Structural and Selective Gas Adsorption Studies of Polyoxometalate

Nov 23, 2010 - (Cl)](DMF)4(H2O) (Co-Mo8-DMF), [R-PW12O40][Co(en)3]36DMF ... DEF), [R-PMo12O40][Co(en)3]35.5DMF (Co-Mo12P-DMF), ...
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DOI: 10.1021/cg101201b

Structural and Selective Gas Adsorption Studies of Polyoxometalate and Tris(ethylenediamine) Cobalt(III) Based Ionic Crystals

2011, Vol. 11 139–146

Chandan Dey, Raja Das, Pradip Pachfule, Pankaj Poddar, and Rahul Banerjee* Physical/Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhaba Road, Pune 411008, India Received August 4, 2010; Revised Manuscript Received November 7, 2010

ABSTRACT: Seven new ionic solids built on polyoxometalate anions and [Co(en)3]3þ cations, namely, [Co(en)3Mo8O26(H3O)(Cl)](DMF)4(H2O) (Co-Mo8-DMF), [R-PW12O40][Co(en)3] 3 6DMF (Co-W12P-DMF), [R-PW12O40][Co(en)3] 3 6DEF (Co-W12PDEF), [R-PMo12O40][Co(en)3] 3 5.5DMF (Co-Mo12P-DMF), [R-PMo12O40][Co(en)3] 3 6DEF (Co-Mo12P-DEF), [R-SiW12O40][Co(en)3]3/2[Cl]1/2 3 6DMF 3 3H2O (Co-W12Si-DMF), and [R-SiW12O40][Co(en)3] 3 6DEF (Co-W12Si-DEF), have been synthesized from nonaqueous (DMF/DEF) media and characterized by single-crystal X-ray diffraction. We attempt to understand if [Co(en)3]3þ cations used in these reaction systems play a crucial role in controlling the assembly of these crystals. These ionic solids, after removal of the DMF or DEF molecules, are found to exhibit size selective H2 adsorption properties over N2. The amount of hydrogen adsorption was influenced by POM anion types and their orientations. CoMo12P-DEF has the highest (0.9 wt %) H2 uptake, and CoW12P-DEF has the lowest (0.4 wt %) uptake among the series when the adsorbate pressure approached 1 atm.

Introduction Polyoxometalate (POM) clusters constitute a wide variety of structures formed via condensation reactions of metal oxide anions of transition metals (e.g., V, Mo, W, etc.) in high oxidation states.1 POMs are used as the building components of supramolecular crystalline solids and provide a multitude of possible applications in catalysis,2 electrochemistry,3 photochemistry,4 magnetochemistry,5 and medicine.6 One of the most interesting aspects of POM chemistry lies with the fact that POM clusters in the crystal lattice can be viewed as supramolecular synthons.7 As a result, it is possible to engineer their assembly inside the polyoxometalate-based ionic crystals to create materials with desired properties.8 However, despite increasingly active research, understanding of the self-assembly processes which govern the resulting polyoxometalate-based ionic crystal structure formation remains limited. This article highlights our recent results and provides a perspective of the use of polyoxometalates: (i) in the rational syntheses of ionic crystals with porous tris(ethylenediamine)cobalt(III) cation, [Co(en)3]3þ, under mild conditions; and (ii) on the self-assembly of the discrete anionic components and on the sorption properties of the resulting supramolecular ionic crystals. Recently, researchers found that, [Co(en)3]Cl3, one of the most fundamental compounds of coordination chemistry,9 can transform its ionic crystal structure to adopt adsorbed guests of various sizes and shapes by weak physisorption.10 Thus, an ionic crystal, unlike rigid metal organic frameworks (MOFs),11 can be considered to possess extremely unrestricted transformability upon absorbing different gases. Both POM based ionic crystals and [Co(en)3]Cl3 are well-known for their absorption of organic solvents and H2O, but POM clusters are greatly underutilized in terms of their capabilities of storing hydrogen or carbon dioxide.12 This made us interested in the rational synthesis of a series of polyoxometalate-based ionic crystals with [Co(en)3]3þ, as this cation had already exhibited absorption

