Polyhedron 78 (2014) 1–9

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Polyhedron

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Multinuclear-based copper coordination architectures constructedfrom pyridyl-1H-benzimidazol-derived flexible tripodal connectorand the magnetic behaviors

http://dx.doi.org/10.1016/j.poly.2014.04.0120277-5387/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 375 2089090.E-mail addresses: hujiyong@hncj.edu.cn (J. Hu), zjinan@zzu.edu.cn (J. Zhao).

Jiyong Hu a, Hongchang Yao b, Yun Bai b, Dandan Zhao b, Shufang Chen a, Jin’an Zhao a,⇑a College of Chemical and Material Engineering, Henan University of Urban Construction, Henan 467036, PR Chinab College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450052, Henan, PR China

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

Article history:Received 22 December 2013Accepted 8 April 2014Available online 18 April 2014

Keywords:Metal–organic architectureTripodal linkersMultinuclear building unitInclined interpenetrationMagnetic behaviors

Consideration of multinuclear-based coordination architecture with specific properties, two new coppercomplexes based on a tripodal flexible ligand 1,3,5-tris[(2-pyridyl-1H-benzimidazol)-1yl-methyl]-2,4,6-trimethylbenzene (TPBT), namely, [CuI

4CuII4(TPBT)4(l-Cl)4Cl8(H2O)17]n (1), [Cu6(TPBT)2(SO4)6(H2O)5]n (2),

have been constructed under the solvothermal reactions. Complex 1 is a 2D reticulate structure with themixed-valence Cu(I,II) centers. While complex 2 features an inclined interpenetration of 2D sheets, whichlead to the 3D supramolecular architecture, and its asymmetric unit contains six distinct Cu(II) centers.In addition, upon decreasing the temperature, the vmT value of 1 increases gradually to a maximum at82 K and then decreases. Whereas the sample of 2 is cooled, the feature of vmT value exhibits antiferromag-netic interactions between Cu(II) centers.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The design and construction of specified structural topologies,in particular, multinuclear-based architectures, which may exhibitvarious metal–metal interactions and lead to higher stability andporosity, has recently drawn much attention in the field of supra-molecular chemistry [1]. The interactions of metal centers mayendow such entities yield unique physical properties, such as mag-netism, catalysis, luminescence, gas adsorption, etc [2]. These func-tions primarily depend on the well-defined structures andcompositions, which are controlled by various factors, such asthe coordination preference of metal centers, the suitable shape,functionality and symmetry of the tailored organic linkers [3].Mainly due to the fact that Cu is an essential bioelement responsi-ble for numerous catalytic processes in living organisms and also,to the possibility of exploring magneto-structural correlationsresulting from the mutual interactions among the metal centers,the copper multinuclear-based assemblies have drawn tremen-dous attention [4]. The structure-specific self-assembly could ten-tatively be favored by the suitable tailored organic connector.

Multidentate N-containing group can bridge adjacent metalcenters by versatile coordinating sites, sustaining a variety of mul-tinuclear building units [5]. Such units are further extended by the

tripodal platform giving rise to multinuclear-based multidimen-sional motifs [6]. The functional arms constructed around centralarene group make the trimethylbenzene spacer become an appeal-ing building block, since these substituents attached to the 1,3,5-positions of 2,4,6-trimethylbenzene oriented on the opposite orsame sides of the benzene plane resulting in diverse architectures[7]. By contrast, the reports about the multidentate N-heterocyclegroups as arms to construct such structures remain practicallyunexplored, whereas the multicarboxylate groups are usually uti-lized as arms around the central arene moiety. For example, severalmotifs based on a series of dendritic hexacarboxylate linkers withthree isophthalate appendages linked to the central platform havebeen reported [8].

The above consideration was exemplified by the preparation of aC3 symmetrical tripods 1,3,5-tris[(2-pyridyl-1H-benzimidazol)-1yl-methyl]-2,4,6-trimethylbenzene (TPBT), constructed around cen-tral arene groups and extended with three 2-pyridyl-1H-ben-zimidazol arms. The three free-rotating arms connected viamethylene entities can fit coordinative geometry of metal centersadaptively. Such flexibility also affords opportunity to investigatehow the self-assembly process is affected by the distinct anionsengaging in strong and specific bonding interactions, and the anionsand TPBT may act synergetically to afford networks with fascinatingaesthetics and promising functions. Consequently, two structuraldiversity copper complexes, namely, [CuI

4CuII4(TPBT)4(l-Cl)4Cl8

(H2O)17]n (1), [Cu6(TPBT)2(SO4)6(H2O)5]n (2), have been constructed.

