A Cryptand Metal-Organic Framework As a Platform for the Selective Uptake and Detection

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A Cryptand Metal-Organic Framework as a Platform for the Selective Uptake and Detection of Group I Metal Cations

Sergey A. Sapchenko,*[a],[b] Pavel A. Demakov,[a],[b] Denis G. Samsonenko,[a],[b] Danil N. Dybtsev,[a],[b] Martin Schröder*[a],[c] and Vladimir P. Fedin[a],[b]

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Abstract:The metal-organic framework (MOF) complex (H3O)2[Zn4(ur)(Hfdc)2(fdc)4] (1, ur = urotropine, H2fdc = furan-2,5-dicarboxylic acid) incorporates cryptand-like cavities, which can be used to separate and detect Rb+ and Cs+ optically. This is the first example of the effective employment of a MOF material for optical detection of these cations.

Alkali metals are important elements, with Na+ and K+ levels influencing multiple physiological processes including the regulation of the Na+/K+-ATPase solute pump.[1] Li+ salts are widely employed in medicine,[2]heavier Rb+cations can replace the electrolytic function of K+and affect plasma biochemical parameters, while Cs+ cations are able to participate in chemical reactions within living cells.[3] Since many biological processes are regulated by these metal cations, any imbalance in concentration can cause severe physiological effects. Given the importance of alkali metal cations for living organisms, there is much interest in developing optical sensors for Group I cations.[4–6]

Water-soluble fluorescent probes, which switch their fluorescence on complexation with metal cations, are often based upon molecular systems incorporating crown-ether rings or cryptands attached to polyaromatic fragments, the latter being responsible for the fluorescent properties of the probe in solution.[7–14] Unlike soluble probes, a solid sensor can be readily removed from the test solution and recovered for the next analysis. Even though there has been some success in attaching optical molecular probes to solid polymeric substrates,[15,16] these tend to detect only one alkaline metal cation, and generally do not demonstrate high selectivity in the presence of other alkali metal ions compared to soluble probes. Metal-organic frameworks (MOFs) are an emerging class of microporous solids, which are being intensively investigated due to their structural and functional diversity,[17–22] and have been used for luminescent detection of various analytes.[23–27] The porosity of these compounds coupled to their luminescence affords a functional chemistry not available to traditional inorganic complexes or organic molecular probes. Changes in

Figure 1.Views of the structure of 1: a) the tetrahedral secondary building unit (Zn atoms are shown green, N blue, O red, C gray, H omitted); b) the framework structure; c) the anionic cavity with hydroxonium counter-cation within (shown pink, the electrostatic contacts are shown as orange lines).

luminescence upon introduction of the d-metal cations to MOFs have been described,[28–31] but despite attempts[32–35] there are still no successful examples of luminescent detection of alkali metals by MOFs.

Little has been reported on the use of MOFs for selective alkali metal ion capture, although the separation of K+ ions from a mixture of Na+, K+, Mg2+ and Ca2+ by the composite Fe3O4@mSiO2@MOF has been reported.[36] We demonstrate herein, for the first time, effective detection of alkali metal cations using a new porous MOF material, (H3O)2[Zn4(ur)(Hfdc)2(fdc)4]·G (1) (ur = urotropine, H2fdc = furan-2,5-dicarboxylic acid, G = 4DMF∙14H2O∙2H2fdc∙2ur). In particular 1 shows high selectivity for Rb+ and Cs+cations and changes its luminescent properties upon the formation of the corresponding inclusion compounds with these cations. It is also worth noting that the isotopes Rb-86 and Cs-137 as uranium fission products in nuclear reactors[37] cause negative impact on the biosphere, their chemical similarity to K+ making them ready substitutes in plants.[38]

Colorless crystals of (H3O)2[Zn4(ur)(Hfdc)2(fdc)4]·G (1) were prepared by heating a mixture of Zn(NO3)3·6H2O, furan-2,5-dicarboxylic acid (H2fdc) and urotropine in N-methylpyrrolidone (NMP) or DMF at 100°C for 24 h. A single crystal X-ray analysis was performed on crystals obtained from DMF. 1 crystallizes in

Figure 2.a) Molar fraction of alkali metal cation in the supramolecular adducts of compound 1 after immersion in 102M MNO3 solution (M = Li, Na, K, Rb, Cs) in NMP; b) Molar fraction of alkali metal cation in a 3∙10–3 M solution of alkali metals nitrates (blue) vs the molar fraction obtained within the supramolecular adduct of 1 (red).

