Design and Synthesis of Polymer-Bound Penta-aza Ligand for

ARTICLE pubs.acs.org/IECR

Design and Synthesis of Polymer-Bound Penta-aza Ligand for Selective Adsorptive Separation of Cobalt(II) from Zirconium(IV) Pradipta Kumar, Rupa S. Madyal, Uttamkumar Joshi, and Vilas G. Gaikar* Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Road, Matunga, Mumbai 400019, India

bS Supporting Information ABSTRACT: A penta-aza ligand was designed using the Density Functional Theory (DFT), synthesized, and then loaded on a polystyrene matrix to prepare a functional polymer as a Co(II) selective adsorbent. A comparative experimental study on adsorption of Co(II) and Zr(IV) revealed that the polymer-supported ligand is more selective toward Co(II) than Zr(IV) from aqueous acidic solutions. The maximum adsorption capacities of the adsorbent for Co(II) and Zr(IV) were 4.1 and 0.5 mg/g, respectively, at pH 1.0. Vibrational frequencies of the metalligand complexes were calculated with the VWN-BP functional to confirm true local minima.

’ INTRODUCTION The use of ion exchange resins for separation of metal ions from aqueous solutions is widespread.13 Separation of heavy metal ions is also commonly encountered in the nuclear industry. The major challenge in these separations is in selectivity toward the desired metal ion when present at ultralow concentrations and in the presence of other charged species. Conventional ion exchange resins function on the basis of charges on the ions and may not significantly differentiate between different ionic species from each other. The resins then get loaded by the ions present at the highest concentrations, reducing many times the efficacy of the separation. The development of functional polymers with metal ion specific ligands is, therefore, increasingly sought for improved separation efficiency. A significant amount of development has already taken place for metal specific ligands in solvent extraction in the nuclear industry because of the ease of handling liquids. However, the concomitant contamination of the solvent in the extraction processes has been a matter of concern apart from radiation stability of the solvents and ligands. Use of a solid matrix with ion-specific ligands has now been attracting attention because the solid adsorbents can be contained easily in case of contamination. The availability of polymer matrices of different types and creativity of polymer chemists to load any ligand on a polymer matrix help in developing newer adsorbents for specific applications. Understanding the molecular interactions between metal ions and specific ligands becomes then imperative. Molecular design of a ligand as an ion exchanger, for ionspecific uptake, has increasingly attracted attention of physicists as well as chemists alike in the past few years.4,5 It is often possible to design a large number of hosts for a given metal ion with the knowledge of donoracceptor interactions and geometry of the coordination structure.6 Yet, very few of them would achieve applications in the field. We attempt here to develop a ligand for separation of Co(II) from its mixtures with Zr(IV) in highly acidic aqueous solutions. High-purity zirconium and its alloys (Zircaloys) have been proven to be the most vital cladding materials in nuclear r 2011 American Chemical Society

industries because of excellent corrosion resistance, high thermal conductivity, low absorption cross section for thermal neutrons, and excellent radiation stability.7,8 Cobalt is a common impurity in this cladding material. The content of cobalt is accurately known for determining the suitability for use in reactors as cobalt produces radioactive nuclides.912 The techniques to separate cobalt from zirconium include precipitation9 and chromatography.13 Precipitation of major constituents is commonly employed with minimum coprecipitation of other impurities from the solution. However, the process shows a disadvantage of loss of a large quantity of prime constituents. The chromatographic purification process involves chlorination of the cladding material to corresponding metal chlorides followed by dissolution in water and then separation of individual components through a chromatography column.14,15 Upon dissolution, ZrCl4 reacts with water to give ZrOCl2 and HCl. The chromatographic separation of ZrOCl2 and CoCl2 at high HCl concentrations in the aqueous solutions by ion exchange has its own limitations.13 Apart from the requirement of a large excess of a diluent, the chromatography system gets loaded by the large amount of ZrOCl2 in the solution because of its stronger affinity toward the ion exchange resins as compared to Co(II) ions. Cobalt forms an impurity in the solutions at very low concentrations, and an appropriate choice of an adsorbent, selective to cobalt ion, would bring down inventory of the solid adsorbent as well as reduce the amount of the aqueous waste that is generated using commonly available ion exchange resins. To the best of our knowledge, no ion-exchange resin that can selectively adsorb cobalt from highly acidic solutions of zirconium is known. Among different ligands that are used for the metal ion separations, the phenanthroline-based ligands form more stable complexes with the metals than those of linear analogues of Received: July 15, 2010 Accepted: May 16, 2011 Revised: May 13, 2011 Published: May 16, 2011 8195

dx.doi.org/10.1021/ie101517j | Ind. Eng. Chem. Res. 2011, 50, 8195–8203

Industrial & Engineering Chemistry Research similar strength and similar chelate rings. The chemistry of aza ligands derived from phenanthroline nucleus and their metal complexes is in a stage of rapid development.1620 We chose to use chelate compounds as they are highly preorganized and follow a lock and key complementarity rule of “best fit” of cavity size for a given metal ion and can take up the desired metal ion selectively. We report here the complete synthesis of a Cospecific ligand and its covalent binding to a polymer support.