2 J. Hu et al. / Polyhedron 78 (2014) 1–9

As the temperatures decrease, the magnetic susceptibility measure-ments exhibit the vmT values increase steadily to reach a maximumaround 82 K for 1, and then rapid decrease. While the sample of 2 iscooled, the feature of vmT value exhibits antiferromagnetic interac-tions between Cu(II) centers.

Table 1Crystal data and structure refinement for complexes 1–2.

Complex 1 2

Formula C192H176Cl12Cu8N36O17 C96H78Cu6N18O29S6

Fw 4193.41 2521.36T (K) 100(2) 113(2)k (Cu, Mo Ka) (Å) 1.54178 0.71073Crystal system orthorhombic monoclinicSpace group Pca21 Cc

a (Å) 30.1144(4) 28.946(8)b (Å) 14.9083(2) 21.063(5)c (Å) 21.9452(2) 23.721(7)a (�) 90 90b (�) 90 116.738(4)c (�) 90 90V (Å3) 9852.4(2) 12916(6)Z 2 4F (000) 4304 5128h Range for data (�) collection

(�)3.56–73.44 1.58–25.00

Final R1a, wR2

b 0.0767, 0.2092 0.0853, 0.2277Goodness-of-fit (GOF) on F2 1.026 1.029

a R1 =P

||Fo| � |Fc||/P

|Fo|.b wR2 = [

Pw(Fo

2 � Fc2)2/

Pw(Fo

2)2]1/2.

2. Experimental

2.1. Materials and general methods

Chemicals were purchased from commercial sources and usedwithout further purification. The 2-pyridyl-1H-benzimidazole wassynthesized according to the literature method [9]. The 1,3,5-tris(2-pyridyl-1H-benzimidazol)-2,4,6-trimethylbenzene (TPBT) wassynthesized according to the reported procedures in the literature[10]. The IR spectra were recorded on a BRUKER TENSOR 27 spectro-photometer with KBr pellets in the region of 400–4000 cm�1. Ele-mental analyses (C, H and N) were performed on a Flash EA 1112elemental analyzer. The powder X-ray diffractions (XRD) were per-formed on a PANalytical X’Pert PRO diffractometer with monochro-matized Cu Ka radiation. The thermogravimetric analysis anddifferential scanning calorimetric analysis were carried out with aNETZSCH STA409 PC/PG thermal analysis instrument over the range30–850 �C by a heating rate of 10 �C/min under air atmosphere. Var-iable temperature magnetic susceptibilities were measured with aSQUID MPMS-XL7 magnetometer, and the temperature is in therange 1.8–300 K.

2.2. Synthesis of [CuI4CuII

4(TPBT)4(l-Cl)4Cl8(H2O)17]n (1)

A mixture of CuCl2 (0.0085 g, 0.05 mmol), TPBT (0.0148 g,0.02 mmol), methanol (6 mL), chloroform (3 mL) and acetonitrile(3 mL) was placed in a 20 mL Teflon-lined stainless steel vessel.The mixture was sealed and heated at 100 �C and kept for 3 days,and then the reaction system was gradually cooled to room tem-perature at a rate of 5 �C h�1. Green block crystals were obtained.Yield: 45%, (based on Cu). Elemental Anal. Calc. for C192H176Cl12Cu8

N36O17: C, 54.99; H, 4.23; N, 12.02. Found: C, 54.78; H, 4.31; N,12.14%. IR(KBr/pellet, cm�1): 3405.51(s), 1600.55(w), 1477.06(s),1384.81(s), 1335.28(w), 1293.50(w), 1181.84(w), 1054.04(s),1015.57(w), 926.87(s), 788.53(w), 745.49(s), 692.88(m),495.88(w), 431.48(w).

