1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Organic & Supramolecular Chemistry Weak and Reversible Binding of Alkali Metal Ions (Na + /K + ) by an Aza-Oxa Cryptand Mayank Gupta, Kapil Tomar, Sarvesh K. Pandey, and Parimal K. Bharadwaj* [a] A laterally asymmetric cryptand incorporating amino N and ethereal O was utilized to bind the hard alkali metal ions (Na + and K + ) in aqueous medium. Single crystal X-ray diffraction studies revealed that both Na + and K + ions were included inside the cryptand cavity. However, the trapped metal ions were weakly bound and could be released from the cavity of the cryptand. This cryptand had been used as a column chromatographic material for trapping Na + and K + ions from water as the trapped metal ion could be released back into water to extract the cryptand in its free state. Metal binding capability of the cryptand was evaluated using computational studies that revealed lower binding energy (B.E.) for the Na + and K + ions compared to first row transition metal ions. Introduction Porous organic cages have emerged as a new class of porous solids. [1] Porous solids include metal–organic frameworks (MOFs), [2] covalent organic frameworks (COFs) [3] and new classes of porous organic polymers. [4] Among these, porous organic cages are a unique class of microporous materials composed of discrete molecules with intrinsic, guest accessible cavities. These cage compounds retain their shape/cavity on removal or addition of guest molecules without collapsing of the intrinsic cavity preventing disruption of the pore network. [5] These cages can be used for capturing cations, anions as well as small molecules according to the cavity size and presence of donor atoms or functional groups at strategic positions. [6] Capturing of the guest is effective when cryptands are immobilized in a solid phase [7] or nanomaterials. [8] Presence of donor atoms such as N, P or S in a cryptand have a strong tendency to bind transition and heavy metal ions inside the cavity. On the other hand, hard ethereal O atoms in the bridges have shown their binding capacity for alkali/alkaline earth metal ions. It is also possible to tune the metal binding abilities of cryptands via derivatization if secondary amino groups are present in the bridges; changing the cavity size as well as modulating the donor abilities of the N donors. [9] A significant variation in the affinity trend towards anions were also observed when the spacer size was changed. [10] Cryptands with ethereal O donors (Scheme 1a) were known to bind hard alkali metal ions while those with amino N donors (Scheme 1b) showed less tendency to do so. We have been actively engaged in the study of metal cryptates for some time now [9b] and reported earlier about the binding abilities of the laterally non-symmetric macrobicyclic aza-oxa (Scheme 1c) cryptand [11] (L o ) towards a variety of first row transition and heavy metal ions that bind inside the cavity with moderate stability. We thought of using this cryptand (L o ) to trap hard metal ions like Na + and K + that would bind inside the cavity weakly (Scheme 1c) affording release of the trapped metal ion back in water when extraction was made. Using this strategy, we could use the cryptand for reversible binding of alkali metal ions. Herein, we have shown that both Na + and K + ions could be included inside the cavity and the trapped metal ions could be released back into water when the metal-cryptate was sonicated with water. Results and discussion The compounds, K + �L 0 and Na + �L 0 in single crystal form suitable for crystallographic studies were obtained using the alkali metal thiocyanate salts and the cryptand (L 0 ) in CHCl 3 : MeOH (1:2, v/v) mixed solvent system. When other salts of these alkali metal ions were used, the crystal quality were poor. Single-crystal X-ray diffraction analysis revealed that both the alkali metal cryptates, K + �L 0 and Na + �L 0 crystallized in the trigonal system with space groups R3 and R-3, respectively. In each case, the asymmetric unit comprised of one third of the cryptand (L 0 ), one third metal ion and a disordered thiocyanate anion (SCN  ). While thiocyanate anion had successfully been located in the potassium complex, L 1 (K + �L 0 ) it was not [a] M. Gupta, Dr. K. Tomar, Dr. S. K. Pandey, Prof. P. K. Bharadwaj Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India E-mail: pkb@iitk.ac.in Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201803353 Scheme 1. Cryptand with (a) all O donors (b) all N donors and (c) mixed O and N donors. Full Papers DOI: 10.1002/slct.201803353 1785 ChemistrySelect 2019, 4,1785–1788 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim