Capillary electrophoretic separation of cis/trans isomers of bis(o-diiminobenzoquinonato)platinum(II) complexes using β-cyclodextrins as the selector

3,4-Diaminobenzenesulfonate (DBS) and Pt-II form a diradical complex (Pt-II-DBS) having cis/ trans isomerism. Before separation of the isomers by using capillary electrophoresis (CE) using b-cyclodextrin (b-CD) derivatives as the selector, the inclusion behavior of PtII-DBS into b-CD and methyl-b-CD (MCD) was investigated using UV-vis and H-1 NMR measurements to confirm the formation of a 1 : 1 inclusion complex at pH 10.0 in aqueous solutions. By employing CE using an MCD-containing electrophoresis buffer, the cis and trans isomers of PtII-DBS were separated. Single crystals of trans-PtII-DBS were obtained to permit the assignment of the resolved peaks. By analyzing the dependence of the electrophoretic mobility of the isomers on the concentration of CDs in the buffer, the thermodynamic inclusion constant was determined. For example, the inclusion complex of PtII-DBS into MCD afforded K-cis = 378 +/- 2 and K-trans = 330 +/- 4 M-1 (at pH 10.0, 298 K). The difference in the Gibbs energy of inclusion, Delta Delta G = -RT ln(K-trans/K-cis), was only 0.34 kJ mol(-1), suggesting that CE can discriminate between marginal differences in energy. The comparison in the electrophoretic mobilities of the inclusion complexes indicated that the cis isomer affords a value 21% greater than that observed for the trans isomer, suggesting that cis-PtII-DBS is tightly bound to the MCD cavity and forms a more compact complex than trans-PtII-DBS. Thus, CE using CDs as the tool not only resolves the isomers but also demonstrates the inclusion manner in solution.