Phosphorus-Containing Macrocyclic Ionophores in Metal Ion

Sep 14, 2000 - Phosphorus-Containing Macrocyclic Ionophores in Metal Ion Separations. Galina G. Talanova. Department of Chemistry and Biochemistry, Te...
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Ind. Eng. Chem. Res. 2000, 39, 3550-3565

Phosphorus-Containing Macrocyclic Ionophores in Metal Ion Separations Galina G. Talanova1 Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061

Application of phosphorus-containing macrocyclic compounds, i.e., crown ethers, lariat ethers, calixarenes, and resorcinarenes, in the separation of metal ions by solvent extraction, membrane transport, and sorption is discussed in this paper. I. Introduction Utilization of macrocyclic ligands for the determination and separation of metal ions is receiving the everincreasing attention of researchers.2 Ionophores with phosphorus-containing donor groups (phosphine oxide, phosphonate, phosphate, etc.) immobilized on the macrocyclic platform are of particular interest because the metal-binding properties of such ligands may integrate the selectivity of macrocycles and the efficiency of organophosphorus separation agents.3 During the last two decades, several review papers on the synthesis of phosphorus-containing crown ethers (CEs), lariat ethers (LEs), and cryptands were published (for example, see ref 4). The very recent surveys5 were dedicated to the preparation of phosphorus-containing calixarenes. In most of these papers,4b,c,5a,b complexation in solutions, solvent extraction, and membrane transport of metal ions with use of particular types of ionophores was outlined as well. However, none of the previously published review papers focused on the application of the organophosphorus macrocycles in metal ion interphase separations. This paper provides a comprehensive survey of the utilization of phosphorus-containing macrocyclic ligands (CEs, LEs, and calixarenes) in the separations of metal ions by solvent extraction, membrane transport (i.e., liquid membranes, poly(vinyl chloride) (PVC) membranes of ion-selective electrodes (ISE), and bilayer lipid membranes (BiLLM)), and sorption. Data on the separation of metal ions by thin-layer chromatography with employment of certain macrocycles are also included. It should be emphasized that, from a wide variety of the reported phosphorus-containing macrocyclic ligands, only hydrolysis-stable ones, in particular, with tetracoordinate phosphorus (e.g., phosphine oxides, phosphonates, phosphates, and their thio-analogues) may be applied in the interphase separation systems. Data on metal ion separations with phosphoruscontaining CEs/LEs and calixarenes are presented in sections III and IV, respectively. Section III, in its turn, is subdivided into subsections “Neutral CEs and LEs” and “Proton-Ionizable CEs and LEs”. Although studies in all three areas have been conducted worldwide, readers may notice some interesting trends. In particular, most of the works on the application of the phosphorus-containing neutral CE and LE ionophores were published by the scientists of the former USSR and other Eastern Europe countries; in the area of separations by proton-ionizable CEs and LEs, scientists of the U.S.A. and Japan made the main contributions, while

researchers of Western Europe were obvious leaders in the exploration of phosphorus-containing calixarenes as metal ion carriers. II. General Characterization of Metal Ion Binding by Macrocyclic Ionophores in the Interphase Separation Systems Complexation-based processes of solvent extraction, membrane transport, and sorption of metal ions consist of metal species transfer from an aqueous solution into an organic phase because of their binding with an ionophore. Interaction of a macrocyclic ligand L containing no proton-ionizable groups (neutral ionophore) with metal salt MXn in the two-phase system water-organic solvent or water-polymeric sorbent proceeds in accordance with eq 1

Mn+aq + nX-aq + mLorg h MLmXn org

(1)

where subscripts aq and org denote species in an aqueous solution and an organic phase, respectively. The thermodynamic stability of the complex formed in the two-phase system is determined by the equilibrium constant K found by eq 2

K ) [MLmXn org]/[Mn+aq][Lorg]m[X-aq]n

(2)

where [MLmXn org], [Lorg], [Mn+aq], and [X-aq] are the equilibrium concentrations of the macrocyclic complex and ligand in the organic phase and the metal cation and anion in the aqueous solution, respectively. Metal binding by a neutral ionophore proceeds via ligandcation donor-acceptor interactions. The process of the interface metal ion transfer with proton-ionizable macrocyclic ionophore HL (for simplicity, the ligand here is assumed to contain only one acidic group) is described by eq 3.

Mn+aq + nHLorg h MLn org + nH+aq

(3)

The thermodynamic stability of the complex (K′) is defined by eq 4.

K′ ) [MLn org][H+aq]n/[Mn+aq][HLorg]n

(4)

In contrast with neutral ligands, complex formation by proton-ionizable macrocycles includes both ligandmetal ion coordinate bonding and a stronger, electrostatic interaction between the anionic form of the ionophore (L-) and the metal cation. As is obvious from

