Engineering selectivity into polymer-supported reagents for transition metal ion complex formation

Reactive & Functional Polymers 70 (2010) 545–554

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Engineering selectivity into polymer-supported reagents for transition metal ion complex formation Amanda N. Pustam, Spiro D. Alexandratos * Department of Chemistry, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, United States

a r t i c l e

i n f o

Article history: Available online 9 May 2010 Keywords: Polymer-supported Selectivity Metal Ligand

a b s t r a c t Polymer-supported reagents are widely employed in the complexation of metal ions for applications in separations science, organic reactions (as catalysts) and in analytical chemistry (for processes requiring metal ion enrichment, detection and quantification). Various strategies for the continued development and optimization of polymer-supported reagents have evolved. This review cites ways in which polymer-supported reagents can be designed to achieve improved metal ion selectivities from the viewpoint of the mechanisms involved in the complexation of transition metal ions from aqueous solutions. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The selective removal of toxic metal ions from the aqueous environment remains a challenging problem. This issue has been addressed by the development of innovative methods for sequestering metal ions. Existing technologies include solvent extraction [1,2], precipitation [3–5], electrochemical methods [6], membrane separation [7,8], and ion exchange using natural [9–11] and synthetic materials [12]. Research in the area of metal ion separations is ongoing, in part, to meet environmental regulations. Further, there is a global impetus towards a reduction of metal ion waste collected from any site for treatment and/or storage at landfills. This can be achieved through recycling and reusing the metal ions. A concern that arises is to identify ways of separating metal ions, especially those that exhibit similar chemical behavior such as trivalent lanthanides from transplutonium actinides [13]. The various technologies noted above are useful for specific applications, though they are not without shortcomings [14]. For instance, while liquid–liquid extractions are efficient for separations involving high concentrations of metal ions, large quantities of secondary waste are generated which require further treatment. Some extractant loss into the aqueous phase is likely due to their finite solubility. Ion-exchange resins are attractive alternatives for separations from dilute solutions since the extractant is bound to a solid phase, simplifying the separation process. This makes it suitable for separations of mixtures with complex matrices and also environmentally safe. However, they can exhibit poor metal ion selectivity and kinetics. The advent of chelating polymers in the 1950s has allowed the synthesis of ion-selective resins [15]. * Corresponding author. E-mail address: [email protected] (S.D. Alexandratos). 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.05.002

Chelating polymers offer enhanced selectivity for the separation and concentration of metal ions [16], while the varying stability constants of metal ion complexes under different solution conditions can allow for greater degrees of separations. Chelating resin complexes oftentimes have higher stability constants compared to their corresponding monomer complexes [17]. This has been attributed to an entropy effect; a large positive change in entropy due to the proximity of functional groups and the high local concentration of ligands within the resin coupled to the release of waters of hydration upon complexation. Chelating resins include a wide range of ligands, including iminodiacetates [18,19], hydroxylamines [20], amides [21], Schiff bases [22], phosphates and phosphonates [23]. Polymer-supported reagents are polymers that have been chemically modified with various ligands. These modified materials are important as ion exchangers because they can be prepared to a desired selectivity with appropriate ionophoric reagents. They also have high mechanical and chemical stability and recovery of metal ions can occur without destruction of the support. Polymer-supported reagents are effective chelants because of high uptake capacities of specific metal ions. There has been much research on improving equilibration rates for these materials. This review focuses on organic polymers; modified inorganic polymers [24], organic–inorganic hybrids [12,25], and impregnated resins [26] will be the subject of future reviews. The need for designing selective polymers by determining ‘‘the basic principles by which a polymeric reagent can recognize a targeted substrate” [27] has been expressed. The objective of this review is to contribute to the understanding of the behavior of polymer-supported reagents with primarily transition metal ions in aqueous solutions. Detailed considerations about the interactions of lanthanide ions and polymer-supported reagents have been reviewed [28]. Resins are organized by classifying them based

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2. Synthesis and characterization Polymer-supported reagents are most commonly prepared by modification of a crosslinked copolymer, usually polystyrene, poly(vinylbenzyl) chloride and poly(glycidyl methacrylate), with divinylbenzne or ethylene glycol dimethacrylate as the crosslinker. Polymers can be prepared as beads by suspension polymerization. By this method, the beads can have different levels of porosity. The bead form of the polymer makes it amenable to continuous processes and enhances its versatility. Different functional groups can be immobilized on the polymer due to the applicability of organic reactions on a solid phase. The success of the functionalization relies largely on the choice of a suitable solvent to swell the beads, especially if they are microporous (gel) beads. Swelling is important so that the reagent can have greater accessibility into the bead, resulting in a high degree of functionalization and thus a high metal ion saturation capacity. To ensure complete reaction, excess reagents are used. Macroporous resins are porous beads with channels for access of reagents into the interior of the bead. Swelling is not as significant for these beads compared to gel beads. Changes in the FTIR and NMR spectra of the polymer can indicate whether a particular reaction sequence was successful. The degree of functionalization can be quantified by elemental analysis. Alternatively, polymer-supported reagents can be prepared from a functionalized monomer bearing a complexing group that is copolymerized with a crosslinking agent. This method is not always amenable to suspension polymerization. Evidence of metal ion interactions with functional groups is gathered from extended X-ray absorption fine structure (EXAFS) [29] spectra, X-ray photoelectron spectroscopy (XPS), electron spin resonance spectra (ESR), solid state NMR and IR spectroscopy. Images of the bead’s texture, size, and shape can be obtained from scanning electron microscopy (SEM). Pore size and surface area can be determined with porosimeters. 3. Ion-complexing polymer-supported reagents Selectivity is a property of ion-complexing polymer-supported reagents that refers to their ability to complex a specific metal ion in the presence of other ions. This is especially necessary where the targeted metal ion is toxic and in much lower concentrations than other metal ions present, as is common in groundwater contamination. Because of the varying environmental conditions in which metal ions are found, different chelating ion exchangers have been studied, their applicability dependent on the situation. Much of the development of ion-complexing resins is based on evidence from experimental work from solvent extraction [30], analysis of X-ray crystallography data [32] and molecular mechanics calculations [31–33]. These studies focus on observing the interaction(s) of small molecule complexants with metal ions. The following survey of the literature illustrates strategies that are used to enhance metal ion selectivity in polymers. 3.1. Immobilizing ligand(s) with two or more coordinating sites Multiple ligand interactions can result in metal ion binding with greater affinity and selectivity. This effect is seen by comparing complexation by resins with carbonyl and phosphoryl coordinating sites (Fig. 1) [34]. The uptake of Cu(II) and Pb(II) from 0.10 N HNO3 solutions was 89.5% and 97.5%, respectively, for the a-ketophos-

