A Highly Selective Compound for Lead: Complexation Studies of

cations. The selectivity factor is >105.5. Thermodynamic quantities (log K, ΔH, and ΔS values) for the interactions of MC5 with metal ions were dete...
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A Highly Selective Compound for Lead: Complexation Studies of Decamethylcucurbit[5]uril with Metal Ions Xian Xin Zhang, Krzysztof E. Krakowiak,† Guoping Xue, Jerald S. Bradshaw,*,‡ and Reed M. Izatt* Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602

Calorimetric and potentiometric studies indicate that a cage-type compound, decamethylcucurbit[5]uril (MC5), shows very high selectivity for Pb2+ over alkali, alkaline-earth, NH4+, and Cd2+ cations. The selectivity factor is >105.5. Thermodynamic quantities (log K, ∆H, and ∆S values) for the interactions of MC5 with metal ions were determined at 25 °C by calorimetric titration in 50% formic acid/water (v/v) and by potentiometric titration in an aqueous solution. The log K(H2O) value for protonation of MC5 was determined to be 9.56. Pb2+ forms both 1:1 and 2:1 (Pb2+/MC5) complexes in 50% formic acid/water and aqueous solutions, while all other cations studied form only 1:1 complexes with MC5 in the 50% formic acid/water solution. However, both 1:1 and 2:1 complexes have been detected in an aqueous solution for Sr2+, indicating that the stoichiometry of MC5 complexes in 50% formic acid and in aqueous solutions could be different. Introduction The cage-type compound cucurbituril was first synthesized by Behrend and co-workers in 1905.1 They believed that they had synthesized a novel compound consisting of macromolecular rings. Seventy-six years later, Freeman, Mock, and Shih reinvestigated the compound and confirmed its cage-shape structure by X-ray diffraction analysis for its Ca2+ complex.2,3 Cucurbituril has structural similarities to calixarenes, which are useful for many applications4-6 and have at least 2000 papers published concerning them.7 However, cucurbituril, which has an upper and a lower rim similar to those found in the calixarenes, has not been functionalized yet.8-10 Only a few compounds,1,11,12 including cucurbit[6]uril (CC6)1 and decamethylcucurbit[5]uril (MC5)11 (Figure 1), have been reported in about 50 papers. The CC6 ligand has an internal cavity with ∼5.5 Å diameter and is accessible from the exterior by two carbonyl-fringed portals with 4 Å diameters.2 The cavity within the CC6 has a dimension equivalent to those of the R-cyclodextrins (∼5.7 Å). Similar to the cyclodextrins, the interior of CC6 is a hydrophobic region capable of encapsulating hydrocarbon molecules. Each portal of CC6 consists of six urea carbonyl oxygens which are able to bind cations. The rigid structure of CC6 is expected to lead to high specificity in its host-guest interactions. However, CC6 is insoluble in all common solvents including water. Its acid-base chemistry has not been studied, and most binding studies have been performed in formic acid/water mixtures. Complexation of CC6 with alkylammonium ions has been studied extensively, and it has been shown that the cationic heads of the alkylammonium or alkyldiammonium ions associate with the negative ends of the carbonyl dipoles of CC6 and the hydrocarbon tails extend into the core of the † Current address: IBC Advanced Technologies, Inc., P.O. Box 98, American Fork, UT 84003. ‡ Phone: (801) 378-2415. Fax: (801) 378-5474. E-mail: [email protected].

Figure 1. Structures of cucurbiturils.

