Inorganica Chimica Acta 370 (2011) 170–174

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Inorganica Chimica Acta

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Synthesis, structures and properties of macrocyclic nickel(II) supramoleculeswith imidazole pendants

Sol Han a, Taehyung Kim a, Alan J. Lough b, Ju Chang Kim a,⇑a Department of Chemistry, Pukyong National University, Busan 608-737, Republic of Koreab Department of Chemistry, University of Toronto, Toronto, ONT, Canada M5S 3H6

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

Article history:Received 22 November 2010Received in revised form 15 January 2011Accepted 17 January 2011Available online 28 January 2011

Keywords:Nickel(II) complexC–H���p interactionp–p interactionSupramolecule

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.01.051

⇑ Corresponding author. Tel.: +82 51 629 5589; faxE-mail address: kimjc@pknu.ac.kr (J.C. Kim).

Two new nickel(II) complexes with the composition [Ni(L+H)(CH3CN)2](ClO4)3 (1) and [Ni(L)(tp)]�6H2O (2),(L = 3,10-bis{3-(1-imidazolyl)propyl}-1,3,5,8,10,12-hexaazacyclotetradecane, tp = terephthalate) havebeen synthesized and structurally characterized by a combination of analytical, spectroscopic and X-raydiffraction methods. The structure of 1 consists of monomeric cations of the formula [Ni(L+H)(CH3CN)2]3+

and perchlorate ions. The nickel(II) ion is six-coordinate with bonds to the four nitrogen atoms of themacrocycle and two nitrogen atoms of the axial acetonitrile ligands. One of the protonated imidazolependants of the macrocycle is hydrogen bonded to the imidazole group of the neighboring nickel(II) mac-rocycle, forming an undulated 1D supramolecule. Then, the two 1D supramolecular chains are furtherinterconnected by C–H���p interactions between the methyl group of the acetonitrile ligand and one ofthe imidazole groups to form a 2D double stranded supramolecular polymer. In the structure of 2, the1D coordination polymer is formed with nickel(II) macrocycles and bridging terephthalate ions, whereeach 1D chain is interconnected with p–p interactions of pendant imidazole moieties of the macrocycles,resulting in the formation of a 2D supramolecule.

� 2011 Elsevier B.V. All rights reserved.

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1. Introduction

Metallosupramolecules self-assembled by metal ion buildingblocks and organic molecules have been of great interest not onlydue to fascinating structural features but also potential applica-tions in a wide variety of areas including catalysis, molecular mag-nets, non-linear optics, gas storage and separation, and molecularswitches and sensors [1–6]. In the self-assembly of metallosupra-molecules, intermolecular forces such as hydrogen bonds, p–pinteractions, C–H���p interactions, and C–H���O interactions are usu-ally involved together with metal–ligand coordination bonds [7–14]. In particular, macrocyclic complexes containing two vacantcoordination sites at axial positions in the square-planar geometrycan act as metal building blocks for organic ligand linkers in theself-assembly process. Furthermore, the functional pendant groupsin the macrocycle are expected to afford a favorable environmentfor possible intermolecular interactions such as hydrogen bonds,C–H���p and p–p interactions [3,9]. Bearing these advantagesof metallomacrocycles in mind for the construction of metallosu-pramolecules, we attempted to self-assemble nickel(II) macrocy-cles containing imidazole pendants as building blocks, andsuccessfully obtained new macrocyclic nickel(II) supramolecules

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1 and 2. Herein, we report the detailed structures and propertiesof 1 and 2.

2. Experimental

2.1. Physical measurements

All chemicals utilized in this investigation were obtained fromcommercial sources, were reagent grade, and were used withoutfurther purification. Distilled water was used for all procedures.Infrared spectra of solid samples were recorded on a Perkin–ElmerParagon 1000 FT-IR spectrophotometer between 4000 and400 cm�1 as Nujol mulls on KBr discs. Solid state electronic spectrawere measured by the diffuse reflectance method using samplesdiluted with BaSO4 with a Shimadzu 2401 PC UV–Vis recordingspectrophotometer. The thermal and elemental analyses wereperformed at the analytical laboratory of Korea Research Instituteof Chemical Technology, Daejeon, Korea.

