Chapter 25
Equilibrium Shift Mechanism for the Pfeiffer Effect Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 12, 2015 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch025
Stanley Kirschner, Thaddeus Gish, and Ulysses Freeman, J r . Department of Chemistry, Wayne State University, Detroit, M I 48202
The equilibria that have been proposed to exist (and to change) during the occurrence of the Pfeiffer Effect are described, along with proposals for the nature of these equilibria. In particular, the effects on the equilibria of changing concentrations and concentrations ratios of the environment substance to the racemic complex have been studied, in an effort to identify the equilibria that actually exist during the appearance of the Pfeiffer Effect.
The Pfeiffer Effect (1) is the change in optical rotation of a racemic mixture of an optically labile complex when it is placed into a solution containing one enantiomer of an optically active compound (known as the "environment substance"). For example, if an aqueous solution of /evo-malic acid is added to a solution of a racemic mixture of an optically labile complex, such as £ > , L - [ N i ( p h e n ) 3 ] C l 2 (phen = orthophenanthroline), a marked change in optical rotation of the system is observed (the "Pfeiffer Effect"), and this rotation continues to undergo change over a period of about 120 hours. It should also be noted here that if the same malic acid solution is added to a racemic mixture of an optically stable complex, e.g., [Co(en)3]Cl3 (en = emylenediamine), no such change in optical rotation is observed. The Equilibrium Displacement Mechanism Dwyer (2) and Kirschner (3) have attributed this change in rotation to a shift in the equilibrium between the dextro- and /evo-enantiomers of the complex, a shift that occurs because of the presence of an optically active "environment" (the /evo-malic acid) around the complex. Such an equilibrium shift is not possible in the case of optically stable complexes because no equilibrium exists between such enantiomers, as evidenced by the tendency of enantiomers of such complexes to resist racemization in solution for very long periods of time. Other proposals have also been put forth for the mechanism of this Effect, which are described in excellent reviews by Gillard and Williams (4) and Schipper (5), but space does not permit discussing these here.
0097-6156/94/0565-0303$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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Further, it has been proposed (6) that a "micro-mechanism" involving hydrogenbonding between the protons on the oxygen atoms of the environment substance and the π-electron clouds of the ori/io-phenanthroline ligands is responsible for this equilibrium shift because Zevo-malic acid (the S -enantiomer) fits preferentially (over Rmalic acid) into the delta propeller formed by the ligands of the (-)£> -complex. This preferential hydrogen-bonding between one enantiomer of the optically active environment substance and the π-electrons of the "propeller blades" formed by the ligands of one enantiomer of the complex inhibits the ability of that enantiomer to isomerize to the opposite enantiomer, thereby altering the equilibrium between the enantiomers, i.e., causing the equilibrium shift to occur. The Enantiomeric Equilibrium in the Presence of the Environment Substance During the work on this equilibrium displacement mechanism, a question arose about whether the shift in the equilibrium (and in the equilibrium constant for this shift) can be altered by changing the concentrations and/or concentration ratios of the complex to the environment substance (7). The equation for this originally-proposed equilibrium shift is: A(+)/> -[Ni(phen) ]2+ = A ( - ) -[Ni(phen) ]2+ (1) 3
D
3
and the equilibrium constant for this reaction is represented by the equation: Κ = [ A ( - ) -[Ni(phen) ]2+]/[ A ( + ) -[Ni(phen) ]2+] D
3
D
3
(2)
A series of Pfeiffer-active systems was prepared, using racemic -[Ni(phen) ]Cl2 (8) and Zevo-malic acid, which differ in concentrations of the complex and the environment substance as well as in the ratios of the complex to the environment substance. For each system the equilibrium shift was observed experimentally, and the equilibrium constant was calculated (9), based on the optical rotations observed for the system and corrected for the presence of the optically active environment substance. 3
The Nature of the Pfeiffer Effect Equilibrium It was noted (Table J) that the equilibrium constants calculated according to equation 2 (for the enantiomeric shift described by equation 1) were not, in fact, constant. Therefore, it is proposed that, while the equilibrium described by equation 1 can hold for solutions of racemic mixtures of optically labile complexes themselves, it does not accurately describe Pfeiffer Effect equilibria for such systems that also contain optically active environment substances. Consequently, equilibrium constant calculations were made using equations to describe the equilibria in Pfeiffer-active systems, which utilized the concentrations of the environment substance as well as those of the complex enantiomers and of the hydrogen-bonded complex enantiomers (those having environment substance enantiomers hydrogen-bonded to them). Among the equations studied for these equilibria are: A(+)D -[Ni(phen) ] + + A(-)D -[Ni(phen) ] + + 2
3
2
3
2
2 S(-)D -malic acid = 2 A(-)D -[Ni(phen) ] +.£(-)£> -malic acid 3
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
(3)
