Zn(II), Cd(II) - American Chemical Society

Oct 9, 2012 - Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221005, India. ‡. Physical/Materials Chemistry Divisio...
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Metal [Zn(II), Cd(II)], 1,10-Phenanthroline Containing Coordination Polymers Constructed on the Skeleton of Polycarboxylates: Synthesis, Characterization, Microstructural, and CO2 Gas Adsorption Studies Pramod Kumar Yadav,† Niraj Kumari,† Pradip Pachfule,‡ Rahul Banerjee,‡ and Lallan Mishra*,† †

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221005, India Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India

‡

S Supporting Information *

ABSTRACT: Four new coordination polymers have been obtained by the reaction of metal [Zn(II), Cd(II)], 1,10phenanthroline separately with two carboxylic acids, [biphenyl4,4′-dicarboxylic acid (4,4′-H2BDA),and 4,4′-azodibenzoic acid (4,4′-H2ADA)]. The crystal structures of the resulting complexes of type {[Zn(4,4′-BDA)(Phen)]2·HCON(CH3)2}n (1), {[Cd(4,4′-BDA)(Phen)]2·H2O}n (2), {[Zn(4,4′-ADA)(Phen)]2}n (3), and [Zn(4,4′-ADA)(Phen)(H2O)2]n (4) have been elucidated using their single-crystal X-ray diffraction analysis. Their thermal stabilities have been investigated using thermogravimetric analysis. Complexes result in a 2D structure with channel formation. SEM studies show a coordinationinduced effect on their morphology. Grain and flowery morphology of uncoordinated ligands 4,4′-H2BDA and 4,4′H2ADA, respectively, changes on coordination with metal salts. SEM micrographs of complexes exhibit ridged surface, cracks of 2 μm width, parallelopiped structure, and feathery appearance for 1, 2, 3, and 4, respectively. Complexes show moderate adsorption of CO2 gas.

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depending on the number of deprotonated carboxylic groups.3 Currently, a large number of porous MOFs based on aromatic carboxylic ligands have been reported in several current review articles.4 In this regard, intelligent combination of bridging carboxylates with 1,10-phenanthroline like chelating ligands and metal ions using solvothermal methods has generated many interesting coordination architectures. The geometries of different carboxylate groups as bridging ligands may play an important role in adjusting the topologies of their transition metal complexes.5 As the length of the linear spacer is increased, three-dimensional (3D) coordination polymers tend to interpenetrate, and if the structure is already interpenetrated, the degree of interpenetration usually increases with the length of the spacer ligand.6 On other hand, a molecule such as 1,10phenanthroline, which commonly acts as terminal bidentate chelating ligand, may also provide supramolecular interactions for molecular recognition or assembly processes.7 In this

INTRODUCTION Current interest in coordination polymers is rapidly expanding due to their intriguing architectures1 and potential applications in the areas of catalysis, separation analysis, gas adsorption, sensors, and in electronic and magnetic devices.2 In this context, aromatic carboxylic ligands have been widely used as building blocks in the construction of metal−organic framework (MOF), as carboxylic groups can be partially or completely deprotonated, and can coordinate with metal ions in multicoordinated ways. Carboxylates are attractive metal binding units in coordination networks due to the negative charge that significantly enhances their ability to bind strongly to metals centers. This feature undoubtedly contributes to the robust nature of coordination networks. Diversified coordination modes (monodentate, chelating, and/or bridging) of carboxylates allow access to a wide variety of structures. Sometimes it becomes difficult to predict correct coordination geometries and node of their connectivities with metal ions. The ligands containing multiple carboxylic acid units have proven to be good candidates because they can be regarded not only as hydrogen acceptors but also as hydrogen donors, © XXXX American Chemical Society

Received: July 2, 2012 Revised: October 4, 2012

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context, metal (Zn, Cd) ions with d10 configuration are particularly promising due to their wider range of coordination numbers together with their applications8,9 in luminescence and biological activities. Considering these aspects, it was found interesting to synthesize complexes of Zn(II) and Cd(II) 1,10phenanthroline reacting separately with two different types of ligand frameworks such as biphenyl-4,4′-dicarboxylic acid (4,4′H2BDA) and 4,4′-azodibenzoic acids (4,4′-H2ADA) without use of a third component as spacer. Microstructure and CO2 gas sorption studies of resulting coordination polymers have also been carried out.

