Journal of Molecular Structure 979 (2010) 214–220

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Three-dimensional hybrid organic–inorganic frameworks based onlanthanide(III) sulfate layers and organic pillars of 1,4-piperazinediacetic acid

Lin Cheng, Shaohua Gou *, Gang XuSchool of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 June 2010Received in revised form 23 June 2010Accepted 23 June 2010Available online 27 June 2010

Keywords:Hybrid organic–inorganic frameworkMagnetic propertiesThermogravimetric properties

0022-2860/$ - see front matter Crown Copyright � 2doi:10.1016/j.molstruc.2010.06.029

* Corresponding author. Tel./fax: +86 025 8327238E-mail address: sgou@seu.edu.cn (S. Gou).

Two types of three-dimensional (3D) hybrid organic–inorganic frameworks [Ln2(H2pda)(SO4)3(H2O)4]n [I,Ln = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4) and Eu3+ (5)] and {[Ln2(H2pda)(SO4)3(H2O)2]�2H2O}n [II, Ln = Gd3+

(6) and Dy3+ (7)], where H2pda is 1,4-piperazinediacetic acid, have been obtained by the reactions ofH2pda, lanthanide oxides and H2SO4 under similar hydrothermal conditions. All the frameworks are con-structed by two-dimensional inorganic lanthanide(III) sulfate layers and organic H2pda pillars. The lan-thanide(III) sulfate layers in both types I and II consist of the similar lanthanide(III) sulfate chains,which are built from lanthanide(III) ions and g3,l3-sulfates, and the different sulfate bridges. The coor-dination modes of sulfate bridges in I and II are g2,l2-bidentate and g4,l2-tetradentate, respectively.Each organic H2pda pillar in two types is connected with four Ln3+ ions in a bis-bidentate syn-anti mode.Compounds 1 and 6 are thermally stable below 180 and 300 �C, respectively. The variable-temperaturemagnetic studies show that the vM values of 2 and 3 increase on cooling, which may be attributed tothe depopulation of Stark levels and/or intramolecular antiferromagnetic coupling between the metalcenters.

Crown Copyright � 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction

In recent years, the synthesis and characterization of hybridorganic–inorganic framework solids has been an area of rapidgrowth. The aim of this intense activity is the deliberate designof materials with intriguing architectures and topologies, as wellas specific properties, for example, electronic, magnetic, optical,catalytic, electrical, magnetic, and microporous materials [1–19].The self-assembly of designed organic ligands and inorganic skele-tons to construct such organic–inorganic aggregations has beenproved to be a basic and powerful strategy. The inorganic skeletonsare divided into two cases as single metallic cations and inorganicchains/sheets. The latter is built up from two parts of metallic cat-ions and oxoanions. Sulfate, as a tetrahedral oxoanion, has been ofsubstantial interest in the construction of hybrid organic–inorganicmaterials with beautiful topologies and interesting properties dueto the diversity of its coordination modes and thermally stability ofthe materials including metal sulfates [20–25]. Meanwhile, lantha-nide elements exhibit unprecedented topological architectures andcoordination chemistry because of the accessibility of highercoordination numbers and the inherent flexibility, which is impos-sible with main group elements [26,27]. However, less attentionhas been paid to hybrid organic–inorganic frameworks including

010 Published by Elsevier B.V. All

1.

inorganic skeletons formed by lanthanide sulfate and bridging or-ganic pillars. In addition, to the best of our knowledge, only twoexamples of three-dimensional hybrid materials based on two-dimensional inorganic skeletons formed by lanthanide sulfateand bridging organic ligands has been reported so far [28,29].

On the other hand, hydrothermal synthesis has recently becomea useful tool for the fabrication of novel metal–carboxylateframeworks [30,31]. Particularly, the hydrothermal reaction cangenerally present relatively compact crystal packing and reducemetal-aqua coordination geometry to induce condensed metal–carboxylate frameworks [32–34]. But this kind of reaction is quiteaffected by a number of factors such as temperature, concentra-tion, and pH for a successful synthesis to obtain a polymeric struc-ture as expected [35].

We have begun to construct hybrid organic–inorganic frame-works by the reactions of lanthanide sulfate with flexible 1,4-pipe-razinediacetic acid under hydrothermal conditions in view of thefollowing characteristics: (a) flexible spacer ligands are much diffi-cult to control their conformations in the architecture of the prod-ucts, but piperazine is much easy to exhibit its chair configuration,compatible to its analogue-cyclohexane, which can reduce its coor-dination modes and strength the predictablility of the correspond-ing polymeric frameworks; (b) the dicarboxylic ligand is easilycoordinated to hard lanthanide ions as a linker; (c) as an artificialamino acid, the proton transfer of the carboxylates to the nitrogenatoms of piperazine (see Scheme 1) has little influence on the

rights reserved.

