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Complexation of the uranyl ion by crown ethers, diaza-crown ethers and analogous acyclic ligands in solution
Polyhedron Vol. 8, No. 18, pp. 2251-2254, Printed in Great Britain
1989 0
0277-5387189 $3.00+.00 1989 Pergamon Press plc
COMPLEXATION OF THE URANYL ION BY CROWN ETHERS, DIAZA-CROWN ETHERS AND ANALOGOUS ACYCLIC LIGANDS IN SOLUTION J. LAGRANGE,
J. P. METABANZOULOU,
P. FUX and P. LAGRANGE*
URA 405 au CNRS, EHICS, 1 rue Blaise Pascal, 67000 Strasbourg, France (Received 1 March 1989 ; accepted 26 April 1989) Abstract-The complexation of uranyl ion, UO:+, with several crown ethers (15C5 ; 18C6 ; 21C7 ; 24C8 ; 3OClO) substituted or not by benzo or carboxylic groups and with several d&a-crown ethers (21; 22 ; 23 coronands) substituted or not by alkyl chains have been investigated in propylene carbonate and in acetonitrile (in the presence of 0.1 M tetraethylammonium perchlorate). The results are compared with those obtained for the complexation of UOi+ by analogous acyclic ligands. The inclusion of UO:+ in the cavity of the 18C6 ligand family is demonstrated. The substitutions of the ligands have, in general, only small effects on the complexation.
The inclusion of the uranyl ion in a crown ether (or a diaza-crown ether) of the appropriate size has been demonstrated in the solid state, by structural determinations.‘-’ In solution, the uranium inclusion is in competition with the solvation of the ligand cavity, and the complexation of uranium itself is also in competition with the solvation. Only weak complexes are observed with crown ethers in water : the coordination between the uranyl ion and the complexing groups of the ligand occurs with hydrogen bond formation between the hydrogen atoms of the water molecules solvating the uranium and the oxygen atoms of the crown ether.’ In water, the uranyl ion is only stable in acidic medium. Thus, the polyaza-crown ethers are not useful, because the amino functions of the ligand are protonated7 in this acidic medium. In this work, we have determined the nature and the stability constants of the complexes formed between the uranyl ion and the ligands mentioned below (Fig. l), in propylene carbonate, principally, and in acetonitrile. These two organic solvents have been chosen for their solvating properties. They both have sufficiently high dielectric constants to provide good dissociation of the strong electrolytes, but the metallic cations are less solvated by these organic solvents than by water. Thus, the complexation and inclusion of the uranyl ion in a macro-
cyclic ligand must be more facile in propylene carbonate or in acetonitrile than in water. The ligands are : 1,4,7,10,13pentaoxacyclopentadecane (15C5) ; 1,4,7,10,13,16-hexaoxacyclooctadecane (18C6); 1,4,7,10,13,16,19-heptaoxacycloheneicosane (21C7); 2,3,11,13,-dibenzo-1,4,7,10, 13,16-hexaoxacyclooctadeca-2,11 -diene (DB18 C6); 2,3,14,15-dibenzo-1,4,7,10,13,16,19,22-o&aoxacyclotetracosa-2,lCdiene (DB24C8) ; 1,4,7,10, 13,16-hexaoxacyclooctadeca-2,3,11,12-tetracarboxylic acid (18C6TCA) ; 1,4,10-trioxa-7,13diazacyclopentadecane (21); 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (22); 1,7,10,13,19pentaoxa4,16diazacycloheneicosane (23) ; 4,13-dimethyl-1,7,10, 16-tetraoxa-4,13-diazacyclooctadecane (22DM) ; 4,13 - didecyl - 1,7,10,16 - tetraoxa - 4,13 - diazacyclooctadecane (22DD); 4,13-dihydroxyethyl-1,7,10, 16tetraoxa-4,13diazacyclooctadecane (22DETOH) ; diethylamine (DEA) ; diethyl ether (DEE) ; 2,5,8,11,14_pentaoxapentadecane (TeEG) ; 1,8diaminooctane (DAO) ; 1,8-diamino-3,6-dioxaoctane (DAOO) ; tris[2-(2-methoxyethoxy)-ethyl]amine (TDA).
