New functionalized macrocyclic and macroacyclic Schiff bases: f-metal ions complexation and separation

Materials

Chemistry

and Physics,

31 (1992)

181-198

181

New functionalized macrocyclic and macroacyclic Schiff bases: f-metal ions complexation and separation E. Bullita, Istituto

P. Guerriero,

di Chimica

J. C. Dupuy,

e Tecnologia

S. Tamburini, dei Radioelementi

Cso

Stati

Uniti, 4, 35020 Padua

(Italy)

M. Prevost

Znstitut National

de I’Environnement

R. Bonora

and L. Marchesini

Istituto

P. A. Vigato

- CNR,

di Chimica

Indusm‘ale,

Industtiel

et des Risques

Universitci di Padova,

(INERIS),

T/is Marrolo

Centre de Recherche,

9, 35100 Padua

91710

Vert-le-Petit

(France)

(Italy)

Abstract New macrocyclic and macroacyclic Schiff bases have been obtained by condensation of 2,6-diformyl-4Z-phenols (Z = Cl, CH,, (CH,),C) with 4-N-dodecyldiethylenetriamine or the polyamines, 1,5-diamino-3-azapentane, 1,5diamino-3-thiapentane and octadecylamine. Their interaction with lanthanide(II1) (Ln(II1) = La, Ce, Sm, Eu, Tb, Dy, Yb) and the uranyl(V1) ions have been studied. The complexes, formulable as Ln(H,L)(X),.nH,O or UOzL (H,L - unionized macrocyclic or macroacyclic ligands; X=N09-, Cl-, n=l, 0), have been synthesized by template procedure or by reaction of the appropriate metal salt with the preformed ligands. They have been characterized by elemental analysis, mass spectrometric measurements, infrared and nmr spectroscopy (decoupling and n.0.e. nuclear Overhauser effect, experiments). A study of the phase transfers of trivalent rare earths from nitrate and/or chloride media through an organic membrane to a receiving aqueous solution by the macrocyclic compounds has been undertaken and the results obtained for the extraction, transport and stripping of dysprosium(III), europium(II1) and lanthanum(II1) ions are reported. It was verified that these ligands are efficient in the extraction of lanthanide(II1) Ions . without the addition of a synergic agent. A high selectivity has been observed for dysprosium(II1) or europium(II1) with respect to lanthanum(II1) from a mixture of La, Eu and Dy.

Introduction The preparation of macrocyclic and macroacyclic ligands and their coordination chemistry has received a great impulse in these years owing to their potential in basic and applied chemistry [l-4]. One goal is represented by the design and synthesis of compounds which can mime the functionality of natural carriers in transporting metal ions. This is useful not only to simulate a biochemical process but also for the development of new methodologies in separation science [5, 61. Most of the work in this area has been concerned with alkaline or alkaline-earth cations [7-141; much less attention has been devoted to the transport processes involving f-ions, particularly lanthanides. f-block coordination chemistry has been studied in detail, although not as extensively as that of d-transition or alkaline or alkaline-earth cations and only recently macrocyclic or macroacyclic ligands, specific for lanthanide or actinide ions, have been synthesized. Among these ligands, Schiffbases

0254-0584/92/$5.00

play a relevant role owing to their versatility, easy preparation and purification and to the possibility of giving rise to stable complexes [15, 161. Such studies are particularly welcome for complexes with f-ions, owing to the recent interest for a deeper knowledge of the properties of lanthanide and actinide compounds in connection with the increasing industrial applications of new f-compounds in catalysis, new materials, solid state chemistry, biology, medicine, etc. [17]. The recent development of specific macrocyclic and macroacyclic ligands has opened new possibilities in the field of selective metal ion recognition. These ligands are generally synthesized by reaction of the appropriate precursors in the presence of a suitable metal ion, which serves as ternplating agent. The subsequent demetallation of the complex produces the metal-free compound which can be extracted into an organic phase and purified by liquid chromatography. A direct synthesis of the ligands from the precursors which normally contain two functional groups each, can give rise

0 1992 - Elsevier

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182

to a mixture of compounds, sometime difficult to separate. However, in the separation processes free ligands are needed, hence it is necessary to set up synthetic methodologies for the preparation of such extractants in an almost pure form and possibly in high yield. Macrocyclic compound are well known as reagents for the selective complexation of metal ions [18-201: they have been considered for the separation of neighbouring lanthanides because of their potential for size selectivity. With a neutral macrocyclic compound like crown-ether, extractability of an ion-pair into an organic solvent is governed by the chemical nature of the counter ion; in general a counter anion with large molar volume is more efficient and consequently the picrate anion is widely used in crown-ether extraction system (for lanthanides see example [21-231). However, industrial aqueous solutions usually contain hard anions (Cl-, NO,-, SO,“-) and distribution ratios of metals between these phases and an organic phase with crownether extractants are too low for commercial applications. There are advantages in the use of mixed donor macrocycles for metal-ion recognition studies. First, the complexes do not show the extreme kinetic and thermodynamic stability often exhibited by other systems. Secondly, with cyclic systems solution speciation tends to be simpler than for open chain ligands; in many instances, 1:l (metal:ligand) complex formation predominates. The trivalent lanthanide ions have some resemblance in their electronic configuration and chemical properties to the alkali and alkaline earth metal ions. The separation of the lanthanide elements from each other remains one of the most difficult challenges. These elements are very similar in their chemical and physical properties, and are not easily separated by methods that rely on such differences. Moreover some separation techniques, such as supported liquid membranes or emulsionated membranes, offer the possibility of a practical application of many of these very expensive ligands. It was recently observed that Schiff bases derived from substituted salicylaldehyde and triamines or diamines containing additional donor atoms in the aliphatic chain (facultative diamines) or macrocyclic ligands containing the same coordination moiety, bind f-ions nearly in their equatorial plane [24-281. However these complexes show a low solubility in the industrial diluents. With the separation techniques mentioned above, high solubility in non-

coordinating solvents (xylenes, aliphatic hydrocarbons, etc.) is needed. The presence of aliphatic chains at the periphery of the coordination moiety of these ligands considerably enhances the solubility of the related complexes in aromatic or aliphatic solvents. Thus we have prepared macrocyclic and macroacyclic Schiff bases by condensation of 2,6-diformyl-4substituted phenols with facultative polyamines and octadecylamine or 4-N-dodecyldiethylenetriamine and studied their interaction with lanthanide(II1) and uranyl(V1) ions. The present paper reports the preparation and properties of these ligands and of the related complexes with f-metal ions, obtained by template procedure or by reaction of the appropriate metal ion with the preformed Schiff base. The present work has been also carried out in order to verify also the feasibility of a rare earth separation process by supported liquid membranes, using these specific Schiff bases. Data on distribution measurements have shown that complementary studies have to be undertaken to determine the conditions of extraction and stripping before carrying out experiments with supported liquid membrane. The results obtained from the extraction and the transport experiments of dysprosium, europium and lanthanum are reported. During the development of the work, the extraction and stripping processes, related to metal separation by membrane techniques, have been studied in detail, focusing the interest on the optimization of best working conditions for both processes (particularly the pH range of the loading and receiving phases).