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1389(vs), 1220(m), 1101(m), 1009(m), 858(s), 794(s), 722(s), 641(m), 504(m), 420(m). [Zn(4,4′-ADA)(Phen)(H2O)2]n (4). Synthetic procedure of complex 4 was the same as that used for the synthesis of complex 3 except that the reaction mixture was heated at 110 °C. The crystals of 4 were obtained after 8 h. Yield: ∼65%. Anal. Calcd for C26H16N4O6Zn (4): C, 57.21; H, 2.95; N, 10.26. Found: C, 56.84; H, 3.54; N, 10.41. IR (KBr, cm−1): 3397(w), 3062(m), 1597(s), 1548(s), 1427(s), 1388(vs), 1218(m), 1090(m), 1009(m), 867(s), 794(s), 726(s), 642(m), 479(m). The corresponding Cd(II) complex of 4,4′-ADA was also synthesized, but its crystals could not be obtained so it was not included here. X-ray Crystallographic Studies. The X-ray diffraction data were collected by mounting a single crystal of the sample on glass fibers. Oxford diffraction XCALIBUR-EOS diffractometer was used for the determination of cell parameters and intensity data collection at room temperature. Monochromating Mo Kα radiation (λ = 0.71073 Å) was used for the measurements. The crystal structures were solved by direct methods using SHELXS-97 Program13 and were refined by full matrix least-squares SHELXL-97.14 Drawings were carried out using MERCURY,15 and special computations were carried out with PLATON.16 Crystallographic data (excluding structure factors) for the structures reported in this Article have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as deposition nos. CCDC 896155 (1), 864674 (2), 850421 (3), and 864673 (4). Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 lEZ, UK (fax, 44 (1223) 336 033; e-mail, [email protected]). Gas Adsorption Measurements. Low pressure volumetric gas adsorption measurements involved in this work were performed at 77 K for N2, maintained by a liquid nitrogen bath, with pressures ranging from 0 to 1 bar on a Quantachrome Quadrasorb automatic volumetric instrument, while CO2 adsorption measurements were done at 273 K in the same pressure range. In all adsorption measurements, ultra highpurity N2 or CO2 was obtained by using calcium aluminosilicate adsorbents to remove trace amounts of water and other impurities before introduction into the system. The micro crystals of each complex were soaked in a 1:1 dry dichloromethane and methanol mixture for 12 h. Fresh 1:1 dry dichloromethane and methanol mixture was subsequently added, and the crystals were allowed to stay for additional 48 h to remove free solvates (DMF, DMSO, and H2O) present in the framework. The sample was dried under a dynamic vacuum ( 2σ(I)]a R indices (all data) GOF on F2 (GOF)a a

1

2

3

C55H39N5O9Zn2 C52H32N4O9Cd2 1044.65 1081.62 monoclinic monoclinic 293(2) 293(2) C2/c C2/c 25.453(3) 26.4771(1) 19.230(3) 19.1903(7) 19.513(3) 19.6020(8) 90 90 106.914(16) 107.784(5) 90 90 9174(2) 9483.9(7) 8 8 1.519 1.515 1.118 mm−1 0.957 mm−1 4288 4320 21 374/10 259 47 852/11 355 0.0754 0.0901 −34 ≤ h ≤ 18 −35 ≤ h ≤ 34 −24 ≤ k ≤ 22 −26 ≤ k ≤ 25 −26 ≤ l ≤ 25 −22 ≤ l ≤ 26 full-matrix, least-squares on F2 R1 = 0.0818, wR2 = R1 = 0.0789 [0.0961], wR2 = 0.1816 0.1889 [0.2040] R1 = 0.1788, wR2 = R1 = 0.1737, wR2 = 0.2196 0.2725 1.023 0.994 [0.891]

