Journal of Molecular Structure 1076 (2014) 280–284

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Journal of Molecular Structure

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

Isostructural 1D coordination polymers of Zn(II), Cd(II) and Cu(II)with phenylpropynoic acid and DABCO as organic linkers

http://dx.doi.org/10.1016/j.molstruc.2014.07.0670022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.

Rajendran Saravanakumar a, Babu Varghese b, Sethuraman Sankararaman a,⇑a Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Indiab Sophisticated Analytical Instrument Facility, Indian Institute of Technology Madras, Chennai 600036, India

h i g h l i g h t s

� Isostructural 1D coordination polymers of divalent Zn, Cd and Cu have been synthesized.� Phenylpropynoic acid and DABCO are used as organic ligand and linker, respectively.� Coordination polymers are characterized by single crystal XRD data.� DABCO links paddle wheel secondary building motifs to form the polymer chain.� Polymer chains are connected by weak p–p and CAH. . .p interactions in the solid.

a r t i c l e i n f o

Article history:Received 9 April 2014Received in revised form 23 June 2014Accepted 28 July 2014Available online 2 August 2014

Keywords:Coordination polymersMetal-organic frameworkPhenylpropynoic acidDABCOPorous solids

a b s t r a c t

Using phenylpropynoic acid (PPA) and 1,4-diazabicyclo[2.2.2]octane (DABCO) as organic spacers, iso-structural coordination polymers of Zn(II), Cd(II) and Cu(II) were synthesized by solvothermal methodand structurally characterized using single crystal XRD, powder XRD, 13C CP-MAS NMR spectroscopy. Sin-gle crystal XRD data revealed four PPA units coordinating with two metal ions forming a paddle wheelsecondary building unit (SBU). The paddle wheel units are connected through coordination of DABCOnitrogen to the metal centers from the axial positions leading to the formation of the 1D coordinationpolymers along the c axis. Intermolecular p stacking and CAH. . .p interactions between the adjacentpolymer chains convert the 1D coordination polymer into an interesting 3D network with the CAH. . .pbonds running along the crystallographic a and b axes. Thermal and nitrogen adsorption studies of thesecoordination polymers are reported.

� 2014 Elsevier B.V. All rights reserved.

Introduction

Porous solids have long been an attractive area of research foracademician and industrialist owing to their wide application [1–5]. Coordination polymers (CP) (1D and 2D) and metal-organicframework (MOF) (3D) materials represent a class of porous mate-rials built from organic spacers and metal ions or metal clusters [6–10]. CPs have attracted the interest of numerous researchers owingto simple synthetic methods, tailorable pore dimensions and func-tionalization of pores. The pore dimensions can be varied system-atically by judicious choice of primary building blocks [11–17]. CPsor MOFs are potentially very useful for gas storage, catalysis andsmall molecule separations and as sensors [18–23]. Carboxylicacids, mostly aromatic ones, are widely used as organic spacerdue to their ability to coordinate with metal ions in a variety of

coordination modes [24–25]. However acetylenic carboxylic acidsare scarcely used as organic spacers in the synthesis of CPs. Oneinstance of a Zinc MOF with acetylenedicarboxylic acid as organicspacer has been reported [26]. The acetylenic spacer unit increasesthe length without altering the direction and thus provide CPs withwider channels. In addition, acetylenic units increase the p elec-tron density of the framework and may act as storage sites forthe adsorbed gas molecules [27–30]. Phenylpropynoic acid, anacetylene homologue of benzoic acid is used as an organic spacerin the present study along with DABCO, a well known linker, toform coordination polymers of Zn(II), Cd(II) and Cu(II) ions. Inclu-sion of acetylene unit into organic linker creates an additionalinteraction site that can lead to CAH. . .p and p. . .p interactions.These weak non-covalent interactions can in turn convert 1D CPto a 3D network. This strategy may find use in preparation ofhigher dimension networks that can be exploited for gas storage(acetylene, CO2, SO2, CH4 etc.), gas purification, guest inclusionapplications etc. The metal ions, namely Zn(II), Cd(II) and Cu(II)

Fig. 1. Structure of CP 1 in the crystal (stick representation). The four fold axis (c-axis) passes through ZnAN bond axis. The four fold symmetry is imposed on theDABCO moiety which leads to its four fold disorder.

R. Saravanakumar et al. / Journal of Molecular Structure 1076 (2014) 280–284 281

ions, were chosen to investigate if structurally diverse CPs could besynthesized by varying the metal ion.

