Polyhedron 69 (2014) 84–89

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Polyhedron

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Selective gas sorption and electrochemical properties of a dicyanamidecoordination polymer: Insight from experimental and theoretical study

0277-5387/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.poly.2013.11.029

⇑ Corresponding author. Tel.: +98 3113913261; fax: +98 3113912350.E-mail addresses: Chinif@cc.iut.ac.ir, l.tabrizi@ch.iut.ac.ir (H. Chiniforoshan).

Leila Tabrizi a, Hossein Chiniforoshan a,⇑, Patrick McArdle b, Hossein Tavakol a, Behzad Rezaei a,Mohammad Mohammadi Dehcheshmeh a

a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iranb School of Chemistry, National University of Ireland Galway, University Road, Galway, Ireland

a r t i c l e i n f o

Article history:Received 16 September 2013Accepted 22 November 2013Available online 1 December 2013

Keywords:Cadmium (II)DicyanamideGas sorptionCyclic voltammetryTheoretical calculation

a b s t r a c t

We report the gas sorption ability of [Cd(l-hmt)(l-dca)2]n (1) (hmt = hexamethylenetetramine,dca = dicyanamide), a coordination polymer with 2D rectangular grid structure known as the porousnon-porous materials, for CO2, CH4, and N2 under different temperatures (295 K, 195 K). It was observedthat polymer adsorbs CO2 and CH4, but not N2 even at low temperature. Also, since electron transferbehavior of 1 has an important role on gas adsorption, electrochemical properties of polymer and itsmetal (cadmium) in absence and presence of hmt, are studied by cyclic voltammetry. In addition, thermalstability of 1 is investigated by thermogravimetric (TGA) analysis. To gain a deeper insight into theobserved gas uptake of mentioned polymer, theoretical calculations have been used to study intermolec-ular interactions and characterize the bonding of gas sorption.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, extensive studies of porous coordination poly-mers (PCPs), also called metal–organic frameworks (MOFs), havebeen reported due to their potential applications in many fields suchas storage, separation, catalysis and ion exchange [1–3]. In contrastto conventional microporous materials such as zeolites and clays,the rational design of coordination polymers is currently of greatinterest in the realm of well-ordered porous structures, flexibleand dynamic behaviors in response to guest molecules [4–6].

In considering the category of porosity ‘‘without pores’’ it seemsunreasonable to suggest that crystals possessing lattice cavities,but no atomic-scale channels leading to these cavities, might bepermeable. It was recently reported that nonporous solid-statestructures could behave as porous materials, even though no large,discrete lattice cavities interlinked by channels wide enough to al-low molecules to migrate between cavities or open channels arepresent in these structures [7–13]. The host molecules cooperatewith one another in a dynamic and concerted motion in order tocreate windows of opportunity, thus may transport a liquid or gas-eous guest into the crystal even in the absence of traditional openchannels [8,9].

Cd(II)-coordination polymers, which extend into one, two orthree dimensions (1D, 2D and 3D, respectively), via more or lesscovalent metal–ligand bonding, have been investigated in many

fields. The d10 configuration and softness of cadmium (II) permita wide variety of geometries and coordination numbers. In thisfield, the effective and facile investigations for the synthesis ofpolymers of cadmium (II) have been exploited by using well-designed organic ligands as bridges or terminal groups (buildingblocks) with cadmium ions as nodes, which, so far, has been atan evolutionary stage with the current focus mainly on under-standing the basic system properties [14–16].

Among the various types of anionic-bridging ligands, dicyanam-ide has been widely used because it is a versatile ligand with threenitrogen donor atoms and it may act as a uni-, bi-, and tridentateligand. The varieties of its coordination modes allow the prepara-tion of large variety of architectures with one-, two-, and three-dimensional networks [17–19].

The use of hexamethylenetetramine (hmt), a simple heterocy-clic compound with a cage-like structure, has also been investi-gated and a considerable number of hmt-driven metal–organicnetworks have been reported in the last few years. Networks ofhmt acting as N-donor bridging ligands can produce a wide varietyof supramolecular architectures in different one-, two- and three-dimensional topological motifs which binds to a metal center ina l2 or l4 fashion [20–22]. Among the applications of hmt poly-mers as functional materials, a number of selected compoundspoint to the interesting properties such as host–guest interactionsinvolving water molecules, high porosity, selective sorption ofgases and solvents, ion exchange, molecular magnetism [22].

