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Thermodynamic parameters of the β-hydroxypropionate complexes of the lanthanides
J. inorg,nucl.Chem., 1969,Vol. 31, pp. 3523to 3526. PergamonPress. Printedin GreatBritain
THERMODYNAMIC PARAMETERS OF THE fl-HYDROXYPROPIONATE COMPLEXES OF THE LANTHANIDES A. D. JONES* and G. R. C H O P P I N Department of Chemistry, Florida State University, Tallahassee, Florida 32306 (First received 20 January 1969; in revised form 7 April 1969) Abstract--The thermodynamic parameters for formation of the fl-hydroxypropionate complexes of the lanthanide ions were measured in an aqueous medium of 2.00M (NaCIO4) ionic strength. The agreement with similar data for propionate complexing in contrast to the lack of agreement with data for a-hydroxy-propionate complexing indicates that the B-hydroxy group is not involved in a chelate ring.
THERMODYNAMIC data have been reported for the formation of lathanide complexes with propionate and ot-hydroxypropionate (lactate) ligands [1, 2]. The lactate complexes are more stable due to formation of chelates. It has been suggested that the chelate ring includes a hydrogen bonded water molecule between the metal and the hydroxyl group[3, 2]. This suggested the possibility of chelation by fl-hydroxypropionate without a water molecule. The present work is concerned, then, with determining the free energy, enthalpy and entropy of complexation of the lanthanides and yttrium with flhydroxypropionic acid in 2.00M ionic strength solutions. EXPERIMENTAL
Lanthanide perchlorates were prepared by dissolving the respective oxides (Baker and Adamson Co., 99.9 per cent) in perchloric acid and evaporating off excess acid so that stock solutions of pH about 2-5 resulted. The stock solutions were standardized by passing aliquots through cation-exchange resins in their hydrogen form, and then titrating the acid released with carbonate-free sodium hydroxide solution. All working solutions were adjusted to an ionic strength of 2"00M using sodium perchlorate. fl-hydroxypropionic acid was prepared by dissolving fl-propiolactone (Aldrich Chem. Co.) in hot water[4]. A buffer solution was prepared by partially neutralizing the acid with carbonate-free sodium hydroxide solution, and the ionic strength was adjusted to 2-00M with sodium perchlorate. The equipment and procedures for the simultaneous pH and calorimetric titrations have been described previously [2]. A sample set of data is given in Table 1. RESULTS
The pH and enthalpy data were processed as described previously [2] using a
CDC 6400 computer. The stability constants, kl, for the reaction Ln+~q~+ L~-~q~=L nL +2 (aq) *Present address: Department of Chemistry, St. Salvator's College, University of St. Andrews, St. Andrews, Fife, Scotland. 1. G. R. Choppin and A. J. Graffeo, lnorg. Chem. 4, 1254 (1964). 2. G. R. Choppin and H. G. Friedman, lnorg. Chem. 5, 1599 (1966). 3. I. Grenthe,Acta. chem. scand, lg, 283 (1964). 4. T. L. Gresham, J. E. Jansen and F. W. Shaver, J. Am. chem. Soc. 70, 998 (1948). 3523
3524
A.D.
J O N E S and G. R. C H O P P I N
Table 1. Data on the thulium/3-hydroxypropionic acid CM = 26'17 mM CL = 971.9 mM Initial v o l u m e = 50 ml Volume pH pH' (ml) 0.50 0"60 0"70 0"80 0"90 1.00 l ' 10 1 '20 1 '30 1 '40 l "50 1"75 2"00 2' 25 2.50 2.75 3"00 3.50 4.00 4-50 5"00 5.50 6.00 6 '50 7.00 7.50 8.00 8-50 9-00 9-50 10-00
3'803 3.859 3"888 3.917 3.933 3"951 3-962 3.975 3.984 3"991 4.001 4.016 4"030 4. 042 4.052 4.061 4.070 4.083 4.095 4.106 4.116 4-125 4.133 4" 139 4.147 4"153 4" 158 4.163 4.168 4.174 4.177
4"258 4"257 4'256 4"256 4.256 4"256 4"256 4"256 4"256 4'256 4.256 4"256 4"257 4. 257 4.257 4.257 4.257 4 '258 4'258 4.259 4.260 4.260 4-260 4.261 4.261 4.263 4.263 4.264 4-265 4-265 4"266
C a = 0"219 m M CHL = 933"4 m M [L] × 10a (M)