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

1 2

Empirical formula C24H45NiN12Cl3O12 C28H54NiN10O10

Formula weight 858.78 749.52T (K) 150(1) 150(2)k (Å) 0.71073 0.71073Crystal system monoclinic orthorhombicSpace group P21/n Pbcna (Å) 14.0250(6) 11.5493(4)b (Å) 13.4289(7) 15.5884(4)c (Å) 19.5756(5) 20.1112(6)b (�) 94.456(2)V (Å 3) 3675.7(3) 3620.72(19)Z 4 4Dcalc (Mg/m3) 1.552 1.375Absorption coefficient

(mm�1)0.820 0.602

F(0 0 0) 1792 1600Crystal size(mm) 0.20 � 0.12 � 0.10 0.20 � 0.12 � 0.08h range for data collection 2.58 to 27.52o 2.61 to 27.48o

Index ranges �18 6 h 6 18 �14 6 h 6 14�16 6 k 6 17 �15 6 k 6 20�23 6 l 6 24 �24 6 l 6 26

Reflections collected 20 580 22 588Independent reflections 8088 [Rint = 0.0582] 4136 [Rint = 0.0748]Completeness to h 98.3% (h = 25.24o) 99.8%(h = 27.48o)Absorption correction Semi-empirical from

equiv.Semi-empirical fromequiv.

Maximum and minimumtransmission

0.923 and 0.888 0.958 and 0.875

Refinement method Full-matrix least-squares on F2

Full-matrix least-squares on F2

Data/restraints/parameters 8088/21/489 4136/0/223Goodness-of-fit (GOF) on F2 0.999 1.008Final R indices[I > 2r(I)] R1 = 0.0535,

wR2 = 0.1171R1 = 0.0508,wR2 = 0.1252

R indices (all data) R1 = 0.1166,wR2 = 0.1446

R1 = 0.1072,wR2 = 0.1527

Largest difference peak andhole (e �3)

0.630 and -0.519 0.772 and -0.514

S. Han et al. / Inorganica Chimica Acta 370 (2011) 170–174 171

Caution! The perchlorate salts are potentially explosive andshould be handled in small quantities.

2.2. Syntheses

2.2.1. Syntheses of precursor complexes [Ni(L+2H)](ClO4)4�2H2O and[Ni(L)](ClO4)2

Precursor complexes [Ni(L+2H)](ClO4)4�2H2O and [Ni(L)](ClO4)2

were prepared according to the literature procedures with slightmodifications by using 1-(3-aminopropyl)imidazole instead of4-(aminomethyl)pyridine [15]. To a stirred methanol (40 mL)solution of Ni(OAc)2�4H2O (3.1 g, 12.5 mmol) were slowly addedethylenediamine (1.7 mL, 25 mmol), paraformaldehyde (1.5 g,50 mmol) and 1-(3-aminopropyl)imidazole (2.98 mL, 25 mmol).The mixture was refluxed for 1 day. The hot solution was filtered,cooled and HClO4 (60%, 10 mL) was added slowly to the filtrate.The solution was stored overnight in the refrigerator until yellowcrystals [Ni(L+2H)](ClO4)4�2H2O formed. The yellow crystals werefiltered, washed with methanol and finally dried in air. Yield:�10%. Anal. Calc. for C20H44N10O18Cl4Ni: C, 26.3; H, 4.8; N, 15.3.Found C, 26.3; H, 4.8; N, 15.2%. IR (Nujol, cm�1): 3319 (mOH),3204 (mNH), 1575 (mC@N), 1089, 1018 (mCl–O), 623 (mCl–O).

To a suspension of [Ni(L+2H)](ClO4)4�2H2O (1 g, 1.14 mmol) inwater was added an excess amount of triethylamine (�1 mL).The mixture was stored overnight in the refrigerator until yellowcrystals [Ni(L)](ClO4)2 formed. The yellow crystals were filtered,washed with water and dried in air. Yield: �90%. Anal. Calc. forC20H38N10O8Cl2Ni: C, 35.5; H, 5.7; N, 20.7. Found C, 35.3; H, 5.6;N, 20.5%. IR (Nujol, cm�1): 3216, 3077 (mNH), 1611 (mC@N),1078, 1035 (mCl–O), 623 (mCl–O).