2.3. Synthesis of [Cu6(TPBT)2(SO4)6(H2O)5]n (2)

A mixture of CuSO4�5H2O (0.0062 g, 0.025 mmol), TPBT(0.0074 g, 0.01 mmol), methanol (0.5 mL), water (0.5 mL), andDMSO (1 mL) was placed in a glass reactor (10 mL), which was per-formed at 85 �C for 3 days and then gradually cooled to room tem-perature at a rate of 5 �C h�1. Green spiculate crystals wereisolated. Yield: 72% (base on Cu). Elemental Anal. Calc. for C96H78-

Cu6N18O29S6: C, 45.73; H, 3.12; N, 10.00. Found: C, 45.51; H,3.24; N, 10.21%. Element analysis was measured after heated com-plex 2 at 240 �C for 2 h. IR(KBr/pellet, cm�1): 3424.93(s),1610.19(s), 1479.57(w), 1429.36(s), 1383.50(s), 1339.16(s),1277.82(w), 1113.21(s), 1018.06(w), 949.91(w), 790.04(m),744.76(s), 618.49(s), 424.52(w).

2.4. Crystal structure determination

A crystal suitable for X-ray determination was mounted on aglass fiber. The data of 1 was collected on a SuperNova with graph-ite monochromated radiation with graphite monochromated CuKa (k = 1.54184 Å) at 100 K, whereas 2 was conducted on a RigakuMM-OO7/Saturn 70 using graphite-monochromated Mo Ka

radiation (k = 0.71073 Å) at 113 K. The structures were solved bydirect methods and refined by full-matrix least-squares. The non-hydrogen atoms were refined with anisotropic thermal parame-ters. All calculations were using the SHELXTL-97 program system.Hydrogen atoms were assigned idealized positions and wereincluded in structure factor calculations. The final cycle of full-matrix least-squares refinement was based on the observed reflec-tions and variable parameters [11]. Besides, the solvent moleculeswere badly disordered and could not be modeled, which wasremoved from the structure of 2 by squeeze option in platon[12]. Table 1 gives the crystallographic crystal data and structureprocessing parameters and the selected bond lengths and anglesof them are listed in the Table 2 for the two complexes.

3. Results and discussion

3.1. Crystal structure of [CuI4CuII

4(TPBT)4(l-Cl)4Cl8(H2O)17]n (1)

Complex 1 belongs to the orthorhombic space group Pca21 andexhibits a 2D sheet structure. As shown in Fig. 1, the asymmetricunit consists of four crystallographically independent copper cen-ters, two TPBT connectors, and six Cl�. All the copper centers arepenta-coordinated. The charge balance indicates that 1 is a cop-per(I,II) mixed-valence compound, although the Cu(II) salt wasapplied in the reaction. Among them, Cu1 and Cu4 bearing distortedtrigonal–bipyramidal coordination sphere are monovalent, whichare balanced by one Cl�, respectively, whereas the Cu2 and Cu3 withsquare-pyramidal geometries are bivalent centers. The Cu1 is coor-dinated by one chloride (Cl1) and four nitrogen atoms (N4, N5, N16and N17) from two pyridyl-1H-benzimidazol moieties of two TPBTconnectors. The coordination environment around the Cu4 is com-pleted by one chloride anion (Cl6) and four nitrogen atoms (N1,N2, N10c, N11c) from two pyridyl-1H-benzimidazol moieties aswell. Cu2 binds two pyridyl-1H-benzimidazol nitrogen atoms(N13b, N14b), and the other three coordination positions matchedwith one terminate Cl anion (Cl4) and two l-Cl anions (Cl2, Cl3).Besides, Cu3 is constructed by two pyridyl-1H-benzimidazol nitro-gen atoms (N7, N8), one terminate chloride (Cl5) and two l-Clanions (Cl2, Cl3). Further, the neighboring Cu2 and Cu3 are heldtogether through somewhat weaker axial interactions (Cu2–Cl2 = 2.748 and Cu3–Cl3 = 2.707 Å), which is slightly longer thanthe sum of the covalent radii, 2.58 Å, similar to previous reports

Table 2Selected bond lengths (Å) and angles (�) for complexes 1–2.

