A Highly Symmetric Ionic Crystal Constructed by Polyoxoniobates and

Aug 28, 2017 - (15) The cationic cobalt complexes form a six-membered ring, and the noncoordinated polyanion resides inside the ring (red broken circl...
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A Highly Symmetric Ionic Crystal Constructed by Polyoxoniobates and Cobalt Complexes for Preferential Water Uptake over Alcohols Jufang Hu, Yanqing Xu,* Dingkun Zhang, Baokuan Chen, Zhengguo Lin, and Changwen Hu* Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China S Supporting Information *

and {Ni(en)3}5{HVNb8V8O44}.13c We speculate that accessibility to more diverse ionic structures based on PONbs would be ascribed to their intrinsically higher negative charges, which may facilitate cocrystallization of the ionic components from aqueous media to single crystals, large enough for X-ray structural analysis under hydrothermal conditions. Herein we propose that the polyanion PNb12O40(VIVO)6, although with a less negative charge of 3−, can feasibly build an ionic structure with tris(1,2diaminopropane)cobalt complexes, [CoII(pn)3]4[PNb12O40(VIVO)6][OH]5·20H2O (1, where pn = 1,2-diaminopropane). In 1, the polyanionic PNb 12 O 40 (V IV O) 6 and cationic [CoII(pn)3]2+ densely arrange and give a void volume as small as 3.10% within the supramolecular framework. Especially, in the high vapor pressure, 1 shows the preferential inclusion of water (19.72 mol mol−1) over alcohols (9.60 and 4.49 mol mol−1 for methanol and ethanol, respectively). Moreover, a gate-opening behavior in the water adsorption isotherm of 1 is observed. Well-formed single crystals of 1 were obtained as dark-brown cubes by the reaction of cobalt acetate, vanadyl phosphate, and potassium hexaniobate in the presence of 1,2-diaminopropane at 140 °C under hydrothermal conditions. The 4+ and 2+ oxidation states of vanadium and cobalt are supported by X-ray photoelectron microscopy analysis (Figures S7 and S8). Singlecrystal X-ray diffraction (XRD) reveals that 1 crystallizes in the cubic Im3̅m space group and possesses a hexacapped Keggin-type PONb, PNb12O40(VO)6 (Figure 1a), ionically interacting with eight surrounding cobalt complexes, [Co(pn)3]2+. In the asymmetric unit of the polyanion, only seven crystallographically independent atoms, labeled as Nb1, V1, P1, O1, O2, O3, and O4, can be positioned (Figure S1). As a result, the transition-metal centers in the polyanion can be simplified as a highly symmetrical structure of the Keplerate type14 consisting of a central Nb12 cuboctahedron and a V6 octahedron (Figure 1b). To the best of our knowledge, the polyanion in 1 holds the highest symmetry hitherto obtained in PONbs. The only crystallographically independent cobalt center is hexacoordinated with octahedral geometry completed by six nitrogen atoms from three 1,2-diaminopropane ligands. Symmetry-imposed disorder of the carbon atoms within the ligands can be clearly found (Figure S2). Compared with the commonly used ethylenediamine as the protective ligand, 1,2diaminopropane with larger steric hindrance would make a difference in the three-dimensional (3D) assemblies.15 The cationic cobalt complexes form a six-membered ring, and the

A BS T R A C T: A n i o n i c c ry s t a l a s s e m b l e d b y PNb12O40(VO)6 and tris(1,2-diaminopropane)cobalt complexes was hydrothermally isolated and structurally characterized by routine methods. The compound exhibits three-dimensional channels with a pore size of 3.68 Å × 2.30 Å and composed of hydrophilic oxygen atoms of polyanions and hydrophobic −CH3 groups of 1,2diaminopropane ligands. With increasing vapor pressure, the compound shows preferable adsorption toward water over alcohols, and a gate-opening behavior was deduced from the water adsorption isotherm.

