Separations of Soft Heavy-Metal Cations by Lower ... - ACS Publications

Chapter 10

Separations of Soft Heavy-Metal Cations by Lower Rim-Functionalized Calix[4]arenes

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 21, 2015 | http://pubs.acs.org Publication Date: July 20, 2000 | doi: 10.1021/bk-2000-0757.ch010

Galina G. Talanova, Vladimir S. Talanov, Hong-Sik Hwang, Nazar S. A. Elkarim, and Richard A. Bartsch Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061

Calix[4]arenes functionalized on the lower rim with various donor groups are used for recognition and separation of soft heavy metal cations, e. g., Ag(I), Au(III), Cd(II), Hg(II), Pd(II), and Pt(II). Recently, calix[4]arenes containing proton-ionizable N-X-sulfonyl carboxamide functionalities on the lower rim have been found to efficiently extract Hg(II), a soft metal ion, from acidic aqueous solutions with excellent selectivity over alkali, alkaline earth, and many transition and heavy metal cations, including Ag(I), Cd(II), Pb(II), Pd(II), and Pt(II). This unexpected favoring of Hg(II) complexation by ionophores containing hard donor groups has been used to obtain the first calixarene-based, mercury-selective fluorogenic reagent by incorporating a dansyl fluorophore moiety as a part of the N-X-sulfonyl carboxamide groups.

During the last decade, remarkable progress in the transition and heavy metal coordination chemistry of the calixarene ligands has been achieved (1-4). An important contributing factor was the introduction of softer donor groups with nitrogen, phosphorus, or sulfur atoms on the calixarene lower rim instead of hard oxygen-containing functions. This gaveriseto new calixarene-based ionophores with enhanced affinity for c/-metal ions, including the soft Pearson's acids of Ag(I), Au(III), Cd(II), Hg(II), and platinides(II), and diminished complexing ability toward hard alkali and alkaline earth metal cations. This feature of lower rim-fimctionalized calixarenes, particularly sulfur-containing derivatives (Figure 1), has been utilized in the separation of soft heavy metal ions. TTius, calix[4]arene tetrathioamides 1 are efficient Ag(I) picrate extractants that show low extraction levels for Cd(II), Co(II), Cu(II), alkali and alkaline earth metal picrates (5). The calixarene-based bis(dithioamide) 2, which is restricted to the 1,3alternate conformation, possesses high selectivity for Cd(II) over Pb(II), Cu(II), Ca(II), and K(I) cations in plasticized PVC membranes of chemically modified field effect transistors (6). For another Cd(II) sensor based upon calix[4]arene tetrathioamide 3 which is fixed in the cone conformation, Cu(II) interference is stronger (7). Ionophores 4 with four pendent dithiocarbamate moieties exhibit highly efficient sol-

© 2000 American Chemical Society

In Calixarenes for Separations; Lumetta, Gregg J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

125


126

10 R = C H S M e 2

Figure 1. Structures of calix[4]arenes with soft sulfur-containing donor groups on the lower rim utilized in soft heavy metal ion separations.