To the best of our knowledge no investigation has been carriedout which demonstrates the gas sorption property of dicyanamide

L. Tabrizi et al. / Polyhedron 69 (2014) 84–89 85

metal frameworks to date. We selected [Cd(l-hmt)(l-dca)2]n (1) aknown coordination polymer [23] with highly permeability despiteits seemingly nonporous solid-state structure (while interstitiallattice cavities are present, the cavities are not interlinked by chan-nels) for gas sorption of CO2, CH4, and N2 under different temper-atures (295 K, 195 K). For investigation of electron transferbehavior of 1, electrochemical properties of polymer and its metal(cadmium) in absence and presence of hmt, are studied by cyclicvoltammetry. To gain a deeper insight into the observed gas uptakeof 1, theoretical calculations such as frequency calculations, atomin molecule (AIM) analyses, natural bond orbital (NBO) and popu-lation analyses have been used to study intermolecular interac-tions and characterize the bonding of gas sorption.

Table 1Crystallographic data for 1.

1

Empirical formula C20H24Cd2N20

Formula weight 769.39T (K) 295.4Crystal system orthorhombicSpace group Pnmaa (Å) 13.1294 (8)b (Å) 9.0851 (5)c (Å) 10.8288 (7)a (�) 90.00b (�) 90.00c (�) 90.00V (Å3) 1291.69 (14)Z 2l (mm�1) 1.70Dcalc (mg m�3) 1.978F(000) 760h Range (�) 3.1–29.1Independent reflections 1260Data/restraints/parameters 1260/0/112Goodness-of-fit (GOF) on F2 1.050Final R indices R1 = 0.0278, wR2 = 0.0676R indices (all data) R1 = 0.0369, wR2 = 0.0727Largest difference peak and hole (e Å�3) 0. 657, �0.562

2. Experimental

2.1. Materials

All the chemicals and solvents were purchased from Merck orSigma–Aldrich compounds and were used without any furtherpurification.

2.2. Physical measurements

Thermogravimetric (TGA) analyses were performed under Ar-gon using STA-503 at a heating rate of 10 �C/min. The powder dif-fraction pattern of compound 1 was performed with an X-raypowder diffractometer (XRD D5000Siemens) with Cu Ka radiation.Measurements were made in a 2h range of 5–50 �C at room tem-perature. The adsorption isotherms were measured at295 K,195 K for N2 and 295 K for CO2 and CH4 using an Autosorb-1 system from Quantachrome and ultra-pure gases (99.999%). Elec-trochemical experiments were performed with l-Auto lab type IIIelectrochemical analyzer instrument. A conventional three elec-trode cell assembly was used with a carbon paste as working elec-trode, saturated calomel reference electrode and Pt rod as thecounter electrode. Electrochemical measurements were carriedout under nitrogen atmosphere in a conventional one-compart-ment cell. A digital pH meter was used for preparing electrolytesolution. Carbon-paste capillary electrode was prepared by mixinggraphite powder with (1 lm particle size) an appropriate amountof mineral oil (Nujol) and thorough hand mixing in a mortar andpestle. A portion of the graphite paste mixture was packed intothe end of a glass capillary (0.5 mm inner diameter). Electrical con-tact was made by forcing a copper wire down the glass capillaryfrom back side. The modified electrode was prepared by mixingthe graphite paste and Cd (II) complex (99:1 ratio).

2.3. Synthesis of [Cd(l-hmt)(l-dca)2]n (1)

To the solution of Cd(OAc)2�2H2O (26.6 mg, 0.1 mmol), in 20 mlethanol, added aqueous solution (5 ml) of hmt (14 mg, 0.1 mmol)followed by the addition of a solution of sodium dicyanamide(17.8 mg, 0.2 mmol) in 5 ml water into the mixture with continu-ous stirring. The mixture was heated for 60 min. at 50 �C with stir-ring and then filtered after cooling to room temperature. Theresulting solution was kept at room temperature. After a few days,shiny single crystals suitable for X-ray determination were ob-tained. Yield 84%.

