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Two organic–inorganic hybrid frameworks with unusual inorganic andorganic connectivity†

Di-Chang Zhong, Xiao-Long Feng and Tong-Bu Lu*

Received 30th September 2010, Accepted 27th January 2011

DOI: 10.1039/c0ce00961j

Two m-Cl� bridged Cd(II) organic–inorganic hybrid frameworks

have been constructed with unusual I1O3 and I2O2 connectivity,

respectively.

In the past two decades, a variety of organic–inorganic hybrid

compounds have been constructed, which were discussed in several

current reviews.1 To well analyze and understand their structures,

Rao and Cheetham et al. pioneeredly divided the frameworks of

organic–inorganic hybrid compounds into two categories: organic

frameworks with M–L–M (L ¼ organic ligands) connectivity and

inorganic frameworks with M–X–M (X¼O, N, Cl, and S) bonding.2

The whole dimensionality of a hybrid framework can be represented

with a notation InOm, where n and m represent the dimensionalities of

inorganic and organic connectivity, respectively. To our knowledge,

almost all the dimensionalities of these hybrid frameworks are in the

range of 0–3 (m + n # 3), and so far only a few examples have been

found with dimensionalities over three (m + n > 3).3

Very recently, we obtained a cadmium hybrid compound based on

tetrazolate-5-carboxylate, Tzc, which shows unprecedented three

dimensional inorganic connectivity (I3, linked by m-OH�) and three

dimensional organic connectivity (O3, linked by Tzc ligand).3b To

extending our research on the connectivity of hybrid frameworks, we

introduced Cl� as inorganic bridge, and successfully synthesized two

m-Cl� bridged Cd(II) organic–inorganic hybrid frameworks,

{[Cd(HBDT)Cl]}n${3DMF$4H2O}n (1$(3DMF$4H2O)n; H2BDT¼1,4-benzeneditetrazole), and {[Cd5(BDT)2Cl6]$2CH3OH}n (2). Their

crystal structures with unusual I1O3 and I2O2 dimensionalities, and

photoluminescent properties are reported in this communication.

Compounds 1 and 2 were obtained by solvothermal reactions of

cadmium salts with H2BDT under different reaction conditions.‡

The infrared spectrum of 1 shows an intense peak at 1659 cm�1 and

a broad peak at 3468 cm�1, indicating the existence of DMF and H2O

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/State KeyLaboratory of Optoelectronic Materials and Technologies/School ofChemistry and Chemical Engineering, Sun Yat-Sen University,Guangzhou, 510275, China. E-mail: lutongbu@mail.sysu.edu.cn; Fax:+86-20-84112921

† Electronic supplementary information (ESI) available:Crystallographic data in CIF, and TG, XRPD results (PDF). CCDCreference numbers 795026 and 795027. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c0ce00961j

This journal is ª The Royal Society of Chemistry 2011

molecules in 1. The peaks at 1435 cm�1 in 1 and 1439 cm�1 in 2 belong

to the absorptions of HBDT� and BDT2� ligands (Fig. S1†).4

The single crystal X-ray diffraction analysis reveals that 1 is an

orthorhombic structure.xAs shown in Fig. 1a, Cd1 is six-coordinated

with four N atoms from four individual HBDT� ligands and two m2-

Cl� anions, forming a distorted octahedral geometry, in which four N

atoms locate the equatorial plane and two Cl� anions occupy the

axial positions. The Cd–Cl distance (2.6186(19) A) is longer than Cd–

N distance (2.337(3) A), indicating an elongated octahedron around

Cd1. The HBDT� anion, employing a symmetrical coordination

mode, bonds to four Cd1 through four terminal N atoms of two

tetrazolyl rings (Fig. 1a). Through the bridging of m2-Cl� anions, a 1D

zigzag chain of [(CdCl)n]n+ is generated (Fig. 1b). These chains are

further connected by HBDT� anions, resulting in a 3D porous

framework containing 1D rhomboidal channels (Fig. 1d). The

dimension of each channel is 10.2 � 10.2 A, and the channels are

occupied by DMF and water molecules. About 50% solvent-acces-

sible volume is estimated by using PLATON software.5 Though m-

Cl� bridges are very common in 0D, 1D chain, and 2D layer coor-

dination compounds, the m2-Cl� bridge mode in 1 is very rare in 3D

organic–inorganic hybrid frameworks.6 To our knowledge, only two

3D organic–inorganic hybrid frameworks containing m2-Cl� bridges

have been reported so far.7

Another interesting feature of 1 is that the organic HBDT� anions

also link the Cd1 atoms to form a 3D organic framework when the

inorganic m2-Cl� bridges are omitted (Fig. 1c and d). Therefore, the

whole dimensionality of the framework of 1 can be described as I1O3.

