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The effect of ligand rigidity on the stability of europium(III) complexes with substituted diglycolic acids
Journal of AUoys and Compounds, 180 (1992) 375-381 JAL 8044
375
The effect of ligand rigidity on the stability of europium(III) complexes with substituted diglycolic acids Kenneth L. Nash, E. Philip Horwitz, Ralph C. Gatrone and Paul G. Rickert Argonne National Laboratory, 9700 S. Cass Ave., Argonne IL 50439-4831 (USA)
Abstract The stability of europium(III) complexes with substituted diglycolic acids is altered by changing the ligand rigidity. The diglycolate analog tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) achieves nearly an order of magnitude stronger binding of europium(III) in dilute acidic solutions, partly as a result of favorable complexation geometry forced by the tetrahydrofuran ring. The analogous aromatic ring systems furan-2,3,4,5-tetracarboxylic acid and furan-2,5-dicarboxylic acid, which force the carboxylate groups to assume a planar configuration, exhibit much lower alfmity for europium(III) in dilute acid. The observed increase in stability for the THY'rCA complex implies that one or two of the six geometric isomers of THY'rCA must be dominant. 1. I n t r o d u c t i o n Fixing the spatial orientation of the binding sites of a chelating ligand often results in an increase in c o m p l e x stability. The lanthanide complexes of t r a n s - l , 2 - d i a m i n o c y c l o h e x a n e t e t r a a c e t i c acid are 2 orders of magnitude s t r o n g er than thos e of ethane-l,2-diaminetetraacetic acid [1]. The t r a n s orientation imposed by the cyclohexane backbone results in a m o r e favorable c o mp lex atio n geometry, and gives stronger c o m p l e x e s because of greater e n t r o p y o f reaction. It is well known that digiycolic acid forms strong com pl exes with the lanthanides in dilute acidic solutions [2]. It is the p u r p o s e of this study to examine the effect of ligand g e o m e t r y on europium(III) c o m p l e x stability for ligands which alter the backbone rigidity of digiycolic acid. A ligand providing potentially improved ligand ge omet ry (tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THVrCA)), and two related ligands possessing less favorable geometries (furan-2,5-dicarboxylic acid (FDCA) and furan-2,3,4,5tetracarboxylic acid (FrCA)) axe examined for their relative acidity and europium(HI) complexing ability. 2. E x p e r i m e n t a l d e t a i l s THFTCA was p u r c h a s e d from Aldrich and purified either by recrystallization from acet one or as the tetrasodium salt from e t h a n o l - w a t e r mixtures.
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
376
FDCA was purchased from Spectrum Chemical Manufacturing Corporation as the diacid and used as received. FYCA was synthesized in our laboratory by the method of Reichstein et al. [3] and assayed b y titrimetric analysis and nuclear magnetic resonance spectrometry. The latter two c o m p o u n d s exhibited somewhat limited a q u e o u s solubility. Protonation constants were determined for each ligand by both forward and reverse titrations at 2.0 M ionic strength (Na-HNO3) and 25.0 °C. For THFrCA, titrations were also performed at 0.5 and 0.2 M, and near-zero ionic strength. Changes in acidity were measured using a Ross combination electrode and Beckman ¢ - 7 1 pH meter, standardized with pH 4.00 and 7.00 buffers. Measured pH values were converted to p[H] using a calibration curve prepared by determining the pH of titrimetrically standardized solutions of HNO3-NaNO3 at I = 2.0 M. Europium stability constants with T H F r C A were determined by the distribution coefficient depression method [4] using bis(2-ethyl/hexyl)phosphoric acid in CC14 as the counter phase for extraction from 2.0 M Na-HNO3. The extraction stoichiometry and log Kex values were as reported previously [5]. All experiments were performed in