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Molecular Design of Agents for Selective Anion Separation T. L. Hubler, J. H. Sukamto, G. M. Anderson,(a) S. A. Bryan,(b,c) G. J. Lumetta(b,c) Supported by Laboratory Directed Research and Development funding (STEP Initiative). (a) Student. (b) EMSL User. (c) Environmental Technology Division. Commercially available materials for separation of anions via solvent extraction (SX) or ion exchange (IX) cannot achieve the high performance characteristics needed for many environmental applications. Such applications generally require the contaminants to be removed to very low levels from very dilute streams or from mixtures of competing anions. New separating agents need to be developed with enhanced selectivity and overall performance. This project has examined new redox-switchable materials that may be used in an ESIX (electrically switched ion-exchange) process for anions. A particular anion of interest to the DOE cleanup mission is the pertechnetate ion, which is present in solutions with high nitrate ion concentration. Hence the materials must have a redox-switchable unit and selectivity for the pertechnetate ion over nitrate ion. One class of redox-active materials that have shown anion selectivity are those reported by Beer et al. (1999). The general type of anion ligands that have been investigated are the ferrocene derivatized calixarenes (Figure 5.1). Several new compounds were prepared and characterized with standard spectroscopic techniques and by use of electrochemical methods. Figure 5.1. Compounds 1-3 are new metallocene-functionalized calixarenes which are candidate electroactive for anion exchange materials. One of the primary methods used to characterize the selectivity of the compounds for one anion over another is observance of a potential shift in the cyclic voltammetry (CV) of the metallocene compound. Hence, compound 1 showed a 30-35 mV shift to lower potential when contacted with solutions containing tetrabutylammonium nitrate, tetrabutyl-ammonium perrhenate and combinations thereof; the shift occurred only upon addition of nitrate ion, seemingly to indicate that 1 exhibits some selectivity for nitrate ion. Experiments were run under rigorous water and air-free conditions, but it was not possible to rule out small amounts of water as being the source of the shift since the CV shifts upon redox cycling of 1 when contacted with solutions containing 0.20% water. Additionally, analytical data on the distribution of elements/ions in the aqueous and organic phases are difficult to obtain for the ferrocene compounds because of the requirement for a supporting electrolyte with which to carry out the redox cycling of the ferrocene compound. Synthesis of the analogous cobalticenium compounds was necessary in order to carry out batch distribution studies for nitrate. For example, compound 3 appears to show selective extraction of perrhenate ion (ReO4-) over nitrate ion (NO3-). Solutions of 3, 4, and 5 were tested by batch contact in a biphasic dichloromethane/water system whereby NaReO4 and NaNO3 were dissolved in the aqueous phase while the organometallic compound was dissolved in the organic phase. In order to test which portions of the molecule may be responsible for the extraction of ReO4- into the organic phase, comparison of the base organometallic unit (in this case cobalticenium hexafluorophosphate) was made with the calixarene-substituted material. In the case of 4, the ionic compound itself moved completely into the aqueous phase; hence, the decamethyl-substituted metallocene 5 was examined instead (remains entirely in the organic phase). Extraction data is listed in Table 5.1, and consists of ICP elemental analysis of the aqueous phase after contact for 135 minutes. The change in ReO4- ion concentration was taken as the change in rhenium concentration determined by ICP. Table 5.1. Relative change in the amounts of rhenium (as ReO4-) and phosphorus (as PF6-) from extraction studies of an aqueous solution of NaReO4 and NaNO3 (each 1 mM). Compound DP (mmole) DRe (m mole) 3 5.78 ± 0.51 -8.15 ± 0.93 5 4.78 ± 0.11 -5.50 ± 0.47 Compound 3 clearly extracts more ReO4- than 5, demonstrating the potential of these materials for use as extractants for SX processes. Less hexafluorophosphate ion (PF6-) moves into the aqueous phase than expected based on a 1:1 ion-exchange rate. In conclusion, it is not yet clear that the calixarene moiety of these organometallic compounds plays any substantial role in the ion-exchange process. Generally, it is thought that hydrogen-bonding from the calixarene portion of the molecule provides the selectivity for anion extractions, although these particular calixarene structures are not optimized for interaction with the particular anions of interest. The extra steric bulk from the permethylated cobalticenium compound may account for the difference in extraction of the ReO4- by weakened interaction of the ions due to an increased distance between the metal center and the ReO4-. However, the results may be another indication that the calixarene portion of the molecule does indeed enhance the selectivity of these molecules. Reference Beer, P. D., P. A. Gale, and G. Z. Chen, "Mechanisms of Electrochemical Recognition of Cations, Anions, and Neutral Guest Species by Redox-Active Receptor Molecules," Coord. Chem. Rev. 185-186, 3-36 (1999). William R. 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