CC6 cage.8-10 Structurally matched alkylammonium and alkyldiammonium ions were found to form the most stable complexes.2,8-10,13-15 Studies on metal-ion complexation with CC6 have not been as fruitful as those on alkylammonium ions. Early data published by Buschmann and co-workers suggested the complexation of two metal ions to one CC6 molecule in an aqueous solution (each portal was occupied by one metal ion).16 Later, 1:1 complexes were observed in

10.1021/ie0001725 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/25/2000

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formic acid/water mixtures.17,18 Recently, Kim and coworkers have found that the stoichiometry of solid complexes of CC6 is different from that in solutions.19-21 For example, it has been found that each CC6 molecule binds four Na+ ions in the solid state in which each portal binds two Na+ ions and five water molecules.19 The molecule of CC6 is quite rigid, and its conformation hardly changes during the complexation. The published crystal structural results show only small distortions of the CC6 molecule in the metal-ion complexes.3,19-21 The second cucurbituril compound, MC5, is less known, and up until now, only one communication regarding it has been published.11 The cavity diameter of MC5 is about 6 Å, and the portals have ∼2.5 Å diameters. No metal-ion complexation data of MC5 have been published. Lead is of environmental concern,22 and studies have been carried out to find highly selective compounds for lead.23 In this paper, we report thermodynamic data for complexation of MC5 with alkali, alkaline-earth, two ammonium, lead, and cadmium cations. The interactions were evaluated in a 50:50 (v/ v) formic acid/water mixture (50% formic acid) by a calorimetric titration method and in an aqueous solution by a potentiometric method. Both studies revealed a very high selectivity for lead by MC5. Experimental Section Materials. Ligands MC5 and CC6 were synthesized as reported.1,11 LiCl (Fisher), NaCl (Mallinckrodt), KCl (Aldrich), KNO3 (Mallinckrodt), RbCl (Fisher), CsCl (Fisher), MgCl2‚6H2O (Aldrich), CaCl2‚2H2O (Spectrum), SrCl2‚6H2O (Mallinckrodt), BaCl2 (Fisher), CdCl2‚ 2.5H2O (Baker), PbCl2 (Spectrum), Pb(NO3)2 (Mallinckrodt), NH4Cl (Mallinckrodt), (CH3CH2)4NCl (Fluka), and formic acid (Fisher, 88% aqueous solution) were used as purchased. The purity of all inorganic salts used was greater than 99%. Calorimetric Titration. log K, ∆H, and T∆S values for the interactions of the metal ions with MC5 and CC6 were determined by titration calorimetry as described earlier24 at 25.0 ( 0.1 °C in 50% formic acid using a Tronac 450 isoperibol titration calorimeter. The initial solution volume in the Dewar was 20 mL. The reliability of the equipment was tested by determining the log K and ∆H values in water for the interaction of 18-crown-6 with Ba2+, and the values obtained were in excellent agreement with the literature values.25 Concentrations of metal and ammonium ion solutions, except Pb2+, were 0.10-0.12 M, and those of MC5 and CC6 ligands were 2.0 × 10-3-3.2 × 10-3 M. The concentration of Pb2+ was 0.12 M and then increased to 0.22 M after a 2:1 (Pb2+/ MC5) complex was observed. The metal-ion solutions were titrated into the ligand solutions. Ionic strength and pH values were constant in a 50% formic acid solution. The titration experiments showed that, except for Pb2+, all host-guest interactions studied had a 1:1 cation/ligand ratio. The method used to process the calorimetric data and to calculate the log K, ∆H, and T∆S values has been described.26 Potentiometric Titration. The protonation and several stability constants (with Pb2+, Cd2+, Sr2+, and Ba2+) of MC5 were determined by potentiometric titration in an aqueous solution at 25.0 ( 0.1 °C. The titrations were carried out at a constant ionic strength of 0.05 M Et4NCl using an automatic microprocessorcontrolled potentiometric titrator.27 A 6 × 10-3 M MC5 solution (5.0 mL) containing either 1 × 10-2 M HCl (for

Table 1. log K, ∆H (kJ‚mol-1), and T∆S (kJ‚mol-1) Values for Interactions of Decamethylcucurbit[5]uril (MC5) and Cucurbit[6]uril (CC6) with Metal Ions Determined Calorimetrically at 25.0 °C in a 50% Formic Acid/Water (v/v) Solution ligand MC5