duplicate. Distribution ratios were determined using 152'X54Eu tracer from laboratory stocks and counted for ~/activity. The distribution experiments were run as a function of total ligand concentration, total metal ion concentration (by adding nonactive Eu(NO3)3 to some replicates), and acidity to determine the stoichiometry of the complexes (M~ Ha L~). This method was also applied in the search for possible F r C A complexes. To assess the complexing ability of FDCA, a potentiometric method was used. A solution of known concentration of FDCA was prepared at pH 3 (dominant ligand species H L - ) and titrated with a standardized Eu('NO3) 3 solution. The decrease in pH was monitored using the above-described electrode system. 3. R e s u l t s
The results of the ligand protonation experiments are presented in Table 1, together with protonation constants from the literature for related systems. Analysis of the T H F r C A data as a function of ionic strength was performed using an expression b a s e d on the Davies equation for ionic interaction:
pKi(I)=pK,°+0.509 Az 2 [Ilm/(1 + I m ) - C I ]
(1)
where pK, ° is the thermodynamic protonation constant, Az 2 is the difference in charge between products and reactants, and C is an adjustable parameter to account for ionic interactions at high concentrations. The average value for C for the four p K values was 0.07 ( ± 0 . 0 4 ) . The distribution data for the E u - T H F r C A system (at constant acidity) are evaluated using
Do~D-- 1 - fl(EuNO~)[N03- ] -- axLf+ a2Lf2
(2)
377 TABLE 1 Protonation constants for diglycolic acid and related compounds
(25.0 °C)
Compound
pK1 ~
pK2
pK3
pK4
Conditions
Reference
THFrCA THFrCA THFTCA THFTCAa THFrCAb FTCA¢ FDCA Furoic Diglycolic Glycolic
1.57(0.03) 1.71(0.02) 1.74(0.01) 2.08 2.07(0.08)
2.86(0.02) 2.93(0.04) 3.06(0.01) 3.68 3.66(0.04) 1.56(0.01) 3.04(0.01)
4.08(0.01) 4.28(0.01) 4.51(0.01) 5.40 5.42(0.04) 3.08(0.01)
5.61(0.04) 5.90(0.03) 6.18(0.01) 7.26 7.41(0.08) 6.78(0.01)
I=2.0 M
Present Present Present Present Present Present Present 6 2 6
-
1.97(0.01) 3.19 2.75 3.74
I=0.5 M I=0.2 M
I= 0 Iffi 0 I=2.0 M I=2.0 M
1=2.0 M I = 1.0 M I = 2.0 M
3.76
work work work work work work work
Values in parentheses represent ± 2q uncertainty limits. aSingle titration near zero ionic strength. bprotonation constants derived from Davies-type equation. cStrong acid, fourth proton not bound under experimental conditions. 400 r |
log 13121= 14.00 (~0.02)
3001
~ M I°gB142 =26"f(±0' i5 04]
2 0 0 ~ M 100
0
o.o
oi
o12
o'.3 o.4
ols
o'.6 o17
o8
o.g
ITHFTCA] t
Fig. 1. Experimental data and calculated least-squares fits for Eu(III)-THFTCA complexation data (I= 2.0 M, Na-HNO3, 25.0 °C). Acidity for each curve and overall fit parameters are indicated in the figure. The fl(EuNOa)[NO3] t e r m c o r r e c t s for the p r e s e n c e o f EuNO32+ [6]; a is a c o n d i t i o n a l stability q u o t i e n t ( c o n s t a n t acidity); Lr is t h e free ligand conc e n t r a t i o n . A fit o f the p a r a m e t e r s a~ as a f u n c t i o n o f acidity gives the s t o i c h i o m e t r y o f the c o m p l e x e s with r e s p e c t t o [H + ]. E x p e r i m e n t s with a d d e d n o n - a c t i v e Eu(NOs)3 r e p r o d u c e d t h o s e with only t r a c e r e u r o p i u m , eliminating the possibility o f p o l y n u c l e a r c o m p l e x e s u n d e r t h e s e conditions. The c o m p l e x e s EuH2L + a n d Eu(H2L)2- are indicated as the m o s t p r o b a b l e species. C o m b i n i n g t h e d a t a u n d e r all conditions, the b e s t fit f o r t h e distribution d a t a is p r o v i d e d b y s u b s t i t u t i n g t h e p a r a m e t e r s fll21[H+] 2 f o r al a n d fl142[H+] 4 f o r a2. The n o n - l i n e a r l e a s t - s q u a r e s p r o g r a m d e s c r i b e d p r e v i o u s l y [5] w a s a p p l i e d t o adjust t h e data. T h e distribution d a t a with c a l c u l a t e d lines a n d the log v a l u e s are s h o w n in Fig. 1.