EXPERIMENTAL Reagents
Uranium(W) per-chlorate was obtained from * Authorto whom c.orrespondenceshould he addressed. Ventron and tetraethylammonium perchlorate 2251
J. LAGRANGE
2252
0 0 0 X
X
X
0 m
0
0
n
0
X
et al.
m=O;n=O;X=H m=l:n=l:X=H m= l;n= l;X=COOH m=l;n=Z;X=H
DBlW6 DB24CS DB3OClO
m=l;n=l m=2;n=2 m=3;n=3
0
1X5 18C6 18C6TCA 21C7
0
X
N
m
011
21 22
m=O;n=l;X=H m=t;n=I;X=H m=Z;n=l;X=H
23
m=l;n=l;X=CH3 22DM m=l;n=l;X=(CH&Q$ 22DD m = 1; n = 1: X = CH2CH20H 22DETOH
DEE
DA0
T
DEA
?
TDA
DA00
TeEG Fig. 1. The ligands.
(Fluka) was recrystallized twice from water and carefully dried in uucuo. The ligands were obtained from Merck, except for 21C7 (from Parish), 22DM and 22DETOH (synthesized by Gramain et al.* Propylene carbonate (Fluka) was purified according to Gosse.’ Acetonitrile, Uvasol, was purchased from Merck. The ionic strength was maintained at 0.1 M by the addition of tetraethylammonium perchlorate, (TEA)ClO+ The residual water content was determined by a Karl Fischer titration. All test solutions contained less than 150 ppm of water.
at 25 f O.l”C. The absorbances of the solutions containing between 10m4 and 10m6 M of uranyl ion and between lop3 and 10T6 M of a given ligand were recorded at 5 nm intervals. Data treatments were conducted using the Sillen’s generalized least-squares method LETAGROPSPEFO.” (The computer was an IBM 308 1.) The calculation allows us to choose the stoichiometry and stability constant of the formed complexes which gives the most appropriate model.
Spectrophotometric measurements
Each formed complex, (UO,),L,z”‘, is characterized by its stoichiometry, (x:y). The logarithms of the stability constants, /I, of the equilibria :
Spectrophotometric measurements were performed with a Kontron Uvikon 860 spectrophotometer between 230 and 350 nm, in quartz cells,
RESULTS
xuo:+
+yLc-
(UG2)XLyzx+,
Complexation
of uranyl ion by crown ethers
Table 1. Complexation by crown ethers in propylene carbonate (- indicates that no complex was observed)
2253
Table 3. Complexation by acyclic iigands in propylene carbonate (- indicates that no complex was observed) Ligand
Complex
Ligand
Complex
15c5 18C6 21C7
-
-
DEE
-
-
1:l I:1
5.29kO.01 3.09 +0.01
TeEG
1:l
2.99f0.20
DEA
DB18C6 DB24C8 DB30ClO
1:l 1:l 1:l
5.51 f0.31 3.63kO.16 2.95kO.12
I:1 1:2
DA0
1:l
3.08 +0.20
DA00
1:l
3.81 kO.20
18C6TCA
1:l
5.61 f0.21
TDA
1:l 1:2
4.41& 0.20 8.19kO.20
log B
log B
with B=
wwxL:“+l w:+lx[Lly
are tabulated
in Table 1 for the complexation of UO$+ by crown ethers in propylene carbonate, in Table 2 by diaza-crown ethers in propylene carbonate, in Table 3 by acyclic ligands in propylene carbonate, and in Table 4 for the complexation in acetonitrile. In acetonitrile, the low solubility of the aza ligands did not allow us to carry out measurements using these aza complexing agents. The uncertainty limits quoted are twice the computed standard deviations.
