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Complexation of lanthanides(III) with Macrocyclic 18-crown-6 in methanol utilizing a colorimetric complexant by spectrophotometry
Polyhedm Vol. 8, No. 12, pp. 1561-1565, Printed in Great Britain
0271-5381/89 S3.00+ .oO 0 1989 Pcrgamon Press
1989
plc
COMPLEXATION OF LANTHAND Es(III)wm MACROCYCLIC l&CROWN-6 IN METHANOL UTILIZING A COLORIMETRIC COMPLEXANT BY SPECTROPHOTOMETRY EMIKO OHYOSHI”
and SUSUMU
KOHATA
Yatsushiro National College of Technology, Hirayamashinmachi Kumamoto 866, Japan
2627, Yatsushiro,
(Received 27 June 1988 ; accepted 15 February 1989)
Abstract-A spectrophotometric procedure using a calorimetric complexant has been developed for the study of the complexation between several lanthanides(II1) (Ln”‘) and 18-crown-6 (18C6, A) in methanol. 2-(2-Thiazolylazo)-4-methylphenol (TAC, L) was used as a calorimetric complexant because of its simple structure with which no protonated complexes may be formed. The absorption spectra of the species in the Ln-TAC-l8C6 system at various concentrations of 18C6 showed no formation of the ternary complex, LnLA. The conditional stability constants of the Ln(TAC) complexes at constant acidity in the absence (KLnL’)and the presence (&,,,,_,) of 18C6 were determined by monitoring the absorbance due to the Ln(TAC) complexes. The competition between TAC and 18C6 for the metal ion resulted in a decrease in KL,,‘L’with increasing 18C6 concentration. From the dependence of l/KLnpLon the concentration of 18C6, the stability constants of the Ln( 18C6) complexes could be determined. The stability constants of these complexes were found to decrease with increasing atomic number. The results are compared with those reported previously. On the basis of the complexation trend observed, 18C6 was used as a competitive ligand to increase the selectivity of chelating agents.
Macrocyclic polyethers of the 18-crown-6 type are unique ligands because of their lower complexingability with the small lanthanides, despite their higher charge density. iv2 This trend in the lanthanides is very interesting because a reverse order has been always noted for most chelating agents. It suggests, therefore, that the ligands of the 18C6 type might be used competitively so as to increase the selectivity of the chelating agent. Since 18C6 is soluble both in aprotic and protic (methanol) solvents, it should be suitable for this purpose. Izatt et al. ’ studied the lanthanide complexation of 18C6 in methanol by calorimetric titrations and showed no formation of 18C6 complexes with any postGd”‘. However, Ahnasio et al. 3 reported that 18C6 formed very stable complexes with Dymr (1ogK = 7.90), Er”’ (1ogK = 7.67) and Yb”’ (1ogK = 7.50) in anhydrous propylene carbonate (aprotic solvent). It thus seemed that further investigations
should be made in methanol. Recently, spectrophotometric procedures”’ involving the competition of the two ligands have successfully been applied to the study of many complexes. In the present work, we have developed a similar spectrophotometric procedure to study the complexation of several lanthanides with 18C6 in methanol, and have compared the results with those reported by Izatt et al. Also, the competitive effect of 18C6 on the selectivity of chelating agents has been explored. EXPERIMENTAL Reagents
All chemicals used were of GR grade. The ligands, TAC (Dojindo Chemical Co.) and 18C6 (Aldrich Chemical Co.) were dissolved in methanol (spectroscopic grade) to prepare 1.0 x lo- 3 M and 0.20 M stock solutions, respectively. Seven types of * Author to whom correspondence should be addressed. LnCl, - nHzO [Ln = La, Ce, Pr (n = 7) ; Ln = Nd, 1561
1562
E. OHYOSHI
and S. KOHATA
Eu, Dy, Yb (n = 6)] were dissolved in methanol to give concentrations of 5-20 x lop3 M, which were standardized by EDTA titration. Perchloric acid and tetraethylammonium perchlorate were used to adjust the acidity and to control the ionic strength, respectively.