O

O O C P OH OH

on the mechanisms by which metal ions interact with the ligands when bound to the support. The mechanisms identified are grouped according to the properties of the metal ions and are the ionic radius, polarizability, and coordinate geometry.

CH2P OH OH

A

B

Fig. 1. Polystyrene-bound a-ketophosphonic acid (A) and phosphonic acid (B) resins.

phonic acid resin (A) compared to 27.8% and 55.6%, respectively, for the phosphonic acid resin (B). Cooperation of the carbonyl and phosphoryl moieties in binding the metal ions results in higher sorption levels. In another study, the sorption capacity of pseudocrown ether resins with pendent functional groups containing nitrogen and sulfur, were determined for Cu(II), Co(II), Pb(II), and Hg(II) in the pH range 1–4 [35]. The resins were identified as NCR-S, NCR-SS and NCR-SN based on the identity of the donor atoms in the R group (Fig. 2). All resins showed the highest affinity for Hg(II), compared to the other metal ions. Of the three resins, NCR-SN had the greatest capacity and NCR-S had the lowest capacity. Note that the pendent functional group of NCR-S has one coordinating sulfur atom per ligand while NCR-SN has two (nitrogen and sulfur) coordinating atoms per ligand. X-ray photoelectron spectroscopy confirmed that the sulfur and nitrogen of the NCR-SN resin were involved in binding the Pb(II), Cu(II) and Hg(II) ions. Macrocyclic ligands such as crown ethers, calixarenes, resorcinarenes, cyclodextrins, and porphyrins, contain several donor sites for metal ion interactions and can be immobilized on polymers. Macrocycles bind metal ions through the macrocycle effect [36]. The immobilization of 25,26,27,28-tetraethoxycarbonylmethoxycalix[4]arene on Amberlite XAD-4 via an azo linkage is pre-organized for Pb2+ and works best at pH 6.5 [37]. This resin interacts with Pb2+ through a coordinative bond to each carbonyl oxygen on the ligand (Fig. 3). The specific attraction for lead ions is also attributed to the nature of the donor groups (see Section 3.4 below) which favors polarizable cations such as lead. Low-molecular weight complexing agents maintain their complexing ability after immobilization on polymer supports. A macroporous resin, PDTA-4 (Fig. 4), was synthesized by attaching 1,3diamine-2-hydroxypropane-N,N,N0 ,N0 -tetraacetic acid to XAD-4 via an ester link [38]. The resin tested by column studies was found to be very selective for uranium(IV), thorium(IV) and zirconium(IV) (over Zn(II), Ni(II), Mn(II), Mg(II), Fe(III), Co(II), Cd(II), and Bi(III)) at pH below pH 2.5. The complexing agent may be chosen as a result of its selectivity for metal ions in biological systems [39]. The goal is to design ligands or biomimics which would perform similarly when immobilized on polymers. Examples of ligands immobilized that are

OCH2CHO CH2

[

CH2CHO]n CH2CHO CH2 CH2

O

O

O

(CH2)2

(CH2)2

(CH2)2

O

R

(CH2)2 OCH2CHO

O (CH2)2

[

CH2CHO]m CH2CHO CH2 O

Fig. 2. Structures of NCR-S (R = SCH2CH3), NCR-SS (R = SCH2CH2SCH2CH3) and NCRSN (R = SCH2CH2N(CH2CH3)2).

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CH2 CH CH2 CH

n

target metal ion. This interaction is accompanied by a chemical reaction such as precipitation, reduction or complexation. 3.3. Ion-imprinted polymers (IIP)

NH3+ClH2C

HC CH2

CH CH2

n

N N

Ion-imprinted polymers [45] operate from the principles of molecular recognition [46,47]. A template of a particular metal ion within a polymer matrix is formed, and the polymer forms a ‘‘memory” of that metal ion upon its removal. When the polymer is later contacted with a solution containing different metal ions, the polymer can extract the templated ion preferentially. Several methods by which these polymers can be prepared have been identified [45]. Further details are provided in a subsequent section under the heading ionic radius. 3.4. Application of hard–soft acid–base theory to immobilized donor groups

O O EtO EtO

O

O O

O OEt O

O

OEt

Fig. 3. 25,26,27,28-Tetraethoxycarbonylmethoxycalix[4]arene supported XAD-4.

H2C HC

n

O O

CH2N CH CH2N

H2C HC

n

The affinity and selectivity a ligand has for metal ions is related to the stability of the metal ion complex formed upon binding. A hydrophilic resin (Fig. 5) with thiol functional groups on a polyacrylamide backbone was prepared [48] and its affinity for several cations was quantified over a pH range of 0–8. The resin had no affinity for alkali and alkaline earth ions while its affinity for first row divalent transition metal ions showed the same order as the Irving–Williams series: Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II).