properties.10 These ionic solids, formulated as [Co(en)3Mo8O26(H3O)(Cl)](DMF)4(H2O) (Co-Mo8-DMF), [R-PW12O40][Co(en)3] 3 6DMF (Co-W12P-DMF), [R-PW12O40][Co(en)3] 3 6DEF (Co-W12P-DEF), [R-PMo12O40][Co(en)3] 3 5.5DMF (Co-Mo12P-DMF), [R-PMo12O40][Co(en)3] 3 6DEF (Co-Mo12P-DEF), [R-SiW12O40][Co(en)3]3/2[Cl]1/2 3 6DMF 3 3H2O (CoW12Si-DMF), [R-SiW12O40][Co(en)3] 3 6DEF (Co-W12Si-DEF) (en = ethylenediamine, DMF = dimethyl formamide, DEF = diethyl formamide) (Figure 1), display selective hydrogen (H2) and carbon dioxide (CO2) sorption over nitrogen (N2). These ionic solids have been determined by single crystal X-ray diffraction and further characterized by IR spectra, PXRD, TGA, and EA. Experimental Section

*E-mail: [email protected]. Fax: þ91-20-25902636. Telephone: þ 9120-25902535.

Synthesis of [Co(en)3Mo8O26(H3O)(Cl)](DMF)4(H2O) (Co-Mo8DMF). Sodium molybdate dihydrate (5.8 g of Na2MoO4 3 2H2O) was taken in a 100 mL beaker with 20 mL of DMF. The mixture was heated at 200 °C for 30 min. After 3 min of cooling, 3.3 mL of 12 M concentrated HCl was added to that solution. The filtrate was collected as a β-octamolybdate solution. To a 0.5 mL solution of β-octamolybdate was added 2 mL of H2O to make the solution dilute. A 0.025 M solution (0.5 mL) of tris(ethylenediamine)cobalt(III)chloride in water was added to that final solution. After two days, golden-yellow crystals formed. FT-IR (KBr 4000-450 cm-1): 3267 (s, νasymNH2), 1646 (s, νstrCdO), 933 (s, νasymModO), 894 (s, νasymMo-Oa), 841 (s, νasymMo-Ob), 704 (s, νasym Mo-Oc). Elem Anal. Calcd: C, 12.18; H, 3.18; N, 7.83. Found: C, 12.09; H, 3.21; N, 7.85. [DMF/H2O = 1:5; Amount of water ∼85%]. Synthesis of [r-PW12O40][Co(en)3] 3 6DMF (Co-W12P-DMF). To 2 mL of a 0.05 M solution of sodium phosphotungstate hydrate (Na3PW12O40 3 xH2O) in DMF was added 9 mL of DMF and then 2.5 mL of a 0.05 M solution of tris(ethylenediamine)cobalt(III)chloride in water with gentle shaking. After 24 h, golden-yellow crystals started forming. FT-IR (KBr 4000-450 cm-1): 3240 (s, νasymNH2), 1650 (s, νstrCdO), 1055 (s, νasymP-O), 952 (s, νasymWdO), 884 (s, νasymW-OcW), 814 (s, νasymW-Oe-W). Elem anal. Calcd: C, 8.10; H, 1.85; N, 4.72. Found: C, 8.12; H, 1.85; N, 4.75. [DMF/H2O = 4:1; Amount of water ∼20%]. Synthesis of [r-PW12O40][Co(en)3] 3 6DEF (Co-W12P-DEF). Two milliliters of a 0.05 M solution of Na3PW12O40 3 xH2O in DEF was taken in a 30 mL culture tube. To that solution was added dropwise

