Chromatographic separation of stereoisomers of ligand-bridged diruthenium polypyridyl species

Cation-exchange chromatographic techniques have been developed to separate stereoisomers of polymetallic complexes, using SP Sephadex C-25 as support. Through the example of the ligand-bridged dinuclear cation [{Ru(dmbpy)(2)}(2)(mu-bipym)](4+)(dmbpy=4,4'-dimethyl-2,2'-bipyridine, bipym=2,2'-bipyrimidine), the isolation and characterisation of the, meso and rac diastereoisomers by elution with aqueous sodium toluene-4-sulfonate solution are demonstrated. The effects of variation in salt concentration and temperature on the efficacy of the separation are discussed. The enantiomeric pair of the rac diastereoisomer (Delta Delta and Lambda Lambda) was subsequently chromatographically resolved on SP Sephadex C-25, relying upon the inherent chirality of the support. Optical resolutions using eluents with chiral anions [aquecus sodium (+)-O,O'-dibenzoyl-D-tartrate and sodium (-)-O,O'-dibenzoyl-L-tartrate] were investigated, with the (-)-enantiomer demonstrating a positive [the (+)-enantiomer a negative] synergistic effect in combination with the Sephadex support. Crystals of the meso form were isolated, allowing an X-ray structural determination.