DISCUSSION The discussion will take into account the following points : the possibility of the uranyl ion inclusion in crown ethers and in diaza-crown ethers, the influence of the presence of substituting groups on the ligands, and the influence of the solvent. Table
2. Complexation by diaza-crown propylene carbonate Ligand
ethers
Complex
log B
21
1:l 1:2
4.96f0.20 8.56f0.25
22
I:1 1:2
7.45 f 0.05 12.40+_0.04
23
1:l 1:2
6.79kO.10 12.96fO.10
22DM
1:l 1:2
6.90+0.10 14.28kO.19
22DD
1:l 1:2
3.88kO.19 7.74kO.20
22DETOH
I:1 1:2
7.08 kO.20 14.5OkO.25
in
Complexation Iigandr
by crown ethers and analogous linear
The uranyl ion complexation by crown ethers shows only the formation of 1 : 1 complexes, when they are observed. The inclusion in the ligand hole is only possible if this cavity has a diameter greater than the ionic diameter of the uranyl ion (2.8 A),” measured in the equatorial plane. Thus, 15C5 which has a cavity diameter of ca 1.8 8, I2 does not complex
the uranyl ion. Furthermore, the ether function is a weak complexing group and the uranyl ion is being complexed only by poly-coordinating ligands having more than one ether group. Diethyl ether does not provide observable complexation with the uranyl ion, but TeEG gives a stable 1: 1 chelate. The complexation by 18C6 shows a complex 200 times more stable than those formed with TeEG in propylene carbonate and in acetonitrile. This effect must be due to the inclusion of the cation in the ligand cavity (macrocyclic effect), but the stability increase is limited by the difficulty encountered by uo:+ to penetrate into the hole of the crown ether ’ 3The best crown ether seems to be 18C6 with its adapted cavity (diameter around 2.9 &.I2
Table 4. Complexation in acetonitrile (- indicates that no complex was observed) Ligand
Complex
log B
DEE TeEG 15c5 18C6
1:l 1:l I:1 I:1
1.47kO.12 3.8OkO.30 6.00 f 0.40 5.16kO.35
DB18C6 DB24C8
2254
J.
LAGRANGE et al.
Complexation by d&a-crown ethers and analogous ligands Amines are better complexing agents than ether and a complexation by diethylamine is observed in propylene carbonate : formation of a stable 1: 2 complex. The metals have a greater affinity for nitrogen atoms than for oxygen atoms. Thus, we can conclude that “exclusive” or “inclusive” complexes can be observed with the diaza-crown ethers. The inclusion of the uranyl ion in the ligand cavity is more complicated with the diaza-crown ethers than with the crown ethers, because 1 : 1 and 1 : 2 complexes are observed with the “diaza” ligands. However, the 1: 1 complexes with 21 or 22 or 23 ligands are 100-1000 times more stable than the analogous complexes formed with the acyclic ligands : DAO, DA00 and TDA. Thus, this macrocyclic effect allows us to conclude that, at least, a partial inclusion of the uranyl ion in the diazacrown ether must be probable. The best ligand is the 22 coronand which is analogous to 18C6. InJuence of the substituting groups of the ligands With the crown ethers, the benzo groups increase the complexing power of the ligands. These substitutions maintain the ligand cavity open and entry for the cation is easier. The carboxylic acid substituents can also assist the entry of the cation into the ligand cavity. In conclusion, these substitutions only have a small effect on the complexation in propylene carbonate, but a more important effect for “benzo” groups in acetonitrile. With the diaza-crown ethers, the complexation is inhibited by the alkyl chains bonded to the ligand 22. This effect increases with the length of the chains. The decyl groups could be inside the coronand cavity and the possibility of inclusion of the uranyl ion in the crown ether decreases strongly. The presence of two methyl groups or two alcohol groups has a very weak influence on the complexation. These observations present limitations in the use of alkyl chains which can immobilize these macrocyclic ligands on a polymer matrix. Influence of the solvent Only the complexation of uranyl ion by crown ethers and analogous linear ether ligands has been carried out in propylene carbonate and in acetonitrile. The “aza” ligands are not soluble enough in acetonitrile. We notice that the ligands without “benzo” groups are better complexing agents of the uranyl ion in propylene carbonate than in acetonitrile. A decrease in the stability constants is
Table 5. Logarithm of the stability constant of the 1 : 1 complexes formed between UO,‘+ and 18C6 in different solvents Propylene carbonate
Acetonitrile
5.29
3.80
MeOH-H 2O (90: 10%) Hz0 3.15”
2.0b
a Reference 14. bReference 6. observed when the solvent has a greater solvating property of the metallic cation, as we can see in Table 5 for the complexation of the uranyl ion by 18C6. The structure of the complexes changes from “inclusive” in propylene carbonate to “outer sphere” in water. The influence of the “benzo” substituting groups of the crown ethers is greater in acetonitrile than in propylene carbonate because they hinder the solvation of the hole in the coronands, which is easier for acetonitrile than for propylene carbonate. A similar effect has been previously observed by Kolthoff et al. ’ 5 for the complexation of K+ and Na+. The crown ether cavities are probably more strongly solvated by the acetonitrile molecules than by the larger propylene carbonate molecules. REFERENCES
Folcher, N. Keller, P. Plurien and P. Rigny, Inorg. Nucl. Chem. L.&t. 1975,11,469. 2. R. M. Costes, G. Folcher, P. Plurien and P. Rigny, 1. R. M. Costes, G.