Complex 1Cu(1)–N(5)#1 2.028(6) Cu(1)–Cl(1) 2.2486(19)Cu(2)–N(13) 2.033(8) Cu(2)–Cl(2) 2.748(2)Cu(3)–N(8) 1.960(6) Cu(3)–Cl(3) 2.705(2)Cu(4)–N(2) 2.165(7) Cu(4)–Cl(6) 2.260(2)N(5)–Cu(1)–N(17)#1 93.3(2) N(5)–Cu(1)–Cl(1)#1 146.07(19)N(14)–Cu(2)–N(13) 172.8(2) N(13)–Cu(2)–Cl(2) 92.1(2)N(8)–Cu(3)–N(7) 78.8(3) N(8)–Cu(3)–Cl(4) 170.5(2)N(10)–Cu(4)–N(2) 94.6(3) N(2)–Cu(4)–Cl(6) 124.83(19)

Complex 2Cu(1)–O(1) 1.939(6) Cu(1)–N(1) 1.948(8)Cu(1)–O(5) 1.979(7) Cu(1)–N(2) 1.996(8)Cu(2)–O(6) 1.963(7) Cu(2)–O(4) 1.970(6)Cu(2)–N(11) 2.000(7) Cu(2)–O(13) 2.266(7)Cu(3)–N(7)#1 1.934(9) Cu(3)–O(2) 2.021(7)Cu(3)–O(25) 2.334(9) Cu(4)–O(7) 1.945(6)Cu(4)–N(16)#2 1.949(8) Cu(4)–O(13) 1.956(7)Cu(4)–N(17)#2 1.981(5) Cu(5)–N(4) 1.871(10)Cu(5)–O(26) 2.125(13) Cu(6)–O(29) 1.940(8)Cu(6)–O(21) 1.945(8) Cu(6)–N(13) 1.956(9)O(1)–Cu(1)–N(1) 91.0(3) O(1)–Cu(1)–O(5) 93.1(3)N(10)–Cu(2)–O(6) 92.4(3) O(6)–Cu(2)–O(4) 92.8(2)N(7)#1–Cu(3)–O(2) 92.0(3) O(2)–Cu(3)–N(8)#1 163.1(4)O(7)–Cu(4)–N(16)#2 90.1(3) O(7)–Cu(4)–O(13) 93.9(3)N(4)–Cu(5)–O(17) 176.3(4) O(17)–Cu(5)–O(27) 93.2(4)O(29)–Cu(6)–O(21) 90.9(4) O(29)–Cu(6)–N(13) 97.0(4)

Symmetry transformation used to generate equivalent atoms: #1 �1/2 + X,�Y, +Z;#2 1/2 + X,�Y, +Z; #3 1/2 + X,�1 � Y, +Z; #4 �1/2 + X,�1 � Y, +Z for 1; #1 x � 1/2,y � 1/2,z; #2 x + ½, y + 1/2,z for 2.

J. Hu et al. / Polyhedron 78 (2014) 1–9 3

[13]. In the dimeric [Cu2(l-Cl2)] entity, the equivalent basal positionof Cu(II) center is occupied by a l-Cl anion, which bridges the axialsite in the adjacent one simultaneously. Thus, two square-pyrami-dal Cu(II) centers are double connected with Cu� � �Cu separation of3.547 Å.

2-Pyridyl-1H-benzimidazole can adopt bridging or chelatingmodes binding one or more metal centers. For example, based onthis rigid molecule, several Cu(I) or Cu(II) coordination architec-tures with unique structural features have been constructed [14].With such moiety as arm, TPBT can bind the metal centers indifferent orientations. In 1, each arm binds one copper center

Fig. 1. The coordination environments of copper centers for 1 (all the hydr

through chelating fashion. The single copper center and dimeric[Cu2(l-Cl)2] units are joined together by TPBT to form 2D layersalong the a,b-plane (Fig. 2), which are further stacked to give a3D molecular structure (Fig. 3).

3.2. Crystal structure of [Cu6(TPBT)2(SO4)6(H2O)5]n (2)

SO42� anions have versatile coordination modes and may assist

the flexible TPBT to construct distinct frameworks. Replacementof CuCl2 in 1 with CuSO4 results in an inclined interpenetrating3D complex 2, which crystallizes in the monoclinic space groupCc. The asymmetric unit contains six independent Cu(II) centers,two TPBT, six SO4

2� anions and five coordinated aqua molecules.As shown in Fig. 4a, six Cu(II) centers show obviously differentcoordinate geometries. The platon-squeeze method was used tosubtract the effect of unlocated solvent molecules. Among them,Cu1, Cu2, Cu3 and Cu4 are bridged by SO4