4

C52H32N8O8 Zn2 1027.60 monoclinic 293(2) C2/c 29.2153(11) 18.7144(6) 20.3431(7) 90 107.516(4) 90 10606.8(6) 8 1.287 0.962 mm−1 4192 37 194/9332 0.0312 −34 ≤ h ≤ 26 −22 ≤ k ≤ 22 −24 ≤ l ≤ 24

C26H16N4O6Zn 545.80 triclinic 293(2) P1̅ 7.5540(5) 9.1794(6) 18.2952(11) 87.596(5) 84.518(5) 76.700(5) 1228.68(14) 2 1.475 1.049 mm−1 556 9403/5693 0.0368 −10 ≤ h ≤ 9 −9 ≤ k ≤ 11 −24 ≤ l ≤ 19

R1 = 0.0428 [0.0789], wR2 = 0.1391 [0.1816] R1 = 0.0544, wR2 = 0.1482

R1 = 0.0538, wR2 = 0.1207 R1 = 0.0835, wR2 = 0.1436 0.929

1.049 [0.994]

Statistics for non-SQUEEZED data are bracketed.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1, 2, 3, and 4 Complex 1 Zn(1)−O(7) Zn(1)−O(5) Zn(1)−O(6) Zn(1)−N(3) Zn(2)−O(3)

1.968(4) 2.027(5) 2.341(5) 2.092(6) 2.034(5)

Zn(2)−O(1) Zn(2)−O(2) Zn(2)−N(1) Zn(2)−O(4)

2.179(6) 2.147(5) 2.111(6) 2.418(6)

O(5)−Zn(1)−O(6) N(3)−Zn(1)−N(4) N(2)−Zn(2)−N(1) O(2)−Zn(2)−O(1) O(3)#1−Zn(2)−O(4)

60.5(19) 79.5(2) 77.8(2) 59.4(2) 57.9(2)

2.260(6) 2.300(5) 2.301(6) 2.413(5) 2.290(7)

O(009)−Cd(01)−O(008) O(007)−Cd(01)−O(004) N(10)−Cd(01)−N(11) O(003)−Cd(02)−O(17) O(006)−Cd(02)−O(13) N(015)−Cd(02)−N(012)

55.95(19) 54.78(2) 73.0(2) 55.9(2) 55.7(2) 71.8(2)

1.995(3) 2.008(2) 2.094(3) 2.103(2) 2.359(3) 2.442(3)

O(003)−Zn(01)−O(004) O(008)−Zn(01)−O(005) N(10)−Zn(01)−N(013) O(3)−Zn(02)−O(4) O(2)−Zn(02)−O(1) N(018)−Zn(02)−N(012)

59.66(8) 59.46(10) 79.49(9) 58.98(9) 58.12(11) 79.01(10)

2.168(3) 2.199(3)

O(006)−Zn(01)−O(004) O(007)−Zn(01)−O(004) N(008)−Zn(01)−N(009)

83.45(10) 86.09(10) 76.69(12)

Complex 2 Cd(01)−O(007) Cd(01)−O(009) Cd(01)−O(008) Cd(01)−O(004) Cd(01)−N(10)

2.214(5) 2.267(5) 2.372(5) 2.510(5) 2.297(6)

Cd(02)−O(006) Cd(02)−O(003) Cd(02)−O(17) Cd(02)−O(13) Cd(02)−N(015)

Zn(01)−O(008) Zn(01)−O(003) Zn(01)−N(10) Zn(01)−N(013) Zn(01)−O(004) Zn(01)−O(005)

2.040(3) 2.049(2) 2.100(2) 2.104(2) 2.311(2) 2.324(3)

Zn(02)−O(2) Zn(02)−O(3) Zn(02)−N(018) Zn(02)−N(012) Zn(02)−O(4) Zn(02)−O(1)

Zn(01)−O(006) Zn(01)−O(007) Zn(01)−O(004)

2.089(3) 2.140(3) 2.151(3)

Zn(01)−N(008) Zn(01)−O(002)

