Chromatographic separation of stereoisomers of ligand-bridged diruthenium polypyridyl species

Abstract

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 (µ-bipym)] 4ϩ (dmbpy = 4,4Ј-dimethyl-2,2Ј-bipyridine, bipym = 2,2Ј-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 (∆∆ and ΛΛ) was subsequently chromatographically resolved on SP Sephadex C-25, relying upon the inherent chirality of the support. Optical resolutions using eluents with chiral anions [aqueous sodium (ϩ)-O,OЈ-dibenzoyl--tartrate and sodium (Ϫ)-O,OЈ-dibenzoyl--tartrate] were investigated, with the (Ϫ)-enantiomer demonstrating a positive [the (ϩ)-enantiomer a negative] synergistic effect in combination with the Sephadex support. Crystals of the meso form were isolated, allowing an X-ray structural determination.The study of polymetallic 'supramolecular' assemblies is a rapidly expanding field of chemistry, largely because of their potential application in materials for such diverse purposes as photochemical molecular devices, 1-4 and as photoprobes of structure and function of polynucleotides such as DNA. 5-9 In particular, much attention has been focused upon the polypyridyl complexes of ruthenium and osmium as the basis for such assemblies as a result of their favourable photochemical and redox characteristics. 1 When bidentate ligands are involved in such centres, stereoisomerism is inherent in the resultant assemblies, but surprisingly this problem has until recently received only tacit attention.In general, studies of chirality in octahedral tris(bidentate ligand)ruthenium() centres have been limited to mononuclear species, and isolation of enantiomers has relied on the sometimes inefficient technique of diastereoisomer formation using chiral auxiliary anions. 10-13 Such complexes can then be used as chiral building blocks for larger assemblies, 12 but this approach requires tedious synthetic procedures where care must be taken to preserve the chiral integrity at each metal centre during every reaction step. Additionally, in the vast majority of the targets studied, crystallisation has proved the most favourable method of diastereoisomeric discrimination. Our aim was to develop general chromatographic techniques for the separation of stereoisomers not only of mononuclear complexes, but also of oligomeric transition-metal assemblies containing polypyridyl ligands. In the latter case the simplest example is that of a dinuclear complex of the type [{Ru(pp) 2 } 2 (µ-L b )] nϩ [pp is a symmetrical bidentate ligand, L b is a bis(bidentate) bridging ligand]. There are the possibilities of the meso (Λ∆) and the rac diastereoisomers, the rac form consisting of an enantiomeric pair (ΛΛ and ∆∆), as illustrat...

Isolation of Enantiomers of a Range of Tris(bidentate)ruthenium(II) Species Using Chromatographic Resolution and Stereoretentive Synthetic Methods

“…The involvement of the support in ficient for resolution of complexes with increased numbers of methyl-substituted ligands, although not with the same the chromatographic resolution of dinuclear complexes has also been reported previously. [30] These simultaneous interpredicability as in the case above {e. 2ϩ may also have its origins in competing steric interactions, as de-the resolution of a particular substrate.…”

Substituted dipyridophenazine complexes of Cr(iii): Synthesis, enantiomeric resolution and binding interactions with calf thymus DNA

“…40 mg) on a C-25 column with 0.1 M sodium (-)-O,O¢-dibenzoyl-L-tartrate as the eluent at 2 ml min -1 flow rate recycled by a peristaltic pump. 32 Within a single pass down a column 1 m in length the two enantiomers were separated as distinct yellow bands. The fractions were collected separately, precipitated as PF 6 -salts by adding KPF 6 , and subsequently isolated by suction filtration or extraction into and subsequent evaporation of CH 2 Cl 2 (this latter technique resulted in lower yields due to the limited solubility of the complexes in CH 2 Cl 2 ).…”

Metal–metal interactions in dinuclear ruthenium complexes containing bridging 4,5-di(2-pyridyl)imidazolates and related ligands

“…EI mass spectrum: calc. m/z for C 19 H 14 .38 x 10 -5 mol) in ethylene glycol (2.5 mL) and 2 drops of conc. NaOH.…”