Temperature-Controlled Synthesis of Metal-Organic Coordination

Aug 13, 2009 - In all cases, 2,6-nds counteranions interact with water and bipy .... Diverse Structures of Metal–Organic Frameworks Based on a New S...
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DOI: 10.1021/cg900386q

Temperature-Controlled Synthesis of Metal-Organic Coordination Polymers: Crystal Structure, Supramolecular Isomerism, and Porous Property

2009, Vol. 9 4147–4156

Prakash Kanoo, K. L. Gurunatha, and Tapas Kumar Maji* Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064 Received April 7, 2009; Revised Manuscript Received June 19, 2009

ABSTRACT: Five new supramolecular metal-organic coordination polymers (MOCPs), {[Ni(bipy) (H2O)4](2,6-nds) 3 4H2O} (1), {[Ni(bipy)(H2O)4](2,6-nds) 3 2H2O} (2), {[Ni (bipy)(H2O)4](2,6-nds)} (3), {[Ni (bipy)(H2O)4](2,6-nds)} (4), {[Cu (bipy)(H2O)4](2,6-nds)} (5) (bipy = 4,40 -bipyridyl; 2,6-nds = 2,6-naphthalenedisulphonate) have been synthesized and structurally characterized. Compounds 1 and 5 were synthesized at room temperature in H2O/EtOH medium, whereas 2-4 were isolated under hydrothermal conditions. Compounds 1-4 were synthesized maintaining the same stoichiometric ratio of metal and ligand under different reaction temperatures, and the different structures of the compounds indicate that the temperature plays a significant role in the construction of the coordination polymers. Structural characterization reveals that the one-dimensional [M(bipy)(H2O)4]2þ cationic chain is a basic building unit for all of the MOCPs, while 2,6-nds remains as a counteranion. In all cases, 2,6-nds counteranions interact with water and bipy molecules through strong hydrogen-bonding and π-π interactions to afford threedimensional supramolecular structures. Compounds 1-4 have the same building unit with different network superstructures and are related as supramolecular isomers. Supramolecular isomerism in 3 and 4 is very interesting since they have the same molecular formula, {[Ni(bipy)(H2O)4](2,6-nds)}, and are polymorphs. Compounds 4 and 5 are isomorphous. The thermogravimetric study suggests that the dehydrated compounds are stable up to 300 C. Furthermore, sorption studies suggest that dehydrated compounds of 1 and 2 are permanently porous.

*To whom correspondence should be addressed. Fax: (þ91)80 2208 2766. E-mail: [email protected].

depending upon the polarity and Lewis basicity of the adsorbates. Recently, Rossenisky et al. and Real et al. have reported a concerted ligand exchange reaction of the metalbound water molecules with an extra framework bridging ligand and concomitant transformation toward the porous structure.12 Ligands with sulfonate groups, like arylsulfonate RSO3-, bear a strong structural analogy to the phosphonate RPO3but are not well-explored due to the perception that sulfonate is a weakly coordinating ligand.13 This fact is largely justified based upon the library of transition metal sulfonate crystal structures obtained under hydrous conditions.14 However, sulfonate ligands form very stable compounds with alkali or alkaline earth metals and even with lanthanide ions.15 Arylsulfonates, like 2,6-naphthalenedisulphonate (2,6-nds), are weakly coordinating to the transition metals and remain as a counteranion. However, as a counteranion, it is able to generate porous stable structures with different topologies through π-π and hydrogen-bonding interactions in the presence of other aromatic organic ligands, for example, 4,40 -bipyridyl (bipy), phenanthroline, and different solvent molecules.16 Moreover, changing the reaction conditions and stoichiometry in the reactions with the same building units, such noncoordinating ligands may take a different orientation in the network, resulting in different network superstructures, a typical phenomena in MOCPs known as supramolecular isomerism or polymorphism.17 Supramolecular isomerism, which has received increasing attention recently,18-22 is defined as the existence of more than one type of network superstructure for the same molecular building blocks. Supramolecular isomerism is important for a better understanding of supramolecular synthons and, by inference, how

r 2009 American Chemical Society

Published on Web 08/13/2009

Introduction Research on porous metal-organic coordination polymers (MOCPs) has attracted enormous attention during the last two decades due to their intriguing network topology, tunable pore size, structural flexibility, and multiple functionality.1 Rigid and robust MOCPs with applications in gas storage,2 size selective separation,3 catalysis,4 and ion exchange5 are well-documented. Flexible frameworks that show dynamic behavior, like guest responsive fitting,6 guest responsive structural asymmetry,7 framework breathing,8 and singlecrystal to single-crystal structural transformation9 are of interest for selective accommodation or molecular recognition properties. Recently, Kitagawa et al. classified such frameworks in different categories based on structural transformation.1b,e,f One of the keys to structural flexibility is to exploit the noncovalent interactions (like H-bonding and π-π interactions) along with the directional covalent bond.1e,f The combination of mixed ligands such as different di- or tricarboxylates and pyridyl-containing linkers allows one to build different networks with different topologies and pore sizes with versatile functionality.10 However, many transition metals produce low dimensional coordination networks in mixed ligand systems, since the weaker ligand remains as an extra framework entity or counterion and octahedral geometry is completed by ancillary coordinating solvent molecules.11 Such low dimensional systems afford unsaturated metal sites after removal of the solvent molecules and may show interesting sorption properties like size selective adsorption