CH2COOH

3.5. Effect of spacer groups

CH2COOH

Spacer groups are moieties that connect the ligand to the polymer support. Introducing ethylene oxide and ethylene sulfide spacers between chloromethylated polystyrene and the di(isobutyl)phosphine sulfide ligand increased the resin’s capacity for gold ions [49]. The synthesis of polymers with different spacer arms (Fig. 6) showed that ethylene oxide and sulfide spacers increase the hydrophilicity of the polymers and also participate in chelating the metal ions. Polymers 3 and 4 (Fig. 6) were the most effective for gold and palladium: the capacity for gold was 2.82 and 6.50 mmol/g for polymers 3 and 4, respectively, while it was 0.40 mmol/g for both polymers 1 and 2. Spacer groups enhance the flexibility of the polymer by having the ligand at a distance from the polymer backbone and improving accessibility of the metal ion to the ligand [50]. In this way, the ligand may show complexation behavior that approaches that of a free ligand in solution.

CH2COOH CH2COOH

Fig. 4. 3-Diamine-2-hydroxypropane-N,N,N0 ,N0 -tetraacetic acid immobilized onto XAD-4 (PDTA-4).

biomimics include hydroxamate [40] and catechol-containing [41,42] moieties. 3.2. The microenvironmental effect The environment around a ligand may be altered by the presence of other functional groups and affect the ligand’s ability to bind metal ions. The selectivity of Cu(II) over Co(II) was studied by synthesizing polystyrene/poly(N-vinylimidazole-co-ethyl acrylate) interpenetrating polymer networks (IPNs) with varying ratios of the N-vinylimidazole and ethyl acrylate [43]. Different ratios of imidazole: ester cause changes in the polymer’s polarity and hydrophilicity. All IPNs containing imidazole had a much higher affinity for Cu(II) than for Co(II). As the percentage of vinylimidazole (%VIm) decreased from 100% to 53%, the Cu(II) affinity increased. This was indicated by an increase in the binding constant from 3130 N1 to 8203 N1. A further decrease in %VIm resulted in a decrease in the binding constant for Cu(II) because the much-reduced polarity caused metal complex formation to be disfavored. Some immobilized ligands may react with metal ions after coordination. They are classified as Reactive Ion Exchangers (RIEX). Dual mechanism bifunctional polymers are included in this category [44]. These polymers contain two different functional groups, in which one ligand enhances the metal ion’s accessibility into the matrix while the other ligand recognizes and interacts with the

4. Factors affecting the affinity and selectivity of polymersupported reagents Many ion exchangers with different functional groups have been reported; some are more selective depending on the condi-

HC CH2

HC CH2

CO

CO HN CH2 HN

HN CH2 HN CO

CH2SH

n

CO CH CH2

m

Fig. 5. Poly [N-((acryloylamino)methyl)mercaptoacetamide].

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5. Correlation of metal ion properties with the selectivities of polymer-supported reagents

n

S CH2

P

iBu

5.1. Ionic radius

iBu

n

Polymer 1

S CH2

O (CH2CH2)4

P iBu iBu

Polymer 2 n

S CH2

(OCH2CH2)3

P

iBu

iBu

Polymer 3 n

S CH2 SCH2CH2

Polymer 4

(OCH2CH2)2

P

iBu

iBu

Fig. 6. Polymers with ethylene oxide and ethylene sulfide spacer groups.

tions in which they will be applied. Sorption studies report metal ion affinities in terms of percent metal ion complexed, distribution coefficients, and selectivity coefficients. However, it is oftentimes challenging to understand the mechanism by which the resins can complex metal ions. Solution conditions can influence the apparent affinities and selectivities. For example, metal ion speciation plays a significant role in complexation to the ligand and it is dependent on solution pH and the presence of counterions such as nitrate, chloride, and sulfate. Organic compounds in the environment may act as complexing ligands and affect speciation [51]. The physical and chemical properties of the polymer matrix are also variables. For example, the extent of crosslinking of polyacrylamide affected the affinities of glycine [52] and ethylenediamine ligands [53]. Polymer hydrophobicity can affect its compatibility with aqueous solutions and uptake capacities. The Cu(II) capacity for a polystyrene-based resin with 2-aminomethylpyridyl ligands was 8 g/L while the capacity of a methacrylate-based resin with the same ligand was twice as great [54]. When external factors are constant, metal ion affinities and selectivities are governed by the electronic compatibility of the ligand and metal ion. To design ligands with enhanced selectivity, the donor properties of the ligand must be matched with the acceptor properties of the metal ion [55]. This concept is expressed in several reviews where ion-selective ligands are identified [56]. It has been noted that ‘‘that no attempt has been made to develop a relationship between selectivity and different properties of a chelating group present in a selective ion-exchange resin” [57]. Additional relevant data are needed to establish a relationship between selectivity and properties of a chelating group. However, the Irving–Williams series is one example of a trend elicited by divalent first row transition metal ions in their interaction with polymer-supported reagents, proteins and small molecules. The affinity order, Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II) is related to the stability of the complex formed [58]. The following sections categorize the properties of metal ions that most strongly affect their sorption by polymer-supported reagents based on literature data. The emphasis is on transition metal ions. The examples of polymer-supported reagents illustrate the relationships between ionic properties and observed selectivities.

