Pfeiffer-effect

Pfeiffer effect The change in rotation of a solution of an optically active substance on the addition of a racemic mixture of an asymmetric compound. 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

The Pfeiffer effect is a term used to describe changes in the optical activity of solutions containing a chiral compound (the environmental substance ) on the addition of a racemic dissymmetric complex. The effect is generally attributed to a shift in the position of the equilibrium between d and l isomers for the racemic complex. The exact mechanism involved in mediating the chiral interaction is unknown. Perhaps surprisingly, both environmental substance and complex may simultaneously be cations. Studies of the Pfeiffer effect usually involve a moderately labile racemic complex [Cr(ox)3]3 is a popular choice for such studies, summarized in Table 82. Other studies of the optical activity of tris oxalates include work on photoinduced optical activity,898 photoracemization899 and the solid-state racemization of K3[Cr(ox)3]. 900 901... 

Paul Pfeiffer discovered a very interesting stereochemical phenomenon, which now bears his name — the Pfeiffer effect this has received a good deal of attention.30 When an optically active substance which is stable in solution is added to a solution of a labile chiral substance, the optical rotation of the solution changes, reaching a new level in some hours. Several theories have been advanced to explain the phenomenon, the most satisfactory based on the supposition that the optically active ion or molecule forms an association with one isomer of the racemic pair of the labile substance and thus shifts the dextro—levo equilibrium. In general it is not possible to use this as a means of resolution, for when the added optically active substance is removed from the labile material, the latter immediately racemizes. 

The basis of the method is akin to the Pfeiffer effect [8] except that, in this instance, the roles of the ligands are reversed and reorganization of the inner sphere and not the outer sphere of the metal is intimately involved. The racemate originates in the solution environment and the enantiomer is part of the coordination compound (vide infra). Calculation of the enantioexcess is most easily done using spectral differences. Figure 5 shows the CD spectrum for the parent complex (lowest curve) where M is Cu(II) and L is L-tartrate in strong base together with a series of curves in which the L-pseudoephedrine concentration has been systematically increased. An isosbestic point at 538 nm is obvious [51]. 

Optical Activity and the Pfeiffer Effect in Coordination Compounds... 

Since Werner s pioneering work on optical activity in complex inorganic compounds there have been many important developments in the field. One of the more interesting 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-a-bromo-camphor-T-sulfonate) upon the addition of solutions of racemic mixtures of certain coordination compounds (e,g, D,L-[Zn o-phen)z](NOz)2, where o-phen = ortho-phenan-throline). Not all combinations of complexes, optically active environments and solvents show the effect, however, and this work attempts to apply optical rotatory dispersion 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,... 

Further studies of the effect were carried out by Pfeiffer and his co-workers 14f 16)y Brasted (5), Dwyer and co-workers (7), Kirschner (P), and others 10y 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 unambiguously the source of the effect. To this end they applied optical rotatory dispersion techniques as a tool in their study. 

All solutions were prepared from reagent grade chemicals and solvents without further purification. Two of the new ligands which were tested for the Pfeiffer effect, 2-(2-pyridyl)-benzimidazoline and 2-(2-pyridyl)-imidazoline, were prepared by the method of Walter and Freiser 18). The other ligand which had to be synthesized, (ethanediylidenetetra-thio)tetraacetate (ETTA), 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 alumi-num(III) iodide [A1(TS2)]I was prepared by the method of Das Sarma and Bailar 6). 

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 complexes 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-7r-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 II. Systems Found Not to Exhibit the Pfeiffer Effect...

The PfeiflFer Effect in Aqueous Solution. During this work several additional systems which do not display the Pfeiffer effect in water were observed. These systems are reported in Table II. 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-bromocamphor-TT-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 complex has an absorption band in 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. wavelength were very similar to that of the resolved complex (Figures 3 and... 

Quantitative Aspects of the Pfeiffer Effect. Kuhajek 10) formulated an equation for calculating the magnitude of the Pfeiffer effect. In 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-a-bromocamphor-TT-sulfonate. The first of these series had the concentration of the optically active environment held constant, while the concentration of the complex (i.e., the tris-(or /io-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. An experiment was... 

Figure 5. A comparison of the Pfeiffer effect with zinc (II), cadmium(II), and mercury (II)...

Source of the Pfeiffer Effect. No completely satisfactory explanation has yet been set forth which accounts for all of the observations associated with the Pfeiffer effect. Dwyer and co-workers (7) have proposed a configurational activity explanation which states that the dextro and levo enantiomers of optically active, labile complexes in solution are in equilibrium (with Keq. = 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. 

During the course of this work several systems did not exhibit the Pfeiffer effect, and these are listed in Table II. 

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