’ EXPERIMENTAL SECTION Materials. All of the chemicals used were of laboratory grade. Special chemicals, such as 1,10-phenanthroline monohydrate, sodium hydride and dimethyl acetamide, dimethyl formamide, xylene, 3,4-dihydro-2H-pyran, cobalt(II) chloride, and ZrOCl2, all AR grade, were used as received from s.d. Fine Chemicals, Spectrochem, Ranbaxy Fine chemicals (RFCL), Molychem and Loba Chemi, Mumbai, without further purification. All reactions were conducted in oven-dried glassware under a N2 atmosphere. The progress of the reaction was monitored by thin-layer chromatography (TLC) using Merck silica gel 60F254 precoated aluminum sheets and visualized through a UV chamber. Column chromatography was carried out using 200400 mesh silica gel. Chloromethylated polystyrene was obtained from Auchtel Ltd. Mumbai, having chloride ion content of 3.5 mequiv/g, pore volume of 0.078 cm3/g, specific surface area of 29 m2/g, and an average pore diameter of 112 Å. Methods. The chemical analyses of different intermediates and the final products for carbon, hydrogen, and nitrogen contents were conducted using a Perkin-Elmer 240B Elemental Analyzer. Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a Perkin-Elmer 1720 using KBr pellets. Phase transition temperature (melting point) of the products was measured by differential scanning calorimetry (SHIMADZUDSC-60). The mass spectra were recorded on a ThermoElectron System (ESI-MS), and the electronic spectra were on a UVvis2700 double beam-spectrophotometer. H NMR spectra were recorded on a Mercury plus 300 MHz NMR spectrometer (Varian, U.S.). The H spectra were calibrated from internal TMS signal (0.0 ppm). The metal ion concentrations were measured on ICP-AES (ARCOS from M/s. Spectro, Germany). ICP-AES is one of the most important techniques for determination of cobalt and zirconium. In the operating condition, a CCD detector was used along with RF generator power of 1400 W, frequency of RF generator of 27.12 MHz, plasma flow rate of 12 L/min, auxiliary gas flow rate of 1 L/min, nebulizer of 0.8 L/min, and the pump speed was 30 rpm. The most sensitive line for cobalt is at 228.616 nm, and that of zirconium is at 339.1 nm with an instrument detection limit (IDL) of 10 ppb each. Density Functional Theory (DFT) Method. The geometries of hexa-aza and penta-aza ligands (L) and their complexes with Co2þ ion and ZrO2þ ion in the vacuum were initially optimized by FORCITE molecular mechanics (MM) module using Universal Force Field (UFF) of Material Studio (MS) (ver. 4.1, Accelerys, Inc.). The optimized geometries were further improved by DFT calculations of DMol3 module of the MS. The basis set DNP (double numerical plus polarization) was used for all of the DFT calculations with all electron core treatment for H, C, N, O, and Cl. For Co and Zr, only effective core potentials were used to save computational time. The DNP basis set includes a polarization p-function on all hydrogen atoms. The exchange correlation energy was calculated using generalized

