Calixarenes for Separations - American Chemical Society

Chapter 16

Metal Ion Complexation and Extraction Behavior of Some Acyclic Analogs oftert-Butyl-calix[4]areneHydroxamate Extractants 1

1

2

Timothy N. Lambert , Matthew D . Tallant , Gordon D . Jarvinen , and Aravamudan S. Gopalan Downloaded by PENNSYLVANIA STATE UNIV on June 29, 2012 | http://pubs.acs.org Publication Date: July 20, 2000 | doi: 10.1021/bk-2000-0757.ch016

1,3

1

Department of Chemistry and Biochemistry, New Mexico State University, LasCruces, NM 88003-8001 Los Alamos National Laboratory, Los Alamos,NM87545 2

Recently, we reported the preparation of two new calix[4]arene based hydroxamate extractants 8 and 9 designed for the selective complexation of actinide(IV) ions and some results of their metal ion extraction studies; however, these ligand systems did not achieve the preferential extraction of the An(IV) over Fe(III). In order to understand the complexation behavior of 8, we have prepared the related acyclic tetrahydroxamate 10, trihydroxamate 11, as well as the monohydroxamate 12 and examined their complexation/extraction of Th(IV) and Fe(III) cations into chloroform from aqueous nitrate solutions. Our results show that the trihydroxamate 11 is a selective extractant of Th(IV) over Fe(III) at p H 1-2 and in fact shows greater promise than the tetrahydroxamate 10. Our results also suggest that not all four hydroxamate moieties of 10 are involved in the actinide complexation process in thepHrange of study.

A variety of separation technologies and processes are currently being explored for the remediation of high level and transuranic radioactive waste (7,2). In connection with these efforts, there is an urgent need to develop cost effective and efficient chelating and extracting agents to selectively remove actinides, such as plutonium and americium, from waste streams as well as from contaminated soil and water (3-7). To design chelators that have the requisite properties of selectivity and high binding constants for a particular metal ion one must consider its charge, size and specific coordination chemistry (8). Because of their larger size, actinide ions Corresponding author (E-mail: [email protected]).

208

© 2000 American Chemical Society

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

Downloaded by PENNSYLVANIA STATE UNIV on June 29, 2012 | http://pubs.acs.org Publication Date: July 20, 2000 | doi: 10.1021/bk-2000-0757.ch016

209 typically have a higher coordination number (eight or more) and a more flexible ligand geometry (dodecahedral, square antiprism). It is generally accepted that selective actinide complexation can be achieved by taking advantage of their larger coordination sphere relative to the smaller transition metals (9-12). Raymond and others have exploited the similarity in the coordination chemistry of Pu(IV) and Fe(III) to develop a number of cyclic and acyclic chelators for the binding of plutonium, some of which are potentially useful for in vivo biodecorporation of this metal ion (13-16). Since the Pu(IV) ion is a hard Lewis acid, the preferred ligands that have been incorporated into these synthetic chelators have been catecholates, hydroxypyridinonates and hydroxamates (17,18). A number of reports have appeared on the properties and applications of calix[n]arenes, a unique class of molecules (19-27). Calixarenes that are immobilized in the cone conformation present an ideal platform for the introduction of various ligand groups onto the same face of the molecule and hence for the construction of selective extractants for actinides (28,29). The use of calixarenes for the complexation of a variety of metal ions including some transition and f-block elements has been reported (30-32). Shinkai and coworkers have developed some calixarene based uranophiles (having carboxylate groups appended to the lower rim) which exhibit remarkable selectivity for the linear uranyl ( U 0 2 ) ion, whose coordination geometry (planar, penta or hexacoordinate) is quite different from that of the An(IV) cations (33-35). They have also synthesized similar p-teri-butylcalix[n]arene derivatives with hydroxamate groups and examined their ability to extract uranium and other transition metal ions from aqueous solution (36,37). Some of the calixarene based extractants for the tetra- and tri valent actinides that have been reported are shown in Figure 1. The calixarene derivatives 1 having groups analogous to C M P O (octyl(phenyl)N,N-diisobutylcarbamoyl methylphosphine oxide) appended to the upper rim have been found to be better extractants for actinides than C M P O itself (38). More recently, it has been shown that calix[4]arenes of this type show selectivity for the trivalent light lanthanides and trivalent actinides in their extraction from highly saline (4 M NaN03) or acidic media (3 Μ H N 0 ) into chloroform (39). Some resorcin[4]arene cavitands of the type 2 have also been examined for the selective extraction of Eu(III) from acidic/saline media into dichloromethane (40-42). Another novel class of calixarene derivatives with phosphine oxide groups 3 attached to the lower rim has been synthesized and shown to have high efficiency in the extraction of Th(IY) and Pu(IV) from simulated nuclear waste (43). The synthesis of a lower rim functionalized C M P O derivative 4, as well as a tetraiminocarboxylate calix[4]arene derivative 5, with some preliminary actinide extraction studies has recently been published (44,45). In addition, the synthesis of some novel 3-hydroxy-2-pyridinone (3,2-HOPO) derivatives of A-tertbutylcalix[4]arenes, 6 and 7, has recenty been disclosed from our laboratory (46). These 3,2-HOPO derivatives show considerable selectivity for the preferential extraction of the Th(IY) ion over Fe(III) under acidic conditions. 2 +

