Inorganic Chemistry Communications 14 (2011) 261–264

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Inorganic Chemistry Communications

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A new three-dimensional cobalt(II) coordination polymer based onbiphenyl-2,2′,6,6′-tetracarboxylic acid and 1,2,4-triazole: Synthesis,crystal structure and magnetic properties

Lin Cheng, Jian-Quan Wang, Shao-Hua Gou ⁎School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

⁎ Corresponding author.E-mail address: cep02chl@yahoo.com.cn (L. Cheng).

1387-7003/$ – see front matter. Crown Copyright © 20doi:10.1016/j.inoche.2010.11.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 October 2010Accepted 3 November 2010Available online 12 November 2010

Keywords:Biphenyl-2,2′,6,6′-tetracarboxylate1,2,4-TriazoleCo(II) complexAntiferromagnetic

A new three-dimensional coordination polymer with the formula [Co(bta)0.5(Htz)(H2O)]n (1) (H4bta=biphenyl-2,2′,6,6′-tetracarboxylic acid and Htz=1,2,4-triazole), has been hydrothermally synthesized and structurallycharacterized. The structure of 1 can be considered as two-dimensional [Co(bta)(H2O)]n layers, consisting ofone-dimensional [Co(COO)]n chains, which are separated by bta ligands, and neutral Htz pillars. The variabletemperaturemagnetic property study indicates that there is aweak antiferromagnetic coupling interaction betweenCo(II) ions, whichmainly arises from the antiferromagnetic coupling interaction in the one-dimensional [Co(COO)]nchains through syn–anti carboxylate bridges.

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

Over the past fewyears, coordination polymers have received awiderange of attention in the field of supramolecular chemistry and crystalengineering, owing to their intriguing aesthetic structures andtopological features aswell as their potential applications inmagnetism,electric conductivity, molecular adsorption, heterogeneous catalysis,nonlinear optics and fluorescent materials [1,2]. The aromaticmulticarboxylate ligands with complicated configurations, such as 1,2-benzenedicarboxylate [3], 1,3,5-benzenetricarboxylate [4], and 1,2,4,5-benzenetetracarboxylate [5], have been proved to be good candidates ofligands in the construction of a rich variety of coordination polymerswith high-dimensional networks and interesting properties because oftheir diverse coordination modes and high structural stability. Incontrast, biphenyl-2,2′,6,6′-tetracarboxylic acid (H4bta) (Scheme 1),as a member of multidentate O-donor ligands, is rarely used [6].However, its following structure features inspire our research interests:(a) it has a rich variety of coordination modes with four carboxylicgroups,whichmaybe completely orpartially deprotonatedupon thepHand help to construct novel coordination polymers with magnetic and/or adsorptionproperties; (b) it is aflexible ligand since twophenyl ringscan be rotated around the C–C single bond, which can be used toconstruct chiral coordination polymers because of the non-coplanarityof two phenyl rings.

On the other hand, five-membered heterocycles such as pyrazole,imidazole, triazole and tetrazole, have been frequently used in the

10 Published by Elsevier B.V. All rig

preparation of coordination polymers because they are very strongN-ligating donors for d-metal ions and can be readily deprotonated toform corresponding azolate anions to bridge metal ions into extendedhigher dimensional (2D and 3D) coordination polymers [7]. In thiswork, we report the synthesis [8], crystal structure [9] and magneticproperties of a new coordination polymer with the formula [Co(bta)0.5(Htz)(H2O)]n (1) (H4bta=biphenyl-2,2′,6,6′-tetracarboxylicacid and Htz=1,2,4-triazole) featuring a 3D network based on

Scheme 1. Structure of H4bta.

hts reserved.

Fig. 1. Local coordination environments of Co1 (a) and Co2 (b) in 1. All the hydrogen atomsare omitted for clarity. Symmetry codes, a: −1/2−x, 1/2−y, −z; b: 1/2 −x, 1/2 −y, −z;c:−1/2−x, 1/2+y, z; d:−x,−y,−z.

262 L. Cheng et al. / Inorganic Chemistry Communications 14 (2011) 261–264

mixed-bridged carboxylate/1,2,4-triazole ligands, in which H4btawere prepared according to the literature [11].

Single-crystal X-ray diffraction shows that 1 is a three-dimensionalframework consisting of two-dimensional [Co(bta)(H2O)]n layers andneutral Htz pillars. There are two crystallographically independent Co(II) ions with different coordination environments in an asymmetricunit, in which Co1 displays a distorted octahedral geometry, beingcoordinated by four carboxylate oxygen atoms from two chelating btaand two nitrogen atoms from two Htz ligands, while Co2 aresurrounded by two carboxylate oxygen atoms from two chelatingbta, two nitrogen atoms from two Htz ligands and two coordinatedwater molecules, forming a distorted CoO4N2 octahedron (Fig. 1).Each bta, with the angle of two benzene rings of 86.80(2)°, is anegative tetravalent anion and coordinated to two Co1 and two Co2

Fig. 2. Coordination modes of bta (a) and Htz (b) in 1.

atoms in a hexadentatemode, while each Htz ligand is neutral, linkingone Co1 and one Co2, as shown in Fig. 2.