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gradient corrected functional VWN-BP. Because no experimental data are available for these complexes, we confirmed the reliability of the functionals using the data on a cobalt(III) hexamine complex.21 Although we had done the optimization with both the quartet and the doublet multiplicity, because these complexes are outer orbital complexes, the quartet multiplicity, that is, high spin complexes, was considered for the optimization. Synthesis of Ligands. The synthesis of ligands is shown in Schemes 1 and 2. 2-Azido-1,10-phenanthroline (II). Sodium azide (1.0 g) and 2-chloro-1,10-phenanthroline (I) (2.0 g) in dimethyl acetamide (DMA) were heated to 120 °C for 6 h. DMA was distilled out under reduced pressure, and then water was added to separate out solid (II) (1.5 g, 75%). N-Acylated 2-Amino-1,10-phenanthroline (III). Sodium hydride (3.0 g) was added slowly to a solution of acetamide (7.5 g) in xylene (60 cm3) at 100 °C and stirred for 45 min at the same temperature. 2-Chloro-1,10-phenanthroline (I) (3.0 g) was then added, and the temperature was raised to 140 °C. The reaction mixture was then stirred at the same temperature for 7 h. The crude solid was filtered and extracted with dichloromethane (DCM) and purified through column chromatography using DCM as the eluent (2.8 g, 85%) 2-Amino-1,10-phenanthroline (IV). Sodium hydroxide (1.05 g) was added to a solution of N-acylated 2-amino-1,10-phenanthroline (III) (2.5 g) in methanol (15 cm3) and water (5 cm3) and then stirred at refluxing for 45 min. Methanol was removed under reduced pressure to separate out solid on addition of water. The solid product was filtered and dried at 90 °C under reduced pressure for 3 h (1.9 g, 95%). In an alternate procedure to get IV, NaBH4 (0.34 g) was added slowly to a solution of 2-azido-1,10-phenanthroline (II) (1.0 g) in dimethyl formamide (DMF) over a period of 20 min at 120 °C and stirred for 2 h. The excess NaBH4 was decomposed with addition of 0.1 N HCl by adjusting to pH 6, and DMF then was distilled out under reduced pressure to separate out solid IV (600 mg, 68%) on addition of water. N,N-Bis-[1,10-phenanthrolin-2yl]-amine (V). Sodium hydride (1.5 g) was added slowly to a solution of 2-amino-1,10phenanthroline (IV) (2.0 g) in DMA (40 cm3) at 100 °C and stirred for 30 min, followed by addition of 2-chloro-1,10-phenanthroline (I) (2.2 g) and stirring for 7 h at refluxing. DMA was distilled out under reduced pressure to separate out solid on addition of water. The solid was then filtered and dried at 90 °C under reduced pressure for 2 h (3.2 g, 84%). N,N-Bis-[1,10-phenanthrolin-2yl]-amino Methylated Polystyrene (VI). Sodium hydride (43 mg) was added to a solution of N,N-bis-[1,10-phenanthrolin-2yl]-amine (V) (150 mg) in DMA (5 cm3) at 120 °C followed by the addition of chloromethylated polystyrene (CMPS) (110 mg), after keeping the polymer beads in DMF for 12 h to swell. The reaction mixture was stirred and refluxed for 12 h. The polymer was then filtered, thoroughly washed with methanol, water, and acetone, in that sequence, and then dried at 90 °C for 6 h under vacuum to get yellow brown beads. 2-([1,10]-Phenanthrolin-2-yl-amino)-ethanol (VII). A mixture of 2-chloro-1,10-phenanthroline (I) (5.0 g) and ethanol amine (5 cm3) was heated together for 4 h at 100 °C. The reaction mixture was then poured into ice-cold water and stirred for 45 min at 10 °C to precipitate out VII as the product. The solid was washed with cold water and crystallized from dichloroethane and dried at 90 °C under vacuum for 1 h (5.0 g, 90%). 8196

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Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis of VI

Scheme 2. Synthesis of Polymer-Supported Penta-aza Ligand with Spacer (XI)

[1,10]-Phenanthrolin-2-yl-(tetrahydro-pyran-2-yl-oxyethyl)amine (VIII). 2,3-Dihydropyran (6.6 cm3) was added to a solution of 2-([1,10]-phenanthrolin-2yl- amino)-ethanol VII (2.0 g) in DCM at 25 °C and stirred for 5 min to get a clear solution. To this mixture was added p-toluene sulfonic acid (PTSA) (2.36 g), and the reaction mixture was stirred at the same temperature. After 45 min, a saturated solution of sodium bicarbonate (50 cm3) was added and stirred for 10 min. The organic layer was separated and dried over anhydrous Na2SO4. DCM was distilled out to get VIII (2.3 g, 85%) Bis-[1,10]-phenanthrolin-2-yl-[2-(tetrahydro-pyran-2-yl-oxy)ethyl]-amine (IX). Sodium hydride (0.43 g) was added to a mixture of [1,10]-phenanthrolin-2-yl-(tetrahydro-pyran-2-yl-

