The stability of some lanthanide complexes with mandelate and atrolactate

,t. inorg, nucl. Chem., 1966, Vol. 28, pp. 1949 to 1954. Pergamon Press Ltd. Printed in Northern Ireland

THE STABILITY OF SOME LANTHANIDE COMPLEXES WITH MANDELATE AND ATROLACTATE H. THUN and F. VERBEEK Laboratory for Analytical Chemistry, Ghent University, Belgium and W . VANDERLEEN

Computer Center, Ghent University, Belgium (Received 22 October 1965 ; in revisedform 13 January 1966) Abstract--The successive formation constants were determined for the mandelate and atrolactate complexes of several lanthanide elements by potentiometric titration. The formation constants increase as a function of the atomic numbers for both ligands. For atrolactate the usual irregularities in the trend in the Eu-Gd region are observed. For a given lanthanide the order of complex stability is atrolactate ), mandelate ~-, ~-hydroxyisobutyric acid/~, lactate > glycolate. The average ligand number per metal ion exceeding a value of three, indicates the formation of ML~- complexes. The acid dissociation constant for each of the ligands was also calculated. IN OUR study a b o u t the stability of the complexes of ~-hydroxycarboxylates <1,2) with lanthanide ions, the mandelates (phenylglycolates) and atrolactates (phenylmethylglycolates) were investigated next. Both ligands were of special interest for examining the influence of the phenylgroup by comparing the stability constants of their complexes with those of the corresponding glycolates and lactates. The stepwise stability constants were determined for the complexes of several lanthanide ions with both ligands, by a potentiometric technique. The atrolactate and mandelate anions form 4:1 complexes with the rare earth elements, giving rise in solution to M L 2 ~, ML2 +, M E a and M E 4- complexes. CALCULATION

PROCEDURE

All formulae used were the same as previously described, except lot the corrections for the change of ionic strength during titrations. The formerly used equation, developed by SONESSON{a) A I = 0-5- Q u . ~q- (7 ¢~) (1) is only valid for trivalent metal ions up to i~ -- 3 and consequently will cause errors in the lower pL-region. Although these errors are very small (-k0"02 mV), it would however be more correct to use the formula represented in its general form as AI=

0.5. CM . ( N - - e ) . ( N + 1 q- e)

where N = valency of the metal ion and

e = IN - ~[. ':~ L. EECKHAUT, F. VERBEEK, H. DEELSTRA and J. HOSTE, Analytica chim. Acta 30, 369 (1964). '~' H. THEN, F. VERBEEK and W. VANDERLEEN, J. inorff, nucl. Chem. 27, 1813 (1965). .a~ A. SONESSON, Acta chem. scan& 12, 165 (1958). 1949

(2)

1950

H. THUS, F. VERBEEKand W. VANDERLEEN

I n case of a n M 3+ metal ion this becomes AI=

0 . 5 . C M . ( 3 - - e ) . (4 q- e)

(3)

and ~=

13 - ~l

A new c o m p u t e r p r o g r a m based o n F r o n a e u s ' integration m e t h o d (4) was developed a n d will be discussed in detail in a later paper. The advantages of this new p r o g r a m m e lie in its greater speed a n d simplicity c o m p a r e d to the least squares t r e a t m e n t used before, (1'2) as it takes only a b o u t 3 to 7 m i n to calculate the four constants a n d their s t a n d a r d deviations a n d as it is n o longer necessary to introduce approximate values of the constants. EXPERIMENTAL Mandelic acid. Fluka's DL Mandelic acid, purissimum, was used. Atrolactic acid. This acid was prepared and purified as described in literatureJ ~ All other reagents as well as procedure, apparatus and definitions of symbols were the same as before, c2~ Throughout titrations temperature was kept at 25-0 ~ 0"I°C and ionic strength / = 1 in NaC1Oa. RESULTS AND DISCUSSION F o r b o t h ligands a n example of a titration is given in Tables 1 a n d 2. Only fifteen m e a s u r e m e n t s are r e p r o d u c e d a l t h o u g h for each titration 30-40 ~i a n d (L) values were calculated a n d used for c o m p u t i n g the f o r m a t i o n constants. The low solubility of the s o d i u m m a n d e l a t e causes a c o r r e s p o n d i n g low CL ° value; for the atrolactate buffer the low CL ° is due to the low solubility of the acid a n d to the desire of o b t a i n i n g a reasonable d-value TABLE 1.---TITRATION OF Nd 3+ WITH MANDELATE. [H]a = 5'784;

223.0mM;

6 = 0.9935;

C~ ° = 12-29mM; 10.0ml; k = 2"0

C~ ° = 5.450mM;

Volume (ml)

E.an (mY)

AEan (mY)

Eao (mY)