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Figure 1. Synthesis and chemical composition of different [Co(en)3]3þ-POM based ionic solids reported in this paper. 1.5 mL of 0.05 M Co(en)3Cl3 in water. Within 10 min, golden-yellow crystals started forming. FT-IR (KBr 4000-450 cm-1): 3263 (s, νasymNH2 ), 1643 (s, νstrCdO), 1079 (s, νasym P-O), 977 (s, νasymWdO), 896 (s, νasymW-Oc-W), 820 (s, νasymW-Oe-W). Elem Anal. Calcd: C, 10.34; H, 0.66; N, 4.67. Found: C, 10.35; H, 0.69; N, 4.66. [DEF/H2O = 4:3; Amount of water ∼42%]. Synthesis of [r-PMo12O40][Co(en)3] 3 5.5DMF (Co-Mo12P-DMF). To a solution of 2 mL of a 20 wt % solution of H3PMo12O40 in ethanol was added dropwise 7 mL of DMF. Then 0.5 mL of 0.05 M Co(en)3Cl3 in H2O was added dropwise. Green crystals form within 48 h. FT-IR (KBr 4000-450 cm-1): 3250 (s, νasymNH2), 1654 (s, νstrCdO), 1061 (s, νasymP-O), 957 (s, νasymModO), 878 (s, νasymMo-Oc-Mo), 820 (s, νasymMo-Oe-Mo). Elem Anal. Calcd: C, 8.03; H, 1.66; N, 4.78. Found: C, 7.99; H, 1.70; N, 4.81. [DMF/EtOH/H2O = 14:4:1; Amount of water ∼6%]. Synthesis of [r-PMo12O40][Co(en)3] 3 6DEF (Co-Mo12P-DEF). To 2 mL of a 20 wt % solution of H3PMo12O40 in ethanol was added 1 mL of DEF. Then 0.5 mL of 0.05 M Co(en)3Cl3 in H2O was added dropwise. Green crystals formed within 48 h. FT-IR (KBr 4000-450 cm-1): 3257 (s, νasymNH2), 1643 (s, νstrCdO), 1061 (s, νasymP-O), 956 (s, νasymModO), 879 (s, νasymMo-Oc-Mo), 804 (s, νasymMo-Oe-Mo). Elem Anal. Calcd: C, 16.19; H, 3.37; N, 7.29. Found: C, 16.22; H, 3.38; N, 7.23. [DEF/EtOH/H2O = 2:4:1; Amount of water ∼14%].

Synthesis of [r-SiW12O40][Co(en)3]3/2[Cl]1/2 3 6DMF 3 3H2O (CoW12Si-DMF). To 2 mL of a 0.05 M solution of H4SiW12O40 3 xH2O in DMF was added dropwise 2 mL of a 0.05 M solution of Co(en)3Cl3 in water. Yellow crystals start growing after 7 days. FT-IR (KBr 4000450 cm-1): 3252 (s, νasymNH2), 1657 (s, νstrCdO), 970 (s, νasymWdO), 921 (s, νasymSi-O), 882 (m, νasymW-Oc-W), 800(s, νasymW-Oe-W). Elem Anal. Calcd: C, 8.72; H, 2.10; N, 5.65. Found: C, 8.75; H, 2.05; N, 5.61. [DMF/H2O = 2:2; Amount of water ∼50%]. Synthesis of [r-SiW12O40][Co(en)3] 3 6DEF (Co-W12Si-DEF). To a solution of 2 mL of 0.05 M H4SiW12O40 3 xH2O in DEF was added dropwise 3 mL of 0.05 M Co(en)3Cl3 in water. Yellow crystals form within 48 h. FT-IR (KBr 4000-450 cm-1): 3260 (s, νasymNH2), 1644 (s, νstrCdO), 969 (s, νasymW = O), 921 (s, νasymSi-O), 882 (m, νasymWOc-W), 795(s, νasymW-Oe-W). Elem Anal. Calcd: C, 10.59; H, 2.08; N, 4.79. Found: C, 10.61; H, 2.11; N, 4.80. [DEF/H2O = 2:3; Amount of water ∼60%]. Single-Crystal X-ray Analysis. All single crystal data were collected on a Bruker SMART APEX three circle diffractometer equipped with a CCD area detector (Bruker Systems Inc., 1999a)13 with Mo KR radiation (λ = 0.71073 A˚). Data were integrated using Bruker SAINT software.14 Data were subsequently corrected for absorption by the program SADABS.15 The space group determinations and tests for merohedral twinning were carried out using XPREP.16 All structures

