Dee., 1959
2055
METALCHELATES OF MERCAPTOSUCCINIC ACID
SOME METAL CHELATES OF MERCAFTOSUCCINIC ACID1 BY GRAEME E. CHENEY, QUINTUSFERNAXDO AND HENRY FREISER Department of Chemistry, University of Pittsburgh, Pittsburgh IS, Pennsylvania Received J u l y 13, 1960
The acid dissociation constants of mercaptosuccinic acid and the stability constants of its chelates with zinc(II), cobalt(I1) and nickel(I1) have been determined. The zinc(I1) chelate is more stable than the nickel(I1) chelate; this is usually the case when a sulfur-metal bond is present in a metal chelate. Mercaptosuccinic acid, like most mercaptans, reduces copper(11) to copper(1).
This investigation was undertaken in order to obtain more information on the manner in which the mercaptide sulfur complexes with divalent metal ions. The Calvinz-Bjerruma potentiometric titration technique has been used to study the behavior of cobalt(II), nickel(II), copper(II), manganese(I1) and zinc(I1) with mercaptosuccinic acid. Previous work on the chelate stabilities of mercaptoacetic 0-mercaptopropionic acid5 and 6-mercaptopurine6 have shown that zinc(I1) is more stable than the corresponding nickel(I1) complex; in addition, these mercaptans tend to reduce copper(I1) to copper(1). As revealed by this study, mercaptosuccinic acid behaves in a similar manner toward these three metal ions. Experimental Apparatus and Materials.-The titration apparatus, and standardization of sodium hydroxide and metal perchlorate solutions have been described pre~iously.~ Potentiometric measurements of pH valucs were. made with a Beckman Model “G” pH meter equipped with an external glass-saturated calomel electrode pair and standardized with Beckman buffers a t pH 4.00 and 7.00 a t 25”. hlercaptosuccinic acid, kindly supplied by Evans Chemetics Inc., New York, was purified by extraction with ether, and dried in a vacuum desiccator. The equivalent weight found by titrating the two carboxylic acid protons was 75.2 (theoretical 75.1). Neocuproine wm obtained from G. F . Smith Chemical Co. and used without further purification. Procedure.-The general procedure employed for the de-
termination of the chelate formation constants in this work was to add 105.0 ml. of water and 5.0 ml. of metal perchlorate to a weighed amount of mercaptosuccinic acid in the titration vessel, degas the solution with prepurified nitrogen, and then titrate with standard sodium hydroxide (0.1 N ) maintaining an atmosphere of nitrogen in the titration vessel throughout the titration. The ratio of reagent to metal ion was varied from 1:1 to 6: 1 and the reported values of chelate formation constants are an average of four determinations for zinc(I1) and nickel(I1) and two for cobalt(I1). In the determination of the acid dissociation constants of mercaptosuccinic acid, 110 ml. of water was added to a weighed amount of reagent and the titration carried out with 0.1 N sodium hydroxide in a nitrogen atmosphere. Calculation of Acid Dissociation Constants.-The acid dissociation constants of mercaptosuccinic acid were calculated from the potentiometric titration data as descrihed. The equilibria involved are (1) Abstracted f r x the thesis submitted by G. E. Cheney in partial fulfillment of the requirements for the Ph.D. degree at the University of Pittsburgh, June, 1959. (2) & Calvin ‘I. and K. Wilson, J . Am. Chem. Soc., 67, 2003 (1945). (3) J. Bjerrum, “Metal Ammine Forniation in Aqueous Solution,” P . Haase and Son, Copenhagen, 1941. (4) D. L. Leussing, J . Am. Chem. Soc., 80, 4180 (1958). ( 5 ) Q. Fernando and H. Breiser, ibid., 80, 4929 (1958). (6) G. Cheney, H. Freiser and Q. Fernando, ibid., 81, 2611 (1959). (7) H.Froiser, R. G . Charles and W . D. Johnston, ibid., 74, 1383 (1952).
