted acylamide MOFs and as a

result resolved several problems in this field such as thermo-

stability and high dimensional MOFs.14 Later, this method was

developed by Du et al., where low-temperature hydrothermal or

even high-temperature hydrothermal synthesis up to 180 uC was

employed to construct carboxylate-assisted acylamide MOFs.15

In the meantime, Cao et al. reported that even with the presence

of alkalis such as NH3?H2O low-temperature hydrothermal

synthesis can also be applied to prepare carboxylate-assisted

acylamide MOFs.16 From the structural viewpoint, all of these

carboxylate-assisted acylamide MOFs show relatively high

thermostability and outstanding topological architecture such

as the coexistence of both polyrotaxane and polycatenane

characters, self-catenated 658-mok nets with three-fold inter-

penetration, eight-fold interpenetration with [4 + 4] mode.

As an ongoing work in this field, we report herein the

synthesis, structure, thermostability and luminescence properties

in detail for a series of new carboxylate-assisted acylamide

metal–organic frameworks (Scheme 1).

Experimental

Materials and physical measurements. Commercially available

reagents are used as received without further purification. The

College of Biology, Chemistry and Material Science, East China Instituteof Technology, Fuzhou, Jiangxi, 344000, China.E-mail: ecitluofeng@163.com{ Electronic supplementary information (ESI) available CCDC referencenumbers 877925–877928. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c2ce25602a

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acylamide ligand used here is synthesized according to literature

methods.14 Elemental analysis for C, H and N was performed on

a Perkin-Elmer 240 analyzer. Steady-state photoluminescence

spectra were measured on a SHIMADZU RF-5301PC spectro-

fluorophotometer.

Synthesis of 1. A mixture of Zn(NO3)2 (0.1 mmol), H2oba

(0.1 mmol), and L(0.1 mmol) in 5 mL DMF was sealed in a

Teflon-lined stainless steel vessel and heated at 120 uC for 3 days,

and then the reaction system was cooled to room temperature.

Colorless crystals were obtained. Yield: 58% based on Zn(NO3)2.

Elemental analysis (%): Calc.: C 58.41, H 3.67, N 8.51. Found: C

58.48, H 3.61, N 8.56. The phase purity of the bulk samples are

confirmed by XRD studies. (Fig. S1, ESI{)

Synthesis of 2. A mixture of Zn(NO3)2 (0.1 mmol),

H2nap(0.1 mmol), and L (0.1 mmol) in 5 mL DMF was sealed

in a Teflon-lined stainless steel vessel and heated at 120 uC for

3 days, and then the reaction system was cooled to room

temperature. Colorless crystals were obtained. Yield: 66% based

on L. Elemental analysis (%): Calc.: C 55.13, H 4.25, N 8.44.

Found: C 55.90, H 4.27, N 8.51. The phase purity of the bulk

samples are confirmed by XRD studies. (Fig. S1, ESI{)

Synthesis of 3. A mixture of Cd(NO3)2 (0.1 mmol), H2ip

(0.1 mmol), and L (0.1 mmol) in 8 mL deionized water was

sealed in a Teflon-lined stainless steel vessel and heated at 160 uCfor 3 days, and then the reaction system was cooled to room

temperature. Colorless crystals were obtained. Yield: 5% based

on Cd(NO3)2. Elemental analysis (%): Calc.: C 45.72, H 3.83, N

8.20. Found: C 45.65, H 3.88, N 8.15. The phase purity of the

bulk samples are confirmed by XRD studies. (Fig. S1, ESI{)

Synthesis of 4. A mixture of Cd(NO3)2 (0.2 mmol),

H2bdc(0.2 mmol), and L (0.2 mmol) in 8 mL deionized water

was sealed in a Teflon-lined stainless steel vessel and heated at

160 uC for 3 days, and then the reaction system was cooled to

room temperature. Colorless crystals were obtained. Yield: 62%

based on Cd(NO3)2. Elemental analysis (%): Calc.: C 49.49, H

3.51, N 8.88. Found: C 49.42, H 3.55, N 8.83. The phase purity

of the bulk samples are confirmed by XRD studies. (Fig. S1,

ESI{)