2.2.2. Synthesis of [Ni(L+H)(CH3CN)2](ClO4)3 (1)To a suspension of [Ni(L+2H)](ClO4)4�2H2O (180 mg, 0.20 mmol)

in CH3CN/water was added seven drops of triethylamine. The mix-ture was stored overnight in the refrigerator until green crystalsformed. The green needle crystals were filtered, washed with waterand dried in air. Yield: �90%. Suitable crystals of 1 were picked upwhile they were wet for X-ray diffraction studies. The microanalyt-ical data for 1 were obtained after the removal of acetonitrile mol-ecules from the complex by thorough vacuum drying due to theinstability of 1. Anal. Calc. for C20H40N10Cl3O12Ni (10, 1 – 2CH3CN):C, 30.9; H, 5.2; N, 18.0. Found C, 30.7; H, 5.1; N, 17.8%. IR (Nujol,cm�1): 3274, 3139 (mNH), 2303, 2275 (mC„N), 1589 (mC@N),1087, 1012 (mCl–O), 622 (mCl–O).

2.2.3. Synthesis of [Ni(L)(tp)]�6H2O (2)To a DMF (5 mL) solution of [Ni(L)](ClO4)2 (67 mg, 0.1 mmol)

was added a stoichiometric amount of sodium terephthalate(21 mg, 0.1 mmol) dissolved in water (5 mL), which was allowedto stand in an open beaker at ambient temperature. After 1 week,pink needle crystals of 2 were obtained. Suitable crystals of 2 werepicked up under a microscope while they were wet for X-ray dif-fraction studies and subsequent spectroscopic measurements.Yield: �90%. Anal. Calc. for C28H54N10O10Ni (2): C, 44.9; H, 7.3; N,18.7. Found C, 45.2; H, 7.0; N, 19.0%. IR (Nujol, cm�1): 3390(mOH), 3149 (mNH), 1585 (masCOO), 1290 (msCOO).

2.3. X-ray crystallography

A summary of selected crystallographic data for 1 and 2 is givenin Table 1. X-ray data were collected on a Nonius Kappa CCD dif-fractometer, using graphite monochromated Mo Ka radiation(k = 0.71073 Å). A combination of 1� u and x (with j offsets) scanswere used to collect sufficient data. The data frames were inte-grated and scaled using the Denzo-SMN package [16]. The struc-tures were solved and refined, using the SHELXTLnPC V6.1 package

[17]. Refinement was performed by full-matrix least squares onF2, using all data (negative intensities included). In 1, hydrogenatoms other than that on N8 were included in calculated positions.The N8–H8N���N10#4 hydrogen bond confirms the protonation atN8, otherwise the N8���N10#4 intermolecular distance would beunusually short. The position of the H atom was refined on N8.One of the perchlorate ions is disordered in 1.

3. Results and discussion

3.1. Synthesis and structure of 1

During the deprotonation of [Ni(L+2H)](ClO4)4�2H2O in CH3CN/H2O with an excess amount of triethylamine, the nickel(II) com-plex 1 in which one of the pendant imidazole groups is deproto-nated and acetonitrile molecules are axially coordinated to thecentral nickel(II) ion was crystallized out. The molecular structureof 1 is shown in Fig. 1 and the selected bond distances and anglesare listed in Table 2. In 1, the nickel(II) ion exhibits a distorted octa-hedral coordination geometry with the four secondary aminenitrogen atoms of the macrocycle and two nitrogen atoms of ace-tonitrile molecules in axial positions. The Ni–N bond distances varyfrom 2.052(3) to 2.069(3) Å, which are typical for a six-coordinatednickel(II) d8 system [9,18]. The Ni–Naxial distances of 2.163(3) and2.126(3) Å lie within the previously reported values in relatedsystems {trans-[Ni(L1)(CH3CN)2](PF6)2 (L1 = 3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,01.18,07.12] docosane); Ni–Naxial =2.1684(15) Å [19], [Ni(cyclam)(CH3CN)2][Ni(dmit)2]2 (cyclam =1,4,8,11-tetraazacyclotetradecane, dmit = isotrithionedithiolate);

Fig. 1. Molecular structure of 1 with atom-labeling scheme. Hydrogen atoms areomitted for clarity other than those participating in hydrogen bonds and onacetonitrile molecules.

Table 2Selected interatomic distances (Å) and angles (�) for 1.