3

The overall goal of our research program is to develop organic chelators capable of the specific binding/removal of actinides, such as Pu(IY), from process waste streams in the presence of more abundant and competing metal ions such as Fe(III), Al(III), alkali and alkaline earth metal ions (47,48). Recently, we reported the preparation of two new calix[4]arene based hydroxamate extractants 8 and 9,



210

Figure J. Calixarene based extractants reported for actinide (III or IV) ions



211 Figure 2, designed for the selective complexation of An(IV) ions and some results of their metal ion extraction studies (49). The primary hydroxamate 8 was the more efficient extractant of Th(IV) at p H 2 removing greater than 90% of this metal ion from the aqueous phase in comparison to approximately 20% for the secondary hydroxamate 9 under identical conditions. However, in competitive studies, hydroxamates 8 and 9 were found to be selective for the extraction of Fe(III) (83 and 98%) over Th(IV) (24 and 3%) at p H 2 from an aqueous 0.10 M NaCl solution. In order to ascertain the importance of the calix[4]arene backbone in selective metal ion binding, and to develop a more systematic understanding of the actinide chelation/extraction properties of this class of ligands, we have now prepared the related acyclic tetrahydroxamate 10, trihydroxamate 11, as well as the monohydroxamate 12, Figure 2. Their complexation/extraction behavior with Th(IV), Fe(III), Eu(III) and Cu(II) has been examined using liquid-liquid extraction and spectrophotometric studies.

RESULTS

A N D DISCUSSION

Syntheses The syntheses of ligands 10-12 were achieved in good yields via synthetic routes analogous to that previously reported for calixarene 8 (49). The synthetic route for the preparation of the acyclic tetrahydroxamate 10 is given in Scheme 1 (47% overall yield).

Single Metal Ion Extraction Studies The ability of chelators 8 and 10-12 to extract Th(IV), Fe(III), Eu(III) and Cu(II) ions from aqueous NaN03 (0.10 M ) at p H 1 and 2 into chloroform has been examined and the results are presented in Table I. The protocol for the metal ion extraction experiments followed procedures described earlier (46). Thorium(IV) and Eu(III) were chosen for these studies, as they are surrogates for Pu(IV) and Am(III) present in radioactive waste streams. A solution of 0.10 M N a N 0 3 / l % HNO3 was adjusted to pH 1 or 2 with concentrated aqueous NaOH, followed by addition of the desired concentration of the metal ion(s) of interest. Equal volumes (4 mL:4 mL) of the aqueous solution containing the metal ion(s) at the specified p H and chloroform containing excess of the ligand (four to sixteen-fold excess, depending on the ligand) were contacted for 2 h at ambient temperature with gentle shaking. In order to have a valid comparison, the molar concentrations of the ligands were adjusted so as to provide the same equivalents (4 mM) of the hydroxamate binding units in each case. The layers were separated carefully by centrifugation and the concentrations of the metal ions in the aqueous layers were determined by ICP analysis. The percent metal ion extracted by the ligands could then be determined. In the case of calix[4]arene tetrahydroxamate 8, a significant amount of precipitation occurred in these extraction experiments (much more at pH 1 compared to p H 2). This precipitation led to results with poor reproducibility and obviously prevents the reliable comparison of calixarene