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eqs 3 and 4, such a process is pH-dependent. In addition, with application of a proton-ionizable ionophore, a metal cation is extracted from the aqueous solution into the organic phase as a neutral, intracomplex compound MLn. Therefore, the metal ion interface transfer proceeds without participation of the aqueous phase anion X-, which may be advantageous for the separation process. Distribution of the metal ion between the aqueous solution and the organic phase is controlled by the value of K (K′). (In solvent extraction, the complex stability constant is called the extraction constant, Kex.) In particular, the more stable the macrocyclic complex, the greater the efficiency of the metal ion solvent extraction/ sorption will be. In contrast, efficient macrocycle-mediated transport of metal ions through a liquid membrane, which involves both the complexation process at the source aqueous solution-organic phase interface and decomplexation at the organic phase-receiving aqueous solution interface, requires an average stability of the metal ion-ionophore complex. In the earlier works, the greatest rates of CE-mediated membrane transport of alkali metal6 and silver7 ions were observed with the macrocyclic complex stability constants in solution6 and in the extraction system7 of 5-6 log units. Under the conditions of competitive metal ion extraction/membrane transport/sorption, selectivity of the macrocyclic ionophore in interphase metal ion separations is determined by the relative stability of the complexes formed by the ligand with the competing cations. III. Phosphorus-Containing CEs and LEs This section covers data on metal ion separations by CEs and LEs having either endocyclic phosphorus atoms or exocyclic phosphorus-containing moieties that are introduced in the benzo substituents of the CE or located in the side arms attached to the macrocycle. Based on the mechanism of metal ion binding, phosphorus-containing CEs and LEs may be subdivided into neutral and proton-ionizable ionophores. This subdivision was adopted for arranging the data in Tables 1 and 4, respectively. Structural formulas of the macrocyclic ligands are presented in Figures 1 and 2. III.1. Neutral CEs and LEs. One of the early reports of solvent extractions of alkali metal cations (AMC) with use of neutral CEs containing endocyclic phosphoryl groups (structures 1 and 2 in Figure 1 and Table 1) was published by Cram and co-workers. 8,9 They found ionophores 1a-c with smaller macrorings to be selective for K+. As shown in Table 2, stability (association) constants of the corresponding CE-KPic complexes varied as 1c > 1a > 1b. For the larger CE, 1d, extraction of AMC enhanced in the order Li+ < Na+ < K+ < Rb+ < Cs+ as the metal ion radius increased. Ligand 2 with two phosphoryl groups exhibited preference for Na+. Interestingly, this CE revealed also a relatively high affinity for Li+. However, the efficiency and selectivity of AMC extractions by ligands 1 and 2 was generally lower than that of the CEs containing no endocyclic phosphoryl groups. Evidently, PdO groups in 1 and 2 were oriented outside of the macrocycle cavity, which made impossible the cooperative interaction of all oxygen donors of the CE with a metal cation and thus decreased the complex stability. Probably, unfavorable direction of the endocyclic phosphonyl group relative the CE cavity affected the

Table 1. Neutral Phosphorus-Containing CEs and LEsa Utilized in Metal Ion Separations metal ionsb

ionophore 1a-d 2 3a 3b 3c

AMC AMC FeIII Na+, K+, Mg2+, Ca2+ Na+, K+, Mg2+, Ca2+ K+, Ag+ Ag+,

HgII,

PdII

3d

K+, Ag+

3e

K+, Ag+

3f

Ag+, HgII, PdII K+, Ag+

4 5a 5b 6 7a 7b

Ag+, HgII, PdII Mg2+, Ca2+ Mg2+, Ca2+ Ca2+ AMC Li+, Na+, K+ Li+, Na+, K+

7c 7d

Li+, Na+, K+ Li+, Na+, K+

8 9a-d 10 11a-f

PdII PdII Na+, K+, Cs+ Na+, K+, Cs+, Ca2+, Sr2+ AMC Na+, K+, Mg2+, Ca2+ Na+, K+, Cs+ Na+, K+ Na+, K+ AMC, Ca2+, Mg2+, Ag+ CuII, CoII, NiII, CdII AMC, Ca2+, Mg2+, Ag+, CuII, CoII, NiII, CdII AMC, Ca2+, Mg2+, Ag+, CuII, CoII, NiII, CdII AMC, Ca2+, Mg2+, Ag+, CuII, CoII, NiII, CdII

12a-c 13a 13b-h 13i 14 15a-g

16a-f 17a-c 18a-e

separation methodc

ref

extrn. (CHCl3) extrn. (CHCl3) BLM (DCE) BiLLM

8 8 10 12

BiLLM

12, 14

extrn. (DCE), BLM (DCE) extrn. (CHCl3) extrn. (DCE), BLM (DCE) extrn. (DCE), BLM (DCE) extrn. (CHCl3) extrn. (DCE), BLM (DCE) extrn. (CHCl3) extrn. (CHCl3) extrn. (CHCl3) BLM (CHCl3) ISE extrn. (CH2Cl2) extrn. (CH2Cl2), ISE extrn. (CH2Cl2) extrn. (CH2Cl2), ISE extrn. (CHCl3) extrn. (CHCl3) BLM (CHCl3) BLM (CHCl3)

7, 13 11 7 7 11 7 11 15 15 17 18 18 18 18 19 19 20 20

extrn. (CHCl3) BiLLM

21 14

BLM (CHCl3) BLM (CHCl3) sorption TLC

22 23 23 24

TLC

25

TLC

25

TLC

25

a For the structural formulas, see Figure 1. b AMC ) alkali metal cations. c BLM ) bulk liquid membrane transport, BiLLM ) bilayer lipid membrane transport, extrn. ) solvent extraction, ISE ) ion-selective electrodes, TLC ) thin-layer chromatography, DCE ) 1,2-dichloroethane.

ability of ionophores 3a and 3b to perform metal ion separations. Fourteen-membered CE 3a showed only moderate capability for transporting FeIII through a bulk liquid membrane (BLM)10 and was ineffective in solvent extractions of alkali, alkaline-earth, and transition metal ions11 and transport of metal ions through BiLLM.12 Similar extraction inactivity was observed for the 17-membered CE 3b with an adamantyl substituent on the phosphorus.11 However, this ligand increased the conductivity of BiLLM in the presence of Na+, K+, Mg2+, and Ca2+ and demonstrated appreciable K+/Na+ selectivity.12 Thiophosphonyl-containing CEs 3c-f performed in metal ion separations much better than their phosphonyl analogues. In particular, the presence of soft PdS donor groups in these ligands provided enhanced affin-

Figure 1. Structural formulas of neutral phosphorus-containing CEs and LEs listed in Table 1.

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Figure 2. Structural formulas of proton-ionizable phosphorus-containing CEs and LEs listed in Table 2.

ity for soft electron acceptors. 3c-f exhibited efficient extraction of AgPic into 1,2-dichloroethane (DCE) with extraction constants of 5-6 log units7 and average levels of extraction into CHCl3.11 For ligands 3c and 3e, a

moderate-to-weak extracting ability toward HgII and PdII was observed.11 Additionally, CEs 3c-f showed high rates of Ag+ transport through BLM6 (see Table 3) and only slight ionophoric activity for K+.7,13 On the

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Table 2. Solvent Extraction of Alkali and Alkaline Earth Metal Picrates by Neutral Phosphorus-Containing CEs and LEs single metal ion extraction

ionophore [ref] Li+ [8]a

1a 1b [8]a 1c [8]a 1d [8]a 2 [8]a 5a [15]b 5b [15]b 7a [18]c 7b [18]c 7c [18]c 7d [18]c 12a [21]a 12b [21]a 12c [21]a

25.6 10 38 41 80

Na+ 189 63.9 466 117 103

K+

Rb+

Cs+

815 233 3.1 × 103 961 50

109 64.6 461 1.04 × 103 15

32.3 18 882 1.9 × 103 7.5

Ca2+ Mg2+

16 32.5 68 13 25 21 33 136 320

36 20 34 50 620 1.16 × 106 8.3 × 103

2 5

25 31 51 30 12 4.5 2.75 8.39 × 105 1.95 × 104 2.52 × 103 5.8 × 105 1.1 × 106 2.32 × 104

a Association constants, K , calculated as K /K where K a ex d ex is the extraction constant and Kd is the distribution constant of the metal picrate between CHCl3 and H2O. b Extraction percentage: aqueous phase, 15 mM metal picrate; organic phase, 15 mM ionophore in CHCl3. c Extraction percentage: aqueous phase, 5 mM metal picrate; organic phase, 10 mM ionophore in CH2Cl2.

Table 3. Transport of Metal Picrates across BLMs Mediated by Neutral Phosphorus-Containing CEs and LEs ionophore [ref] 3c [7] 3d [7] 3e [7] 3f [7] 10 [20]b 11a [20]b 11b [20]b 11c [20]b 11d [20]b 11e [20]b 13b [22]b 13c [22]b 13d [22]b 13e [22]b 13f [22]b 13g [22]b 13h [22]b 13i [23]b

single metal ion transport rate, V × 108, mol/h Na+

1.5 2.1 2.4 5.4 5.7 6.3 2 2 2 2 3 3 9.5

K+ 0.8a 0.6a 0.6a 0.7a 0.65 2.5 16.1 3.5 11.0 3.8 25 23 26 23 25 35 11 69.8

Cs+

Ca2+

Sr2+

Ag+a 48.5 44.0 40.0 49.1

0.22 1.0 3.6 0.5 3.4 2.8

0.6 2.2 1.0 1.9 0.8

0.3 6.6 0.7 11.1 2.2

2 4 2 3 20

a Metal ion flux: J × 108, mol/(h‚cm2). Source phase: 1.0 mM aqueous metal picrate. Membrane: 0.6 mM ionophore in DCE. Receiving phase: deionized H2O. b Source phase: 2.0 mM aqueous metal picrate. Membrane: 1.0 mM ionophore in CHCl3. Receiving phase: deionized H2O.

other hand, 3c was found to induce permeability of BiLLM for K+, Na+, and Ca2+.12,14 The percentage of extraction of Ag+, HgII, and PdII into CHCl3 by CE 4 containing two endocyclic thiophosphonyl groups was reported11 to be essentially the same as those for ligands 3c and 3e. This might indicate participation of only one PdS group of 4 in metal ion coordination. Recently developed15 macrocycles 5 with incorporated “dioxazaphosphocane” moieties possessed good extraction selectivity for CaPic2 over MgPic2 (Table 2). The percentage of Ca2+ and Mg2+ extraction into CHCl3 by 5b was found to be somewhat greater than that for 18crown-6. 5b was studied also as a CaPic2 carrier through a CHCl3 BLM. Another approach to macrocyclic ionophore phosphorylation consists of the introduction of exocyclic phosphorus-containing moieties. A pendant PdO group in