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Figure 2. Crystal structure of Co-Mo8-DMF. (a) Hydrogen-bonding interactions between β-[Mo8O26]4- and [Co(en)3]3þ. N-H 3 3 3 O hydrogen bonded interactions have been shown as red broken lines. (b) The two-dimensional structure of Co-Mo8-DMF consists of β-[Mo8O26]4- and [Co(en)3]3þ clusters. (c) 2D supramolecular layers formed by β-[Mo8O26]4- and [Co(en)3]3þ clusters stack on top of each other in ABAB fashion. (d) Space filling model of these 2D supramolecular layers. DMF, H2O, and Cl- molecules occupy the void space between the layers. Color code: Mo (green), N (blue), O (red), C (black), Co (magenta), H (light cyan), Cl (light green). were solved by direct methods and refined using the SHELXTL 97 software suite.17 The absorption coefficient ( μ) ranged between 1 and 11 for all ionic crystals reported in this paper. Since all the ionic crystals contain Mo/W, there could be a possibility of high absorption of Mo KR X-ray radiation. Data were collected at 293(2) K for all Co(en)3-POM ionic crystals except CoW12P-DMF where data collection took place at 100(2) K. All structures were examined using the Adsym subroutine of PLATON.18 Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the CCDC as Deposition Nos. CCDC 777221, 777222, and 777224-777228. Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 lEZ, U.K. (fax: þ 44 (1223) 336 033; e-mail: [email protected]. ac.uk). Gas Adsorption Experiments. To remove the guest species from the frameworks and prepare the evacuated forms of Co(en)3-POM ionic crystals for gas-sorption analysis, the as-synthesized Co(en)3POM samples were immersed in dry CH2Cl2 at ambient temperature for 48 h and evacuated at ambient temperature for 5 h and then at an elevated temperature (100 °C) for 2 h. Co(en)3-POM samples thus obtained were optimally evacuated, as evidenced by the long plateau (25-300 °C) in their TGA traces.

Results and Discussion Synthesis. In a specific crystallization process, many factors can affect the formation and crystal growth of products. These are the starting concentrations, pH values, temperature, and nature of solvents. Selection of the POM anions was an important factor for the complexation of the polyoxometalates with the [Co(en)3]3þ. Among a series of POM anions, we found that β-octamolybdate (β-Mo8O264-) and R-Keggin (R-PMo12O403-, R-PW12O403-, R-SiW12O404-) yielded ionic complexes with