fi
Q × 101 (cal)
] - x 10 -~ (cal/mole)
3.162 4.170 5'052 6.040 6'921 7.895 8'787 9'763 10.680 11 '606 12.600 14'976 17.403 19' 877 22.345 24"892 27"433 32 '413 37.513 42"713 47"844 53'083 58.248 63 '305 68.518 73'468 78 '436 83.426 88.441 93.480 98.103
0.090 0" 107 0"129 0.147 0"169 0' 187 0"208 0"226 0'246 0-266 0'282 0"327 0.369 0-408 0.446 0"479 0.511 0.578 0.634 0-681 0.727 0.763 0"798 0.834 0:858 0'890 0.917 0.939 0"955 0"966 0.994
1 '697 2' 126 1.931 2.316 1'815 2' 144 1.423 2.003 1.642 I "373 1-918 3-757 3-834 3. 307 3.297 3.304 2.907 5.421 5-202 4.443 4.074 3.806 3.240 3.126 2'716 2.280 2 '060 2.103 2.087 1.624 1-433
-- 3'957 -- 5'812 -- 7.498 --9-519 -- 11"102 -- 12.973 -- 14"214 -- 15.962 -- 17-394 -- 18"592 -- 20"266 -- 23"544 -- 26.890 -- 29"776 -- 32"653 -- 35"536 -- 38"073 -- 42 '803 --47.342 --51"219 -- 54.775 --58"096 --60-923 -- 63 "651 --66"020 --68.010 -- 69"807 --71-642 -- 73.463 -- 74.881 -- 76-131
T = 25-0°C;/x = 2.00M (NaCIO4). k a = 27.9; AH = 5.28 kcal/mole; AS = 24.3 cal/mole.
are listed in Table 1. The values of AH1 and A S 1 a r e also presented in Table 2. Figure 1 compares the values of AH1 and TAS1 with the corresponding data for the propionate and lactate complexes. The measurements were made in solutions of total ionic strength 2.00M (NaCIO4), and since the concentrations of reacting species were always small (30mM) compared to the total ionic strength, it was assumed that activity coefficients remained constant. The thermodynamic values obtained then relate to a standard state where the solvent medium is 2.00M (NaCIO4). Only data for the monoligand complex LnL 2+ are reported because the errors involved in determining the parameters for subsequent complexes are too large to allow meaningful interpretation. The order of magnitude of k2 and AH2 is
Thermodynamic parameters
÷1 ÷1 ÷1 ÷1 ÷1
3525
÷1 ÷1÷1 ÷1 ÷1 ÷1.÷1÷1 ÷1
"r~ O ~J
O O
~D e~
~4 ¢,1
¢. O
¢4
~
~
i~'~
i i ~ "~
~-
re',
z
[~.
.=_ O
3526
A . D . JONES and G. R. CHOPP1N
÷lO +8
0
E
0
- 4
" - I
"'=..=x__x__x_.~.
-x ....
x
t-.t_.~__i_.¥__t._~L___t
-6 -8 -10
, i Lo Ce
i Pr
,
,
Nd Pm
, , , Sm Eu Gel
, = , = T b D y H o Er
= , = Tm Y b Lu
Fig. I. Plot of thermodynamic parameters for formation of L n L =+ at 25°C and p. = 2.00M (NaCIO4). Propionate: TAS1 (~), - - A H 1 (~, fl-hydroxypropionate: TAS1 ~), --AH~
O; Lactate: TDS1(~),--AH~(~). ca. 10 and 2.0 kcal. mole -1, respectively. T h e error limits in Fig. 1 represent the
standard deviations from the m e a n of 3 - 5 titrations of each metal-ligand combination. DISCUSSION T h e data given in Fig. 1 show that the stability of the lanthanide/3-hydroxypropionate complexes is due to the positive e n t r o p y of c o m p l e x a t i o n since the enthalpy values are endothermic for all the complexes, and hence oppose c o m p l e x formation. This is similar to the behavior reported previously for the lanthanide propionate and isobutyrate c o m p l e x e s [ l ] , but at variance with that of the lactate s y s t e m where both AH and AS favor complexation [2]. M o r e o v e r , both the magnitude of the values and their trends with lanthanide atomic n u m b e r are similar for the propionate and /3-hydroxypropionate but not for the a - h y d r o x y - and /3-hydroxypropionate. F r o m ' t h i s we conclude that fl-hydroxypropionate complexes are similar in nature to t.he propionates rather than the chelated lactates. A similar conclusion has b e e n reached for the lanthanide or- and/3-mercaptopropionate complexes [6]. Acknowledgements-This research was supported by the United States Atomic Energy Commission
under Contract AT-(40-1)-1797. The NSF assisted via a grant to the Florida State University Computer Center. 6. G. R. Choppin and L. A. Martinez-Perez, lnorg. Chem. 7, 2657 (1968).