127 vent extraction of Au(III) and Pd(II) with somewhat lower affinity for Hg(II) and Ag(I) (8). In contrast, structurally related calixarenes 4 are ineffective binders of soft Cd(II), Pt(II), and Pt(IV) ions, as well as harder Pb(II), Sn(II), Fe(II), Co(II), Νί(Π), Zn(II), and Mn(II) cations. Calix[4]arene 5 with four thiol groups on the lower rim shows a moderate extracting ability toward the soft metal ions of Ag(I), Au(III), Hg(II), and Pd(II). For the related tetrathioether 6a, highly efficient Au(III) separation, moderate Ag(I) and Pd(II) extraction and very weak Hg(II) binding are observed (8). Ligand 6b which does not contain t-Bu groups on the upper rim exhibits only low differentiation among Ag(I), Au(III), Hg(II), and Pd(II) (8). Ionophores 7-9 that each possess four pendent thioether groups on the lower rim were utilized in silver ion-selective electrodes (9). Calixarenes 7 and 8 are restricted to the partial cone (paco) conformation and exhibit excellent selectivity for Ag(I) over a wide range of other metal ions, including Na(I), Hg(II), and Pb(II). Paco 8 provides more selective Ag(I) recognition than the cone isomer of the related calixarene 9. Ligand 10 ( 7) is another example of a calixarene-based sensor for Ag(I). Calix[4]arenes tunctionalized on the lower rim with harder oxygen-containing donor groups, e. g., amide, phosphine oxide, ester, or ketone (Figure 2), also have been explored in separations of soft (mostly precious) metal ions. Thus, tetraamide 11 efficiently extracts Au(III) and Ag(I) ions from acidic nitrate solutions with discrimination over Pd(II) and Pt(II) (10). Tetraamide ligands 12 provide high levels of Ag(I) and Cd(II) picrate extraction from neutral aqueous solutions (5), but show poor selectivity for these metal salts over Na(I), K(I), Mg(II), Ba(II), and Pb(II) picrates. The calix[4]arene tetraketone 13 allows selective separation of Ag(I) present in small amounts from excess Pd(II) in highly acidic aqueous nitrate solution (77). Within the series of calix[4]arenes 14 which have various oxygen-containing donor groups, the ionophore with two pendent amide and two phosphine oxide groups appears to be the most efficient Ag(I) extractant and allows removal of trace amounts of Ag(I) in the presence of a large excess of Cu(II) ( 12). Calixarenes 15 with four u-coordinating allyl groups on the lower rim were applied as carriers in silver ion sensors (13). Ionophore 15a exhibits high Ag(I) selectivity while the ester group-containing analogue 15b provides only poor Ag(I) recognition with severe Na(I) interference. To the best of our knowledge, no report of Hg(II), Cd(II), Pd(II) or Pt(II)selective calix[4]arenes functionalized on the lower rim with oxygen-containing donor groups has been published. Recently, we prepared the series of calix[4]arene derivatives 16-19 (Figure 3) which contains on the lower rim two proton-ionizable iV-X-sulfonyl carboxamide groups of "tunable" acidity (14) (see also the chapter of this monograph entitled "Calix[4]arenes with a Novel Proton-ionizable Group: Synthesis and Metal Ion Separations"). These ionophores were found to efficiently extract Pb(II) from acidic aqueous nitrate solutions with good-to-excellent selectivity over many transition, alkali, and alkaline earth metal cations. However, the soft heavy metal ions of Ag(I), Pd(II) and especially Hg(II) produced significant interference with Pb(II) extraction by the calix[4]arene N-X-sulfonyl carboxamides. This unexpected favoring of soft metal cations over harder metal ions by ligands containing hard donor groups encouraged us to investigate the solvent extraction of Ag(I), Cd(II), Hg(II), Pd(II), and Pt(II) by ionophores 16-19.


128


R

11 R = C(CH )2C(CH3)3;X = NEt2 3

t

12 R = B u ; X = N E t , 2

13 R = C(CH )2C(CH )3; X = M e 3

3

14 R = CH C(0)NEt2, C H P ( 0 ) P h , CH C0 EtorMe 2

2

2

2

2

15 Ε 15a C H = C H 15b C 0 2 C H C H = C H 2

2

2

Figure 2. Structures of calix[4]arènes with hard oxygen-containing donor groups on the lower rim utilized in soft heavy metal ion separations.


129


OMe

OCH C(0)NHS02X 2

X 16 C F 17 M e 18 Ph 19 C é H ^ O ^ 3

Figure 3. Structures of calix[4]arene di(N-X-sulfonyl carboxamides).

Separations of Soft Heavy Metal Ions with Calix[4]arene iV-X-Sulfonyl Carboxamides Solvent Extraction of Silver(I) Extractions of Ag(I) from acidic and neutral aqueous nitrate solutions into chloroform by calix[4]arene di(iV-X-sulfonyl carboxamides) 16-19 (75) showed that their propensity for Ag(I) binding is controlled by their acidities in accordance with the electron-withdrawing ability of the substituents X, e.g., 16 (CF ) » 19 (CeHLiNQz4) > 18 (Ph) > 17 (Me). A similar trend was noted for solvent extractions of Pb(II) (14) and of alkali and alkaline earth metal cations by 16-19. Ligand 16 efficiently separates Ag(I) from dilute nitric acid (pH < 3), while for the weaker NH-acids 17-19, a higher pH is required. Calixarene 16 was found to interact with Ag(I) in a water-chloroform extraction system to yield complexes with a 2:1 metal-to-ligand stoichiometry, as described by following equation: 3