Co1 and Co2 are alternately linked by single syn–anti carboxylatebridges into an infinite [Co(COO)]n chain, which is stabled bycoordinated carboxylate oxygen atoms coming from another benzenerings of bta ligands (Fig. 3). The intrachain Co1⋯Co2 distance is 5.040(3)Å, being comparable with those in the Co(II) complexes linked bysingle syn–anti carboxylate bridges (4.900–5.420 Å) [12]. Thesechains are further extended by bta ligands to build a two-dimensional[Co(bta)(H2O)]n layer (Fig. 3) with the shortest interchain Co⋯Codistance of 8.708(1)Å, which are finally pillared by neutral bidentateμ1,4-Htz ligands into a three-dimensional network (Fig. 4). Theshortest Co⋯Co distance across Htz ligands is 6.304(1)Å, being a littlelonger than that of a three-dimensional Co(II) compound bridged byneutral Htz (6.239 Å) [13].

The thermogravimetric analysis of powder samples of 1 wascarried out from 29 to 596 °C under a nitrogen atmosphere at aheating rate of 10 °C min−1, as shown in Fig. 5. The TGA curve for thecompound shows that there is no weight loss between 29 and 350 °C,which indicates that the framework of 1 can remain stable up to350 °C. Decomposition of the polymer began at 350 °C; in thetemperature range of 350–412 °C the removal of coordinated watermolecules and Htz ligands occurred with a loss of 28.1% (calc. 23.1%).The second weight loss of 46.4% between 412 and 596 °C correspondsto the pyrolysis of bta ligands and the final residual weight was 30.5%(calc. 24.2%) corresponding to CoO.

The magnetic properties of the crystalline samples of 1 weremeasured at an applied field of 2 kOe from 1.8 to 300 K, as shown inFig. 6, in the forms of χM and χMT versus T. At room temperature, χMTis equal to 2.87 cm3 Kmol−1, which is much higher than the spin-onlyvalue of 1.875 cm3 Kmol−1 based on amagnetically isolated Co(II) ion(g=2 and S=3/2) due to the prominent orbital contribution. Uponlowering the temperature, χMT continuously decreases and reaches0.69 cm3 K mol−1 at 1.8 K. The magnetic properties of complex 1 arefairly the same as that reported previously [14]. Above 10 K, themagnetic properties can mainly be ascribed to the free Co(II) ionbehavior. Below 10 K, the large spin-orbital coupling constant (λ=−180 cm−1) [15] makes three electrons only occupy the ground statethat split from 4T1 of the Co(II) ion, further leads to a lower spin stateof S′=1/2. So the value of χMT sharply decreases below thistemperature. According to the single-crystal structure of complex 1,there are two bridges between CoII ions of carboxylate and neutral Htzligands, and the magnetic coupling interaction mediated by the latteris neglectable because the Co(II) atoms are well separated by theneutral Htz bridges. So, complex 1 can be viewed as a magnetic chain(Fig. 7). For estimating the magnetic coupling interaction between Coions, the treatment method reported by Rueff et al. [14] and thesimple phenomenological Eq. (1) [16] can be used here.

χMT = A exp −E1 = kTð Þ + B exp −E2 = kTð Þ ð1Þ

where A+B equals the Curie constant, and E1 and E2 represent the“activation energies” corresponding to the spin-orbit coupling and tothe magnetic exchange interaction, respectively. The best fittingresults give:−E1/k=−61(2) K and−E2/k=−2.1(1) K with C=3.15(2) (=1.53(2)+1.62(2))cm3 K mol−1. Thus, the magnetic couplingconstants between Co ions are−4.2 K according to the relationship ofχMT∝exp(+ J/2kT) [15,17]. This indicates that the weak antiferro-magnetic exchange interaction between metal ions dominate themagnetic properties in 1.

In conclusion, compound 1 represents a unique example of three-dimensional coordination polymers that can be considered as two-dimensional [Co(bta)(H2O)]n layers and neutral Htz pillars. Themagnetic property study indicates that 1 shows a weak antiferro-magnetic behavior, which mainly arises from the antiferromagnetic

Fig. 3. Structure of the two-dimensional [Co(bta)(H2O)]n layer along the ac plane.

263L. Cheng et al. / Inorganic Chemistry Communications 14 (2011) 261–264

coupling interaction in the one-dimensional [Co(COO)]n chainsthrough syn–anti carboxylate bridges.

Acknowledgements

The authors are grateful to the financial support from the NationalNatural Science of Foundation of China (project No. 20801011 and No.

Fig. 4. Structure of the three-dimensional framework pillared by Htz ligands of 1.

6507040021) and the funding from the Southeast University (No.4007041121 and No. 9207040016).

Appendix A. Supplementary material

CCDC reference number 795118 contains the supplementarycrystallographic data for this paper. These data can be obtained free ofcharge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from theCambridge Crystallographic Data Centre, 12, Union Road, CambridgeCB2 1EZ,UK; fax: (internat.)+44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk]. Supplementary data to this article can be found online atdoi:10.1016/j.inoche.2010.11.009.

100 200 300 400 500 600

30

45

60

75

90

105

TG

/ %

Temperature /°C

Fig. 5. TG plot for 1.

Fig. 7. One-dimensional magnetic chain via syn–anti carboxylate bridges in 1.

0 50 100 150T / K

200 250 300

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.1

0.2

0.3

0.4

χ MT

/ cm

3 K

mo

l-1 χM

/ cm3 m

ol -1

Fig. 6. Temperature dependence of magnetic properties in the forms of χM and χMT for1. The dots are experimental values and the red solid lines represent the best fits.

264 L. Cheng et al. / Inorganic Chemistry Communications 14 (2011) 261–264

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