CC6

cation Li+

Na+ K+

Rb+ Cs+ NH4+ (CH3CH2)4N+ Mg2+ Ca2+ b Sr2+ Ba2+ Cd2+ Pb2+ c Li+ d Na+ d K+ d Rb+ d Cs+ d Mg2+ Ca2+ d Sr2+ d Ba2+ Cd2+ Pb2+

log K 1.99 ( 0.07 2.54 ( 0.05 3.48 ( 0.04 3.37 ( 0.03 2.36 ( 0.04 1.57 ( 0.06 2.93 ( 0.04 a a 2.40 ( 0.07 3.40 ( 0.07 3.02 ( 0.02 1.34 ( 0.08 >9 2.38 3.23 2.79 2.68 a a 2.80 3.18 3.08 ( 0.04 2.83d a 3.51 ( 0.07

∆H

T∆S

anion

ClClClNO3ClClClClCl17.1 ( 0.8 30.8 Cl-23.1 ( 0.4 -3.7 Cl-37.4 ( 0.3 -20.2 Cl108 ( 3 116 Cl-23.5 ( 0.8 >27.9 NO3-3.4 10.1 Cl-5.9 12.5 ClO4-2.3 13.6 NO3-1.5 14.2 ClClCl-6.5 9.4 NO3-10.6 7.5 NO3-13.2 ( 0.3 4.4 Cl-17.4d -1.3d ClO4Cl-13.6 ( 0.5 6.4 NO3-14.4 ( 0.8 -5.4 ( 0.3 -10.0 ( 0.5 -10.7 ( 0.3 -12.5 ( 0.6 -18.4 ( 0.5 -3.2 ( 0.1

-3.0 9.1 9.9 8.5 1.0 -9.4 13.5

a No measurable heat other than the heat of dilution indicating that ∆H and/or log K is small. b Determined by competitive calorimetric titration with SrCl2. c For 1:1 interaction. The log K value was evaluated by a competitive calorimetric titration with KNO3. Both direct and competitive titrations showed the formation of 1:1 and 2:1 (Pb2+/MC5) complexes. d From ref 17.

protonation constant measurements) or 1 × 10-2 M HCl and 6 × 10-3-1.2 × 10-2 M metal ions (for stability constant measurements) was titrated by an 8 × 10-2 M Et4NOH solution. Temperature was controlled within (0.1 °C by using a jacketed cell through which water from a constant-temperature bath was circulated. Potentials to within (0.1 mV were measured using an Orion model 701A digital ion analyzer in conjunction with a Cole-Parmer combination electrode. The electrode was calibrated as a hydrogen concentration probe by titrating known amounts of HCl with CO2-free Et4NOH solutions and determining the equivalence point by Gran’s method,28 which gives the standard potential E° and the ionic product of water Kw. Calculations were performed with the SUPERQUAD program29 using an IBM computer, and species distribution plots were obtained using the program SPE.30 The instrument was calibrated according to the methods recommended by IUPAC,31 and the protonation constants of glycine and stability constants for glycine-Ni2+ interactions in water obtained with our potentiometer were in excellent agreement with the recommended values.31 All thermodynamic parameters in this paper are the averages taken from two to four determinations. Uncertainties are given as standard deviations. Results and Discussion Log K, ∆H, and T∆S values for interactions of MC5 and CC6 with metal and ammonium ions in 50% formic acid are listed in Table 1. Also listed in Table 1 are like values for CC6-metal ion interactions determined by Buschmann and co-workers for comparison. The proto-