Experimental Materials

4N-dodecyldiethylenetriamine and the other polyamines are commercial products (Kodak or Aldrich) and have been used without further purification. The other products and the solvents were reagent grade and used as available. The ketoprecursors 2,6-diformyl-4Z-phenols (Z = Cl, CH3, (CH&-C) have been prepared according to the literature [29-321. The ligands HZLG and HzL, have been obtained by condensation, in methanol, of 2,6-diformyl-4-chlorophenol with 1,5-diamino-3-azapentane or 1,5-diamino-3-thiapentane respectively, in a 2:l molar ratio [33]. UO1(LG) and U02(L) have been prepared by reaction in methanol of uranyl(V1) salts with the preformed ligands HzLG or H& in a 1.1 molar

183

ratio or by condensation of the formyl precursor and the appropriate amine, in the presence of uranyl(V1) salts as ternplating agents [33]. The purity of the ligands and complexes has been tested by elemental analysis, IR, nmr and mass spectrometry measurements. Preparation of the macrocycles

Method A. To a chloroform solution (500 ml) or 2,6-diformyl-4-Z-phenol (5 mmol), 4-N-dodecyldiethylenetriamine (5 mmol) in chloroform (50 ml) was added. The resulting yellow-orange solution was stirred for 2 h, then maintained under stirring over anhydrous sodium sulphate overnight. It was filtered and the solvent removed at reduced pressure. The oil or solid obtained was dried in vacua. Method B. 4-N-dodecyldiethylenetriamine (5 mmol) in ethanol (50 ml) was added to a stirring solution of 2,6-diformyl-4Z-phenol dissolved in 250 ml of ethanol. After 1 h the yellow solution was evaporated to dryness and the amber oil obtained was dissolved in diethylether (100 ml). The stirring was continued for 1 h and then the solvent was evaporated; the residue was dried in vacua and the oil or the sticky solid obtained was used without further purifications. Preparation of lanthanide(III) complexes Ln(H2L,)(N03)3,

(Ln = La, Ce, Sm, Eu, Tb, Dy,

fij

To an ethanolic solution (100 ml) of the ligand H2L, (1 mmol) prepared as above, La(NO& - 6H20 (1 mmol) or Dy(NO,), .6H,O was added. The pale yellow solution obtained was stirred for 1 h; the solvent was evaporated and the residue was dried in vacua. The sticky solid obtained was treated with n-heptane and stirred until a fine yellow solid was formed. It was filtered and dried in vacua.

Ln2(H,L) W&h

(Ln = La,

DY)

The same procedure employed for the mononuclear compounds was used. A ligand/metal molar ratio 1:2 was used instead of 1:l. Ln (Hz L,,) Cl, (Ln = La, Dy) The synthesis was carried out in the same way as for Ln(H,L,)(NO,),. LnCl, .6H,O salts were used instead of Ln(NO,), .6H,O. La (Hz LE) (NO,), (C = S, Z = Cl) H2L, (0.5 mmol) in 50 ml of chloroform was reacted with octadecylamine (1 mmol) dissolved in 30 ml chloroform. The suspension obtained was refluxed for 2 h and then filtered. The yellow solution, after addition of La(NO,), .6H,O) (0.5 mmol) in methanol (30 ml), was refluxed for 1 h, the solvent was removed and the residue was dissolved in dichloromethane, dried overnight over anhydrous Na,SO, and precipitated with n-pentane. The orange compound obtained was filtered, washed with n-pentane and dried in vacua. Preparation of uranyl(FIj complexes

Method A. To a chloroform (100 ml) solution of 2,6-diformyl-4-Z-phenol (2 mmol), 4-N-dodecyldiethylenetriamine (2 mmol) in chloroform was added. To the resulting yellow solution, a methanolic solution (20 ml) of Ln(NO& *xI&O (1 mmol) was added. The resulting pale yellow solution was stirred for 1 hour. The residue obtained by evaporation of the solvent was treated with methanol (La, Ce) or diethylether (Sm, Eu, Tb, Dy, Yb), filtered, washed with methanol or diethylether and dried in vacua.

UOz (L,) (for X=NH) and UO,(L,) (for x=s; Z=Cl) To an ethanolic solution (50 ml) of octadecylamine (1 mmol), UOZ(LG) or UOz(LH) (0.5 mmol) in dimethylformamide (20 ml) were added. The resulting solution was refluxed for 1 h, then reduced in volume under reduced pressure. The orange precipitate obtained by the addition of methanol was filtered, washed with methanol and dried in vacua.

Method B. To a chloroform (100 ml) solution of the appropriate macrocyclic ligand prepared as above (2 mmol), a methanolic solution of the appropriate Ln(NO& .xH,O (2 mmol) was added; the resulting solution was treated following the same procedure described in the method A.

The same procedure employed for the preparation of the lanthanide complexes was used. U02(N03)2 - 6H,O was used instead of the lanthanide(II1) nitrate salts, and LiOH (2 mmol) in methanol (30 ml) was added after the addition of the uranyl(V1) salt.

184

uo,

(LF)

To a dimethylformamide solution (20 ml) of UOz(LG) (0.5 mmol), 4-N-dodecyldiethylenetriamine (0.5 mmol) in methanol (50 ml) was added. The solution was refluxed for 1 h, then reduced in volume. On addition of methanol an orange precipitate was formed, it was filtered, washed with methanol and dried in WZCUO. Analytical data for the prepared complexes are in Table 1. Extraction and sttipping equipments and tests

In the liquid-liquid extraction experiments, a multi-stirrer equipped with a timer has been used. Isopar G (Exxon Chemicals), is an industrial solvent with the following physico-chemical properties: boiling point 155-173 “C; composition 2% C9, 72% ClO, 23% Cll, 3% naphthenics. Solvesso 150 (Exxon Chemicals) has the following physicochemical properties: Flash point 66 “C; boiling point 190-210 “C; viscosity 1.12 cps (25 “C); density 0.895. Aqueous nitric or hydrochloric solutions of the appropriate lanthanide ions (10e3 M or 10m4 M) and organic solution of the macrocycles (in the range 10-2-10-3 M) have been prepared and used. Physico-chemical measurements

The IR spectra were carried out as potassium bromide pellets using a Perkin-Elmer 580 model TABLE

1. Analytical

data for the prepared

EtOH

La,(H&c)(NW, . Hz0 C~(H~LJ(NO&.HZO Bm(HaLJ(NW3 Eu(H,L,)(NO,),*H,O T’J(H,L,)(NO,), . Hz0 JYH,L,)(NO,), Dy(H,L,)(NW,

DY(H&)(NW,

.4HzO

Dy(H&)(Cb-EtOH DY,(H&)(NO&-HZO La(HzLa)(NWx. HZ0 UO*(Ln) UO,(Ln) u”Z(LA) u”2(LF)

compounds Found

Calc.

