Clay column chromatography for optical resolution: separation and asymmetric induction of cyclometalated iridium(III) complexes

Abstract Chromatographic separation was attempted by eluting iridium(III) complexes on a high-performance liquid chromatography (HPLC) column packed with an ion-exchange adduct of synthetic hectorite and Δ-tris(1,10-phenanthroline)ruthenium(II). Affinity was adjusted by altering the loading of Δ-[Ru(phen)₃]²⁺ in the hectorite. Seven kinds of neutral iridium(III) complexes were resolved, with two peaks assigned to the Δ- and Λ- enantiomers. Three kinds of positively charged iridium(III) complexes were eluted, yielding Δ-enantiomers with optical purities of 60% to 72% as less strongly adsorbed isomers. The opposite enantiomers were too firmly adsorbed for elution. When a five-coordinated iridium(III) complex (trans-[Ir(iPrBQ)₂Cl]) (iPrBQ = mono-deprotonated 2-isopropylbenzo[h]quinoline) was injected, the entire complex was adsorbed and could not be eluted. Adding a negatively charged chelating ligand (disodium salt of 4,7-diphenylphenanthroline disulfate, Na₂BPS) eluted a negatively charged six-coordinated complex (trans-[Ir(iPrBQ)₂BPS]−). The transformation from Δ- or Λ-trans-[Ir(iPrBQ)₂Cl] to trans-[Ir(iPrBQ)₂BPS]− occurred in the column. Notably, the eluted complex exclusively took the Δ-conformation from their ECD spectra. This is the first example of the stereoselective transformation of a coordinatively robust iridium(III) complex by passing it through a short column of 1 cm in length. The column uniquely afforded enantiomerically neutral and positively and negatively charged iridium(III) complexes.