2.4. Crystallographic data collection and structural refinements

Relevant data about the collections and structure solutions aresummarized in Table 1. Crystals of 1 for X-ray crystallography weregrown by slow evaporation of solution. An Oxford Diffraction

Xcalibur system was used to collect X-ray diffraction data. Thecrystal structures were solved by direct methods (SHELXS-97) andrefined by full matrix least squares using SHELXS-97 within the Osc-ail package [24,25]. Non-hydrogen atoms were refined anisotropi-cally. Hydrogen atoms were included in calculated positions withthermal parameters 30% larger than the atom to which they wereattached.

2.5. Computational details

The geometry optimizations and frequency calculations(including energies) were performed using ab initio calculationsemploying GAUSSIAN 09 program package [26]. Atoms in molecule(AIM) analyses were performed using AIM200 program [27]. Thismethod has been presented useful information about intermolecu-lar interactions and characterization of bonds through the analysisof the electron density [28]. In addition, natural bond orbital (NBO)theory is another method to estimating the values of intermolecu-lar interactions [29], which its method is implemented in theGAUSSIAN program package. All calculations were performed at HF/6-311G level of theory and the theoretical data were obtained afteroptimization of the molecular geometry and carried out at temper-ature of 295 K.

3. Results and discussion

3.1. Structural description of [Cd(l-hmt)(l-dca)2]n (1)

The structure of 1 has been redetermined (R1 = 0.0278,wR2 = 0.0727 compared to original report [23] which hadR1 = 0.0858 and wR2 = 0.1743) that has 2D grid structure whichthe cadmium atom is six-coordinated with a distorted octahedralgeometry. ORTEP diagram is shown in Fig. S1 in supplementarymaterial. The dca ligand is in l1,5-mode and forms [Cd(l–dca)]n

chain running along parallel to c-axis. While the hmt is also in l-mode and bridges two Cd atoms in adjacent chains along with an-other l1,5-dca chain propagated in sinusoidal fashion along a-axis,to give the 2D sheet. It can be considered to consist of alternatinglayers of metal and one dca alternating with layers of the bulkierhmt and another dca. The Cd–Cd distance of hmt linkage is6.5656(7) Å and that of dca is 9.3846(6) Å. The distance between

Fig. 2. A TGA curve for 1.

Fig. 3. Cyclic voltammograms of polymer (1), Nadca and hmt.

86 L. Tabrizi et al. / Polyhedron 69 (2014) 84–89

two adjacent layers is ca. 5.9782(53) Å. The metal atoms and onedca ligands lie strictly in ac-plane and bulkier hmt along with otherdca ligand protrude from both sides of the plane (Fig. S2 in supple-mentary material) [23]. It is worth mentioning that cavities formedby l–dca and l–hmt can be suitable for investigation of gassorption.

The powder X-ray diffraction (PXRD) patterns (see Fig. 1) indi-cate that PXRD pattern of the as-synthesized 1 closely matchesthe simulation of 1 based on the single-crystal structure as a refer-ence. All peak positions in the PXRD pattern of the as-synthesized 1are identical to the simulated results, implying that the structure ofthe powder samples is exactly the same to that of the single-crystalstudy. Meanwhile, some differences exist between both structuresas revealed by the intensity of some peaks that can be explained bythe presence of an interpenetrated structure.

3.2. Thermogravimetric analyses of 1

Thermogravimetric analysis (TGA) of 1 (Fig. 2) have been per-formed to elucidate the solid state thermal property, identify theend products and thereby the composition of the original species.Upon heating, crystal 1 start to lose hmt at 190 �C and completeloss of hmt ligand takes place at 400 �C. The experimental massloss is 34.50% [Calc. 36.39%]. Finally 1 undergoes under decompo-sition with formation of some unidentified species.

3.3. Electrochemical properties of 1

Since electron transfer behavior of the synthesized polymer hasan important role on gas adsorption at the surface of modified sup-port, firstly, the electrochemical properties of polymer and its me-tal (cadmium) in absence and presence of hmt was investigated. Itwas shown that presence of hmt enhanced the charge transferactivity of polymer. On the other hand, hmt plays an important roleto control surface charge where some adsorbate may be adsorbed.The cyclic voltamogram (CV) were investigated in phosphate buf-fer solution (pH 4) at the carbon paste electrode with scan rate50 mV/s and potential window from �1.5 to +1.5 V at room tem-perature. The CV of polymer (Fig. 3) shows three irreversible peaksat +0.47 and �0.25 and �0.53 V. Therefore, the electron transferprocess of polymer may involve the following steps [30]. Thus,the CdII(l-dca)(l-hmt)CdII linkage is still retained in one oxidationand two reduced forms.