THERMODYNAMIC PARAMETERS OF THE fl-HYDROXYPROPIONATE COMPLEXES OF THE LANTHANIDES A. D. JONES* and G. R. C H O P P I N Department of Chemistry, Florida State University, Tallahassee, Florida 32306 (First received 20 January 1969; in revised form 7 April 1969) Abstract--The thermodynamic parameters for formation of the fl-hydroxypropionate complexes of the lanthanide ions were measured in an aqueous medium of 2.00M (NaCIO4) ionic strength. The agreement with similar data for propionate complexing in contrast to the lack of agreement with data for a-hydroxy-propionate complexing indicates that the B-hydroxy group is not involved in a chelate ring.
THERMODYNAMIC data have been reported for the formation of lathanide complexes with propionate and ot-hydroxypropionate (lactate) ligands [1, 2]. The lactate complexes are more stable due to formation of chelates. It has been suggested that the chelate ring includes a hydrogen bonded water molecule between the metal and the hydroxyl group[3, 2]. This suggested the possibility of chelation by fl-hydroxypropionate without a water molecule. The present work is concerned, then, with determining the free energy, enthalpy and entropy of complexation of the lanthanides and yttrium with flhydroxypropionic acid in 2.00M ionic strength solutions. EXPERIMENTAL
Lanthanide perchlorates were prepared by dissolving the respective oxides (Baker and Adamson Co., 99.9 per cent) in perchloric acid and evaporating off excess acid so that stock solutions of pH about 2-5 resulted. The stock solutions were standardized by passing aliquots through cation-exchange resins in their hydrogen form, and then titrating the acid released with carbonate-free sodium hydroxide solution. All working solutions were adjusted to an ionic strength of 2"00M using sodium perchlorate. fl-hydroxypropionic acid was prepared by dissolving fl-propiolactone (Aldrich Chem. Co.) in hot water[4]. A buffer solution was prepared by partially neutralizing the acid with carbonate-free sodium hydroxide solution, and the ionic strength was adjusted to 2-00M with sodium perchlorate. The equipment and procedures for the simultaneous pH and calorimetric titrations have been described previously [2]. A sample set of data is given in Table 1. RESULTS
The pH and enthalpy data were processed as described previously [2] using a
CDC 6400 computer. The stability constants, kl, for the reaction Ln+~q~+ L~-~q~=L nL +2 (aq) *Present address: Department of Chemistry, St. Salvator's College, University of St. Andrews, St. Andrews, Fife, Scotland. 1. G. R. Choppin and A. J. Graffeo, lnorg. Chem. 4, 1254 (1964). 2. G. R. Choppin and H. G. Friedman, lnorg. Chem. 5, 1599 (1966). 3. I. Grenthe,Acta. chem. scand, lg, 283 (1964). 4. T. L. Gresham, J. E. Jansen and F. W. Shaver, J. Am. chem. Soc. 70, 998 (1948). 3523
3524
A.D.
J O N E S and G. R. C H O P P I N
Table 1. Data on the thulium/3-hydroxypropionic acid CM = 26'17 mM CL = 971.9 mM Initial v o l u m e = 50 ml Volume pH pH' (ml) 0.50 0"60 0"70 0"80 0"90 1.00 l ' 10 1 '20 1 '30 1 '40 l "50 1"75 2"00 2' 25 2.50 2.75 3"00 3.50 4.00 4-50 5"00 5.50 6.00 6 '50 7.00 7.50 8.00 8-50 9-00 9-50 10-00
3'803 3.859 3"888 3.917 3.933 3"951 3-962 3.975 3.984 3"991 4.001 4.016 4"030 4. 042 4.052 4.061 4.070 4.083 4.095 4.106 4.116 4-125 4.133 4" 139 4.147 4"153 4" 158 4.163 4.168 4.174 4.177
4"258 4"257 4'256 4"256 4.256 4"256 4"256 4"256 4"256 4'256 4.256 4"256 4"257 4. 257 4.257 4.257 4.257 4 '258 4'258 4.259 4.260 4.260 4-260 4.261 4.261 4.263 4.263 4.264 4-265 4-265 4"266
C a = 0"219 m M CHL = 933"4 m M [L] × 10a (M)
fi
Q × 101 (cal)
] - x 10 -~ (cal/mole)