Compound

&LA H2Lr.,. 2H20 H,Lc. lEtOH La(HaL,)(NU La(H,Lc)(NO& La(HzLc)(C13).

infrared spectrometer equipped with a data station. The ‘H and 13C n.m.r. spectra were recorded at 200.132 MHz on a Bruker AC 200 spectrometer equipped with an Aspect 3000 computer. CDC13 was used as solvent. All mass spectrometric measurements were performed on a VG ZAB 2F instrument operating in electron impact (EI) (70 eV, 200 mA, ion source temperature 200 “C) and fast atom bombardment (FAB) (8 KeV Xe atom bombarding sulpholane acidic solution of the sample) conditions. Quantitative analyses of lanthanide elements have been carried out by flame emission spectroscopy in order to estimate correctly concentration at ppm level. Lanthanum and dysprosium have been determined at 442 nm (La) and at 526.5 nm (Dy) (spectral band pass =0.2 nm) using a reducing acetylene-nitrous oxide flame following a standard procedure. Metal contents in aqueous solutions were determined also by Inductively Coupled Plasma EmisDy: 353.17 nm and La: sion Spectroscopy. 408.67 nm. The analytical data are in agreement with those obtained with other methods, particularly with those obtained by a SIMS (Secondary Ions Mass Spectrometry) apparatus. This method does not show interferences and has a higher sensitivity [34-361.

C%

H%

N%

C%

H%

N%

68.63 71.94 75.00 49.49 55.67 59.03 43.36 8.69 49.01 48.21 48.65 40.08 48.34 51.58 58.14 42.08 51.06 55.72 55.00 52.03 45.14

9.11 10.31 10.99 6.57 7.83 8.74 6.23 6.63 6.51 6.57 6.46 6.38 6.42 7.87 8.40 6.05 7.34 7.59 7.40 6.72 5.47

10.00 10.07 9.05 10.82 10.43 6.88 10.53 10.64 10.72 10.54 10.64 10.51 10.58 9.66 7.01 10.51 7.44 5.80 4.57 7.58 8.77

68.42 71.91 74.62 49.58 55.98 58.63 43.30 48.67 49.68 48.25 48.40 48.08 48.20 51.27 57.89 42.03 50.68 55.8 55.28 51.7 44.72

9.37 10.64 11.33 6.70 8.09 7.16 6.25 6.86 6.86 6.76 6.82 6.80 6.57 7.07 8.22 6.28 7.24 8.01 7.24 6.97 5.39

9.88 9.40 9.09 10.81 10.10 6.67 10.10 10.42 10.61 10.37 10.59 10.40 10.80 9.04 6.92 10.11 7.65 5.49 4.47 7.39 8.46

185

Results

and discussion

Lanthanide(III) and uranyl(K) complexes with macrocyclic and macroacyclic ligands 2,6-diformyl-4Zsubstituted phenols (Z = Cl, CH3, (CH,),C) are useful precursors in the preparation of Schiff bases especially designed for fmetal ions. Accordingly the symmetric macrocyclic HZLA-HzLC and macroacyclic HZLD, H2LE and the asymmetric macrocyclic H,L, ligands, have been synthesized by condensation of the appropriate keto- and amino-precursors, according to Schemes 1 and 2. In particular H2LA-H,Lc have been prepared, in one step, by the reaction of the proper 2,6diformyl-4Z-substituted phenol and 4N-dodecyldiethylenetriamine, in a 1:l molar ratio and in a dilute methanol solution. The complete condensation with the consequent formation of the macrocycle is evidenced by the disappearance in their IR spectra of bands at about 1680 cm-’ due to formyl groups and of the bands due to NH,? groups, present in the spectra of the keto- and amineprecursors (Scheme 1). Strong bands, due to zC=N groups, confirm that the cyclization reaction has occurred. Electron impact mass spectrometry evidences the parent peak [Ml+’ at the appropriate value for H-L, and H,L, while it does not lead to the molecular ion for HZLA, although some fragmentation ions can be related to the Cl-compound structure. In recent years fast atom bombardment (FAB) has proved to be a good analytical tool in

organometallic, coordination and complex organic compounds. Consequently we performed FAB experiments on HZLA, comparing the results with those obtained for H,L, and H2Lc. For all three macrocycles a well detectable molecular peak is present at the appropriate m/z value, which match perfectly with the theoretical ones for [M-H]+‘. It originates through a proton extraction which leads to a single charged molecular cation. Symmetric cleavages, leading to [M/2]+’ ions are present in the EI spectra of the compounds as observed in macrocyclic poly-ether-ester compounds. The species [M/2-H]+‘, common in crown ether mass spectrometry, are absent in EI but are clearly detectable in FAB spectra of all three compounds. Lanthanide(II1) complexes, soluble in aromatic or aliphatic hydrocarbons, can be obtained either by the reaction of the preformed ligand H2LAHzLc with the appropriate lanthanide(II1) ion or by template procedure. They are yellow or yellow orange solids which can be conveniently prepared in a methanol/chloroform solution. Some of the lighter complexes prepared contain a water molecule which is not present for the heavier ones. Lanthanum(II1) and cerium(II1) complexes are almost insoluble in methanol while the other complexes are soluble. More important, the complexes are soluble in the common organic solvents, also aromatic hydrocarbons. In these complexes the Schiff base behaves as a neutral pentadentate ligand. Their infrared spectra parallel with those reported above for the complexes with the macroacyclic ligands; two zC=N bands are present

z = cl (H&4) -T

Z = CH3 (H$B) Z = (CH3)3C (Is94

Scheme

1. Preparation

of macrocyclic

ligands and related

complexes.

186

+

u%z+

+ LiOH

X=NH;

Z

x=s;

Z

= Cl

(H&i)

:

X=NH;

Z= Cl

@2LF)

+ LiOH

2NH,(CHzhCH,

01 x -- Lo(h’OJ3

c/\

8, P”R ‘yf

\

7.

