Trapping of Ag+ and Cs+ by a crown-ether polymer studied by EXAFS

1. Phys. Chem. Solids Vol. 53, No. 3, pp. 44-57, Printed in Great Britain.

TRAPPING

1992

0022.x97/92 ss.00 + 0.00 Pergamon Pm.3 plc

OF Ag+ AND Cs+ BY A CROWN-ETHER POLYMER STUDIED BY EXAFS

F. BENIERE,~N. Bmmu,t

C. R. A. CATLOW,~ M. COLE,$ J. SIMON@ and L. ANGELY!~

YLaboratoire de Science des Matbriaux, U.A. CNRS 040804, Universitt de Rennes, 35042 Rennes, France $The Royal Institution of Great Britain, London, U.K. §Laboratoire d’Electrochimie Organique, U.A. CNRS 439, UniversitC de Rennes, 35042 Rennes, France (Received

23 April 199 1; accepted 24 July 1991)

Abstract-Polymeric crown-ethers are expected to exhibit the remarkable complexing properties of the crown ether molecules with the further advantage of being able to be fabricated as a plastic membrane. The structure of poly(dibe.nzo-18-crown-6) doped with the metallic cations Ag+ and Cs+ has been examined by EXAFS. The silver ion is found at the middle of the ring formed by the six oxygen and 12 carbon atoms. The cesium ion is slightly off this position on a line perpendicular to the crown. In both cases the six oxygen nearest neighbours are split into two sub-shells of two and four atoms. A three-dimensional structure is suggested for the neutral polymer. Keywords: Crown-ether complexation.

polymer,

ion extraction,

1. INTRODUCTION The two classes of cyclic molecules:

crown-ethers

[l]

and cryptates [2] present remarkable structure-specific interactions of high selectivity (recognized by the award of the 1987 Nobel Prize for Chemistry to C. J. Pedersen and J. M. Lehn). Their organic solutions exhibit a strong and selective complexing power towards the alkali metal cations and other ions. The selectivity is related to the match between the diameter and the cavity size of the ring formed by the oxygen (crown ethers) and nitrogen and oxygen (cryptates) atoms. Solid mixtures of the crown ethers with phosphomolybdic or similar acids have been prepared for ion-exchange resins or selective membranes [3]. Condensation resins of crown ethers with fonnaldehyde have been used to separate alkali metal cations [4,51. However, the major forward step in the attempt to transform these macrocyclic ionophores under a solid form has been achieved by Simonet et al. [6] who electrochemically synthesized polymeric films from the monomer in an organic solution. The extraction of Na+ and Rb+ in such a membrane of poly-(dibenzo-1%crown-6) has revealed the potential of the material for ion-exchange applications [7]. A surprising result was the measurement of the capacity: one metallic cation can be trapped for statistically three or four oxygen rings whereas exactly one cation can be complexed for every crown in the monomer solutions. It appeared essential to 449

EXAFS

of metal ions, crown-ether

structure,

locate the ions trapped inside the polymeric network. We report in the present work the structural study of the poly(dibenzo-18crown-6) doped with Ag+ and Cs+ cations. These polymeric materials do not possess the long range order which would have allowed conventional diffraction techniques. On the other hand, the Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is ideally suited to investigate the local environment of the trapped ions relative to the heterocyclic ring. Indeed, a number of studies have already utilised the EXAFS technique to probe the local structure surrounding a dopant cation in polymer systems [8].

2. EXPERIMENTAL

2.1. Preparation of material The poly(dibenzo-18-crown-6) (hereafter designated as PDBl8C6) was prepared in the Laboratory of Electrochemistry at the University of Rennes. The polymeric material is formed on the plane surface of a platinum anode in an organic electrochemical cell [6] in the form of -20 microns films. The electrochemical polymerization provides the polymer under the form P+, aBF; (a is the doping ratio, i.e. the electron hole ratio per polymerized unit). The positive charge of the oxidized form P”+ is compensated by insertion of some of the BFi anions of the supporting salt NBu, BF., used for the synthesis. A planar representation of the polymer is shown in Fig. 1.