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they develop and occur in other solid phases and solution.17 In this context, several examples of structural isomerism,23,24 conformational isomerism,25 and topological isomerism26,27 have recently appeared in the literature. Supramolecular isomers also show different magnetic and optical properties based on their different crystal packing in the framework.21c,d Herein, we report the synthesis, structural characterization, and structural flexibility of five 3D supramolecular MOCPs, {[Ni(bipy)(H2O)4](2,6-nds) 3 4H2O} (1), {[Ni(bipy)(H2O)4](2,6-nds) 3 2H2O} (2), {[Ni(bipy)(H2O)4](2,6-nds)} (3), {[Ni(bipy)(H2O)4](2,6-nds)} (4), and {[Cu(bipy) (H2O)4](2,6-nds)} (5), constructed from 2,6-nds and bipy with transition metals Ni(II) and Cu(II). The structure determination reveals that in all cases, one-dimensional (1D) [M(bipy)(H2O)4]2þ cationic chains interact with the 2,6-nds counteranions by π-π and Hbonding interactions to afford three-dimensional (3D) supramolecular structures. Compounds 1-4 have the same building blocks but different network superstructures and are related as supramolecular isomers. All of the MOCPs show high thermal stability and powder X-ray diffraction (PXRD) patterns of the thermally treated compounds, suggesting structural transformation upon removal of metal-bound water molecules. N2 and CO2 sorption studies with dehydrated samples of 1 and 2 suggests the porous nature of the MOCPs. Experimental Section Materials. All of the reagents and solvents employed were commercially available and used as supplied without further purification. Ni(NO3)2 3 6H2O, Cu(NO3)2 3 2.5H2O, bipy, and disodium salt of 2,6-napthalenedisulfonic acid were obtained from Aldrich Chemical Co. Physical Measurements. The elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer. IR spectra of the compounds were recorded on a Bruker IFS 66v/S spectrophotometer using the KBr pellets in the region 4000 to 400 cm-1. Thermogravimetric analysis (TGA) was carried out on Mettler Toledo TGA850 instrument in the temperature range of 25650 C under nitrogen/oxygen atmosphere (flow rate of 50 mL/min) at a heating rate of 5 C/min. PXRD patterns in different states of the samples were recorded on a Bruker D8 Discover instrument using Cu KR radiation. Adsorption Measurements. N2 and CO2 adsorption studies with the dehydrated samples of 1 and 2, prepared at 493 K under high vacuum, were carried out using Quantachrome Autosorb-1C analyzer at 77 and 195 K, respectively. Synthesis. (a) {[Ni(bipy)(H2O)4](2,6-nds) 3 4H2O} (1). An aqueous solution (25 mL) of Na2-2,6-nds (0.166 g, 0.5 mmol) was mixed with an ethanolic solution (25 mL) of bipy (0.078 g, 0.5 mmol) and stirred for 20 min to mix well. Ni(NO3)2 3 6H2O (0.145 g, 0.5 mmol) was dissolved in 50 mL of water. Two milliliters each of ligand solution and metal solution was slowly and carefully layered employing 1 mL of water/EtOH buffer solution. Sky blue block-shaped crystals were obtained after 3 weeks. The crystals were separated and washed with an ethanol-water (1:1) mixture and dried. The bulk amount of the material was obtained by direct mixing of the ligand solution (mixture of bipy and Na2-2,6-nds) to an aqueous solution of Ni(II) with appropriate ratios, and the phase purity of the sample was checked by elemental and PXRD analysis. Yield, 77%, relative to Ni. Anal. calcd for C20H30NiN2O14S2: C, 37.19; H, 4.64; N, 4.34. Found: C, 37.31; H, 4.49; N, 4.52. IR (KBr, cm-1): ν(O-H), 3120-3600; νas(OSO) ∼1145-1230; νs(OSO), 1030. (b) {[Ni(bipy)(H2O)4](2,6-nds) 3 2H2O} (2). In a typical synthesis Ni(NO3)2 3 6H2O (0.145 g, 0.5 mmol), Na2-2,6-nds (0.166 g, 0.5 mmol) and bipy (0.078 g, 0.5 mmol) were mixed well in water (10 mL), and the reaction mixture was placed in a 23 mL Teflonlined stainless steel autoclave. The reaction mixture was subsequently heated to 140 C for 24 h, and then, the autoclave was cooled over a period of 12 h at room temperature. The final product was filtered and washed several times with water and EtOH. Light