CdIIIðl-dcaÞðl-hmtÞCdII �������e;þ0:47CdIIðl-dcaÞðl-hmtÞCdII

� ������!þe;�0:25CdIðl-dcaÞðl-hmtÞCdII

������!þe;�0:53CdIðl-dcaÞ

� ðl-hmtÞCdI

Fig. 1. Powder X-ray diffraction (PXRD) patterns of 1 (a) 1 simulation based on thesingle-crystal structure, (b) the as-synthesized 1.

Also, the cyclic voltamogram of dicyanamide ligand has not anypeak current and hmt has a very weak current at �0.7 V. For moreinvestigations, the redox behavior of Cd (II) in absence and pres-ence of hmt in different mole ratios under the same conditions ofpolymer was examined (Fig. 4). The results show that the Cd(II)voltamogram in absence hmt displays two irreversible peaks withweak currents (DE = 405 mV). In the presence of hmt, the observedpeaks are shifted to higher potential and current, the correspond-ing of them in polymer peaks. Also the higher mole ratios of hmtlead to increase peak current. Therefore it is obvious that the poly-mer prepared from hmt produces more current, due to the widerpotential window under the current–potential curves; and itshows higher specific capacitance and better capacitive behaviorof polymer [31,32].

Fig. 4. Cyclic voltammograms of Cd(II) in absence and presence of hmt in differentmole ratios.

Fig. 6. Gas adsorption isotherms of N2 at 195 and 295 K.

L. Tabrizi et al. / Polyhedron 69 (2014) 84–89 87

3.4. Aspects of gas sorption characterization

3.4.1. Comments to the application of the BET methodHere we want to discuss some important aspects of the applica-

tion of the widely accepted BET (Brunauer–Emmett–Teller) meth-od [33] for surface area analysis of microporous materials such asMOFs. Usually, two stages are involved in the evaluation of the BETarea. First, it is necessary to transform a physisorption isotherminto the ‘BET plot’ and from there to derive the value of the BETmonolayer capacity, Xm. The second stage is the calculation of thespecific surface area, S, which requires knowledge of the molecularcross-sectional area. The monolayer capacity Xm is calculated fromthe adsorption isotherm using the BET equation 1/[X((P0/P) � 1)] = (1/XmC) + [(C � 1)/XmC] (P/P0), where X and P/Po are thedata the surface area analyzer measures, and that Xm relates di-rectly to surface area and C is an empirical constant which givesan indication of the order of magnitude of the attractive adsor-bent–adsorbate interactions.

The computer program takes over and a least-squares linearregression is used to fit the best straight line through a trans-formed data set consisting of the following pairs of values: 1/VSTP

(P0/P) � 1 and P/P0. The monolayer capacity, Vm, is calculated fromthe slope, s, s = C � 1/Vm.C and the intercept, i, of the straight linei = 1/Vm�C. Solving for Vm, Vm = 1/s + i can be calculated. Thereafter,the specific surface area S can be obtained from the monolayercapacity Xm by the application of the simple equation: S = VmLAVAm/Mv where LAv is Avagadro’s number, Am is the so-called cross-sectional area (the average area occupied by each molecule in a com-pleted monolayer) Mv is molar volume for the number of moles.

3.4.2. Gas sorption properties of 1The host framework, 1, was subjected to gas sorption of CO2,

CH4, and N2. It was observed that 1 adsorbs CO2 and CH4, but notN2 (Fig. 5). The BET (Brunauer–Emmett–Teller) surface areas esti-mated from the CO2 and CH4 sorption isotherms are 213 and183 m2/g, respectively (see Figs. S3 and S4 in supplementary mate-rial). This variation should be related to difference in the intermo-lecular interaction force of the guest molecules. The framework ofhost, which contains Cd2+ ions, the polar residues and the p-elec-trons of the dca ligand, gives rise to an electric field that inducesa dipole in CO2.

Besides such dipole-induced dipole interactions, the quadruplemoment of CO2 (3.3–3.4 � 10�26 e.s.u.) would interact with theelectric field gradient, providing a further contribution to the po-tential energy of adsorption [3,34]. In addition, there may be somedonor–acceptor affinity between the CO2 molecules and the Lewisacidic Cd2+ ions [3].

