Discriminating Effect of Adsorbed Asymmetric Film Electrode toward Electrsactive Metal Co mplex Enantio mers SIR: The planned production of differences in the chemical or physical behavior of optical enantiomers requires an interaction with an optically asymmetric environment. Diastereomer compound formation is one well known approach. Interaction with an optically active surface, without distinct diastereomer formation, is another, and optically active column substrates have served as a basis for the chromatography of enantiomers. By analogy with the latter approach, utilization of an optically active electrode surface could provide a route to the electrochemical differentiation of electroactive enantiomeric substances. The present communication describes a convenient method for preparation of a useful asymmetric electrode surface and demonstrates, with the chronopotentiometric technique, that the electrochemistry of electroactive enantiomers at such surfaces can be different, The presence of an adsorbed film on a metal electrode surface (2) is commonly observed to exert a retarding (kinetic) action on electrode processes, producing rate control, variously, by chemical reaction rates, electron transfer rates, or film penetration rates. If the adsorbed film is also optically asymmetric, it can exert a kinetic effect toward a reacting asymmetric substance of a magnitude potentially different from that exerted on the enantiomeric mirror image of the reacting substance. Thus, an optically asymmetric electrode surface providing possibilities of kinetic discrimination between reacting enantiomers can be prepared by adding to the system of enantiomers a suitable chemically- and electrochemically-inert, optically-active surfactant. Such electrodes will herein be termed adsorbed asymmetric film electrodes (AAF’E). The surfactant selected for this preliminary report is brucine, which is optically active, electrochemically inert over a wide potential span, strongly adsorbed over this entire potential region (as shown by electrocapillary data), and attains adsorption equilibrium a t a stationary electrode within a short, stirred, waiting period, The electro-
masked by the brucine catalytic hydrogen wave. By means of the dependence of the descending i ~ 1 / 2- i kinetic slopes on ligand concentration, and other data, it has been ascertained that the general electrode reaction mechanism representing the first two waves for copper tartrate and the single wave for cadmium alanate is
7.0
1 e
IO
I2
14
16
CURRENT, ja.
Figure 1 . Current-transition time behavior of first chronopotentiornetric wave for copper tartrate at the brucine-AAFE [CUI = 2.00mM pH =
[brucine] = 6.00mM
4.80
Hanging mercury drop electrode area = 0.0332 cm.2
0
0.250M d-tartrate
0 0.250M I-tartrate
active species are the enantiomer metal complexes formed in the following media: copper(I1) in tartrate medium where the tartrate ligands are exclusively either of the d- or I-form, and cadmium(I1) in basic a-alanine medium where the alanate ligands are either of the d- or I-form. Chronopotentiometric reductions of copper tartrate and cadmium alanate occur in single, diffusion-controlled waves on mercury in the absence of brucine. At the brucine-AAFE, however, copper(I1) in tartrate medium exhibits (I) three distinct waves a t -0.15, -0.42, and -0.70 volt us. S.C.E. Measuring each r from t = 0, the i+ values for the first two waves decrease with increasing applied current; the first wave also attains a limiting ir11/2 a t sufficiently high currents. The third wave represents a diffusion-controlled ~ Cadmium condition ( i ~ a l ’ constant). in alanate medium displays a single wave a t the brucine-AAFE (at about -0.95 volt), the isl/zvalue for which decreases with increasing applied current. The balance of the cadmium wave system is
For the first copper tartrate wave,
n
- m is 2 and m has tentatively also been
assigned B value of two. The stoichiometry of Reaction 1 for the second copper tartrate and the cadmium alanate waves, and also the exact mechanism by which brucine invokes kinetic control by Reaction 1, have not yet been fully clarified. The details of a partial analysis of the copper tartrate system will be discussed elsewhere (1). The chronopotentiometric behaviors of the copper tartrate and cadmium alanate d- and Z-enantiomers are identical in the absence of brucine. When brucine is present, however, measurable differences in i+l2 values are observed within each enantiomer pair. Figure 1 compares the transition time characteristics of the first wave for copper tartrate d- and 1-enantiomers; the difference shown was ascertained as being larger than the experimental error of transition time measurement. The enantiomeric difference exists only in the region of decreasing irl1l2,showing that its source is a difference in the rate with which the two enantiomers undergo Reaction 1 a t (or near) the brucineAAFE. A similar comparison of the enantiomeric kinetic behaviors of the second copper tartrate wave a t the brucine-AAFE shows that there also the I-tartrate isomer exhibits a smaller Reaction 1 rate. The d/l ratios of the i+‘ - i slopes are 1.3 and 1.7 for the first and second waves, respectively. The diffusion controlled third wave 1 / ~ in the dproduced identical i ~ ~ values and I-tartrate media which also agreed VOL. 37, NO. 13, DECEMBER 1965
