Optical Activity and the Pfeiffer Effect in Coordination Compounds

24 Optical Activity and the Pfeiffer Effect in Coordination Compounds S T A N L E Y K I R S C H N E R and K E N N E T H R. M A G N E L L

Downloaded by SUNY STONY BROOK on October 12, 2014 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch024

Department of Chemistry, Wayne State University, Detroit, Mich.

Since Werner's pioneering work on optical activity in complex inorganic compounds, there have been many im portant developments in the field. One of the more in teresting of these is known as the "Pfeiffer effect" which is a change in the optical rotation of a solution of an optically active substance (e.g., ammonium d-α-bromo camphor-π-sulfonate) upon the addition of solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Zn(o-phen) ](NO ) , where o-phen = ortho-phenan throline). Not all combinations of complexes, optically active "environments," and solvents show the effect, how ever, and this work attempts to apply optical rotatory dis persion techniques to the problem, as well as to determine whether solvents other than water may be used without quenching the effect. Further, the question of whether systems containing metal ions, ligands, and optically active environments other than those already used will show the effect has been studied also. 3

3

2

C i n c e Werner's early paper (19, 20) on the optical activity he postulated and discovered i n coordination compounds, there have been several discoveries closely related to this work. One of these is the observation by Pfeiffer and Quehl (15) that the optical rotation of an aqueous solu tion containing an optically active substance (e.g., ammonium d-abromocamphor-7r-sulfonate, later referred to as the optically active "en vironment") may be changed b y adding solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Zn(o-phen) ](N0 )2, where o-phen = or^io-phenanthroline). This effect has been referred to as the "Pfeiffer effect" in honor of its discoverer (3, 4) who first observed it dur ing an attempted resolution of the racemic complex mentioned above (15). 3

3

366

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

24.

KIRSCHNER AND MAGNELL

367

Optical Activity

Further studies of the effect were carried out by Pfeiffer and his co-workers (14,16), Brasted (5), Dwyer and co-workers (7), Kirschner (9), and others (10, 11). Table I shows the systems which exhibited the Pfeiffer effect. In this work the authors have attempted to expand the scope of the Pfeiffer effect to other systems and solvents and to determine unambigu ously the source of the effect. To this end they applied optical rotatory dispersion techniques as a tool i n their study.


Table I. Complex

Optically Active Compound or Ion

0

[Zn(o-phen) ](N0 ) [Zn(o-phen) ] 3

3

3

2

++

>>

[Zn(dipy) ] 3

++

[Zn(8-amq) ] + [Cd(o-phen) ]S0 [Cd(dipy) ]++ [Cd(o-phen) ](N0 ) [Ni(o-phen) ]S0 [Cd(o-phen) ]++ [Ni(o-phen) ] +

2

3

4

3

3

3

3

4

3

++

3

[Ni(dipy) ]S0 [Co(o-phen) ]S0 [Co(o-phen) ] + [Mn (o-phen) ] [Cu(edta)]" 3

4

3

4

3

3

Systems Exhibiting the Pfeiffer Effect

++

+ +

2

2

d-camphorsulfonate d-bromoeamphorsulfonate d-cinchonine hydrochloride /-strychnine sulfate d-cinchonine monochloromethylate d-quinic acid /-nicotine d-camphorsulfonate d-bromocamphorsulfonate d-bromoeamphorsulfonate d-camphorsulfonate d-bromocamphorsulfonate d-bromocamphorsulfonate d-bromocamphorsulfonate Z-nicotine c?-camphorsulfonate Z-nicotine c?-bromocamphorsulfonate c?-bromocamphorsulfonate d-bromocamphorsulfonate d-camphorsulfonate Z-quinine hydrobromide

Solvent Reference

Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water

(15) (15) (15) (15) (16) (16) (16) (15) (16)

(ID (16) (16) (16)

(14) (16)

(14) (14) (14) (14) (5) (16) (9)

Abbreviations: o-phen = orJ/io-phenanthroline; edta = ethylenediaminetetraacetate; 8-amq = 8-ammoquinoline; and dipy = 2,2 -dipyridyl. 0

/

Experimental A l l solutions were prepared from reagent grade chemicals and sol vents without further purification. Two of the new ligands which were tested for the Pfeiffer effect, 2-(2-pyridyl)-benzimidazoline and 2-(2pyridyl)-imidazoline, were prepared by the method of Walter and Freiser (18). The other ligand which had to be synthesized, (ethanediylidenetetrathio)tetraacetate ( E T T A ) , was prepared by the method of Longo et al. (12). Tris(ethylenediamine)nickel(II) chloride was prepared by the method of State (17). Bis (salicylidene) triethylenetetramine aluminum(III) iodide [A1(TS )]I was prepared by the method of Das Sarma and Bailar (6). Necessary spectra were determined on a Cary Recording Spectro photometer, M o d e l 14. Optical rotations were determined with a Perkin2