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Table 2. Logarithms of the Protonation Constant of MC5 (L) and of Stability Constants of Its Complexes with Metal Ions (M2+) Determined Potentiometrically at 25.0°C in an Aqueous Solution (µ ) 0.05 M, Et4NCl) reaction

log β

H+ + L h HL+

9.56 ( 0.01 Cd2+

M2+ + L h ML2+ 2M2+ + L h M2L4+ M2+ + L + H2O h M(OH)L+ + H+ 2M2+ + L + 2H2O h M2(OH)2L2+ + 2H+ a

9. There are no reported ligands containing all oxygen heteroatoms that have selectivities this large for lead over any of the alkali or alkaline-earth metal ions.32 The high stability of the Pb2+/MC5 complex and the high selectivity of MC5 for Pb2+ are also found in an aqueous solution (Table 2). Data in Table 2 show that both 1:1 and 2:1 Pb2+/MC5 complexes are much more stable than the MC5 complexes with Cd2+, Sr2+, and Ba2+. These results indicate that MC5 could be a practical reagent for selective separation and determination of lead and could be used under both neutral and acidic conditions. One possible application would be to immobilize MC5 on a polymer in order to selectively adsorb Pb(II) from aqueous solutions. The lead could be removed from the polymer using high concentrations of acids. The amount of lead recovered could be determined by routine analytical methods.

The high lead selectivity by MC5 is probably related to two factors: size match between Pb2+ and MC5 portals and rigidity of the MC5 molecule. The diameter of Pb2+ (2.4 Å)33 is very close to that of each of the MC5 portals (∼2.5 Å).11 In addition, the rigid structure of MC5 allows it to bind most tightly with a cation whose size best fits the portals. If a cation is too big (such as K+, Rb+, Cs+, or Ba2+) or too small (such as Li+, Na+, Ca2+, or Sr2+), the stability constants are several orders of magnitude lower than that of the lead complex. It is not surprising that Pb2+ binds CC6 much more weakly than MC5 because the diameter of CC6 portals (4 Å)2 is significantly larger than that of Pb2+. Another possible reason for high lead selectivity by MC5 is probably a better match of the Pb(II) coordination sphere by forming Pb(OH)/MC5+ or (PbOH)2/MC52+ complexes (see below) than that of the other cation complexes. Protonation Constant and Potentiometric Study. Because of the limited solubility of CC6 in water, interaction of CC6 with H+ was evaluated by Buschmann et al.16 using a UV spectroscopic method. They reported a log β value of 6.04 and assumed a 1:2 CC6/ (H+)2 complex. The higher water solubility of MC5 allowed us to determine the protonation constant for MC5 in an aqueous solution using a potentiometric technique. We found only one protonation constant for MC5 (Table 2). The log K value, 9.56, indicates that MC5 behaves as a base in an aqueous solution. The difference in protonation constants between MC5 and CC6 molecules could be a result of different distances between the portals and different sizes of the portals.2,11 Each MC5 molecule should be strongly protonated with one H+ in a 50% formic acid solution. It is possible that one portal of an MC5 molecule is strongly bound with a proton and the other portal may be only solvated or interact very weakly with H3O+, leaving it available to bind a metal ion. Therefore, 1:1 complexes of MC5 and CC6 were observed for all metal ions studied (except Pb2+) in 50% formic acid. Only Pb2+, which interacts very strongly with MC5, can replace H+ to form a 2:1 complex. In an aqueous solution, MC5 forms highly stable complexes with Pb2+ but interacts weakly with Cd2+, Sr2+, and Ba2+, as mentioned above. The stabilities of Cd2+ and Ba2+ complexes are too weak to be determined accurately. The stoichiometries of MC5 complexes in 50% formic acid and in an aqueous solution could be different. One example is seen for Sr2+. Sr2+ forms both 1:1 and 2:1 (Sr2+/MC5) complexes in water but forms only a 1:1 complex in 50% formic acid. Once the acidic conditions (such as 50% formic acid) are removed, the cations are able to replace the proton from the cucurbiturils, resulting in 2:1 complexes. Pb2+ forms both 1:1 and 2:1 Pb2+/MC5 complexes in an aqueous solution, but they exist as hydroxy forms of Pb(OH)/MC5+ and (PbOH)2/MC52+. The “normal” complexes of Pb/MC52+ and Pb2/MC54+ have not been detected. In the Pb2+/MC5 complexes, a hydroxy group (OH-) not only may act as a strong binder occupying the lead coordination sites opposite the MC5 portal but also effectively neutralizes the high positive charges of the complexes. Therefore, formation of hydroxy Pb2+/MC5 complexes is most likely an efficient way to lower the free energies of complexation. The species distribution of the Pb2+/MC5 system as a function of pH is shown in Figure 2. In acidic regions, the complex (PbOH)2/MC52+ and protonated MC5 are