Kanoo et al. green needle-shaped crystals were obtained as a single phase. Good quality single crystals were picked out for X-ray diffraction analysis. Yield, 81%, relative to Ni. Anal. calcd for C20H26NiN2O12S2: C, 39.39; H, 4.26; N, 4.59. Found: C, 39.57; H, 4.06; N, 4.46. IR (KBr, cm-1): ν(O-H), 3120-3600; νas(OSO) ∼1140-1250; νs(OSO), 1031. (c) {[Ni(bipy)(H2O)4](2,6-nds)} (3). Compound 3 was synthesized using the same hydrothermal technique and the same molar ratio adopted for 2, only changing the reaction temperature to 120 C. A sky blue crystalline product was obtained in good yield as a single phase. The solid was filtered and washed with water and EtOH several times. Yield, 82%, relative to Ni. Anal. calcd for C20H22NiN2O10S2: C, 41.86; H, 3.84; N, 4.88. Found: C, 41.62; H, 3.51; N, 4.59. IR (KBr, cm-1): ν(O-H), 3342; νas(OSO) ∼1141-1228; νs(OSO), 1031. (d) {[Ni(bipy)(H2O)4](2,6-nds)} (4). The same hydrothermal technique of 2 was employed for the synthesis of compound 4 using the same molar ratio, only changing the temperature to 100 C and increasing the reaction to 36 h. A single phase blue crystalline product was obtained upon cooling the autoclave over a period of 12 h. The crystals were filtered and washed with water and EtOH several times. Yield, 83%, relative to Ni. Anal. calcd for C20H22NiN2O10S2: C, 41.86; H, 3.84; N, 4.88. Found: C, 41.49; H, 3.73 N, 4.63. IR (KBr, cm-1): ν(O-H), 3351; νas(OSO) ∼1139-1226; νs(OSO), 1031. (e) {[Cu(bipy)(H2O)4](2,6-nds)}n (5). Blue single crystals of compound 5 were synthesized adopting the similar procedure employed for 1. Cu(NO3)2 3 2.5H2O was used as a metal source instead of Ni(NO3)2 3 6H2O. The bulk amount of compound 5 was obtained by the direct mixing of Cu(NO3)2 3 2.5H2O, bipy, and Na2-2,6-nds in ethanol-water medium, and phase purity was confirmed by the elemental and PXRD analysis. Yield, 81%, relative to copper. It is worth mentioning that the compound 5 can also be obtained as a single phase crystalline product by adopting the similar reaction condition as of 4, that is, hydrothermal technique at 100 C. Anal. calcd for C20H22CuN2O10S2: C, 41.52; H, 3.81; N, 4.84. Found: C, 41.27; H, 3.98; N, 4.63. IR (KBr, cm-1): ν(O-H), 3100-3600; νas(OSO) ∼1140-1231; νs(OSO), 1037. Single Crystal X-ray Diffraction. Suitable single crystals of compounds 1-5 were mounted on a thin glass fiber with commercially available super glue. X-ray single crystal structural data were collected on a Bruker Smart-CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source with graphite monochromated Mo KR radiation (λ = 0.71073 A˚) operating at 50 kV and 30 mA. The program SAINT was used for integration of diffraction profiles, and absorption correction was made with SADABS program. All of the structures were solved by SIR 9228 and refined by a full matrix least-squares method using SHELXL.29 For all of the compounds, the nonhydrogen atoms were refined anisotropically. All hydrogen atoms were located by Fourier analysis except in compound 1 where some of the hydrogen atoms were fixed by HFIX and placed in ideal positions. A potential solventaccessible area or void space was calculated using the PLATON30 multipurpose crystallographic software. The coordinates, anisotropic displacement parameters, and torsion angles for all of the compounds are submitted as Supporting Information in CIF format. All crystallographic and structure refinement data of the compounds are summarized in Table 1. Selected bond distances and angles are shown in Table 2. Hydrogen-bonding parameters of 1-5 are given in Tables S1-S5 of the Supporting Information, respectively. All calculations were carried out using SHELXL 97,29 PLATON,30 SHELXS 97,31 and WinGX system, Ver 1.70.01.32

Results and Discussion Crystal Structure Description. {[Ni(bipy)(H2O)4](2,6-nds) 3 4H2O} (1). Compound 1 crystallizes in monoclinic system with P21/n space group. Structure determination reveals that 1 is built up from 1D coordination chains of [Ni(bipy)(H2O)4]2þ stabilized by 2,6-nds counteranions and four crystalline water molecules (Figure 1). The Ni(II) center is located on an inversion center, while the 2,6-nds lies along a 2-fold axis. The four oxygen atoms (O1, O1a, O2, and

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Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1-5 parameter

1

2

3

4

5

C20H22NiN2O10S2 573.23 triclinic P1 8.4893(2) 8.6526(2) 9.1010(2) 101.3480(10) 90.2660(10) 117.0750(10) 580.20(2) 1 293 1.076 1.641 296 1612 1794 4853 0.024 1.04 0.0267 0.0690 0.38, -0.30

C20H22NiN2O10S2 573.23 monoclinic C2/c 13.6515(3) 11.2360(3) 15.9913(4) 90 108.613(2) 90 2324.58(10) 4 293 1.074 1.638 1184 1982 2682 14104 0.066 1.02 0.0397 0.0862 0.36, -0.47

C20H22CuN2O10S2 578.09 monoclinic C2/c 13.5231(4) 11.0694(3) 16.0659(5) 90 108.101(2) 90 2285.93(12) 4 293 1.199 1.680 1188 2437 3276 10942 0.039 1.07 0.0407 0.0963 0.46, -0.47

)

)

C20H26NiN2O12S2 empirical formula C20H30NiN2O14S2 formula weight 645.29 609.20 crystal system monoclinic triclinic P1 space group P21/n a (A˚) 7.8267(2) 7.6427(2) b (A˚) 11.3064(3) 11.3085(3) c (A˚) 15.6709(5) 14.3244 (4) R (deg) 90 78.570(2) β (deg) 98.114(2) 83.034 (2) γ (deg) 90 84.684 (2) 1372.86(7) 1201.51(6) V (A˚3) Z 2 2 T (K) 293 293 -1 0.929 1.050 μ (mm ) 3 1.561 1.665 Dcalcd (g/cm ) F (000) 672 618 reflections [I > 2σ(I)] 2344 2835 unique reflections 3274 3948 measured reflections 11828 12298 0.039 0.048 Rint 1.07 1.05 GOF on F2 a 0.0393 0.0524 R1[I > 2σ(I)] b 0.1213 0.1945 Rw[I > 2σ(I)] 0.41, -0.46 0.64, -0.55 ΔF max/min (e A˚-3) P P P P a R= Fo| - |Fc / |Fo|. b Rw= [ {w(Fo2 - Fc2)2}/ {w(Fo2)2}]1/2.