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he perspective of bioalcohols as sustainable alternatives for fossil fuels prompts the research on new purification techniques capable of eliminating water from bioalcohols.1 In this regard, sorption separation has been established as one promising method, and typical adsorbents include zeolites,2 activated carbons,3 polymer materials,4 and nanoparticles.5 Recently, metal−organic frameworks (MOFs) are proven to be sorbent materials with excellent water capture properties.6 Different from those rigid materials with intrinsic pores, ionic compounds, constructed by molecular anions and cations, usually possess variable molecular-sized space and strong electric fields at the internal surface, which is unique with respect to guest inclusion.7 Polyoxometalates (POMs) are anionic metal oxide clusters with stable metal−oxo structures8 and are well condsidered as building blocks to construct ionic structures as hosts for guest inclusions. The cationic components generally involve (i) inorganic monovalant cations (NH4+, K+, Cs+, or Ag+),9 (ii) organometallic cations ([Co(en)3]3+, [Co(tacn)3)]3+, or [Ni(tacn)3)]2+, where en = ethylenediamine and tacn = 1,4,7triazacyclononane), 10 and (iii) microcations ([TM 3 O(OOCC6H5)6], where TM = Cr, Rb, or Ru).11 Compared with the widely explored ionic solids based on POMs and large microcations, the ones constructed by POMs and simple organometallic cations are much more challenging because of their rapid precipitation upon mixing in aqueous solution. Moreover, their properties of size-selectively distinguishing water from methanol and ethanol have been much less explored. Polyoxoniobates (PONbs) are an emerging subclass of POMs12 and are proven to be a new kind of POM-based supramolecular synthon to construct ionic solids, including {Cu(en) 2}6 {XNb12O40 (VO) 2} (X = Ge or Si),13a {Cu(en)2(H2O)2}4{HnXNb12O40Sb2} (X = Si/Ge, P/As, V),13b © XXXX American Chemical Society

Received: May 26, 2017

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DOI: 10.1021/acs.inorgchem.7b01360 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. (a) Drawing of the 3D supramolecular framework constructed by PNb12O40(VO)6 and cobalt complexes in 1. (b) Enlarged onedimensional channel composed of surface oxygen atoms of the PONbs and −CH2 groups of the cobalt complex cations. Symmetry-imposed disorder of the carbon atoms within organic ligands is found; hydrogen atoms are omitted for clarity. Color code: Nb, cyan; V, pink; Co, olive green; P, green; O, red; C, gray; N, blue.

Figure 1. (a) Polyanionic cluster in 1. (b) Representation of the Keplerate-type structure of the polyanion PNb12O40(VO)6 formed by a central Nb12cuboctahedron and a V6 octahedron. (c) Supramolecular layer formed by PNb12O40(VO)6 and cobalt complexes along the (1,1,1) plane in 1. Symmetry-imposed disorder of the carbon atoms within organic ligands is found; hydrogen atoms are omitted for clarity. Color code: Nb, cyan; V, pink; Co, olive green; P, green; O, red; C, gray; N, blue.

in 1 are formed in situ by hydrothermal treatment, thus allowing the formation of an unusual crystalline arrangement. Clearly, 1 is structurally different from our recently reported cobalt-linking PONb-based compounds, where the PONb polyanions were covalently connected to the cobalt complexes, giving rise to rigid 3D architectures.15b,16 Furthermore, compound 1 is thermally stable below 150 °C, as confirmed by TGA (Figure S9) and variable-temperature powder XRD analysis (Figure S10). The gas adsorption isotherm for N2 was measured on desolvated samples of 1, which reveals that 1 was nonporous to N2 (Figure S11). Vapor adsorption studies for H2O, CH3OH, and CH3CH2OH were carried out on desolvated samples of 1 at 298 K to investigate its polar guest inclusion properties. As shown in Figure 4, 1 adsorbs water molecules gradually at low

noncoordinated polyanion resides inside the ring (red broken circle in Figure 1c). Typically, the trianionic POM would be neuralized by the equimolar amount of 3+ organometallic cations to form the 1:1 complexes with low symmetry. 10 In 1, the polyanions PNb12O40(VO)6 with a charge of 3−, are packed in a bodycentered-cubic cell (Figure 2), which is typically found in the

Figure 2. Unit cell of 1. Color code: Nb, cyan; V, pink; Co, olive green; P, green; O, red; C, gray; N, blue.

trianionic POM-based solids compensated for by monovalant cations.17 Differently, eight organometallic cations exist around one polyanion at the center of the unit cell, resulting in an anionto-cation site ratio of 1:4 in 1. Five hydroxyl anions are additionally needed to complete the compensation of anion− cation charges in 1. The alkaline conditions for PONb assemblies render the hydroxyl anions formation-ready, which is quite common in PONb systems.18 Alternatively, in 1, the two components are arranged via weak N−H···Ob (Ob = bridge oxygen, 3.144 Å) hydrogen-bonding interactions, leading to a 3D channel composed of surface oxygen atoms of the PONbs and −CH3 groups of the cobalt complexes (Figure 3). The close packing of polyanions and cobalt complexes leads to a reduction in the pore size (with a size of 3.68 × 2.30 Å of van der Waals radii of atoms taken into consideration) and space for accessible guest molecules, consistent with a small space volume of 3.10% on the basis of PLATON calculations for 1. According to thermogravimetric analysis (TGA; Figure S9), a total of 20 water molecules are filled in the voids. We speculate that the ionic components PNb12O40(VO)6 and tris(1,2-diaminopropane)cobalt complexes