To our knowledge, only two organic–inorganic hybrid frameworks

with I1O3 dimensionalities have been reported so far,3a,e and 1 is the

third example exhibiting 1D inorganic and 3D organic connectivity.

Compound 2 crystallizes in P�1 space group.x The structure

contains three crystallographically independent Cd(II) ions (Fig. 2a).

Cd1 coordinates with two N atoms from two individual BDT2�

anions, three m3-Cl� anions and one methanol O atom, forming

a distorted octahedral geometry. Cd2 and Cd3 also show distorted

octahedral geometries, in which Cd2 coordinates with two N atoms

from two individual BDT2� anions, three m3-Cl� anions and one m2-

Cl� anion, and Cd3 bonds to four N atoms from four individual

BDT2� anions and two m2-Cl� anions. It is worth to noting that two

types of m-Cl� bridges are observed in 2, one is m3-Cl� (Cl(1) and

Cl(2)), connecting three Cd atoms, the other is m2-Cl� (Cl(3)), linking

two Cd atoms (Fig. 2a). Through the bridging of m3-Cl�, Cd1 and

Cd2 are connected together to form a 1D ladder-like ribbon along the

CrystEngComm, 2011, 13, 2201–2203 | 2201

Fig. 2 (a) The coordination environments of Cd1, Cd2 and Cd3, the

asymmetric bridging mode of BDT2� anion, and two types of m-Cl�

bridges. (b) The 2D inorganic connectivity linked by m-Cl� bridges. (c)

The 2D organic connectivity linked through BDT2� anions. (d) The

structure of 2 containing 2D inorganic (green) and 2D organic (gray)

connectivity (the coordinated methanol molecules are omitted for

clarity).

Fig. 1 (a) The coordination environment of Cd1, the symmetric

bridging mode of HBDT� anion and the m2-Cl� bridge in 1. (b) The m2-

Cl� bridged1D zigzag inorganic chain. (c) The 3D organic connectivity

linked by HBDT anions. (d) The structure of 1 containing 1D inorganic

(green) and 3D organic (gray) connectivity.

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a-axis. These ribbons are further bridged by Cd3 and m2-Cl� (CdCl2unit) along the c-axis to generate a 2D inorganic framework (Fig. 2b).

To date, some organic–inorganic hybrid compounds containing 2D

inorganic frameworks have been presented in literatures,8 however,

these inorganic frameworks are connected by m-O (or m-OH�)

bridges. Among numerous organic–inorganic hybrid compounds, the

one containing 2D inorganic frameworks linked by m-Cl� bridges has

not been reported so far.6

The BDT2� anion in 2 employs an asymmetric coordination mode,

bonding to two Cd1, two Cd2, and two Cd3 atoms through six N

atoms of two tetrazolyl rings (Fig. 2a). It is interesting to note that the

BDT2� anions connect three crystallographically independent Cd

atoms together to form a 2D organic framework when the inorganic

m-Cl� bridges are omitted (Fig. 2c). Thus the whole dimensionalities

of the framework of 2 can be represented as I2O2. To the best of our

2202 | CrystEngComm, 2011, 13, 2201–2203 This journal is ª The Royal Society of Chemistry 2011

Fig. 3 Fluorescent emission spectra for H2BDT, 1, and 2 in the solid

state at room temperature (lex¼ 288, 284, and 283 nm for H2BDT, 1, and

2, respectively).

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knowledge, organic–inorganic hybrid framework containing 2D

inorganic and 2D organic connectivity has not been presented yet.2a,b

Therefore, 2 is an unprecedented hybrid framework, not only for its

2D inorganic framework linked by m-Cl� bridges, but also for its I2O2

dimensionalities.

The result of thermogravimetric (TG) analysis indicates that

1$(3DMF$4H2O)n is unstable in air, with continuous weight loss in

the temperature range from 30 to 600 �C (Fig. S2†). The results of

variable temperature powder X-ray diffraction (VT-PXRD)

measurements also imply the poor stability of the framework

(Fig. S3a†). The TG curve of 2 shows an initial weight loss of 5.2% in

the 30–325 �C temperature range, corresponding to the removal of

two coordinated methanol molecules per formula unit (calcd 5.1%),

then began to decompose with continuous weight loss upon further

heating (Fig. S2†). The results of VT-PXRD measurements of 2 show

the framework of 2 is stable over 320 �C (Fig. S3b†), which is

consistent with the result of thermal analysis.