Table 2. Selected Bond Distances (A˚) and Angles (deg) for Compounds 1-5a

Ni1-O1 Ni1-N1 Ni1-O2#1 O1-Ni1-O2 O1-Ni1-O1#1 O1-Ni1-N1#1 compound 2 Ni1-O1 Ni1-O3 Ni1-N1 O1-Ni1-O2 O1-Ni1-O4 O1-Ni1-N2 compound 3 Ni1-O1 Ni1-N1 Ni1-O2#2 O1-Ni1-O2 O1-Ni1-O1#2 O1-Ni1-N1#2 compound 4 Ni1-O1 Ni1-N1 Ni1-O1#3 O1-Ni1-O2 O1-Ni1-N2 O1-Ni1-O2#3 compound 5 Cu1-O1 Cu1-N1 Cu1-O1#4 O1-Cu1-O2 O1-Cu1-N2 O1-Cu1-O2#4 a

2.070(2) 2.098(2) 2.071(2) 90.49(9) 180.00 89.47(7)

Ni1-O2 Ni1-O1#1 Ni1-N1#1 O1-Ni1-N1 O1-Ni1-O2#1

compound 1 2.071(2) O2-Ni1-N1 2.070(2) O2-Ni1-O2#1 2.098(2) O1#1-Ni1-N 90.54(7) N1-Ni1-N1#1 89.51(9) O1#1-Ni1-N1#1

86.91(7) 180.00 89.47(7) 180.00 90.54(7)

O1#1-Ni1-O2 O2-Ni1-N1#1 O2#1-Ni1-N1 O1#1-Ni1-O2#1 O2#1-Ni1-N1#1

2.064(5) 2.090(4) 2.102(5) 90.46(18) 178.27(16) 93.89(18)

Ni1-O2 Ni1-O4 Ni1-N2 O1-Ni1-O3 O1-Ni1-N1

2.068(6) 2.073(4) 2.106(5) 87.93(16) 88.11(18)

O2-Ni1-O3 O2-Ni1-N1 O3-Ni1-O4 O3-Ni1-N2 O4-Ni1-N2

178.12(17) 88.33(17) 92.08(16) 89.67(18) 87.85(16)

O2-Ni1-O4 O2-Ni1-N2 O3-Ni1-N1 O4-Ni1-N1 N1-Ni1-N2

2.0658(19) 2.0783(19) 2.1069(17) 91.82(8) 180.00 89.06(9)

Ni1-O2 Ni1-O1#2 Ni1-N1#2 O1-Ni1-N1 O1-Ni1-O2#2

2.1069(17) 2.0658(19) 2.0783(19) 90.94(9) 88.18(8)

O2-Ni1-N1 O2-Ni1-O2#2 O1#2-Ni1-N1 N1-Ni1-N1#2 O1#2-Ni1-N1#2

92.20(7) 180.00 89.06(9) 180.00 90.94(9)

O1#2-Ni1-O2 O2-Ni1-N1#2 O2#2-Ni1-N1 O1#2-Ni1-O2#2 O2#2-Ni1-N1#2

88.18(8) 87.80(7) 87.80(7) 91.82(8) 92.20(7)

2.112(2) 2.081(2) 2.112(2) 88.32(9) 90.42(6) 91.69(9)

Ni1-O2 Ni1-N2 Ni1-O2#3 O1-Ni1-N1 O1-Ni1-O1#3

2.066(2) 2.064(2) 2.066(2) 89.58(6) 179.16(9)

O2-Ni1-N1 O1#3-Ni1-O2 N1-Ni1-N2 O2#3-Ni1-N1 O2#3-Ni1-N2

90.31(5) 91.69(9) 179.98(2) 90.31(5) 89.69(5)

O2-Ni1-N2 O2-Ni1-O2#3 O1#3-Ni1-N1 O1#3-Ni1-N2 O1#3-Ni1-O2#3

89.69(5) 179.38(7) 89.58(6) 90.42(6) 88.32(9)

2.004(2) 2.000(2) 2.004(2) 92.70(9) 90.62(5) 87.28(9)

Cu1-O2 Cu1-N2 Cu1-O2#4 O1-Cu1-N1 O1-Cu1-O1#4

2.441(2) 2.003(2) 2.441(2) 89.38(5) 178.76(8)

O2-Cu1-N1 O1#4-Cu1-O2 N1-Cu1-N2 O2#4-Cu1-N1 O2#4-Cu1-N2

89.31(5) 87.28(9) 180.00(3) 89.31(5) 90.69(5)

O2-Cu1-N2 O2-Cu1-O2#4 O1#4-Cu1-N1 O1#4-Cu1-N2 O1#4-Cu1-O2#4

90.69(5) 178.63(8) 89.38(5) 90.62(5) 92.70(9)

89.51(9) 93.09(7) 93.09(7) 90.49(9) 86.91(7) 89.50(18) 91.4(2) 90.65(15) 90.16(16) 177.99(18)

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

O2a; a = 1 - x, 1 - y, 1 - z) from the water molecules sit at the equatorial positions and two nitrogen atoms (N1 and N1a; a = 1 - x, 1 - y, 1 - z) occupy the axial positions, forming a slightly distorted octahedral geometry around each Ni(II) center (Figure 1a). The degree of distortion from the ideal octahedron is reflected in the cisoid angles [86.91(7)-93.09(7)]. Ni-O bond

distances are 2.070(2) and 2.071(2) A˚, and the Ni-N bond distance is 2.098(2) A˚. One-dimensional linear chains of [Ni(bipy)(H2O)4]2þ stack along the crystallographic c-axis in -AB- fashion (Figure S1a of the Supporting Information). The counteranions 2,6-nds are intercalated between the 1D chains along the a-axis through π-π interactions with

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Figure 1. Crystal structure of 1: (a) view of the asymmetric unit. (b) View of the 2D sheet formed by 1D chains [Ni(bipy)(H2O)4]2þ and 2,6-nds anions by H-bonding (dashed lines) and π-π interactions along the c-direction. (c) Three-dimensional supramolecular network constructed by the 1D [Ni(bipy)(H2O)4]2þ chains with a 2,6-nds counteranions and guest water molecules through π-π and H-bonding interactions (dashed lines) viewed along the a-direction.