Inorg. Nucl. Chem. Lett. 1976, 12, 13. 3. R. a.
Costes, G. Folcher, P. Plurien and P. Rigny,
Znorg. Nucl. Chem. Lett. 1976, 12,491. 4. A. Navaza,
F. Villain and P. Charpin, Polyhedron
1984,3, 143. 5. A. Dejean, P. Charpin, G. Folcher, P. Rigny, A. Navaza and G. Tsoocaris, Polyhedron 1987,6,189. 6. M. Brighli, P. Fux, J. Lagrange and P. Lagrange, Znorg. Chem. 1985,24,80. 7. B. Spiess, F. Arnaud and M. J. Schwing-Weill, J. Znorg. Nucl. Chem. 1979, 15, 13. 8. P. Gramain and Y. Frere, New J. Chim. 1979,3, 53. 9. B. Gosse and A. Denat, J. Electroanal. Chem. Interfat. Electrochem. 1974,56, 129. 10. L. G. Sillen and V. Warnqvist, Arkiv. Kemi 1968,31, 377. 11. L. Cattalini, U. Croatto, S. Degetto and E. Tondello,
Inorg. Chim. Acta 1971,5, 19. 12. N. K. Dalley, in Synthetic Multidentate Macrocyclic Compounds (Edited by R. M. Izatt and J. J. Christensen), Academic Press, New York (1978). 13. P. Fux, J. Lagrange and P. Lagrange, J. Am. Chem.
Sot. 1985, 107,5927. 14. C. Luca and H. A. Azab, Analyt. Lett. 1984,17,1937. 15. I. M. Kolthoff and M. K. Chantooni, Analyt. Chem. 1980,52, 1039.
1989 0
0277-5387189 $3.00+.00 1989 Pergamon Press plc
COMPLEXATION OF THE URANYL ION BY CROWN ETHERS, DIAZA-CROWN ETHERS AND ANALOGOUS ACYCLIC LIGANDS IN SOLUTION J. LAGRANGE,
J. P. METABANZOULOU,
P. FUX and P. LAGRANGE*
URA 405 au CNRS, EHICS, 1 rue Blaise Pascal, 67000 Strasbourg, France (Received 1 March 1989 ; accepted 26 April 1989) Abstract-The complexation of uranyl ion, UO:+, with several crown ethers (15C5 ; 18C6 ; 21C7 ; 24C8 ; 3OClO) substituted or not by benzo or carboxylic groups and with several d&a-crown ethers (21; 22 ; 23 coronands) substituted or not by alkyl chains have been investigated in propylene carbonate and in acetonitrile (in the presence of 0.1 M tetraethylammonium perchlorate). The results are compared with those obtained for the complexation of UOi+ by analogous acyclic ligands. The inclusion of UO:+ in the cavity of the 18C6 ligand family is demonstrated. The substitutions of the ligands have, in general, only small effects on the complexation.