368

WERNER

CENTENNIAL

Elmer Photoelectric Polarimeter, Model 141. Optical rotatory dispersion measurements were made either with the Perkin-Elmer instrument equipped with a Bausch and Lomb High Intensity Grating Monochromator, or with a manual spectropolarimeter constructed by Kirschner and co-workers (1) at Wayne State University. This instrument has been modified by replacing the calcite polarizing and analyzing prisms with quartz prisms and changing light source to a 500-watt Hanovia Xenon Arc Lamp.


Discussion and Results The Pfeiffer Effect in Nonaqueous Solvents. ALCOHOLS. Because inner complexes are usually not soluble in water, and because the Pfeiffer effect has not yet been demonstrated to occur with an inner complex, the effect was studied in solvents other than water. One obvious choice is the lower alcohols because these solvents will dissolve most inner com plexes and are suitable for polarimetric studies. However, Landis (11) has reported that the Pfeiffer effect does not take place in methanol with tris(l,10-phenanthroline)zinc(II) ion and d-a-bromocamphor-T-sulfonate (BCS) as the optically active environment. He reports that the final solutions were cloudy; therefore, their optical rotatory properties would be difficult to study, and the question was still open. During the course of this work the authors were not able to observe the Pfeiffer effect in ethanolic solution. The optically active environments employed were d-camphor or 3-d-bromocamphor, and the complexes which

Table I I .

Systems Found Not to Exhibit the Pfeiffer Effect

Complex

Optically Active Environment

Solvent

Zn(bzac) Ni(en) Cl Zn(TTA) Ni(dipy) Cl Ni(dipy) Cl 1 NiCl :3 dipyam 1 Zn(N0 ) :2 terpy Fe(o-phen) (C10 ) Zn(Bzac) CurETTA Ni:3 pybim Cu(terpy) A1(TS ) I

d-camphor d-a-bromocamphor-7r-sulfonate d-camphor 3-d-bromocamphor d-( -)-camphoric acid 3-d-bromocamphor d-c*-bromocamphor-7r-sulfonate d-SbOtart 3-d-bromocamphor o?-a-bromocamphor-7r-sulfonate d-( — )-camphorie acid d-a-bromocamphor-7r-sulfonate d-a-bromocamphor-7r-sulfonate

Ethanol Water Ethanol Ethanol Ethanol Ethanol Water Water Ethanol Water Ethanol-Water Water Water

0

2

3

2

2

3

2

3

2

2

3

2

3

4

2

2

2

2

° Abbreviations: Bzac = benzoylacetonate; en = ethylenediamine; T T A = thenoyltrifluoroacetonate; dipy = 2,2'-dipyridyl; dipyam = 2,2'-dipyridylamine; terpy = 2,2',2"-terpyridyl; o-phen = 1,10-phenanthroline; E T T A = (ethanediylidenetetrathio)tetracetate; pybim = 2-(2-pyridyl)-benzimidazoline;" d-SbOtart = dantimonyltartrate; and T S = bis(salicyfidene)triethylene-tetramine. 2


24.


Optical

369

Activity


were used are listed in Table I I . The fact that the substances utilized for optically active environments are essentially nonionic may have been the reason for not observing the effect. Of course, it is possible that the effect does occur, but that the magnitude of the change in rotation is not large enough to be observed with the present instrumentation. A M I D E S A N D A C I D S . The authors also found that the Pfeiffer effect can be observed in both Af,iV-dimethylformamide and glacial acetic acid. Figures 1 and 2 show the effect of changing the solvent composition from pure water to either glacial acetic acid or iV,iV-dimethylformamide. These figures show that the magnitude of the effect is greatly reduced in both solvents compared with water and that the sign of the effect changes in the acetic acid-water system. Once these solvents had been established as suitable, they were applied to other systems besides the [Zn(o-phen) ](N03)2-BCS system. One of the ligands of interest is 2-(2-pyridyl)-benzimidazoline, and preliminary work showed that the ligand would not form stable aqueous solutions with zinc(II) ion. However, ligand and zinc nitrate were successfully dissolved in glacial acetic acid. Several solutions, each containing both zinc nitrate and ammonium d-bromocamphorsulfonate, were prepared and were treated with the following ligands (one to each solution): or/Ao-phenanthroline, 2,2 -dipyridyl, and 2-(2-pyridyl)benzimidazoline. The resulting solu tions were 0.01M. in zinc(II) ion and 0.03M. in ligand. Polarimetric measurements on the solutions both with and without the ligands showed that or£/io-phananthroline produced the largest Pfeiffer effect and 2,2'dipyridyl the smallest, with 2-(2-pyridyl)-benzimidazoline being between the two. 3