=
+ +
HsR H + H?R-, Z i i = [H+][HzR-]/[HaRl (1) H 2 R - Z H C HR-, Kz = [H+][HR‘]/[H2R-l (2) HR= Jr H + RE, K3 = [H+][R’]/[HR’I (3)
+
where H3R represents mercaptosuccinic acid and parentheses denote molar concentrations. In the titration of mercaptosuccinic acid with sodium hydroxide, the first two buffer regions. overlap; the third buffer region occurs a t a sufficiently hlgh.pH so that the equilibrium involving K 3may be considered independently of KI and Kz. Considering the equilibria involving only KI and KZit can be shown that
where T H ~ is R the total concentration of mercaptosuccinic acid and if ( H + N a + - OH-) = S, TH~R
+
+ H+.K1 (S E -2 ) + K1/Kz = 0 0 (H+)2s
a plot of (H+)z,fJ/(S- 2) against H+[(S - 1)(8 - 2)l will give a straight line of slope equal to KI and intercept equal to KIK2. K 3 was evaluated from the part of the neutralization curve pertaining to the species HR- in the usual manner.* Calculation of Stepwise Formation Constants.-If IC1 and Iiz are the acid dissociat,ion constants of the carboxyllc acld functionalities and Ka is that pertaining to the mercaptan dissociation in mercaptosuccinic acid, it can be shown that E, the average number of aniovs (OZ-CHS-CH~COZ)-~ bound to a divalent metal ion is given by
where T M = total metal ion concentration, T H ~ R = total mercaptosuccinic acid concentration, (J = 3TH3R H+ Na+ OH-. The stepwise formation constants for the chelates of zinc(II), nickel( 11) and cobalt(I1) were obtained by plotting 5 against - log R where R = (O~C-CHS-CHZ-CO~)-~
-
+
The values of -log R at ? =i0.5 and 1.0 gave the first stepwise formation constant as log K1 and the useful comparative constant log Knv,respectively. The second stepwise formntion constant, log Kz, was evaluated from the relation 2 log K., = log Ki log Ziz since the maximum vnlue of E obtainable below the metal hydrolysis region was 1.3.
+
Results and Discussion Titration Curves.-The titration curve of mcrcaptosuccinic acid against sodium hydroxide indicates that the two carboxylic acid groups have “overlapping” dissociation constants since there are only two buffer regions, oiie between pH 3 and 5, and the other beyond pI-1 10. Further, a strong (8) A. E. Martell and M. Calvin, “Chemistry of the Metal Chelate Compounds,” Prantice-Hall Inc., New York, N. Y., 1952.
2056
GRAEME E. CHENEY,QUINTUSFERNANDO AND HENRYFREISER
Vol. 63
inflection point, between pH 6 and 9 corresponds to the formation of the disodium salt of mercaptosuccinic acid. [Thus, the first buffer region is attributed to the two carboxylic acid groups and the second to the sulfhydryl group. The acid dissociation constants determined on this basis are given in Table I together with values for similar carboxylic acids for purposes of comparison; p K a 1 and p K a z refer to carboxylic acid group dissociations, P K a a refers to the mercaptan group dissociation. A comparison of the dissociation constant,s of the sulfhydryl groups in mercaptosuccinic, mercaptoacetic and P-mercaptopropionic acids reveals that p K a a of mercaptosuccinic acid is somewhat lower than might be expected on the basis that the dissociation involved is from a dinegatively charged anion.
lated from the values of log K1 and log K , since the maximum value of E obtainable below the metal hydrolysis region was 1.3. Formation constants of structurally similar compounds have been added for comparison. As would be expected if mercaptosuccinic acid is tridentate, there is a large difference, A, between log K1 and log K2 (Table 11) for the nickel(I1) and zinc(I1) chelates. This value is much larger than the corresponding values for the mercaptoacetic and /3-mercaptopropionic acid chelates. Further log K 1 for mercaptosuccinic acid is larger than the corresponding values for the other carboxylic acids, and since Chaberek and Martell" have suggested that aminosuccinic acid is tridentate by comparing formation constant values of it and other amino carboxylic acids with various transition metal ions, it seems probable that mercaptosuccinic TABLE I acid behaves as a tridentate ligand with nickel(I1) ACID DISSOCIATION CONSTANTSOF SOMECARBOXYLICand zinc(I1). In this connection i t is noteworthy ACIDSAT 25.0' IN WATER that Sidgwick12 has compared the relative donor pK.1
Mercaptoacetic acid' 6-Mercaptopropionicacids Mercaptosuccink acid Aminosuccinic acids Malic acid10 Succinic acid10 Refers to amino group.
3.60 4.38 3.30 1.94 3.26 4.07
pKsa
.. ..
4.94 3.70 4.68 5.28
PKS,
10.55 10.38 10.64 9.62'
... ...