NH NH

H2pda

H2C

H2CC C

O

O

O

O

Scheme 1.

L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220 215

coordination capability of carboxylate, and is capable of makingthe organic pillar neutralized, which is much helpful to form a neu-tral lanthanide sulfate chain/layer and a neutral framework. Wehave recently synthesized the first hybrid organic–inorganicframeworks based on lanthanide(III) sulfate chains, in situ gener-ated oxalate and organic pillars of 1,4-piperazinediacetic acid un-der the hydrothermal conditions at 160 �C [36]. Herein reportedare two types of three-dimensional hybrid organic–inorganicframeworks based on lanthanide(III) sulfate layers and the sameorganic pillars, [Ln2(H2pda)(SO4)3(H2O)4]n [I, Ln = La3+ (1), Pr3+

(2), Nd3+ (3), Sm3+ (4) and Eu3+ (5)] and {[Ln2(H2pda)(SO4)3(H2O)2]�2H2O}n [II, Ln = Gd3+ (6) and Dy3+ (7)] (H2pda = 1,4-piperazinedi-acetic acid), by changing the reaction temperature.

2. Experimental section

2.1. Materials and methods

All solvents and reagents were obtained commercially and usedwithout further purification. Infrared (IR) spectroscopic studieshave been carried out in the mid-IR region as a KBr pellet (BrukerVector22 FT-IR). C, H, N and S microanalyses were carried out witha Perkin–Elmer 1400C analyzer. Thermal analysis (TG) was per-formed on a TGA Q500 thermal analyzer under nitrogen atmo-sphere at a scan rate of 10 �C min�1. Magnetic susceptibility dataof powder samples were collected in the temperature range of1.8–300 K with the use of a Quantum Design MPMS XL-7 SQUIDmagnetometer.

2.2. Synthesis of compounds

The synthesis procedures of 1–7 are similar and a typical syn-thetic procedure is described below.

Synthesis of [La2(H2pda)(SO4)3(H2O)4]n (1). A mixture ofLa2O3(0.033 g, 0.1 mmol), H2pda�2HCl (0.027 g, 0.1 mmol) and50% H2SO4 (0.1 mL) and water (6 mL) was heated in a 15-mLTeflon-lined vessel at 120 �C for 3 days, followed by slow cooling(5 �C h�1) to room temperature. After filtration and washing withH2O, pale-yellow block crystals were collected and dried in air(0.012 g, yield ca. 28% based on H2pda). Anal. Calcd. for C8H22La2-

N2O20S3: C, 11.43; H, 2.64; N, 3.33; S, 11.45. Found: C, 11.59; H,2.29; N, 3.05; S, 11.58%. Main IR (KBr, cm�1): 3351(s), 3214(s),2976(m), 1727(vs), 1655(s), 1594(vs), 1460(w), 1437(s), 1414(vs),1382(w), 1276(m), 1134(m).

2.3. X-ray crystallography

Diffraction intensities for the compounds were collected at291(2) K on a Bruker Apex CCD area-detector diffractometer (MoKa, k = 0.71073 Å). Absorption corrections were applied by usingmultiscan program SADABS [37]. The structures were solved withdirect methods and refined with full-matrix least-squarestechnique using the SHELXTL program package [38]. Anisotropic

thermal parameters were applied to all the non-hydrogen atoms.The organic hydrogen atoms were generated geometrically (C–H:0.96 Å). Crystal data as well as details of data collection and refine-ments for the complexes are summarized in Table 1. Selected bonddistances and bond angles are listed in Table S1, and hydrogenbonding parameters are given in Table S2.

3. Results and discussion

3.1. Synthesis

Compared with the synthesis of the first hybrid organic–inor-ganic frameworks employing H2pda�2HCl (H2pda = 1,4-piperazin-ediacetic acid), 50% H2SO4 in the presence of different lanthanideoxide at 160 �C that we have reported [36], compounds 1–7 wereprepared under rather milder hydrothermal conditions and thereaction temperature was reduced to 120 �C. Surprisingly, no oxa-late were formed in these coordination polymers, which hintedthat the decomposition of the carboxylic groups of the ligand onlyoccurs in a rather higher temperature and pressure under hydro-thermal conditions. Meanwhile, compounds 1–5 have differentinorganic layers of lanthanide sulfate and different frameworksfrom those of 6–7 under the similar conditions, which may beattributed to the lanthanide contraction effect. All the resultingcompounds are not soluble either in H2O or common organic sol-vents such as CH3OH, CH3CN, and CH3COCH3.