Fig. 5. Gas sorption isotherms of N2, CH4 and CO2 at 295 K; adsorption (filled) anddesorption (open).

Hysteresis was observed in the gas sorption isotherm of 1 forCH4 at 295 K. Such hysteresis behavior was also observed for vari-ous PCPs [3]. An in-depth understanding of the mechanism for hys-teresis in the adsorption/desorption isotherms of PCPs is notpossible readily. However, guest-directed framework rearrange-ments have been suggested for some coordination networks[3,35–37]. Such observation is a result of the increased adsor-bate–adsorbent interactions as the gas molecules access pore re-gion around the metal centers [35].

Though the discrimination of CH4 and CO2 is challenging be-cause of the similar polarizability of CH4 (2.60 Å3) [38] and CO2

(2.63 Å3), however, the experimental result clearly shows that amolecular sieving effect contributes to the observed high selectiv-ity for CO2. In other words, although the crystal structure of 1 isseemingly nonporous, thermal motion would create a transient‘‘window’’ welcoming the molecules of CO2, which would thuspenetrate and diffuse into the crystal owing to its small kineticdiameter (3.3 Å) [39] than the other gases such as N2 (3.64 Å),and CH4 (3.8 Å). Although the kinetic diameter of N2 is much smal-ler than CH4, however, N2 sorption was not observed at 295 K andeven at 195 K (Fig. 6). A similar behavior has been recently re-ported for N2 [3,40] and is rare for microporous coordinationpolymers.

3.5. Computational studies

To gain a deeper insight into the observed highly selective CO2

uptake of 1, theoretical calculations were performed for CO2, CH4,

Fig. 7. Optimized structure of cavity in polymer framework.

Table 2Heat of formation and interaction energies between compound 1 (host) and adsorbed molecule (guest: CH4, CO2 and N2) using natural bond orbital (NBO) and atom in molecule(AIM) analysis.

Complex DHf (kcal/mol) NBO calculations AIM analysis

Second order perturbation energies (in kcal/mol) Charges (aua) Intermolecular critical points

Host to guest Guest to host Total Host Guest P (a.u.) Laplacian of q (a.u.)

1–CH4 �3.96 1.09 2.45 3.54 �0.001 0.001 3.27 � 10�3 �4.03 � 10�3

1–CO2 �4.66 0.36 2.01 2.37 0.230 �0.230 4.10 � 10�3 �5.30 � 10�3

1–N2 �2.26 0.30 1.42 1.72 �0.001 0.001 1.62 � 10�3 �3.09 � 10�3

a Each a.u. (or e) is 1.60 � 10�19 coulombs (in SI units).

Table 3Results of population analysis for compound 1 and its complexes with CH4, CO2 andN2. The dimension of reported energies is electron volt (eV).

Population analysis

HOMO LUMO Ega

1–CH4 0.01187 0.14317 0.13131–CO2 0.01332 0.13369 0.120371–N2 0.01225 0.14036 0.128111 0.01401 0.14426 0.13025

a Eg = ELUMO � EHOMO.

88 L. Tabrizi et al. / Polyhedron 69 (2014) 84–89

and N2 sorption state of 1. To calculate the minimum cavity dimen-sions, the structure of 1 was optimized using ab initio calculations(as described in computational method, Section 2.5). Then, usingGAUSSVIEW 5 program [26], the distance between the nuclei of inneratoms (DN) were determined from the output of optimization pro-cess. The exact dimensions were calculated from subtraction van-der-waals atomic radii (in both sides of the cavity) from DN. As itshown in Fig. 7, the calculated cavity dimensions are 5.80 and4.06 Å (each cavity dimension = DN � 2 � RC, RC is van-der-Waalsradii of carbon). In addition, heat of formations for gas sorptionof 1 are reported (calculated from the enthalpy values using fre-quency calculations) (Table 2) along with the interaction energiesbetween compound 1 (host) and adsorbed molecule (guest: CH4,CO2 and N2) and natural bond orbital (NBO) and atom in molecule(AIM) analysis. Heat of formation for complexes between 1 and ad-sorbed molecules found to be as follows: 1–CO2 > 1–CH4 > 1–N2.These results show that 1-CO2 complex is more stable than othersand CO2 is more easily adsorbed on 1. Meanwhile, small values forDHf confirm the reversibility of the adsorption process. This obser-vation is derived from the higher ability of compound 1 to uptakeCO2. The decreasing order of the heat of formation, 1–CO2 > 1–CH4 > 1–N2 shows the dominance of dipole–dipole and dipole-in-duced dipole interactions towards the total heat of formation.NBO analyses were made to obtain charges and estimate the valuesof intermolecular interactions. Second order perturbation energiesof host to guest (1 to adsorbed gas) are found to be as follows: 1–CH4 > 1–CO2 > 1–N2. However, the absolute value of charge for 1–CO2 complex is larger than that of the others. These data provideanother proof for existence of more effective interaction betweenthe compound 1 and CO2.