2� anion to form a tetra-nuclear building unit, whereas the remaining Cu5 and Cu6 are cou-pled in pair by SO4

2� anion giving rise to another binuclear unit. Forthe tetranuclear unit, the Cu1 adopts a distorted square-pyramidalgeometry defined by two nitrogen atoms (N1, N2) from one pyri-dyl-1H-benzimidazol moiety, three oxygen atoms (O1, O5, O9)from three SO4

2� anions. Cu2 is in a distorted square-pyramidalcoordination geometry as well, which is completed by two nitro-gen atoms (N10, N11) from another pyridyl-1H-benzimidazol moi-ety, and three oxygen atoms (O4, O6, O13) from three differentSO4

2� anions. Contrastingly, Cu3 has a distorted octahedral coordi-nation geometry, with two oxygen atoms (O2, O9) and two nitro-gen atoms (N7, N8) lying on the equatorial plane, and the O5 aswell as the water oxygen (O25) in the apical positions. Also, thesix-coordinated Cu4 bears distorted octahedral coordinationsphere, where the equatorial coordination sites are occupied bytwo nitrogen atoms (N16, N17) and two SO4

2� oxygen atoms (O7,O13), and the axial positions are supplied by two SO4

2� oxygenatoms (O4, O14). As shown in Fig. 4b, the adjacent four Cu(II) cen-ters are linked by four SO4

2� anions (g1:g1:g2:l4, g1:g1:g2:l4,g2:l2, g1:g2:l3 coordination fashions for S1, S2, S3 and S4 con-taining SO4

2�) to form a tetranuclear building unit.Cu5 adopts trigonal–bipyramidal coordination environment,

with one SO42� oxygen atom (O17), two nitrogen atoms (N4, N5)

ogen atoms, solvent molecules and part labels are omitted for clarity).

Fig. 2. The 2D infinite layer motif of complex 1 parallel to a,b-plane (all hydrogen atoms and labels have been omitted for clarity).

Fig. 3. View of the 3D crystal packing of the structure of 1 and stacking layers of the 2D layers (all hydrogen atoms and solvent molecules have been omitted for clarity).

4 J. Hu et al. / Polyhedron 78 (2014) 1–9

from one pyridyl-1H-benzimidazol moiety lying in the equatorialplane, and two water oxygen atoms in the axis positions. Cu6 adoptssquare-pyramidal coordination geometry, and the equatorial plane

is completed by one terminal SO42� oxygen atom (O21), two nitrogen

atoms and one water oxygen atom. As shown in Fig. 4c, Cu5 and Cu6centers are bridged by one SO4

2� anion (g1:g1:l2 coordination

Fig. 4. (a) The Cu(II) coordination environment of 2. (b) The tetranuclear building unit of 2. (c) The binuclear entity of 2 (all hydrogen atoms are omitted for clarity).

J. Hu et al. / Polyhedron 78 (2014) 1–9 5

mode) to form another binuclear building unit, in which the SO42�

bridge occupies a basal position in the coordination sphere of Cu5and an apical position in coordination geometry of Cu6. Also, theslightly longer apical Cu–O distance of 2.665 Å lies in the range ofother copper complexes [15]. The tetranuclear and binuclear build-ing units are joined together by tripodal TPBT resulting in a 2Dsheet, in which the purple spheres are the existence of pores(Fig. 5). Consequently, to reduce the pore size, the potential voidsare filled via mutual twofold inclined interpenetration of identical2D networks resulting in a 3D architecture (Fig. 6).

3.3. XRD patterns and Thermal analyses

To confirm the phase purity of these complexes, Powder XRDpatterns were recorded for complexes 1 and 2, and as-synthesizedcompound matched well with simulated patterns calculated fromthe single-crystal diffraction data (Figs. S1 and S2 of the Supportinginformation), indicating the phase purity of each bulk sample. Theair-stable complexes 1–2 can retain their crystalline integrity atambient temperature. Thermogravimetric analysis of 1 showed

an initial weight loss of 6.38% taken place between 30 and198.1 �C, accorded with the loss of corresponding solvent mole-cules (calcd 7.30%). It is confirmed by the DSC, as the spectrumof DSC exhibits an endothermic peak near 78.1 �C. The complexis stable up to 278.4 �C, and there is an exothermic peak at284.3 �C, demonstrating the decomposition (Figs. S3 and S4). For2, the first weight loss of 15.51% until 244.5 �C, this can be assignedto the loss of coordinated and solvent water molecules. The com-plex 2 is stable before 244.5 �C, with an exothermic peak at265 �C, which indicates the decomposition.