These facts indicate that the dissociation of the YbL complex only takes place in procedure (b), i.e. there is no formation of ternary complex, YbLA. Similar results were obtained for other systems of Ln-L and Ln-L-A studied. Determination of the conditional stability constants of Ln-TAC complexes
Procedure (a) The metal solution was added in small steps, 25 x low3 cm3 each time, using a microliter pipet, to 5.0 cm3 of a methanol solution containing TAC (2.0 x lo- 5 M), at a constant acidity and ionic strength, to give concentrations of 2.5-40 x lo- ’ M. (b) To the same solution, prepared by procedure (a), 18C6 solution was further added in small steps to give concentrations of 1.0-12x 10V3 M. After each addition of the metal and 18C6 solution, the absorbance due to the Ln(TAC) complex (570 nm) was measured using a Shimadzu UV-260 spectrophotometer.
The absorbance D due to the Ln-TAC complex was used to determine the conditional stability constant at a given acidity, KLnL’: D was measured at a constant acidity and various Ln concentrations in the binary Ln-TAC(L) system [procedure (a)]. The constant defined as KLnL’= [LnL]/([Ln][L’]), where [L’] = [L] +[HL] (the charges are omitted for simplicity), was determined by the method of Lang’ [eq. WI : CL&L/D
=
(CL,+CL-(D/&L~L)}(II&L~L) +(WLnL'~LnL)I9
where CL, and CL are the total concentrations of Ln and L, and sLnLis the molar absorptivity of LnL. Plots of CL&,/D vs CL,,+ CL - (D/eLnL), where eLnL is assumed to be D,,/C, (D,, is the maximum absorbance), were almost linear and show that only the 1: I complex is formed. The reciprocal of the slope of the line yielded the corrected value of aLnL which was utilized to replot the above relation. The slope/intercept ratio of the resulting plot gave the K LnL’value. The results obtained for different lanthanides are listed in Table 1. From the values obtained at different acidity for La”‘, Ce”’ and Ybru, it is found that KLnL decreases due to the side reaction, H + L = HL occurring. In the ternary Ln-TAC(L)-18C6(A) system [procedure (b)], D was observed to decrease with increasing concentration of 18C6 (C,) (see
RESULTS AND DISCUSSION Absorption spectra
Figure 1 shows the variation in absorption spectra of the TAC(L) solutions with varying Yb concentration, CYb, at [HClO,] = 4 x lop4 M and [18C6] = 0 (procedure (a), solid lines l-4), or varying the 18C6(A) concentration, C,, at [HClO,] = 4x 1O-4 M and C, = 1.0x 10m4 M (procedure (b), dotted lines 5-10). In each procedure, (a) or (b), the Yb(TAC) complex formation (A,, = 572 nm) or its dissociation occurs, respectively. It is seen that the solid and dotted curves have the same isosbestic point (488 nm). Moreover, the two curves, 2 and 8, are identical.
0
300
400
(1)
600
500
Wavelength
700
I nm
) and Yb-TAC-18C6 (---) in a 4x 10e4 M HC104Fig. 1. Spectral changes of YbTAC (methanol solution at /J = 0.025. TAC only : 0, 2.1 x lo-’ M. C,, in procedure (a) : 1, 0.25; 2, 0.5 ; 3, 0.75; 4, 1.0x 1O-4 M. C, in procedure (b): $0.2; 6, 0.6; 7, 1.0; 8, 1.4; 9, 1.8; 10,2.6x lo-* M.