This interaction energy is so low for N2 regarding to its chargeand energy. Although, this energy for CO2 is lower than CH4, butits charge transfer (obtained from the total charges of host andguest) is higher than the others which shows interaction in 1–CO2 complex can be considered as electrostatic attraction. As a re-sult, these observations confirm our previous estimate about thehigher uptake of CO2 versus the CH4 and N2.

AIM analyses were performed for host–guest interaction to ob-tain intermolecular interactions and characterization of bondsproperties. In this part, q (electron density) and its Laplacianmay be very useful parameters to estimate the strengths of thehost–guest interaction. The values calculated for the charge den-sity q at the host–guest intermolecular bond critical points lie be-tween 1.62 � 10�3 and 4.10 � 10�3.These low values reflect theweak character of this bond between host and guest moleculesand also the guest is trapped in the interstitial lattice cavity andmakes only van der Waals contact with the host.

It is noticeable that 1–CO2 has the largest values of the electrondensity, confirming the higher efficiency between 1 and CO2.

Results of population analyses (the HOMO and LUMO energiesand the band gaps) for compound 1 and its complexes with CH4,CO2 and N2 are summarized in Table 3. Obviously, changes in theLUMO–HOMO gap (i.e. Eg = ELUMO � EHOMO) could be a cause of

gas uptake effects. It is a typical quantity to describe the stabilityof the host–guest. For all three gases, the energy of the LUMOs isdecreased when they uptake by 1. However, this energy is verylow for 1–CO2 among others. The comparison of the energy ofthe LUMOs of 1–CO2 and 1 can provide an important evidencefor the higher ability of this compound to uptake CO2. The calcula-tions reveal that the energy of the LUMO of 1–CO2 is lower thanthat of 1. Therefore, electron acceptance character of 1 increasessignificantly by uptake of CO2.

4. Conclusion

In conclusion, we have demonstrated that [Cd(l-hmt)(l-dca)2]n

(1) a seemingly nonporous crystal exhibiting highly selective up-take of CO2. However, the N2 sorption of 1 do not occurred evenat low temperature, not because of size selectivity but because ofthe different nature of the host–guest interactions. Theoretical re-sults also show the lowest interaction of N2 among other gases.Since electron transfer behavior of 1 has an important role ongas adsorption, its electrochemical properties are studied by cyclicvoltammetry. Its voltammogram shows the current peaks arestrongly dependent on the hmt concentration and presence ofhmt enhanced the charge transfer activity of polymer. Also, theexperimental and theoretical results obtained in this study clearlyindicate that CO2 among the examined gases could penetrate anddiffuse highly even into the seemingly nonporous crystal of 1.

On the basis of the observation, it has been suggested that thehost molecules might cooperate with one another in a dynamicand concerted fashion in order to create windows of opportunity,thus allowing gaseous guest molecules to freely traverse the crys-tals even in the absence of traditional open channels. Also, the re-sults showed that 1 can be also a good candidate material for fluegas separation and carbon dioxide removal from the air.

Acknowledgements

We are grateful for the financial support from the Departmentof Chemistry, Isfahan University of Technology (IUT). Also, theauthors would like to thank National Iranian Gas Company Re-search Center for gas sorption tests.

L. Tabrizi et al. / Polyhedron 69 (2014) 84–89 89

Appendix A. Supplementary data

CCDC 941180 contains the supplementary crystallographic datafor compound 1. These data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road, Cam-bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this articlecan be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2013.11.029.

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