3.2. Type I structure of 1–5

According to the single-crystal XRD study, complexes 1–5 crys-tallize in the monoclinic system, C2/c space group. Since the crys-tals of 1–5 are isomorphous according to the X-ray structureanalysis, only the structure of 1 is depicted in detail. 1 is a three-dimensional framework consisting of two-dimensional inorganic[La(SO4)(H2O)]1 layers and organic H2pda pillars. As shown inFig. 1a, each La(III) ion is surrounded by eight oxygen atoms, inwhich four oxygen atoms are from four sulfates, two carboxylateoxygen atoms from two H2pda ligands and two from two coordi-nated water molecules, forming a LaO8 polyhedron of distortednub disphenoid geometry. The La–O bond distances of LaO8 arein the range 2.419(3)–2.586(3) Å (av. 2.52 Å), in which the shortestLa–O separation resulted from the La1–O4 g bond of tridentate sul-fate, while the longest La1–O1W from the coordinated water.Other distances of La–O(sulfate) vary in the range of 2.419(3)–2.545(4) Å. La–O1 and La–O2f bonds from H2pda are 2.562(4)and 2.419(3) Å, respectively.

The three-dimensional structure can be considered as an organ-ic–inorganic hybrid framework constructed by two-dimensionalinorganic layers and organic H2pda pillars. The inorganic layersconsist of inorganic [La(SO4)(H2O)]1 chains running along theb-axis and l2-oxobridged SO2�

4 . All the sulfates in compound 1have two types of La–O coordination modes: l2-oxobridged sulfateand l3-oxobridged sulfate (Fig. 2b and c). In the [La(SO4)(H2O)]1chains (Fig. 3a and b), two La3+ groups are capping on two g3,l3-SO2�

4 ions formed into eight-membered [La2(O–S–O)2]1 rings,which is similar to those in {[Ln2(H2pda)(C2O4)(SO4)2(H2O)2]�2H2O}n [II, Ln = Pr3+, Ce3+ and Eu3+] [36]. There are also two kindsof eight-membered rings in one chain: one is that the bridges be-tween the La3+ ions are only two SO2�

4 anions; the other is thatthe bridges are two syn-anti carboxylates, respectively, comingfrom two H2pda ligands, besides two bridging SO2�

4 anions. TheLa–La distances in the rings are 4.472 and 5.642 Å, respectively.These two inorganic rings are alternately arranged into an inor-ganic [La(SO4)(H2O)2]1 chain by sharing their two apexes (La3+

ions) with two coordinated H2O molecules in one apex, which is

Table 1Crystal data and structure refinement for 1–7.

Compound 1 2 3 4

Empirical formula C8H22La2N2O20S3 C8H22N2O20Pr2S3 C8H22N2Nd2O20S3 C8H22N2O20S3Sm2

Formula weight 840.31 844.31 850.97 863.21Crystal system Monoclinic Monoclinic Monoclinic MonoclinicSpace group C2/c C2/c C2/c C2/ca (Å) 24.565(2) 24.436(4) 24.366(3) 24.187(3)b (Å) 6.7033(6) 6.6606(11) 6.6443(8) 6.5870(7)c (Å) 16.7768(15) 16.737(3) 16.718(2) 16.6304(19)a (deg) 90 90 90 90b (deg) 128.827(1) 128.881(6) 128.889(4) 128.946(4)c (deg) 90 90 90 90V (Å3) 2152.2(3) 2120.6(6) 2106.7(5) 2060.7(4)Z, Dcalcd. (Mg/m3) 4, 2.593 4, 2.645 4, 2.683 4, 2.782T/K 291(2) 291(2) 291(2) 291(2)F(000) 1624 1640 1648 1664l (mm�1) 4.310 4.940 5.277 6.054hmin/hmax �30/25 �29/21 �30/27 �29/22kmin/kmax �8/5 �8/8 �8/8 �7/8lmin/lmax �19/20 �17/20 �14/20 �16/20Ref. collected/unique 5460/2108 (Rint = 0.042) 5456/2078 (Rint = 0.038) 5390/2061 (Rint = 0.076) 5286/2005 (Rint = 0.047)Parameters 163 163 163 159R1

a (I > 2r) 0.0317 0.0236 0.0386 0.0297wR2

b (all data) 0.0851 0.0563 0.0938 0.0825GOF 1.10 1.04 1.027 1.091Max/min (Dq) (e �3) 1.05/�2.18 0.77/�0.77 2.13/�1.64 1.69/�1.81