oxyethyl)-amine (VIII) (2.0 g) and 2-chloro-1,10-phenanthroline (I) (1.33 g) in xylene at 130 °C. The reaction mixture was then stirred at the same temperature for 2 h. Water was added, after distilling out xylene under reduced pressure, to get a yellowbrown solid (2.5 g, 80%). 2-(Bis-[1,10]-phenanthrolin-2-yl-amino)-ethanol (X). Aqueous 0.1 M HCl (10 cm 3 ) was added to a solution of bis[1,10]-phenanthrolin-2-yl-[2-(tetrahydro-pyran-2-yl-oxy)ethyl]-amine (IX) (2.5 g) in methanol (30 cm3) at 30 °C and stirred for 30 min. Methanol was distilled out, and the remaining solution was neutralized with NaHCO3 solution (5 wt %) to precipitate out X. The solid product was filtered and dried at 90 °C for 2 h (1.6 g, 72%). Polymer-Bound Penta-aza Ligand with Spacer (XI). Sodium hydride (17 mg) was added to a solution of 2-(bis-[1,10]-phenanthrolin-2yl-amino)-ethanol (X) (150 mg) in DMA (3 cm3) at 120 °C and stirred for 30 min. CMPS (100 mg) beads were then added to the reaction mixture and stirred at 130 °C for 10 h. The polymer was then filtered, washed with methanol, water, and acetone, and then dried at 90 °C for 6 h under vacuum to get yellow brown beads. Preparation of Cobalt Complexes. The cobaltligand complexes were obtained as red-brown precipitates (m/z 431.33 with V and m/z 475.04 with X) after refluxing a methanolic solution of the ligands with cobalt chloride 3 6H2O in a 1:1 ratio for 23 h. The precipitates were removed, washed with ethanol, recrystallized from methanol, dried at 6070 °C under vacuum for 1 h, and analyzed for elemental composition. Table 1 shows the estimated and experimental values of ligands and their metal complexes. Adsorption Experiments. Adsorption of cobalt and zirconium by the ligand loaded polymer was measured by adding 0.2 g 8197



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Table 1. Elemental Compositions of Ligands and Their Cobalt Complexes C-content (%) ligands and metal complexes

H-content (%)

N-content (%)

calcd

exp

calcd

exp

calcd

exp

V

77.2

76.81

4.05

4.75

18.76

18.38

VCo complex

57.25

56.25

3.0

3.59

13.91

13.51

X

74.82

74

4.55

4.86

16.78

XCo complex

57.03

56.48

3.5

3.91

12.79

of the adsorbent to 10 cm3 aqueous solutions of the mixtures having acidities ranging from 0 to 1 mol/dm3 of HCl and over a pH range of 07. Small amounts of dilute HCl and NaOH solutions were added to adjust the pH of the metal salts solutions, if necessary. The batch adsorption was conducted in a shaker bath with a reciprocal shaking of 100 strokes/min for 24 h at ambient temperature of 298 ( 2 K. The equilibrium was determined by taking samples of the solution at fixed time intervals. The samples were filtered and analyzed by ICP-AES. All studies were carried out in duplicate. The standard deviation observed was less than 2%. Deionized water was used throughout the experiments. The equilibrium adsorption capacity Qav (mg/g) and the distribution coefficients Kd (dm3/g) for both of the metal ions were calculated from eqs 1 and 2, respectively. Qav ¼

Co C V Ws

ð1Þ

Kd ¼

Co C V C Ws

ð2Þ

where Qav is the average amount of adsorbed Co(II) or Zr(IV) per unit adsorbent, C and Co are the equilibrium and initial concentrations in mg/dm3, V is the volume of the solution in dm3, and Ws is the weight of the adsorbent in g. The selectivity of the separation between Co and Zr is estimated by the ratio of their Kd’s.

’ RESULTS AND DISCUSSION 1. Selective Ligand Designing. The selectivity of a metal toward a ligand lies in its coordination properties and chemical hardness and softness. We felt that by applying the HSAB principle it is possible to design a ligand with selectivity toward the desired metal ion with the help of computational tools. Co2þ is classified as a borderline acid (between hard and soft). A nitrogen atom, either aromatic or directly bonded to an aromatic ring, is classified as a borderline base and is a suitable donor atom for Co2þ ions. Aromatic nitrogen (sp2 hybridized) would be the best choice in highly acidic solutions. There is more “s” character in the sp2 orbital than in the sp3 orbital containing a lone pair of electrons, and hence it is closer to the nucleus and somewhat hidden relative to the sp3 orbital with more “p” character. Also, the lone pair of aromatic nitrogen is involved in the resonance, and therefore it is relatively difficult to have protonated as compared to the sp3 hybridized lone pair. To place four aromatic sp2 N donor atoms at the four corners of the central rhombus, we needed four pyridine molecules or two 2,20 -bipyridine molecules or two 1,10-phenanthroline molecules. The latter takes care of the first choice. 2,20 -Bipyridine has the disadvantage of steric