the side arm attached to the macroring (as in the LEs) or in the benzo/naphtho substituent of the CE may serve as an additional metal binding site. When the LE side arm is oriented over the macrocycle, the ligand is said to be preorganized17 for cooperative metal ion coordination by the CE cavity and the side-arm donor atoms. This may enhance ligand selectivity for particular metal cations and, therefore, its potential in metal ion separations. For example, LE phosphate 6 applied in the PVC membranes of ISE exhibited improved selectivity for Li+ over Na+ and K+.17 N-pivot LEs 718 with phosphonoalkyl (7a-c) and phosphine oxide (7d) groups in the side arms were found to be more selective (although somewhat less efficient) extractants of alkali metal picrates from aqueous solutions into CH2Cl2 (see Table 2) than the parent monoaza-15-crown-5 (percentage of LiPic, NaPic, and KPic extraction under otherwise identical conditions of 65, 56, and 67%, respectively). Interestingly, introduction of the phosphorus-containing side arm in the aza-CE changed the order of the extraction selectivity for AMC. Specifically, in contrast with the selectivity range of K+ ∼ Li+ > Na+ observed for monoaza-15crown-5, the extraction percentage of alkali metal picrates varied as Li+ > Na+ > K+ for 7a, K+ > Na+ > Li+ for 7b and 7c, and Na+ > K+ > Li+ for 7d. However, both 7b and 7d showed a slight preference for Li+ when utilized as ionophores in the PVC membranes of ISE.18 N-pivot LEs 8 and 9 with pendant phosphine oxide groups were studied as extractants of Pd(II) into chloroform.19 Ligands 9 provided a considerable improvement of Pd(II) distribution into the organic phase relative to the “parent” diaza-18-crown-6. Also, ionophores 9 were much more efficient Pd(II) extractants than compound 8 with a monoaza-15-crown-5 framework. Within the series of ligands 9, the distribution coefficients of Pd(II) varied as 9d > 9b . 9c > 9a. This trend was explained in terms of the contrasting ability of different length PdO-containing side arms to participate in the metal ion chelation cooperatively with the CE moiety. Phosphorylation of the benzo group in ligand 1020 resulted in a significant decrease of its ionophoric efficiency for AMC (Table 3) relative to the “parent” benzo-15-crown-5, which showed NaPic, KPic, and CsPic transport rates of 4.7 × 10-8, 2.1 × 10-8, and 0.96 × 10-8 mol/h, respectively, under otherwise identical conditions. Meanwhile, Na+ selectivity of benzo-15crown-5 in the BLM transport remained essentially unchanged with introduction of the phosphoryl group. Evidently, the phosphorus-containing moiety in 10 did not participate in complexation. However, via the electron-withdrawing effect, it diminished the electrondonor capacity of the CE oxygens. The related bis-CEs 1120 provided greater rates of NaPic, KPic, and CsPic transport across a CHCl3 BLM than 10 (Table 3). Ligands 11 were capable of carrying CaPic2 and SrPic2 through the BLM as well. The rates and selectivity of membrane transport of the metal ions with bis-CEs 11a-f varied significantly as the structure of the bridging fragment between two CE units was altered. Macrocycle 12a21 with two pendant phosphonate groups attached to the aromatic moieties appeared to be a weak, but selective, extractant for NaPic (Table 2). Two related neutral LEs, 12b and 12c,21 with larger polyether cycles exhibited a dramatic increase of the extracting ability toward alkali metal picrates compared

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with that of 12a and revealed the selectivity for Na+ and Rb+, respectively. (It is interesting that LE 12b not only overcame significantly the AMC extraction efficiency of its analogue containing no pendant phosphonate groups but also showed a contrasting selectivity order.) Disubstituted dibenzo-18-crown-6 and dibenzo-24crown-8 derivatives (13) with phosphorus-containing exocyclic moieties introduced in both of the benzo groups of the CE were studied as carriers for alkali and alkaline earth metal cations (AEMC) in BiLLM and BLM.14,22,23 In particular, ligand 13a was found to increase the permeability of the BiLLM for Na+, K+, Mg2+, and Ca2+ ions.14 A series of diphosphorylated dibenzo-18-crown-6 compounds 13b-g22 and 13i23 showed similar ionophoric properties for Na+, K+, and Cs+ (Table 3). As was observed for the phosphorus-containing benzo-CE 10, the rates of BLM transport of alkali metal picrates with 13b-g and 13i were significantly lower relative to those with nonphosphorylated dibenzo-18-crown-6 (NaPic, KPic, and CsPic transport rates of 1.1 × 10-7, 14.6 × 10-7, and 1.9 × 10-7 mol/h, respectively). However, the introduction of two electron-withdrawing phosphoruscontaining groups in these ligands did not affect the K+ selectivity of BLM transport characteristic for dibenzo18-crown-6. Diphosphorylation of dibenzo-24-crown-8 (13h) altered neither the AMC carrier efficiency of the “parent” CE nor its selectivity for Cs+.22 Polymer 14 incorporating diphosphorylated dibenzo18-crown-6 moieties immobilized on silica gel gave rise to an efficient K+/Na+-selective sorbent of alkali metal cations from aqueous solutions. 23 A recently developed24,25 group of CEs, bis-CEs, and related ligands containing cyclophosphazene units (compounds 15-18) was explored in separations of metal ions (AMC, Mg2+, Ca2+, Ag+, CuII, CoII, NiII, and CdII) by thin-layer chromatography (TLC) on silica gel using hexane-THF (2:3) as an eluent. All of the compounds 15b-g and 16-18 provided stronger metal ion binding than their “parent” CE 15a. As the macrocycle structure varied, the ionophores showed a wide spectrum of selectivities. For instance, 15b, 16b, 18a, and some other ligands exhibited preference for K+ and 17c was selective for Li+, while 16c, 17a, and 18b were especially efficient in Ag+ separation. III.2. Proton-Ionizable CEs and LEs. For this group of ionophores, complex formation includes a step of their acidic proton dissociation. Therefore, unlike the described above metal ion separations using neutral CE and LE carriers, interface transfer of metal cations mediated by proton-ionizable macrocycles must be pHcontrolled (see eqs 3 and 4). Proton-ionizable phosphorus-containing macrocyclic ionophores utilized in metal ion separations are listed in Table 4 with structural formulas given in Figure 2. Different-sized lipophilic, proton-ionizable CEs 19 containing an endocyclic dialkylhydrogenphosphate moiety were examined by Izatt and co-workers26 as carriers of AMC (see Table 5), Ag+, Pb2+, ZnII, CdII, and NiII in BLM. The ionophoric effectiveness of ligands 19b-d was found to be the greatest when the pH of the aqueous phase source of metal ions was increased to 13.5-14, while 19a showed the best results with the source phase pH 12. CEs 19d and 19a provided the highest and the lowest rates of AMC transport, respectively. 19a with a smaller macroring showed preference for Na+, while larger CEs