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i.e., both ligands form the most stable complexes with Sr2+. However, MC5 demonstrates a complexation behavior different from that of CC6 in two aspects. First, MC5 shows an excellent selectivity for Pb2+, as has been mentioned, but the binding constant of Pb2+ with CC6 is only a little higher than those of other cations. Second, MC5 forms the most stable complex with K+, among the alkali cations, while CC6 forms the most stable complex with Na+. Acknowledgment Support of this work by the Department of Energy, Office of Basic Energy Sciences (Grant DE-FG02-86ER 13463), is gratefully acknowledged. Figure 2. Species distribution of the Pb2+/MC5 (L) system as a function of pH.

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the predominant species in the solution. The formation of binuclear hydroxy species of lead in the low pH solutions is due to the high stability constant (log β2 ) 17.5). The complex Pb(OH)/MC5+ begins to form at pH 5, and its amount increases with increasing pH. In alkaline regions (pH 9-11), Pb(OH)/MC5+ is the most important species (∼60%), but the amounts of the complex (PbOH)2/MC52+ and free ligand MC5 are also significant (both ∼20%). Thermodynamics of Complexation. The data in Table 1 indicate that, in most cases, complexation of MC5 and CC6 with metal ions is favorable in terms of both enthalpy and entropy changes (negative ∆H and positive ∆S values). The favorable T∆S values are consistent with the rigid structures of the ligands. During the complexation, the conformation of MC5 and CC6 molecules does not change by any significant degree, resulting in very small or no conformational entropy loss. As a result, desolvation of the ligands and metal ions leads to positive T∆S values. The large binding constant of Pb2+ with MC5 originates from large favorable enthalpy and entropy changes. On the other hand, the enthalpy and entropy changes for Pb2+/CC6 complexation are much lower than those for Pb2+/MC5 complexation. Among the cations studied, Ba2+ interactions with both ligands show the most favorable enthalpy gains. However, the largely unfavorable entropy change (in the case of MC5) or small T∆S values (in the case of CC6) result in Ba2+ complexes which are not highly stable. Both the T∆S and ∆H values for MC5/ Cd2+ complexation are large probably because of extensive desolvation of the ligand and the metal ion. Anions have an effect on thermodynamic quantities for the cucurbituril interactions with the metal ions. We examined the anion effect by a calorimetric titration of MC5 with K+ and of CC6 with Ba2+ using Cl-, NO3-, and ClO4- as the counterions (see Table 1). Larger log K and less negative ∆H values are found for the chloride anion as compared with nitrate and perchlorate anions. Therefore, we used chlorides where possible for calorimetric titrations. In the case of Pb2+, the nitrate was used because a > 0.1 M 50% formic acid solution of Pb2+ could not be prepared because of the limited solubility of PbCl2. Selectivities by MC5 and CC6. Stability constants for the binding of alkali cations by MC5 have a sequence K+ > NH4+ > Na+ > Rb+ > Li+ > Cs+, and those for the binding of alkaline-earth cations have a sequence Sr2+ > Ba2+ > Ca2+ > Mg2+. Among the alkaline-earth cations, MC5 shows the same selectivity order as CC6;

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Received for review February 7, 2000 Revised manuscript received June 23, 2000 Accepted July 4, 2000 IE0001725