The inclusion of the uranyl ion in a crown ether (or a diaza-crown ether) of the appropriate size has been demonstrated in the solid state, by structural determinations.‘-’ In solution, the uranium inclusion is in competition with the solvation of the ligand cavity, and the complexation of uranium itself is also in competition with the solvation. Only weak complexes are observed with crown ethers in water : the coordination between the uranyl ion and the complexing groups of the ligand occurs with hydrogen bond formation between the hydrogen atoms of the water molecules solvating the uranium and the oxygen atoms of the crown ether.’ In water, the uranyl ion is only stable in acidic medium. Thus, the polyaza-crown ethers are not useful, because the amino functions of the ligand are protonated7 in this acidic medium. In this work, we have determined the nature and the stability constants of the complexes formed between the uranyl ion and the ligands mentioned below (Fig. l), in propylene carbonate, principally, and in acetonitrile. These two organic solvents have been chosen for their solvating properties. They both have sufficiently high dielectric constants to provide good dissociation of the strong electrolytes, but the metallic cations are less solvated by these organic solvents than by water. Thus, the complexation and inclusion of the uranyl ion in a macro-
cyclic ligand must be more facile in propylene carbonate or in acetonitrile than in water. The ligands are : 1,4,7,10,13pentaoxacyclopentadecane (15C5) ; 1,4,7,10,13,16-hexaoxacyclooctadecane (18C6); 1,4,7,10,13,16,19-heptaoxacycloheneicosane (21C7); 2,3,11,13,-dibenzo-1,4,7,10, 13,16-hexaoxacyclooctadeca-2,11 -diene (DB18 C6); 2,3,14,15-dibenzo-1,4,7,10,13,16,19,22-o&aoxacyclotetracosa-2,lCdiene (DB24C8) ; 1,4,7,10, 13,16-hexaoxacyclooctadeca-2,3,11,12-tetracarboxylic acid (18C6TCA) ; 1,4,10-trioxa-7,13diazacyclopentadecane (21); 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (22); 1,7,10,13,19pentaoxa4,16diazacycloheneicosane (23) ; 4,13-dimethyl-1,7,10, 16-tetraoxa-4,13-diazacyclooctadecane (22DM) ; 4,13 - didecyl - 1,7,10,16 - tetraoxa - 4,13 - diazacyclooctadecane (22DD); 4,13-dihydroxyethyl-1,7,10, 16tetraoxa-4,13diazacyclooctadecane (22DETOH) ; diethylamine (DEA) ; diethyl ether (DEE) ; 2,5,8,11,14_pentaoxapentadecane (TeEG) ; 1,8diaminooctane (DAO) ; 1,8-diamino-3,6-dioxaoctane (DAOO) ; tris[2-(2-methoxyethoxy)-ethyl]amine (TDA).
EXPERIMENTAL Reagents
Uranium(W) per-chlorate was obtained from * Authorto whom c.orrespondenceshould he addressed. Ventron and tetraethylammonium perchlorate 2251
J. LAGRANGE
2252
0 0 0 X
X
X
0 m
0
0
n
0
X
et al.
m=O;n=O;X=H m=l:n=l:X=H m= l;n= l;X=COOH m=l;n=Z;X=H
DBlW6 DB24CS DB3OClO
m=l;n=l m=2;n=2 m=3;n=3
0
1X5 18C6 18C6TCA 21C7
0
X
N
m
011
21 22
m=O;n=l;X=H m=t;n=I;X=H m=Z;n=l;X=H
23
m=l;n=l;X=CH3 22DM m=l;n=l;X=(CH&Q$ 22DD m = 1; n = 1: X = CH2CH20H 22DETOH
DEE
DA0
T
DEA
?
TDA
DA00
TeEG Fig. 1. The ligands.