450

F. BENIEREet al.

However, the material acquires a large complexing power only after it is reduced to the neutral state P. This can be achieved by immersing the films either in an electrolytic solution containing the electrogenerated super-oxide anion 0; or in a chemical solution of NBu, OH in acetonitrile [9]. The OH- ions are substituted for the BF; anions, their negative charge being compensated by NBu$ cations. The material is then ready for the fixation of the metal cations [I. This work is concerned with the complexing nature of the polymer with Ag+ and Cs+. These cations have been lixed by immersing the film samples in an aqueous solution of 0.1 M AgNO, (or CsNO,) for 8 h at room temperature. The concentration of cesium within the material was approximately one Cs+ cation per four oxygen rings, a little more for silver. This is the maximum capacity of absorption of the polymer. It corresponds for both silver and cesium to a concentration of nearly 10% in weight. The polymeric films are assembled in piles of six samples between glass plates. This arrangement allows the X-ray beam to cross a total thickness of about 100 microns of material.

2.2. EXAFS The EXAFS technique exploits a rather subtle aspect of X-ray absorption. When X-rays pass through a material, it absorbs some of the radiation. It is found that the degree of absorption varies smoothly

Fig. 1. Planar representation

as the energy of the targeted X-rays is increased, superposed to the sharp increases known as absorption edges. EXAFS is manifested by the oscillations observed on the high-energy side of an X-ray absorption edge. These oscillations arise as a result of the interference effects between the “outgoing” photoelectron wave (invoked by means of core electron excitation by the X-ray beam) and the “backscattered” wave (caused by the reflection of the incident photoelectron wave by neighbouring atoms). The intensity and frequency of the EXAFS can be interpreted in terms of the number, nature and distance of the backscattering atoms adjacent to the absorbing atom. The EXAFS technique clearly offers a very powerful tool to probe the local structure surrounding a particular type of atom or ion, i.e. EXAFS is “atomspecific”. X-rays from a synchrotron are used because of the very strong intensity available in a wide-range continuous spectrum. The mathematical representation of the oscillatory EXAFS function x(k) for a K-edge absorption, assuming that there is only single scattering of the photoelectron and that the spherical photoelectron wave is approximated by a plane wave, can be described by the following equation: X(k)=F&$V;(.)lexp(-2glk? I xexp(-2R,/1)sin(2kRl+26;+$,).

of poly(dibenzo-IB-crown-6).

(1)

Crown-ether polymer studied by EXAFS We have used a more sophisticated curved wave theory of EXAFS upon which are based the EXAFS curve fitting programs developed at Daresbury in the U.K. that we have applied in the present analysis. The curved wave surface calculations are done using the angle-averaged curve wave theory and the angle dependent factor from the small atom approximation [lo, 111. The programs also calculate approximate phase shifts for a specified central/neighbouring interaction. k is the momentum of the photoelectron, Nj is the number of scattering atoms at a distance Rj, each with a backscattering factorA( 1 is the elastic mean free path of the photoelectron. The DebyeWaller term, exp( -2afk2), is a measure of the thermal and static displacements within each shell, where IJ~ is the mean square variation in interatomic distance between the emitting and scattering atoms. u; and tij take into account the phase shifts experienced by the photoelectron wave on passing through the potentials of the absorbing atom and thejth shell atoms. The strategy adopted in analysing EXAFS spectra of unknown structures is to fit the experimental data to a parameterized model. The unknown parameters Nj, Rj and trj were refined by least squares iteration using the Daresbury EXAFS curve fitting programs EXCURV88 (cesium) and EXCURV90 (silver). Both programs are based on the same basic equations, EXCURV90 being the most recent version in use at Daresbury. The Ag(K) edge (25,500eV) EXAFS data were collected on the Ag-doped samples on EXAFS station 9.2 at the Synchrotron Radiation Source at Daresbury. The measurements were taken under beam conditions of 2 GeV and 170 mA. The standard transmission mode EXAFS procedure was adopted, with the data being collected under conditions of ambient temperature and pressure where the intensity of the X-ray beam is measured before and after the sample. The CsL,,, edge (5000 eV) data were also collected on the Cs-doped samples on station 7.1 under beam conditions of 2 GeV and 200 mA at ambient temperature and pressure. The standard transmission mode EXAFS set-up was again adopted. Collection of the data at room pressure and temperature is a marked advantage of the EXAFS technique in the present situation. The polymeric network is stabilized by NBu,OH. Low pressures and/ or high temperatures may entail the decomposition of the amine and, therefore, we avoid the modification of the material, which could occur with other techniques like Auger Spectroscopy, X-ray induced photoelectron spectroscopy, or scanning electron microscopy. On the other hand, low temperatures might have produced silver aggregates.