I

\A

(”

Z

Scheme

2. Preparation

\A/

bH N-(CHJ,,-CH,

X

of macroacyclic

ligands and related

X=NH;

Z = Cl &L,)

X=NH;

Z = CH3 (H2Ld

complexes.

in the 1635-1655 cm-’ region while the bands due to nitrate groups support the presence of both bidentate and ionic groups. If a large amount of Ln(NO& - 6H20 is used with the macrocyclic ligands HZLA, HzLB and H,L,, yellow-orange species can be obtained, which, on the basis of elemental analysis, could be formulated as binuclear complexes Ln,(H,Le)(NO,),. The solubility of these binuclear compounds in xylene is quite low so

that they can be purified, from the mononuclear analogues, by using this solvent. A comparison of the infrared spectra of 3 3 and La(H2Lc)(C1,), allows the La(H&)(NO ) identification of the absorption bands due to the nitrate. The VC=N lies at the same frequency 1648 cm-l for both compounds while the free ligand HzLc shows a sharp K=N at 1633 cm-‘. The spectra of the nitrate complexes are similar

187

to those obtained for the analogous complexes with the acyclic ligands HzL, and HZLE. In both series of complexes (vide postea) bidentate coordinated and ionic nitrate groups are present. Dy(H,L,)(NO,), shows an infrared pattern very similar to that of La(H,L,)(NO,),; the zC=N lies at 1642 cm-‘, while the strong absorption at 1384 cm-‘, due to the nitrate group is strongly enhanced. The spectrum of La2(H2LC)(N03)6 is similar to those of the other nitrate complexes; the VC=N lies at 1645 cm-l. Some differences appear in the range of absorption of the nitrate ions; the complexity of the compounds however does not allow any certain correlation between these differences and the coordinating behaviour of these groups. The macrocyclic uranyl(V1) complexes have been prepared, as a yellow-orange solid, similarly to the lanthanide(II1) analogues, by template procedure or by the reaction of UO,(NO,),* 6H20 with the preformed ligand in methanol/chloroform and in presence of LiOH (Scheme 1). Their IR spectrum show only one VC=N band at about 1630 cm-’ while the presence of the uranyl group is shown by the v3 O-U-O at about 903 cm-‘. A FAB mass spectrometry investigation on the lanthanide(II1) and uranyl(V1) complexes adds useful information. For the uranyl(V1) complexes the parent peak at the appropriate m/z value has been detected. and La(H,L,)(NO& La(H,L,)(NO,), do not show such a peak, however the peak of the organic ligands is clearly detectable, this being an additional evidence of their coordination to the central metal ion as macrocyclic entities. The solubility of the macrocyclic ligands and related lanthanum(II1) or uranyl(V1) complexes has allowed a nmr investigation, which was carried out in CDC13 as solvent. The crude condensation macrocycles, obtained by evaporation to dryness of a methanolic solution containing the reactants (see experimental) have been purified by chromatography (using silica gel and CHC& as eluent) to eliminate possible undesired impurities. The ligands remain yellow oils also after the chromatographic purification. The ligand H2LB was discussed in detail owing to the better resolution of the peaks. The peaks present in the nrnr spectra of HZLA and H,L, have been assigned by comparison with those obtained for HZLB (Table 2). In addition the shape of spectra obtained and the assignments of the peaks were also compared with those found in the uranyl(V1) and lanthanum(III) complexes with the same ligands, the most remarkable differences

TABLE

2. ‘H nmr

data

Int

E~FWII Ar-t but Ar-CH__3 (CHk CH,-CI+

6 18 6 36 4

(C&I,--(C&P-W&I,C&-N= Aromatic HC=N

4

$I

2

8 8 4 4

for

the macroqclic

ligands

H2LA

&La

H&

@pm)

@pm)

@pm)

0.86

0.83 1.37

0.87 -

1.15-1.50 1.48 2.30-2.70 2.70-3.10 3.20-3.80 6.70-7.65 10.50 8.20

2.15-2.30 1.15-1.40 1.48

1.15-155 1.50

2.42-2.63 2.64-3.16 3.40-3.70 7.15-7.80 10.15-10.50

2.1c2.70 2.70-3.00 3.35-3.80 7.05-7.90 10.50

8.20

8.30

being a higher compression of the aliphatic peaks (3.5 ppm) and a shift toward higher fields of the phenolic proton of the free ligand with respect to the values found in the complexes where the phenolic oxygen is coordinated to the central metal ion. The nmr spectrum of H,L, reveals a remarkable complexity which needs some justifications. Three single peaks attributable to CH,-(xylene) could be due to the presence of different isomers and to an asymmetry in the molecule. From the contour plot of a COSYPH experiment, coupling between the CH,-(xylenic) singlet peaks (2.15-2.30 ppm) and the three doublets, due to aromatic protons (7.10-7.80 ppm) occurs. Thus the asymmetry of the molecule can be due (in agreement with fluorescence, elemental analyses and mass spectrometric data) to the coordination of the Na+ ion (coming from Na,SO, used for a further drying of the oils) to the phenolic oxygen, the iminic nitrogen-C=N and the close aminic nitrogen. This coordination could be responsible for the different electronic distribution of the two benzene rings and hence to the multiplicity of the peaks. Long range cross-peaks, between the phenolic (8.20 ppm) and the methylenic protons (3.60 ppm) strongly support the asymmetry of the ligand, suggesting the presence of a strong hydrogen bond between the iminic nitrogen (bound to this methylene group and not coordinated to the Na+ ion) and the phenolic oxygen bound to such a metal ion. The complexity of the peaks due to the N-CH,-CH,-N chains parallels the patterns found in the lanthanide complexes; this is also due to the coordination of Na+ to the ligand. All the monodimensional nmr spectra of the complexes La(H,L,)(NO&, UO,(L*), and La(H,L,)(Cl), show a conLa(H,L,)(NO& siderable complexity in the aliphatic region, due

188

to the high number of aliphatic protons and to the as~met~ of the macrocycles caused by the coordination of the f-metal ion to one of the two possible coordination chambers. The clearest results on monodimensional spectra have been obtained for La(H,L,)(NO,), where only one isomer is present. It must be underlined that in these systems it is possible to obtain different isomers which clearly affect the nmr patterns and hence an easily readibility of the spectra. For instance the monodimensional spectra of UOz(LA), La(H2Lc)(N0,), and La(H2L,)(C1)3 are similar to that of La(H,L,)(NO,), but they show the presence of isomers. For the complexes with the ligand H,L, the 2D NMR correlation spectroscopy allows the interpretation of the obtained spectra, which are unclear in the unidimensional spectra, and hence the correct assignments of the peaks for all the samples under investigation (Table 3). Thus the 2D NMR contour plot of La(H,L,)(Cl), (Fig. 1) evidences the peaks due to the -C(CH,) groups, completely covered in the unidimensional NMR by the aliphatic CH2 groups. The cross peaks between 7.8 ppm and 1.18 ppm show a scalar coupling between the t-but@ and the aromatic protons (line ‘a’ of Fig. 1). In the 2D nmr contour plot of La(H,L,)(NO& a scalar coupling between the OH (11.9 ppm), the HC=N(7.9 ppm) and the aliphatic -CH2-N= (4.4 ppm) has been detected (Fig. 2, line a). In addition to these ‘long-range couplings’, it is possible to suggest, again on the basis of bidimensional results, an asymmetry of the macrocycles, caused by the coordination of the f-ion only to one chamber of the compartmental cavity. This coordination, making the two parts of the ligand magnetically non-equivalent, causes a multiplicity of peaks not correlated to each other. It is possible to obtain, by COSY contour plots, the scalar couplings within the two different aliphatic chains, from the CH,--N= to the (CH& protons TABLE