The titration curves in the presence of metal ions reveal the over-all stoichiometry of the reactions between these ions and mercaptosuccinic acid. I n order to neutralize the protons released by the two carboxylic acid groups of the reagent, 6.4 ml. of 0.1 N sodium hydroxide are required; consequently, the displacement of the reagent ion curve a t 6.4 ml. of base and pH 8.5 from the reagent curve must be due to proton release, which in the absence of metal hydrolysis must depend on the reaction between the mercaptan group and the metal ion. Since nickel(I1) and zinc(I1) show an equivalent weight corresponding to one-half the atomic weight a t this pH, whereas cobalt(I1) shows an equivalent weight corresponding to its atomic weight, i t may be concluded that the nickel and zinc chelates are 2:1, mercaptosuccinic acid to metal ion, while the cobalt chelate is 1 :l. The titration curve of mercaptosuccinic acid in the presence of manganese(I1) does not correspond to a stoichiometry of either 1:l or 2:l mercaptosuccinic acid to manganese(I1). Further, the displacement from the mercaptosuccinic acid curve occurred in the metal hydrolysis region; thus, this reaction was not further considered. The titration curve with copper(I1) and mercaptosuccinic acid shows the same displacement from the reagent curve as that of the similar curves for nickel(I1) and zinc(I1) a t pH 8.5, however, this is misleading and the reaction is discussed below. Chelate Formation Constants.-Table I1 gives the stepwise formation constants for zinc(I1) and nickel(I1) with mercaptosuccinic acid, and log K1 for cobalt(I1). Values of log Ka are calcu(9) R. F. Lumb and A. E. Martell, THISJOURNAL, 57, 690 (1953). (10) El. K. Cannan and A. Kibrick, J . A m . Chem. SOC.,60, 2314
(1938).
TABLE I1 FORMATION CONSTANTS AT 25.0' IN WATER FOR MERCAPTOSUCCINIC AND STRUCTURALLY RELATEDCARBOXYLIC ACIDS
CHELATE
Metal ion
Co(I1) log Ki log Kz 2 log K a v Ni(11) log Ki log Kz 2 log Kav
,&MerAminoMercapto- capto- MerclLqto?upacetic propionic succinic ainicm acid4 acidg add acid"
6.31
12.15
... ... ...
... ...
5.90 4.28 10.18
6.98 6.55 13.51
5.21 4.39 9.60
7.97 4.90 12.87
7.12 5.27 12.39
7.86 7.18 15.04
6.75 6.05 12.80
8.47 5.28 13.75
5.84 4.31 10.15
0.43 0.68
0.82 0.70
3.07 3.19
1.85 1.53
5.84
...
Zn(11) log Ki log Ks 2 log Kav
A
log K1 - log Kz Ni( I1) Zn(I1) At 30' in water.
properties of oxygen and sulfur and in general showed that the donor properties of sulfur are dependent to a greater extent on the acceptor properties of the metal; on the other hand, according to Sidgwick, oxygen and nitrogen may be considered quite similar with respect to their tendency to complex with metal ions. Therefore, it is not surprising that the mercaptosuccinic acid-zinc(I1) chelate is more stable than the aminosuccinic acid-zinc(I1) chelate, nor is the fact that there is a reversal in order of stabilities between the corresponding nickel(1I) and zinc(I1) chelates of mercapto- and aminosuccinic acids, since zinc(I1) would appear to accept sulfur as the donor atom (11) S, Chaberek and A. E. Martell, J . A m . Chem. SOC.,74, 6021 (1952). (12) N. V. Sidgwick, J . Chem. Soc., 433 (1941). (13) 6. Chaberek and A. E. Martell, J . A m . Chem. Soe., 74, 0021 (1952).