3.4. Magnetism behaviors

The temperature dependence of the magnetic susceptibility ofcomplex 1 was measured from 1.8 to 300 K in an applied magneticfield of 2 kOe. Fig. 7 shows the vm and vmT versus T plots. The val-ues of vmT at 300 K is 0.751 cm3 K mol�1, which is consistent withtwo isolated spin S = 1/2 (0.75 cm3 K mol�1 for g = 2). Consideringthe structure of 1, the value is reasonable because two Cu(I) ionsare included in the structure unit. Upon cooling, the vmT values

Fig. 5. The 2D structure of complex 2 (all hydrogen atoms are omitted for clarity).

Fig. 6. The doubly inclined interpenetration 2D?3D dimensional expansion of complex 2.

6 J. Hu et al. / Polyhedron 78 (2014) 1–9

increase gradually from 0.751 cm3 K mol�1 at 300 K to reach amaximum value of 0.76 cm3 K mol�1 at 82 K. Below this tempera-ture, the vmT values rapidly decrease to 0.70 cm3 K mol�1 at 1.8 K.The increase of vmT values are the characteristic of the presence of

ferromagnetic coupling between the magnetic centers, while adrop below 82 K may be due to the inter-molecular antiferromag-netic exchange, zero-field splitting (ZFS), or a mixture of both. Con-sidering that complex 1 has a dinuclear structure in which the

Fig. 7. Magnetic susceptibility of 1 plotted as vm (D) vs. T (left axis) and vmT (e) vs. T (right axis) with the fit to Eq. (1) (solid line).

Fig. 8. Plot of isothermal (1.8 K) magnetization data vs. field, H, for 1.

J. Hu et al. / Polyhedron 78 (2014) 1–9 7

Cu(II) ions are bridged by two l-Cl, the susceptibility data wereanalyzed by Bleaney–Bowers expression based on a HeisenbergHamiltonian H = �2JS1S2 [16]:

v0m ¼Ng2b2

kT� 1

3þ e�2J=kTð1� qÞ þ Ng2b2

4kTq ð1Þ

vm ¼v0m

1� zJ0v0mð2Þ

where |2J| is the singlet–triplet energy gap and N, g, b and k havetheir usual meanings. q is a variable fraction of paramagnetic impu-rities. zJ0 accounts for the interdimer exchanges. A good fit resultedin the solid line in Fig. 7, with the parameters g = 2.18, J = 5.39 cm�1,q = 0.17% and zJ0 = �0.26 cm�1 with a coefficient of determinationof R2 = 0.99326. The obtained J value can be comparable to thoseof similar Cu(II) complexes [Cu2(dpt)2Cl2]X2 [17].

The field dependence of the magnetization (0–70 kOe) mea-sured at 1.8 K shows a rapid increase of magnetization. The magne-tization value at 70 kOe is 1.84 Nb, very close to the Brillouinfunction with S = 2 Nb for two uncoupled Cu(II) magnetic moments(Fig. 8). The lower value indicates the presence of intermolecularantiferromagnetic coupling and/or a ZFS effect in complex 1. Theresult shows that the lowest state of complex 1 is ST = 1. The sameground state is also designated to the complexes [Cu2(dpt)2Cl2]X2

[17].The vm and vmT versus T plots for complex 2 are given in Fig. 9.

The observed vmT value at 300 K is 2.20 cm3 K mol�1, which is ingood agreement with that expected value (2.25 cm3 K mol�1) forsix independent Cu(II) ions (g = 2.0, S = 1/2). As the temperaturedecreases, the vmT value keeps constant above 90 K and then,decreases monotonously from 2.20 cm3 K mol�1 at 90 K to1.19 cm3 mol�1 K at 1.8 K. The profile is indicative of the existenceof antiferromagnetic interactions between Cu(II) centers.