Ka.10*

0"20 0"30 0"40 0.45 0"60 0.80 1.20 1"80 2.40 3.30 4.40 5.70 9.00 13.00 17-00

4"80 8-08 10.89 12.23 15.59 19.31 25.01 30.98 35.17 39.52 43'05 45-84 49.92 52.41 53-88

0.026 0"034 0"042 0.046 0.057 0"070 0.090 0.108 0.116 0.119 0-117 0.111 0.094 0.079 0.067

61-66 59'50 58"34 57.95 57.15 56.55 56-02 55.77 55-75 55.86 56'06 56'31 56-91 57.44 57-82

6.73 6"85 6'91 6.93 6.95 6.96 6.95 6.90 6.85 6.78 6'69 6.61 6.43 6-28 6.18

t4, S. FRONAEUS, Acta chem. scand. 4, 72 (1950); 5, 139 (1951). ~5~Organic Syntheses, Vol. 33, p. 7, J. Wiley, New York (1953).

[L] (mM) 0.654 1"180 1.776 2.105 3'158 4.740 8.376 14'635 21'387 31-660 43.692 56.645 83.305 106-316 122.944

Cz°= v0 =

t~ 0-263 0'355 0-452 0.499 0-644 0'831 1'169 1"583 1.901 2-252 2.547 2.786 3-136 3-389 3'549

1951

The stability of some lanthanide complexes with mandelate and atrolactate TABLE 2,---TITRATION OF Yb 3 WITH ATROLACrAIE. [H]A := 5'784; CL~ = 159"65 raM; d =0"4418; C rt'~ = 12.29mM; C/~° == 1.450mM; vo-- 10.0ml; k = 1-0 Volume (ml)

EA~

AE~a

EAe

(mY)

(mY)

(mY)

A%'IO~

[L] (mM)

0.30 0-40 (1.70 0.90 1-10 ~-30 ~.50 1-70 1.90 2-20 2.50 3-00 5.00 8-00 11-00

29.43 29.20 29.65 30-63 31.93 33-50 3531 37.29 39-33 42.58 45-74 50.67 64-02 72.47 76.46

0.015 0-019 0.030 0-036 0-041 0.045 0.048 0.050 0-052 0-054 /).055 0.056 0,046 6.036 0.029

87.03 86.90 85.46 85-13 84-91 84.77 84.69 84.65 84.60 84.69 84.62 84.67 84.92 85.15 8554

5-09 4.94 502 5,1)2 5.02 5.02 5.01 5,90 5.00 4.96 4.97 4-94 4.86 4.80 4,72

0,450 0601 1-143 1.546 1.997 2,507 3-088 3.750 4.495 5.779 7-297 10.210 24.440 44.441 59.907

0.388 0-508 0.850 1.069 1-281 1.484 1,678 1.861 2-035 2.275 2-486 2.783 3.451 3.814 3-984

A s in p r e v i o u s e x p e r i m e n t s w i t h h o m o l o g o u s acids, n o p o l y n u c l e a r c o m p l e x e s w e r e to be e x p e c t e d . From

t h u s o b t a i n e d ri a n d (L) d a t a , a p p r o x i m a t e K,, v a l u e s w e r e d e r i v e d by

BJERRUM'S g r a p h i c a l h a l f a m e t h o d CG) a n d t h e e x a c t c o n s t a n t s w e r e c a l c u l a t e d b y c o m puter treatment.

T h e r e s u l t i n g v a l u e s a r e listed in T a b l e s 3 a n d 4.

TABLE 3.--STABILITY CONSTANTS OF LANTHANIDE-MANDELATE

Element La Ce Pr Nd Sm

Log K t

2.18 2.24 2.48 2.59 2.56

~: 0.0l I: 0.01 i 0.01 h 0.01 ! 0.01

Log R72 1.44 1.51 1.62 1.70 1.76

± 0.01 2- 0.0l ± 0.01 -- 0-02 ± 0.02

Log K3 1-23 i 0.01 1-27 ± 0.01 1.35 2- 0.01 1.32 2- 0.02 1.42 2_ 0.02

TABLE 4.--STABILITY CONSTANTS OF LANTHANIDE-ACTROLACTATE

Element Pr Nd Sin Eu Gd Ho Er Yb

Log Kt 2.40 2.55 2.57 2.55 2.54 2.97 3-03 3.05

:k 0.01 ± 0.0l 0.01 ~ 0.01 !: 0-01 L: 0-01 :: 0-01 i 0-01

Log K~ 1-56 ~ 0.02 1.64 % 0-02 1.89 ~: 0.02 2.17 ± 0.01 2.07 ± 0.01 2.38 ~_ 0.01 2.48 ± 0.02 2.56 ± 0-02

Log K:~ 1.36 ± 0-01 1.42 i 0.02 1.54 i: 0.02 -1.70 ± 0.02 1.92 -- 0.01 2.01 ± 001 2.07 ": 0.01

COMPLEXES (FRONAEUS' TECHNIQUE)