3. ‘H nmr data of the complexes

Assignment =3-

Int 6

t-but

18

Kw9

36

WkCH3

-&WY-N -0&--N= aromatic HC=N OH

(Fig. 1, line ‘b’ and ‘c’; Fig. 2, line ‘a’ and ‘b’). To obtain HzL,,-H,L, a step by step procedure is necessary. For instance 2,6-difo~yl-4-chlorophenol gives, with facultative diamines, 1,5-diamino-3-azapentane or 1,5-diamino-3-thiapentane, the acyclic potentially pentadentate ligands HzLG and HZLH (which contain C=O groups) which can be involved in further condensation reactions with primary amines. Using NHz-(CH,),,-CH, or 4-Ndodecyldiethylenetriamine the macroacyclic and macroacyclic ligands H,L,-H,L, of scheme 2 were obtained. These Iigands form, by reaction with ianthanide(II1) nitrates in a 1:l molar ratio, the related complexes soluble in the common organic solvents and also in hydrocarbons. The same complexes can be obtained by a template procedure (scheme 2). In these complexes the Schiff bases behave as neutral pentadentate ligands, the remaining coordination site, about the central metal ion, being filled by bidentate nitrate groups. The presence of the nitrate groups is shown by strong absorption bands at about 1460-1470, 1381-1383, 1282-1290, 1034-1030 and 818812 cm-‘. It is not possible to assign the correct coordination behaviour of the nitrate groups from IR data only. However it is to be noted that the complexes [Ln(H,L,)(NO,),]NO, (Ln = Eu, Tb), which have the same Schiff base coordination moiety and where two nitrates are bidentate and one is ionic, show IR bands comparable in their intensity and frequency with those reported above. Consequently similar coordination may be suggested for the two series of complexes. H2L, is the macrocyclic Schiff base obtained by condensation of 2-6-difo~yl-4-chlorophenol with 1,5-diamine-3-~apentane. According to this view, the band at about 1380 cm-l could be assigned to the ionic N03- ion, while those at 1460-1470, 1282-1290, 1034-1030 and 818-812 cm-’ could be attributed to bidentate nitrate groups. The strong bands at 1630, 1660 cm-’ are due to VC=N.

4

12 8 4 4 2

LaWkW)3 0.86 1.18 1.15-1.3s 1.55 2.03-3.42 3.52-5.41 7.00-8.21 7.00-8.21 11.17, 12.15

La(H&&NWs 0.83 1.20-1.32 1.1.5-1.40 1.57 2.15-3.35 3.40-4.72 7.01-7.54 7.95, 8.15 10.74, 11.91

u&@-A)

0.84 0.95-1.41 1.44-1.71 2.28-3.94 4.00-6.12 7.36-7.74 7.94, 8.13 -

~(HzLA)(NO,),

0.83 1.01-1.35 1.56 2.21-3.43 3.04-7.52 7.04-7.52 7.81-7.92 10.35, 12.2

-

--

-

_-

.y

_

__ _-_

.-.

-.--_ -_

_

-_-

h~,...i..*~~~...,...;,,.,,,.,,,,..,.

-0 _

__-

it-.

0.

_-

_-

-_

:

L-

- -. _-;

_

.

_.

_

_

r

_ - ____-

:-

_ -

-=

_

-

7

-.

_-

_

x.-a. _---_

-

-

-:

.+

-

_’

.__

_

.,,,

_

‘-

*; z

-

.,,,

-_:-

_

,,.,

__

-=--;

-

_

--

- _

.

..,,

_.

i

.._,,

-

,.,,

-

_,,..

,,,

_ __._I-.-

190

0.0

I.

00

2.00

3.00

4.00

5.00

6.00

1.00

6.00

9.00

10.00

It.00

12.00

m

12.00

II.00

IO.00

1.00

I

6.00

PPn

’

I

5.00

I b.00

*

I

3.00

*-

2.00

1.00

Line ‘a’. - - - - - Line ‘b’, Fig. 2. 200 MHz 2D ‘H nmr Correlation Spectroscopy (COW) contour plot of La(H&)(NO&

0.0

'8

191

Using uranyl(V1) salts a similar reaction pathway was obtained. The apical positions of the coordination polyhedra about uranium are occupied by the almost linear O-U-O group; consequently the preparation of the complexes of the type and UO,(L,)-UO,(L,) UO,(L,) or U02(L~) (where the ligands coordinate as dianionic, pentadentate to the central metal ion) LiOH was used to pull away the anions and to make the uranyl complexes analogous to the lanthanum ones. It must be noted that, in boiling methanol or using U02(CH3C00)2. 2H,O as starting material, the same complexes, without a counter anion, are easily obtained. This is a remarkable difference between uranium and lanthanide ions. These ions maintain their counter anions if the base is not used. The infrared spectra of UOZ(LG) or UOZ(LH) and UO,(L,)-UO,(L,) show a strong peak at 889 (X =NH) and 894 (X = S) cm-’ due to antisymmetric v30 -U0 stretching. For X = NH a sharp band at 3210 cm-‘, due to vNH is also detectable. A comparison of the nmr spectra of UOZ(LG) and UOZ(L,,) (Scheme 2) reveals the same pattern for the -CH*--CH2-NH-CH,-CH,groups, suggesting that the coordination and the structure around the uranium ion was maintained also after the condensation of the aliphatic chains to the free C=O groups. ‘H nmr data for the complex UOZ(L,,) are reported in Table 4. The assignment of aromatic and aliphatic protons was determined by decoupling and n.0.e. experiments. Figure 3 reports a comparison of the ‘H nmr of UOZ(LG) and UOz(LD) for the aliphatic region. The N-H proton of the starting complex, a triplet which lies at 7.35 ppm in DMSO D6 solution, is missing in the bottom spectrum. Open cryptands In order to improve selectivity, new tridimensional ligands have been synthesized. Planar macrocycles have been widely studied and applied but often do not offer good discrimination for different metal ions; three-dimensional ligands, instead, may offer good selectivity. From the synthetic viewpoint, preparation of three-dimensional ligands is often difficult and the yields are low, whereas planar macrocycles are easier to prepare and obtainable in higher yield. A good balance of all these factors is represented by the open cryptands (Scheme 3) where the two pendant arms Y can coordinate to a metal ion in its apical positions. The mobility of these arms may allow a highly dynamic complexation and decomplexation of several ions, as required for efficient transport.