As already described, the unit cell of compounds 2 consists of atetranuclear {Cu4O4} clusters and a {Cu2} dimer. The antiferromag-netic interactions between the tetramer are mainly propagatedthrough two types of pathways. One is between the Cu(II) ions(Cu1 and Cu3, Cu2 and Cu4) bridged by l-O atoms. The other isthrough the O–S–O bridge of two capping sulfate units. It is wellknown that in comparison to oxide bridges, three-atom bridgessuch as O–P–O only support negligible exchange interactions [18].Similarly, the sulfate bridging ligands may contribute to the mag-netic exchanges, but the couplings mediated through the O–S–O

units are much weaker than those through the l-O bridge. There-fore, the tetranuclear unit may be viewed approximately as twosubdinuclear {Cu2} units, connected by four l-O (O4, O5, O9 andO13). Based on the similar consideration, the {Cu2} dimer linkedby O–S–O bridge may be regarded as two isolated Cu(II) centers,and similar result has been reported in the complex [Cu(PAH-OX)(SO4)]2�2H2O, [Cu((R)-4)-(l2-SO4)]2 and [Cu2((R)-4)((S)-4)(l2-SO4)2] [19]. Thus, a simplified equation can be derived by neglectingthe exchange of O–S–O bridges:

vm ¼ 2vmðCu2Þ þ 2vmðCuÞ þ Na ð3Þ

where vm(Cu2) and vm(Cu) are the equations for the {Cu2} dimmerand mononuclear Cu(II) species, respectively. The best fittingparameters are: J = �16.4 cm�1, g = 2.12, Na = �0.00526 cm3 mol�1

with a coefficient of determination of R2 = 0.99916. The exchangecoupling constants over the l-O bridges obtained for complex 2may be compared with those reported for other oxo-bridged dicop-per(II) complexes. For example, the complexes [(Cu2(HAP)2-

IPA)(OH)(H2O)](ClO4)2�H2O, [Cu4(l3-OH)2(dtbp)6 (py)2] exhibitantiferromagnetic behavior via exchange pathway provided by theO-bridge [20]. The observed J values can be correlated to the molec-ular structure by considering the available exchange path ways

Fig. 9. Magnetic susceptibility of 2 plotted as vm (D) vs. T (left axis) and vmT (e) vs. T (right axis) with the best fit result (solid line).

8 J. Hu et al. / Polyhedron 78 (2014) 1–9

between the Cu(II) centers. In general, the exchange coupling isantiferromagnetic for the Cu–O–Cu angles greater than 97.5�.Cu1� � �Cu3 are connected via two l2-O bridge with the bridgingangle 103.4� and 97.6�, and therefore show a antiferromagneticinteraction. Cu2� � �Cu4 are connected via two l2-O bridge with thebridging angle 105.6� and 95�, it can be concluded that the antifer-romagnetic interactions are related to the predominant interactionthrough O bridge with higher angle value.

The fitting results are consistent with extensive experimentaland theoretical studies performed on the spin exchange withinanalogous. The magnetic susceptibility obeys the Curie–Weisslaw, vm = C/(T � h), with the Curie constant C = 2.24 cm3 K mol�1,h = �2.78 K in the range of 1.8–300 K (Fig. S5). The small negativeWeiss constant confirms the occurrence of overall weak antiferro-magnetic interactions between the neighboring Cu(II) centers inthe crystal lattice.

4. Conclusions

The flexible multidentate connector may endow the anions toplay crucial roles in the coordination network assembly via engagingin diverse and specific bonding interactions. Consequently, two dis-tinct frameworks with multinuclear copper building units have beenconstructed. With the decrease of temperature, the vmT values ofcomplex 1 first increase to a maximum at 82 K, and then decrease.When the sample of complex 2 is cooled, the feature of vmT valuesconfirms the existence of antiferromagnetic interactions.

Acknowledgments

We gratefully acknowledge the financial support by the He’nankey science and technology research (Nos. 122102310061), andthe Training and Funding Program for young key teacher of HenanUniversity of Urban Construction, the funding program for youngkey teacher of He’nan colleges and universities (No. 2013GGJS-175).

Appendix A. Supplementary data

CCDC 945192–945193 contain the supplementary crystallo-graphic data for 1�2. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road, CambridgeCB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.

cam.ac.uk. The information of Powder X-ray patterns, the curvesthermogravimetric (TG) and differential scanning calorimetric(DSC), and the magnetic susceptibility of 1 in the form of vm

�1 versusT for 1�2. Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.04.012.

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