Pfeiffer effect in coordination compounds

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

OPTICAL ROTATORY DISPERSION, CIRCULAR DICHROISM, AND THE PFEIFFER EFFECT IN COORDINATION COMPOUNDS... 

Cince Werner s early paper 19, 20) on the optical activity he postulated and discovered in 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 solution containing an optically active substance (e.g., ammonium d-a-bromocamphor-TT-sulfonate, later referred to as the optically active en-vironment O be changed by adding solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Zn(o-phen)3](N03)2, where o-phen = or /io-phenanthroline). This effect has been referred to as the Tfeiffer effect in honor of its discoverer 3, 4) who first observed it during an attempted resolution of the racemic complex mentioned above 15). 

Stan s some one hundred articles dealt with the synthesis, structure, stereochemistry, and biological properties of coordination compounds, including the anticancer activity of platinum complexes optical rotatory dispersion circular dichroism the Pfeiffer Effect in metal complexes inorganic nomenclature and the application of computer techniques to chemical and information problems. A prominent educator, he edited three books on inorganic and coordination chemistry. 

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]. 

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,... 

The Pfeiffer Effect (1) is defined as the change in optical rotation of an optically active system (usually a solution of one enantiomer of an optically active compound, called the "environment substance", dissolved in an optically inactive solvent) upon the addition of a racemic mixture of a dissymmetric, optically labile coordination compound. Much work has been done on this Effect (2 - 8) and several mechanisms have been proposed to explain it, which are described in a review by Schipper (2). It is of interest to note that the Effect can occur with racemic mixtures of certain optically labile complex cations (e.g., D.L-[Zn(o-phen)3]2+) whether the environment substance is anionic (d- -bromo-camphor- -sulfonate), neutral (levo-nicotine), or cationic (d-cinchoninium), The most frequently used solvent for the Pfeiffer Effect is water (10), although the Effect is known to occur in other solvents as well (l.it.6). 

During the past decade there has been a considerable resurgence of interest in the optical rotatory dispersion and circular dichroism of coordination compounds. In addition, there also has been a considerable renewal of interest in the Pfeiffer effect during this same period of time. This coincidental renewal of interest in these fields is most fortunate, because it is possible to apply optical rotatory dispersion and circular dichroism techniques to the study of the Pfeiffer effect with fruitful results vide infra). 

In 1931, Pfeiffer and Quehl observed that the optical rotation of a solution of an optically active compound (e.g., ammonium ti-a-bromo-camphor-TT-sulfonate, hereinafter called an environment compound) changes significantly upon the addition of solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Ni(o-phen)3]Cl2). Brasted and his students, and Dwyer and his co-workers have studied this effect in some detail with particular reference to the source and nature of the effect. Kirschner and his co-workers have also studied the effect and have found considerable evidence in support of the mechanism for this effect described by Dwyer and his co-workers. Kirschner and Magnell" have developed quantitative expressions for the rotation change due to the Pfeiffer effect, and have defined a positive Pfeiffer rotation as an enhancement of optical... 

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