Table 4. Proton-Ionizable Phosphorus-Containing LEsa Utilized in Metal Ion Separations ionophore 19a-d

metal ionsb Ag+,

20 21a-d 22 23

AMC, Pb2+, Zn2+, CdII, NiII AMC AMC AMC Li+, Na+, K+

24a-c

Li+, Na+, K+

25a-c 26a-c 27 28a

Li+, Na+, K+ Li+, Na+, K+ Li+, Na+, K+ AMC

28b 28c 28d

AMC AEMC AMC

28e 28f 28g

AMC AEMC AMC

28h 28i 28j

AMC AEMC AMC

28k

AMC

28l 28m

AEMC AMC

28n 28o

AEMC AMC

28p 29

AMC, AEMC AMC

30a-c 31a,b 32a-e

Li+, Na+, K+ AMC Pb2+, Zn2+

separation methodc

ref

BLM (CH2Cl2)

26

extrn. (CHCl3) extrn. (CHCl3) PSLM extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3), BLM (CHCl3) BLM (CHCl3) BLM (CHCl3) BLM (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3), BLM (CHCl3) extrn. (CHCl3) extrn. (CHCl3) BLM (CHCl3) extrn. (1,2-Cl2C6H4) sorption sorption

8 27 28 4c, 29 4c, 29 4c 4c 4c 30 33, 34 30 31 30, 31 33, 34 30 31 30, 31 33, 34 30 31 30, 31 33, 34 30, 33, 34 31 30, 31 33, 34 31 30, 31 33, 34 32 30 33, 34 35 36 37

a For the structural formulas, see Figure 2. b AMC ) alkali metal cations, AEMC ) alkaline earth metal cations. c extrn. ) solvent extraction, BLM ) bulk liquid membrane transport, BiLLM ) bilayer lipid membrane transport, PSLM ) polymersupported liquid membranes.

19b-d were selective for K+ or Rb+ (depending on the source phase pH). Also, all four ligands 19 appeared to be good carriers of Ag+ and Pb2+ and allowed only low fluxes of ZnII, CdII, and NiII in the BLM. Compound 208 containing endocyclic phosphorus with a 2-carboxyphenyl substituent, although potentially acidic, was utilized in alkali metal picrate extractions from neutral aqueous solutions where it acted as a neutral ionophore. Under such conditions, 20 did not exhibit improved AMC extraction relative to the nonionizable analogue 1c (see Table 2). A series of lipophilic LE derivatives of dibenzo-16crown-5 21 containing a phosphonic acid monoalkyl ester moiety in side arms of varied length were reported by Bartsch and co-workers.27 In competitive solvent extraction of AMC from neutral and basic aqueous solutions into CHCl3, these proton-ionizable ionophores showed higher efficiency than the analogous LE carboxylic acids. Surprisingly, the extraction selectivity of ligands 21 varied as the side-arm length increased from Na+ for 21a and 21b to Li+ for 21c and 21d. Such a change was attributed to the alteration of the metal ion coordination mode. Thus, for 21a and 21b with shorter

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Table 5. Transport of Alkali Metal Cations across BLMs Mediated by Proton-Ionizable Phosphorus-Containing CEs and LEs metal ion transport ionophore [ref] [26]a

19a 19b [26]a 19c [26]a 19d [26]a 23 [4c]b 24a [4c]b 24b [4c]b 24c [4c]b 25a [4c]b 25b [4c]b 25c [4c]b 26a [4c]b 26b [4c]b 26c [4c]b 27 [4c]b

Li+

Na+

K+

Rb+

Cs+

0.43 0.34 0.25 0.88 2.82 1.29 9.41 13.42 0.79 0.33 0.04 0.81 0.35 0.31 0.14 Sr2+ . Mg2+, Li+, Na+, K+, Rb+, Cs+ with markedly different efficiencies in binding divalent alkaline earth and monovalent alkali metal ions. The AMC extractability of phosphoric acid groupcontaining, photoresponsive benzo-CEs 30 was found to vary when the ligands underwent light-induced cistrans isomerization.35 Thus, the extraction percentage of Li+, Na+, and K+ with cis isomers of 30a and 30c was generally higher than that with the corresponding trans isomers. Conversion of cis-30b into the trans