TABLE 4. ‘H nmr data for the uranyl(V1)

complex UO,(L,)

Chemical shift p.p.m.

Integral

Assignment

9.38 9.30 8.31 7.53 4.72 4.60 4.27 3.79 3.73 1.71 1.39 1.25 0.87

2s 2s 2d 2d 2m 2d 2d 4t 2m 4m 4m 56 s 6t

m n

0

1\!

8, ,' \ I-7 \

‘9

h

\

0

P x g E d b C

a

NXH,-C%-(CHJ,,CH,-C%

I

\ 1

,H;N -U02

In order to prepare these macrocycles with pendant arms, it is necessary to synthesize suitable functionalized polyamines of the type NH,-(CH,),NH-(CH&-NH-CH2CSH4N or NH2-(CH.J,-NR(CH&-NH2, where n and R can be changed to obtain a matrix of compounds. The variation of n produces more or less flexible ligands, while the variation of R can be tailored in agreement with the solvent used or with the metal ions to be separated. By reaction of these polyamines with the appropriate keto-precursors (i.e. 2,6-diformyl4-substituted phenols, 2,6-diformyl- or 2,6-diacetylpyridine, etc.) the corresponding macrocycles can be synthesized (Scheme 4). Thus the condensation in ethanol or acetonitrile of 2,6-diformyl-4-chlorophenol and [NH*-(CH2)],-N(CH,)-OCH, produces the macrocycle HzLL as yellow-brown oil. It was characterized by elemental analysis, IR and nmr spectroscopy. FAB mass spectrometry data (in 3-nitrobenzylalcohol) agree with the proposed structure. The parent peak at the appropriate m/z value together with the fragmentation species,

192

a

b

uo;?(LD)

50 Fig. 3. Comparison

4.5

I

I

410

of the ‘H NMR spectra

3.5

of UO,L,

1

I.S

2’0

and UOzLD for the aliphatic

1.0

region.

confirms the macrocyclic nature of the ligands prepared and the ability of the side arms to bind the central metal ion in a peculiar fashion for each metal ion used. The yield of open cryptands was very low. It must be considerably increased in order to propose the use of these compounds in separation techniques. We have decided to focus our attention on macrocyclic compounds, because they offer the advantage to be prepared in one condensation step. In addition their macrocyclic nature could offer a better selectivity towards lanthanides with respect to the cyclic ones. Scheme 3. Schematic representation of open cryptands (X = pyridine, furan, thiophene, pyrrole, phenol; Y = pyridine, secondary amine-NR, OR).

related to the macrocyclic compounds, are clearly detectable. In addition the X-ray structure of the barium(I1) and copper(I1) complexes with the ligand Hzb

Distribution

studies

During the extraction tests an excess (ten times) of the appropriate macrocycle was used in order to obtain the mononuclear complex in the organic phase. The solubility of the mononuclear macrocyclic or macroacyclic complexes depends on the substituent Z and follows the order -Cl < -CH3 < -C(CH,),. The ligands HZLB and HzLc have been employed for the extraction ex-

193

R-N

I

,N CC%),

\ N-R

\ (CHzJn ..--N

N--R

\ (W,,

N’

(C&k \

I R-p N

I (C”Z)n 011

N,

/ (CQ”

R”

Scheme 4. Open cryptands

derived from substituted

diformyl or diacetylphenols

periments due to their higher solubility and to the fact that they can be synthesized in ‘one pot’ reaction. Xylene, solvesso 150 or a mixture of xylene and aliphatic hydrocarbons (isopar G) have been used as diluents, because they do not cause damage to the polymeric support and thus the results obtained can be transferred to pilot plants or can be tested in experiments with supported liquid membranes. Tests with xylene as diluent The tests started with a series of liquid-liquid

extractions, using equal volumes (10 ml) of the organic and the feed phase at various pH values, stirred for long periods of time (24 h), in order to obtain the phase equilibrium. The best results have been obtained in the pH range 4-6. For lanthanum(II1) the percentage of extraction increases with the lowering of its initial concentration in the feed solution from 10m3 to 10m4 M. There is also a protonation of the ligand which changes the initial pH of the feed solution to about 5.5 in the absence of buffered solutions.

and pyridine.

It was observed that using times up to 4 h there is a partial redissolution of the metal ions into water. Thus a series of experiments using times up to 30 min and pH 3-6 with a ligand concentration in the organic phase 10-z-10-3 M have been carried out. At pH 3 the results are not satisfactory either as D or as selectivity; at pH 4, Dy3+ is extracted in higher percentage than La3+ for contact time longer than 20 min. The most significant results have been found at pH about 5 where dysprosium is extracted in higher percentage with respect lanthanum. This behaviour is enhanced for a ligand concentration lop3 M. Tests have been also carried out at basic pH. Rare earth solutions were prepared by dissolving the nitrate salts in NaOH solution at pH=8 to give a concentration 10m3 in Ln3+ while HzLc in xylene lo-’ M was the organic phase. Equal volumes of aqueous and organic phases were mechanically shaken at room temperature for a contact time from 5 to 120 minutes. A rapid disengagement of the phases was obtained after shaking and both phases were clear after a few

194

minutes. Moreover, no precipitate was observed in the interphase. The percentage of lanthanum extraction is greater than that at pH =5, while dysprosium is extracted approximately in the same amount. During the stripping tests equal volumes of xylenic solutions of the lanthanide ions as complexes (10e3 M) have been shaken with aqueous solution at pH 2, 3, 4, 5. It was found that the back-extraction distribution coefficient D increases with the contact time and with lower pH. At pH < 2 a very high D value is obtained; this is also due to the hydrolysis of the ligand. The complex is almost completely destroyed and metal ion and the precursors of the ligand are transferred to the aqueous solution, which becomes yellow. A comparison of the D values obtained for La3+ and Dy3+ under the same conditions, shows La3+ is extracted in a higher percentage; under the best back-extraction conditions (pH = 3, Ln3+ = 10e3 M) La3+ is extracted at a value of about 70% and Dy3+ at about 50%. We have studied also the possibility of making the extraction and stripping processes sequential with the aim to achieve a better understanding of the whole process. Consequently the experiments were carried out in two sequential steps; the first consisted in an extraction from the feed solution into a xylenic phase and the second was a stripping from the obtained organic phase into an aqueous solution. These two processes, combined together, allow the metal transfer between two aqueous phases through an organic one. The experiments were performed by using these conditions: pH =5 (HN03) and 136 mg/ml and 168 mg/ml of La3+ or Dg+ respectively in the aqueous feed solution, a 10m3 M xylenic solution of H,L,, pH=3 (HN03) in the aqueous stripping solution. The results show that lanthanum was extracted in a lower amount and completely stripped whereas dysprosium was extracted in a considerably bigger amount and stripped in a percentage lower than that of lanthanum. This result show that, in the whole process, dysprosium is preferentially transferred to the receiving phase.

