Journal of Inclusion Phenomena and Macrocyclic Chemistry 36: 21–37, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 21 Analysis of Equilibria in the Extraction of Cesium Nitrate by Calix[4]arene-bis(t -Octylbenzo- Crown-6) in 1,2-Dichloroethane TAMARA J. HAVERLOCK, PETER V. BONNESEN, RICHARD A. SACHLEBEN and BRUCE A. MOYER ⋆ Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6119, U.S.A. (Received: 21 September 1998; in final form: 19 February 1999) Abstract. The extraction of CsNO 3 by the highly lipophilic calixarene-crown ether calix[4]arene- bis(t -octylbenzo-crown-6) (CABOBC6) in 1,2-dichloroethane (1,2-DCE) at 25 ◦ C has been shown to be consistent with the formation of both 1 : 1 and 2 : 1 metal : ligand species. Variation of the aqueous-phase CsNO 3 concentration up to 1.0 M and variation of the organic-phase calixarene concentration up to 0.10 M was modeled by the program SXLSQI. Formation of the organic-phase species CsBNO 3 (B = calixarene) was confirmed as well as the organic-phase dissociation products CsB + and NO − 3 . Good evidence for the 2 : 1 metal : ligand organic-phase species Cs 2 B(NO 3 ) 2 was also found, although the dissociation of nitrate from this complex was not observed. Binding of the second Cs + cation by the ligand is approximately two orders of magnitude weaker than binding of the first Cs + cation. The logarithm of the apparent partition ratio (log P B ) of the calixarene between water and 1,2-DCE was found by 1 H-NMR techniques to be > 5.1. Key words: Calixarene-crown ether, Cs + complexation, solvent extraction, SXLSQI, equilibrium constants, partition ratio, modeling. 1. Introduction New cesium-selective extractants from the calix[4]arene-crown-6 family have re- cently been introduced by collaborating European investigators [1–15]. As shown in Figure 1, these compounds represent a hybrid of the calix[4]arene and crown ether families, in which one or two crown-6 fragments bridge the phenolic positions of the calix[4]arene in the 1,3-alternate conformation. The facing pairs of aromatic rings of the calixarene in this conformation form two cavities that are each preor- ganized for partial insertion of a Cs + ion. Crystal structures of model complexes with cesium salts [1, 10–13] have demonstrated a significant pi-interaction between these facing aromatic rings and the guest Cs + ion, which is also coordinated by the six ether oxygen atoms of the polyether chain. This unique structure leads to strongly enhanced selectivity and strength in the extraction of cesium [1, 7, ⋆ Author for correspondence.