Ni–N1 2.062(3) Ni–N2 2.064(3)Ni–N3 2.052(3) Ni–N4 2.069(3)Ni–N11 2.163(3) Ni–N12 2.126(3)

N1–Ni–N2 85.76(11) N1–Ni–N4 94.14(11)N1–Ni–N11 91.22(11) N2–Ni–N11 86.66(11)N2–Ni–N4 178.47(10) N2–Ni–N12 91.13(11)N3–Ni–N1 179.65(11) N3–Ni–N2 94.51(11)N3–Ni–N4 85.59(11) N3–Ni–N12 90.95(11)N3–Ni–N11 88.58(11) N4–Ni–N11 91.81(11)N4–Ni–N12 90.39(11) N12–Ni–N11 177.70(11)

Symmetry transformations used to generate equivalent atoms.

Fig. 2. (a) View of an undulated 1D supramolecular structure of 1 by hydrogenbonds ( ). (b) View of C–H���p interactions ( ) occurring between the hydrogenatoms of the acetonitrile ligands and imidazole pendant groups [24]. Hydrogenatoms are omitted for clarity other than those participating in hydrogen bonds andC–H���p interactions.

Table 3Hydrogen bonds for 1 (Å and �).

D–H���A d(D–H) d(H���A) d(D���A) <(DHA)

N1–H1C���O1#1 0.93 2.21 3.121(4) 168.1N2–H2C���O4 0.93 2.24 3.112(4) 155.4N3–H3C���O7#2 0.93 2.20 3.060(4) 153.2N4–H4C���O6#3 0.93 2.32 3.079(5) 138.8N8–H8N���N10#4 1.05(5) 1.67(5) 2.688(5) 163(4)

Symmetry transformations used to generate equivalent atoms: #1 �x + 1/2,y + 1/2,�z + 1/2 #2 x,y � 1,z #3 �x + 3/2,y � 1/2,�z + 1/2 #4 x + 1/2,�y + 1/2,z + 1/2.

Fig. 3. View of the 2D double stranded supramolecular structure of 1 by hydrogenbonds ( ) and C–H���p interactions ( ) between the 1D supramolecular chains[24]. Hydrogen atoms are omitted for clarity other than those participating inhydrogen bonds and C–H���p interactions.

172 S. Han et al. / Inorganica Chimica Acta 370 (2011) 170–174

Ni–Naxial = 2.137(5) Å [20], b-trans-[Ni(L2)(CH3CN)2](ClO4)2�H2O(L2 = 1,4,8,11-tetrakis(2-cyanoethyl)-1,4,8,11-tetraazacyclotetra-decane); Ni–Naxial = 2.109(5) Å [21]}. The skeleton of the macrocy-clic unit in 1 adopts the classical trans III (R,R,S,S) conformationwith two chair-form six-membered and two gauche-form five-membered chelate rings [22,23]. One of the pertinent structural

features of 1 is the formation of a 2D hydrogen bonded supramol-ecule by C–H���p interactions. Therefore, one of the imidazolependant groups which is protonated experiences hydrogen bond-ing interaction with a deprotonated imidazole group belonging toan adjacent nickel(II) macrocycle {D–H���A = N8–H8N���N10#4;d(D–H) = 1.05(5) Å, d(H���A) = 1.67(5) Å, d(D���A) = 2.688(5) Å,\(DHA) = 163(4)�, symmetry code #4 x + 1/2, �y + 1/2, z + 1/2},resulting in the formation of an undulated 1D hydrogen bondedsupramolecule (Fig. 2(a), Table 3). Then, the two 1D supramolecu-lar chains are further interconnected by C–H���p interactions(Fig. 2(b)) between the methyl group of the acetonitrileligand and one of the imidazole groups to form a 2D doublestranded supramolecular polymer {C–H���p = C24–H24A���Imcentroid;d(H���p) = 2.952 Å, d(C���p) = 3.607 Å, \(CHp) = 125.21�, Im = imid-azole} (Fig. 3). Another structural feature found in 1 is the coordi-nation of acetonitrile molecules to the nickel(II) ion, forming adistorted octahedral geometry around the nickel(II) ion. It has beengenerally understood that the square-planar macrocyclic nickel(II)complex with acetonitrile molecules in non-coordinating solventundergoes a square-planar and octahedral species in equilibrium.The equilibrium favors in the square-planar side, therefore, a yel-low square-planar nickel(II) complex is usually an isolated productin the solid state [23,25]. However, we were able to isolate andcharacterize the green octahedral nickel(II) complex 1 from theequilibrium although it rapidly loses axial acetonitrile ligands inair at ambient temperature.