/

The Pfeiflfer Effect i n Aqueous Solution. During this work several additional systems which do not display the Pfeiffer effect i n water were observed. These systems are reported in Table I I . The ligands involved in these systems are: ethylenediamine, (ethanediylidenetetrathio)tetra acetate, bis(salicylidene)triethylenetetramine, and 2,2 ,2 -terpyridyl. Another ligand, 2-(2-pyridyl)-imidazoline, exhibited the Pfeiffer effect in water. The effect was observed with zinc (II) as the central metal ion and d-a-bromoeamphor-7r-sulfonate as the optically active environment. However, the effect was not observed for this ligand and nickel(II) in water, but the evidence for complex formation was not conclusive in this case. Because the Pfeiffer effect is exhibited by tris(l,10-phenanthroline)nickel(II) ion and d-a-bromocamphor-7r-sulfonate and, because the com plex has an absorption band i n the visible region, this system was studied using optical rotatory dispersion techniques. The study revealed that the optical rotatory dispersion curves showing Pfeiffer rotation vs. wave length were very similar to that of the resolved complex (Figures 3 and 4) (8). /

//


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WERNER

CENTENNIAL

The observed Pfeiffer rotation, P b . , is defined as: 0

Pobs.

B

0. O l M Z n ( N O j ) 0. 0 3 M

2

=

8

± ( a « + e

—

(1)

Ot ) 0

+

o-phen


0. 0 2 M N H 4 d - B C S

20

40

60

Volume Percent N,N-Dimethylformamide

Figure 1.

80 (in water)

The Pfeiffer effect in N\N-dimethylfarmamide

20

40 Volume P e r c e n t A c e t i c A c i d

Figure 2.

60

80 (in Water)

The Pfeiffer effect in acetic acid



371

400

450

500

1

1

550

600

WAVELENGTH (MILLIMICRONS)

Figure 3.

Comparison of the optical rotatory dispersion and the Pfeiffer rotation of [Ni(o-pheri) ]++ t

where a^. is the observed rotation of the solution containing both the environment and complex, and a is the observed rotation of the solution containing the optically active environment compound alone. The sign c

e


372

WERNER

CENTENNIAL

to use in front of the parentheses is the same as the sign of rotation of a . Calculations involving the molar rotation of the resolved complex and the observed Pfeiffer rotation showed that the equilibrium between the optical antipodes is shifted by only about two and one-half per cent (7). This indicates that the effect may not be observed in some cases where it would be expected because of very small changes in the optical rotations.


e

The Pfeiflfer Effect with Different Metal Ions. The Pfeiffer effect has been reported for complexes of zinc(II) and cadmium(II) (see Table I) but not for mercury(II). It was found during the course of this work that the effect also occurs with mercury(II). Solutions which were 0.005M in metal nitrate and 0.010M i n ammonium d-a-bromoeamphor-7r-sulfonate were studied for the Pfeiffer effect upon adding sufficient orZ/io-phenanthroline to make the solutions 0.015M in ligand. It was found that the Pfeiffer effect was largest with mercury(II) and smallest with zinc(II). Increasing the ligand :metal ratio to six increased the Pfeiffer effect with mercury(II) the most. The behavior is not unexpected when the stability constants of the three complexes are examined, log K = 5.20, 4.10, and 3.7 for Zn(II), Cd(II), and Hg(II), respectively, with 1,10-phenanthroline (2). Table III shows the results of the experiments. F r o m this table and 3

Table III. Observed Pfeiffer Rotation (Degrees) for Systems with Different Ligand: Metal Ratios at 546 Millimicrons 0

Metal Ion Zn(II) Cd(II) Hg(II)

L/M = 3 0.045 0.074 0.081

L/M = 0.056 0.075 0.109

Ligand is 1,10-phenanthroline.

Figure 5 it can be seen that changing the metal ion of the complex from Zn(II) to Cd(II) to Hg(II) enhances the observed Pfeiffer rotation. Quantitative Aspects of the Pfeiffer Effect. Kuhajek (10) formu lated an equation for calculating the magnitude of the Pfeiffer effect. I n practice this equation is difficult to handle because some of the terms are difficult to define. In order to test Kuhajek's relationship, two series of solutions were examined for the magnitude of the Pfeiffer effect. These solutions were composed of zinc (II) nitrate, 1,10-phenanthroline, and ammonium d-abromocamphor-7r-sulfonate. The first of these series had the concentra tion of the optically active environment held constant, while the concen tration of the complex (i.e., the tris-(or^o-phenanthroline) zinc(II) ion) was varied. In the second series the concentration of the complex was held constant, while that of the environment was varied. A n experiment was



24.