Results and discussion

Synthesis of coordination polymers 1–3

Addition of an aqueous solution of DABCO to a suspension ofPPA in water at 80 �C furnished the corresponding salt. A whiteprecipitate was instantly formed upon addition of this salt solutionto an aqueous solution of zinc nitrate at 80 �C (Scheme 1). The pre-cipitate was allowed to stand at room temperature for a day uponwhich colorless crystals of 1 suitable for single crystal XRD analysiswere obtained. Similar observations were made when PPA-DABCOsalt was reacted with cadmium acetate. Single crystals of 2 werethus obtained. In case of copper nitrate reaction with PPA-DABCOsalt gave a green precipitate and it did not yield single crystalsupon standing at room temperature. Therefore an alternativemethod was adapted to obtain the corresponding copper deriva-tive, 3. PPA and DABCO were dissolved in cyclohexanol and thissolution was carefully layered over an aqueous solution of coppernitrate in such a way that the two layers were seen distinctly. Itwas left undisturbed for 15 days upon which green crystals of 3were formed at the interface. These crystals were suitable for sin-gle crystal XRD analysis. CPs 1–3 were insoluble in water and allcommon organic solvents such as CH2Cl2. CHCl3, EtOAc, MeOH,DMF and DMSO. The CPs 1–3 were characterized by XRD, powderXRD, 13C CP-MAS NMR.

Crystal structure description of coordination polymer 1

Single crystal XRD data revealed that CPs 1–3 are isostructuralbelonging to tetragonal crystal system. While CP 1 and 2 belongto I4/m space group, CP 3 belongs to P42/n space group. Hence onlythe structure of 1 is discussed in detail here. Compound 1 crystal-lized in tetragonal space group I4/m. Two zinc atoms are bridgedby four carboxylates to form the well-known paddle wheel second-ary building unit (SBU) (Fig. 1) with ZnAZn distance of 3.003 Å andZnAO distance of 2.059 Å. The axial positions of the paddle wheelare occupied by nitrogen atoms of DABCO units with ZnAN dis-tance of 2.058 Å. The coordination geometry around zinc is squarepyramidal with four carboxylate oxygens and a nitrogen from DAB-CO. The DABCO units connect one paddle wheel SBU with anothersuch unit and extends along the c-axis to form 1D coordinationchain (Fig. 1). The observed structure is similar to the CP obtainedfrom benzoic acid, pyrazine and rhodium (II) [31–33]. However,

COOH

+ NN metal salts

H2O, 80 oC M M

O

O O

O

O

OO

NNN NO

metal salts = Zn(NO3)2.6H2O or Cu(NO3)2.3H2Oor Cd(OAc)2.2H2O

M = Zn (1), Cd (2), Cu (3)

n

Scheme 1. Synthesis of 1D coordination polymers 1–3.

the presence of acetylenic unit in PPA promotes p–p interactionsand CAH. . .p interactions in 1 that make the structure very inter-esting. These weak non-covalent interactions between CP chainsconvert the 1D coordination polymer to a 3D network structure(Fig. 2). The CAH. . .p interactions are observed between the parahydrogen atoms of the phenyl rings of one coordination polymerchain and the triple bonds of the acetylene unit of adjacent coordi-nation polymer chain. The CAH. . .p distances are 2.87 and 2.686 Å,respectively (Table 1). The CAH. . .p interactions link the adjacentcoordination polymer chains along the a and b axes to form a por-ous structure. The pore is diagonally 12.7 Å wide and face to face9.27 Å wide. The phenylethynyl groups of the adjacent moleculeslie in parallel planes within the p–p interaction distance of3.461 Å (Fig. 3). These weak p stacking and CAH. . .p interactionsconvert the 1D coordination polymer into an interesting 3D net-work with the CAH. . .p bonds running along the a and b axesand the coordination polymer chain running along the c axis.

Synthesis of CPs 1–3 gave solid precipitate in addition to singlecrystals suitable for XRD. The solid precipitate (presumably micro-crystalline solid) was analysed by powder XRD and the powderXRD data was compared with the powder XRD pattern generated

Fig. 2. Formation of 3D network in CP 1 through CAH. . .p interactions, representedby dash lines, between the para hydrogen atom of the phenyl ring and the triplebond of the acetylene unit of adjacent molecule.

Table 1Metal–metal (M–M) distance and other bond lengths in CPs 1–3 (all values inangstroms).

CP M–M M–N M–O p–pa CAH. . .pa

1 3.003 2.058 2.059 3.461 2.87, 2.6862 3.029 2.252 2.234 3.479 2.807, 2.7063 2.833 2.20 1.934 3.463 2.856, 2.703

a p stacking distance and CAH. . .p distance as defined in Fig. 3.

Fig. 3. Intermolecular p–p and CAH. . .p interactions, represented by dash lines, inthe crystal of CP 1. The distance between the centroid of the phenyl ring to thecentroid of the acetylenic bond is 3.461 Å and the CAH. . .p distances are 2.87 and2.686 Å. The centroids are indicated by brown dots in the figure. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

Fig. 4. Comparison of experimental (solid line) and simulated (dotted line) powderXRD patterns of CP 1.