Complex 3

Complex 4

spectra of the same complexes showed a peak at 1420 cm−1 assigned to the ν(NN) vibration.18 The bands observed at 500−480 and 420−380 cm−1 were assigned to ν(M−N) and ν(M−O) vibrations, respectively.19

ever, from the spectra of complexes 3 and 4, peak separations between νasym(−COO) and νsym(−COO) were found as 79 and 212 cm−1, which supported bis-bidentate and monodentate coordination modes of carboxylate groups, respectively. The C

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Table 3. Selected Parameters for Weak Interactions in Complexes 1, 2, 3, and 4 D−H···A Complex 1 C(029)−H(029)···O(6) C(111)−H(11A)···O(100) C(039)−H(039)···O(7) C(042)−H(042)···O(3) C(056)−H(056)···O(8) C(059)−H(059)···O(8) Complex 2 C(019)−H(019)···O(007) C(019)−H(019)···O(003) C(046)−H(046)···O(007) C(046)−H(046)···O(003) C(061)−H(061)···O(004) Complex 3 C(025)−H(025)···O(005) C(065)−H(065)···O(004) C(068)−H(068)···O(005) Complex 4 O(002)−H(1W)···O(005) O(002)−H(2W)···O(003) O(007)−H(3W)···O(002) O(007)−H(4W)···O(003) C(028)−H(028) ..O(004) C(030)−H(030)···O(005) C(033)−H(033)···N(010)

D−H (Å)

H···A (Å)

D···A (Å)

DHA (deg)

symmetry code

0.93 0.96 0.93 0.93 0.93 0.93

2.42 1.94 2.45 2.58 2.48 2.52

2.743(9) 2.31(4) 2.764(9) 3.377(12) 3.160(10) 3.150(9)

100 100 100 144 130 125

0.93 0.93 0.93 0.93 0.93

2.44 2.51 2.56 2.52 2.57

2.765(10) 3.367(10) 3.299(10) 3.133(9) 3.320(12)

100 153 137 123 137

−x, y, 1/2 − z 1/2 − x, 3/2 − y, −z −1/2 + x, −1/2 + y, z −1/2 + x, 1/2 + y, z

0.93 0.93 0.93

2.56 2.44 2.60

3.407(4) 3.152(5) 3.464(5)

152 134 155

1 − x, y, 3/2 − z −1/2 + x, 1/2 − y, −1/2 + z −1/2 + x, 1/2 − y, −1/2 + z

0.99 0.97 1.00 0.92 0.93 0.93 0.93

1.59 1.74 1.74 1.81 2.46 2.52 2.62

2.569(3) 2.689(1) 2.734(3) 2.664(0) 2.768(5) 3.246(5) 3.517(2)

169 167 174 153 100 135 163

−x, 2 − y, 1 − z −1/2 + x, −1/2 + y, z

1 + x, y, z −x, 2 − y, 1 − z

−x, 2 − y, 1 − z −x, 2 − y, −z

Figure 1. (a) Molecular structure (ORTEP) of 1 drawn at 30% probability level. (b) Coordination environment around zinc(II). (c) The stacking of 2D networks showing the formation of channel. (d) Space-filled model showing the presence of space. All hydrogen atoms are omitted for clarity.

Because complexes were found insoluble in common solvents, UV−vis spectra of both ligands and their complexes were recorded in the solid state (Supporting Information S1). The spectral bands of free ligands arise mainly due to intraligand n→π* and π→π* transitions. However, in the

spectra of their complexes, spectral features are changed with respect to band position due to their coordination with metal ions.20 The intraligand n→π* transition is blue-shifted at λmax 331 nm and λmax 340 nm in the spectra of complexes 1 and 2, respectively. The UV−vis spectrum of H2ADA exhibited an D