Evidence for ionic size as a factor in complexation was found in studies of small molecule complexes, particularly with macrocyclic polydentates [59–63]. Macrocycles encapsulate metal ions. Ionic radius determines which metal ion gets complexed and the stability of the complex. For example, the lanthanide(III) complexes of 1,4,7,10-tetrakis(carbamoylmethyl)–1,4,7,10-tetraazacyclodecane showed an increase in stability constants across the series with optimum stability found with Sm(III) followed by a decrease to Lu(III) [64]. The cyclic tetramer p-t-octylcalix[4]arene tetracarboxylate has a high selectivity for Pb(II) over Fe(III), Al(III), Cu(II) and Zn(II) ions [65]. Immobilization of this extractant on polyallylamine, PAA, produced a lead-selective resin [66]. The resin, PAA-Calix (Fig. 7), exhibited a selectivity order Pb  Cu  Zn = Ni = Co, where the maximum sorption of lead occurs at a pH of 4.5. Sorption studies using unfunctionalized PAA confirmed that the sorption behavior of PAA-Calix was due to the calix[4]arene component and not the free amino groups of PAA. A similar selectivity for lead ion by PAA-Calix and by the soluble tetramer was found. It was postulated that the size of the lead ion most closely matches the size of the coordinating sites of the calixarene derivative. While this is a significant factor, the preference for lead was also shown by other macrocyclic ligands bearing oxygen donor sites such as crown ethers [67]. A sandwich complex of Zn(II) with the 4-vinylbenzyl derivative of 1,4,7-triazacyclononane (TACN) was copolymerized in the presence of divinylbenzene [68]. A 2:1 ratio of TACN:Zn in the polymer matrix was obtained. The polymer (Fig. 8) was then treated with 6 N HCl to remove the Zn(II) ions. The demetallated resin was selective for Zn2+ in the presence of Ni2+, Co2+ and Mn2+ (Mn2+ < Ni2+ < Co2+ n Zn2+) at pH 4.5. The 2:1 ratio of TACN:Zn was retained. The Zn(II) thus acted as a template to produce cavities within the polymer that were of the same size as the ion and those cavities led to the recognition of Zn(II) in the subsequent loading cycle. However, when Cu(II) ions were present, the resin had a higher affinity for Cu(II) compared to Zn(II) at the same pH. While Cu(II) and Zn(II) ions have similar ionic radii, the results indicate that ionic radius is important but other factors also affect the selectivity (vide infra). The competition between Cu(II) and Fe(III) was studied at pH 2. A high selectivity was noted for Cu(II) relative to Fe(III) and this was attributed to the difference in ionic radii between the two ions. A complex similar to the one described above was prepared using mercury ions instead of zinc [69]. The demetallated polymerized sandwich complex of [{mono-N-(4-vinylbenzyl)-1,4,7-triaza-

H 2C

CH CH2 NH CH2

n

CH2

4

OCH2COOH Fig. 7. Calix[4]arene tetracarboxylic acid immobilized on polyallylamine (PAACalix).

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Zn2+ NH

N N H

Fig. 8. Polymerized Zn2+ sandwich complex.

cyclononane (TACN)}2 Hg](OTf) for Hg(II) at pH 2.  2 was selective  High selectivity coefficients KHg2þ =Mnþ were found for mercury in the presence of metals with smaller ionic radii (Fe3+, Cu2+, Cd2+) and larger ionic radii (Pb2+, Ag+). In the absence of Hg(II), the demetallated resin complexes low levels of other ions (0.87, 41.2 24.1 and 130.6 lmol/g Pb2+, Fe3+, Ag+ and Cu2+, respectively). These observations suggest the prevailing factor determining selectivity is ionic size. However, the ratio of TACN:Hg in the polymer was 1:1; when a metal ion with a large ionic radius (such as Hg2+) is used as the template, the large distance between the TACN ligands may allow them to coordinate metal ions in a manner independent of the sandwich arrangement. The polymer may not be recognizing the metal ions based on the cavity size created from imprinting. The random arrangement of TACN ligands have a low affinity for Cu2+, Co2+, Ni2+, Zn2+ and Mn2+ and a high affinity for Hg2+. Thus the polymerized sandwich arrangement of TACN ligands achieves good selectivity for metal ions with smaller ionic radii by the template method through spatial recognition. An ion-exchange resin with a high capacity and selectivity for the uranyl ion was prepared by imprinting. The synthesis involved the copolymerization of styrene and uranyl vinylbenzoate crosslinked with divinylbenzene [70]. When the uranyl ion was removed, the binding sites for recognition of the uranyl ion were retained by the polymer. When the polymer was contacted with a solution containing a mixture of ions, there was a preference 2+ 2+ 2+ 3+ for UO2þ at pH 3.5. A comparison 2 compared to Ni , Cd , Cu , Fe of sorption properties of the imprinted polymer to a non-imprinted polymer (polymer prepared without the uranyl ion) for the uranyl ion demonstrated that imprinting had improved its affinity and selectivity. The rebinding capacity (lmol/g) was 0.407 for the uranyl-imprinted polymer and 0.021 for the non-imprinted polymer at pH 3.5. Furthermore, a Ni(II)-imprinted polymer was prepared in a manner similar to the uranyl-imprinted polymer. There was lower sorption of uranyl by the nickel-imprinted polymer at pH 3.5 and 5.3 (0.058 and 0.321 lmol/g, respectively). This was due to a difference in the size of the cavity created by the nickel ion due to the orientation of the functional groups from the size of the uranyl ion. A polar support, poly (glycidyl methacrylate-co-ethylene glycol dimethacrylate), (p(GMA-O)), and its thiirane analog (p(GMA-S)) were prepared (Fig. 9) and modified by the incorporation of aza crown ether-type ligands; [15]aneNO4, [15]aneN2O3 and [18]aneN2O4, (Fig. 10A) [71]. The metal ion affinity of p(GMA-O)-[15]aneN2O3 generally decreased with decreasing metal ion radii: Ag+ > Pb2+ > Cd2+ > Cu2+ > Co2+ > Zn2+  Ni2+ (exceptions were Ag+ and Cu2+). The pendent hydroxy groups of the p(GMA-O)-[15]ane-