O-content (%) calcd

exp

16.57

3.83

4.5

12.84

3.0

4.81

cobalt-content (%) calcd

exp

11.72

12.15

10.78

11.27

clash between 3 and 30 hydrogen atoms after complexation, which reduces the complex stability. In the case of 1,10-phenanthroline, this problem is avoided, and the two molecules of 1,10phenanthroline can be bridged by either one or two amine group(s) to have four sp2 N donor atoms. In this way, the ligand was considered as containing five and six nitrogens named pentaaza and hexa-aza, respectively, for the complexation of Co(II) ions. To shed light on the effect of structure on binding preferences, we decided to study further both of these structures by DFT calculations. Normal frequencies were evaluated for the optimized structures, indicating a true local minimum. As far as a global minimum is concerned, it is very difficult to determine for these complexes. We guessed a few structures, in an attempt to find the global minima, by keeping the metal ion in different positions near the plane of the ligand. The electronic energy values suggest that the structure with the metal ion in the cavity seems to be the most stable structure. Therefore, we assume that the optimized structure is close to the global minimum. 1.1. Uncomplexed Ligand. Figure 1 shows the DFT-optimized geometries of hexa-aza and penta-aza ligands with the atomic numbering. The electrostatic surface potential (ESP) (Figure 2) shows a central cavity as an electron-rich center where the metal ion can find a place to interact. The structural analysis of the ligands and their complexes with the metal ions is reported in Table 2. In the hexa-aza ligand, both of the phenanthroline rings lie in the same plane as indicated by the improper torsion angle C3N2N20 C30 of 0°, whereas, in the penta-aza ligand, one of the phenanthroline units is twisted with an improper torsion angle of 74.37° showing more flexibility than the hexa-aza ligand to adopt the required orientation. In comparison to the hexa-aza structure, NN distances are slightly stretched and CNC bond angles are shrinked in the penta-aza structure. Mulliken charge distribution on the coordinating atoms (nitrogen) in the hexa-aza system showed the charge density of 0.25e, which was less than the bridging N atoms (0.37e). A similar trend was observed in the penta-aza system with an increase in the charge by 0.1e. 1.2. CoCl2:L Complex. The DFT-optimized geometry of the complexes of CoCl2 with the hexa-aza and penta-aza structures is also shown in Figure 1. The ligands in the present work have bridging at either end or one end of two phenanthroline rings. Also, the presence of acetate groups on the phenanthroline rings in the work of Moghimi et al.23 repels the two phenanthroline rings slightly away from each other as they are free rings. As a consequence, the distance between the metal ion and ring nitrogens of the ligand is increased. The hexa-aza ligand in the present work has two fused planar phenanthroline rings and provides suitable cavity size for Co(II) to fit in. The bridging N atoms do not take part in the coordination, but phenanthroline N atoms are forced to 8198



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Figure 1. DFT-optimized geometry of ligands and their CoCl2 and ZrOCl2 complexes. (a) Hexa aza, (b) penta aza, (c) CoCl2hexa-aza complex, (d) CoCl2penta-aza complex, (e) ZrOCl2hexa-aza complex, and (f) ZrOCl2penta-aza complex.

Figure 2. Electrostatic surface potential of ligands: (a) hexa-aza and (b) penta-aza. 8199



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Table 2. Comparison of Geometry Parameters of the Ligand and Its Complexes with Metal Ion bond length, (Å)

bond angle, (deg)

N1N10

N1N2

hexa-aza

2.67

2.68

128.55

penta-aza

2.97

2.76

127.84

hexa-aza CoCl2

2.78

2.59

penta-aza CoCl2

2.8

hexa-aza ZrOCl2 penta-aza ZrOCl2

system

energy, (kcal/mol)

C2N3C20

IE

SE

Mulliken charges on MCl2

130.68

79.37

4.34

0.87

2.62

130.98

64.83

1.94

0.76

2.72

2.66

127.14

67.68

8.15

0.44

2.81

2.73

126.83

64.61

1.9

0.42

Free Ligand

Complex

Table 3. Comparison with Experimental X-ray Structures bond distances, (Å)

a

bond angles, (deg)

CoN1

CoN2

CoN20

CoN10

N1CoN2

N10 CoN20

2 phenanthroline units þ 1 N bridging

2.010

1.947

1.949

1.997

82.95

82.93

present work

2 phenanthroline units þ 2 N bridging 3 phenanthroline units

1.899 1.942

1.898 1.942

1.899 1.936

1.898 1.945

86.05 84.40

86.06 84.77

present work Sharma et al.22a

2 phenanthroline acetate units

2.158

2.267

2.113

2.041

75.16

76.31

Moghimi et al.23

X-ray crystallographic data.