Figure 2. Calix[4]arene tetrahydroxamates and acyclic analogs


213


HQ,

HQ

Ç=0

OH

-

OH

OH

γ

2

4

= 0

2

K C 0 , CH3CN, reflux, 79% 2

3

0

0

0

/-Bu

f-Bu

(j \ ^

/-Bu

/-Bu

Ç

l.I(CH ) C0 Et,

. KOH, EtOH, rt, HC1, 91%

/-Bu

HQ^

C=0

/-Bu

n= 2

n=2

1. ( C O C D 2 , reflux

2. NH OBn-HCl, C H C 1 , pyridine, rt, 70% 2

HOHN

x

HOHN

Ç=o

N

BnOHN

HOHR

c=o

N

BnOHN

C=Q

c-o

2

x

2

BnOHN^

c =o

Ç

5% Pd/C, H , EtOH, rt, 94% 2

/-Bu ): =2 n

/-Bu

η = 2, Bn = C H P h 2

Scheme 1. Synthetic route to tetrahydroxamate extradant 10


= 0


214 8 to extractants 10-12 at p H 1, and to a certain extent at p H 2. Hence, the discussion that follows focuses to a large extent on the acyclic analogs 10 and 11, and the monohydroxamate 12. As seen in Table I, at p H 1 the acyclic tetrahydroxamate 10 and trihydroxamate 11 are very effective for the extraction of Th(IV) with 99% removal. The monohydroxamate 12 only extracts 13% under these conditions. A t p H 2, all the hydroxamate ligands are highly efficient in the extraction of Th(IV) into chloroform with greater than 99% of the available metal being removed. The extraction capabilities of these ligands for Fe(III) are quite different than that found for Th(IV) and show a strong p H dependence, Table I. Surprisingly, in the presence of excess ligand concentration, the monohydroxamate 12 is the best extractant for Fe(III) with 64% removal at p H 1. In contrast, the tetrahydroxamate 10 and trihydroxamate 11 were less efficient (30 and 33% respectively). A t p H 2, the efficiency for the removal of Fe(III) for all ligands increases. For example, the tetrahydroxamate 10 and trihydroxamate 11 are able to extract 83% and 85% respectively while the monohydroxamate 12 still remains the most effective extractant (92%). The ability of these ligands to extract Eu(III) and Cu(II) into chloroform under similar conditions (0.10 M NaNC>3, p H 2) was also examined. None of the ligands extracted Eu(III) or Cu(II) to any appreciable amount under these conditions. In fact, the extraction of Eu(III) was less than 2% while the removal of the cupric ion was less than 3% in all cases.

Table I. Th(IV) and Fe(III) Single Metal Extraction Studies - % Extracted by Hydroxamate Ligands* Initial Th(IV) Fe(III) 7

1* 2

a

C

899

>99

85

83

85

92

>99

into chloroform from 0.10 M NaN0 [8] = 1.00 mM, [10] = 1.00 mM, [11] =1.33 mM, [12] = 4.00 mM; [Th ] = 0.23 mM, [Fe ] = 0.23 mM; [Th *] = 0.22 mM, [Fe ] = 0.18 mM. precipitation occurred with this ligand at pH 1. Precipitation was less obvious but still present at pH 2. 3)

b

4+

3+

c

4

d

Competitive Extraction Studies - Th(IV) vs. Fe(III) Clearly the selective extraction of Th(IV) or Fe(III) in the presence of trivalent lanthanides or divalent copper can be achieved using any of the hydroxamate ligands 10-12 at p H 2 or lower; however, it was more pertinent to our goals to ascertain whether the selective extraction of Th(IV) could be achieved in the presence of Fe(III)