O

n

N

HN

CH3

C

E

O

O

n

H N

N2O3 resin destabilized the complexes of small metal ions compared to those of larger metal ions which explained the preference of p(GMA-O)-[15]aneN2O3 for larger ions. Metal nitrate solutions were used for Ag+ and Pb2+ ions experiments. The order described above, (Cd2+ > Cu2+ > Co2+ > Zn2+  Ni2+), was not observed for p(GMA-S)-[15]aneN2O3, which showed a reversal in the affinity of Cd2+ and Cu2+ from buffered metal chloride solutions at pH 3–7, under non-competitive conditions. This difference in the trend was attributed to a replacement of hydroxy groups with thiol groups, which have a stronger binding affinity. Smaller ions such as Cd2+, Cu2+ and Zn2+ could not fit in the large cavity of p(GMA-O)-[18]aneN2O4 and had a low uptake capacity. For larger metal ions such as Ag+ and Pb2+, metal uptake by p(GMA-O)-[18]aneN2O4 and p(GMA-O)-[15]aneN2O3 resins were similar. A Hg(II)-imprinted polymer is prepared by polymerizing methacrylic acid and trimethylolpropane trimethacrylate in the presence of Hg(II)-1-(2-thiazolylazo)-2-naphthol (TAN-Hg, Fig. 10B) [72]. Extraction efficiencies of Hg(II) are compared for the imprinted polymer, P(TAN-Hg), a blank P(B) that was prepared without the template, and a polymer prepared with the ligand only

CH3

C

L

O EH

Fig. 9. Structure of p(GMA-O) (E = O) and p(GMA-S) (E = S) before and after immobilization by ligand L.

H N O

O O

O N H

7, 16-diaza-1, 4, 10, 13-tetroxacyclooctadecane, [18]aneN2O4

H

O N O

N

H

O

7, 13-diaza-1, 4, 10-trioxacyclopentadecane, [15]aneN2O3

N H

O O

O O

13-aza-1,4,7,10-tetroxacyclopentadecane, [15]aneNO 4 Fig. 10A. Aza crown ethers.

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Fig. 10B. Hg(II)-1-(2-thiazolylazo)-2-naphthol.

P(TAN). At pH 7, almost 100% extraction of Hg(II) is obtained with P(TAN-Hg), while P(B) and P(TAN) showed 70% and 40%, respectively. High selectivity for Hg(II) is confirmed by competitive studies of Hg(II) sorption in the presence of CH3Hg(I), Cd(II), Co(II), Cu(II), Ni(II), Pb(II) and Zn(II). The selectivity coefficient for Hg(II) is large in all cases, with the highest when compared to organic mercury. The distribution coefficient (D) for Hg(II) with P(TANHg) is the highest (49) while the corresponding D values for the other ions range from 0.67 for Pb(II) to 0.07 for Zn(II).

5.2. Coordination geometry of the metal ion The coordination number and geometry of transition metal ions have been found to affect the apparent selectivities of polymersupported reagents. This holds for oligomeric chelators [73] as well as for larger molecules such as proteins [74]. Evidence was obtained from single crystal X-ray diffraction (XRD) and EXAFS spectra of complexes in the solid state and from computer modeling calculations of complex formation [75]. Infrared spectra also identify the ligands that participate in complexation from shifts in the band frequencies. The coordination number is defined as the number of ligand donor atoms connected to the central metal ion. Coordination geometry refers to the spatial arrangement of the ligands around the central ion. It is possible for a metal ion with a particular coordination number to have more than one geometrical arrangement. The ionic selectivity of polymer-supported reagents can be enhanced by creating pre-organized binding sites or fixing the position of the binding site so that a given metal ion can adopt a preferred coordination number and geometry. One way this is achieved is through the preparation of ion-imprinted polymers [76,77]. In the preparation of these polymers, a cavity is created in which the specific geometric configuration for a particular metal ion is retained. When the imprinted polymer is contacted with a solution of metal ions, the ion which best fits the geometric requirements of the template ion will be complexed preferentially. Covalent and non-covalent interactions make it possible to fix binding sites and improve selectivities and stabilities of metal ion binding. The imprinted polymer is more selective for the template ion compared to the non-imprinted polymer for the same metal ion. The geometry around an ion and its size are related. As discussed earlier, ionimprinted polymers also show selectivity based on ionic radius. A surface-templated resin was prepared by emulsion polymerization using monomers, dioleylphosphoric acid and divinylbenzene in the presence of copper acetate; L-glutamic acid dioleylester ribitol was added as a surfactant [78]. The resin was washed with 1 M HCl to remove the bound copper and its affinity for Cu(II) investigated. A Zn2+ imprinted polymer and a non-imprinted polymer were also made and their affinity for Cu2+ examined. All resins showed an increase in %Cu2+ ion sorbed upon increasing solution pH. The sorption increase was most significant for the Cu2+ imprinted polymer: it removed 100% of the copper ions compared to the non-imprinted and Zn2+-imprinted polymers (both sorbed about 50% Cu2+ at pH 6). Imprinting with Cu2+ arranged the dioleylphosphoric acid groups so that a favorable coor-