22,23

coordinate, forming shorter distances and appropriate bond angles. We have further compared the predicted values with the experimental values reported by Sharma et al.22 with three free phenanthroline units without any substituent and those with acetate substituted phenanthroline reported by Moghimi et al.23 in Table 3. It is obvious that if the ligands are free or have substituents, the distances between interaction centers are increased. Similarly, the bond angles from the present work are very close to those reported by Sharma et al.22 In the optimized complexes, cobalt ion sits at the center of the ligand cavity maintaining nearly at the same position in both of the hosts, irrespective of the number of the bridging nitrogen atoms. The other parameters of the complex geometry (distances, angles, etc.) are compared to those of ligand geometry in Table 2. The hexa-aza ligand remains somewhat planar even after the complexation. The changes in the other parameters can be explained by close observation of the ligand and complexes’ geometries. The comparison of CoN bond distances in the hexa-aza and penta-aza systems shows that CoN distances are shorter by 0.08 Å in the hexa-aza structure as compared to the penta-aza system, which is, however, insignificant. Apart from slight deviation in the bond length, the angles made by Co with 4 sp2 N suggest insignificant differences in the orientations. These structural features lead qualitatively to nearly the same selectivity of the ligands for cobalt. A notable change was observed, however, in the complexes upon coordination with the cation, along with the counterions in NN distances and CNC bond angles. The N1N2 distance is shortened by 0.09 Å, and the N1N10 distance is lengthened by 0.1 Å, whereas the CNC angle is increased by 2.13° in the hexa-aza system to compensate for the offset. Yet, in the penta-aza system, all of the coordinating atoms of the ligand came closer to the cation at the cost of shrinking of bonds by 0.2 Å and angles within ∼3°, indicating enough flexibility of the ligand to encapsulate a Co(II) ion. Mulliken charge analysis

(Table 2) shows that electron transfer from the hexa-aza ligand toward Co(II) is slightly more (0.87e) as compared to that in the penta-aza system (0.76e), indicating a slight preference over the penta-aza ligand. 1.3. ZrOCl2:L Complex. Being larger in size, zirconium cation cannot enter into the ligand cavity and lies above the plane of hexaaza ligand along with the counterions as shown in Figure 1e. The ZrO bond makes a small angle with the plane of the ligand with Zr close to the ring nitrogen atoms of the phenanthroline at a distance of 2.59 and 2.68 Å in the hexa-aza and penta-aza systems, respectively (Table 2). The Zr atom also tries to attract the ring nitrogen atoms of the other phenanthroline ring through strong charge interactions. As a consequence, the hexa-aza ligand is no more planar, and the two phenanthroline rings of the macrocycle make an angle of about 157° and 147° in hexa-aza and penta-aza, respectively, with each other. However, the penta-aza ligand has more flexibility than the hexa-aza ligand to accommodate the Zr(IV) ion, which lies out of the plane of the later ligand. Importantly, this shows an unfavorable orientation for ZrO2þ. From a structural changes point of view, trends were observed in distances and angles for both of the ligands upon coordination with ZrO2þ cation that are similar to those found in Co(II) complexes. Electron transfer from both of the ligands to the cation is nearly the same, that is, at ∼0.4e. 1.4. Selectivity and Preorganization Analysis. The selectivity of the ligands toward Co(II) over Zr(IV) was analyzed by comparing interaction energies (IE) of the ligands with the metal ions. The ligandcation interactions were studied by constructing 1:1 complexes in the gas phase, in the absence of competing interactions, which otherwise can induce polarization. The metalligand interaction energies were calculated using eq 3: IE ¼ TEcomplex ðTEligand þ TEmetal ion Þ 8200

ð3Þ



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Table 4. Experimental Results of Cobalt and Zirconium Adsorption from Hydrochloric Acid Solutions Using Polymer-Supported Penta-aza Ligand cobalt(II) [HCl] mol dm3

before adsorption after adsorption

zirconium(IV)

% of

Qav

Kd

before adsorption after adsorption

adsorption (mg/g) (cm3/g)

% of

Qav

Kd

adsorption (mg/g) (cm3/g)

separation factor, R

(ppm)

(ppm)

7.6

183

165

9.83

0.9

5.46

1.38

14.8 39.7

175 171

158 161

9.71 5.84

0.85 0.5

5.4 3.0

2.74 13.24

(ppm)

(ppm)

0

204

181

11.27

1.15

0.05 0.1

202 185

156 103

22.77 44.32

2.3 4.1

0.5

187

122

34.75

3.25

26.6

1

185

168

9.18

3.35

5.02

170

166

2.35

0.2

1.2

22.16

162

159

1.85

0.15

1.0

5.02

where TE is the total energy of the respective optimized structures. To form a complex with a metal ion, the ligand molecule rearranges its geometry to have the maximum interaction. This is achieved at the cost of stretching of some bonds, angles, torsion angles, etc. As a result, the ligand molecule is strained after the complex formation. This strain at a quantitative level is termed as strain energy (SE) and is calculated as: SE ¼ VEligand af ter complex VEf ree ligand