1759
with the value observed in the absence of brucine. An analogous comparison, Figure 2, of the reduction of cadmium d- and 1alanate complexes a t the brucine-AAFE again reveals a difference in behavior ascribable to enantiomeric rate differences in Reaction 1, and again the disomer exhibits the faster ligand dissociation rate. The d / l ratio of i# - i kinetic slopes is 1.3. A small background wave due to adsorbed brucine a t - 1.0 volt interferes a t high currents, but contributes to the cadmium alanate transition time to a minor extent a t lower currents where the enantiomer difference is still quite apparent. Polarographic comparisons of the metal complex enantiomer systems likewise reveal asymmetric effects of brucine in that the depressions of the copper tartrate and cadmium alanate polarographic waves by brucine are more severe for the 2-isomers. Coordination of nonadsorbed brucine to the electroreducible complex ML,-, or to its kinetic precursor ML, to form mixed brucine-tartrate or brucinealanate complexes in the bulk solution constitutes a potential source of these differing behaviors of the enantiomeric complexes, inasmuch as such mixed complexes would be diastereomers rather than simple enantiomers. ii variety of chronopotentiometric evidence, however, clearly rules out this possibility. The adsorption spectrum of copper in tartrate medium is independent of brucine concentration. The value of the high current limiting i ~ (Figure ~ ~ 1) ’ for copper tartrate is independent of brucine concentration a t GmM and lower and is identical for the d- and 1isomers under all conditions. For both the copper and cadmium systems the extrapolated zero current (diffusion controlled) i ~ 1 ’ 2values are independent of brucine concentration and are within experimental error the same for the dand 1-isomers. Brucine coordination to copper should be facilitated a t higher pH, yet an increase in pH ultimately obliterates the copper enantiomer differences, In both copper and cadmium systems, a t constant brucine concentration the i ~ 1 1 2- i slope is independent of the metal/brucine ratio. It must be concluded that the observed differences do have their source in an asymmetric interaction between the metal complex enantiomers and ad-
1760
ANALYTICAL CHEMISTRY
CURRENT, +.
Figure 2. Current-transition time behavior for cadmium alanate at the brucine-AAFE [Cd] = 10.0mM [brucine] = 2.00mM pH = 9.40 Hanging mercury drop electrode area = 0.0332 cm.2 @ 0.300M d-alanate 0 0.300M I-alanate 0 no cadmium, 0.300M d- or I-alanate
sorbed brucine-Le., that the brucinetion time ratio in both copper and cadAAFE exhibits characteristics of an mium systems. These and other asasymmetric electrode surface. A definipects of enantiomer differentiation, tive explanation of the nature of these including the importance of the surfactasymmetric surface interactions must, ant type, the significance of the sign of however, await a more detailed underrotation of the reacting enantiomer, the standing of the mechanism by which basic mechanism of the differentiating adsorbed brucine invokes the kinetic effect, the use of other types of kinetic presence of Reaction 1 than has as yet effects and electrochemical techniques, been attained. and the general analytical and synthetic The absolute difference between the (destructive and preparative) aspects of transition times observed for a pair of the ilAFE, are under continuing study enantiomers can be quite dependent on ~ and will be more fully described in future the species undergoing electron transfer. reports. Larger differences in T could be produced in the cadmium alanate system owing to LITERATURE CITED an apparently very small bulk concen( 1 ) Kodama, M., Murray, R. W., ANAL. tration of the electroreducible CdAl,-, CHEM.37, 1638 (1965). species as compared to the appreciable (2) Reilley, C. N., F,tumm, W., “Progress in Polarography, Vol. I, P. Zuman, concentration of CuTz-2 in the copper I. M. Kolthoff, eds., Interscience, New tartrate system. This permits use of York, 1962. larger currents to spread the d- and 1ROYCE W. MURRAY i+iZ - i curves farther apart prior to METSUOKODAMA a t high currents. attainment of (i~l/~),,~, Department of Chemistry For example, the ratio r ( d ) / T ( E ) for the University of North Carolina cadmium alanate data of Figure 2 is 3.5 Chapel Hill, N. C. 27515 a t 90 Ma. and 13.9 a t 120 pa.; even larger RECEIVED for review July 14, 1965. Acratios could be obtained if the brucine cepted September 27, 1965. Division background wave did not become domiof Analytical Chemistry, 150th Meeting, ACS, Atlantic City, September 1965. nant a t higher currents. The solution Work supported in part by the Advanced conditions with respect to metal and Research Projects Agency (Contract SDligand concentrations, pH, and the 100) and by the Directorate of Chemical brucine surface excess are also important Sciences (Air Force Office of Scientific Research Grant No. AF-AFOSR-58464.) variables in maximizing the d / l transi-