378 Similar experiments were performed for the Eu(III)-FrCA system. At 0.01 M acid, the observed distribution ratios were as follows (FrCA concentrations i n parentheses): Do -- 8.56 ( _ 0.52); D(0.01 M) -- 11.78 ( ± 0.66); D ( 0 . 0 1 8 M ) = 9 . 9 5 ( ± 0 . 1 4 ) ; D(0.032 M ) = 8 . 4 1 ( ± 0 . 1 5 ) . At higher ligand concentrations, F r C A was precipitated on contact with the organic solution. A plot of 1/19 vs. Lt (excluding the Do value) gives a weighted linear fit with slope [3/Do=1.39 ( ± 0 . 0 0 0 3 ) and intercept 1/Do=0.0750 ( ± 0 . 0 0 5 7 ) . The stability constant derived as slope divided by intercept is 18.6 ( ± 1.4) while the Do value determined as the reciprocal of the intercept is Do(calc)= 13.3 ( ± 1 . 1 ) . The discrepancy between the fit and experimental values for Do indicates that the complex (presumed to be EuH3L 2 +) is partially extracted into the organic phase. This condition violates one of the principal requirements of the Schubert analysis [4 ]. As a result the derived ~ value must be considered as a lower limit. In the Eu(IID-FDCA system, the Lt:Mt ratio was varied from 11.8 to 0.79 by titrating a solution of FDCA with a standardized solution of Eu(NOs)3 while monitoring pH. As the total metal ion concentration was increased, the pH dropped from 3.27 to 3.20. These data were analyzed by solving the metal, ligand and proton mass balance equations using three assumptions: no complex formation; formation of an EuHL 2+ complex having lg fi= 1.67 (as reported for Eu-furoate complex [7]); formation of an EuL ÷ complex with lg fllO1=4.6 (see below). The third possibility, which requires full deprotonation of the ligand, can be positively eliminated, as the predicted final pH value from this model is 2.67. Both the first (no complex) and second (EuHL 2+) reproduce the observed pH variation, though the EuHL 2+ model exhibits a systematic deviation. If a complex is formed in this system, it must be of the form EuHL 2+, and likely is characterized by lg f l < l . 0 (fi = [EuHL 2+ ]/[Eu 8+ ][HL-]).
4. D i s c u s s i o n
The protonation constants summarized in Table 1 provide several interesting points for discussion. First, it should be noted that the free energy for the addition of the first proton to either the FDCA-furoic acid (AG = - 17.7 kJ mo1-1) or the diglycolic-glycolic (AG=--21.4 kJ m o l - ' ) acid pairs is nearly identical for each set, that is, the p K values are constant. The constant affiuity of the ligand for proton is observed despite the doubling of the formal charge, and a statistical factor for the presence of two potential binding sites. The similarity of lg flloI values for furoic acid and FDCA noted above is consistent with the p K v a l u e s and suggests independence of the carboxylate groups in FDCA. Comparing furoic-glycolic and FDCA-diglycolic acids, we see that introduction of the aromatic ring decreases proton affinity by 8.6 kJ mol-1. This could represent either the result of an inductive effect of the fumn ring or a geometric effect caused by the planar arrangement of the carboxylates.
379 For addition of the first proton to the tetracarboxylates, the unsaturated ring system provides an order of magnitude stronger binding of H + (at 2.0 M ionic strength). This effect is reversed for the addition of subsequent protons, that is, the binding of second through fourth protons to FrCA is weaker than that to THFTCA. The large difference between binding strength of first and second protons in a polycarboxylic acid is also observed in a-dialkylsubstituted malonic acids wherein the difference between stability of HL- and H2L species is reported as pKI ~<2.0, pK2>~ 7.0 (ref. 8, pp. 103-106). Ostacoli et al. [9] suggest that the anomalously large values for the first protonation constant of dialkylmalonates result from steric factors forcing the carboxylate groups closer together, thus promoting the sharing of the first H + by the two carboxylate groups. Minimum energy calculations (using the program A L C H E M Y , use of which was described in an earlier publication [10]) for each of the ligands of this investigation indicate an approximately 0.2/~ decrease in the separation of the ether oxygen from the carboxylates in F r C A relative to the same separation for the THFrCA isomers and diglycolate. This observation is consistent with the suggestion of Ostacoli et al. Since the driving force for complex formation and ligand protonation is primarily electrostatic attraction, it is typically observed that lg fl values are linearly correlated with ligand protonation constants [11, 12 ]. With this relationship in mind, we can calculate an expected value for hypothetical EuH2FI~C + and EuFDC + chelate complexes. If we compare a modified fl(EuH2THI~C +) = [EuH2THF]?C + ]/[Eu a + ] [H2THFrC 2- ] = 10 4.31 with the data for diglycolic acid and related etherdicarboxylates (ref. 8, pp. 133-137), and assume a linear relationship between lg fl and pK2 [ 11 ] for the structurally similar ligands, we can estimate fl(EuFDC +) = 104.6 and fl(EuH21~rc +) = 102'5. Results presented above for both Eu-FTCA and Eu-FDCA indicate much weaker binding of europium(HI). The experimental results suggest that these two ligands, like furoic acid [7], probably bind europium(HI) as simple carboxylic acids. To gain a better understanding of the probable dominant geometric isomers of THF]?CA, and to obtain greater insight into the reasons for the higher stability of the Eu(IH)-THF]?CA complexes, we modeled these complexes using the molecular modeling program A L C H E M Y . For the present task, the minimum conformational energy was calculated for each of the fully protonated ligands, including the six geometric isomers of THFrCA. The six isomers, described in terms of the relative positions of the carboxylic acid groups (above or below the ring) are as follows: 2, 3, 4, 5 up; 2, 4, 5 up; 3, 4, 5 up; 2, 5 up; 2, 3 up; 2, 4 up. The minimum potential energy was also calculated for each of the isomers/ligands when bound at the 2 and 5 positions to a generic octahedral metal ion (called iron by the program). The difference between the calculated energies is a measure of how much the ligand must reorganize its structure on complexation (solvation effects and metal ion bonding to the ether oxygen
380 1,0]0.9 -
Eu3+
0.8 -
~
0.7
X = digtycolote L = THFTCA EuH4L2" EuH2L+
.~-.~0],6,.,-6
• 0.5
-
0.4 0.3
020.1-
p[H] Fig. 2. Calculated fractional speciation of tracer europium in the presence of 0.2 M diglycolic acid and 0.2 M THFTCA.
cannot be a c c o m m o d a t e d by the program). The complexes formed by the T H F r C A (2,5-¢/s) isomers are the most stable species ( E r ~ = 0 - 1 1 kJ tool- 1), fonowed b y diglycolate (E~--61 kJ tool-l), THFrGA (2,5-trans) isomers ( E ~ = 1 4 3 - - 1 6 6 kJ tool-l), and F r c A (E~d~----362 kJ mol-1). This result indicates that one or more of the T H F r C A (2,5-c/s) species are the most probable as the TI-IFrCA complexes are stronger than those of diglycolate. It also supports the argument that no chelate complexes are formed by FrGA. The relative binding strength of diglycolic acid and T H F r C A in solutions at pH < 2 is shown in Fig. 2. The calculated speciation diagram is b a s e d on the present data and literature data for the EuCIII)-diglycolate system (Iffi 1.0 M [2]). Above pH 2, the comparative calculations are not considered valid because of the high probability of further deprotonation of EuH2(THI~rC) + and Eu(H2THFrC))2 -. Although an appreciable difference in relative complex stability is noted below pH 1.8, at pH 2 the two ligands appear to be of nearly comparable complexing strength. The enhanced stability of E u - T H F r C A complexes in acidic media is superficially correlated with the increased acidity of the ligand. Although the present data do not permit definite conclusions to be drawn, the favorable arrangement of binding groups in the THFTCA ligand p r o b a b l y c o n t r i b u t e s to a more positive complexation entropy to account for the greater complex strength.
Acknowledgment Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US Deparh~tent of Energy, under Contract W-31-109-ENG-38.
References 1 A. E. Martell and R. M. Smith, Critical S t a b i l i t y Constants, Vol. 1, Plenum, New York, 1976, pp. 204, 236.
381 2 I. Grenthe and H. Ots, Acta Chem~ Scand., 26 (1972) 1217, 1229. 3 T. Reichstein, A. Grussner, K. Schindler and E. Hardneier, Heir. Chim. Acta, 16 (1933) 276. 4 J. Schubert, J. Phys. Colloid Chem., 52 (1948) 340. 5 K. L. Nash and E. P. Horwitz, Inorg. Chim. Acta, 169 (1990) 245. 6 G. R. Choppin, D. A. Kelly and H. E. Ward, in D. Dyrssen, J. O. Liljenzin and J. Rydberg (eds.), Solvent Extraction Chemistry, North-Holland, Amsterdam, 1967, p. 46. 7 S. S. Yun, G. R. Choppin and D. Blakeway, J. Inorg. Nucl. Chem., 38 (1976) 587. 8 A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3, Plenum, New York, 1977. 9 G. Ostacoli, A. Vanni and E. Roletto, Gazz. Chim. Itak, 100 (1970) 350. 10 R. C. Gatrone and E. P. Horwitz, Solvent Extr. Ion Exch., 6 (1988) 937. 11 K. L. Nash, Eur. J. Solid State Inorg. Chem., 28 (1991) 389. 12 G. R. Choppin, Radiochim. Acta, 32 (1983) 43.