1563
Complexation of lanthanides(II1) Table 1. Conditional stability constants of Ln(TAC) complexes in methanol at 25°C and p = 0.025
Ln
(2~10-~M)
La Ce Pr Nd En DY Yb
3.71(11) 4.21(8)
Ln-TAC log&S @=W) (4~10-~M) (8~10-~M) 3.53(9) 4.06(6) 4.22( 11) 4.28(9) 4.30(10) 4.21(7) 4.73(6)
Id
Nd
4.58(7)
The numbers in parentheses are uncertainties of the last decimal place. 0
curves 5-10 in Fig. 1). The conditional stability constant at a given C,., and at the same acidity as that in the binary system, KLn,L’,which is defined as KL,,z' = bWWn'l~'l),whe~[Ln'l= iUd+[LnAl + [LnAd = CL,,- [LnL] and IJ.,] = [L] + WL] = CL - [LnL], was obtained by eq. (2) :
[Al 5 x 103
10
Fig. 2. Plots of the reciprocal conditional stability constants 1/KLo,L’ vs [18C6].
Figure 2 shows the results of the plots of eq. (4) for the Ln-TAC-18C6 systems (Ln = Ndrn, Eu”’ K Ln'L' = ~I[(&L,LCL-~){CL,-(~/ELnL)~l. (2) and Yb”‘) at [HClOJ = 4 x lop4 M. Similar linear plots were obtained for Ce”‘, Pr”’ and Dy”‘. The KLnSLvalues decreased with increasing C,. However, the acidity was too high for La”’ to obtain reliable values of KLnrL’.A lower acidity, Determination of the stability constants of Ln-18C6 [HClO,] = 2 x 10e4 M, was thus used for La”‘, and complexes the linear plot of eq. (4) was obtained. At higher K LnSL’ values obtained at constant acidity and acidity, [HClO,] = 8 x lop4 M used for Ybrrr, or a various C, can be related to the stability con- lower one, [HClO,] = 2 x lop4 M used for Ce”‘, the plots of eq. (4) were also found to be linear. The stants of the 1: 1 and 1: 2 Ln-18C6 complexes, facts indicate that these lanthanides only form the K~n~andK~ti*, where KLnA= [LnA]/([Ln][A]) and 1: 1 complex with 18C6 in the range of [18C6] used. K LnA,= [LnA,I/([Ln][A]*), as follows : The KL~L values given by the reciprocal intercept K LnTL’ = [LW([Ln'l[~I) of the plots generally agreed with,the corresponding values obtained in the binary system (Table l), but = ~~~~I/{~~~I~~+~L~~~I+~L~~~~I~~~~~~ were less accurate than the latter. Thus, we used the = KL~L'I(~+KL~A[AI+KL~A,[AI~) (3) latter value for calculation of KLnAwhich correl/KLnfL’= (~/KL,,)(~+KL~A[AI+KL~A~[AI~), (4) sponds to the product of the slope and KLnL’.The results for different lanthanides are listed in Table where [A] = CA since C, is usually in sufficient 2. By varying the acidity, the KLnAvalues for Ce”’ excess compared with CL,. According to eq. (4), a and Yb”’ remain unchanged, indicating no effect of plot of l/KL,,pL vs [A] (A : 18C6) at relatively low the hydrogen ion on complexation between Ln and values of [A] and constant acidity yields a straight 18C6. line, and KLnAas the slope/intercept ratio. When formation of LnA2 occurs with increasing [A], Dependence of the stability of Ln( 18C6) on the K can also be obtained from the slope of the atomic number of the Ianthanides pl?&cording to eq. (5), obtained by rearrangeFigure 3 shows the variation in stability constants ment of eq. (4) : of Ln-18C6 complexes (KLnA)with atomic number, {(~I~L~~L')-(~I~L~L')}/[AI together with the values reported by Izatt et al.,’ which were obtained by calorimetric titrations = (~IKL~L')(KL~A+KL~A~[AI). (5)
1564
E. OHYOSHI and S. KOHATA
Table 2. Stability constants of Ln(18C6) complexes in methanol at 25°C and p = 0.025
Ln La Ce Pr Nd ELI DY Yb
Ln-TAC-18C6 log &“A (WlO,I) (~xIO-~M) (4~10-~M) (8~10-~M) 3.89(8) 3.67(5)
3.72(6) 3.60(8) 3.40(6) 2.85(10) 2.17(6) 1.91(8)
TEAP, existing in solution, released the solvent molecules’H,O and/or CH 30H coordinated to the Ln3+ ion and favoured to react the Ln3+ ion with 18C6. This effect was con&med by the fact that the stability of Eu( 18C6) increases by increasing the ionic strength, i.e. log K,, = 2.54,2.85 and 3.04 at p = 0.013,0.025 and 0.05, respectively.