dination (most likely square planar) around the Cu2+ ion was formed upon contact. This spatial arrangement was not met by the Zn2+ imprinted (which favors a tetrahedral arrangement) nor the reference resin, the latter having a random arrangement of groups. Another Cu(II)-selective imprinted polymer containing methacryloylamidohistidine crosslinked with ethylene glycol dimethacrylate was successfully developed [79]. In the presence of Zn(II), Ni(II) and Co(II) at pH 7 and 25 °C, Cu(II) was preferentially sorbed: 42.1 mg/g Cu(II) was sorbed compared to 18.3, 11.7 and 8.2 mg/g of Zn(II), Ni(II) and Co(II), respectively. The uptake of metal ions on the non-imprinted polymer was comparable: 16.8 and 14.4 mg/g for Cu(II) and Zn(II), 10.8 and 9.3 mg/g for Ni(II) and Co(II). Distribution coefficients, Kd, indicated that the imprinting was successful – Kd for Cu(II) was 5329 while the next highest Kd was 577 for Zn(II), while the Kd for non-imprinted polymer was 506 for Cu(II) and 405 for Zn(II). Analysis of ratios of selectivity coefficients for Cu(II) in the presence of Zn(II), Ni(II), or Co(II) showed that Cu2+ uptake was 7.4, 9.5 and 12.3 times more selective from a mixture of Cu2+/Zn2+, Cu2+/Ni2+ and Cu2+/Co2+, respectively, for imprinted than non-imprinted polymers. Metal ion specific binding sites can be designed using acyclic multidentate ligands. Cu(II) selectivity was obtained by synthesis and polymerization of a Cu(II)–triethylenetetramine complex [80]. The ligand was a mono- or di-subsituted vinylbenzyl chloride triethylenetetramine complex (mVb-TETA or dVb-TETA) which binds Cu(II) through its nitrogen donor sites (mVb-TETA–Cu or dVb-TETA–Cu). Copolymerization of the copper complex with 2ethyl-2-(hydroxymethyl)propane-1,3-diol trimethacrylate (TRIM) formed a macroporous polymer. The binding of Cu(II) to the ligand gives a conformation that will be specific for an ion with the geometric requirements of Cu(II) and, when polymerized, this conformation will be ‘‘locked-in”. Ultraviolet–visible (UV–Vis) spectra indicated that the geometry of the Cu(II)–ligand complex was retained when polymerized; for the monosubstituted TETA–Cu(II) complex, a square planar geometry (polymer and monomer had absorption peaks at similar wavelengths); a distortion of the planar geometry was observed for the divinylbenzyl-TETA–Cu(II) complex. Polymers with 5% and 10% mVb-TETA–Cu and dVb-TETA–Cu were prepared. The Cu(II) uptake was 17 mg/g for 10% mVb-TETA–Cu and dVb-TETA–Cu, 13 mg/g for 5% mVb-TETA–Cu and 7 mg/g for 10% dVb-TETA–Cu The corresponding values of Cu(II) complexed for polystyrene-TRIM and poly(dVb-TETA–TRIM) prepared without the copper template, were 1–3 and 1.9–1.3 mg/g. The selectivity for Cu(II) was determined with the 10% mVb-TETA–Cu resin by contacting it with a solution of Zn(II) ions followed by a solution of Cu(II) ions or a solution of Zn(II) and Cu(II) ions in a 1:1 ratio. The amounts of Cu(II) bound were the same in both cases; the maximum binding capacity was 13 mg Cu/g polymer. Polymer-supported catechol ligands were selective for Fe3+ at pH 1–3 in the presence of divalent (Cu2+, Zn2+, Mn2+, Ni2+, Mg2+) and trivalent (Al3+, Cr3+) metal ions [81]. The selectivity was 11 times greater (see Table 1) when the sulfonated catechol ligand (Fig. 11) was changed to a sulfonated 3,3-linear tris(catechol) amide (PS-3,3-LICAMS), a ligand predisposed for octahedral geometry (Fig. 12). It was evident that the catechol ligands were involved in chelate formation since the sulfonated and unsulfonated polymers (PS-CATS and PS-CAT) had similar selectivities for Fe3+ from aqueous solution. The FT-IR spectrum of the complex, [(PS-CATS)2Fe] showed free sulfonic acid bands at 1220 and 1175 cm1 while for the [(PS-3,3-LICAMS)Fe]3 complex, free sulfonic acid bands were observed and the C–O stretch shifted from 1270 cm1 to 1257 cm1. Immobilization of the sulfonated bis(catechol) linear amide (PS2–6-LICAMS) ligand, (Fig. 13), also showed a high selectivity for tri-

551

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554 Table 1 Km values for the sulfonated catechol polymers. Relative equilibrium selectivity coefficients, Km with Mg2+ as standard, i.e. (Mn+/Mg2+)

PSCATS

PS-3,3LICAMS

PS-2–6LICAMS

Fe3+ Cr3+ Al3+ Mn2+ Cu2+ Zn2+ Ni2+ Mg2+

171 66 43 5.0 0.8 0.8 0.3 1

1817 443 121 6 4 2 3 1

65 28 6 1 1 1 1 1

n

CH2 OH

NaO3S

OH

echol groups in PS-2–6-LICAMS had no effect on the order of selectivity. Derivatives of a tetradentate ligand, N,N0 -bis(2-pyridylmethyl)0 2,2 -diaminobiphenyl, and a tridentate ligand, N-(2-pyridylmethyl)-2,20 -diaminobiphenyl were bonded to crosslinked chloromethylated polystyrene [82]. Small molecule chelate complexes of the tetradentate ligand were prepared with Cu(II) and Pd(II), confirming the rationale that this ligand is predisposed to form complexes with a square or tetrahedral geometry due to arrangement of its donor nitrogen atoms. Solvent extraction studies in chloroform solutions of the tetradentate and tridentate ligands (Figs. 14 and 15) confirmed that Cu(II) is selectively chelated from a mixture of Cu(II) and Fe(III). Iron(III) commonly forms 6-coordinate octahedral complexes. Phenolic tridentate and tetradentate analogs of the above ligands were immobilized via the phenolic group (Figs. 16 and 17). The O-alkylated polymers had good binding affinities for Cu(II). For the tetradentate group, when the polymer was attached ortho to the methylamino groups, the Cu(II) uptake capacity averaged 1 mmol/g. When methyl groups were introduced ortho to the pyridine donor atoms (in both rings), affinity for Cu(II) was reduced. This could result from a change of ligand stereochemistry

SO3Na Fig. 11. Polystyrene-supported sulfonated catechol, (PS-CATS).