X-ray structural studies. Suitable single crystals of 1–4 were

selected and mounted in air onto thin glass fibers. Accurate unit

cell parameters were determined by a least-squares fit of 2h

values, and intensity data were measured on a Bruker Smart

Breeze CCD area diffractometer with Mo-Ka radiation (l =

0.71073 A) at room temperature. The intensities were corrected

for Lorentz and polarization effects as well as for empirical

absorption based on multi-scan techniques, all structures were

solved by Direct methods and refined by full-matrix least-

squares fitting on F2 by SHELX-97. All non-hydrogen atoms

were refined with anisotropic thermal parameters. All hydrogen

atoms except for that on acylamide groups and water molecules

in 2–4 where they are first found and then refined freely or with

the restriction of O–H = 0.85 ¡ 0.01 A. Crystallographic data

for the three compounds are summarized in Table 1.

Results and discussion

Crystal structure

Zn2(L)2(oba)2?2H2O (1). The single crystal X-ray diffraction

reveals that polymer 1 crystallizes in the acentric Pca21 space group

with the Flack factor of 0.199(18). This big Flack factor suggests the

partial racemic twinning of the crystal studied. The asymmetrical

unit contains two crystallographically-independent Zn(II) sites,

both of which are four-coordinated in the tetrahedral geometry

finished by two L nitrogen atoms [Zn–N = 2.051(6)–2.063(7) A] and

two oba22 oxygen atoms [Zn–O = 1.900(5)–1.967(5) A]. (Fig. 1)

The oba22 ligand adopts the V-shape configuration with the folded

angle of ca. 118.4u or 117.0u (calculated from C43–O7–C29 and

C49–O10–C61). The two carboxyl groups of the oba22 ligand show

the monodentate coordination mode. The L ligand takes the cis-

configuration. (Fig. 1)

As shown in Fig. 2, along the b axis L ligands organize the

Zn(II) ions in-turn to give rise to the 1D helical chain, and the

repeat distance of it is about ca. 11.3 A. In addition, each oba22

ligand acts as a bridge to combine two identical helical chains

together, while each 1D helical chain connects to four identical

helical chains via oba22 spacers, resulting in the overall 3D net.

(Fig. 2) Moreover, although three-fold interpenetration is

observed in the mode of Class Ia, microporous structure still

exists in 1. And the solvent-accessible volume occupied by free

water molecules estimated by Platon program is ca. 935.7 A3,

equal to 15% of the cell volume.17

The topology analysis is carried out by means of Topos40

program.18 The short and long Schlafli symbol is given as 66 and

6.62.6.62.6.62. The short Schlafli symbol is the same as that

observed in dia, gsi, lcs, lon, mmt, msp, nep, una, wfa net, however

its long Schlafli symbol is very different from them, indicating a

new four-connecting topological prototype.19 Nevertheless, this

topology matrix can be deduced from the five-connecting noz net

Scheme 1 The acylamide ligands and its coordination configuration, as

well as the coordination fashion of organic carboxylate ligand used in

this work.

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when one linker of it is omitted. Thereby, this special net is

defined as noz-4-Pcca.18 (Fig. 3) To our knowledge, there was

only one case that is classified to be noz-4-Pcca net, but it is

composed of both metal node and organic ligand node, and it

also does not show any interpenetration. Thus, our case should

present the first noz-4-Pcca net showing interpenetration.20,21

Zn2(L)(nap)2?(DMF)1.5H2O(2). When the rigid H2nap ligand

is used, then a 2D net is generated. As depicted in Fig. 4, the 2D

net is built on a Zn2(CO2)4 paddle-wheel substructure, where the

Zn(II) ion holds the ZnNO4 pyramidal geometry finished by four

nap22 oxygens [Zn–O = 2.030(3)–2.092(3) A] and one L nitrogen

[Zn–N = 2.027(4) A for Zn1 site, and 2.025(4) A for Zn2 site].