Eflect of another ligand on the selectivity of a chelating agent 1.96(11)
The numbers in parentheses are uncertainties of the last decimal place.
From eq. (4) simplified by neglecting the last term, the ratio of the conditional stability constants of the chelates for the two lanthanides (Ln, and LnJ, Ln,L and Ln,L, at a particular [A] is given by
under identical experimental conditions to this work except for the ionic strength. As is expected, the stability constants of the LnA complexes decrease through the lanthanides series despite the increase in charge density. It is noted that the values of La’II to Cd” reported by Izatt et al.’ are about one order of magnitude smaller than ours, and those after Tb”’ decrease to zero. This difference may arise from the difference in the ionic strength of the solution. The electrolyte (C2H&NC104 (TEAP) was used to adjust the ionic strength to 0.025 in our study, while they performed at p = 0.005, suggesting no supporting electrolyte being used. As Massaux et al. * pointed out, the presence of protic solvents such as water or methanol gives rise to a significant reduction of the stability of metal-macrocycle complexes because of solvation effects. It thus seems that the electrolyte
It is evident from eq. (6) that when the stability order of LnL for Ln, and Lnz is opposite to that of LnA, the ratio, KLn’+/KLniL’,becomes larger than &+%.n,~‘. This is the case for the present system, L = TAC and A = 18C6. However, the former is smaller than the latter if the stability of LnL and LnA for the two lanthanides varies in the same order. To make sure of the effect of A on the selectivity of L, we plotted the conditional stability constants of Ln-TAC complexes at [HClO,] = 4 x 10e4 M in the absence (KLnL’)and in the presence (KLnrL’)of 18C6 (C, = 1.0 x lo- 3 M) vs atomic number (Fig. 4). The stability of Ln-TAC complexes, either in the absence or presence of 18C6, increases with charge density on the cation, as is characteristic of
y” ++ik&p% Atomic Number
Fig. 3. Plots of stability constants log KLnAvs the atomic number of lanthanides : this work (0) and ref. 1 (A).
I
I
I
I
I
I
CeNd!Sm Dy Atomic
I
Yb
Number
Fig. 4. Plots of conditional stability constants log KLaL and log KLnsL' vsthe atomic number of lanthanides. LnTAC (a) and Ln-TAC-18C6(A) (C, = 1 x lop3 M) (0).
1565
Complexation of lanthanides(II1) usual chelates. However, a comparison of the two curves shows a greater slope in the ternary system than in the binary system. Consequently it is evident that 18C6 reacts competitively with the Ln3+ ion to increase the selectivity of TAC. Hence, in order to increase the separation factor of the two lanthanide chelates it is important to choose another ligand which shows an opposite coordinating trend toward lanthanides. This discussion is based on the assumption that no ternary complex, LnLA, is formed. Acknowle&ement-This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (No. 62540450).
REFERENCES 1.
R. M. Izatt, J. D. Lamb and J. J. Christensen, J. Am.
Chem. Sot. 1977,99,8344. 2. J. Massaux, J: F. Desreux, C. Delchambre and G. Duyckaerts, Inorg. Chem. 1980,19, 1893. and M-J. Schwing3. M-C. Almasio, F. Amaud-Neu Weill, Helv. Chim. Acta 1983,66, 1296. 4. V. M. Loyola, R. Pizer and G. Wilkins, J. Am. Chem. Sot. 1977,99,7185. 5. E. Ohyoshi, Analyt. Chem. 1985,57,446. 6. E. Ohyoshi, Polyhedron 1986,5,2101. 7. S. W. Thompson and R. H. Byrne, Analyt. Chem. 1988,&J, 19. 8. R. P. Lang, J. Am. Chem. Sot. 1962,84,1185.
