No abstract

Although polarimetric or circular dichroism measurements are of critical importance for the characterization of optically active molecules, their value as probes of enantiomeric purity is compromised by the necessity for access to data for optically pure reference samples. For transition metal complex systems, this problem has been most successfully addressed via highfield NMR spectroscopic investigations in the presence of chiral shift reagents. 1-3 However, with the increasing availability of modestly priced and user-friendly capillary electrophoresis (CE) instrumentation, an alternative direct CE method for determining enantiomeric purity offers several practical advantages over conventional NMR procedures. For example, less sample is required, no deuterated solvents are employed, and the method is not restricted to diamagnetic analytes. Despite the growing use of CE for chiral separations in the analytical field, 4-6 only a few studies have involved the separation of transition metal complex systems. Fanali et al. 7 achieved isomeric separation of some ethylenediamine/amino acid complexes of Co 3+ using sodium (S)-(+)-tartrate in the buffer, while more recently Bushey and co-workers 8 employed micellar electrokinetic chromatography to resolve enantiomers of several Fe 2+ complexes containing tridentate quinoline-type ligands. We report here the versatility and convenience of capillary electrophoresis (CE) as an alternative procedure for the determination of enantiomeric purity of a range of transition metal complexes containing R-diimine ligands.At operating voltages of 10-20 kV, injection of millimolar aqueous solutions of racemic M(R-diimine) 3 2+ species (M ) Ru 2+ , Ni 2+ , Fe 2+ ; R-diimine ) 1,10-phenanthroline or 2,2′bipyridine) into a capillary containing 25 mM phosphate buffer (pH 7) and 100 mM potassium antimonyl d-tartrate results in effective separations of the respective Λ and ∆ optical isomers. 9,10 The electropherogram of racemic Ru(phen) 3 2+ is shown in Figure 1A. 11 Excellent baseline enantiomeric separation is apparent, with the two peaks yielding identical integra-tions. The enantiomeric order of migration was established by coinjecting the racemic analyte with a sample of the ∆ isomer, which resulted in the selective growth of the peak at longer migration time. We conclude, therefore, that the ∆ isomer has the greater interaction with the antimonyl d-tartrate anion in the electrophoretic buffer. The corresponding electropherogram of a resolved sample 13 of ∆-(-) D -Ru(phen) 3 2+ (∆ 264 ) -600 M -1 cm -1 ) is provided in Figure 1B. Consistent with the prior spiking experiment, the dominant peak is observed at the longer time. From the relative areas of these two peaks, the enantiomeric purity of our ∆-(-) D -Ru(phen) 3 2+ sample is assessed to be 98.5%. It is noteworthy that the most widely reported literature circular dichroism value for ∆-(-) D -Ru(phen) 3 2+ is ∆ 264 ) -540 M -1 cm -1 , 12,14 which based on our data corresponds to an enantiomeric excess of 89%.Representative electropher...