(Fig. 5) The metal-to-metal span in this paddle-wheel substruc-

ture is 2.9451(7) A. The bi(bidentate) coordination mode is

observed for nap22 ligands. The regular square grid constructed

by Zn2(CO2)4 substructures and nap22 spacers has the size of ca.

1.1 6 1.1 nm. Along the c axis, the L ligands with the anti-

configuration are grafted into this 2D net via Zn–N coordination

bonds. Further, these 2D nets are organized together by typical

N–H…O hydrogen bonds [N3–H2M…O8 = 3.134(5) A] in the

ABAB stacking mode, finally resulting in the 3D supramolecular

net. Note that, as shown in Fig. 6, a clear porous channel

occupied by both DMF and water molecules along the c axis is

observed in this 3D supramolecular net, which is estimated by

the Platon program giving a solvent-accessible volume of

1805.1 A3, equal to 36.6% of the cell volume.17

Cd(H2O)(L)(ip)?3H2O(3). The symmetrical unit of 3 is shown

in Fig. 7, where the Cd(II) site bonds to three ip22 oxygens [Cd–

O = 2.288(2)–2.526(2) A], one terminal water molecule [Cd–O =

2.337(2) A], and two L nitrogens [Cd–N = 2.304(2)–2.335(2) A],

creating the CdO4N2 octahedral geometry.

Two L ligands in the anti-configuration integrate with two

Cd(II) ions to generate a 28-membered ring of Cd2(L)2. Further,

through ip22 spacers these Cd2(L)2 loops are in-turn combined

Table 1 Crystallographic data and structure refinement details for 1–4

Polymers 1 2 3 4

Chemical formula C64H48N8O16Zn2 C49.5H45.5N6.5O13.5Zn2 C26H26CdN4O11 C26H22CdN4O8

Formula weight 1315.92 1078.24 682.91 630.86T/K 296(2) 296(2) 296(2) 296(2)Space group Orthorhombic, Pca21 Monoclinic,P21/c Triclinic, P1 Monoclinic,P21/ca/A 15.8517(5) 10.9501(4) 10.1561(6) 8.2505(9)b/A 11.2658(4) 20.6804(9) 10.2773(6) 30.086(3)c/A 34.8339(9) 21.8474(9) 14.4890(9) 10.3031(11)a (u) 90 90 85.432(4) 90b (u) 90 94.217(2) 72.012(3) 99.9130(10)c (u) 90 90 83.560(3) 90Z 4 4 2 4Reflections collected/unique 25 243/10 753 34 960/8636 20 618/5011 19 129/4433Rint 0.0612 0.0443 0.0424 0.0323Dc (g cm23) 1.401 1.423 1.589 1.658V/A3 6220.7(3) 4934.0(3) 1427.70(15) 2519.3(5)S 0.983 1.124 1.094 1.248Flack factor 0.199(18)R1, wR2 (all data) 0.0925, 0.1824 0.0775, 0.1448 0.0384, 0.0791 0.0393, 0.0875

Fig. 1 View of the symmetrical unit of 1 and the coordination

surrounding the metal ions. The hydrogen atoms except on the acylamide

group are omitted for clarity. Color code: Zn/purple, C/green, N/blue, O/

red, H/brown.

Fig. 2 View of the 1D helical chain, the connectivity pattern between

helical chains, as well as the 3D net built on the substructure of helical

chains of 1.

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Fig. 3 Schematic description of the noz-4-Pcca net and its three-fold interpenetration (colored in purple, red, green, respectively) observed in 1.

Fig. 4 View of the 2D net built on the Zn2(CO2)4 paddle-wheel

substructure of 2.

Fig. 5 View of the symmetrical unit of 2 and the coordination

surrounding the metal ions. The hydrogen atoms except on the acylamide

group are omitted for clarity. Color code: Zn/purple, C/green, N/blue, O/

red, H/brown.