0271-5381/89 S3.00+ .oO 0 1989 Pcrgamon Press
1989
plc
COMPLEXATION OF LANTHAND Es(III)wm MACROCYCLIC l&CROWN-6 IN METHANOL UTILIZING A COLORIMETRIC COMPLEXANT BY SPECTROPHOTOMETRY EMIKO OHYOSHI”
and SUSUMU
KOHATA
Yatsushiro National College of Technology, Hirayamashinmachi Kumamoto 866, Japan
2627, Yatsushiro,
(Received 27 June 1988 ; accepted 15 February 1989)
Abstract-A spectrophotometric procedure using a calorimetric complexant has been developed for the study of the complexation between several lanthanides(II1) (Ln”‘) and 18-crown-6 (18C6, A) in methanol. 2-(2-Thiazolylazo)-4-methylphenol (TAC, L) was used as a calorimetric complexant because of its simple structure with which no protonated complexes may be formed. The absorption spectra of the species in the Ln-TAC-l8C6 system at various concentrations of 18C6 showed no formation of the ternary complex, LnLA. The conditional stability constants of the Ln(TAC) complexes at constant acidity in the absence (KLnL’)and the presence (&,,,,_,) of 18C6 were determined by monitoring the absorbance due to the Ln(TAC) complexes. The competition between TAC and 18C6 for the metal ion resulted in a decrease in KL,,‘L’with increasing 18C6 concentration. From the dependence of l/KLnpLon the concentration of 18C6, the stability constants of the Ln( 18C6) complexes could be determined. The stability constants of these complexes were found to decrease with increasing atomic number. The results are compared with those reported previously. On the basis of the complexation trend observed, 18C6 was used as a competitive ligand to increase the selectivity of chelating agents.
Macrocyclic polyethers of the 18-crown-6 type are unique ligands because of their lower complexingability with the small lanthanides, despite their higher charge density. iv2 This trend in the lanthanides is very interesting because a reverse order has been always noted for most chelating agents. It suggests, therefore, that the ligands of the 18C6 type might be used competitively so as to increase the selectivity of the chelating agent. Since 18C6 is soluble both in aprotic and protic (methanol) solvents, it should be suitable for this purpose. Izatt et al. ’ studied the lanthanide complexation of 18C6 in methanol by calorimetric titrations and showed no formation of 18C6 complexes with any postGd”‘. However, Ahnasio et al. 3 reported that 18C6 formed very stable complexes with Dymr (1ogK = 7.90), Er”’ (1ogK = 7.67) and Yb”’ (1ogK = 7.50) in anhydrous propylene carbonate (aprotic solvent). It thus seemed that further investigations
should be made in methanol. Recently, spectrophotometric procedures”’ involving the competition of the two ligands have successfully been applied to the study of many complexes. In the present work, we have developed a similar spectrophotometric procedure to study the complexation of several lanthanides with 18C6 in methanol, and have compared the results with those reported by Izatt et al. Also, the competitive effect of 18C6 on the selectivity of chelating agents has been explored. EXPERIMENTAL Reagents
All chemicals used were of GR grade. The ligands, TAC (Dojindo Chemical Co.) and 18C6 (Aldrich Chemical Co.) were dissolved in methanol (spectroscopic grade) to prepare 1.0 x lo- 3 M and 0.20 M stock solutions, respectively. Seven types of * Author to whom correspondence should be addressed. LnCl, - nHzO [Ln = La, Ce, Pr (n = 7) ; Ln = Nd, 1561
1562
E. OHYOSHI
and S. KOHATA
Eu, Dy, Yb (n = 6)] were dissolved in methanol to give concentrations of 5-20 x lop3 M, which were standardized by EDTA titration. Perchloric acid and tetraethylammonium perchlorate were used to adjust the acidity and to control the ionic strength, respectively.