Moreover the contribute to a process. A mixture of as organic phase.

change of the organic phase can better yield in the ion transport

Isopar G and xylene was chosen The solubility values of the ligands HzLc and of the complexes ~r$HzL~$~3)3 and Ln,(H,Lc)(NO,), (Ln = La Dy) were tested. HzLc is the more soluble ligand in both xylene and Isopar G, the values being 800 gr/l and 620 gr/l respectively. The solubility values of the Ln(H,L,)(NO,), are about 425 g/l in xylene and almost insoluble in Isopar G. The complexes of the type Ln2(H2Lc)(N03)6 have a low solubility in xylene and are insoluble in Isopar G. According to these results, a xylene 60%-Isopar G 50% mixture, which allows a solubility > 10m3 mol/l for Ln(H2Le)(N03)3 complexes, was used as organic phase for the different tests. i) Lanthanum and dysprosium extraction: The experimental conditions were the same a those used for the extraction tests in 100% xylene. The extraction results from a feed containing both lanthanum and dysprosium (lop3 M) with this organic phase show that Dy3+ is extracted in a bigger amount (40%) with respect to La3+ (5%), and showing also a decrease of DDY by using this organic phase with respect to the xylenic one. ii) Lanthanum and dysprosium stripping: The experimental conditions are the same as those used for the stripping tests with 100% xylene as the organic phase. The results show that this organic phase favours the stripping of both metal ions. iii) Sequential extraction and stripping: Experiments, carried out by employing the same conditions used with the xylenic phase, show that, in the xylene 60%-Isopar G 40% mixture, the dysprosium and the lanthanum transfer reach 2% and 0.6% respectively. When the metal ions are simultaneously present the transfer values reach 3.9% for dysprosium and drop to 0% for lanthanum.

Tests with Solvesso 150 as diluent Tests with xylenellsopar

G as diluent

It was observed that, during preliminary mass transport tests with supported liquid membranes, some problems, concerning impregnation and membrane life-time, were encountered; therefore we thought to modulate the properties of the organic phase in order to make it more compatible with the polymeric support.

In solvent extraction processes the general requirements of a diluent are: high solvency, low volatility, high flash point, readily available; in addition for a supported liquid membrane a low surface tension (good wetting of support) and an increased viscosity (membrane stability) are required. Among the typical diluents used in solvent extraction processes, Solvesso 150 has been chosen.

195

Four different unbuffered feed solutions (A-D) . , have been used: A: a solution of Dy3+ and La3’, 5~ 10e4 M each; nitrate medium; pH 5.5; B: a solution of Df+ and La3+, 5~ lop4 M each; nitrate medium; pH 5.5 excess of nitrate anion (0.3 M NH4N03); c: a solution of D$ + and La3’, 5 x lop4 each; chloride medium; pH 5.5 excess of chloride anion (0.3 M LiCl+O.3 M NH,Cl); D: a solution of Dy3+ and La3+, 5 x 10M4 each; chloride medium; pH 5.5 excess of chloride anion (0.3 M NH,Cl). H-LB or H,L, (1 x 10P2 M) in Solvesso 150 was the organic phase while an aqueous unbuffered solution (pH 3.5, HNO,) was the stripping solution. Equal volumes of aqueous and organic phases (O/A= 1) were mechanically shaken at room temperature. After a contact time of 5 or 30 min, both phases were clear after a few minutes. This series of experiments has been carried out with no preequilibration of the organic phase. It was observed that dysprosium is preferentially extracted with respect to lanthanum and for a contact time of 30 mn, the H,L, loading capacity was slightly higher than the H,L, one. For H,Lc the metal % extractions were Dy = 90-95 and La =35; for HZL, the metal % extractions were Dy = 75-90 and La = 30. The stripping of the loaded organic phase was difficult with acid solutions (pH 3.5) and, for a contact time of 30 mn, Dy was stripped to a maximum value of 70% (Table 5). TABLE Aq. Sol. nitrate

5 Aq. Sol. chloride

Extraction Dy

(*)(**) La

Stripping DY

Dy and La extraction A(***) B(***) c(***) D(***)

and stripping 89.6 96 89.7 93.8

with H?Lc 19.1 35.5 5.6 22.9

62.9 42.15 73.5 18.3

Dy and La extraction A(***) B(***) c(***) D(***)

and stripping 76.8 89.9 82.2 78.4

with HILa 19 27.9 3.7 4

52.5 49.1 72.7 59.5

(*)Extraction t =30 minutes; stripping t = 30 minutes. (**)%Metal extracted (stripped). (***)A: A solution of Dy’+ and La3+ t X 10m4 M each; nitrate medium; pH 5.5. B. A solution of D$+ and La3 + 5 x 10m4 M each; nitrate medium; pH 5.5 excess of nitrate anion (NH.,N03 0.3 M). C: A solution of Df+ and La3+ 5 x 10e4 M each; chloride medium; pH 5.5 excess of chloride anion (LiCl 0.3 M+NH&I 0.3 M). D: A solution of Dya+ and La3+ 5 X 10e4 M each; chloride medium; pH 5.5 excess of chloride anion (NH,Cl 0.3 M).

Dysprosium extraction is time dependent; a contact time of at least 20 mn was necessary to obtain an extent of about 90% (with H,L,) which is characteristic of a very slow kinetic, whereas lanthanum extraction appears not to be time-dependent. In the stripping tests contact times of 30 mn were used, the formation of a third solid phase was observed, and the aqueous solutions remained cloudy after settling. Stripping with an acidic solution at pH 3.5 is not efficient enough and appears not to be a kinetic effect. As shown from previous results, the percentage of stripped metal increased with the lowering of pH (pH-2) but in these conditions the ligand was hydrolyzed and the precursors were transferred with the metals to the aqueous solutions. The ligands, which contain easily protonable N atoms, extract H’ and the pH of the aqueous solution increases up to a value incompatible with the required stripping reaction conditions. Moreover, if the protonated organic phase is not scrubbed with water before recycling at the extraction step, a very low lanthanide extraction amount is observed and phase disengagement becomes very slow and difficult. The best results have been obtained in nitrate medium in the presence of an excess of ammonium nitrate (which might act as a slating-out agent, decreasing the water activity) (Table 6); moreover this proves that the ammonium ion is not extracted with the ligand. For example D, increases from 8.6 to 24 and DLa from 0.24 to 0.55 (contact time 30 mn) with an excess of NH4N03 in the feed. In the same conditions (excess NH4+, solutions B and D), Dy and La were extracted to a lower extent from chloride medium and, in the presence of lithium cations, both Dy and La extraction decreased, this due to a probable competitive extraction of Li+ with H2L, or H,L,. The separation factor S,,L, is superior to 10 from all the feed solutions; the highest values were obtained from chloride aqueous solutions in the presence of Li+ ions; (S > 100 for 30 min contact time); it means that the probable lithium co-extraction preferentially decreases the lanthanum extraction. Finally, tests in sequential conditions show the metal (i.e. Dy) is distributed between the three phases, this being (Table 7) due, in part, to the low extent of the stripping step.