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isomer increased notably extraction of Li+, while binding of Na+ and K+ was diminished for the same isomer transformation. For 30c, the isomerization affected also the extraction selectivity order. Specifically, cis-30c was selective for K+, whereas trans-30c showed a slight preference for Na+ binding. Polymeric dibenzo-18-crown-6 phosphonic acid 31a and its monoethyl ester 31b allowed separation of AMC from acidic, neutral, and basic aqueous solutions.36 The sorption capacity of these materials exceeded that observed for the analogous CE carboxylic acid resins. Compound 31a exhibited appreciable selectivity for Li+ in sorption from aqueous solutions with pH above 7. Polymer 31b was a K+-selective sorbent from acidic-toneutral AMC solutions. A series of proton-ionizable LE resins 32 with phosphorus-containing functions in the side arm attached in the geminal position of dibenzo-16-crown-5 were applied to sorption of heavy metal ions, Pb2+ and Zn2+, from highly acidic aqueous solutions.37 All of them demonstrated notable selectivity for Pb2+, which was the greatest for LE phosphonic acids 32d and 32e. These two polymers as well as LE phosphonic acid monoethyl ester 32a provided quantitative removal of Pb2+ from an aqueous solution at pH 2. The sorption capacity of the monoethyl ester resins diminished significantly as a bulky Pr group was introduced in the geminal position of dibenzo-16-crown-5 (32b) or the side-arm length was increased (32c). IV. Phosphorus-Containing Calixarenes and Resorcinarenes Calixarenes38 are a distinctive group of macrocyclic, multidentate ligands with variable shapes of the π-electron-rich cavities and spatial orientation of pendent functionalities. In contrast with phosphorus-containing CEs and LEs that were applied mostly in separations of alkali and alkaline earth cations and only sometimes for transition metal ions, calixarene-based ligands with phosphorus-containing moieties (Figures 3 and 4 and Table 7) revealed a much broader spectrum of ionophoric propensities. In particular, such ligands were found to be efficient extractants and trans-membrane carriers for rare-earth and actinide metal ions, which stimulated their exploration as potential separation agents for nuclear waste management. Research teams of France, Northern Ireland, and Ireland studied ionophoric properties of the series of ligands 3339-42 and 3439,43-45 with pendant phosphine oxide groups immobilized on the lower rim of the calix[n]arene moieties (where n ) 4, 5, 6, and 8) (for the structural formulas, see Figure 3). The principal difference between clusters 33 and 34 was the length of the side arms bearing PdO donor centers. The long-armed calix[4]arenes 33a and 33b showed only very insignificant, low-selective extraction of alkali metal picrates.39 Meanwhile, in solvent extractions of AEMC, these ionophores were appreciably selective for Ca2+ 39 (Table 8). 33a was found to be capable of discriminating Ca2+ vs Mg2+ and AMC in the PVC membranes of ISE.41 In addition, calixarene-based phosphine oxides 33a, 33c, and 33d were studied as sensors of Pb2+ in the ISE.42 All three ionophores exhibited excellent Pb2+ selectivity over many other metal cations. The most important interference produced by Ca2+ diminished on going from calix[4]- to calix[5]- to calix[6]arene.

The short-armed calixarenes 34a and 34b provided greater percentages of AMC39,43 and AEMC38 extraction into CH2Cl239 (or DCE)43 than the related ionophores 33 with longer PdO-containing pendants (see Table 8). With both AMC and AEMC, calix[5]arene 34b possessed an improved ionophoric efficiency relative to that of the calix[4]arene homologue 34a. 34a applied in solvent extraction39,43 and BLM transport43 of alkali metal picrates showed selectivity for K+ over other AMC. In contrast, the extraction percentage of metal picrates with 34b39 tended to increasing consistently on going along the AMC group, from Li+ to Cs+. In solvent extractions of alkaline earth metal picrates,39 calixarene tetramer 34a demonstrated a preference for Ca2+, while the pentamer analogue 34b provided high and equally efficient extraction for Ca2+ and Ba2+. 34a allowed also moderate levels of AgPic extraction into CH2Cl2 and transport across the BLM.44 The long-armed calixaryl phosphine oxides 33a, 33b, and 33d-g exhibited high degrees of extraction of EuIII, ThIV,39,40 NpIV, PuIV, and AmIV 40 from simulated nuclear wastes into CH2Cl239 or 1,2-nitrophenylhexyl ether (NPHE)40 (see Table 9) and transported the actinide ions through the PSLM.40 These ligands (except the octamer 33f) were found to be more efficient extractants of EuIII and ThIV than the classical f-metal ion extractants, trioctylphosphine oxide (TOPO) and carbamoylmethylphosphine oxides (CMPO).46 All of the ionophores 33a, 33b, and 33d-g exhibited selectivity for ThIV over EuIII and for PuIV over NpIV and AmIV. Calix[6]arenes 33d and 33e provided much higher levels of ThIV, EuIII, NpIV, and PuIV extraction than their calix[4]- and calix[8]arene analogues. The extraction propensities of compounds 33 toward the f-metal cations showed a general tendency toward decreasing when t-Bu substituents were introduced on the upper rim of the calixarene moiety as in 33b, 33e, and 33g. Calix[4]arene-based phosphine oxide 34a (which was found to exist in the cone conformation)47 with shorter side arms demonstrated a significantly enhanced percentage of ThIV extraction and even a larger increase of EuIII extraction capacity compared with those observed for 33a39 (Table 9). Moreover, 34a appeared to be the most efficient ionophore of EuIII within the examined series of calixaryl phosphine oxides 33 and 34. In contrast, calix[5]arene 34b was determined to be an ineffective extractant of the f-metal ions under otherwise identical conditions. The cone-shaped 34a also provided high extraction percentages for a variety of trivalent rare earth metal ions (REMC), i.e., La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, and Y, from an aqueous solution containing Al(NO3)3 into DCE with a good selectivity for PrIII, NdIII, and SmIII.45,48 (34a was found to be capable of extracting only insignificant quantities of Al3+ and Na+ ions that might cause interference with the REMC separations.45) For the competitive extractions of 11 REMC with 34a, the separation factor between the most extracted (NdIII) and the least extracted (YbIII) species was 16.50. This “intragroup separation” factor value was the largest one among those determined for the series of calix[4]arene-based ionophores in which the number of pendant phosphoryl groups varied from two as in 35a to three as in 38 (these ligands will be discussed vide infra) to four as in 34a.48 Extraction of EuIII by 34a was performed with use of different diluents and found to decrease in the order nitrobenzene > DCE > CH2Cl2 > CHCl3.45

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Figure 3. Structural formulas of phosphorus-containing calixarenes listed in Table 3.

Ionophoric capacities of calix[4]arene bis(phosphine oxide)s 35 containing two other o-substituents on the lower rim were compared with the characteristics of calix[4]arene tetrakis(phosphine oxide) 34a. In particular, ligand 35a was tested for competitive extraction of REMC into DCE.48 Its extractive efficiency appeared to

be much lower than that of 34a. It should be noted that the levels of REMC extraction by calixaryl phosphine oxides were found to enhance in the order 35a < 38a ≈ 38b < 34a as the number of pendant PdO groups increased from two to three to four.48 However, 35a showed appreciable discrimination of 11 REMC with a

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3559

Figure 4. Structural formulas of phosphorus-containing resorcinarenes listed in Table 3.

separation factor between the best extracted ion, SmIII, and the least extracted species, LaIII, of 9.77. Di-O-methylated bis(phosphine oxide) 35b, as well as the related bis(ethyl oxyacetate) 35c, exhibited a moderate extraction capacity for AgNO3.44 In contrast, the logarithm of the Ag+-extraction constant determined for calix[4]arene 35e with two phosphine oxide and two diN-ethyloxyacetamide groups (see Table 8) was 1-1.5 log units larger than the corresponding values for 34a, 35b, and 35c, and also for the commercially available macrocyclic ionophore dicyclohexano-18-crown-6.44 Additionally, 35e provided a 3-fold greater rate of AgNO3 transport through a BLM (CH2Cl2) relative to that for dicyclohexano-18-crown-6. Bis(phosphine oxide)diamide 35e showed high levels of alkali metal picrate extraction into CH2Cl2 with selectivity toward Na+.39 It is interesting that, with DCE as a diluent, 35e behaved as a Li+-selective extractant of alkali metal picrates43 (Table 8). Ligand 34e containing mixed functional groups appeared to be a generally more efficient ionophore (extractant and trans-membrane carrier) of AMC than the analogous tetrakis(phosphine oxide) 34a.39,43 A preference for NaPic extraction into CH2Cl2 was also observed for the bis(phosphine oxide)bis(tert-butyl ester) 35d.39 However, the percentage of AMC extractions with 35d was significantly lower relative to those with 35e and 34a. In contrast with the tendency observed for AMC extraction by 35e, the extraction propensities of this

ligand (as well as of its diester analogue 35d) for alkaline-earth metal picrates were diminished compared with those of tetrakis(phosphine oxide) 34a39 (Table 8). The bis(phosphine oxide)diamide showed plateau selectivity for Ca2+, Sr2+, and Ba2+ over Mg2+, while the bis(phosphine oxide) diester did not discriminate between the AEMC under otherwise identical extraction conditions. The two isomers of bis(phosphine oxide)diamide 35e, cone 36 and partial cone 37, were utilized in separations of REMC by extraction from an aqueous solution into DCE (see Table 9) and transport through PSLM.49 Ligand 36 restricted to the cone conformation provided higher levels of REMC extractions and greater rates of membrane transport than the partial cone isomer 37. At the same time, both of the isomeric bis(phosphine oxide)diamides exhibited decreased efficiency and selectivity of the REMC separations relative to those of the tetrakis(phosphine oxide) 34a. These observations confirmed the important role of phosphoryl groups in the metal ion coordination by the calix[4]arenes. Additionally, it should be mentioned that both of the isomers 36 and 37, because of their appreciable affinity for Na+, were unable to extract REMC from the sodiumcontaining aqueous solutions. Another large group of phosphoryl-containing calixarenes utilized predominantly in separations of f-metal ions is represented by structures 39-42.49-53 This type of macrocyclic ligand containing pendant acetami-

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Table 7. Phosphorus-Containing Calixarenesa and Resorcinarenesb Utilized in Metal Ion Separations ionophore 33a 33b 33c 33d 33e-g 34a

34b 35a 35b,c 35d 35e 36e 37f 38a,b 39a 39b 39c 39d 39e 39f 39g,h 39i 40a-c 41a-f 42a-c 42d 43 44a-c 45a,b 46 47 48 49 50a,b 51a-c 51d 52a-d

metal ionsc

separation methodd

ref

EuIII,

extrn. (CH2Cl2) extrn. (NPHE), PSLM ISE extrn. (CH2Cl2) extrn. (NPHE), PSLM ISE extrn. (CH2Cl2) extrn. (NPHE), PSLM ISE extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CH2Cl2) extrn. (DCE), BLM (DCE) extrn. (DCE) extrn. (CH2Cl2), BLM (CH2Cl2) extrn. (CH2Cl2) extrn. (DCE) extrn. (CH2Cl2) extrn. (CH2Cl2) extrn. (DCE), BLM (DCE) extrn. (CH2Cl2), BLM (CH2Cl2) extrn. (CH2Cl2) extrn. (DCE, nitrobenzene, CH2Cl2, CHCl3), PSLM extrn. (DCE), PSLM extrn. (DCE) extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CHCl3) extrn. (CHCl3) extrn. (CH2Cl2) extrn. (NPHE) extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CH2Cl2) extrn. (NPHE) extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CH2Cl2) extrn. (NPHE) extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CH2Cl2) extrn. (NPHE), PSLM extrn. (CH2Cl2, NPHE) extrn. (CH2Cl2, NPHE) extrn. (CHCl3) extrn. (CHCl3) extrn. (CH2Cl2), BLM (CH2Cl2) extrn. (CH2Cl2, CH3CO2Et, P(O)(OBu)3-hexane) extrn. (CH2Cl2, CH3CO2Et, P(O)(OBu)3-hexane) extrn. (CH2Cl2, CH3CO2Et, P(O)(OBu)3-hexane) BLM (CHCl3) extrn. (CHCl3) extrn. (CH2Cl2) extrn. (CH2Cl2) extrn. (CH2Cl2) extrn. (CH2Cl2) extrn. (CH2Cl2)