3.2. Synthesis and structure of 2

By the reaction of nickel(II) complex [Ni(L)](ClO4)2 with sodiumterephthalate in DMF/H2O, a six-coordinate nickel(II) complex 2was isolated. In 2, the skeleton of the macrocyclic unit adoptsthe classical trans III (R,R,S,S) conformation as is found in 1. Thestructure of 2 exhibits a 1D coordination polymer and the coordi-nation environment around the central nickel(II) ion can be de-scribed as a distorted octahedron with four Ni–N and two Ni–Obonds (Fig. 4). The central nickel atom is located on an inversion

Fig. 4. Molecular structure of 2 with atom-labeling scheme. Hydrogen atoms areomitted for clarity other than those participating in hydrogen bonds.

Table 4Selected interatomic distances (Å) and angles (�) for 2.

Ni–N1 2.055(2) Ni–N2 2.066(2)Ni–O1 2.1222(19)

N2–Ni–N1 86.52(9) N2–Ni–N1#1 93.48(9)N2–Ni–O1 89.53(7) N2#1–Ni–O1 90.47(7)N1–Ni–O1 88.41(8) N1#1–Ni–O1 91.59(8)

Symmetry transformations used to generate equivalent atoms: #1 �x�1,�y +1,�z + 1.

Table 5Hydrogen bonds for 2 (Å and �).

D–H���A d(D–H) d(H���A) d(D���A) <(DHA)

N1–H1���O2#1 0.93 2.09 2.940(3) 152.1N2–H2���O1W#3 0.93 2.18 3.057(3) 157.3O1W–H1Wa���N5#4 0.84 1.94 2.772(3) 170.4O2W–H2Wa���O3W 0.84 2.02 2.815(4) 158.9O1W–H1Wb���N3 0.83 2.18 2.981(3) 161.3O2W–H2Wb���O2#5 0.84 2.03 2.848(3) 165.3O3W–H3Wb���O1W 0.84 1.99 2.827(3) 177.0O3 –H3Wa���O2W#6 0.84 2.01 2.854(4) 179.6

Symmetry transformations used to generate equivalent atoms: #1 �x � 1, �y + 1,�z + 1 #2 �x � 2,�y + 1,�z + 1 #3 x � 1/2,�y + 1/2,�z + 1 #4 �x � 1,y,�z + 1/2 #5x + 1,y,z #6 �x,y, � z + 1/2.

Fig. 5. View of the 1D coordination polymer along crystallographic a axis and the2D supramolecular structure by p–p interactions ( ) between the imidazolependant groups in 2. Dashed lines ( ) indicate hydrogen bonds. Hydrogen atomsare omitted for clarity other than those participating in hydrogen bonds.

S. Han et al. / Inorganica Chimica Acta 370 (2011) 170–174 173

center. The important bond distances and angles of the structure 2are listed in Table 4. The Ni–N bond distances vary from 2.055(2) to2.066(2) Å, which are typical for a six-coordinated nickel(II) d8

system [9,18,26]. The Ni–O distance of 2.1222(19) Å is similarto those previously reported values in closely related exam-ples {[Ni(L3)(isonicotinate)2] (L3 = 3,10-bis(4-pyridinemethyl)-1,3,5,8,10,12-hexaazacyclotetradecane); Ni–O = 2.135(4) Å [9],[Ni(L3)(BQDC)]�3H2O (BQDC = 2,20-biquinoline-4,40-dicarboxylate);Ni–O = 2.110(4) Å [9], [Ni(L4)(4,40-bpdc)2]�3H2O (L4 = 3,10-bis(2-hydroxyethyl)-1,3,5,8,10,12-hexaazacyclotetradecane, 4,40-bpdc =4,40-biphenyldicarboxylate); Ni–O = 2.0976(16) Å [26], [Ni(L4)-(2,6-ndc)]�2CH3CN (2,6-ndc = 2,6-naphthalenedicarboxylate; Ni–O = 2.142(2) Å [26]}. In the structure of 2, the imidazole pendantgroups of the nickel(II) hexaazamacrocycle involve in p–p interac-tions between the 1D polymers {d(Imcentroid���Imcentroid) = 4.175 Å},which leads to a supramolecule propagating along the crystallo-graphic a axis. The distance of 4.175 Å between the imidazole ringsof the macrocycles is comparable to those of the previouslyreported values in related complexes {[Ni(L3)(isonicotinate)2];d(p���p) = 3.673 Å, 4.847 Å, [Ni(L3)(BQDC)]�3H2O; d(p���p) = 5.237 Å[9]}. Together with the p–p interactions of pendant imidazole