good crystallinity. We anticipated the formation of 1:1 ionic complex, as the -3/-4 charged POM anions would be neutralized by the equimolar amount of the 3þ charge of [Co(en)3]3þ. Two important aspects that we considered during the synthesis are as follows: (i) The pH value of the reaction system should be carefully maintained so as to stabilize the structure and avoid early precipitation of microcrystalline powder. All these ionic crystals were obtained at a pH of ∼7. (ii) DMF and DEF should be used as solvent for crystallization.19 The complexation of [Co(en)3]3þ with POM anions was performed by mixing of aqueous solutions of Co(en)3Cl3 to a DMF/DEF solution of POM anion, which resulted in the crystallization of block/needle shaped crystals. It has been observed that in DEF media crystallization is faster (10-48 h) than in DMF media (24-160 h). Crystal Structure of Co(en)3-POM Based Ionic Crystals. Co-Mo8-DMF, first among the seven Co(en)3-POM based ionic crystals, crystallizes in the P1 space group and consists of two β-[Mo8O26]4- anionic clusters,20 cationic [Co(en)3]3þ complexes,9 Cl- anions, H3Oþ, free water, and DMF molecules in the crystal lattice (Figure 2a). The most interesting structural feature of Co-Mo8-DMF is the 2D supramolecular H-bonded network comprising alternating [Co(en)3]3þ and β-[Mo8O26]4clusters along the ac plane (Figure 2b). [Co(en)3]3þ and β-[Mo8O 26]4- clusters are connected to each other via strong N-H 3 3 3 O [D, 3.080(2) A˚; d, 2.249(3) A˚; θ, 149.7°] hydrogen bonds.21 These 2D supramolecular layers stack on top of each other in the AAAA fashion, with the shortest distance between two layers being 5.432 (2) A˚ (Figure 2c). Cl- anions, H3Oþ, free

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Table 1. Crystal Data and Structure Refinement for All POM Based Ionic Crystals Reported in This Paper CoMo8-DMF empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions

volume Z density (calculated) goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

CoMo12P-DEF

CoMo12P-DMF

C18H57ClCoMo8N10O32 1787.64 100(2) K 0.71073 A˚ triclinic P1 a = 10.198(4) A˚

C36H90CoMo12N12O46P 2668.38 293(2) K 0.71073 A˚ orthorhombic P212121

C45H112Co2Mo24N23O91P2 6725.4(14) 293(2) K 0.71073 A˚ triclinic P1 a = 12.100(2) A˚

C31H24CoN12O46PW12 3596.72 293(2) K 0.71073 A˚ orthorhombic P212121

C24H66CoN12O46PW12 3554.99 293(2) K 0.71073 A˚ monoclinic P21/c

C98H232Co4N38O139Si3W36 11105.44 293(2) K 0.71073 A˚ monoclinic P21/n

b = 12.330(5) A˚ c = 19.818(8) A˚ R = 81.068(6)° β = 86.281(6)° γ = 84.809(7)° 2448.3(17) 2 2.425

a = 14.395(3) A˚ b = 19.836(4) A˚ c = 28.009(5) A˚

a = 14.538(3) A˚ b = 19.929(4) A˚ c = 28.151(6) A˚

a = 15.550(3) A˚ b = 19.059(3) A˚ c = 23.026(4) A˚ β = 97.121(3)°

a = 14.0765(11) A˚ a = 13.1948(11) A˚ b = 41.183(3) A˚ b = 20.9672(18) A˚ c = 22.7696(18) A˚ c = 29.020(3) A˚ β = 93.7130(10)° β = 97.373(2)°

7998(3) 4 7998(3)

b = 23.4858(13)A˚ c = 24.259(3) A˚ R = 83.935(3)° β = 79.012(3)° γ = 86.859(3)° 6725.4(14) 2 2.441