Compound 5 6 7

Empirical formula C8H22Eu2N2O20S3 C8H18Gd2N2O18S3�H2O C8H18Dy2N2O18S3�H2OFormula weight 866.41 858.97 869.47Crystal system Monoclinic Triclinic TriclinicSpace group C2/c P-1 P-1a (Å) 24.212(3) 6.7326(13) 6.694(3)b (Å) 6.5900(7) 12.392(2) 12.313(6)c (Å) 16.664(2) 13.035(2) 12.952(6)a (deg) 90 75.963(4) 76.308(9)b (deg) 128.944(4) 76.038(4) 76.254(8)c (deg) 90 74.337(3) 74.460(9)V (Å3) 2068.0(4) 997.7(3) 981.8(8)Z, Dcalcd. (Mg/m3) 4, 2.783 2, 2.859 2, 2.941T/K 291(2) 291(2) 291(2)F(0 0 0) 1672 820 828l (mm�1) 6.420 7.008 7.977hmin/hmax �29/20 �7/8 �8/7kmin/kmax �8/7 �14/15 �15/11lmin/lmax �17/20 �10/16 �15/15Ref. collected/unique 5277/2010 (Rint = 0.047) 5382/3810 (Rint = 0.044) 5322/3744 (Rint = 0.034)Parameters 159 287 287R1

a (I > 2r) 0.0328 0.0634 0.0445wR2

b (all data) 0.0779 0.1380 0.0832GOF 1.083 1.012 1.029Max/min (Dq) (e �3) 1.41/�1.37 3.91/�1.92 1.50/�2.01

a R1 =P

Fo � Fc/P

Fo.b wR2 = [

PwðF2

o � F2c Þ

2/P

w(F2o )2]1/2.

216 L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220

further linked by g2,l2-SO2�4 to construct an unusual two-dimen-

sional [La2(SO4)3(H2O)4]1 layer along the a-axis with the shortestinterchain La���La distance 6.24(4) Å (Fig. 3c). Meanwhile, inter-layer strong hydrogen bonds are found between the coordinatedwater molecules and the oxygen atoms of sulfate groups[O1W���O8 m 2.787(5), O1W���O7a 3.112(5), O2W���O5k 2.794(6)and O2W���O5g 2.708(5) Å] to stabilize the layer. Finally, thesetwo-dimensional inorganic layers are pillared by electroneutraltetradentate H2pda ligands in a bidentate syn-anti mode into athree-dimensional hybrid organic–inorganic framework (Fig. 3d).The intralayer distance is ca. 8.4 Å.

The H2pda pillars also interact with the adjacent layers viaN–H���O, O–H���O and weak C–H���O hydrogen bonds (Table S2),which contributes to the additional stability of the structure. TheN–H���O hydrogen bond originates from the protonated N atomsof the pillars and the O atoms of SO2�

4 with a N���O distance of2.756(5) Å. The O–H���O hydrogen bond involves the coordinated

waters from the layers and the carboxylate oxygen atoms fromthe adjacent pillars, and the O1W���O1a distance is 2.911(5) Å.The C–H���O hydrogen bonds come from the piperazine C–H andthe oxygen atoms of SO2�

4 and coordinated water molecules, theC���O distances ranging from 3.048(7) to 3.363(5) Å.

3.3. Type II structure of 6 and 7

The structure of type II, represented by compound 6, is differentfrom those of type I, though it can also be described as a three-dimensional framework consisting of inorganic [Ln2(SO4)3(H2O)4]1layers and organic H2pda pillars. Firstly, there are two crystallo-graphically independent Gd(III) ions owning the same coordinationmode with similar bond lengths and bond angles in an asymmetricunit. Secondly, the local coordination environment around a Ln3+

ion is a little different. As shown in Fig. 1b, both Gd1 and Gd2are eight-coordinated and exhibit a distorted nub disphenoid

Fig. 1. Local coordination environment of compounds 1 (a) and 6 (b) with 30% thermal ellipsoids. All the hydrogen atoms are omitted for clarity. Symmetry codes for 1, f: 3/2 � x, 1/2 � y, 2 � z; g: 3/2 � x, 3/2 � y, 2 � z; for 6, a: �x, 2 � y, 2 � z; d: 1 � x, 2 � y, 2 � z.