(Fluka) was recrystallized twice from water and carefully dried in uucuo. The ligands were obtained from Merck, except for 21C7 (from Parish), 22DM and 22DETOH (synthesized by Gramain et al.* Propylene carbonate (Fluka) was purified according to Gosse.’ Acetonitrile, Uvasol, was purchased from Merck. The ionic strength was maintained at 0.1 M by the addition of tetraethylammonium perchlorate, (TEA)ClO+ The residual water content was determined by a Karl Fischer titration. All test solutions contained less than 150 ppm of water.
at 25 f O.l”C. The absorbances of the solutions containing between 10m4 and 10m6 M of uranyl ion and between lop3 and 10T6 M of a given ligand were recorded at 5 nm intervals. Data treatments were conducted using the Sillen’s generalized least-squares method LETAGROPSPEFO.” (The computer was an IBM 308 1.) The calculation allows us to choose the stoichiometry and stability constant of the formed complexes which gives the most appropriate model.
Spectrophotometric measurements
Each formed complex, (UO,),L,z”‘, is characterized by its stoichiometry, (x:y). The logarithms of the stability constants, /I, of the equilibria :
Spectrophotometric measurements were performed with a Kontron Uvikon 860 spectrophotometer between 230 and 350 nm, in quartz cells,
RESULTS
xuo:+
+yLc-
(UG2)XLyzx+,
Complexation
of uranyl ion by crown ethers
Table 1. Complexation by crown ethers in propylene carbonate (- indicates that no complex was observed)
2253
Table 3. Complexation by acyclic iigands in propylene carbonate (- indicates that no complex was observed) Ligand
Complex
Ligand
Complex
15c5 18C6 21C7
-
-
DEE
-
-
1:l I:1
5.29kO.01 3.09 +0.01
TeEG
1:l
2.99f0.20
DEA
DB18C6 DB24C8 DB30ClO
1:l 1:l 1:l
5.51 f0.31 3.63kO.16 2.95kO.12
I:1 1:2
DA0
1:l
3.08 +0.20
DA00
1:l
3.81 kO.20
18C6TCA
1:l
5.61 f0.21
TDA
1:l 1:2
4.41& 0.20 8.19kO.20
log B
log B
with B=
wwxL:“+l w:+lx[Lly
are tabulated
in Table 1 for the complexation of UO$+ by crown ethers in propylene carbonate, in Table 2 by diaza-crown ethers in propylene carbonate, in Table 3 by acyclic ligands in propylene carbonate, and in Table 4 for the complexation in acetonitrile. In acetonitrile, the low solubility of the aza ligands did not allow us to carry out measurements using these aza complexing agents. The uncertainty limits quoted are twice the computed standard deviations.
DISCUSSION The discussion will take into account the following points : the possibility of the uranyl ion inclusion in crown ethers and in diaza-crown ethers, the influence of the presence of substituting groups on the ligands, and the influence of the solvent. Table
2. Complexation by diaza-crown propylene carbonate Ligand
ethers
Complex
log B
21
1:l 1:2
4.96f0.20 8.56f0.25
22
I:1 1:2
7.45 f 0.05 12.40+_0.04
23
1:l 1:2
6.79kO.10 12.96fO.10
22DM
1:l 1:2
6.90+0.10 14.28kO.19
22DD
1:l 1:2
3.88kO.19 7.74kO.20
22DETOH
I:1 1:2
7.08 kO.20 14.5OkO.25
in
Complexation Iigandr
by crown ethers and analogous linear
The uranyl ion complexation by crown ethers shows only the formation of 1 : 1 complexes, when they are observed. The inclusion in the ligand hole is only possible if this cavity has a diameter greater than the ionic diameter of the uranyl ion (2.8 A),” measured in the equatorial plane. Thus, 15C5 which has a cavity diameter of ca 1.8 8, I2 does not complex
the uranyl ion. Furthermore, the ether function is a weak complexing group and the uranyl ion is being complexed only by poly-coordinating ligands having more than one ether group. Diethyl ether does not provide observable complexation with the uranyl ion, but TeEG gives a stable 1: 1 chelate. The complexation by 18C6 shows a complex 200 times more stable than those formed with TeEG in propylene carbonate and in acetonitrile. This effect must be due to the inclusion of the cation in the ligand cavity (macrocyclic effect), but the stability increase is limited by the difficulty encountered by uo:+ to penetrate into the hole of the crown ether ’ 3The best crown ether seems to be 18C6 with its adapted cavity (diameter around 2.9 &.I2
Table 4. Complexation in acetonitrile (- indicates that no complex was observed) Ligand
Complex
log B
DEE TeEG 15c5 18C6
1:l 1:l I:1 I:1
1.47kO.12 3.8OkO.30 6.00 f 0.40 5.16kO.35
DB18C6 DB24C8
2254
J.