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direction. Significant π interactions were also implicated for the reinforcement and overall stability of the crystal lattice. Complex 1 exhibited face-to-face π−π interactions between phenanthroline rings at a centroid-to-centroid distance of 3.418 and 3.584 Å (Supporting Information S3). The crystal packing along the c axis provided 2D structure showing the formation of a channel as depicted in Figure 1(c). Out of two phenanthroline rings of a molecular unit of the channel, one is pointed outward of the channel whereas the other is pointed inside the channel. The phenanthroline rings of two banks of the channel showed face-to face interaction and are separated at a distance of 3.418 and 3.584 Å, and divide the channel showing the formation of space of area 22.34 Å × 24.04 Å as depicted in Figure 1(d). Crystal Structure of {[Cd(4,4′-BDA)(Phen)]2·H2O}n (2). In complex 2, each Cd(II) center is surrounded in a distorted octahedral geometry consisting of two N donors from the 1,10phenanthroline ring and four oxygen atoms from two carboxylate groups of two different 4,4′-biphenyldicarboxylic acid molecules, as depicted in Figure 2(a). Both carboxylates present in biphenyl-4,4′-dicarboxylic acid are coordinated in a chelating bidentate mode as depicted in Scheme 1 (coordination mode (b)). Chelating the 1,10-phenanthroline group makes a bite angles of 73.0° (N10−Cd01−N11) and contributes distortion of the coordination geometry around the Cd(II) ion. The dihedral angles between two phenyl rings of two crystallographically distinct biphenyldicarboxylate dianions 1′ and 2′ (Supporting Information S4) are 38.5° and 11.5°, respectively. The neighboring Cd(II) centers are separated at a distance of 15.33 Å and are bridged by a biphenyldicarboxylate dianionic group. Three consecutive Cd(II) ions are bent at 121.83°. As compared to complex 1, a larger distance of separation and larger bending between Cd(II) ions could be attributed to higher atomic weight and larger size of Cd(II) ions as compared to Zn(II) ion. The molecular structure of the complex showed that, unlike complex 1, both terminal 1,10-phenanthroline rings in complex 2 lie trans to each other. Yet they are again not parallel to each other; hence a zigzag chain is formed. The independent chains are linked through π−π interaction between 1,10-phenanthroline rings at a distance of 3.60 Å, which results in the formation of channels as presented in Figure 2(b). The structure of complex 2 possesses large voids containing a considerable number of diffuse electron density peaks that could not be adequately modeled as a solvent. The SQUEEZE routine of PLATON16 was applied to the collected data, which resulted in reductions in R1 and particularly wR2. PLATON analysis shows that the number of void grid points is 304 and percentage filled space is 71.5%. Total potential solvent accessible void volume is 885.7 Å3. Total (positive) electron count in voids/cell is 123. It shows ∼3.0 N,N′-dimethylformamide molecules. Crystal Structure of {[Zn(4,4′-ADA)(Phen)]2}n (3). Complex 3 also exists as a neutral one-dimensional infinite zigzag coordination chain with Zn(II) centers bridged by 4,4′-ADA ligands as depicted in Figure 3(a). Each Zn(II) center is coordinated by two asymmetrically chelating carboxylate groups of azobenzene-4,4′-dicarboxylic acid molecule and a chelating 1,10-phenanthroline ligand. Thus, each Zn(II) center is hexacoordinated in a distorted octahedral geometry consisting of two N donors from the 1,10-phenanthroline ring and four oxygen atoms from two carboxylate groups of two different 4,4′-ADA ligands. The carboxylate group present in

additional peak at λmax 460 nm. It was assigned to the n→π* transition21 from azo (NN) group. This transition arises at λmax 465 nm in the spectra of complexes 3 and 4. However, additional bands observed at λmax 333 and 317 nm in the spectra of respective complexes arise due to intra-ligand transition.21,22 Structural Description of the Complexes. Crystallographic data and refinement details for the structural analyses of the complexes are summarized in Table 1. Selected bond lengths and bond angles with their estimated standard deviations are presented in Table 2, while selected parameters for weak interactions are listed in Table 3. Crystal Structure of {[Zn(4,4′-BDA)(Phen)]2·(HCON(CH3)2}n (1). The X-ray single-crystal structure determination of complex 1 suggested that it was crystallized in monoclinic crystal system with space group C2/c. Of the two Zn(II) centers, Zn(1) is pentacoordinated by two N atoms (N1, N2) from the 1,10phenanthroline ring, and three oxygen atoms (O5, O6, O7) from two carboxylate groups of two different biphenyl dicarboxylic acid molecules. It exhibited a distorted squarepyramidal geometry. The second Zn(II) ion, Zn(2), is surrounded by two N atoms (N2, N3) from the 1,10phenanthroline ring and four oxygen atoms (O1, O2, O3, O4) from two carboxylate groups of two different biphenyl dicarboxylic acid molecules in a distorted octahedral geometry, as depicted in Figure 1(a,b). The distances Zn(1)−O(7) (1.955(6) Å) and Zn(2)−O(4) (2.416(8) Å) fall in the reported range.23 Thus, there are two crystallographically independent Zn(II) ions along with the phenanthroline group and two 4,4′-BDA anions in the asymmetric unit of complex 1. The two 4,4′-BDA anions exhibit differences in their connectivity with Zn(II) ions (Supporting Information S2). Out of two carboxylates present in one biphenyl-4,4′dicarboxylic acid, one carboxylate group is coordinated in a chelating bidentate mode, whereas the other is coordinated in a monodentate mode as depicted in Scheme 1 (coordination Scheme 1. Crystallographically Established Coordination Modes of Carboxylic Groups in Complexes 1−4