ð4Þ

where VE is the valence energy term in FORCITE MM energy of the ligand. The preorganization of the designed ligand is analyzed from the strain energy values. Table 2 shows the interaction energies of the complexes of Co2þ and ZrO2þ ions with the ligands along with the strain induced in the ligand after the complexation. The hexa-aza complex of Co2þ is more stable by 11.69 kcal/mol than the ZrO2þ complex, whereas the penta-aza complex showed better stability for Co2þ by 0.22 kcal/mol. Overall, from an energetic point of view, Co(II) is selectively complexed in both of the ligands. As far as the interaction of Co2þ ion with hexa-aza macrocycle is concerned, it is preferred by 14.54 kcal/mol over the penta-aza ligand, and also the strain induced after complex formation is very low, showing the high degree of preorganization of the ligand for Co2þ ion. This important result is useful in understanding the competitive nature of co-ordination of Co(II) in the presence of Zr(IV). In that way, the hexa-aza molecule is more promising in its use in the selective separation of Co(II) from Zr(IV). In view of the importance of the simulation results of coordination in HCl solutions and interaction in the gas phase, further confirmation by experiments is desirable, and, therefore, we decided to develop a functional polymer loaded with the ligands. The present experimental study is limited to penta-aza ligand only because of the ease of its synthesis and its metal complexes. The general scheme (Scheme 1) for the synthesis of polymersupported penta-aza ligand (VI) involves treatment of N,Nbis-[1,10-phenanthrolin-2yl]-amine (V) with chloromethylated polystyrene (CMPS), where V was first converted to its sodium salt by using NaH in DMA followed by adding swollen CMPS to it. In the FTIR spectra of the ligand loaded polymer VI, a stretching vibration for the CdN and CdC appeared at 1653 and 1606 cm1, respectively. Also, the absence of CCl stretching at 674 and 1264 cm1 suggested that the loading of the ligand on the polymer matrix was complete. For converting 2-chloro- to 2-amino-1,10-phenanthroline, a lot of work has been done up to date.24,25 The reactions have been carried out at high temperature and pressure conditions using high boiling point solvents

Figure 3. Experimental distribution coefficients of cobalt and zirconium over penta-aza resin as a function of pH.

such as phenol, acetamide, etc., which have some inherent disadvantages.26 Distillation of these high boiling solvents is an important step in the isolation of the product, but in most cases that results in lower yields and difficulty in operating conditions. Herein, we report N-acylated 2-amino-1,10-phenanthroline (III) and 2-azido-1,10-phenanthroline (II) as novel sources for IV. The starting material for the synthesis of VI is 2-chloro-1,10phenanthroline, which has previously been synthesized.27 The electronic spectra (Figure S1 of the Supporting Information) of the penta-aza ligand V were studied using a UV vis spectrophotometer in different solvents. The spectrum of the penta-aza ligand in acetonitrile was similar to that in methanol but was different from that in chloroform, showing the existence of an amineimine equilibrium.28,29 In polar protic or polar aprotic solvents such as methanol, acetonitrile, DMF, and DMSO, the amine form is more favored due to hydrogen bonding, whereas in CHCl3, DCM, and CCl4, the imine formation is favored. Synthesis of the penta-aza ligand was further simplified with Scheme 2. The advantage of using the penta-aza ligand with a spacer unit is that it is convenient to prepare using mild reaction conditions and requires less reaction time than Scheme 1. 2. Separation Study. Cobalt and zirconium were not adsorbed at all on CMPS. Only in the presence of chelating ligands on the polystyrene matrix did both metals get adsorbed, but to different extents, indicating selectivity toward Co. The adsorption on the modified polymeric resin was a slow process, and equilibrium was reached in 24 h. Table 4 shows the 8201



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aqueous acidic solutions. The adsorption capacity of the adsorbent for cobalt was found to be 4.1 mg g1 at 25 °C.