400

450 Wavelength

Figure 5.

373

Optical Activity

500 (Millimicrons)

550

A comparison of the Pfeiffer effect with zinc cadmium(II), and mercury (II)

600

(II),

also performed using a series similar to the first one mentioned above, but using 2,2'-dipyridyl as the ligand. The results of these are shown in Table I V . It was found that the Pfeiffer rotation is nearly linear with regard to the concentration of either of two of the major constituents of the system (the complex or the environment) if the other is held constant. This linearity holds best at low concentrations of the solutes. Since the Pfeiffer rotation is linear with respect to the concentration of each of the major constituents, it follows that it would be linear with respect to the product of the concentrations of these constituents. Figure 6 shows a plot of the Pfeiffer rotation vs. the product of the molar concen tration of tris(l,10-phenanthroline)zinc(II) ion and the molar concentra tion of ammonium d-a-bromocamphor-7r-sulfonate. As can be seen, the deviation from linearity is very slight. It should be also noted that at that high end of the curve, the relationship between the concentration of the two constituents is not so favorable as at other parts of the curve. The linearity of this plot suggests that an equation of the type utilized for the calculation of molar rotation might apply to this phenomenon also. R e writing the specific rotation equation in standard form results in the following: a = [a]-c-d

(2)

where a = observed optical rotation; [a] = specific rotation; c = concen tration of optically active (environment) in g . / m l . ; and d = path length i n decimeters.


374

WERNER

Table IV.

Variation of Pfeiffer Rotation (P b .) with Concentration 0

A.

8

[Zn(o-phen)z]++ and d-BCS


w 0.005 0.010 0.010 0.010 0.010 0.010 0.010 0.005 0.010 0.020 0.030

0.010 0.019 0.019 0.010 0.020 0.030 0.040 0.010 0.010 0.010 0.010

0.281 0.532 0.528 0.276 0.564 0.846 1.125 0.257 0.280 0.280 0.280 B.

0.010 0.010 0.010 0.010

CENTENNIAL

0.010 0.020 0.030 0.040

0.321 0.686 0.695 0.369 0.743 1.099 1.449 0.300 0.367 0.421 0.461

Px

IPM]X

0.039 0.154 0.167 0.093 0.179 0.253 0.324 0.043 0.087 0.141 0.181

7800 8100 8800 9300 8950 8450 8100 8600 8700 7050 6050

0.010 0.018 0.027 0.036

1000 900 900 900

[Zn(di y)z] and d-BCS ++

V

0.284 0.565 0.847 1.122

0.294 0.593 0.874 1.158

[c] = molar concentration of complex; [e] = molar concentration of environment; = observed optical rotation of environment (in degrees); a — observed optical rotation of environment plus complex (in degrees); and [PM] — molar Pfeiffer rotation (in degrees). a

at

e+€

The usual expression for molar optical rotation is:

m = Mty->

(3)

This can be rewritten as:

^ - wt

(4)

where [c] = molar concentration and d = path length i n meters. Since Pfeiffer rotation is also a linear function of concentration, a similar equa tion can be written to express i t : m

W

=

( 5 )

where [P]{ = specific Pfeiffer rotation at a given wavelength, X, and tem perature, t; c = concentration of complex ion in g . / m l . ; e = concentra tion of environment i n g . / m l . ; and d = cell path length in decimeters. The equation for "molar Pfeiffer rotation" ([PM]\) can now be developed analogously as follows:


24.


375

Optical Activity

where P b s . = observed Pfeiffer rotation at a wavelength, X , and tempera ture, t; [c] = molar concentration of complex ion; [e] = molar concentra tion of environment; and d = cell path length in meters. The use of the molar Pfeiffer rotation expression permits a comparison of the Pfeiffer effect i n different systems. 0


m

W

0.300

[ Z n (fi-phen) ] 3

Figure 6.