mode (a)). However, in another biphenyl-4,4′-dicarboxylic acid, both carboxylate groups are coordinated in a chelating bidentate mode, Scheme 1 (coordination mode (b)). The neighboring Zn(II) centers are bridged by a biphenyldicarboxylate dianion and are separated at a distance of 15.19 Å. Three consecutive Zn(II) ions are bent at 119.62°. The dihedral angles between two phenyl rings of two crystallographically distinct biphenyldicarboxylate anions 1 and 2 (Supporting Information S2) are 40.3° and 15.5°, respectively. Both terminal 1,10-phenanthroline rings lie in a cis configuration but are not parallel to each other; hence they make a zigzag chain and restrict the growth of the chain in a straightforward E

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Figure 2. (a) Molecular structure (ORTEP) of 2 drawn at 40% probability level. (b) The stacking of 2D networks showing the formation of channel. All hydrogen atoms are omitted for clarity.

solvent accessible void volume is 2156.1 Å3. Total (positive) electron count in voids/cell is 164. Crystal Structure of [Zn(4,4′-ADA)(Phen)(H2O)2]n (4). Complex 4 is formulated as Zn(4,4′-ADA)(bpy)(H2O)2. Each Zn(II) center exhibits octahedral geometry involving oxygen atoms of two asymmetrically monodentate carboxylate groups as shown in Scheme 1 (coordination mode (d)) along with two water molecules in a cis position and two nitrogen atoms from phenanthroline molecule (Figure 4a). There are again two distinct types of bridging mode of 4,4′-ADA ligands. In the first type of bridging, it is coplanar with the phenanthroline ring and separated two Zn(II) centers at 16.373(2) Å, with the azo group lying at the inversion center of the complex. The non planar 4,4′-ADA bridges two Zn(II) centers at a distance of 17.609(1) Å apart, Figure 4(b). The coordinated water molecules play an important role in the formation of complementary hydrogen bonding with the carboxylate group of the adjacent molecule. The crystal packing of the complex is a composite of intra- and intermolecular hydrogen-bonding interactions (Figure 5). The most interesting feature of the structure is that it contains strong hydrogen bonding between coordinated water molecules [O007 and O002] and oxygen atoms [O003] of carboxylate groups to form infinite 1D water chains containing cyclic water cluster. In the water chain, the average O−O distance is ∼2.70 Å. Two such H-bonded networks cross-link to each other and lead to the formation of 2-D sheet structure (Supporting Information S7).