451

The monochromation is obtained by two parallel single crystals of the best quality (silicon or germanium according to the energy range). The primary X-ray beam undergoes a double diffraction upon the two crystals at the given same angle 6. A monochromatic beam is obtained at the energy (wave length) selected by Bragg’s law. The angle 0 is progressively varied in order to tune the beam. The use of this double monochromator together with the crystallographic quality of the silicon and germanium allows a resolution of 10e4 to be obtained. Harmonic rejection is achieved by off-setting the first crystal out of parallel with respect to the second by a desired amount and employing a servo control system to hold it there as the scan proceeds. The servo acts to match the I, current reading to a reference voltage which can be provided by a constant internal supply in the servo unit, or from an external input. The external input allows scans at constant harmonic rejection to be performed by varying the reference voltage as the scan proceeds to compensate for the variation in monochromatic beam intensity with monochromator angle. To do this, the peak Z, intensities at the start and finish positions of the scan range must be measured. With this information input, the EXAFS station program interpolates the peak I, intensities at each point in the scan between the end values, and subtracts a percentage corresponding to the desired amount of harmonic rejection. This amounted to 50% at the Cs L,,, edge. 2.3. Precision and limits of the method The basic data are the measurements of the EXAFS function xtk, obtained with the monochromator of 10m4 resolution. This gives approximately a resolution of 1 eV at Ag(K) edge. The fit index is the product:

with the weighting power of 3. Several models of the unknown structure are attempted. For each of them, the program computes the calculated ycalFas a function of k. The quality of the fit of y,,rc to y,,, is evaluated through the R-factor:

i

R =i=’

IYicxp -YicalcI/Yiexp

n

which is multiplied by 100 to change it into a percentage. In the present analysis of the structural environment of Ag and Cs in the PDB18C6, the final fits have implied seven adjustable parameters:

F. BENIEIIE et al.

452

-three bond distances between the metal and two sub-shells of oxygen closest neighbours and a shell of carbon closest neighbours, -the three corresponding Debye-Waller factors -the energy term AR0 (numerically given below). The coordination numbers were fixed. All other parameters were also not refined. Several analyses were carried out by changing the coordination numbers, which were compared according to the R and Debye-Waller factors. The accuracy of the bond distances is + 0.02 A. 3. RESULTS 3.1. Ag-PDBlK6 Figure 2 shows the experimental EXAFS spectrum of the Ag-doped polymer over the energy range of 25,200-26,100 eV. The background absorption features which are superimposed on the EXAFS oscillations were removed by fitting polynomial functions to both the pre-edge and post-edge regions of the experimental spectrum. Figure 3 compares the experimental EXAFS oscillations with the calculated curve corresponding to the atomic configuration discussed below which gave the best fit. The energy window used in this data analysis was E,,

= 33.00 eV

and

E,,,,, = 572.00 eV.

The data analysis involved a k3 weighting being used, so as to enhance the features at high k where single scattering is most accurate. Inelastic scattering of the photoelectron was accounted for by an

imaginary potential, V, , which is related to the mean free path of the photoelectron, 1, by the following equation: 3, = -k(h/2n)* 2mV,

’