H2C NH HN CH2 NaO3S

n

OH

N

N

OH CO CH2N(CH2)3

Fig. 14. N,N0 -bis(2-pyridylmethyl)-2,20 -diaminobiphenyl (tetradentate ligand).

N(CH2)3

CO

CO OH OH

NaO3S

NH

OH NaO3S

NH2 HN CH2

OH

Fig. 12. Polystyrene-supported sulfonated 3,3-linear tris(catechol)amide, (PS-3,3LICAMS).

N

n

Fig. 15. N-(2-pyridylmethyl)-2,20 -diaminobiphenyl (tridentate ligand).

valent ions. When the divalent ion affinity of PS-2–6-LICAMS, a potentially square planar ligand, was compared to PS-3,3-LICAMS and PS-CATS, PS-2–6-LICAMS was found to be the most selective for divalent ions, although no selectivity was observed with any divalent ion (Table 1). Also, changing the cavity size (from two to six CH2 moieties) on the linear amide chain between the two cat-

CH2O

CH2

NH HN CH2 N

n Fig. 16. O-alkylated polymer of N-(2-pyridylmethyl)-2,20 -diaminobiphenyl.

CH2N

(CH2)2-6 NH CO

CO HO

n

OH

CH2O

NaO3S

OH HO

SO3Na

Fig. 13. Polystyrene-supported sufonated bis(catechol) linear amide, (PS-2–6LICAMS).

H2C NH

NH CH2

N

N

Fig. 17. O-alkylated polymer of N,N0 -bis(2-pyridylmethyl)-2,20 -diaminobiphenyl.

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or steric effects restricting access to binding sites. There was no affinity for Fe(III) by any of the polymers. 5.3. Metal ion polarizability Polarizability, a, refers to the ability of an atom or ion to be distorted by an electric field [83]. Polarizability is related to softness: atoms which hold onto their electrons less firmly are more polarizable and are described as ‘‘soft”. ‘‘Hard” indicates low polarizability and the electron cloud of the atom is less easily deformed. According to the Lewis concept of acids and bases, the metal ion in a complex or coordination compound is the electron pair acceptor or Lewis acid while the ligand is the electron pair donor or a Lewis base. Soft acids prefer to coordinate with soft bases while hard acids prefer hard bases. Hard–soft acid–base (HSAB) theory is used to predict the stability of compounds [84]. HSAB theory has been correlated with frontier orbital energies of the reactants. When soft acids and soft bases interact, the energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO) is small, so that electron transfer occurs and covalent bonding results [85]. A large HOMO–LUMO band gap is found in hard–hard interactions and the bonding is electrostatic. The first classification of Lewis acids and bases into class ‘‘a” and class ‘‘b” was based on their preferential binding. Later, class ‘‘a” donors and acceptors were defined as ‘‘hard” and class ‘‘b” as ‘‘soft”. Molecules and ions of intermediate character were termed ‘‘borderline”. Polarizability is related to the electronegativity of an atom: highly electronegative atoms are hard; softness decreases from carbon to fluorine across the second row of the periodic table. Polarizability is also a function of atomic number/size: larger atoms are softer than smaller atoms of similar electronegativity. The charge on an atom affects its polarizability: metal cations become harder as the oxidation number increases. Table 2 gives examples of metal ions and atoms/molecules categorized by HSAB theory. There have been attempts to quantify hardness/softness with some physical parameter(s). A parameter known as absolute hardness, g, was identified [86]. Absolute hardness was calculated from ionization potential and electron affinity values. However, determining absolute hardness for polyatomic ions was not accurate, making it difficult to rank acids and bases according to the degree of hardness. In the following examples, a correlation is

Class b or soft

Borderline

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

Cu+, Ag+, Au+, Tl+, Hg+, Cs+ Pd2+, Cd2+, Pt2+, Hg2+, CH3Hg+

Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+,

Tl3+, Tl(CH3)3, BH3

B(CH3)3, SO2, NO+

2+

2+

2+

3+

3+

Be , Mg , Ca , Sr , Sn , Al , Sc , Ga3+, In3+, As3+, Ir3+, Si4+, Ti4+, Zr4+, Th4+, Pu4+, VO2+ 2+ UO2þ 2 , (CH3)2Sn , BeMe2, BF3, BCl3, B(OR)3, Al(CH3)3,

þ þ þ þ RPOþ 2 ; ROPO2 ; RSO2 ; ROSO2 ; SO2 ; R3 C , RCO+, CO2, NC+

RS+, RSe+, RTe+

S

S

N H

10-aza-1, 4- dioxa-7, 13- dithiacyclopentadecane, [15]aneNO2S2 O S

S

O NH

O

S

HN

7, 13-diaza-1,4-dioxa-10-thiacyclopentadecane, [15]aneN2O2S

I+, Br+, HO+, RO+, I2, Br2, ICN, etc. Trinitrobenzene, etc. Chloranil, quinones, etc. Tetracyanoethylene, etc. O, Cl, Br, I, R3C(?) M0 (metal atoms) Bulk metals

R = alkyl or aryl group.

O

7-aza-1-oxa-4, 10-dithiacyclododecane, [12]aneNOS2

Class a or hard

2+

O

N H

Table 2 Classification of Lewis acids.