Figure 2. Crystal structure of 2: (a) view of the asymmetric unit. (b) View of the 2D sheet formed by 1D chains [Ni(bipy)(H2O)4]2þ and 2,6-nds anions by H-bonding (dashed lines) and π-π interactions along the c-direction. (c) Three-dimensional supramolecular network constructed by the 1D [Ni(bipy)(H2O)4]2þ chains with a 2,6-nds counteranions and guest water molecules through π-π and H-bonding (dashed lines) interactions viewed along the a-direction.

bipy (centroid 3 3 3 centroid distances are in the range of 4.05425.1725 A˚) and H-bonding interactions with the coordinated water molecules (—O1-H2 3 3 3 O3, 170(3); O1-H2 3 3 3 O3, 2.797(3) A˚; —O2-H3 3 3 3 O4, 175(3); O2-H3 3 3 3 O4, 2.782(3) A˚] forming a two-dimensional (2D) sheet in the ab plane (Figure 1b). Stacking of 2D sheets progressed in -AB- fashion along the crystallographic c-axis assisted by H-bonding interaction of the crystalline water molecules with coordinated water molecules and 2,6-nds [—O1-H1 3 3 3 O6, 174(3); O1-H1 3 3 3 O6, 2.657(3) A˚; —O6-H12 3 3 3 O3, 172(3); O6-H12 3 3 3 O3, 2.828(3)A˚] resulting in a 3D supramolecular structure (Figure 1c and Figure S2 of the Supporting Information). It is worth mentioning that alignment of the 2,6nds is in the opposite direction in two adjacent 2D sheets offset by 0.5(bþc) to avoid steric repulsion, resulting in small channels (Figure S3a of the Supporting Information) along the a-axis occupied by guest water molecules. The potential

solvent-accessible volume calculated using Platon30 suggests 14.9% void space to the total crystal volume; however, this increases to 20.5% after removal of the coordinated water molecules. The dimension of the triangular type channel is about 2.4  2.1 A˚2.33 The assembly of 2,6-nds and the crystalline water molecules, aided by hydrogen bonding, forms a supramolecular host, where 1D [Ni(bipy)(H2O)4]2þ chains are aligned as a guest (Figure 3a). This host-guest interaction is believed to be an important factor for stabilizing the compound at higher temperature. The intrachain Ni 3 3 3 Ni separation is 11.306 A˚, while the interchain Ni 3 3 3 Ni distance is 10.001 A˚. {[Ni(bipy)(H2O)4](2,6-nds) 3 2H2O)} (2). Compound 2 crystallizes in triclinic P1 space group. Structure determination reveals a 3D supramolecular network similar to 1, generated by 1D coordination chain of [Ni(bipy)(H2O)4]2þ, 2,6-nds counteranions, and two crystalline water molecules (Figure 2). Unlike 1, the Ni(II) center is located on a general

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position as are the atoms of the 2,6-nds. The four oxygen atoms (O1, O2, O3, and O4) from four different water molecules sit at the equatorial positions, and the axial positions are occupied by two N-atoms (N1 and N2) from two bipy, forming a slightly distorted octahedron around each Ni(II) with NiN2O4 chromophore (Figure 2a). Ni-O and Ni-N bond distances are in the range of 2.064(5)-2.090(4) and 2.102(5)-2.106(5) A˚, respectively. The degree of distortion from the ideal octahedral geometry is reflected in cisoid [87.85(16)-93.89(18)] and transoid angles [177.99(18)-178.12(17)]. Similar to 1, the 1D [Ni(bipy)(H2O)4]2þ chains extend along the crystallographic b-axis where 2,6-nds molecules are sandwiched between the chains through π-π (centroid-centroid distances are 3.755 and 4.798 A˚) and H-bonding interactions with coordinated water molecules [—O1-H20 3 3 3 O5, 173(8); O1-H20 3 3 3 O5, 2.800(6) A˚; —O1-H21 3 3 3 O10, 2.725(6); O1-H21 3 3 3 O10, 2.725(6) A˚] forming 2D sheet in the ab plane (Figure 2b). Unlike 1, here, 2,6-nds molecules are aligned in the same direction throughout the network (Figure S1e of the Supporting Information). To facilitate stronger H-bonding interaction, the 2D sheets stack irregularly in c-direction with the generation of a 3D supramolecular network (Figure 2c and Figure S4 of the Supporting Information) housing small 1D water-filled channels along the a-axis (Figure S3b of the Supporting Information). The approximate dimension of the rectangular type channel after removal of all of the water molecules is 3.2  2.7 A˚2.33 The potential solvent-accessible void volume, calculated using PLATON,30 suggests 7.1 and 11.2% void volume to the total crystal volume after removal of the guest as well as coordinated water molecules, respectively. Like 1, 2,6-nds and guest water molecules generate a supramolecular host with bigger cavity where two separate 1D [Ni(bipy)(H2O)4]2þ chains can be accommodated (Figure 3b). This is due to the different orientation and packing of 2,6-nds as compared to 1. The intrachain Ni 3 3 3 Ni separation in 2 is similar as 1, 11.309 A˚, while the interchain Ni 3 3 3 Ni distance is 9.224 A˚. [Ni(bipy)(H2O)4](2,6- nds)} (3). After isolating two different compounds, 1 at room temperature and 2 at high temperature, two more reactions were carried out at 120 and 100 C, maintaining the same stoichiometric ratio, which yielded 3 and 4. Compound 3 crystallizes in the same space group P1 as compound 2, but the arrangement of building units in the unit cell is entirely different. The Ni(II) center is located on an inversion center, while the 2,6-nds lies across an inversion center. The basic building unit and coordination environment of Ni(II) (Figure 4a) are same as for compounds 1 and 2; however, there is no water of crystallization present in the structure. Ni-O bond lengths are 2.0658(19) and 2.1069(17) A˚, and the Ni-N bond length is 2.0783(19) A˚. Unlike 1 and 2, in 3, 1D [Ni(bipy)(H2O)4]2þ chains run along an axis, which passes through the face diagonal of the bc plane (Figure S1g of the Supporting Information). These chains stack along a-direction in -AA- fashion at a unit distance of a-axis (8.4893 A˚) to make a 2D sheet (Figure 4b), where 2,6-nds is intercalated between the chains. The stacking is favored due to π-π interaction (centroid-centroid distances are in the range of 3.6707-4.8224 A˚) of the bipy aromatic rings with 2,6-nds, which are arranged in -AA- fashion along the a-direction. Utilization of the sulfonyl O-atoms of 2,6-nds in H-bonding [—O1-H2 3 3 3 O3, 174(4); O1-H2 3 3 3 O3, 2.680(3); —O1H1 3 3 3 O5, 165(3); O1-H1 3 3 3 O5, 2.756(2)] with coordinated water molecules of the nearest 2D sheets affords a 3D supramolecular network (Figure 4c). To facilitate stronger

