Application of capillary electrophoresis to stereochemical conversions. A case study: the photoracemization/hydrolysis of Cr(diimine)3 3+ species

Capillary electrophoresis (CE) is shown to be a powerful probe of the stereochemical changes accompanying photoinitiated racemization and ligand substitution of optically active Cr(diimine)3 3+ complexes (diimine=2,2′-bipyridine or 1,10-phenanthroline) in aqueous solution. With dibenzoyl-l-tartrate as a chiral additive in the CE capillary buffer, the electropherograms of photolyzed samples of Λ-Cr(diimine)3 3+ species display excellent separations of the Λ and Δ optical isomers of both the parent Cr(diimine)3 3+ complex and the hydrolysis products. Therefore, we have utilized CE analysis as a direct probe of the extent of racemization of the parent Λ-Cr(diimine)3 3+ complexes, while simultaneously determining the optical purity of the hydrolysis product. For pH 2 irradiations, racemization of both Cr(diimine)3 3+ complexes proceeds without concomitant cis-Cr(diimine)2(H2O)2 3+ formation. The corresponding data at pH 7.4 reveal that racemization and hydrolysis are concurrent processes, with formation of the hydrolysis product proceeding with complete loss of optical configuration. For photolyses at pH 11.6, extensive base hydrolysis is apparent (again with no retention of absolute configuration), but is accompanied by the absence of direct Λ-Cr(diimine)3 3+racemization. These results provide support for racemization and hydrolysis proceeding via a common intermediate.