Fig. 5. Solid state 13C CP-MAS NMR spectrum of CP 1. The spinning side bands areindicated with *.

282 R. Saravanakumar et al. / Journal of Molecular Structure 1076 (2014) 280–284

from the single crystal XRD data using Mercury (2.4) program. Thecomparison for CP 1 is shown in Fig. 4. The one to one correspon-dence of peaks in the experimental and the simulated powder XRDpatterns confirmed the phase homogeneity of solid precipitatewith the single crystal.

The solid state 13C CP-MAS NMR spectra of the powdered solidsof CPs 1 and 2 were recorded to confirm the presence of the aro-matic linkers. In case of CP 1 a peak at 158.3 ppm correspondedto the carboxylate carbon, a bunch of peaks at 134–121 ppm tothe aromatic carbons, peaks at 84.4 and 78.7 ppm to the acetyleniccarbons and a peak at 45.2 to the carbon atoms in the DABCO unit(Fig. 5).

Thermal stability and N2 adsorption studies

Thermal gravimetric analysis of CPs 1–3 revealed no significantweight loss up to 150–165 �C. A weight loss of 76% for CP 1in thetemperature range of 150–700 �C corresponded to the loss of phe-nylpropynoic acid and DABCO units, calculated on the basis of thecorresponding molecular formula of the complex. CP 2 displayed aweight loss of 72% in the temperature range of 130–600 �C due tothe loss of both phenylpropynoic acid and DABCO units. The exper-imental percentage weight of residue left almost matches with thecalculated (empirical formula) percentage weight of oxides of thecorresponding metals, ZnO (CP 1), CdO (CP 2) and CuO (CP 3). A

Table 2Porosity data from N2 adsorption–desorption isotherms for CPs 1–3.

CP BET surface area(m2g�1)

BJH average porediameter (Å)

Pore volume(cm3 g�1)

1 15 158.5 0.0272 23 116.6 0.0703 22 148.8 0.82

structurally similar rhodium-benzoate-pyrazine 1D coordinationpolymer [34] exhibited higher thermal stability (247 �C) than CPs1–3.

The porous nature of CPs 1–3 was evident from the single crys-tal XRD analysis. The nature of the porosity of these CPs was fur-ther studied by N2 adsorption isotherms. Prior to N2 adsorptionstudies the samples were degassed at 50 �C for 12 h. The adsorp-tion isotherm of these CPs corresponded to Type-III isothermaccording to the IUPAC classification [35]. The characteristic fea-tures of Type-III isotherm are (a) adsorbate-adsorbate interactionis stronger than the adsorbate-adsorbent interaction, and (b) theheat of adsorption is close to zero. From the adsorption–desorptionisotherm studies the BET surface area, BJH average pore size andpore volume were calculated. CPs 1–3 exhibited very low BET sur-face area (15–23 m2 g�1) in comparison to the 3D MOFs. The aver-age pore diameter was between 116 and 158 Å (see Table 2). Basedon the IUPAC classification [35] these CPs are best described asmesoporous solids.

R. Saravanakumar et al. / Journal of Molecular Structure 1076 (2014) 280–284 283

Experimental section

Physical measurements

IR spectra were recorded on Nicolet 6700 FT-IR spectrometer.Powder X-ray diffraction measurements were made on Bruker D8advance diffractometer equipped with graphite monochromaticCu(Ka) radiation (k = 1.54180 Å), 2h ranging from 5� to 90� with0.1� step width. TGA analysis were performed in TA instrumentsQ500, Hi-RES TGA under nitrogen atmosphere with temperatureincrement of 10 �C/min. Nitrogen adsorption measurements wereperformed at 77 K in Micromeritics ASAT 2020 porosimeter. Priorto sorption process, samples were degassed for 12 h at 50 �C. 13C-CP-MAS were measured on Bruker Avance 400 MHz NMR spec-trometer operating at 100 MHz for 13C. Single crystal X-ray diffrac-tion data were collected on Bruker AXS (Kappa Apex II)diffractometer equipped with a graphite monochromator, Mo(Ka)(k = 0.7107 Å) and fitted with a CCD detector. The structures weresolved using SIR92 [36] program. The structures were refined usingSHELXL-97 [37] program. Phenylpropynoic acid was synthesized asreported earlier [38].