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Figure 3. Perspective view of the supramolecular host-guest interaction assisted by H-bonding (green dashed lines) in 1 (a) and 2 (b).

H-bonding interaction, each 2D sheet slides to about 6.1 A˚ from its neighboring sheet, and the sliding is maintained throughout the network. The intrachain and interchain Ni 3 3 3 Ni separations within the 2D sheet are 11.256 and 8.489 A˚, respectively. [Ni(bipy)(H2O)4](2,6- nds)} (4). Compound 4 crystallizes in monoclinic C2/c space group, and the asymmetric unit is comprised of one Ni atom, one bipy, two water molecules, and one 2,6-nds counteranion (Figure 5a). The Ni(II) center is located on a 2-fold axis, while the 2,6-nds lies across an inversion center. Like 1 and 2, bipy linearly bridges the Ni(II) centers forming 1D [Ni(bipy) (H2O)4]2þ chains where each Ni(II) is bound to four water molecules (O1, O1a, O2, and O2a; a = 2 - x, y, 3/2 - z) at equatorial positions. Slight distortion in octahedral geometry is reflected in cisoid [88.32(9)-91.69(9)] and transoid angles [179.16(9)179.98(2)]. The Ni-O and Ni-N bond distances are in the range of 2.066(2)-2.112(2) and 2.064(2)-2.081(2) A˚, respectively. The 1D [Ni(bipy)(H2O)4]2þ chains stack in -AB- fashion forming a 2D sheet in the bc plane (Figure 5b). These 2D sheets pack along the crystallographic c axis where 2,6-nds molecules are intercalated through π-π interactions with bipy (centroid-centroid distances are in the range of 3.6252-4.8838 A˚) and H-bonding interaction with coordinated water molecules [—O2-H2B 3 3 3 O5, 176(3); O2-H2B 3 3 3 O5, 2.685(3) A˚; —O1-H1B 3 3 3 O3, 165(3); O1-H1B 3 3 3 O3, 2.788(3) A˚] forming a 3D supramolecular

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Figure 4. Crystal structure of 3: (a) view of the asymmetric unit. (b) View of the 2D sheet formed by 1D chains [Ni(bipy)(H2O)4]2þ and 2,6-nds anions by H-bonding (dashed lines) and π-π interactions. (c) Three-dimensional supramolecular network constructed by the 1D [Ni(bipy)(H2O)4]2þ chains with 2,6-nds counteranions and guest water molecules through π-π and H-bonding interactions (dashed lines) viewed along the a-direction.

Figure 5. Crystal structure of 4 and 5: (a) view of the asymmetric unit. View of the 2D sheet formed by 1D chains [M(bipy)(H2O)4]2þ (M = Ni, Cu) and 2,6-nds anions by H-bonding (dashed lines) and π-π interactions along the a-direction. (c) Three-dimensional supramolecular network constructed by the 1D [M(bipy)(H2O)4]2þ chains with 2,6-nds counteranions and guest water molecules through π-π and H-bonding interactions (dashed lines) viewed along the a-direction.

network (Figure 5c and Figure S5 of the Supporting Information). Unlike 3, although they have same molecular formula, 2,6-nds in 4 is more densely packed in -AB- fashion along the c-direction (Figure S1k of the Supporting Information), which consequences a nonporous dense structure. The intrachain and interchain Ni 3 3 3 Ni separations are 11.306 and 7.827 A˚, respectively. Compounds 3 and 4 represent polymorphism phenomena in supramolecular networks. [Cu(bipy)(H2O)4](2,6-nds)} (5). Compound 5 is isostructural with compound 4, and only important structural parameters will be described. As in 4, the copper(II) center is located on a 2-fold axis, while the 2,6-nds lies across an inversion center. In the coordination chain of [Cu(bipy)(H2O)4]2þ, each Cu(II) is in 4 þ 2 coordination environment

with two water molecules in the axial positions (Figure 5a). The Cu-O axial distance is 2.441 A˚, while the equatorial Cu-O distance is 2.004 A˚. The Cu-N bond distances are 2.000(2) and 2.003(2) A˚. Slight distortion in octahedral geometry in 5 is reflected in cisoid [89.31(5)-90.69(5)] and transoid angles [178.63(8)-178.76(8)]. The construction of a 3D supramolecular network (Figure 5c and Figure S5 of the Supporting Information) is assisted by a π-π interaction between 2,6-nds and bipy (centroid-centroid distances are in the range of 3.6731-4.8224 A˚) and H-bonding interactions between 2,6-nds and coordinated water molecules [—O1-H2 3 3 3 O5, 174(2); O1-H2 3 3 3 O5, 2.661(3) A˚; —O2-H4 3 3 3 O4, 172(3); O2-H4 3 3 3 O4, 2.822(3) A˚]. The intrachain Cu 3 3 3 Cu separation is 11.069 A˚, slightly lower