39 40 41, 42 39 40 42 39 40 42 39 40 39 43 45, 48 44 39 48 44 39 43 44 39 48 48 48 49

ThIV

AMC, AEMC, EuIII, ThIV, PuIV, NpIV, AmIV Ca2+, Pb2+ AMC, AEMC, EuIII, ThIV EuIII, ThIV, PuIV, NpIV, AmIV Pb2+ EuIII, ThIV EuIII, ThIV, PuIV, NpIV, AmIV Pb2+ EuIII, ThIV EuIII, ThIV, PuIV, NpIV, AmIV AMC, AEMC, EuIII, ThIV AMC REMC, Al3+, Na+ Ag+ AMC, AEMC, EuIII, ThIV REMC Ag+ AMC, AEMC AMC Ag+ AMC, AEMC REMC, Al3+, Na+ REMC, Al3+, Na+ REMC EuIII, ThIV actnds REMC, AmIII, CmIII REMC, AmIII, CmIII EuIII, ThIV actnds EuIII, ThIV actnds EuIII, ThIV actnds EuIII, ThIV actnds EuIII, ThIV actnds EuIII, ThIV actnds EuIII, ThIV actnds REMC, ThIV, AmIII, CmIII REMC, ThIV, AmIII EuIII, ThIV REMC Li+, Na+, K+ REMC REMC REMC Na+, K+, Cs+, Mg2+, Ca2+, Sr2+ AMC Na+, Cs+, Sr2+, UO22+, FeIII, EuIII Na+, Cs+, Sr2+, UO22+, FeIII, EuIII REMC Na+, Cs+, Sr2+, UO22+, FeIII, EuIII Na+, Cs+, Sr2+, UO22+, FeIII, EuIII

50 50 49 49 49 49 49 49 49 51 52 53 54 55 56 56 56 57 58 59 59 61 59 60

a For the structural formulas, see Figure 3. b For the structural formulas, see Figure 4. c AMC ) alkali metal cations, AEMC ) alkaline earth metal cations, REMC ) rare earth metal cations, and actnds ) actinide ions. d extrn. ) solvent extraction, BLM ) bulk liquid membrane transport, PSLM ) polymer-supported liquid membrane transport, DCE ) 1,2-dichloroethane, and NPHE ) 1,2-nitrophenylhexyl ether. e The cone isomer of calixarene 35e. f The partial cone isomer of calixarene 35e.

dophosphine oxide moieties (so-called “CMPO-substituted” calixarenes) was introduced by a united team of scientists of France and Germany. A variety of calix[4]- and calix[5]arenes 3949,50 and 4049 with multiple CMPO moieties immobilized on the upper (wider) rim was applied in solvent extraction systems for EuIII and actinide ions (Th, Np, Pu, and Am) (Table 9). All of these macrocyclic ionophores provided a highly productive distribution of ThIV between the acidic nitrate aqueous solution and CH2Cl2 which was much greater than that

afforded by the classical CMPOs and demonstrated preference for ThIV over EuIII. No significant, regular variation of the extraction efficiencies of ligands 39 and 40 with ThIV and EuIII was observed for the alteration of the lower rim O-alkyl substituents or metacyclophane moiety size. Ionophores 39 and 40 possessed improved ionophoric properties for actinide ions relative to those of the traditional CMPO extractants. In particular, calix[4]arenes 39a and 39d-f with linear O-alkyl chains exhibited very high levels (>99% for the ligand concen-

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3561 Table 8. Solvent Extraction of Alkali, Alkaline Earth, and Some Other Metal Ions by Phosphorus-Containing Calixarenes and Resorcinarenes extraction, % ionophore [ref]

Li+

Na+

K+

Rb+

Cs+

Mg2+

Ca2+

Sr2+

Ba2+

33a [39]c 33b [39]c 34a [39]c [43]d [44] 34b [39]c 35b [44] 35c [44] 35d [39]c 35e [39]c [43]d [44] 44a [55]e 44b [55]e 44c [55]e 50a [59]f 50b [59]f 51a [59]f 51b [59]f 51c [59]f 51d [59]f 52a [60]f 52b [60]f 52c [60]f 52d [60]f

0.7 3.5 7.6 14.5

0.5 3.7 6.0 5

0.3 3.8 22.3 27

0.5 3.6 17.0 21

0.4 3.6 6.4 6.5

0.6 3.0 5.6

2.6 5.0 23

0.8 3.3 9.8

0.5 3.1 8.4

28.5

37.9

45.0

50.7

56.9

18.2

62.5

52.2

62.3

Ag+a

FeIII b

UO22+

24 51 90 55 66 83 25 16 64 74

82 25 98 92 97 54 57 95 93 98

1.32 1.64 1.06 8.5 53.4 64.5

17.9 65.1 58

13.7 42.6 45

60 63 55

57 66 57