2+

observed between the polarizabilities of ions and ligands based on HSAB theory. The immobilization of mixed crown ethers containing N, S and O atoms (Fig. 18) on copolymers of glycidyl methacrylate and ethylene glycol dimethacrylate (p(GMA-O), (Fig. 9) produced resins that extracted Ag+ in the presence of divalent ions (Pb2+, Cu2+, Cd2+, Zn2+) with negligible affinities. For example, (p(GMA-O)[15]aneNO2S2) had a Ag+ capacity of 1.04 mmol/g [87] and (p(GMA-O)-[16]S4N) had a capacity of 1.39 mmol/g [88] in the pH range of 1–6. Resins containing macrocycles with a higher number of sulfur atoms showed higher Ag+ complexation: (p(GMA-O)-[15]aneN2O2S), with one sulfur atom, had a maximum Ag+ capacity of 0.58 mmol/g at pH 4.7. The ability of crown ethers to complex Ag+ is related to the polarizability of the donor atoms [89]. Note that (p(GMA-O)-[12]aneNOS2), with a smaller ring size, had a capacity of 1.05 mmol Ag+/g resin [87]. Ag+ is a soft Lewis acid and will have an affinity for soft bases with sulfur atoms. Hence the observed trend is expected based on the relative polarizabilities. Further evidence for the affinity of Ag+ for sulfur ligands was the observation that the hydrolysed resin (p(GMA-S)), (Fig. 9), with thiol groups had a capacity of 1.5 mmol Ag+/g resin, while p(GMA-O), when treated similarly, sorbed only 0.01 mmol Ag+/g resin. Chelating resins containing bis(sulfonamide) groups (Fig. 19) were immobilized on Amberlite XAD-2, a commercial styrene– divinylbenzene macroporous resin [90]. Metal sorption was studied from pH 1–7 and was optimum at pH 5.5 for Cu(II) and Zn(II) and pH 6.0 for Cd(II) and Pb(II). Copper and zinc uptakes were greater than cadmium and lead. Generally, the metal capacity increased, though not significantly, with III > II > I. IR spectra suggest

S

S S

H N

S

7-aza-1, 4, 10, 13-tetrathiacyclohexadecane, [16]aneS4N Fig. 18. Crown ether ligands with sulfur, nitrogen and oxygen donor atoms.

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554

553

n

the metals that were sorbed are soft acids and show preference for resins bearing soft bases such as sulfur groups in the polymers.

SO2NH R NHSO2 n

I : R= (CH2)2 II : R= (CH2)3 III : R= (CH2)2NH(CH2)2 Fig. 19. Bis(sulfonamide) resins.

n

that metal ions coordinate to the resin via the amino-nitrogens. There were shifts in the IR spectra for N–H and C–N bands while bands due to C–H and SO2 were not greatly affected. The IR data were comparable for metal complexes with diamine or polyamine groups. Of the metals studied, Cd(II) and Pb(II) are more polarizable than Cu(II) and Zn(II). Nitrogen-containing ligands are considered borderline, preferring metals that are of intermediate hardness/softness. Amines are known for their affinity for Cu(II) [91]. Resin III has the greatest ability to separate Cu(II) from mixtures of Cu(II) and Pb(II) compared to other binary mixtures with Cu(II). The separation factors, K, are: 38.74 (resin III), 22.36 (resin II) and 12.81 (resin I) for the Cu(II)–Pb(II) binary mixture. This is as a consequence of the high affinity for Cu(II) and lower affinity for Pb(II) by the donor nitrogen atoms. Poly(2-hydroxyethylmercaptomethylstyrene–diethanolamine) or (PSME–EDA) is a novel chelating resin that contains S, N, and O atoms (Fig. 20) [92]. This resin has a high affinity for mercury(II) ions from a solution of mercury nitrate buffered at a pH of 5.4. Uptake of Hg(II) ions involved both N and S atoms on the resin as seen by X-ray photoelectron spectroscopy (XPS) (after complexation with Hg(II), the binding energy for N1s and S2p increased while there was almost no change for O1s). A new peak for N1s suggested that Hg(II) was sorbed as Hg(NO3)2. This is consistent with Hg(II), a highly polarizable metal, will have strong affinities for ligands with the polarizable sulfur and nitrogen. Polymers of vinyl 2-hydroxyethyl sulfide (PVHES) and vinyl 2hydroxyethyl sulfide–acrylamide (PVHES–AA) were prepared with the cross linker N, N-methylenebisacrylamide (Fig. 21). Their efficiency as sorbents for metals such as Au(III), Hg(II), Pd(II), Pt(IV), Cu(II), Fe(III) were examined from dilute and concentrated HCl and HNO3 solutions [93]. Copper and iron were not sorbed in appreciable amounts. Among the highest distribution coefficients were, for PVHES, Au(III) 19800; Pd(II) 11606; Hg(II) 1913; and for PVHES–AA, Au(III) 12000; Pt(IV) 200; Pd(II) 168; Hg(II) 167. All

S CH2 CH2 OH

C O NH2

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

CH2CH2OH

[30]

Fig. 20. Poly(2-hydroxyethylmercaptostyrene–diethanolamine) PSME–EDA.

(CH2CH)y

Much is known about polymer-supported reagents and they have been intensely studied. The key variables by which transition metal ions are selectively complexed based on their properties have been outlined: ionic radius, geometry, and polarizability. Solution conditions (pH, counter ions, solvent polarity) often influence the apparent sorption. While relying on one property to predict affinities can be an oversimplified approach, it does oftentimes provide a good first approximation to the observed selectivities. The results do indicate that the polarizability properties of the ion and the ligand can dominate the final results. An understanding of the mechanisms of interaction via property-function relations can be a useful tool in the design of polymer-supported reagents.

CH2CH2OH CH2SCH2CH2N

(CH2CH)x

6. Concluding remarks

(CH2CH)z HN

C O

CH2 HN C O

(CH2CH)n

Fig. 21. Copolymer of vinyl (2-hydroxyethyl) sulfide and acrylamide crosslinked with methylenebisacrylamide.

[31] [32] [33] [34] [35] [36] [37]

[38] [39] [40] [41] [42]

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