375
The effect of ligand rigidity on the stability of europium(III) complexes with substituted diglycolic acids Kenneth L. Nash, E. Philip Horwitz, Ralph C. Gatrone and Paul G. Rickert Argonne National Laboratory, 9700 S. Cass Ave., Argonne IL 50439-4831 (USA)
Abstract The stability of europium(III) complexes with substituted diglycolic acids is altered by changing the ligand rigidity. The diglycolate analog tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) achieves nearly an order of magnitude stronger binding of europium(III) in dilute acidic solutions, partly as a result of favorable complexation geometry forced by the tetrahydrofuran ring. The analogous aromatic ring systems furan-2,3,4,5-tetracarboxylic acid and furan-2,5-dicarboxylic acid, which force the carboxylate groups to assume a planar configuration, exhibit much lower alfmity for europium(III) in dilute acid. The observed increase in stability for the THY'rCA complex implies that one or two of the six geometric isomers of THY'rCA must be dominant. 1. I n t r o d u c t i o n Fixing the spatial orientation of the binding sites of a chelating ligand often results in an increase in c o m p l e x stability. The lanthanide complexes of t r a n s - l , 2 - d i a m i n o c y c l o h e x a n e t e t r a a c e t i c acid are 2 orders of magnitude s t r o n g er than thos e of ethane-l,2-diaminetetraacetic acid [1]. The t r a n s orientation imposed by the cyclohexane backbone results in a m o r e favorable c o mp lex atio n geometry, and gives stronger c o m p l e x e s because of greater e n t r o p y o f reaction. It is well known that digiycolic acid forms strong com pl exes with the lanthanides in dilute acidic solutions [2]. It is the p u r p o s e of this study to examine the effect of ligand g e o m e t r y on europium(III) c o m p l e x stability for ligands which alter the backbone rigidity of digiycolic acid. A ligand providing potentially improved ligand ge omet ry (tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THVrCA)), and two related ligands possessing less favorable geometries (furan-2,5-dicarboxylic acid (FDCA) and furan-2,3,4,5tetracarboxylic acid (FrCA)) axe examined for their relative acidity and europium(HI) complexing ability. 2. E x p e r i m e n t a l d e t a i l s THFTCA was p u r c h a s e d from Aldrich and purified either by recrystallization from acet one or as the tetrasodium salt from e t h a n o l - w a t e r mixtures.
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
376
FDCA was purchased from Spectrum Chemical Manufacturing Corporation as the diacid and used as received. FYCA was synthesized in our laboratory by the method of Reichstein et al. [3] and assayed b y titrimetric analysis and nuclear magnetic resonance spectrometry. The latter two c o m p o u n d s exhibited somewhat limited a q u e o u s solubility. Protonation constants were determined for each ligand by both forward and reverse titrations at 2.0 M ionic strength (Na-HNO3) and 25.0 °C. For THFrCA, titrations were also performed at 0.5 and 0.2 M, and near-zero ionic strength. Changes in acidity were measured using a Ross combination electrode and Beckman ¢ - 7 1 pH meter, standardized with pH 4.00 and 7.00 buffers. Measured pH values were converted to p[H] using a calibration curve prepared by determining the pH of titrimetrically standardized solutions of HNO3-NaNO3 at I = 2.0 M. Europium stability constants with T H F r C A were determined by the distribution coefficient depression method [4] using bis(2-ethyl/hexyl)phosphoric acid in CC14 as the counter phase for extraction from 2.0 M Na-HNO3. The extraction stoichiometry and log Kex values were as reported previously [5]. All experiments were performed in duplicate. Distribution ratios were determined using 152'X54Eu tracer from laboratory stocks and counted for ~/activity. The distribution experiments were run as a function of total ligand concentration, total metal ion concentration (by adding nonactive Eu(NO3)3 to some replicates), and acidity to determine the stoichiometry of the complexes (M~ Ha L~). This method was also applied in the search for possible F r C A complexes. To assess the complexing ability of FDCA, a potentiometric method was used. A solution of known concentration of FDCA was prepared at pH 3 (dominant ligand species H L - ) and titrated with a standardized Eu('NO3) 3 solution. The decrease in pH was monitored using the above-described electrode system. 3. R e s u l t s
The results of the ligand protonation experiments are presented in Table 1, together with protonation constants from the literature for related systems. Analysis of the T H F r C A data as a function of ionic strength was performed using an expression b a s e d on the Davies equation for ionic interaction:
pKi(I)=pK,°+0.509 Az 2 [Ilm/(1 + I m ) - C I ]
(1)
where pK, ° is the thermodynamic protonation constant, Az 2 is the difference in charge between products and reactants, and C is an adjustable parameter to account for ionic interactions at high concentrations. The average value for C for the four p K values was 0.07 ( ± 0 . 0 4 ) . The distribution data for the E u - T H F r C A system (at constant acidity) are evaluated using
Do~D-- 1 - fl(EuNO~)[N03- ] -- axLf+ a2Lf2
(2)
377 TABLE 1 Protonation constants for diglycolic acid and related compounds
(25.0 °C)
Compound
pK1 ~
pK2
pK3
pK4
Conditions
Reference
THFrCA THFrCA THFTCA THFTCAa THFrCAb FTCA¢ FDCA Furoic Diglycolic Glycolic
1.57(0.03) 1.71(0.02) 1.74(0.01) 2.08 2.07(0.08)
2.86(0.02) 2.93(0.04) 3.06(0.01) 3.68 3.66(0.04) 1.56(0.01) 3.04(0.01)
4.08(0.01) 4.28(0.01) 4.51(0.01) 5.40 5.42(0.04) 3.08(0.01)
5.61(0.04) 5.90(0.03) 6.18(0.01) 7.26 7.41(0.08) 6.78(0.01)
I=2.0 M
Present Present Present Present Present Present Present 6 2 6
-
1.97(0.01) 3.19 2.75 3.74
I=0.5 M I=0.2 M
I= 0 Iffi 0 I=2.0 M I=2.0 M
1=2.0 M I = 1.0 M I = 2.0 M
3.76
work work work work work work work
Values in parentheses represent ± 2q uncertainty limits. aSingle titration near zero ionic strength. bprotonation constants derived from Davies-type equation. cStrong acid, fourth proton not bound under experimental conditions. 400 r |
log 13121= 14.00 (~0.02)
3001
~ M I°gB142 =26"f(±0' i5 04]
2 0 0 ~ M 100
0
o.o
oi
o12
o'.3 o.4
ols
o'.6 o17
o8
o.g
ITHFTCA] t
Fig. 1. Experimental data and calculated least-squares fits for Eu(III)-THFTCA complexation data (I= 2.0 M, Na-HNO3, 25.0 °C). Acidity for each curve and overall fit parameters are indicated in the figure. The fl(EuNOa)[NO3] t e r m c o r r e c t s for the p r e s e n c e o f EuNO32+ [6]; a is a c o n d i t i o n a l stability q u o t i e n t ( c o n s t a n t acidity); Lr is t h e free ligand conc e n t r a t i o n . A fit o f the p a r a m e t e r s a~ as a f u n c t i o n o f acidity gives the s t o i c h i o m e t r y o f the c o m p l e x e s with r e s p e c t t o [H + ]. E x p e r i m e n t s with a d d e d n o n - a c t i v e Eu(NOs)3 r e p r o d u c e d t h o s e with only t r a c e r e u r o p i u m , eliminating the possibility o f p o l y n u c l e a r c o m p l e x e s u n d e r t h e s e conditions. The c o m p l e x e s EuH2L + a n d Eu(H2L)2- are indicated as the m o s t p r o b a b l e species. C o m b i n i n g t h e d a t a u n d e r all conditions, the b e s t fit f o r t h e distribution d a t a is p r o v i d e d b y s u b s t i t u t i n g t h e p a r a m e t e r s fll21[H+] 2 f o r al a n d fl142[H+] 4 f o r a2. The n o n - l i n e a r l e a s t - s q u a r e s p r o g r a m d e s c r i b e d p r e v i o u s l y [5] w a s a p p l i e d t o adjust t h e data. T h e distribution d a t a with c a l c u l a t e d lines a n d the log v a l u e s are s h o w n in Fig. 1.