8156(3) 4 2.929

6772(2) 4 3.487

13172.3(18) 2 2.800

7905.6(12) 2 3.120

1.069

0.944

1.127

1.021

1.185

1.185

1.027

R1 = 0.0335, wR1 = 0.0944 R2 = 0.0413, wR2 = 0.1231

R1 = 0.0604, wR1 = 0.1041 R2 = 0.0944, wR2 = 0.1125

R1 = 0.0704, wR1 = 0.1696 R2 = 0.0790, wR2 = 0.1746

R1 = 0.0907, wR1 = 0.1571 R2 = 0.1062, wR2 = 0.1701

R1 = 0.0984, wR1 = 0.2309 R2 = 0.1127, wR2 = 0.2423

R1 = 0.0804, wR1 = 0.1893 R2 = 0.0964, wR2 = 0.1987

R1 = 0.0782, wR1 = 0.1546 R2 = 0.0979, wR2 = 0.1779

water, and DMF molecules reside in the interlayer region (Figure 2d). Co-Mo12P-DEF, Co-W12P-DEF, and Co-W12Si-DEF consist of [PMo12O40]3-, [PW12O40]3-, and [SiW12O40]4- R-Keggin clusters,22 [Co(en)3]3þ complexes,9 and free DEF molecules in the crystal lattice. Co-Mo12P-DEF and Co-W12P-DEF are isostructural (Table 1), and in the crystal lattice [PM12O40]3- (M= Mo/W) and [Co(en)3]3þ clusters arrange in a helical fashion along the c axis. These parallel helixes form 2D corrugated layered structures comprising alternating [Co(en)3]3þ and [PM12O40]3- R-Keggin clusters along the bc plane (Figure 3a). DEF molecules fill the void space between the [Co(en)3]3þ and [PM12O40]3- clusters. We were surprised to see this type of isostructurality, considering the number of solvent molecules (24 DEF) present in the unit cell. Co-W12Si-DEF adopts a 2D sheet structure along the crystallographic ab plane comprising alternating [SiW12O40]4- and [Co(en)3]3þ clusters (Figure 3b). These 2D supramolecular layers stack on top of each other in ABAB fashion, with the shortest distance between two layers being 2.754(2) A˚ (Figure 3c). Co-W12P-DMF, Co-W12Si-DMF, and Co-Mo12P-DMF, apart from common [Co(en)3]3þ, contain [SiW12O40]4-, [PMo12O40]3-, and [PW12O40]3- type of R-Keggin anionic clusters, respectively. Co-W12Si-DMF contains Cl- anions and free water molecules in the crystal lattice and adopts a 2D network comprising alternating [Co(en)3]3þ and [SiW12O40]4- clusters and Cl- anions along the diagonal of the bc plane. This network could be described as an assembly of six [SiW12O40]4- clusters that encompasses four [Co(en)3]3þ and two Cl- anions (Figure 3d). Co-Mo12P-DMF adopts a 2D sheet structure along the diagonal of the crystallographic bc plane comprising alternating [PMo12O40]4- and [Co(en)3]3þ clusters (Figure 3e). These 2D supramolecular layers stack on top of each other in the ABAB fashion, with the shortest distance between two layers being 3.668 (2) A˚. DMF molecules fill the void space between the layers. Co-W12P-DMF, on the other hand, adopts a 2D layer structure along the crystallographic bc plane comprising alternating [PW12O40]4- clusters and [Co(en)3]3þ complexes and stacks along the crystallographic a axis (Figure 3f).