LAGRANGE et al.
Complexation by d&a-crown ethers and analogous ligands Amines are better complexing agents than ether and a complexation by diethylamine is observed in propylene carbonate : formation of a stable 1: 2 complex. The metals have a greater affinity for nitrogen atoms than for oxygen atoms. Thus, we can conclude that “exclusive” or “inclusive” complexes can be observed with the diaza-crown ethers. The inclusion of the uranyl ion in the ligand cavity is more complicated with the diaza-crown ethers than with the crown ethers, because 1 : 1 and 1 : 2 complexes are observed with the “diaza” ligands. However, the 1: 1 complexes with 21 or 22 or 23 ligands are 100-1000 times more stable than the analogous complexes formed with the acyclic ligands : DAO, DA00 and TDA. Thus, this macrocyclic effect allows us to conclude that, at least, a partial inclusion of the uranyl ion in the diazacrown ether must be probable. The best ligand is the 22 coronand which is analogous to 18C6. InJuence of the substituting groups of the ligands With the crown ethers, the benzo groups increase the complexing power of the ligands. These substitutions maintain the ligand cavity open and entry for the cation is easier. The carboxylic acid substituents can also assist the entry of the cation into the ligand cavity. In conclusion, these substitutions only have a small effect on the complexation in propylene carbonate, but a more important effect for “benzo” groups in acetonitrile. With the diaza-crown ethers, the complexation is inhibited by the alkyl chains bonded to the ligand 22. This effect increases with the length of the chains. The decyl groups could be inside the coronand cavity and the possibility of inclusion of the uranyl ion in the crown ether decreases strongly. The presence of two methyl groups or two alcohol groups has a very weak influence on the complexation. These observations present limitations in the use of alkyl chains which can immobilize these macrocyclic ligands on a polymer matrix. Influence of the solvent Only the complexation of uranyl ion by crown ethers and analogous linear ether ligands has been carried out in propylene carbonate and in acetonitrile. The “aza” ligands are not soluble enough in acetonitrile. We notice that the ligands without “benzo” groups are better complexing agents of the uranyl ion in propylene carbonate than in acetonitrile. A decrease in the stability constants is
Table 5. Logarithm of the stability constant of the 1 : 1 complexes formed between UO,‘+ and 18C6 in different solvents Propylene carbonate
Acetonitrile
5.29
3.80
MeOH-H 2O (90: 10%) Hz0 3.15”
2.0b
a Reference 14. bReference 6. observed when the solvent has a greater solvating property of the metallic cation, as we can see in Table 5 for the complexation of the uranyl ion by 18C6. The structure of the complexes changes from “inclusive” in propylene carbonate to “outer sphere” in water. The influence of the “benzo” substituting groups of the crown ethers is greater in acetonitrile than in propylene carbonate because they hinder the solvation of the hole in the coronands, which is easier for acetonitrile than for propylene carbonate. A similar effect has been previously observed by Kolthoff et al. ’ 5 for the complexation of K+ and Na+. The crown ether cavities are probably more strongly solvated by the acetonitrile molecules than by the larger propylene carbonate molecules. REFERENCES
Folcher, N. Keller, P. Plurien and P. Rigny, Inorg. Nucl. Chem. L.&t. 1975,11,469. 2. R. M. Costes, G. Folcher, P. Plurien and P. Rigny, 1. R. M. Costes, G.
Inorg. Nucl. Chem. Lett. 1976, 12, 13. 3. R. a.
Costes, G. Folcher, P. Plurien and P. Rigny,
Znorg. Nucl. Chem. Lett. 1976, 12,491. 4. A. Navaza,
F. Villain and P. Charpin, Polyhedron
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