25. KIRSCHNER ET AL.
Equilibrium Shift Mechanism for the Pfeiffer Effect
305
for which: -[Ni(phen)3]2+.S(-)£> -malic acid]
2
Κ =
(4) 2+
2+
[A(+)D -[Ni(phen) ] ][A(-)Z) -[Ni(phen)3] ][£(-)D -malic acid]
2
3
and:
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A(+)D -[Ni(phen)3]
2+
+
2
A(-)D -[Ni(phen)3] + +
2 S(-)£) -malic acid = A(+)D -[Ni(phen)3] -S(-)£) -malic acid + 2+
(5)
Δ(-)Ζ) -[Ni(phen)3] -S(-)/) -malic acid 2+
for which: [Λ(+)Ζ) -[Ni(phen)3] -S(-)£> -malic acid][A(-)/) -[Ni(phen)3] -S(-)£> -malic acid] 2+
2+
Κ = 2+
2
[A(+)D -[Ni(phen)3] ][A(-)£) -[Ni(phen)3] +][S(-)/) -malic acid]
2
(6) Experimental The racem/c-[Ni(phen)3]Cl2 was prepared and resolved according to the method of Kauffman and Takahashi (8). The Pfeiffer systems were observed for optical activity using a Perkin-Elmer Model 241 Photoelectric Polarimeter. The concentrations and concentrations ratios of complex and environment substance used are indicated in the tables given below. The methods used for the calculations of the equilibrium constants are described elsewhere (9). Results and Discussion Table I shows the effects on the equilibrium constant calculated from equations 1 and 2. It should be noted that, not only does the "equilibrium constant" not remain constant as the concentrations of both the environment substance and the racemic complex increase (while keeping their ratio constant), but this "constant" also increases when the ratio of the concentrations of the environment substance to the complex increases. This implies that it may be possible for more than one molecule of the environment substance to undergo hydrogen-bonding simultaneously to one molecule of the complex, which is a matter that is currently undergoing careful scrutiny in this laboratory. For the conditions under which the Pfeiffer Effect is observed, equation 3 produces a set of equilibrium constants that are constant within a relative standard deviation of 9.5%, as calculated according to equation 4. Table II shows the concentrations of the complexes and environment substances, as well as the equilibrium constants obtained using equation 4. Further, it was observed that the relative standard deviation of the equilibrium constant calculated using equations 5 and 6 was 14.1%, which is a significantly larger deviations than that calculated using equations 3 and 4.
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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COORDINATION CHEMISTRY
Table I The Effects of Changes i n Concentrations and Concentration Ratios of Complex to Environment Substance on the Equilibria of a Pfeiffer Effect System
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a
Enantiomer Concentrations Complex : Envir.
Observed Pfeiffer Rotation ( °)
Equilibrium Constant
0.020 M : 0.040 M
-0.210
1.018
0.020 M : 0.080 M
-0.381
1.040
0.040 M : 0.020 M
-0.205
1.009
0.040 M : 0.040 M
-0.434
1.019
0.040 M : 0.080 M
-0.760
1.033
0 . 0 4 0 M : 0.160M
-1.579
1.069
a
2+
2+
For the equilibrium: Λ(+)£) -[Ni(phen)3] = Δ(-)ζ) -[Ni(phen)3] , for which the equilibrium constant: Κ = [A(-)D -[Ni(phen)3] ]/[A(-)D -[Ni(phen) ] ]; complex: Δ,Λ-(-)ΐ) -[Ni(phen)3]Cl2; environment substance: S(-)l> -malic acid; wavelength: 589 nm; temperature: 21° C. 2+
2+
3
Table Π Concentrations, Concentration Ratios of Complex to Environment Substance, and Equilibrium Constants Obtained from Equation (4) a
Enantiomer Concentrations Complex : Envir.