Fig. 6 View of the hydrogen bond mode, the 3D supramolecular net,

and the channel in this 3D supramolecular net observed in 2.

Fig. 7 View of the symmetrical unit of 3 and the coordination

surrounding the metal ions. The hydrogen atoms except on coordinated

water molecules and acylamide groups are omitted for clarity. Color

code: Cd/purple, C/green, N/blue, O/red, H/brown.

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together to create the 1D ladder-like structure along the b axis.

The two carboxyl groups of the ip22 ligands bear chelate and

monodentate coordination modes. It is notable that the small

void space occupied by free water molecules is observed in

this 1D ladder-like structure and the solvent-accessible volume is

ca. 270.4 A3, equal to 18.9% of the cell volume.17 (Fig. 8)

Furthermore, by means of hydrogen bonds between coordinated

water molecules and ip22 oxygens [O1–H1w…O4 = 2.759(2) A,

O1–H1w…O2 = 2.727(2) A] these 1D ladder structures are

linked together, creating a 2D supramolecular net. (Fig. 9)

Furthermore, the 3D supramolecular net is governed by

hydrogen bonds [N3–H3…O6 = 2.840(2) A] between 2D

supramolecular layers.

Cd(L)(ip)?2H2O (4). Interestingly, the formation of 3 and 4 is

sensitive to the concentration of the initial materials. In contrast

to 3, twice the amount of the initial materials is used to

synthesize polymer 4. The overall structure of 4 is a 3D six-

connecting a-Po net with two-fold interpenetration, indicative of

distinct structural diversity between 3 and 4.

The coordination surrounding the metal ion and the main

composition of polymer 4 is depicted in Fig. 10. Similar to that

observed in 3, the octahedral geometry for the Cd(II) site in 4 is

also made up of two nitrogens [Cd–N = 2.342(3)–2.373(3) A] and

four oxygens [Cd–O = 2.226(3)–2.401(3) A]. However, one

difference is the site of the coordinating water molecule in 3,

which is replaced by one carboxylate oxygen in 4. Moreover, the

arrangement of these coordination atoms towards the Cd(II) ion

in 3 and 4 is also different. All oxygens except for the terminal

water molecule in 3 are located on one plane and the N–Cd–N

angle is ca. 90u, suggesting that the L ligands are arranged in a

vertical fashion, respectively, whereas in 4 all oxygens are seated

on one plane and the N–Cd–N angle is ca. 167u, indicating that

the L ligands are arranged in the opposite fashion. Besides, the

difference also contains the coordination mode of both ip22 and

L ligands: chelate plus bidentate mode for the two carboxyl

groups of ip22 and cis-configuration for L is observed in 4.

The a-Po net is built on a six-connecting Cd2(CO2)4 secondary

building block. Such Cd2(CO2)4 units are different from the

common paddle-wheel M2(CO2)4(M = 3d metal ions) in that the

bridge for this structure is two carboxyl groups rather than four

carboxyl groups for common paddle-wheel structures. The

metal-to-metal distance of 3.882(3) A in Cd2(CO2)4 unit is

thereby longer than that observed in common paddle-wheel

structure.20 Moreover, in contrast to the common paddle-wheel

structure that only holds two additional coordination sites to

Fig. 8 In 3, view of the 28-membered ring of Cd2(L)2, the 1D ladder structure, the porous 1D ladder structure, as well as the AA packing fashion of

these 1D porous structure (viewed down the c direction).

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accommodate the N-donor pillar-like ligands or terminal solvent

molecules, such Cd2(CO2)4 unit owns more coordination

potential. As shown in Fig. 11, four N-donor L ligands are

ligated into this Cd2(CO2)4 unit through Cd–N coordination

bonds. Giving each Cd2(CO2)4 unit as a secondary building

blocks (SBU), then in 4 each Cd2(CO2)4 SBU connects to six

identical Cd2(CO2)4 SBUs through four L linkers and four ip22

linkers, creating six-connectivity of this SBU.