CoW12P-DEF

CoW12P-DMF

CoW12Si-DEF

CoW12Si-DMF C54H156ClCo3N30O95Si2W24 7426.67 293(2) K 0.71073 A˚ triclinic P1

As mentioned previously, it is challenging to systematically design and synthesize supramolecular ionic crystals based on POMs, because many factors, such as initial reactants, concentrations, solvent of crystallization, pH value, reaction time, and temperature, could influence the formation of the resulting structure. The results here show that the nature of the solvent (DMF/DEF) influences the structures of these ionic crystals as it guides the orientation of the [Co(en)3]3þ and POM anions in the resulting crystal lattice. We have also noticed that, among all the ionic crystals reported in the paper, Co-Mo12PDEF and Co-W12P-DEF crystallize in the P212121 space group although racemic [dl-Co(en)3]3þ has been used in synthesis.23 All Co(en)3-POM ionic crystals adopt a 2D sheet structure comprising alternating anionic POM clusters and cationic [Co(en)3]3þ complexes. Interestingly, in DEF media, these layers stack on top of each other in the AAAA fashion whereas in DMF media they stack in ABAB fashion. Thermal Properties and X-ray Powder Diffraction Analysis. In order to confirm the phase purity of the bulk materials, powder X-ray diffraction (PXRD) experiments were carried out on those ionic complexes. Experimental and computer-simulated PXRD patterns of Co(en)3-POM ionic crystal are shown in the Supporting Information (see Section S1, for all data regarding experimental and simulated PXRD of Co(en)3-POM ionic crystals).24 Thermal gravimetric analysis (TGA) was performed on as-synthesized Co(en)3-POM ionic crystals (Figure 4a) in order to understand the behavior of the solvent with respect to heating (see Section S3 in the Supporting Information). The TGA traces of compound CoMo12P-DMF demonstrate a flat plateau until 150 °C followed by a loss of 18% weight (from 150 to 225 °C) due to removal of six DMF molecules and one H2O molecule from the lattice. For CoMo12P-DEF, CoW12P-DMF, and CoW12P-DEF, initial loss of 20% (125-250 °C) or 10% (70-250 °C) weight indicates removal of six DMF/DEF plus one water and 16.5% (100-250 °C) weight indicates loss of six DEF molecules, respectively. CoW12Si-DMF and CoW12SiDEF lose their solvent molecules in two steps. The initial weight loss of 7% (150-175 °C) is followed by a 7% (200-250 °C) weight loss, which indicates removal of six DMF and three water molecules for CoW12Si-DMF. A sharp weight loss in

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Figure 3. (a) Helical arrangement of [PM12O40]3- and [Co(en)3]3þ clusters along the c axis in Co-Mo12P-DEF. (b) 2D supramolecular layers formed by [SiW12O40]4- and [Co(en)3]3þ along the bc plane in Co-W12Si-DEF (c) Stacking of 2D layers in Co-W12Si-DEF in the ABAB fashion. (d) 2D corrugated layered structures comprising alternating [Co(en)3]3þ and [SiW12O40]4- clusters along the bc plane in Co-W12Si-DMF. (e) 2D layered structures comprising alternating [Co(en)3]3þ and [PMo12O40]3- clusters along the bc plane in Co-Mo12P-DMF. (f) 2D supramolecular layers formed by [PW12O40]4- and [Co(en)3]3þ along the bc plane in Co-W12P-DMF. Color code: Mo (green), N (blue), W (red), C (black), Si (cyan), Co (magenta), P (yellow), Cl (light green).

between 300 and 350 °C indicates decomposition of the host lattice for all Co(en)3-POM ionic crystals reported in this paper. CO2 and H 2 Adsorption on Co(en)3 -POM Based Ionic Crystals. Only very few ionic crystals have demonstrated permanent porosity,10,12 since intermolecular Coulombic/ van der Waals interactions are usually too weak for sustaining an open framework. Vapor adsorption properties of [Co(en)3]Cl3 and POM clusters have been well documented in order to understand the increase of cell volume with respect to different gas uptake.12 But POM clusters are greatly underutilized on their capabilities of storing hydrogen and carbon dioxide.12a We focused to examine the porosity of

Co(en)3-POM ionic crystals and prepared it at the gram scale to allow detailed investigation of the aforementioned properties. An important structural feature of these Co(en)3-POM ionic crystals is that they possess voids which are less than 3.0 A˚ in diameter. All Co(en)3-POM based ionic crystals were nonporous to nitrogen because its void size (∼3.0 A˚) was less than the kinetic diameter of nitrogen (3.6 A˚); however, it was able to take up hydrogen (H2) and carbon dioxide (CO2) (the kinetic diameters of hydrogen and carbon dioxide are 2.89 A˚ and 3.3 A˚, respectively). All these Co(en)3-POM based ionic crystals showed reversible hydrogen sorption behavior (Figure 4b). The amount of hydrogen sorption varies, as the POM anion changes.

