pubs.acs.org/crystal Published on Web 11/23/2010 r 2010 American Chemical Society DOI: 10.1021/cg101201b 2011, Vol. 11 139–146 Structural and Selective Gas Adsorption Studies of Polyoxometalate and Tris(ethylenediamine) Cobalt(III) Based Ionic Crystals 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) 3 Mo 8 O 26 (H 3 O)- (Cl)](DMF) 4 (H 2 O) (Co-Mo 8 -DMF), [R-PW 12 O 40 ][Co(en) 3 ] 3 6DMF (Co-W 12 P-DMF), [R-PW 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-W 12 P- DEF), [R-PMo 12 O 40 ][Co(en) 3 ] 3 5.5DMF (Co-Mo 12 P-DMF), [R-PMo 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-Mo 12 P-DEF), [R-SiW 12 O 40 ]- [Co(en) 3 ] 3/2 [Cl] 1/2 3 6DMF 3 3H 2 O (Co-W 12 Si-DMF), and [R-SiW 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-W 12 Si-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 H 2 adsorption properties over N 2 . The amount of hydrogen adsorption was influenced by POM anion types and their orientations. CoMo 12 P-DEF has the highest (0.9 wt %) H 2 uptake, and CoW 12 P-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 oxida- tion 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 photo- chemistry, 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 supramo- lecular 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 crys- tals 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 ]Cl 3 , 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 transform- ability upon absorbing different gases. Both POM based ionic crystals and [Co(en) 3 ]Cl 3 are well-known for their absorption of organic solvents and H 2 O, but POM clusters are greatly under- utilized in terms of their capabilities of storing hydrogen or carbon dioxide. 12 This made us interested in the rational syn- thesis 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) 3 Mo 8 - O 26 (H 3 O)(Cl)](DMF) 4 (H 2 O) (Co-Mo 8 -DMF), [R-PW 12 O 40 ]- [Co(en) 3 ] 3 6DMF (Co-W 12 P-DMF), [R-PW 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-W 12 P-DEF), [R-PMo 12 O 40 ][Co(en) 3 ] 3 5.5DMF (Co-Mo 12 P-DMF), [R-PMo 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-Mo 12 - P-DEF), [R-SiW 12 O 40 ][Co(en) 3 ] 3/2 [Cl] 1/2 3 6DMF 3 3H 2 O (Co- W 12 Si-DMF), [R-SiW 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-W 12 Si-DEF) (en = ethylenediamine, DMF = dimethyl formamide, DEF = diethyl formamide) (Figure 1), display selective hydrogen (H 2 ) and carbon dioxide (CO 2 ) sorption over nitrogen (N 2 ). These ionic solids have been determined by single crystal X-ray diffraction and further characterized by IR spectra, PXRD, TGA, and EA. Experimental Section Synthesis of [Co(en) 3 Mo 8 O 26 (H 3 O)(Cl)](DMF) 4 (H 2 O)(Co-Mo 8 - DMF). Sodium molybdate dihydrate (5.8 g of Na 2 MoO 4 3 2H 2 O) 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 concen- trated 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 H 2 O 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, ν asym NH 2 ), 1646 (s, ν str CdO), 933 (s, ν asym ModO), 894 (s, ν asym Mo-O a ), 841 (s, ν asym Mo-O b ), 704 (s, ν asym Mo-O c ). Elem Anal. Calcd: C, 12.18; H, 3.18; N, 7.83. Found: C, 12.09; H, 3.21; N, 7.85. [DMF/H 2 O = 1:5; Amount of water ∼85%]. Synthesis of [r-PW 12 O 40 ][Co(en) 3 ] 3 6DMF (Co-W 12 P-DMF). To 2 mL of a 0.05 M solution of sodium phosphotungstate hydrate (Na 3 PW 12 O 40 3 xH 2 O) 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, ν asym NH 2 ), 1650 (s, ν str CdO), 1055 (s, ν asym P-O), 952 (s, ν asym WdO), 884 (s, ν asym W-O c - W), 814 (s, ν asym W-O e -W). Elem anal. Calcd: C, 8.10; H, 1.85; N, 4.72. Found: C, 8.12; H, 1.85; N, 4.75. [DMF/H 2 O = 4:1; Amount of water ∼20%]. Synthesis of [r-PW 12 O 40 ][Co(en) 3 ] 3 6DEF (Co-W 12 P-DEF). Two milliliters of a 0.05 M solution of Na 3 PW 12 O 40 3 xH 2 O in DEF was taken in a 30 mL culture tube. To that solution was added dropwise *E-mail: r.banerjee@ncl.res.in. 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