215 under these conditions. These results prompted us to conduct competitive extraction studies with these two metal ions in order to more accurately evaluate the potential of these new ligands to serve as actinide selective extractants. A t p H 1, when an aqueous mixture of equimolar quantities (0.22 mM) of Th(IV) and Fe(III) was contacted with a slight molar excess of the ligand of interest (all balanced to 1 m M total hydroxamate) in chloroform, both the tetrahydroxamate 10 and trihydroxamate 11 were found to selectively extract Th(IV) (53 and 75% respectively) over Fe(III) (18 and 28% respectively), Table II. The extraction efficiency of the monohydroxamate 12 drops significantly under these conditions but it appears to extract slightly more Fe(III). Again, significant precipitation of the calix[4]arene tetrahydroxamate 8 occurred at this p H preventing its reliable comparison to the other ligands.

Table II.

Ligand

Competitive Extraction Studies at pH 1 - Th(IV) vs. Fe(III) % Extracted by Hydroxamate Ligands* %E %E D D S Th

Fe

10

53

18

1.1

0.22

5.0

11

75

28

3.0

0.39

7.7

12

16

20

0.18

0.25

0.72

η+

D = (Σ[Μ ] /Σ[Μ ] ), S ο

w

a/D 4+

Th

Fe

Th/Fe

= D /D , %E = [D/(D+1)J xl00%, into chloroform a

D 3+

from 0.10 M NaNCK [Th ]=0.22 mM, [Fe ]=0.22 mM, [10] = 0.25 mM, [111 = 0.33 mM, [12] = 1.00 mM Competitive metal ion extraction studies conducted at p H 2 gave similar results, Table III. The tetrahydroxamate 10 and trihydroxamate 11 were still found to selectively extract Th(IV) (56 and 83% respectively) over Fe(III) (40 and 44% respectively). A t p H 2, one can see that the monohydroxamate 12 indeed has the opposite selectivity with 60% of the Fe(III) being removed in contrast to 31% for the removal of Th(IV). A t p H 2, the calixarene 8 also appears to be selective for Fe(III) extraction, consistent with earlier results (49), although the numbers must be regarded with some caution due to the observed precipitation. It was surprising to find that the trihydroxamate 11 was an efficient and more selective extractant for Th(IV) over Fe(III) than the corresponding tetrahydroxamate 10 ( Th/Fe = 5.9 vs. Sxh/Fe = 1-9, respectively at p H 2). This suggested that Th(IV) was being extracted as a trihydroxamate species by both these ligands, with the trihydroxamate 11 forming a more stable actinide complex. To investigate this further, a competitive study between these two metal ions with equal molar concentrations (0.25 m M ) of 10 and 11 was performed, Table I V . In this experiment, the concentration of hydroxamate groups available for complexation was higher for tetrahydroxamate 10 than trihydroxamate 11 (1.00 m M and 0.75 m M respectively). The extraction behavior of 10 and 11 was quite similar under these s


216 conditions once again suggesting that the fourth hydroxamate arm of 10 provided no advantage in enhancement of the actinide extraction process.

Table III.


Ligand

Competitive Extraction Studies at p H 2 - Th(IV) vs. Fe(III) % Extracted by Hydroxamate Ligands* S D D %E %E Th/Fe

Fe

Th

Th

Fe

8*

37

51

0.58

1.0

0.58

10

56

40

1.3

0.67

1.9

11

83

44

4.7

0.80

5.9

12

31

60

0.45

1.5

0.30

D = (Σ[Μ ] /Σ[Μ ] ), S ο

w

= D ΙΌ , %Ε = [D/(D+1)1 χ100%, into chloroform

a/b

a

b

4+

3+

from 0.10 M NaNO . [Th ]=0.17 mM, [Fe ]=0.18 mM, [81 = 0.25 mM, [10] = 0.25 mM, [11] = 0.33 mM, [12] = 1.00 mM; some precipitation observed b