Considering the large solvent-accessible volume in 1, the N2 sorp-

tion measurements have been carried out at 77 K and 1 atm. The result

indicates that 1 cannot adsorb N2 (Fig. S4†), implying that 1 becomes

non-porous after removing the guest DMF and water molecules.

Interestingly, when the desolvated 1 was immersed in 1 : 1 (v/v) DMF/

H2O for one day, it can readsorb DMF and H2O molecules and return

back to 1 (Fig. S5†), demonstrating a reversible course.

The fluorescent emission properties of 1, 2, and the free ligand

H2BDT in the solid state were investigated at room temperature. As

depicted in Fig. 3, 1 and 2 exhibit photoluminescence upon excitation

at 284 and 283 nm, respectively. As the free ligand H2BDT shows

broad emission peak at 459 nm upon excitation at 288 nm, the

fluorescent emission maximum at 462 and 465 nm for 1 and 2 can be

tentatively ascribed to the ligand-to-ligand charge transfer (LLCT).9

In conclusion, we obtained two m-Cl� bridged Cd(II) organic–

inorganic hybrid frameworks based on 1,4-benzeneditetrazole ligand,

which show unusual I1O3 and I2O2 dimensionalities, respectively.

Photoluminescent measurements show that both hybrid frameworks

emit blue-light at room temperature.

Acknowledgements

This work was supported by NSFC (20625103, 20831005, 20821001)

and 973 Program of China (2007CB815305).

This journal is ª The Royal Society of Chemistry 2011

Notes and references

‡ Synthesis: Cd(NO3)2$4H2O (1.0 mmol, 0.308 g) and 1,4-benzenedite-trazole (H2BDT, 0.5 mmol, 0.107 g) were dissolved in 8 mL of DMF–H2O (v/v ¼ 1 : 1). With continuous stirring, 0.5 mL of HCl solution (36–37%) was added. After further stirring for ten minutes, the resultingsolution was transferred into a Teflon-lined autoclave and heated at 110�C for 72 h. The autoclave was cooled over a period of 16 h at a rate of 5�C h�1. Block-shaped light yellow crystals of 1$(3DMF$4H2O)n werecollected by filtration. Yield: 67%. Anal. Calcd for C17H34N11O7ClCd: C,31.30; H, 5.25; N, 23.62. Found: C, 30.81; H, 5.26; N, 23.75%. IR (KBr,cm�1): 3468(s), 1659(vs), 1435(s), 1465(w), 1390(m), 1346(w), 1255(w),1230(w), 1178(w), 1148(w), 1102(m), 1059(w), 1011(w), 860(m), 752(m),666(m), 540(w), 491(m). A mixture of CdCl2$2.5H2O (0.5 mmol, 0.114 g),H2BDT (0.5 mmol, 0.107 g), and methanol (8 mL) was stirred at roomtemperature for 30 min, and then transferred into a Teflon-lined auto-clave and heated at 160 �C for 72 h. The autoclave was cooled overa period of 26 h at a rate of 5 �C h�1. Block-shaped dark yellow crystals of2 were collected by filtration. Yield: 52%. Anal. Calcd forC18H16N16O2Cl6Cd5: C, 17.11; H, 1.28; N, 17.74. Found: C, 17.16; H,1.26; N, 17.70%. IR (KBr, cm�1): 1607(w), 1556(w), 1439(s), 1369(m),1284(w), 1256(w), 1229(w), 1169(w), 1120(m), 1043(w), 1007(w), 988(m),834(w), 748(m), 551(w), 486(m), 462(m).

x Crystallographic data for 1 (C8H5N8ClCd): M¼ 361.05, orthorhombic,Imma, a¼ 23.253(6), b ¼ 7.6111(19), c ¼ 14.573(4) A, V¼ 2579.1(11) A3,Z ¼ 4, m ¼ 0.948 mm�1, Dc ¼ 0.930 Mg m�3, F(000) ¼ 696, 1257 unique(Rint ¼ 0.0591), R1 ¼ 0.0538, wR2 ¼ 0.1459 (I > 2s(I)), GOF ¼ 0.994.Compound 2 (C18H16N16O2Cl6Cd5), triclinic, P�1, a ¼ 8.7396(14), b ¼9.1057(15), c ¼ 10.0556(16) A, a ¼ 74.388(2), b ¼ 82.547(2), g ¼77.733(2)�, V ¼ 750.9(2) A3, Z ¼ 1, m ¼ 4.070 mm�1, Dc ¼ 2.793 Mg m�3,F(000) ¼ 594, 2614 unique (Rint ¼ 0.0186), R1 ¼ 0.0201, wR2 ¼ 0.0496 (I> 2s(I)), GOF ¼ 1.104. CCDC 795026 (1) and 795027 (2).

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