the azobenzene-4,4′-dicarboxylic acid molecule acts in the chelating bidentate mode as shown in Scheme 1 (coordination mode c). There are two distinct types of bridging of 4,4′-ADA ligands. The first type is essentially coplanar to phenanthroline ring and bridges two Zn(II) centers separated at 16.944 Å. The second type is nonplanar to phenanthroline ring and acts as a bridge between two Zn(II) centers separated at 16.914 Å. The dihedral angles between two phenyl rings of two crystallographically distinct 4,4′-ADA dianions 1″ and 2″ (Supporting Information S5) are 19.4° and 40.4°, respectively. Thus, the presence of azo group led to more bending as well as larger separation of two metal centers as compared to separation between two Zn(II) centers in complex 1. Three consecutive Zn(II) ions bent at 120.04° become a building unit of zigzag chain with a distance of 29.403 Å (Supporting Information S6) between Zn(II) ions lying at both ends. The packing of the chains is dominated by face-to-face π−π interaction between phenanthroline rings existing inside the chains and separated at a distance of 4.08 Å. Such interactions led the formation of a channel as shown in Figure 3(b). Space-filled model along the b axis also supported the formation of crossed link 2D network having space of cross-sectional area 25.39 Å × 25.67 Å as shown in Figure 3(c). The structure of complex 3 also possesses large voids containing a considerable number of diffuse electron density peaks that could not be adequately modeled as a solvent. The SQUEEZE routine of PLATON16 was applied to the collected data. This complex has a Kitagorodskii packing index (KPI) of 57.8%, and total potential F

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Figure 3. (a) Molecular structure (ORTEP) of 3 drawn at 40% probability level. (b) The stacking of 2D networks of 3 showing the formation of channel. (c) Space-filled model showing the presence of space. All hydrogen atoms are omitted for clarity.

Thermogravimetric (TG) Analysis. Thermogravimetric analysis (TGA) of complexes 1−4 is performed under nitrogen atmosphere to determine the thermal stabilities of complexes. The thermogram of complex 1 shows a weight loss (8.5%) from 99 to 302 °C (S8). It corresponds to the loss of one cocrystallized DMF molecule. The host framework then collapses rapidly. However, in complex 2, a weight loss from 97 to 146 °C is observed as 1.67%, which corresponds to the loss of one cocrystallized water molecule. This corroborates the calculated weight loss of 1.62%. A second weight loss of 44.60% occurs from 345 to 426 °C that may be due to loss of two 4,4′BDA groups. For complex 3, the major weight loss takes place between 300 and 550 °C due to loss of 4,4′-ADA and 1,10phenanthroline. Similarly, in complex 4, a weight loss of 6.30% was observed from 50 to 333 °C, which corresponds to the loss of two coordinated water molecules. This corroborates the calculated weight loss of 6.60%. Microstructural Studies. The surface morphology of the ligands and complexes was investigated by scanning electron microscopy (SEM) (Supporting Information S9). The SEM images of ligand 4,4′-H2BDA (a) clearly showed high agglomeration, while the morphology of ligand 4,4′-H2ADA (d) was flowery. The SEM image of complex 1 showed a ridged surface, and complex 2 showed the presence of cracks of 2 μm width and various lengths. The SEM micrograph of complex 3

showed the presence of parallelopiped crystals with an average size of 20 × 4 × 3 μm, while the morphology of complex 4 was feathery. Thus, the coordination-induced effect on the surface morphology of the complexes was quite evident. Gas Adsorption Studies. Encouraged by the 2D channel structure, and the high thermal stability and structural rigidity of all four complexes, we determined the gas uptake capacities of the desolvated (activated) framework. The samples activated by the aforementioned procedure (∼80 mg) were used to study the gas adsorption properties of these complexes. From the crystal structures of 1−4, it is clear that the pore sizes of these complexes are comparable with the kinetic diameter of nitrogen (3.65 Å). All of these four complexes are nonporous to N2 as they have a comparable aperture size with the kinetic diameter of N2; however, these complexes are able to take the CO2 (3.4 Å) as it has a lesser kinetic diameter. Furthermore, the low kinetic energy of the N2 molecules at 77 K results in N2 molecules being unable to effectively enter small pores.24 In contrary, at relatively higher temperature (273 K), due to high kinetic energy, CO2 molecules efficiently enter into the framework cavity and pores. At 273 K, all four complexes adsorb CO2 effectively. Particularly, complexes 1−3 having similar structure with almost equivalent pore size adsorb nearly the same CO2. In this series, complex 3 adsorbs 1.28 mmol/g of CO2 as the pressure approaches 1 atm, which is highest G

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Figure 4. (a) Molecular structure (ORTEP) of 4 drawn at 40% probability level. (b) The stacking of 2D networks showing the formation of channel. All hydrogen atoms are omitted for clarity.