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Figure 4. (a) Overlay of TGA traces of as-synthesized Co(en)3-POM based ionic solids reported in this paper indicating their thermal stability. The TGA trace of compound Co(en)3Cl3 has also been plotted as a reference. (b) Hydrogen absorption isotherms at 77 K of Co(en)3-POM based ionic solids. Although absorption and desorption is completely reversible, only the absorption isotherm has been shown for clarity. (c) CO2 absorption data of Co-W12Si-DEF, Co-Mo12P-DMF, and Co-Mo12P-DEF at 298 K. The filled and open shapes represent adsorption and desorption, respectively. P/P0, relative pressure at the saturation vapor pressure of the adsorbate gas. (d) Selective H2 (black, circle) adsorption isotherms of Co-Mo12P-DEF over N2 (red, circle) at 77 K.

CoMo 12P-DEF has the highest (0.9 wt %) H2 uptake, and CoW12P-DEF has the lowest (0.4 wt %) uptake among the series when the adsorbate pressure approached 1 atm. Although the H2 adsorptions are somewhat moderate, they compare well with the value of 0.7 wt % obtained for the highest capacity zeolite ZSM-5 and some MOFs reported in the literature.25 As shown in Figure 4c, the framework exhibits a typical type-II profile for CO2 (3.4 A˚) for CoMo12P-DMF, Co-Mo12P-DEF, and Co-W12Si-DEF at 298 K. Hysteretic sorption with CO2 molecules suggests that the gases diffuse into aggregated [Co(en)3]3þ cations and POM anions with some structural transformations, due to positional displacements of organic/inorganic moieties. Activated Co(en)3-POM based ionic crystals exhibit very interesting selective adsorption of H2 over N2. The gas uptake of CoMo12P-DEF for H2 and N2 is 95.0 and 2.0 cm3 g-1 and at P/P0 of about 0.9, respectively, underlying the potential of these ionic crystals for selective separation of H2 over N2 (Figure 4d). Such preferential H2 uptake over nitrogen might be attributed to a size-exclusive effect in which the channels are accessible to hydrogen but not to nitrogen because of their differential kinetic diameters of 2.8 and 3.6 A˚, respectively. To our knowledge this is the first report of selective hydrogen adsorption in POM based ionic crystals, and we believe such selective hydrogen storage capacity could be efficiently utilized for the H2/N2 separation.

Conclusion In this report we synthesized POM based ionic crystals by complexation of Keggin-type POMs with [Co(en)3]3þ in DMF/ DEF media that yielded seven crystalline ionic solids. The crystal structures of these ionic solids showed the sandwich type layered packing. The hydrogen and carbon dioxide sorption properties of these ionic solids were influenced by the types and charges of the polyoxometalates and their orientation in the crystal lattice. We believe that the successful synthesis of these seven new ionic solids and the discovery of their selective H2 adsorption properties, as a result of the cooperative behavior between their cationic/anionic component species, will open up new vistas in the search for polyoxometalate based materials with useful absorptive properties. We are engaged in designing porous ionic crystals with functionalized interiors for enhanced gas adsorption capacity and higher selectivity. Further investigation of the magnetism and CO2 sequestration mechanism of these types of ionic solids is ongoing. Acknowledgment. C.D. acknowledges CSIR, New Delhi, India, for fellowship support. P.P. acknowledges CSIR for a project assistantship (PA-II) from CSIR’s XIth Five Year Plan Project (NWP0022-H), R.B. acknowledges Dr. S. Sivaram, Director, NCL, for an in house grant (MLP020626) and CSIR’s XIth Five Year Plan Project (Grant No: NWP0022-H) for funding and also Dr. B. D. Kulkarni, Dr. S. Pal, and

Article

Crystal Growth & Design, Vol. 11, No. 1, 2011

Dr. K. Vijaymohanan for their encouragement. Financial assistance from the DST (SR/S1/IC-22/2009) is acknowledged. Supporting Information Available: CIF files and description of experimental details, including synthetic methods, crystallography, and supplementary figures, including TGA, powder XRD profiles, tables of crystallographic data, and thermal ellipsoids for all Co(en)3-POM based ionic crystals reported in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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