Table IV. Competitive Extraction Studies at pH 2 - Th(IV) vs. Fe(III) (Equimolar Extractant Concentration) % Extracted by Hydroxamate Ligands S D D Ligand %E %E g

a

Fe

Th

Th/Fe

Th

Fe

10

54

48

1.2

0.93

1.3

11

57

47

1.3

0.88

1.5

η+

η+

D = (Σ[Μ ] /Σ[Μ ] ), S o

w

= D /D , %E = [D/(D+1)] xl00%, into chloroform

a/b

a

b

from 0.10 M NaNCK [Th ]=0.25 mM, [Fe ]=0.24 mM, [10] = 0.25 mM, 4+

3+

[11] = 0.25 mM One would have predicted 10 to be a better extractant for Th(IV) than 11. The tetrahydroxamate 10 was expected to form a strong neutral 1:1 M L tetrahydroxamato complex with Th(IV) in the extraction process by loss of four protons, one from each of the ligand moieties. If extractant 10 only uses three of its hydroxamate groups in the coordination of the actinide(IV) ion to give a M L species, either dissociation of a non participating hydroxamate group leading to a neutral complex or co-extraction of an anion, such as nitrate, must occur in order to maintain electrical neutrality. The 1:1 Th-11 complex, on the other hand, cannot be neutral and an anion must be transported into the organic layer to ensure charge neutrality, making this a less


217 favorable process. One would also predict that both 10 and 11 would have similar effectiveness for the extraction of Fe(III) and the results are consistent with this expectation.


Ligand Molar Variation Studies Ligand molar variation studies were performed to understand the surprising extraction preference of the trihydroxamate 11 for the Th(IV) cation, Figure 3. The intent was to determine the stoichiometry of the extracted species for Th(IV) and Fe(III) and to establish the extent of hydroxamate coordination/participation in the ligand-metal complex for extractant 11. A t p H 2, traditional breakthrough curves, Figures 3a and 3b, for the trihydroxamate 11 with Fe(III) and Th(IV) both gave saturation break-points of approximately 1.4 - 1.5 suggesting the formation of a neutral (M2L-$) extracted species for both metals. A log-log plot analysis of the trihydroxamate 11 with Fe(III), Figure 3c, also showed that the extracted species appeared to be a ( M 2 L 3 ) species (m = 1.5, r = 0.992). On the other hand, the corresponding plot for Th(IV), Figure 3d, does not show a linear relationship suggesting that at least two complexes of different stoichiometries are involved in the extraction process. N

2

n

If 11 extracts Fe(III) at p H 2 as a neutral trishydroxamato ( M 2 L 3 ) species, this would indicate that only two of the hydroxamic acids per ligand molecule are complexing with the cations while the third remains undissociated. Such a preference may be due to geometric constraints of the ligand or due to the high pK 's (8-10) of the hydroxamic acids(50). For the complexation of Th(IV) it appears that on average, three hydroxamic acid groups in each ligand molecule are involved in the binding of the cations. n

a

Spectroscopic

Analyses

3+

As Fe -hydroxamate complexes are colored and well characterized in the literature (51) the nature of the extracted Fe -ligand complexes from these studies were investigated using U V - V I S spectroscopy, Figure 4 (52). For each of the ligands studied, the predominant complex in chloroform at p H 1 and 2 that was observed from the Fe(III) extraction studies (Table I) appeared to be an iron-trishydroxamato species with λ ~430nm, Figure 4a. U V - V I S spectra of the chloroform layers from the ligand variation study of 11 with Fe(III) also support the formation of a trishydroxamato complex, even at extremely low ligand concentrations, Figure 4b. Based on literature precedents, we propose this species to be a neutral ferrictrishydroxamato complex. The preference to extract neutral complexes has been noted in the extraction of the uranyl ion by calix[n]arenes (36). In general, such behavior is anticipated for the extraction of a metal ion into a hydrophobic layer by lipophillic acidic chelating agents (53). One would also expect Th(IV) to be extracted as a neutral complex with these ligands. Unfortunately, Th(IV)-hydroxamato complexes are not colored and therefore cannot be examined using U V - V I S spectroscopy. 3+