These facts indicate that the dissociation of the YbL complex only takes place in procedure (b), i.e. there is no formation of ternary complex, YbLA. Similar results were obtained for other systems of Ln-L and Ln-L-A studied. Determination of the conditional stability constants of Ln-TAC complexes
Procedure (a) The metal solution was added in small steps, 25 x low3 cm3 each time, using a microliter pipet, to 5.0 cm3 of a methanol solution containing TAC (2.0 x lo- 5 M), at a constant acidity and ionic strength, to give concentrations of 2.5-40 x lo- ’ M. (b) To the same solution, prepared by procedure (a), 18C6 solution was further added in small steps to give concentrations of 1.0-12x 10V3 M. After each addition of the metal and 18C6 solution, the absorbance due to the Ln(TAC) complex (570 nm) was measured using a Shimadzu UV-260 spectrophotometer.
The absorbance D due to the Ln-TAC complex was used to determine the conditional stability constant at a given acidity, KLnL’: D was measured at a constant acidity and various Ln concentrations in the binary Ln-TAC(L) system [procedure (a)]. The constant defined as KLnL’= [LnL]/([Ln][L’]), where [L’] = [L] +[HL] (the charges are omitted for simplicity), was determined by the method of Lang’ [eq. WI : CL&L/D
=
(CL,+CL-(D/&L~L)}(II&L~L) +(WLnL'~LnL)I9
where CL, and CL are the total concentrations of Ln and L, and sLnLis the molar absorptivity of LnL. Plots of CL&,/D vs CL,,+ CL - (D/eLnL), where eLnL is assumed to be D,,/C, (D,, is the maximum absorbance), were almost linear and show that only the 1: I complex is formed. The reciprocal of the slope of the line yielded the corrected value of aLnL which was utilized to replot the above relation. The slope/intercept ratio of the resulting plot gave the K LnL’value. The results obtained for different lanthanides are listed in Table 1. From the values obtained at different acidity for La”‘, Ce”’ and Ybru, it is found that KLnL decreases due to the side reaction, H + L = HL occurring. In the ternary Ln-TAC(L)-18C6(A) system [procedure (b)], D was observed to decrease with increasing concentration of 18C6 (C,) (see
RESULTS AND DISCUSSION Absorption spectra
Figure 1 shows the variation in absorption spectra of the TAC(L) solutions with varying Yb concentration, CYb, at [HClO,] = 4 x lop4 M and [18C6] = 0 (procedure (a), solid lines l-4), or varying the 18C6(A) concentration, C,, at [HClO,] = 4x 1O-4 M and C, = 1.0x 10m4 M (procedure (b), dotted lines 5-10). In each procedure, (a) or (b), the Yb(TAC) complex formation (A,, = 572 nm) or its dissociation occurs, respectively. It is seen that the solid and dotted curves have the same isosbestic point (488 nm). Moreover, the two curves, 2 and 8, are identical.
0
300
400
(1)
600
500
Wavelength
700
I nm
) and Yb-TAC-18C6 (---) in a 4x 10e4 M HC104Fig. 1. Spectral changes of YbTAC (methanol solution at /J = 0.025. TAC only : 0, 2.1 x lo-’ M. C,, in procedure (a) : 1, 0.25; 2, 0.5 ; 3, 0.75; 4, 1.0x 1O-4 M. C, in procedure (b): $0.2; 6, 0.6; 7, 1.0; 8, 1.4; 9, 1.8; 10,2.6x lo-* M.
1563
Complexation of lanthanides(II1) Table 1. Conditional stability constants of Ln(TAC) complexes in methanol at 25°C and p = 0.025
Ln
(2~10-~M)
La Ce Pr Nd En DY Yb
3.71(11) 4.21(8)
Ln-TAC log&S @=W) (4~10-~M) (8~10-~M) 3.53(9) 4.06(6) 4.22( 11) 4.28(9) 4.30(10) 4.21(7) 4.73(6)
Id
Nd
4.58(7)
The numbers in parentheses are uncertainties of the last decimal place. 0
curves 5-10 in Fig. 1). The conditional stability constant at a given C,., and at the same acidity as that in the binary system, KLn,L’,which is defined as KL,,z' = bWWn'l~'l),whe~[Ln'l= iUd+[LnAl + [LnAd = CL,,- [LnL] and IJ.,] = [L] + WL] = CL - [LnL], was obtained by eq. (2) :
[Al 5 x 103
10
Fig. 2. Plots of the reciprocal conditional stability constants 1/KLo,L’ vs [18C6].