Figure 5. Hydrogen-bonding motif of coordinated water molecules in complex 4.

is the presence of free −N atoms from the azo (−NN−) functionality present in the structure, which usually favors the adsorption of the polar gas molecules. As complex 4 is having small pores with very limited gas accessible voids, it adsorbs less CO2 (0.85 mmol/g) in this series. Although the gas uptake shown by these 2D complexes is moderate, they are still comparable with some of the MOFs, ZIFs, zeolites, and porous carbons (Table S10). The CO2 uptake properties show these MOFs are very limited as compared to the well-known MOFs such as Mg-DOBDC, HKUST, BIO-MOF-11, etc., due to small pore size and negligible surface area.

among this series of complexes as depicted in Figure 6. However, complexes 1 and 2 adsorb 1.11 and 1.17 mmol/g CO2 at a similar set of conditions as they have similar structure. The probable reason for the higher gas uptake in the complex 3

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CONCLUSION The reaction of metal nitrate containing 1,10-phenanthroline [metal = Zn(II),Cd(II)] separately with biphenyl-4,4′-dicarboxylic acid and azobenzene-4,4′-dicarboxylic acid yields four new 2D coordination polymers. These complexes have been characterized by their single-crystal X-ray diffraction studies. They are thermally stable as characterized by their TGA. The SEM micrographs of the complexes show a coordination-

Figure 6. CO2 adsorption isotherms below 1.0 bar for complexes 1 (green), 2 (red), 3 (sky blue), and 4 (brown) at 273 K. H

dx.doi.org/10.1021/cg301277q | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

(11) Mitra, S. S.; Kundu, P.; Saha, M. K.; Kruga, C.; Bruckmann, J. Polyhedron 1997, 6, 2475. (12) Guo, X.; Zhu, G.; Li, Z.; Sun, F.; Yang, Z.; Qiu, S. Chem. Commun. 2006, 3174. (13) Sheldrick, G. M. SHELXS-97 Program for the Solution of Crystal Structures; University of Gottingen: Gottingen, Germany, 1997. (14) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467. (15) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M. K.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. MERCURY, New software for searching the Cambridge structural database and visualizing crystal structures. Acta Crystallogr. 2002, B58, 389. (16) Spek, A. L. PLATON. J. Appl. Crystallogr. 2003, 36, 7. (17) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1958. (18) Chen, Z.-F.; Zhang, Z.-L.; Tan, Y.-H.; Tang, Y.-Z.; Fun, H.-K.; Zhou, Z.-Y.; Abrahams, B. F.; Liang, H. CrystEngComm 2008, 10, 217. (19) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; John Wiley & Sons: New York, 1997. (20) Chen, W.; Wang, J.-Y.; Chen, C.; Yue, Q.; Yuan, H.-M.; Chen, J.-S.; Wang, S.-N. Inorg. Chem. 2003, 42, 944. (21) Chang, C.-W.; Lu, Y.-C.; Wang, T.-T.; Diau, E. W.-G. J. Am. Chem. Soc. 2004, 126, 10109. (22) Bhattacharya, S.; Sanyal, U.; Natarajan, S. Cryst. Growth Des. 2011, 11, 735. (23) Clegg, W.; Cressey, T.; McCamley, A.; Straughan, B. P. Acta Crystallogr., Sect. C 1995, 51, 234. (24) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477.

induced effect on the morphology of the complexes. Complexes show moderate CO2 uptake capacity at 273 K and 1 atmospheric pressure.

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ASSOCIATED CONTENT

S Supporting Information *

Description of experimental details, including synthetic methods and crystallography, supplementary figures, including TGA and SEM images, and CIF files for complexes 1, 2, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from CSIR and UGC, New Delhi, India is gratefully acknowledged. N.K. acknowledges DST, New Delhi, for financial support in the form of INSPIRE FACULTY AWARD. P.P. acknowledges CSIR for a Senior Research Fellowship. R.B. acknowledges Dr. S. Pal, Director NCL, for funding via an in-house project (MLP020626) and Clean Coal Technology Project (NWP0021-A).

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