(2)

where h represents Plan&s constant and m represents the mass of the electron. The amplitude reduction of the EXAFS oscillations caused by multiple excitations at the central atom/ion was also accounted for by using a reduction factor Ati. These two parameters, Vi, and Ad were assigned the tixed values of - 3.9 eV and 0.9, respectively. This last value which multiplies the elements of the scattering matrix is just empirical. It has been adopted in the program to give some measure of shake-up and shake-off events. The dataset was Fourier-filtered at R,, = 1.85 A and R,, = 3.10 A. For the excited atom approximation, a relaxed 1s core hole phaseshift was used. The refined atomic parameters obtained from this fit (indicating the local structural environment of the Ag+ species in the material) are presented in Table 1. This best fit is derived from the model based on six oxygen nearest neighbours and 12 carbon secondnearest neighbours, where the Ag+ species are situated at the centre of the crown. The introduction of the splitting of the oxygens into two sub-shells with two nearest (2.40 A) and four more distant (2.70 A) 0 atoms caused the R-factor to fall from >40.0% down to 10.4%, thus clearly showing that a splitting does occur. The finally low value of the R-factor (R = 10.4%) indicates the quality of the fit, which may also be appreciated graphically through the close agreement between the experimental and computed

Energy W)

Fig. 2. EXAFS spectrum of Ag-PDB18C6.

Crown-ether polymer studied by EXAFS

453

Fig. 3. EXAFS oscillations of Ag-PDBlK6: experimental (full line) and theoretical (dotted line). A k’ weighting is used. Table 1. The EXAFS parameters of Ag-PDB 18C6 Refined structural parameters: R-factor = 10.4% Bond Debye-Waller Coordination Atom distance factor no. type (A) (A2) 2.0 4.0 12.0

0 0 C

2.39 2.68 3.08

0.001 0.004 0.006

fine structure of the EXAFS oscillations in Fig. 3, as well as between the experimental and computed Fourier transform in Fig. 4.

3.2. Cs-PDB18C6 The energy of the K level of cesium of 35 985 eV is beyond the range of the X-ray spectrum of the Daresbury synchrotron. Therefore, the data were collected at the CsL,,, edge (- 5011.9 ev). In addition, data collection was restricted to a narrow energy window because the CsL,, occurred at 5359.4eV. A typical scan involved data being collected from 300eV below the absorption edge to 340eV above the edge. Figure 5 shows the experimentally recorded photon transmission spectrum of the caesium-complexed

1.51.4I.31.2 I.11.00.9 0.6 FT 0.7 0.6 0.5 0.4 0.3 0.2 0. I

Radial distance th

Fig. 4. Fourier transform of the EXAFS spectrum of Ag-PDB18C6: experimental theoretical (dotted line). (Fourier transforms of the data of Fig. 3.)

(full line) and

F. BENIERE et al.

454

Input energy (eV)

Fig. 5. EXAFS spectrum of Cs-PDBlK6.

poly(dibenzo-1%crownd) over the energy range of 4700-5350eV. The EXAFS data fitting was done using the EXCURV88 program which uses a curved wave approximation. The bond distances and DebyeWaller factors together with a correction term AE, (which accounts for any inaccuracies in defining the position of the CsL,,, edge), were then refined by a least squares iteration process. The coordination numbers (six 0 and 12 C atoms) were fixed at their preset values, with no extra refinement being involved. Several fitting strategies have been attempted, involving two peaks, three peaks and four peaks, successively, with various combinations of oxygen and carbon neighbours. We give our final best fit on Fig. 6 where both the EXAFS oscillations and the corresponding Fourier transform are shown. The data are analysed according to a k3 weighting. The upper part gives thus the product k3 . x(k). There is a discrepancy in the 5.5-7.5 A-’ region between the experimental spectrum and the calculated one which remains unexplained. The other parameters were given the following values:

2.0 1.5 1.0

0.22 0.20 0.16 0.16 0.14 lb-

0.12 0.10

V,, = -2.0 eV,

A,, = 0.72.

0.06 0.06 0.04

Table 2. CsL,,, edge EXAFS fit of Cs-PDB18C6

0.02 0

123456789

Rodiot distance (A)

Fig. 6. EXAFS oscillations (upper part) and Fourier transform (lower part) of Cs-PDB18C6. A k’ weighting is used.

Shell

Nearest neiahbour

coordination no.