Figure 2 shows the results of the plots of eq. (4) for the Ln-TAC-18C6 systems (Ln = Ndrn, Eu”’ K Ln'L' = ~I[(&L,LCL-~){CL,-(~/ELnL)~l. (2) and Yb”‘) at [HClOJ = 4 x lop4 M. Similar linear plots were obtained for Ce”‘, Pr”’ and Dy”‘. The KLnSLvalues decreased with increasing C,. However, the acidity was too high for La”’ to obtain reliable values of KLnrL’.A lower acidity, Determination of the stability constants of Ln-18C6 [HClO,] = 2 x 10e4 M, was thus used for La”‘, and complexes the linear plot of eq. (4) was obtained. At higher K LnSL’ values obtained at constant acidity and acidity, [HClO,] = 8 x lop4 M used for Ybrrr, or a various C, can be related to the stability con- lower one, [HClO,] = 2 x lop4 M used for Ce”‘, the plots of eq. (4) were also found to be linear. The stants of the 1: 1 and 1: 2 Ln-18C6 complexes, facts indicate that these lanthanides only form the K~n~andK~ti*, where KLnA= [LnA]/([Ln][A]) and 1: 1 complex with 18C6 in the range of [18C6] used. K LnA,= [LnA,I/([Ln][A]*), as follows : The KL~L values given by the reciprocal intercept K LnTL’ = [LW([Ln'l[~I) of the plots generally agreed with,the corresponding values obtained in the binary system (Table l), but = ~~~~I/{~~~I~~+~L~~~I+~L~~~~I~~~~~~ were less accurate than the latter. Thus, we used the = KL~L'I(~+KL~A[AI+KL~A,[AI~) (3) latter value for calculation of KLnAwhich correl/KLnfL’= (~/KL,,)(~+KL~A[AI+KL~A~[AI~), (4) sponds to the product of the slope and KLnL’.The results for different lanthanides are listed in Table where [A] = CA since C, is usually in sufficient 2. By varying the acidity, the KLnAvalues for Ce”’ excess compared with CL,. According to eq. (4), a and Yb”’ remain unchanged, indicating no effect of plot of l/KL,,pL vs [A] (A : 18C6) at relatively low the hydrogen ion on complexation between Ln and values of [A] and constant acidity yields a straight 18C6. line, and KLnAas the slope/intercept ratio. When formation of LnA2 occurs with increasing [A], Dependence of the stability of Ln( 18C6) on the K can also be obtained from the slope of the atomic number of the Ianthanides pl?&cording to eq. (5), obtained by rearrangeFigure 3 shows the variation in stability constants ment of eq. (4) : of Ln-18C6 complexes (KLnA)with atomic number, {(~I~L~~L')-(~I~L~L')}/[AI together with the values reported by Izatt et al.,’ which were obtained by calorimetric titrations = (~IKL~L')(KL~A+KL~A~[AI). (5)
1564
E. OHYOSHI and S. KOHATA
Table 2. Stability constants of Ln(18C6) complexes in methanol at 25°C and p = 0.025
Ln La Ce Pr Nd ELI DY Yb
Ln-TAC-18C6 log &“A (WlO,I) (~xIO-~M) (4~10-~M) (8~10-~M) 3.89(8) 3.67(5)
3.72(6) 3.60(8) 3.40(6) 2.85(10) 2.17(6) 1.91(8)
TEAP, existing in solution, released the solvent molecules’H,O and/or CH 30H coordinated to the Ln3+ ion and favoured to react the Ln3+ ion with 18C6. This effect was con&med by the fact that the stability of Eu( 18C6) increases by increasing the ionic strength, i.e. log K,, = 2.54,2.85 and 3.04 at p = 0.013,0.025 and 0.05, respectively.