The 3D net of 4 is shown in Fig. 12. The topology analysis is

performed by Topos40 program, giving six-connecting a-Po net

with 41264 topology symbol and two-fold interpenetration in the

mode of Class Ia.18

In addition, a small void space occupied by water molecules is

observed in 4. The solvent-accessible volume estimated by the

Platon program is ca. 244.9 A3, equal to 9.7% of the cell

volume.17

Properties. The thermostability of these polymers except for

polymer 3 that shows very low yield is explored by TG at

30–800 uC (Fig. 13). For 1, the first weight loss, 3.3%, at 80–130 uCis ascribed to the loss of two guest water molecules (calc. 2.7%).

The chemical decomposition temperature of 1 is estimated around

230 uC. For 2, the continuous weight loss without any platform

until 800 K indicates low thermostability of 2 and the loss of

2.5 DMF molecule plus one water molecule (calc. 18.6%) is

estimated at 30–260 uC (exp. 18.2%). As for 4, its thermostability

is similar to that observed for 1, the loss of two guest molecules at

30–160 uC (calc. 5.7%, exp. 5.3%) and the chemical decomposition

temperature around 280 uC. The very weak thermostability of 2 is

mainly due to the 1D porous structure.

The acylamide of L in the state solid shows strong

luminescence at 430 nm. The polymers 1, 2, 4 in the solid also

afford strong luminescence at 394 nm, 400 nm, 458 nm and if

excited at 354 nm, 351 nm, 316 nm, respectively, implying blue-

light emission for 1, 2, and green-light emission for 4 (Fig. 13).

The corresponding stokes shift is 40 nm, 49 nm and 142 nm. In

contrast to the luminescence of the L ligand, the luminescence of

1, 2 shows a blue-shift of ca. 36 nm and 30 nm, whereas a red-

shift of ca. 28 nm is observed in polymer 4. The p–p* transition

in conjunction with the metal-to-ligand or ligand-to-metal

charge transfer should be the origin for the luminescence of

polymers 1, 2, 4.21,6e The exceptional luminescence properties of

4 such as the green-light emission and big Stokes shift is mainly

due to the extensive p–p interactions (3.681(3) A) existing in 4.

Fig. 9 View of the 2D supramolecular net of 3.

Fig. 10 View of the symmetrical unit of 4 and the coordination

surrounding the metal ions. The hydrogen atoms except on acylamide

groups are omitted for clarity. Color code: Cd/purple, C/green, N/blue,

O/red, H/brown. Fig. 11 View of the six-connecting Cd2(CO2)4 SBU of 4.

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Conclusion

In this work, we report the synthesis, structure, thermostability,

and luminescence properties of a series of carboxylate-assisted

acylamide metal–organic frameworks in detail. Through struc-

ture studies, a new four-connecting topological prototype,

defined as noz-4-Pcca, is disclosed in polymer 1, while a rare

1D porous structure is observed in 3. At the same time, various

degrees of porosity are observed in polymers 1–4, suggesting

potential porous material of them. Note that the structure

diversity controlled by the concentration of the initial materials

exists between 3 and 4, which will to some extent give us some

insight into the crystal engineering of the controllable synthesis

of MOFs. Moreover, the potential of luminescence properties

observed in polymers 1, 2, and 4 is capable for the application as

luminescence materials.

Acknowledgements

This work was supported by the Doctoral Start-up Fund of the

East China Institute of Technology, the Foundation of Jiangxi

Educational Committee (no. GJJ11153), the Natural Science

Foundation of Jiangxi Province of China (no. 2010GQH0005),

the China Postdoctoral Science Foundation (no. 20100480725),

and the Foundation of Key Laboratory of Radioactive Geology

and Exploration Technology Fundamental Science for National

Defense (2010RGET07).

References

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Fig. 12 View of the 3D net of polymer 4 and the schematic description of the a-Po net and its two-fold interpenetration.

Fig. 13 The TG and luminescence studies.

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This journal is � The Royal Society of Chemistry 2012 CrystEngComm, 2012, 14, 6182–6189 | 6189

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