’ ASSOCIATED CONTENT

bS

Supporting Information. Characterization of all of the intermediates and ligands using different spectroscopic techniques, Figure S1 showing the amine imine tautomerism, Table S1 as the comparison of maximum adsorbent capacities (qm) of different adsorbents for Co(II) with the present study, and Figures S2 and S3 respectively illustrating the mass spectra of VCo complex and V. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Figure 4. Experimental separation factor (R) between cobalt and zirconium as a function of pH.

concentrations of both of the metal ions, before and after adsorption experiments, along with the percentages of adsorption and respective Kd values. The pH of the solutions plays an important role in the adsorption process. The Co(II) adsorption is optimum, that is, 44.3% at 0.1 M HCl with Zr coadsorption limited to 5.8%. With increase in pH, the adsorption of both metal ions decreased. In all runs, higher % adsorption of cobalt as compared to that of zirconium proves the selectivity of the resin for Co. Figure 3 shows the pH dependence of the distribution coefficients of both metal ions on the penta-aza resin at equilibrium. The adsorption of Zr reached an equilibrium state with Kd values up to 5.4 cm3/g. On the other hand, cobalt showed strong adsorption on the resin with Kd values of 539 cm3/g. The low distribution coefficient of Co(II) ion at low pH values may result from protonation of the basic sites. The separation factor between the two metals, as a measure of selectivity of the ligand toward Co, was calculated as the ratio of the distribution coefficients of the two metals: R¼

KdCoðIIÞ KdZrðIVÞ

ð5Þ

Figure 4 also shows the equilibrated pH dependence of separation factor of Co(II) with regard to Zr(IV), when the ratio of the volume of aqueous phase to the mass of resin was set at 50:1. In all cases, the separation factor values are larger than that indicating selective uptake of the Co, in the presence of Zr. In particular, the separation factor of cobalt with regard to Zr reached a high value of 22 at pH 0.3.

’ CONCLUSION The molecular simulation has been effectively used to understand the interaction between the metal ions with ligands and hence to predict the selectivity of the ligands toward cobalt. The hexa-aza and the penta-aza ligands are firmly coordinated to Co(II) and are well adapted to form stable complexes. The experimental results of present study reveal that phenanthrolinebased penta-aza ligand covalently loaded on polystyrene matrix can be constructively utilized as an effective adsorbent for selective removal of cobalt in the presence of zirconium from

Corresponding Author

*Tel.: þ91-022-33612013. Fax: þ91-022-33612013. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Department of Atomic Energy, GOI. P.K. acknowledges a fellowship award from the University of Grant Commission (UGC). R.S.M. acknowledges an award of JRF from the Council for Scientific and Industrial Research (CSIR), India. We wish to thank SAIF, IIT- BOMBAY for analytical support. ’ REFERENCES (1) Amara, M.; Kerdjoudj, H. Separation of Metallic Ions Using Cation Exchange Resin in the Presence of Organic Macrocation: Mechanism of Surface Layer Formation. Anal. Chim. Acta 2004, 508, 247. (2) Abolt, R. B.; Knepper, G. D. Separation of Metal Ions on an Anion Exchange Resin by Chromatographic Elution. U.S. Patent 5,246,591, 1993. (3) Legradi, L. Separation of Metals on Ion-Exchange Resins by Means of R-Hydroxyisobutyronitrile as Complexing agent. J. Chromatogr., A 1974, 102, 319. (4) Sharma, K. R. Design, Synthesis, and Application of Chelating Polymers for Separation and Determination of Trace and Toxic Metal ions. A Green Analytical Method. Pure Appl. Chem. 2001, 73, 181. (5) Van de Water, L. G. A.; Hoonte, F.; Driessen, W. L.; Reedijk, J.; Sherrington, D. C. Selective Extraction of Metal Ions by Azathiacrown Ether-Modified Polar Polymers. Inorg. Chim. Acta 2000, 303, 77. (6) Alexander, V. Design and Synthesis of Macrocyclic Ligands and their Complexes of Lanthanides and Actinides. Chem. Rev. 1995, 95, 273. (7) DOE Fundamentals Hand Book Material Science; DOE-HDBK1017/2-93; U.S. Department of Energy:: Washington, DC, 1993; Vol. 2, Module 5, p 11. (8) Boltz, C. L. The Evolution of Nuclear Fuels and Reactors. Phys. Technol. 1973, 4, 223. (9) Ernesto, D. C. Decontamination Studies of Simulate PWR Primary Coolant System Components. M.Sc. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1988. (10) Cundy, A. B.; Croudace, I. W.; Warwick, P. E.; Bains, M. E. D. Decline of Radionuclides in the Nearshore Environment Following Nuclear Reactor Closure. A U.K. Case Study. Environ. Sci. Technol. 1999, 33, 2841. (11) Argonne National Laboratory/EVS. Human Health Fact Sheet; http://www.ead.anl.gov/pub/doc/cobalt.pdf. (12) Cobalt, Radioactive: Nuclear Power Plant Emissions; http:// www.frankmckinnon.com/cobalt.htm. 8202



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Design and Synthesis of Polymer-Bound Penta-aza Ligand for

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