NOTEB
Dec., 1959 in its coordination sphere more readily than would nickel(I1). It is suggested that this may be attributed to a steric effect since sulfur is larger than nitrogen. It is interesting to note that values of 2 log K a v for mercaptosuccinic acid with nickel(I1) and zinc(11) are less than the corresponding values for mercaptoacetic acid (Table 11). This is readily understandable in view of the fact that the 2:l chelate with mercaptosuccinic acid requires that there be four negative charges on it; consequently, it would be expected to be less stable, over-all, than a dinegatively charged chelate; further, the values of 2 log K a v for mercaptosuccinic acid are greater than those of P-mercaptopropionic acid. This is understandable since it has been shown6 that six-membered sulfur-containingc helate rings are much less stable than five-membered sulfurcontaining chelate rings. The Copper(I1) Reaction.-When copper(I1) was added rapidly to a solution of mercaptosuccinic acid previously degassed with nitrogen, a transient blue-purple color was observed initially, and the solution developed a yellow color when the addition of the copper(I1) was completed. The
2057
development of the yellow color indicated that copper(I1) was reduced to copper(1) and the presence of the latter in solution was confirmed by reaction with neocuproine and subsequent extraction into chloroform.I4 A consideration of the titration curve of copper(11) and mercaptosuccinic acid with sodium hydroxide shows that two protons are released from the mercaptan functionality per mole of copper(I1) present. These data would fit the overall stoichiometry represented by these equations postulated by Klots, et ~ 1 . ' ~ 2RSH'
Over-all 4RSH-
+ 2Cu++
+ +
(4) (5)
+ 2RSCu-* + 4H+
(6)
+
RSSR-4 2Cu+ 2Hf RSH- $- CU+ = RSCu+ H+
+ 2Cu++
RSSR-4
where RSH represents mercaptosuccinic acid and RSSR represents the corresponding disulfide. Acknowledgment.-The authors gratefully acknowledge the financial assistance of the U. S. Public Health Service. (14) G. H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry," John Wiley and Sons, Inc., New York, N. Y., 1957. (16) I. M. Klotz, G. H. Czerlinski and H. A. Fiess. J . Am. Chem. Soc., 80, 2920 (1958).
NOTES SEPARATION OF BORON ISOTOPES BY ION EXCHANGE BYYUKIOYONEDA, TOSHIO UCHIJIMA' AND SHOJI MAEISHIMA Department of Applied Chemistry, Faculty of Engineering, University of Tokyo, Tokyo, Japan Received January 1.4, 1968
Various methods have been tried for separating boron isotopes,2some of which are used industrially. In this work we determined the separation factor of boron isotopes in the exchange reaction between an aqueous solution of boric acid and an anion-exchange resin. Such an ion-exchange method has been used for the separation of nitrogen isotopes.8
replaced by a certain form of borate ion due to the large reaction constant. When all the OH- ion on the resin has been replaced by borate ion, boric acid solution begins to flow from the resin bed. The front boundary is somewhat diffuse because of the weak acidity of boric acid solution, even in the presence of glycerol. The amount of boric acid and the isotopic ratio of boron in each fraction of the effluent were analyzed. hotopic analyses of boron isotopes were carried out by the usual method; B203, prepared from the sample, was fluorinated wit.h CoF$ to give BFaand the ratio of boron isotopes was measured with a mass spectrometer designed for isotopic analysis.6 In Fig. 1 the atomic fraction of 'OB (N%) in successive fractions of the effluent is plotted against the sum of the
Experimental Two ion-exchange columns (about 120 cm. in height) were repared from 100-200 mesh spheres of Amberlite CG-400-1 polystyrene-quaternary amine type, strong base for chromatographic use); the ca acities of the columns were about 650 and 900 me of H2B&-, respectively. After the usual pretreatment, %e bed was restored to the OH--cycle. 15 Experimental rocedures and the method of analysis have been describeg by Speddin et a1 .a An a ueous 0.03 M HsB& solution (I) or an aqueous 0.1 M HsB8a solution containing 8 wt. % ' purified glycerol (11) 0 2 4 6 8 10 12 14 16 18 20 was passed through a resin column a t a flow rate of 0.5-0.7 Equiv. in effluent/capacity of column (%). ml./min. a t room temperature. The OH- ion is completely Fig. 1.-Atomic fraction of 'OB in effluent us. equivalents in effluent. (1) Tokai Laboratory, Japan Atomic Energy Research Inatitute,
P
.
Tokai-mura, near Mito, Japan. (2) See, 8. Makishima, Y. Yoneda and T. Tajima, T ~ r JOURNAL, s 61, 1618 (1957); or "Proc. of the Intern. Symposium on Isotope Separation." North-Holland Publ. Go., Amsterdam, 1958. (3) F. H. Spedding, J. E. Powell and H. J. Svec, J . Am. Chsm. SOC., 77, 6125 (1955).
kqo
milliequivalents of HBOa- in the effluent expressed as percentage of the column capacity. The form of the adsorbed (4) I. Kirschenbaum, U. S. Patent 2,622,014 (Dec. 16, 1952). ( 6 ) A report of this mass spectrometer will appear soon.