3.162 4.170 5'052 6.040 6'921 7.895 8'787 9'763 10.680 11 '606 12.600 14'976 17.403 19' 877 22.345 24"892 27"433 32 '413 37.513 42"713 47"844 53'083 58.248 63 '305 68.518 73'468 78 '436 83.426 88.441 93.480 98.103
0.090 0" 107 0"129 0.147 0"169 0' 187 0"208 0"226 0'246 0-266 0'282 0"327 0.369 0-408 0.446 0"479 0.511 0.578 0.634 0-681 0.727 0.763 0"798 0.834 0:858 0'890 0.917 0.939 0"955 0"966 0.994
1 '697 2' 126 1.931 2.316 1'815 2' 144 1.423 2.003 1.642 I "373 1-918 3-757 3-834 3. 307 3.297 3.304 2.907 5.421 5-202 4.443 4.074 3.806 3.240 3.126 2'716 2.280 2 '060 2.103 2.087 1.624 1-433
-- 3'957 -- 5'812 -- 7.498 --9-519 -- 11"102 -- 12.973 -- 14"214 -- 15.962 -- 17-394 -- 18"592 -- 20"266 -- 23"544 -- 26.890 -- 29"776 -- 32"653 -- 35"536 -- 38"073 -- 42 '803 --47.342 --51"219 -- 54.775 --58"096 --60-923 -- 63 "651 --66"020 --68.010 -- 69"807 --71-642 -- 73.463 -- 74.881 -- 76-131
T = 25-0°C;/x = 2.00M (NaCIO4). k a = 27.9; AH = 5.28 kcal/mole; AS = 24.3 cal/mole.
are listed in Table 1. The values of AH1 and A S 1 a r e also presented in Table 2. Figure 1 compares the values of AH1 and TAS1 with the corresponding data for the propionate and lactate complexes. The measurements were made in solutions of total ionic strength 2.00M (NaCIO4), and since the concentrations of reacting species were always small (30mM) compared to the total ionic strength, it was assumed that activity coefficients remained constant. The thermodynamic values obtained then relate to a standard state where the solvent medium is 2.00M (NaCIO4). Only data for the monoligand complex LnL 2+ are reported because the errors involved in determining the parameters for subsequent complexes are too large to allow meaningful interpretation. The order of magnitude of k2 and AH2 is
Thermodynamic parameters
÷1 ÷1 ÷1 ÷1 ÷1
3525
÷1 ÷1÷1 ÷1 ÷1 ÷1.÷1÷1 ÷1
"r~ O ~J
O O
~D e~
~4 ¢,1
¢. O
¢4
~
~
i~'~
i i ~ "~
~-
re',
z
[~.
.=_ O
3526
A . D . JONES and G. R. CHOPP1N
÷lO +8
0
E
0
- 4
" - I
"'=..=x__x__x_.~.
-x ....
x
t-.t_.~__i_.¥__t._~L___t
-6 -8 -10
, i Lo Ce
i Pr
,
,
Nd Pm
, , , Sm Eu Gel
, = , = T b D y H o Er
= , = Tm Y b Lu
Fig. I. Plot of thermodynamic parameters for formation of L n L =+ at 25°C and p. = 2.00M (NaCIO4). Propionate: TAS1 (~), - - A H 1 (~, fl-hydroxypropionate: TAS1 ~), --AH~
O; Lactate: TDS1(~),--AH~(~). ca. 10 and 2.0 kcal. mole -1, respectively. T h e error limits in Fig. 1 represent the
standard deviations from the m e a n of 3 - 5 titrations of each metal-ligand combination. DISCUSSION T h e data given in Fig. 1 show that the stability of the lanthanide/3-hydroxypropionate complexes is due to the positive e n t r o p y of c o m p l e x a t i o n since the enthalpy values are endothermic for all the complexes, and hence oppose c o m p l e x formation. This is similar to the behavior reported previously for the lanthanide propionate and isobutyrate c o m p l e x e s [ l ] , but at variance with that of the lactate s y s t e m where both AH and AS favor complexation [2]. M o r e o v e r , both the magnitude of the values and their trends with lanthanide atomic n u m b e r are similar for the propionate and /3-hydroxypropionate but not for the a - h y d r o x y - and /3-hydroxypropionate. F r o m ' t h i s we conclude that fl-hydroxypropionate complexes are similar in nature to t.he propionates rather than the chelated lactates. A similar conclusion has b e e n reached for the lanthanide or- and/3-mercaptopropionate complexes [6]. Acknowledgements-This research was supported by the United States Atomic Energy Commission
under Contract AT-(40-1)-1797. The NSF assisted via a grant to the Florida State University Computer Center. 6. G. R. Choppin and L. A. Martinez-Perez, lnorg. Chem. 7, 2657 (1968).