Distance (A)

1 2 3

0 0 C

4.0 2.0 12.0

2.87 3.11 3.91

Debye-Waller factor (AZ) 0.020 0.005 0.152

Crown-ether polymer studied by EXAFS The data analysis involved the data being Fourierfiltered within the approximate radial distance window of 2.0-4.5 A. The theoretical parameters obtained from this data fitting are presented in Table 2 in addition to the result AE,,= 16 eV. Good agreement between the experimental data and the proposed theoretical model has been obtained. The model is again based on a splitting of the six oxygen nearest neighbours that form the coordinate bonds to the Cs+ cation into two groups. That is to say, the nearest neighbour oxygen atoms to the Cs+ cation are situated in two different environments. This splitting is once more in a 2/l ratio but now four oxygen atoms occur at a shorter Cs-G bond distance of 2.87 A, with two more oxygens being sited at a slightly further distance of 3.11 A. The adjoining carbon atoms (12 in total) are found to be situated at an average distance of 3.91 A from the Cs+ cation. The DebyeWaller factor for this carbon shell is extremely large therefore indicating the inefficiency of the model of 12 equidistant C atoms. The limits of the EXAFS technique prevented the testing of more disordered carbon ring models. It just seems reasonable that some disordering in the carbon shell may follow the disordering in the oxygen shell.

4. DISCUSSION 4.1. Location of the metal cations relative to the

oxygen ring The only structural information concerning the doped polymer before the present study related to the stoichiometry of the fully doped material (a maximum of one metal cation for two oxygen rings) suggesting a stacking of planes of pure polymer and intermediate planes of metallic cations. In such a structure, by analogy to, intercalated graphite, a metallic cation would have been bonded to 12 oxygen atoms. In the extreme stoichiometry of l/2, every second alternate site would have been occupied by a dopant. However, this intercalated structure is ruled out by the present results for the two different species considered: Ag+ and Cs+. An important conclusion is that the metal cations tend to be bonded to a single oxygen ring. The atomic distances show the foreign cation located in the plane of the crown at its center when enough space is available. This is the case of Ag+ whose diameter of 2.56A fits the cavity of 2.60A [12] of dibenzo-18-crown-6. Taking as a very approximate estimate that the closest possible silver-oxygen bond length is the sum of the oxygen Van der Waals radius (1.40 A) and the Pauling ionic radius of silver [ 131:

rAp++ r. = 1.28 + 1.40 = 2.68 A.

Fig. 7. Structure of AgPDB18C6: (a) aerial and (b) lateral view of an oxygen crown (I, = 2.40 A, 1, = 2.69 A).

Assuming the ionic size of 02-, namely 1.32 A, one obtains a shorter distance of 2.60A. This distance is close to our experimental bond distances: 2.40 A (2 atoms) and 2.69 A (four atoms) reported in Table 1. A schematic representation of Ag-PDB18C6 as obtained by EXAFS is given in Fig. 7 locating the foreign ion within the crown plane at the centre of the cavity. On the other hand, significantly bigger ions lie outside the oxygen plane in the off-centre position adjacent to the oxygen as demonstrated by the results for Cs+ whose diameter is 3.24A. Within the same

a

Fig. 8. Structure of Cs-PDB18C6: (a) aerial and (b) lateral view of the oxygen crown (I, - 2.87 A, 1,- 3.11 A, 1,=1.2A).

F. BENIEREet al.

456

range of approximation distance is:

the cesium-oxygen

closest

rc.+ + r, = 1.62 + 1.40 = 3.02 A. This theoretical value is quite close to the experimental distances in Table 2: 2.87 A (four atoms) and 3.11 A (two atoms). The structure is given in Fig. 8. A geometrical calculation of the distance at which the cesium is displaced from the centre of the crown gives for the distance above the crown lJ = 1.2 + 0.2 A. We conclude therefore that the dopant is sited exactly in the plane of the crown at its center, or as close as possible to this position. These experimental results do not support any layered structure where the metallic ions would have been intercalated in the polymeric material. In this respect, the polymer behaves like the monomer. The structural studies of the dibenzo18trown-6 monomer complexed with alkali metal cations [ 141 also locate the foreign cations Li+, Na+ and K+ in the plane at the center of the ring. The thermodynamical measurements of the complexation constant [15, 161give a maximum of stability for K+, whose diameter (2.66&, like Ag+ (2.56 A) almost perfectly matches the cavity (2.60 A) inside the oxygen ring of the dibenzo-18-crown-6. When the foreign cation diameter exceeds the cavity size, the complexation constant then decreases. This is quite understandable because of the decrease of the electrostatic energy between the positive charge of the cation and the negative charges beared on the oxygens due to the increased distance. Similarly, among a number of smaller and larger ions, Ag+ has been found to be the most adsorbable