Fig. 2. Coordination modes of H2pda and SO2�4 anions observed in compounds 1–7.

L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220 217

geometry, being coordinated by five oxygen atoms from three sul-fates, two carboxylate oxygen atoms from two H2pda ligands andone water molecule. The Gd–O distances range from 2.41(4) to

2.52(4) Å with the average value 2.47 Å, comparable with that inthe Gd3+ complex coordinated by eight oxygen atoms from sul-fates, carboxylates and coordinated water molecules (2.41 Å)

Fig. 3. Structures of 1D inorganic [La(SO4)(H2O)]1 chain (a), its polyhedral form (b, green: SO4 polyhedra; purple: LaO8 polyhedra), inorganic layer linked via SO2�4 anions (c),

and 3D framework pillared by H2pda (d) in 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

218 L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220

[28]. Thirdly, all the sulfates in 6 adopt two types of coordinatedmodes: one is g3,l3-tridentate coordination mode, which is iden-tical to that in 1, and the other is g4,l2-tetradentate coordinationmode, different from the g4,l2-bidentate coordination mode ob-served in 1.

Two kinds of crystallographically independent Gd1 and Gd2in 6, respectively, compose of the analogous one-dimensional[Gd(SO4)]1 chains with g3,l3-sulfates, which is the same as thatin 1. The intrachain Gd1���Gd1 distances are 4.530 and 5.506 Å,while the corresponding Gd2���Gd2 distances are 4.505 and5.592 Å. These two kinds of analogical inorganic chains are alter-nately linked by g4,l2-tetradentate sulfates to form a two-dimen-sional inorganic grid (Fig. 4c). The repeating unit in the 2D gridexhibits a rectangular configuration consisting of four Gd(III)atoms with a size of 6.183 � 6.733 Å2. Meanwhile, intralayerstrong hydrogen bonds are found between the coordinated watermolecules and the oxygen atoms of SO2�

4 groups [O1W���O14k2.806(14), O2W���O4j 2.795(14) and O2W���O14a 2.777(15) Å] tostabilize the layers.

The 3D organic–inorganic framework structure is generated bythe connections of the 2D inorganic layers via the organic H2pda li-gands, as shown in Fig. 4d. Just like [Ln2(H2pda)(C2O4)(SO4)2

(H2O)2]n [Ln = Nd3+, Y3+], {[Ln2(H2pda)(C2O4)(SO4)2(H2O)2]�2H2O}n

[Ln = Pr3+, Ce3+ and Eu3+] [36] and those of type I, each carboxylateof H2pda links two Ln3+ ions from one chain in a bidentate syn-antimode and consequently each H2pda ligand connected with fourLn3+ ions from the adjacent layers. The shortest intralayer Gd���Gddistance is 7.71 Å, being a little shorter than La���La distance of8.34 Å in 1. The 3D structure is also stabilized by N–H���O hydrogenbonds coming from the protonated nitrogen atoms of H2pda andthe oxygen atoms of sulfate with the distance of 3.020(17) Å, to-gether with a great deal of weak C–H���O hydrogen bonds originat-ing from the carbon atoms of H2pda pillars and the oxygen atomsfrom the SO2�

4 groups. The C���O distances range from 2.926(17) to3.344(18) Å. It is noteworthy that there are ‘‘empty” spaces in the3D network, which are filled with lattice water molecules. Theseuncoordinated water molecules are sustained in the apertures ofthis 3D construction by the O–H���O hydrogen bonds involving

the oxygen atoms from sulfate ions [O3w���O10 2.848(17) Å,O3w���O9s 2.716(18) Å and O3w���O16a 3.074(16) Å].

3.4. Thermogravimetric properties

The thermogravimetric analyses of crystal samples 1 and 6 werecarried out from 22 to 784 �C under nitrogen atmosphere at theheating rate of 10 �C min�1, as shown in Fig. 5. For compound 1,it is stable below 188 �C, and an initial weight loss of 9.1% occurredbetween 188 and 220 �C corresponds to the removal of the coordi-nated water molecules (8.6% calculated). The weight loss of 44% ina range of 220–784 �C indicates the decomposition of the wholestructure. The TGA curve of 6 shows that it is stable below300 �C, which indicated that the crystal lattice water moleculesare extraordinary stable due to the strong hydrogen bonding inter-actions. But there are no obvious phenomena corresponding to theremoval of the uncoordinated water (2.1% calculated) and the coor-dinated water (4.2% calculated) in the TGA curve, proving that theremoval of the water molecules and the decomposition of thewhole framework almost occur in the same time.