the cubic space group F4132, with the asymmetric unit comprising one crystallographically independent tetrahedral Zn(II) centre bound to three O-donors from the carboxylategroups of furan-2,5-dicarboxylic acid and one N-centre from a urotropine. The latter bridges four Zn(II) centres to form {Zn4(ur)(O2CR)12} clusters (Figure 1a), which are bridged by mono-protonated Hfdc ligands to give a porous 3D framework with 61% of a free accessible volume.[39] As synthesised1 contains two types of pores: one is hydrophobic filled with guest molecules (DMF, water, urotropine and H2tdc) (Figure 1b), and the other is decorated with O-donors from fdc2– ligands and incorporate hydroxonium counterions, which bind to the framework via hydrogen bonding (Figure 1c). The arrangement of the O-centres within this second cavity resembles the inner environment of cryptands and is formed by three bridging fdc2– ligand capped by the N-donor from the urotropine ligand. Each fdc2– bridge generates 3 non-coordinated O-centres, and thus the cavity has in total 9 free O-donors from 3 furan ring (Ofdc) and 6 carboxylic groups (OCOO). Since the cavity has D3v symmetry, all the non-coordinated oxygen atoms of each bridge fragment are arranged in the form of a triangle, Ofdc…Ofdc = 5.162(3), OCOO…OCOO = 3.566(3) Å. In the center of the cavity oxygen atom OH3O of the hydroxonium cation forms hydrogen bonds with the O-sites of the cavity, OH3O…OCOO = 3.076(2), OH3O…Ofdc = 2.908(2) Å. This motivated us to investigate the exchange of these hydoxoniumcation with Group I cations.

1 was treated with 0.01 M solutions of alkali metal nitrates in N-methylpyrrolidone for 1 day. PXRD data (Figure S1) confirmed the stability of the framework during the ion exchange. In order to determine the alkali metals content in the obtained adducts, we dissolved them in the H2O2 solution in DMF and determined the concentration of the alkali metal cations by atomic emission

Figure 3.Solid-state photoluminescence spectra of compound 1 and its adducts with alkali metal cations.

spectroscopy (Figure 2a). Observed preferential binding of the Rb+ and Cs+ over Li+, Na+ and K+ was observed. Soaking the crystals of the framework 1 in saturated solutions of KNO3 or CsNO3 for 3 days led to the formation of the complexes M2[Zn4(ur)(Hfdc)2(fdc)4] [M=K+ (2) Cs+ (3)], the single crystal X-ray structures of which confirm that the hydroxonium cations in the cryptand cavities of 1 have been replaced by K+ and Cs+, respectively.

The structures of the frameworks in K2[Zn4(ur)(Hfdc)2(fdc)4]·5.5NMP, 2 and Cs2[Zn4(ur)(Hfdc)2(fdc)4]·9NMP, 3, are identical to 1, even though the exchange of H3O+ cations to K+ and Cs+ slightly affects the geometry of the carboxylic groups within the framework (Figure S2). The structures confirm binding of K+ and Cs+ to the O-donors within the cryptand, K…Ofdc= 2.8588(0)Å, K…OCOO = 3.0243(0)Å, Cs…Ofdc 3.100(0)Å and Cs…OCOO = 3.1404(1)Å. The data show that during the reaction with Group I nitrates only H3O+ is substituted, while the protons of Hfdc- fragments remain intact.

To monitor its ability for size-selective sorption, 1 was immersed in a solution of five alkali metal nitrates in NMP, with the concentration of each metal cation [M+] being 3.0·10–3 M.The chemical analytical data of the resulting adduct clearly confirm the strong affinity of the framework towards the heavy alkali metal cations: even though all alkali metal cations were found to substitute H3O+ species within the pore, only 0.6% and 9.0% of the initial hydroxonium cations were replaced by Li+ and Na+, respectively, while almost half of H3O+ cations (41.3%) were substituted by Cs+. Thus, the resulting molar ratio between all five alkali metal cations within the framework 1 was Li:Na:K:Rb:Cs=1:15:34:47:69 (Figure 2b).

From the above experiments we confirmed that alkali metal cations interact differently with the free O-donors of the pores in 1. 1 exhibits photoluminescent behavior and under excitation at 340 nm it shows a broad band at λ = 470 nm in its solid-state emission spectrum, which can be attributed to the intra-ligand electron state transitions. The introduction of alkali metal cations however does not significantly change the position of this emission band, but affects the intensity and the quantum yield of the photoluminescence (Figure 3). The experimental data are given in Table 1.