by a solid derivative of dibenzo-18-crown-6 precipitated with phosphomolybdic acid and entrapped in polyacrylamide [ 171.

4.2. Splitting of the oxygen atoms Another clear result of the present structural study is the splitting of the six oxygens of the crown into two sub-groups. The EXAFS oscillations attributable to backscattering by the six oxygen atoms could never be described by a six-fold symmetry axis. That symmetry is expected to occur in the simple complexation of the 18-crown-6 monomer. However, the addition of the two phenyl rings in dibenzo-18-crown-6 already breaks the symmetry. Furthermore, the association of the phenyl rings between themselves to give the poly-(dibenzo-18crown-6) may also add to the departure from symmetry. It is therefore not surprising to observe that all the six oxygen first neighbours to the metal cations are not at exactly same distance. However, it is not yet easy to interprete the subtle role of the fixed cation. The four oxygens connected to the phenyls are further from Ag+ than the other two oxygens, whereas the reverse situation is observed with Cs+. More complexants have to be studied. Figures 7 and 8 show that the splitting of the oxygen atoms necessarily entails some splitting of the 12 nearest carbon atoms. This is experimentally confirmed by the large Debye-Waller factor on the assigned average carbon position. Unfortunately, we are here at the limit of the experimental information derived from the EXAFS data. Thus, no attempt was made to investigate the sub-structure of the metal-carbon interaction.

Fig. 9. Cross-section of the egg-box model of PDB18C6 complexed with metallic cations M* encaged in the oxygen rings.

Crown-ether polymer studied by EXAFS

4.3. Planar or non -planar configuration? The electron micrographs [18] of the material before and after reduction already suggested a microscopic swelling of the polymer. The measurements of the diffusion coefficient [7] of the metallic cations inside the polymeric network showed that there was a fast hopping mechanism. The sites through which the dopants migrate within the polymer have been characterized by the present study. These independent pieces of information suggest for the neutral polymer an ‘egg-box’ structure like the one schematically represented in Fig. 9. Some of the NBu,+ cations which have not been exchanged with metallic cations are shown inside the big cavities. The counter-anions are not represented. This hypothetical structure allows the large tetrabutylammonium to be accommodated in the large cavities and the metallic cations in the small oxygen rings.

5. CONCLUSION The structure of the polymeric crown-ether PDB18C6 has been studied by EXAFS with Ag+ and Cs+ as dopants. A good fit of the observed spectra has been obtained, particularly for Ag+ for which the K-edge could be studied. The sites of the dopants in the polymer are the same as in the crown-ether monomers as well as in the cryptates [19]. Every metallic cation is trapped inside an oxygen crown. Ag+, whose size exactly fits the oxygen cavity, is encaged at the centre of the oxygen crown. The larger Cs+ ion is trapped on top of the complexing crown. This conclusion derived from the structure logically leads us to conclude that the polymer would offer the extracting capacity and the selectiveness of the monomer in solution, In fact, the situation is complicated by the ion exchange mechanism between NB: and the metallic cations [7] and also by the uncertain role of the anion. The six oxygen atoms are split into two sub-shells of two and four atoms. Cs+ has a preference for

451

the four oxygens linked to the phenyl rings; Ag+ prefers the other two. A three-dimensional structure is favoured by the present results instead of a twodimensional intercalated structure. Acknowledgements-This work is part of a programme supported by the Commission of the European C&munities. We are very grateful to Profs J. Corish (Dublin) and J. Petiau (Paris) for useful discussions.

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