Figure 4. Water (circles), methanol (triangles), and ethanol (squares) vapor adsorption isotherms of desolvated samples of 1 at 298 K.

vapor pressure, suggesting that water molecules cannot easily enter into the small pores under such conditions. With increasing vapor pressure, the water uptake abruptly reaches 6.16 mol mol−1 at P/P0 ≈ 0.34 (marked by the red arrow in Figure 4), higher than that of the alcohols studied. Upon a further increase in the vapor pressure, the amount of guest sorption increases, and the uptake capacities are 19.72 (P/P0 = 1.0), 9.60 (P/P0 = 1.0), and 4.49 mol mol−1 (P/P0 = 0.92) for water, methanol, and ethanol, respectively. It is worth noting that the amount of water (a total of 20 water molecules per formula unit) filled in the crystal lattice of 1 induced from TGA is in accordance with its water adsorption behavior. The polyanions with an oxygen-enriched B

DOI: 10.1021/acs.inorgchem.7b01360 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Some structural figures, selected bond lengths, powder XRD patterns, IR spectra, XPS, and TGA (PDF)

surface, especially the completely coordinated free terminal oxygen atoms directed toward the channel pore, are expected to be the binding sites for hydrogen-bonding formation with those polar molecules. For alcohol molecules, the coexistence of hydrophilic and hydrophobic interactions within the channel in 1 should lead to faster sorption at the initial stage, compared with water molecules. With increasing vapor pressure, the hydrophilic interactions between the host and guest molecules should be dominated in the adsorption process. The amounts of saturation sorption decrease in the order of water (kinetic diameter = 2.68 Å) > methanol (kinetic diameter = 3.6 Å) > ethanol (kinetic diameter = 4.5 Å), underlying the potential of 1 for the selective separation of water over alcohols at high vapor pressure. The small pore size of 3.68 Å × 2.30 Å in 1 would be responsible for the preferential sorption of smaller water molecules in high vapor pressure. It is worth noting that a gate-opening behavior (this is a type of adsorption behavior by adsorption-induced lattice rearrangement, affected by the extent of rearrangement of the framework and the affinity between the host and guest6b) was observed in the water adsorption isotherm, which should be attributed to the flexibility of the methyl group in the 1,2diaminopropane ligands in 1. That is, the structural rearrangement of 1 may be observed with the uptake of water, and the adsorption energy for water compensates for rearrangement of the framework; therefore, gate-opening adsorption occurred for water adsorption. Strong host−guest interaction would account for hysteresis toward low relative pressure.19 Powder XRD measurements on samples after the sorption process confirmed structural integration (Figure S12), suggesting that 1 is some kind of robust adsorbent. Under our conditions using 3 Å zeolite as the adsorbent, the adsorption capacities are 2.78 (P/P0 = 0.96) and 1.29 (P/P0 = 0.93) mmol g−1 for water and ethanol, respectively (Figure S13). The uptake capacity ratio of water and ethanol, 2.16, is much smaller than that (4.41 and 6.18 mmol g−1 for water and 1.40 mmol g−1 for ethanol) of 1. Obviously, compared with the 3 Å zeolite, the titled compound shows much more selective adsorption of water over alcohol, which suggests that 1 might be a kind of excellent adsorbent in eliminating water from bioalcohols. In summary, we have successfully isolated an ionic solid based on PONbs and cobalt complexes via a simple hydrothermal method. Interestingly, the PONb-based ionic compound is proven to be a kind of dynamic absorbent with size-selective guest inclusion depending on the vapor pressure, that is, in the P/ P0 region from 0 to 0.34, 1 is more likely to adsorb alcohols than water molecules, whereas for P/P0 above 0.34, 1 exhibits preferential water uptake over alcohols. The 3D channels with both hydrophilic and hydrophobic properties should account for the selective sorption behavior, whereas the flexibility of the methyl group in the 1,2-diaminopropane ligands in 1 would contribute to the gate-opening behavior of the isotherm for water molecules. The reported well-characterized ionic material brings forth new opportunities for developing PONb-based functional materials.

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Accession Codes

CCDC 1533379 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Changwen Hu: 0000-0002-9026-1145 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21671019 and 21231002), 111 Project (Grant B07012), and 973 Program (Grant 2014CB932103).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01360. C

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DOI: 10.1021/acs.inorgchem.7b01360 Inorg. Chem. XXXX, XXX, XXX−XXX