196 TABLE

6. Selectivity of extraction

Aq. Phase nitrate

Aq. Phase chloride

Dy/La

(H&c) A B

t=5

Dy/La

min**

t=30

min.*

DDY

DLa

S

DDY

DLa

S

1.1 1.05

2.03 6.3 2.15 4.9

0.20 0.39 0.07 0.14

10 16 31 35

8.6 24 8.7 15

0.24 0.55 0.06 0.2

36 44 145 50

1.1 1.05

1.14 2.5 1.13 15

0.24 0.35 0.04

3.3 8 4.6 2.6

0.24 0.40 0.04 0.04

14 20 115 90

1.2 1.1

W&B)

A B

1.2 1.1

4.75 7 33

*For the meaning of A, B. C, D see Table 5. **t=contact time of extraction. TABLE 7. Distribution phases(**) Aq. Phase nitrate

of Dy between

Aq. Phase chloride

organic

and aqueous

%Dy

Aq. I (***)

Org. (***)

Aq. II (***)

C D

10.4 4 10.3 6.2

33 46.5 23.7 76.65

56.4 49.5 66 17.15

C D

23.2 11.1 17.8 21.6

36.5 45.3 22.4 31.7

40.3 43.6 59.8 46.7

(I-GLc) A B

(KLa) A B

*For the meaning of A, B, C, D see Table 5. **Contact time of extraction: 30 minutes; contact time of stripping: 30 minutes. Aq. I = feed aqueous solution. Org. = stripped organic phase. Aq. II = strip product solution.

complex in the organic phase, and finally a poor ligand-cavity fit. The selectivity of the prepared macrocyclic ligands for Dy and Eu over La is very high and from mixtures it is possible to preferentially extract either Dy or Eu over La. For example: feed solution B, ligand: H,Lc, separation factors: SDy,La= 44, !&u/La= 37. On the other hand both extractants exhibit a slightly higher selectivity for Dy over Eu and in the same conditions as reported above, the value of the separation factor SW,, equals only 2. From these competitive extraction experiments, the macrocyclic ligands exhibit the following order of affinity for the three rare-earths tested: Dy > Eu Z+La. The distribution ratio DL,, versus the corresponding ionic radius shows that the larger the ionic radius, the lower the D value: D,> 3 for r < = 0.85 8, (in nitrate medium with excess NH,+). Conclusions

Selectivity of the extraction The different extraction experiments from nitrate and chloride mixtures of dysprosium and lanthanum and preliminary tests on mitures of Dy/ Eu and Eu/La (with an excess of NH4+, solution B and D) have shown that in all cases the heaviest metal tested (Dy) was the most readily extracted. The lightest lanthanide (La) with larger ionic radius does not seem to be extracted with these two ligands. The distribution coefficients for La are always lower than 1. Three reasons could explain this result: a stronger bond between the central metal ion (La) and the coordinated water molecule in the feed, a poorer solubility of the H,L/La

Several macrocyclic and macroacyclic ligands (H2LA-H2Lr_) have been prepared, characterized and used in complexation reactions with f-ions. It was found that they are capable of forming well defined lanthanide(II1) complexes of the type where H2L are HZLA-HZL,_ and Ln(H,L)X,; X = N03-, Cl-. In these complexes the Schiff bases behave as neutral pentadentate ligands through the donor set N302. The macrocyclic or macroacyclic ligands coordinate the central metal ion almost in its equatorial plane, while the counter anions fill the coordination sphere above and below this equatorial plane. In the analogous uranyl(V1)

197

complexes, the Schiff bases behave as polydentate dianionic ligands and coordinate in the equatorial plane of the almost linear O-U-O group. The structures of the ligands and related complexes have been inferred by IR, ‘H and 13C nmr spectra and EI and FAB mass spectrometry. The results obtained from these measurements parallel with those recorded with very similar Schiff base macrocyclic and macroacyclic f-containing complexes, where the structures have been determined by means of X-ray diffractometry. The presence of long aliphatic chains in the periphery of the ligands does not influence their coordination ability and increases their solubility and that of the related lanthanide complexes in aromatic hydrocarbons (i.e. xylenes) or mixtures of aromatic and aliphatic hydrocarbons. Hence these macrocyclic or macroacyclic compounds have been tested in the selective extraction of lanthanides. Both series of ligands are soluble in organic solvents and are usable for the separation of lanthanides. We have decided to focus our attention on macrocyclic compounds, because they offer the advantage of preparation in one condensation step. In addition their macrocyclic nature could offer a better selectivity towards lanthanides. From distribution measurements in static conditions, the new macrocyclic ligands, HZLB and H2Lc, have been shown to be good extractants for some trivaIent lanthanides (Dy-La) from a nitrate medium. But the small difference in pH between feed and stripping solutions require preequilibration of the organic phase to miminize the proton extraction particularly during the stripping step. It was shown that it is also possible to use a mixture containing aliphatic and aromatic hydrocarbons as the organic phase. The results obtained in this case are in line with those obtained with xylene or Solvesso 150. Several problems arise from the acid transportation through the membrane. It must be emphasized that the ligands are efficient in the extraction of lanthanide(II1) ions without addition of a synergic agent as required with the neutral crown ethers. A high selectivity has been observed for dysprosium with respect to lanthanum (from a mixture of La+ Dy). Some disadvantages are: slow kinetics (contact during more than 20 minutes is required for 90% metal extraction) and too moderate efficiency of the stripping process. Moreover, a scrubbing step (with water) is necessary before recycling the organic phase in order to wash out the co-extracted acid.

Due to the single stage operation of SLM technology, the use of these ligands as carriers seems to be difficult for three reasons, in the present working conditions: slow kinetics (and different kinetics for extraction and stripping stages), and need of a scrubbing stage.

Acknowledgements We thank the European Economic Community (contract MAlRl-0013C) and Progetto Finalizzato C.N.R. “Chimica Fine 2” for the financial support. We thank also Mrs. 0. Biolo and Mr. A. Aguiari for technical assistance.

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