Ι Γ 1 & χ


218

100-1 80H


£

6oH

* 40H

20H 04 Τ

1

1

0

1

I

1

2

1

1

I

1

1

1

I

1

1

4

1

ι ι I I I I I I I I I I j I I I

I

6

8

-4.0

-3.0 ^^111

2.5-

100H

• • •• •

d)

2.0 1.5-:

1 .0-^ 0.5-

m

0.0 -o.5-4a 2 4 [i i]/[Th i

6

ι ; ι ι ι ι ι ι ι ι ι 4.0

-3.6

4+

log

3+

3+

-3.2

[113

4+

4+

Figure 3. Plots of a) %Fe extracted vs. [ll]/[Fe ] b) %Th vs. [ll]/[Th ] c) log Df 3+ vs. log [11] d) log Oj 4+ vs. log [11]; [Fe ] = 0.19 mM, [Th ] = 0.20 mM, pH 2, 0.10 Μ NaN0 ; where m = slope. 3+

e

n

4+

3



Figure 4. UV-VIS spectra in CHClj of the extracted species of a) Fe(III) complexes pH 2 (Table I) and b) ll-Fe(III) complexes from ligand molar variation study


220


CONCLUSIONS In contrast to the teri-butylcalix[4]arene tetrahydroxamate 8, the acyclic tetrahydroxamate 10 and trihydroxamate 11 preferentially extracted Th(IV) over Fe(III) in the p H range of 1-2. This suggests that the calix[4]arene platform may not be vital to achieve selective complexation of the larger actinide cation in these systems. Furthermore, the trihydroxamate 11 was surprisingly a more efficient extradant than tetrahydroxamate 10 for the actinide ion. This indicates that all four hydroxamate groups of 10 do not bind to the same actinide ion to impart a greater extraction efficiency with this ligand. It is important to point out that in contrast to 10 and 11, the monohydroxamate 12 extracted Fe(III) preferentially in competitive studies and was the most efficient extractant of Fe(III) at p H 1 and 2. Given this fact, it is not clear why the monomer is a much less efficient extractant of Th(IV) at p H 1. For the extraction of Fe(III) with all of these ligand systems, spectroscopic studies clearly indicate that the iron-trishydroxamato species (kmax ~ 430nm) is dominant in the organic layer under a wide range of conditions. Given that the pK^s °f hydroxamic acids are usually in the range of 8-10, it may be difficult to ensure full ligand participation in actinide binding with tetrahydroxamate systems at this p H range. Also, one cannot ignore the possibility of geometric constraints that inhibit the formation of a 1:1 complex and hence lead to formation of aggregates, such as ( M 2 L 3 ) , to achieve efficiency in the extraction process. Of course, this makes it difficult to predict the metal ion selectivity in the extraction processes involving these hydroxamate ligand systems. Further studies are necessary i f we hope to understand the complexation and stoichiometry of extraction of these fascinating metal-ligand systems. n

ACKNOWLEDGMENTS Portions of this work were supported by the Waste-Management Education and Research Consortium of New Mexico. Dr. Hollie Jacobs (NMSU) is thanked for helpful discussions. Dr. Gary Rayson (NMSU), M r . Patrick Williams (NMSU) and the N M S U - S W A T lab are thanked for their assistance in metal ion analyses.

LITERATURE CITED 1.

Proceedings of the First Hanford Separation Science Workshop, PNL-SA-21775, Pacific Northwest Laboratory, Richland, W A , July 23-25, 1991. 2. U. S. Department of Energy Office of Environmental Management Technology Development, Efficient Separations and Processing Crosscutting ProgramTechnology Summary, DOE/EM-0249, June 1995. 3. Gopalan, Α.; Huber, V.; Jacobs, H . In Waste Management from Risk to Remediation; Bhada, R., Ed.; ECM Press: Albuquerque, NM, 1994, pp 227-246. 4. Cecille, L . ; Casarci, M.; Pietrelli, L. New Separation Chemistry Techniques for Radioactive Waste and Other Specific Applications; Elsevier: London, 1991.


221 5.

6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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Calixarenes for Separations - American Chemical Society

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