Table 1.Photoluminscent properties of 1 and its adducts with alkali metals.
Compound / Quantum Yield φ, % / Quantum yield decrease against 1 (φ0), Δφ/φ0, %
H3O@1
Li@1
Na@1
K@1
Rb@1
Cs@1 / 15.95±0.06
12.02±0.05
15.26±0.05
10.30±0.05
11.11±0.05
12.88±0.06 / –
25
4
35
30
19

All the metal cations quench the photoluminescence of 1, but with different efficiencies. The most distinct effect is observed on introduction of K+ (35% decrease of the quantum yield) and Rb+ (30% quenching). Taking into account the size-selectivity of the framework towards the large alkali metal cations, the framework 1 can be employed as a selective sensor for Rb+cation. 1 also demonstrates high preference to Cs+, and the substitution of H3O+ cations to Cs+ causes a reduction in a fluorescence quantum yield of 19%.

Size-selective optical chemosensors for Cs+cations are less numerous than for light alkali metal cations and are mostly derivatives of crown ethers, calixarenes and cyclophanes active in the solution phase.[40–43] For Rb+, we were unable to find any example of an effective single-molecular or polymeric sensor to this cation. Hence, the obtained framework material (H3O)2[Zn4(ur)(Hfdc)2(fdc)4]·G represents an unique example of solid-state heavy alkali metals selective sensor. We have to note though, that since the luminescence intensity of the framework 1 is the only measurable output, the system can be effectively used for the sensing of alkali metals in a single component solution or from a mixture of alkali metals of notably different binding constants (e.g., Li+vs Cs+).The current MOF material can thus serve as a facile platform for the development of more effective sensors. After processing thematerials can potentially be used in tablet form or serve as the active substance within a mixed-membrane, which can be useful for monitoring biological and environmental events.

In summary, (H3O)2[Zn4(ur)(Hfdc)2(fdc)4]·G incorporates cryptand-like cavities that can participate in cation-exchange reactions, which allowed us to effectively separate and detect the alkali metal cationic species. This is the first example of the use of metal-organic frameworks for the environmentally challenging problem of detection and extraction of alkali metal cations, in particularly Rb+ and Cs+. The presented work opens the wide perspectives for the further development of the MOFs with superior sensing and binding properties towards alkali and other metal cations.

Experimental Section

Experimental details are provided in the Supporting Information.

Acknowledgements

The work was supported by the Russian Megagrant Project № 14.Z50.31.0006 (leading scientist M. Schröder). MS also acknowledges support from EPSRC, the ERC (Advanced Grant), and the University of Manchester. Authors are thankful to Mr. E. Saparbayev, Dr. A. Ryadun and Dr. N. Beisel for their help and suggestions.

Keywords:Metal-organic framework • sensing • cryptand • luminescent detectors • alkali metals recognition

[1]T. Clausen, Physiol. Rev.2003, 83, 1269.

[2]P. E. Keck, S. L. McElroy, S. M. Strakowski, C. A. Soutullo, J. Clin. Psychiatry2000, 61 Suppl 4, 33.

[3](a) D. G. Davis, E. Murphy, R. E. London, Biochemistry1988, 27, 3547; (b) K. Yokoi, M. Kimura, Y. Itokawa, Biol. Trace Elem. Res.1996, 51, 199.

[4]J. Yin, Y. Hu, J. Yoon, Chem. Soc. Rev.2015, 44, 4619.

[5]S. Sahana, P. K. Bharadwaj, Inorg. Chim.Acta2014, 417, 109.

[6]G. R. C. Hamilton, S. K. Sahoo, S. Kamila, N. Singh, N. Kaur, B. W. Hyland, J. F. Callan, Chem. Soc. Rev.2015, 44, 4415.

[7]M. K. Kim, C. S. Lim, J. T. Hong, J. H. Han, H.-Y. Jang, H. M. Kim, B. R. Cho, Angew. Chem. Int. Ed.2010, 49, 364; Angew.Chem.2010, 122, 374.

[8]M. Magzoub, P. Padmawar, J. A. Dix, A. S. Verkman, J. Phys. Chem. B2006, 110, 21216.

[9]L. Lochman, J. Svec, J. Roh, V. Novakova, Dye. Pigment.2015, 121, 178.

[10]T. Schwarze, R. Schneider, J. Riemer, H.-J. Holdt, Chem. Asian J.2016, 11, 241.

[11]Y. Gao, R.-L. Zhong, H.-L.Xu, S.-L.Sun, Z.-M. Su, RSC Adv.2015, 5, 30107.

[12]Y. Gao, S.-L. Sun, H.-L.Xu, L. Zhao, Z.-M. Su, RSC Adv.2014, 4, 24433.

[13]X. Li, X. Gao, W. Shi, H. Ma, Chem. Rev.2014, 114, 590.

[14]A. R. Sarkar, C. H. Heo, M. Y. Park, H. W. Lee, H. M. Kim, Chem. Commun.2014, 50, 1309.