+

+

x[

d-BCS ] -

x 10

4

(molarity ) 2

Variation of Pfeiffer effect with concentration

Source of the Pfeiflfer Effect. N o completely satisfactory explana tion has yet been set forth which accounts for all of the observations asso ciated with the Pfeiffer effect. Dwyer and co-workers (7) have proposed a "configurational a c t i v i t y " explanation which states that the dextro and levo enantiomers of optically active, labile complexes in solution are i n equilibrium (with K . = 1), but that in the presence of an optically active "environment" the equilibrium shifts in favor of one of the enantiomers, resulting in a change in optical rotation. However, this proposal does not account for the fact that the effect is observed for some labile complexes and not for others. In an interesting thesis, Nordquist (13) notes some correlation between the occurrence of the effect and hydrophobic bonding, but this proposal does not account for the effect appearing in nonaqueous solvents. It is of interest to note that the effect has so far been observed only on systems containing ligands which have unsaturated ring systems. eq

Summary of New Systems Exhibiting the Pfeiflfer Effect. In the course of this work several systems were studied to determine whether they


376

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exhibited the Pfeiffer effect. Some new systems were found which do ex hibit the effect, and these are listed in Table V . Table V. Complex

Optically Active Environment

0

Solvent

DMF Acetic Acid Acetic Acid Acetic Acid Water Water Water Acetic Acid ° Abbreviations: pyim = 2-(2-pyridyl)-imidazoline; pybim = 2-(2-pyridyl)benzimidazoline; o-phen = or^/io-phenanthroline; dipy = 2,2'-dipyridyl; and D M F = N, A/-dimet hy If ormamide. [Zn(o-phen) ] [Zn(o-phen) ]+ [Zn(dipy) ] [Zn(o-phen) ] [Hg(o-phen) ]+ [Zn(o-phen) ]++ 1 Z n - 4 pyim 1 Zn+ -3 pybim 3

3

++

+

3


New Systems Exhibiting the Pfeiflfer Eflfect

++

3

3

++

+

3

++

+

rf-a-bromocamphor-7r-sulfonate d-a-bromocamphor-sulfonate d-a-bromocamphor-7r-sulfonate d-cinchonine rf-a-bromocamphor-7r-sulfonate (— )-dibenzoyl-d-tartrate rf-a-bromocamphor-7r-sulfonate rf-a-bromocamphor-7r-sulfonate

Summary of New System Found Not to Exhibit the Pfeiflfer Eflfect. During the course of this work several systems did not exhibit the Pfeiffer effect, and these are listed i n Table II. Acknowledgment The authors wish to express their sincere appreciation to the National Science Foundation for a research grant (No. NSF-GP-5399) which con tributed significantly to the progress of this work. Literature

Cited

(1) Albinak, M. J., Bhatnagar, D. C., Kirschner, S., Sonnessa, A. J., "Advances in the Chemistry of the Coordination Compounds," S. Kirschner, ed., pp. 154 ff. The Macmillan Co., Inc., New York, N. Y., 1961. (2) Anderegg, G., Helv. Chim. Acta 46, 2397 (1963). (3) Basolo, F., Pearson, R., "Mechanisms of Inorganic Reactions," p. 286, Inter science Publishers, New York, New York (1959). (4) Brasted, R., personal communication to S. Kirschner, 1966. (5) Brasted, R., Ph. D. Thesis, University of Illinois, 1942. (6) Das Sarma, B., Bailar, J. C., Jr., J. Am. Chem. Soc. 77, 5476 (1955). (7) Gyarfas, E . C., Dwyer, F. P., Rev. Pure Appl. Chem. 4 (1), 73 (1954). (8) Harkins, T. R., Jr., Walter, J. L., Harris, O. F., Freiser, H., J. Am. Chem. Soc. 78, 260 (1956). (9) Kirschner, S., J. Am. Chem. Soc. 78, 2372 (1956). (10) Kuhajek, E .J.,Ph. D. Thesis, University of Minnesota, 1962. (11) Landis, V.J.,Ph. D. Thesis, University of Minnesota, 1957. (12) Longo, F. R., Ventresca, A., Jr., Drach, J. E., McBride, J. E., Sauers, R. F., Chemist-Analyst 54, 101 (1965). (13) Nordquist, P. E. R., Jr., Ph. D. Thesis, University of Minnesota, 1964. (14) Pfeiffer, P., Nakasuka, Y., Ber. 66, 410 (1933). (15) Pfeiffer, P., Quehl, K., Ber. 64, 2667 (1931).


24.


Optical Activity

377

(16) Ibid. 65, 560 (1932). (17) State, H. M., "Inorganic Syntheses," Vol. V I , E . G. Rochow, ed., p. 200, McGraw Hill Book Co., Inc., New York, New York, 1960. (18) Walker, J. L., Freiser, H . , Anal. Chem. 26, 217 (1954). (19) Werner, Α., Ber. 44, 1887 (1911). (20) Ibid. 47, 3087 (1914).


RECEIVED July 11, 1966.


Optical Activity and the Pfeiffer Effect in Coordination Compounds

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