ORIGINAL ARTICLE New 5,17-(di-ionizable)-25,26,27,28-tetraalkoxycalix[4]arene ligands: synthesis and selected divalent metal ion extractions Pillhun Son • Gaurav Arora • Jennifer D. Crawford • Eun Kyung Lee • Louisa J. Hope-Weeks • Kazimierz Surowiec • Richard A. Bartsch Received: 10 December 2014 / Accepted: 5 January 2015 / Published online: 31 January 2015 Ó Springer Science+Business Media Dordrecht 2015 Abstract Four new series of 5,17-(di-ionizable)- 25,26,26,28-tetraalkoxycalix[4]arene ligands are prepared. The ionizable groups are carboxylic acid and N-(Z)sulfonyl formamide functions with Z = –Me, –Ph, –C 6 H 4 –4-NO 2 , and –CF 3 . When the four alkoxy groups are methoxy, the ligand conformation is flexible. With four butoxy groups, the ligands are locked in cone, partial-cone, or 1,3-alternate conformations. Complexation abilities of the new ligands are evaluated in solvent extractions of selected divalent metal ions from aqueous solutions into chloroform. Both the conformation of the calix[4]arene unit and identity of the acidic group are found to influence the divalent metal ion extraction selectivity and efficiency. Keywords Calix[4]arene ligands Á Crown compounds Á Metal ion extraction Á Ionic recognition Calixarenes are one of the major building block categories for supramolecular chemistry [1–6]. Comprised of four phenolic units, tetrameric calix[4]arenes (with the carbon numbering system shown in Fig. 1) are particularly well- suited for this purpose due to their ready synthetic acces- sibility, ease of structural modification, and variable host– guest binding properties. Much of our metal ion separations research program has involved the synthesis and evaluation of calix[4]arene ligands with acidic groups attached to carbons 25 and 26 or 25 and 27 of the tetrameric macrocycle [7]. Such ligands are designed to effectively extract divalent metal ions from aqueous solutions into organic diluents. Extraction pro- ceeds via an ion-exchange mechanism to form an electro- neutral, di-ionized ligand-divalent metal ion complex. This markedly enhances the metal ion extraction efficiency when the aqueous phase anion is hydrophobic chloride, nitrate, or sulfate. Depending upon the substituents attached to the phe- nolic oxygens, calix[4]arene ligands may be conforma- tionally mobile or may exist in different locked conformations (cone, partial-cone, 1,3-alternate) [1–6]. 1 H and 13 C NMR spectroscopy provide useful insight into the conformations of such ligands [8]. We now describe the preparation of new di-ionizable calix[4]arene ligands (Fig. 2) with methoxy or butoxy groups attached to carbons 25, 26, 27, and 28 and –C(O)X groups on carbons 5 and 17. The acidic groups are X = –OH (carboxylic acid) and –NHSO 2 Z [N-(Z)sulfonyl formamide] with Z = –Me, –Ph, –C 6 H 4 –4-NO 2 , and –CF 3 in which the acidity is varied by changing the electron- withdrawing ability of Z [9]. The latter have been found to exhibit surprising selectivity for certain heavy and transi- tion metal cations [10, 11]. The ligands in the present investigation differ from our previous studies of di-ionizable 25,26- and 25,27-(dialk- oxy)calix[4]arenes extractants in that the new ligands are di-ionizable 25,26,27,28-tetraalkoxycalix[4]arenes. The influence of the structural changes on the com- plexation of selected divalent metal ion species is evaluated by solvent extraction from aqueous solutions into chloro- form. The divalent metal ions studied include hard [12] alkaline earth metal cations (AEMC), intermediate [12] Pb 2? , and soft [12] Hg 2? . P. Son Á G. Arora Á J. D. Crawford Á E. K. Lee Á L. J. Hope-Weeks Á K. Surowiec Á R. A. Bartsch (&) Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409, USA e-mail: richard.bartsch@ttu.edu 123 J Incl Phenom Macrocycl Chem (2015) 81:451–464 DOI 10.1007/s10847-015-0474-2