Eflect of another ligand on the selectivity of a chelating agent 1.96(11)
The numbers in parentheses are uncertainties of the last decimal place.
From eq. (4) simplified by neglecting the last term, the ratio of the conditional stability constants of the chelates for the two lanthanides (Ln, and LnJ, Ln,L and Ln,L, at a particular [A] is given by
under identical experimental conditions to this work except for the ionic strength. As is expected, the stability constants of the LnA complexes decrease through the lanthanides series despite the increase in charge density. It is noted that the values of La’II to Cd” reported by Izatt et al.’ are about one order of magnitude smaller than ours, and those after Tb”’ decrease to zero. This difference may arise from the difference in the ionic strength of the solution. The electrolyte (C2H&NC104 (TEAP) was used to adjust the ionic strength to 0.025 in our study, while they performed at p = 0.005, suggesting no supporting electrolyte being used. As Massaux et al. * pointed out, the presence of protic solvents such as water or methanol gives rise to a significant reduction of the stability of metal-macrocycle complexes because of solvation effects. It thus seems that the electrolyte
It is evident from eq. (6) that when the stability order of LnL for Ln, and Lnz is opposite to that of LnA, the ratio, KLn’+/KLniL’,becomes larger than &+%.n,~‘. This is the case for the present system, L = TAC and A = 18C6. However, the former is smaller than the latter if the stability of LnL and LnA for the two lanthanides varies in the same order. To make sure of the effect of A on the selectivity of L, we plotted the conditional stability constants of Ln-TAC complexes at [HClO,] = 4 x 10e4 M in the absence (KLnL’)and in the presence (KLnrL’)of 18C6 (C, = 1.0 x lo- 3 M) vs atomic number (Fig. 4). The stability of Ln-TAC complexes, either in the absence or presence of 18C6, increases with charge density on the cation, as is characteristic of
y” ++ik&p% Atomic Number
Fig. 3. Plots of stability constants log KLnAvs the atomic number of lanthanides : this work (0) and ref. 1 (A).
I
I
I
I
I
I
CeNd!Sm Dy Atomic
I
Yb
Number
Fig. 4. Plots of conditional stability constants log KLaL and log KLnsL' vsthe atomic number of lanthanides. LnTAC (a) and Ln-TAC-18C6(A) (C, = 1 x lop3 M) (0).
1565
Complexation of lanthanides(II1) usual chelates. However, a comparison of the two curves shows a greater slope in the ternary system than in the binary system. Consequently it is evident that 18C6 reacts competitively with the Ln3+ ion to increase the selectivity of TAC. Hence, in order to increase the separation factor of the two lanthanide chelates it is important to choose another ligand which shows an opposite coordinating trend toward lanthanides. This discussion is based on the assumption that no ternary complex, LnLA, is formed. Acknowle&ement-This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (No. 62540450).
REFERENCES 1.
R. M. Izatt, J. D. Lamb and J. J. Christensen, J. Am.
Chem. Sot. 1977,99,8344. 2. J. Massaux, J: F. Desreux, C. Delchambre and G. Duyckaerts, Inorg. Chem. 1980,19, 1893. and M-J. Schwing3. M-C. Almasio, F. Amaud-Neu Weill, Helv. Chim. Acta 1983,66, 1296. 4. V. M. Loyola, R. Pizer and G. Wilkins, J. Am. Chem. Sot. 1977,99,7185. 5. E. Ohyoshi, Analyt. Chem. 1985,57,446. 6. E. Ohyoshi, Polyhedron 1986,5,2101. 7. S. W. Thompson and R. H. Byrne, Analyt. Chem. 1988,&J, 19. 8. R. P. Lang, J. Am. Chem. Sot. 1962,84,1185.