378 Similar experiments were performed for the Eu(III)-FrCA system. At 0.01 M acid, the observed distribution ratios were as follows (FrCA concentrations i n parentheses): Do -- 8.56 ( _ 0.52); D(0.01 M) -- 11.78 ( ± 0.66); D ( 0 . 0 1 8 M ) = 9 . 9 5 ( ± 0 . 1 4 ) ; D(0.032 M ) = 8 . 4 1 ( ± 0 . 1 5 ) . At higher ligand concentrations, F r C A was precipitated on contact with the organic solution. A plot of 1/19 vs. Lt (excluding the Do value) gives a weighted linear fit with slope [3/Do=1.39 ( ± 0 . 0 0 0 3 ) and intercept 1/Do=0.0750 ( ± 0 . 0 0 5 7 ) . The stability constant derived as slope divided by intercept is 18.6 ( ± 1.4) while the Do value determined as the reciprocal of the intercept is Do(calc)= 13.3 ( ± 1 . 1 ) . The discrepancy between the fit and experimental values for Do indicates that the complex (presumed to be EuH3L 2 +) is partially extracted into the organic phase. This condition violates one of the principal requirements of the Schubert analysis [4 ]. As a result the derived ~ value must be considered as a lower limit. In the Eu(IID-FDCA system, the Lt:Mt ratio was varied from 11.8 to 0.79 by titrating a solution of FDCA with a standardized solution of Eu(NOs)3 while monitoring pH. As the total metal ion concentration was increased, the pH dropped from 3.27 to 3.20. These data were analyzed by solving the metal, ligand and proton mass balance equations using three assumptions: no complex formation; formation of an EuHL 2+ complex having lg fi= 1.67 (as reported for Eu-furoate complex [7]); formation of an EuL ÷ complex with lg fllO1=4.6 (see below). The third possibility, which requires full deprotonation of the ligand, can be positively eliminated, as the predicted final pH value from this model is 2.67. Both the first (no complex) and second (EuHL 2+) reproduce the observed pH variation, though the EuHL 2+ model exhibits a systematic deviation. If a complex is formed in this system, it must be of the form EuHL 2+, and likely is characterized by lg f l < l . 0 (fi = [EuHL 2+ ]/[Eu 8+ ][HL-]).
4. D i s c u s s i o n
The protonation constants summarized in Table 1 provide several interesting points for discussion. First, it should be noted that the free energy for the addition of the first proton to either the FDCA-furoic acid (AG = - 17.7 kJ mo1-1) or the diglycolic-glycolic (AG=--21.4 kJ m o l - ' ) acid pairs is nearly identical for each set, that is, the p K values are constant. The constant affiuity of the ligand for proton is observed despite the doubling of the formal charge, and a statistical factor for the presence of two potential binding sites. The similarity of lg flloI values for furoic acid and FDCA noted above is consistent with the p K v a l u e s and suggests independence of the carboxylate groups in FDCA. Comparing furoic-glycolic and FDCA-diglycolic acids, we see that introduction of the aromatic ring decreases proton affinity by 8.6 kJ mol-1. This could represent either the result of an inductive effect of the fumn ring or a geometric effect caused by the planar arrangement of the carboxylates.
379 For addition of the first proton to the tetracarboxylates, the unsaturated ring system provides an order of magnitude stronger binding of H + (at 2.0 M ionic strength). This effect is reversed for the addition of subsequent protons, that is, the binding of second through fourth protons to FrCA is weaker than that to THFTCA. The large difference between binding strength of first and second protons in a polycarboxylic acid is also observed in a-dialkylsubstituted malonic acids wherein the difference between stability of HL- and H2L species is reported as pKI ~<2.0, pK2>~ 7.0 (ref. 8, pp. 103-106). Ostacoli et al. [9] suggest that the anomalously large values for the first protonation constant of dialkylmalonates result from steric factors forcing the carboxylate groups closer together, thus promoting the sharing of the first H + by the two carboxylate groups. Minimum energy calculations (using the program A L C H E M Y , use of which was described in an earlier publication [10]) for each of the ligands of this investigation indicate an approximately 0.2/~ decrease in the separation of the ether oxygen from the carboxylates in F r C A relative to the same separation for the THFrCA isomers and diglycolate. This observation is consistent with the suggestion of Ostacoli et al. Since the driving force for complex formation and ligand protonation is primarily electrostatic attraction, it is typically observed that lg fl values are linearly correlated with ligand protonation constants [11, 12 ]. With this relationship in mind, we can calculate an expected value for hypothetical EuH2FI~C + and EuFDC + chelate complexes. If we compare a modified fl(EuH2THI~C +) = [EuH2THF]?C + ]/[Eu a + ] [H2THFrC 2- ] = 10 4.31 with the data for diglycolic acid and related etherdicarboxylates (ref. 8, pp. 133-137), and assume a linear relationship between lg fl and pK2 [ 11 ] for the structurally similar ligands, we can estimate fl(EuFDC +) = 104.6 and fl(EuH21~rc +) = 102'5. Results presented above for both Eu-FTCA and Eu-FDCA indicate much weaker binding of europium(HI). The experimental results suggest that these two ligands, like furoic acid [7], probably bind europium(HI) as simple carboxylic acids. To gain a better understanding of the probable dominant geometric isomers of THF]?CA, and to obtain greater insight into the reasons for the higher stability of the Eu(IH)-THF]?CA complexes, we modeled these complexes using the molecular modeling program A L C H E M Y . For the present task, the minimum conformational energy was calculated for each of the fully protonated ligands, including the six geometric isomers of THFrCA. The six isomers, described in terms of the relative positions of the carboxylic acid groups (above or below the ring) are as follows: 2, 3, 4, 5 up; 2, 4, 5 up; 3, 4, 5 up; 2, 5 up; 2, 3 up; 2, 4 up. The minimum potential energy was also calculated for each of the isomers/ligands when bound at the 2 and 5 positions to a generic octahedral metal ion (called iron by the program). The difference between the calculated energies is a measure of how much the ligand must reorganize its structure on complexation (solvation effects and metal ion bonding to the ether oxygen
380 1,0]0.9 -
Eu3+
0.8 -
~
0.7
X = digtycolote L = THFTCA EuH4L2" EuH2L+
.~-.~0],6,.,-6
• 0.5
-
0.4 0.3
020.1-
p[H] Fig. 2. Calculated fractional speciation of tracer europium in the presence of 0.2 M diglycolic acid and 0.2 M THFTCA.
cannot be a c c o m m o d a t e d by the program). The complexes formed by the T H F r C A (2,5-¢/s) isomers are the most stable species ( E r ~ = 0 - 1 1 kJ tool- 1), fonowed b y diglycolate (E~--61 kJ tool-l), THFrGA (2,5-trans) isomers ( E ~ = 1 4 3 - - 1 6 6 kJ tool-l), and F r c A (E~d~----362 kJ mol-1). This result indicates that one or more of the T H F r C A (2,5-c/s) species are the most probable as the TI-IFrCA complexes are stronger than those of diglycolate. It also supports the argument that no chelate complexes are formed by FrGA. The relative binding strength of diglycolic acid and T H F r C A in solutions at pH < 2 is shown in Fig. 2. The calculated speciation diagram is b a s e d on the present data and literature data for the EuCIII)-diglycolate system (Iffi 1.0 M [2]). Above pH 2, the comparative calculations are not considered valid because of the high probability of further deprotonation of EuH2(THI~rC) + and Eu(H2THFrC))2 -. Although an appreciable difference in relative complex stability is noted below pH 1.8, at pH 2 the two ligands appear to be of nearly comparable complexing strength. The enhanced stability of E u - T H F r C A complexes in acidic media is superficially correlated with the increased acidity of the ligand. Although the present data do not permit definite conclusions to be drawn, the favorable arrangement of binding groups in the THFTCA ligand p r o b a b l y c o n t r i b u t e s to a more positive complexation entropy to account for the greater complex strength.
Acknowledgment Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US Deparh~tent of Energy, under Contract W-31-109-ENG-38.
References 1 A. E. Martell and R. M. Smith, Critical S t a b i l i t y Constants, Vol. 1, Plenum, New York, 1976, pp. 204, 236.
381 2 I. Grenthe and H. Ots, Acta Chem~ Scand., 26 (1972) 1217, 1229. 3 T. Reichstein, A. Grussner, K. Schindler and E. Hardneier, Heir. Chim. Acta, 16 (1933) 276. 4 J. Schubert, J. Phys. Colloid Chem., 52 (1948) 340. 5 K. L. Nash and E. P. Horwitz, Inorg. Chim. Acta, 169 (1990) 245. 6 G. R. Choppin, D. A. Kelly and H. E. Ward, in D. Dyrssen, J. O. Liljenzin and J. Rydberg (eds.), Solvent Extraction Chemistry, North-Holland, Amsterdam, 1967, p. 46. 7 S. S. Yun, G. R. Choppin and D. Blakeway, J. Inorg. Nucl. Chem., 38 (1976) 587. 8 A. E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3, Plenum, New York, 1977. 9 G. Ostacoli, A. Vanni and E. Roletto, Gazz. Chim. Itak, 100 (1970) 350. 10 R. C. Gatrone and E. P. Horwitz, Solvent Extr. Ion Exch., 6 (1988) 937. 11 K. L. Nash, Eur. J. Solid State Inorg. Chem., 28 (1991) 389. 12 G. R. Choppin, Radiochim. Acta, 32 (1983) 43.