Log K~ 0.74 0.64 0.89 1.20 1.26

-: £: =: 2~

0-01 0.01 0.01 0.03 0.03

Log t3~ 5.59 5.66 6.34 6.81 7.00

2- 0.01 2- 0.01 ± 0.02 2- 0-04 ± 0-04

COMPLEXES (CHoPPIN'S TECHNIQUE)

Log K4 0-92 5_ 0.0l 1.21 :: 0-03 1-31 :~ 0-02 -1.33 i 0.01 1.76 ± 0"02 1.90 ~ 0-02 1.86 ! 0.02

Log I~ 6.24 ± 0.09 6-82 ± 0-04 7.31 ± 0.03 -7.64 ± 0-02 9-03 ± 0-02 9-42 ± 0.03 9-53 - 0.02

I n Figs. 1 a n d 2 t h e g - ( L ) r e l a t i o n is p l o t t e d f o r b o t h l i g a n d s a n d s e v e r a l l a n t h a n i d e ions. T h e li v a l u e s l a r g e r t h a n t h r e e give a d d i t i o n a l e v i d e n c e f o r t h e e x i s t e n c e o f M L 4- c o m p l e x e s . ~'J. BJERRUM, Metal Ammine Formation in Aqueous Solution, P. Haase, Copenhagen (1941).

H. THUN, F. VERBEEK and W. VANDERLEEN

1952

Nd

Sm

1

2

3

FIG. l . - - F o r m a t i o n curves of the mandelate-lanthanide complexes.

Ho Yb

Nt 3 Pr Er

1

2

3

~, pL

FIG. 2.--Formation curves of the atrolactate-lanthanide complexes.

The stability of some lanthanide complexes with mandelate and atrolactate

1953

Titrations with mandelate buffer of elements higher than Sm could not be performed as precipitation occurred from the very beginning. For atrolactate titrations with more lanthanide ions were performed, though it should be noticed that for Eu and in minor degree for Gd some precipitation occurred. For calculation only potentials obtained before precipitation were used. Only for Eu the obtained fi values did not permit calculation of all four constants.

11og134

10!~,

_ /

9;

z /

/

./r'~/~/" o/

,

,/'//

8

¢~-xt

.

7

,¢.-hydroxYiSObutyrate

I / //

e-

atrolactate rnandelate

3 La Oe Pr Nd

atomic number Pm Srn Eu cad Tb Dy Ho Er Tm Yb Lu

FiG. 3 .- - T he log/3~ values plotted vs. the atomic number of the lanthanide elements.

Figure 3 represents the log f14 values plotted vs. the atomic number of the rare earth elements, not only for the ligands investigated but also for ~-hydroxyisobutyrate. (v-9) When considering the magnitude of the f14 values, it is clear that mandelate and atrolactate rare earth complexes are comparable to the ~-hydroxyisobutyrates and far more stable than the corresponding glycolates (7.1°,u) or even lactates. (7,81 This ~7, G. R. CHOPPIN and J. A. CHOPOORIAN,Jr. gnome,nucL Chem. 22, 97 (1961). is) H. DEELSTRA and F. VERBEEK,Analytica chim. Acta 31, 251 (1964). (a) W. R. STAGG and J. E. POWELL,Inor(. Chem. 3, 242 (1964). (to) A. SONESSON,Acta chem. scand. 13, 998, 1437 (1959). (ll) [. GRENTHE,Acta chem. scand. 16, 1695 (1962).

1954

H. THUN, F. VERBEEKand W. VANDERLEEN

definitely proves that the inductive effect does not play a predominant role amongst the factors that govern the complexing behaviour of ~-hydroxycarboxylate ligands. Indeed, where the well known series of inductive effect < H seems to reappear in the sequence of acid dissociating constants mandelic acid > atrolatic acid > glycollic acid a~ > lactic acid ~12'1a~ > 0~-hydroxyisobutyric acid, n2.1~) it fails to explain the sequence of formation constants of rare earth 0~-hydroxycarboxylate complexes: glycolate < lactate < 0~-hydroyxisobutyrate -~ mandelate < atrolactate. When considering the slope of the curves in Fig. 3, it can be said that for the practical separation of lanthanide ions by column experiments mandelate would give about the same result as 0~-hydroxyisobutyrate, while an improvement should be expected from atrolactate. Studies about the determination of KD values and about the separation of rare earth ions by ionic exchange using atrolactate, are in progress. Acknowledgements--The authors wish to thank Prof. Dr J. HOSTEfor his kind interest and suggestions during this work. They also wish to thank Mrs F. VAN t)EN ASEELEand Ms. B. YZEWIJNfor their technical assistance. ~12~j. E. POWELLand Y. SuzuKI, Inorg. Chem. 3, 690 (1964). ~13~F. VEgBEEKand H. Tr~UN,Analytica chim. Acta 33, 378 (1965).