EquiUbrium Constant
0.020 M : 0.040 M
0.1779
0 . 0 2 0 M : 0.080M
0.1486
0 . 0 4 0 M : 0.020M
0.1694
0 . 0 4 0 M : 0.040 M
0.1917
0.040 M : 0.080 M
0.1491
0.040 M : 0.160 M
0.1659
0.050 M : 0.050 M (10)
0.1785
a
For the equilibrium given in equation 3; complex: Δ,Λ-(-)Ζ) -[Ni(phen)3]Cl2; environment substance: S(-)/) -malic acid; wavelength: 589 nm; temperature: 21° C.
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
25. KIRSCHNER ET AL.
Equilibrium Shift Mechanism for the Pfeiffer Effect 307
Conclusions
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Table I shows that, as the concentrations of the complex are held constant and the concentrations of the environment substance are increased, the magnitude of the Pfeiffer Effect increase, i.e., there is conversion of more of the A(+)-enantiomer to the A(-)-enantiomer, which is hydrogen-bonded by the environment substance. Also, as the concentration of the environment substance is held constant and the concentrations of the complex are increased, the magnitude of the Pfeiffer Effect decreases (i.e., there is a smaller conversion of the A(+)- enantiomer of to the A(-)-enantiomer). These observations support the proposed hydrogen-bonding mechanism (6) for the equilibrium displacement, since increasing the concentration of environment molecules (while keeping the concentration of complex ions constant) would be expected to result in a larger number of the 5-environment molecules becoming hydrogen-bonded to the ligands comprising the delta propeller configuration of the complex ( the preferred configuration for attachment of the S-environment substance). This would further stabilize this configuration and inhibit its return to the opposite enantiomer. Further, it is proposed that, whereas equation 1 represents the enantiomeric equilibrium for fast-racemizing complexes in the absence of an environment substance, equation 3 most accurately describes the equilibrium in the presence of such a substance. Dedication The authors are most pleased to dedicate this paper to Professor George B. Kauffman on the occasion of his receiving the American Chemical Society George C. Pimentai Award in Chemical Education, sponsored by Union Carbide Corporation. Literature Cited 1.
Pfeiffer, P.; Quehl, K. Ber. 1931, 64, 2667; 1932, 65, 560.
2.
Dwyer, F. P.; Gyarfas, E. C.; O'Dwyer, M. F., Nature 1951, 167, 1036.
3. Kirschner, S.; Serdiuk, P. In Stereochemistry of Optically Active Transition Metal Compounds; Saito, K.; Douglas, B., Eds.; American Chemical Society: Washington, DC, 1980;p239. 4.
Gillard, R. D.; Williams, P. A. Intl.Revs.in Phys. Chem. 1986, 5, 301.
5.
Schipper, P. E. J. Am. Chem. Soc. 1978, 100, 1079.
6. Kirschner, S.; Ahmad, N.; Munir, C.; Pollock, R. Pure & Appl. Chem. 1979, 51, 913. 7. Kirschner, S.; Freeman, Jr. U. Abstracts of Papers, 203rd National Meeting, American Chemical Society, San Francisco, CA, April 5-10, 1992, INOR 664. 8.
Kauffman, G. B.; Takahashi, L. T. Inorganic Syntheses 1966, 8, 227.
9.
Freeman, Jr. U.; Gish, T.; Kirschner, S. J. Indian Chem. Soc. 1992, 69, 510.
10. Ahmad, N. The Pfeiffer Effect in Transition Metal Complexes; Ph.D. Dissertation, Wayne State University: Detroit, Michigan, 1969. RECEIVED February 14, 1994
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