[15]H. Yu, X. Ju, R. Xie, W. Wang, B. Zhang, L. Chu, Anal. Chem.2013, 85, 6477.

[16]Y. Hiruta, C. Sato, Y. Takahashi, K. Kubobuchi, Y. Shichi, D. Citterio, K. Suzuki, RSC Adv.2013, 3, 6499.

[17]Z. Gu, C. Yang, N. Chang, X. Yan, Acc. Chem. Res.2012, 45, 734.

[18]B. Li, H. Wang, B. Chen, Chem. Asian J.2014, 9, 1474.

[19]Functional Metal-Organic Frameworks: Gas Storage, Separation and Catalysis (Ed.: M. Schröder), Springer, 2010.

[20]J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.-Y. Su, Chem. Soc. Rev.2014, 43, 6011.

[21]B. Li, M. Chrzanowski, Y. Zhang, S. Ma, Coord. Chem. Rev.2016, 307, 106.

[22]B. Chen, S. Xiang, G. Qian, Acc. Chem. Res.2010, 43, 1115.

[23]L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp, Chem. Rev.2012, 112, 1105.

[24]M. D. Allendorf, C. A. Bauer, R. K. Bhakta, R. J. T. Houk, Chem. Soc. Rev.2009, 38, 1330.

[25]A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong, J. Li, Angew. Chem. Int. Ed.2009, 48, 2334; Angew. Chem.2009, 121, 2370.

[26]H. Li, W. Shi, K. Zhao, Z. Niu, H. Li, P. Cheng, Chem. Eur. J.2013, 19, 3358.

[27]Y. Cui, Y. Yue, G. Qian, B. Chen, Chem. Rev.2012, 112, 1126.

[28]L.-Z. Zhang, W. Gu, B. Li, X. Liu, D.-Z. Liao, Inorg. Chem.2007, 46, 622.

[29]L. Chen, K. Tan, Y.-Q. Lan, S.-L. Li, K.-Z.Shao, Z.-M. Su, Chem. Commun.2012, 48, 5919.

[30]S. Liu, Z. Xiang, Z. Hu, X. Zheng, D. Cao, J. Mater. Chem.2011, 21, 6649.

[31]Z. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev.2014, 43, 5815.

[32]S.-S. Zhao, J. Yang, Y.-Y. Liu, J.-F. Ma, Inorg. Chem.2016, 55, 2261.

[33]R.-M. Wen, S.-D. Han, G.-J. Ren, Z. Chang, Y.-W. Li, X.-H. Bu, Dalton Trans.2015, 44, 10914.

[34]L. Wen, X. Zheng, K. Lv, C. Wang, X. Xu, Inorg. Chem.2015, 54, 7133.

[35]J. Wang, S. Yao, G. Li, Q. Huo, L. Zhang, Y. Liu, RSC Adv.2015, 5, 102525.

[36]W. Wu, A. M. Kirillov, X. Yan, P. Zhou, W. Liu, Y. Tang, Angew. Chem. Int. Ed.2014, 53, 10649; Angew.Chem., 2014, 126, 10825.

[37]J. Severa, J. Bár, Handbook of Radioactive Contamination and Decontamination, Elsevier, 1991.

[38]J. F. Cline, F. P. Hungate, Plant Physiol.1960, 35, 826.

[39]A. L. Spek, ActaCrystallogr. Sect. D Biol. Crystallogr.2009, 65, 148.

[40]N. Kumar, I. Leray, A. Depauw, Coord. Chem. Rev.2016, 310, 1.

[41]S. Chopra, N. Singh, P. Thangarasu, V. K. Bhardwaj, N. Kaur, Dye. Pigment.2014, 106, 45.

[42]M. H. Lee, D. T. Quang, H. S. Jung, J. Yoon, C. Lee, J. S. Kim, J. Org. Chem.2007, 72, 4242.

[43]E. D. Roper, V. S. Talanov, M. G. Gorbunova, R. A. Bartsch, G. G. Talanova, Anal. Chem.2007, 79, 1983.

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Entry for the Table of Contents

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(H3O)2[Zn4(ur)(Hfdc)2(fdc)4] (1, ur = urotropine, H2fdc = furan-2,5-dicarboxylic acid) incorporates cryptand-like cavities, which can be used to separate and detect Rb+ and Cs+ optically. This is the first example of the effective employment of a MOF material for optical detection of these cations. / / Sergey A. Sapchenko,* Pavel A. Demakov, Denis G. Samsonenko, Danil N. Dybtsev, Martin Schröder* and Vladimir P. Fedin
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A Cryptand Metal-Organic Framework as a Platform for the Selective Uptake and Detection of Group I Metal Cations