2A g

+ aq

+ HLorg = Ag Loig + 2H 2

2

+ aq

where H L is the di(proton-ionizable) ligand 16 and the subscripts aq and org denote species in the aqueous and organic phases, respectively. The log of the equilibrium constant for this reaction (e. g., the extraction constant) is 2.81. To probe for the interference of Ag(I) extraction by other metal cations, particularly Na(I), Cu(II), Pb(II), Pd(II), Hg(II), and ΊΊ(Ι), competitive extractions of Ag(I) from aqueous equimolar mixtures of two metal nitrates at pH 2.5 into chloroform by 16 was studied. Relative to the single species Ag(I) extraction under otherwise identical conditions, the level of Ag(I) extraction was unaffected by the presence of Na(I), Cu(II), Pb(II), and Pd(II), while with Hg(H) and T1(I) it decreased by 20-25%. Thus, calix[4]arene mXA^-ti^uoromemylsulfonyl carboxamide) 16 was 2


130 shown to efficiently and selectively separate Ag(I) from aqueous acidic nitrate solutions.


Extraction of Palladium(II) and Platinum(II) Calixarenes 16-19 were also tested in single species extractions of Pd(II) and Pt(II) nitrates from acidic and neutral aqueous solutions. These ionophores were found to be much less efficient complexants for Pd(II) than for Ag(I). Only the most acidic ligand 16 was found to be capable of extracting Pd(II) at a moderate level into chloroform from aqueous solutions with pH < 6. Under otherwise identical conditions, the Pd(II) loading of 16 was about three times lower than that of Ag(I). The percentage of Pd(II) extraction by 16 further decreased when the aqueous phase anion was changed from nitrate to chloride. This was attributed to formation in the aqueous phase of the anionic complex species PdCfe" and PdCl/". None of the calix[4]arene di(N-X-stilfonyl carboxamides) 16-19 gave significant extraction of Pt(II) from aqueous solutions into chloroform.

Extraction of Hg(IT) and Cd(II) Although calixarenes 16-19 were found to be unable to extract Cd(II) from aqueous nitrate solutions into chloroform, all of them exhibited strong affinity for Hg(II) ions (75). The pH profiles for extractions of mercuric nitrate from aqueous solutions into chloroform by 16-19 (Figure 4) demonstrate that the calix[4]arene di(N-

Figure 4. pH profiles for Hg(II) extraction from aqueous 0.50 mM mercuric nitrate solution into the equal volume of chloroform by 0.50 mM calix[4]arene di(N-Xsulfonyl carboxamides) 16 (Φ), 17 (O), 18 (M), and 19 (Π).


131 X-sulfonyl carboxamides) provide efficient Hg(II) separation from acidic aqueous media. Unlike the extractions of soft Ag(I) and Pd(II) described above and those of harder metal ions reported earlier, Hg(II) loadings of these ligands vary only slightly when the identity of the sulfonyl group substituent X is changed. Moreover, the propensities of this calixarene series for Hg(II) extraction decrease in the order: 16 (CF ) > 17 (Me) > 18 (Ph) > 19 (C6H4NO2-4). This ordering differs appreciably from that observed for other metal ions and indicates that the size of X, rather than its electron-withdrawing ability, is important for Hg(II) extraction Due to this unique feature of the calix[4]arene di(/V-X-sulfonyl carboxamides), the lower acidity ligands 17-19 provide extremely selective Hg(II) separation from acidic aqueous solutions since they completely extract Hg(II) at pH < 4 with negligible loadings of other metal cations, e.g., alkali, alkaline earth, and many transition and heavy metal ions, including Ag(I), Cd(II), Cu(H), Pd(II), and Pt(II). To the best of our knowledge, compounds 16-19 are the first representatives of mercuryselective calixarene-type ionophores which contain no soft donor functions. Calixarenes 16-19 have been found to interact with Hg(II) ions forming complexes of either 1:1 or 2:1 metal-to-ligand stoichiometry. In particular, the complexation reaction in an extraction system with an excess of Hg(II) over the iono phore proceeds in accordance with the following equation:


3

2Hg

2+ aq

+ Η Ι ^ + 4N03~aq = [(HgNC^LJarg + 2H 2

+ aq

Evaluated extraction constants for such complexes exceed 13.5 log units and decrease in the order: 16 > 17 > 18 > 19. To provide insight into the mode of Hg(II) coordination with the calix[4]arene di(AA-X-sulfonyl carboxamides), U V spectra of 16-19 in chloroform solutions before and after mercuric nitrate extraction were studied (75). Hg(II) coordination caused changes in the spectra of all four of the calixarenes. The absorption bands for the substituted benzene rings of the ligands at 270-279 nm showed hypsochromic shifts of 17-23 nm in the spectra measured after Hg(II) extractioa Analogous spectral changes were not observed after lead nitrate extraction or after contact of the calixarene solutions in chloroform with 1.0 M aqueous NaOH. The results presented above suggest a significant contribution of the π-electronrich aromatic units in 16-19 to the coordination of Hg(II). Although π-cation interactions were previously observed in some of the calixarene complexes with soft heavy metal ions (see references 2 and 16 for examples), they have not been advocated before in the Hg(II) complexes formed by calixarene-based ionophores utilized for Hg(II) separations. Evidently in coordination of Hg(II) by calix[4]arenes functionalized in the lower rim with soft sulfur-containing donor groups for which strong sulfur-mercury(II) bonding plays a dominant role, the weaker interaction of the π-donor aromatic units with the metal cation is unimportant However for calixarenes that do not contain soft donor groups, participation of the π-electron-rich calixarene cavity in the coordination arrangement for Hg(II) ion may increase dramatically.


132


A Fluorogenic Calix[4]arene JV-X-Sulfonyl Carboxamide for Selective Hg(IT) Recognition Optical chemosensors for the determination of heavy metal ions are receiving ever-increasing attention (17, 18). A convenient approach to the preparation of fluorogenic reagents for selective metal ion recognition consists of attaching fluorophore moieties to the platform of a macrocyclic or chelating complexant. This methodology was applied to obtain the recently reported optical sensors for the detection of Hg(II) (19-21). However, calixarene platforms have not been utilized previously in the synthesis of fluorogenic reagents for Hg(II) recognition. We incorporated a dansyl moiety as a part of the iV-X-sulfonyl carboxamide groups in calix[4]arene derivative 20 (Figure 5) and explored the properties of this ligand as a Hg(II) extractant (22). Bu

l

OMe

Bu

l

OCH C(0)NHS0 2

2

NMe

2

Figure 5. Structure of fluorogenic calix[4]arene di(N-X-sulfonyl carboxamide).

Similarly to its structural analogs 16-19, ionophore 20 efficiently extracted Hg(II) from acidic aqueous mercuric nitrate solutions into chloroform. As shown in Figure 6, Hg(II) uptake by 20 was accompanied by quenching of its fluorescence emission at 520 nm. The relative emission intensity l/lo (where Io and I are emission intensities observed in the spectrum of 20 before and after the Hg(II) extraction, respectively) diminished as the Hg(II) loading increased. With a 50.0 μΜ chloroform solution of the fluorogenic calixarene, the detection limit of Hg(II) in the aqueous phase at pH 2.5 was determined to be 5.00 μΜ. Ionophore 20 exhibited excellent extraction selectivity for Hg(II) over a wide variety of alkali, alkaline earth, transition, and heavy metal ions. Its fluorescence quenching due to the Hg(II) coordination was unaffected by the presence of 100-fold excesses of Na(I), Ag(I), T1(I), Ca(II), Cd(II), Co(II), Cu(H), Νΐ(Π), Pb(II), Pd(II), Zn(II), or Fe(III) (22). Ligand 20 is the first example of a calixarene-based fluorogenic sensor for selective Hg(II) recognition.



133

Ο

0.1

0.2

0.3

0.4

0.5

C (mM) Hg

Figure 6. Dependence of the mercury loading (%) and relative emission intensity (O) of fluorogenic calixarene 20 in a 50.0 μ M chloroform solution after Hg(II) extraction on the initial aqueous phase mercuric nitrate concentration (pH 2.5).

Acknowledgment This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG0394ER14416).

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