Synthesis of CP 1

To a suspension of phenylpropynoic acid (PPA) (20 mg,0.137 mmol) in water (3 mL) an aqueous (1 mL) solution of DABCO(19 mg, 0.17 mmol) was added and heated at 80 �C for few minutesresulting in a clear solution. Addition of this solution to an aqueous(3 mL) solution of Zn(NO3)2�6H2O (20 mg, 0.068 mmol) at 80 �Cfurnished white precipitate immediately, which upon standingfor a day resulted in the formation of colorless crystals along withwhite solid (14.6 mg) 52%. The crystals and white solid werewashed with water, methanol and dichloromethane to removeunreacted starting materials. Mp: 209–250 �C (dec); FT-IR (Neat):2208 (mC„C), 1620 (masymC@O), 1488 (msymC@O) cm�1; 13C-CP-MASNMR (100 MHz) d: 158.3, 134.4, 130.7, 121.5, 84.5, 78.7, 45.3; Anal.calcd for C42H32N2O8Zn2 (MW 822.76) C (61.26), H (3.89), N (3.40);found C (61.00), H (3.56), N (3.36).

Synthesis of CP 2

To a suspension of phenylpropynoic acid (PPA) (20 mg,0.137 mmol) in water (3 mL) an aqueous (1 mL) solution of DABCO(19 mg, 0.17 mmol) was added and heated at 80 �C for few minutesresulting in a clear solution. Addition of this solution to an aqueous(3 mL) solution of Cd(OAc)2�2H2O (18 mg, 0.068 mmol) at 80 �Cfurnished white precipitate immediately, which upon standingfor a day resulted in the formation of colorless crystals along withwhite solid (17 mg) 55%. Solid-crystals mixture was washed withwater, methanol and dichloromethane to remove unreacted start-ing materials. Mp: 173–189 �C (dec); FT-IR (neat): 2211 (mC„C),1609 (masymC@O), 1488 (msymC@O) cm�1; 13C-CP-MAS NMR(100 MHz, ppm) d: 159.1, 134.4, 130.5, 121.4, 83.8, 79.5, 46.6; Anal.calcd for C42H32N2O8Cd2 (MW 916.82) C (54.97), H (3.49), N (3.05);found C (54.88), H (3.33), N (3.43).

Synthesis of CP 3

To a suspension of phenylpropynoic acid (PPA) (20 mg,0.137 mmol) in water (3 mL) an aqueous (1 mL) solution of DABCO(19 mg, 0.17 mmol) was added and heated at 80 �C for few minutesresulting in a clear solution. Addition of this solution to an aqueous(3 mL) solution of Cu(NO3)2�3H2O (16.4 mg, 0.068 mmol) at 80 �Cfurnished green crystalline solid(18 mg, 65%) immediately whichupon standing did not yield single crystals suitable for X-ray anal-

ysis. The solid thus obtained was washed with water, methanoland dichloromethane to remove unreacted starting materials.

To obtain single crystals of CP 3 for X-ray analysis, PPA (20 mg,0.137) and DABCO (19 mg, 0.17 mmol) in cyclohexanol solution(3 mL) was carefully layered over an aqueous solution (1 mL) ofCu(NO3)2�3H2O (16.4 mg, 0.068 mmol) and kept undisturbed for15 days. Green crystals formed at the interface were isolated andfound to be suitable for single crystal X-ray analysis. Mp: 140–150 �C (dec); FT-IR (Neat): 2206 (mC„C), 1610 (masymC@O), 1487(msymC@O) cm�1; Anal. calcd for C42H32N2O8Cu2 (MW 819.8) C(61.47), H (3.90), N (3.41); found C (61.88), H (3.55), N (3.65).

Conclusions

Coordination polymers with acetylenic acids as building blocksare rare in literature. Three isostructural 1D coordination polymerswith Zn2+, Cd2+. Cu2+ as metal ions and phenylpropynoic acid andDABCO as organic linkers have been synthesized and structurallycharacterized. All the three CPs were isostructural with paddle-wheel geometry around the metal centers formed by four carbox-ylate units. The axial positions are linked by DABCO along the crys-tallographic c axis resulting in the formation of the 1D structure.Weak p–p and CAH. . .p interactions between the adjacent coordi-nation polymer chains running along the crystallographic a and baxes results in 3D network structures for these polymers. Thermalstability of the CPs were determined by TGA and they were foundto be stable up to 150–165 �C. From the nitrogen adsorption–desorption isotherms the pore size, pore volume and surface areawere determined. The average pore diameter was between 116and 158 Å for CPs 1–3. Based on the IUPAC classification theseCPs are best described as mesoporous solids.

Acknowledgments

We thank CSIR and DST, New Delhi for financial support, UGC,New Delhi for scholarship to RSK, Department of Chemistry andSAIF, IIT Madras for infrastructure. We thank CCDC, Cambridge,UK for Mercury 2.4 program.

Appendix A. Supplementary material

CCDC number 737661, 737662 and 747437 contain the supple-mentary crystallographic data for CPs 1-3, respectively. These datacan be obtained free of charge from The Cambridge Crystallo-graphic Data Centre via www. ccdc.cam.ac.uk/data_request/cifhttp://dx.doi.org/10.1016/j.molstruc.2014.07.067.

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