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Figure 6. Simplistic view of the arrangement of 1D [Ni(bipy)(H2O)4]2þ chains (pink color) and 2,6-nds counteranions (green color) in supramolecular isomers 1-4: (a) compound 1, (b) compound 2, (c) compound 3, and (d) compound 4 viewed along the crystallographic a-axis.

than the Ni(II) compounds, while the interchain Cu 3 3 3 Cu distance is 8.583 A˚. Supramolecular Isomerism. Five MOCPs 1-5 of transition metal(II) (Ni and Cu) with bipy and 2,6-nds were successfully synthesized and structurally characterized. Crystal building essentially takes place because of a supramolecular interaction34 between [M(bipy)(H2O)4]2þ 1D chains, 2,6-nds, and water molecules along with directional covalent bonds. In all cases, M(II) and bipy bridging along one direction favors the occurrence of extended 1D polymers. To neutralize these positively charged coordination chains, 2,6-nds acts as counteranions in the assembly. In all cases, hydrogenbonding and π-π interactions of 2,6-nds with water molecules and bipy result in 3D supramolecular architectures. Compounds 1-4 were synthesized, maintaining the same stoichiometric ratio of metal and ligand under different reaction temperatures, and the different architectures of the compounds indicate that the temperature plays a significant role in the construction of the MOCPs. The coordination polymers 1-4 are interesting from the crystal engineering point of view since they are supramolecular isomers with the basic structural building unit being [Ni(bipy)(H2O)4]2þ and 2,6-nds (Figure 6). They differ only with the number of crystalline water molecules; 1 and 2 have four and two crystalline water molecules, respectively, whereas 3 and 4 have no crystalline water molecules. It is worth noting that 3 and 4 have the same molecular formula, {[Ni(bipy)(H2O)4](2,6-nds)}, which would lie at the heart of the concept of supramolecular isomerism. It is important to note that, structurally, the 1D coordination chain of [Ni(bipy)(H2O)4]2þ and 2,6-nds is common for all of the MOCPs. It is the effect of temperature that induces a different supramolecular interaction and hence dictates the structure. The isomerism in 1-4 ensues because of the different crystal packing of 1D [M(bipy)(H2O)4]2þ chains and 2,6-nds (Figure 6), and they are categorized as structural supramolecular isomers. Compound 5 is isomorphous with 4, as the metal being replaced by Cu(II). PXRD, Thermogravimetric (TG), and Spectral Chracterizations. The PXRD patterns of compounds 1-5 are shown in Figures S6-S10 of the Supporting Information. The

Figure 7. TGA curve of compounds 1 (black), 2 (blue), 3 (red), 4 (pink), and 5 (wine) in the temperature range 25-650 C (heating rate, 5 C/min under nitrogen/oxygen).

positions of the diffraction peaks in simulated and assynthesized patterns correspond well, which indicate that the as-synthesized samples 1-5 are pure. TGA of compounds 1-5 were performed in the temperature range 25-650 C under nitrogen/oxygen atmosphere (Figure 7). Compound 1 shows release of the four crystalline water molecules in the temperature range of 40-90 C. In the next two overlapping steps (temperature range, 90-185 C), four coordinated water molecules are released. Compound 2 also shows two overlapping steps (first step, 65-150 C; and second step, 150-260 C) to release two guest and four coordinated water molecules. The dehydrated compounds of 1 (10 ) and 2 (20 ) remain stable up to 385 C without any further weight loss, suggesting high thermal stability of the compounds. Compounds 3-5 show stepwise loss of four coordinated water molecules. The first step (130-180 C for 3, 100-170 C for 4, and 80-110 C for 5) corroborates three coordinated water molecules, and the second step (200-290 C for 3, 200-270 C for 4, and 150-190 C for 5) suggests removal of the fourth coordinated water molecule. The dehydrated compounds of 3 (30 ), 4 (40 ), and 5 (50 ) are stable up to ∼400, 385, and 330 C, respectively (hereafter, all of the dehydrated compounds of 1-5 will be designated as 10 -50 ).

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Figure 8. N2 sorption isotherms of 10 (a) and 20 (b) at 77 K (P0 is the saturated vapor pressure of N2).