groups of the macrocycles between the 1D polymeric chains, thecomplex 2 shows a multidimensional structure containing chan-nels, where the lattice water molecules are confined by hydrogenbonds (Table 5, Fig. 5).

3.3. Analytical, physical, spectroscopic, and thermal properties

Due to the instability of 1 even at room temperature in air, themicroanalysis was performed with 10 (1 – 2CH3CN) which was ob-tained by the removal of acetonitrile ligands from the complex 1under thorough vacuum drying, and gave a satisfactory result.The microanalysis for 2 was consistent with the composition ofsix molecules of water, a terephthalate ligand and a nickel(II)macrocycle.

The presence of perchlorate ions was suggested by the strongabsorptions at 1087 and 622 (mCl–O) cm�1 in the IR spectrum in1. In addition, weak bands at 3274 and 3139 (mNH) cm�1, a med-ium band at 1589 (mC@N) cm�1 and weak bands at 2303, 2275(mC„N) cm�1 indicate that the macrocyclic ligand, imidazolegroups and acetonitrile ligands are present in the complex 1. Inthe IR spectrum of 2, strong bands were observable at 1585(masCOO) and 1290 (msCOO) cm�1, a weak band at 3149 (mNH)cm�1 and a broad band at 3390 (mOH) cm�1.

The solid state electronic spectrum of 10 shows an absorptionband at 472 nm which is typical for a square-planar nickel(II)macrocycle. The solid state electronic spectrum of 2 shows threeabsorption bands (312, 505, and 621 nm). These are a characteris-tic spectrum expected for a high-spin d8 nickel(II) ion in a distortedoctahedral environment, and can be attributed to d–d transitions,respectively (3B1g ?

3Eg, 3B1g ?3Eg, 3B1g ?

3B2g + 3B1g ?3A2g)

[27,28].The thermal analysis for the complex 1 was not attempted due

to the presence of explosive perchlorate ions in the complex aswell as its instability even at room temperature. The TGA curvefor 2 under N2 stream shows a quick first weight loss of 15.0%(calcd. 14.4%) between 40 and 70 �C, corresponding to the loss ofsix lattice water molecules. On further heating, gradual weight losswas observed in 210–450 �C range with the loss of terephthalateand macrocyclic ligands. No residue was remained above 450 �C.

4. Conclusions

Two new hexaazamacrocyclic nickel(II) supramolecules 1 and 2were prepared and fully characterized. In the structure of 1, thecentral nickel(II) ion of the macrocycle is six-coordinate with axial

174 S. Han et al. / Inorganica Chimica Acta 370 (2011) 170–174

acetonitrile molecules. One of the imidazole pendant groups of themacrocycle is protonated and hydrogen bonded to the imidazolegroup of the neighboring nickel(II) macrocyclic molecule, formingan undulated 1D hydrogen bonded supramolecule. The 1D supra-molecule is further interconnected by C–H���p interactions be-tween the methyl group of the acetonitrile ligand and one of theimidazole groups, forming a 2D double stranded supramolecularpolymer. The structure of 2 exhibits a 1D coordination polymerwith nickel(II) macrocycles and bridging terephthalate ions, whereeach imidazole pendant group in the 1D chain is interconnectedwith p–p interactions, resulting in the formation of a 2D supramo-lecular polymer.

Appendix A. Supplementary material

CCDC 798983 and 798984 contain the supplementary crystallo-graphic data for 1 and 2, respectively. These data can be obtainedfree of charge from The Cambridge Crystallographic Data Centrevia www.ccdc.cam.ac.uk/data_request/cif.

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2011.01.051.

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