3.5. Magnetic properties

The magnetic susceptibility of 2 and 3 have been measured in thetemperature range of 1.8–300 K at an applied field of 2000 Oe andthe plots of vM and vMT vs. T are shown in Figs. 6 and 7, respectively.For 2, when the temperature decreases from 300 K, the vM valuegradually increases from 0.005 cm3 mol�1 to 0.024 cm3 mol�1 at1.8 K. Meanwhile, the vMT value per Pr(III) ion is 1.47 cm3 K mol�1

at 300 K, which is slightly less than the theoretical value of1.60 cm3 K mol�1 for magnetically isolated Pr(III) ions, then de-creases to 0.048 cm3 K mol�1 at 1.8 K on falling. The variable-temperature magnetic property of a free rare earth ion generallyshows strong deviations from the Curie–Weiss law, and vMTdecreases with the cooling temperature owing to the depopulationof Stark levels [39]. Therefore, the magnetic behavior of 2 may be as-cribed to intramolecular antiferromagnetic coupling between themetal centers and/or the orbital coupling contribution.

Fig. 4. Structures of 1D [Gd(SO4)(H2O)]1 inorganic chain (a), its polyhedral form (b, green: SO4 polyhedra; blue: GdO8 polyhedra), 2D inorganic layer linked via SO2�4 anions

(c), and the 3D framework pillared by H2pda (d) in 6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

100 200 300 400 500 600 700 80040

50

60

70

80

90

100

Temperature ( °C)

Wei

ght %

6 1

Fig. 5. TGA plots for 1 and 6.

0 50 100 150 200 250 300

0.005

0.010

0.015

0.020

0.025

0.0

0.4

0.8

1.2

1.6

T / K

χ m /

cm3 m

ol-1

χm T / cm

3 K m

ol -1

Fig. 6. Temperature dependence of vM (j) and vMT ( ) for 2.

0 50 100 150 200 250 300-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.6

0.8

1.0

1.2

1.4

1.6

T / K

χm T / cm

3 K m

ol -1

χ m /

cm3 m

ol-1

Fig. 7. Temperature dependence of vM (j) and vMT ( ) for 3.

L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220 219

In 300 K, the vMT value of 3 (1.52 cm3 K mol�1) per Nd(III) ion issomewhat lower than that expected for an isolated Nd(III) ion

(1.64 cm3 K mol�1). On cooling, vMT falls to 0.50 cm3 K mol�1 at1.8 K, and vM increases with the temperature down and the valueof vM for 3 ranges from 0.003 cm3 mol�1 at 300 K to 0.28 cm3

mol�1 at 1.8 K. This may also be attributed to the depopulationof Stark levels and/or intramolecular antiferromagnetic couplingbetween the metal centers.

4. Conclusions

In summary, we have successfully synthesized and character-ized two series of inorganic–organic hybrid frameworks [Ln2

(H2pda)(SO4)3(H2O)4]n [I, Ln = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4)and Eu3+ (5)] and {[Ln2(H2pda)(SO4)3(H2O)2]�2H2O}n [II, Ln = Gd3+

(6) and Dy3+ (7)] based on H2pda, lanthanide oxides and H2SO4 un-der similar hydrothermal conditions. All the frameworks are basedon organic H2pda pillars and two-dimensional inorganic layers,which are constructed by similar [Ln(SO4)(H2O)]1 chains anddifferent sulfate bridges. Compounds 1 and 6 are thermally stable

220 L. Cheng et al. / Journal of Molecular Structure 979 (2010) 214–220

below 180 and 300 �C, respectively. The variable-temperature mag-netic studies show that the vM values of compounds 2 and 3 in-crease on cooling, which may be attributed to the depopulation ofStark levels and/or intramolecular antiferromagnetic coupling be-tween the metal centers.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (No. 20271026) and the Foundation of SoutheastUniversity (Nos. 4007041121 and 9207040016).

Appendix A. Supplementary material

CCDC 686352–686358 contain the supplementary crystallo-graphic data for compounds 1–7. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223–336-033; or e-mail:deposit@ccdc.cam.ac.uk. The detailed synthesis procedures of 2–7and additional tables for the complexes are in PDF format as sup-plementary materials.

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2010.06.029.

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