Stepwise release of water molecules can be correlated by the different H-bonding strengths in the compounds. The IR spectrum of the compounds displays characteristic absorption peaks of water molecules, bipy, and 2,6-nds ligands. A broad band in the region of 3100-3600 cm-1 in all compounds corresponds to the ν(O-H) mode of water molecules. Characteristic symmetric νs(OSO) and antisymmetric νas(OSO) modes for 2,6-nds were observed in the region of 1028-1038 and 1140-1250 cm-1. The bands in the range 3047-3190 and 1410-1660 cm-1 are related to ν(C-H) and ν(CdC) or ν(CdN) of bipy and 2,6-nds ligands, respectively. Porous Properties. Interestingly, different network superstructures induce different functionality as revealed in adsorption experiments with 10 and 20 . N2 and CO2 sorption experiments were carried out at 77 and 195 K with 10 and 20 to study their porous properties. In both of the cases, N2 (kinetic diameter, 3.6 A˚)35 sorption isotherms show a type II profile without saturation, suggesting only surface adsorption (Figure 8a,b). Noninclusion of N2 in both 10 and 20 can be realized by the smaller pore size as compared to the kinetic diameter of N2. This may be due to the structural transformation of the supramolecular networks upon removal of all of the water molecules, which reduces the pore size and hence does not allow N2 molecules to easily pass through at low P/P0. CO2 (3.4 A˚) sorption studies of compounds 10 and 20 reveals gradual uptake of CO2 with increasing pressure (Figure 9a,b). The inclusion of CO2 at a low pressure region in both cases suggests the microporous nature of 10 and 20 . Lin and Shimizu et al. have previously reported a similar type of CO2 adsorption profile in supramolecular frameworks.36 The saturated amounts of CO2 (33.5 mL/g, ∼0.75 mol/mol) for 10 and (26.1 mL/g, ∼0.55 mol/mol) for 20 at P/P0 = 1 are comparable. The CO2 isotherms were analyzed by the DR equation37 and suggest 179 and 139 m2/g surface areas for 10 and 20 . The lower surface area of 20 as compared to 10 is correlated by the smaller void space and the structural contraction after removal of the water molecules in the case of 20 , which is opposite in the case of 10 . In contrast to N2 adsorption in 10 and 20 , CO2 sorption revealed large hysteresis, which is believed to occur via dipole-induced dipole interactions where the quadruple moment of CO2 interacts with coordinatively unsaturated Ni(II) sites. The high value of the heat of adsorption as calculated by DR eq (26.45 and 26.59 kJ/mol for 10 and 20 , respectively) also suggests strong interaction of CO2 molecules with unsaturated Ni(II) sites.

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Figure 9. CO2 sorption isotherms of 10 (a) and 20 (b) at 195 K (P0 is the saturated vapor pressure of CO2).

Figure 10. Comparison of PXRD patterns of asynthesized, dehydrated, and rehydrated samples of 2-4. The PXRD pattern of dehydrated samples suggests structural transformation with retention of crystallinity. Upon rehydration, all of the compounds transform to 2.

Transformation of the MOCPs. All of the MOCPs exhibit structural transformation after removal of the guest and coordinated water molecules as evident from the PXRD patterns (Figures S6-S10 of the Supporting Information). It is presumed that the stability of the dehydrated compounds results from the coordinative interaction of the sulfonyl oxygens of 2,6-nds with unsaturated metal centers and π-π interaction of naphthalene ring with bipy that allows the structures to relax in a stable configuration upon removal of water molecules. This was further confirmed by an IR spectrum of compound 2 heated at 200 C (Figure S11 of the Supporting Information). The IR spectrum shows a decrease in the S-O stretching frequency from 1248 to 1229 cm-1, which suggests weakening of the S-O bond. This feature is indicative of some coordinative interaction of 2,6-nds with unsaturated Ni(II) in the dehydrated compound. The dehydrated compounds, 30 and 40 , exhibit similar PXRD patterns, suggesting similar network topologies (Figure 10). When the dehydrated solids are exposed to water vapor, the water molecules get coordinated with M(II) and again reach octahedral coordination, which is revealed by the similarity in the PXRD patterns of rehydrated and assynthesized samples of 1, 2, and 5 (Figures S6, S7, and S10 of the Supporting Information). This suggests structural reversibility with respect to the dehydration-rehydration process.

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Scheme 1. Schematic of the Dehydration-Rehydration Processes in Compounds 1-5a

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Supporting Information Available: X-ray crystallographic data, PXRD patterns, and molecular figures. This material is available free of charge via the Internet at http://pubs.acs.org.

References

a

Compounds 1, 2, and 5 are reversible with respect to dehydration-rehydration, while dehydrated 3 and 4 transform to 2.

It is worth mentioning that water-exposed PXRD patterns of 30 and 40 are the same and are not similar to as-synthesized compounds 3 and 4. Careful observation of the patterns indicates that they are similar to as-synthesized compound 2, suggesting structural transformation of 30 and 40 to 2 upon water exposure (Figure 10 and Scheme 1). This transformation may be rationalized based upon the increased H-bonding interaction in 2 without any loss of significant π-π interaction (minimum ring centroid distances in 2, 3, and 4 are 3.755, 3.6707, and 3.6252 A˚, respectively) as compared to 3 and 4 with the introduction of two crystalline water molecules. However, they do not transform to 1 because the strength of the π-π interaction is less in 1 as compared to 3 and 4 (minimum ring centroid distances in 1, 3, and 4 are 4.0542, 3.6707, and 3.6252 A˚, respectively). Conclusion In conclusion, we have successfully synthesized and structurally characterized five supramolecular MOCPs employing the same building blocks, bipy and 2,6-nds, with metal ions Ni(II) (1-4) and Cu(II) (5). Structural analyses of 1-5 exhibit that a metal bipyridine 1D chain, [M(bipy)(H2O)4]2þ, is common for all of the compounds, which serve as a primary building block for all of the MOCPs. The basic structural unit [M(bipy)(H2O)4]2þ and 2,6-nds counteranions assemble through strong hydrogen-bonding and π-π interactions, resulting in 3D supramolecular architectures in all cases. Compounds 1-4 are related as supramolecular isomers. The isomerism ensues due to the different orientations of 1D metal-bipyridine chains and 2,6-nds in the MOCPs and can be categorized as structural supramolecular isomer. Among them, supramolecular isomerism in 3 and 4 is interesting since they have same molecular formula but different crystal packing. Moreover, compounds 3 and 4 are polymorphs. The presence of structural supramolecular isomerism through the different orientations of [M(bipy)(H2O)4]2þ and 2,6-nds induces different functionality as revealed by the sorption properties with the dehydrated samples of 1 and 2. Acknowledgment. T.K.M. gratefully acknowledges the financial support from DST, Govt. of India (Fast Track Proposal). P.K. is grateful to CSIR, India, for a JRF fellowship.

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