Optical Resolution of Carboxylic Acid Derivatives of Homoleptic Cyclometalated Iridium(III) Complexes via Diastereomers Formed with Chiral Auxiliaries

We report on a facile method for the optical resolution of cyclometalated iridium­(III) (Ir­(III)) complexes via diastereomers formed with chiral auxiliaries. The racemic carboxylic acids of Ir­(III) complexes (fac-4 (fac-Ir­(ppyCO₂H)₃ (ppy: 2-phenylpyridine)), fac-6 (fac-Ir­(tpyCO₂H)₃ (tpy: 2-(4′-tolyl)­pyridine)), and fac-13 (fac-Ir­(mpiqCO₂H)₃ (mpiq: 1-(4′-methylphenyl)­isoquinoline))) were converted into the diastereomers, Δ- and Λ-forms of fac-9 (from fac-6), fac-10 (from fac-4), fac-11 (from fac-6), and fac-14 (from fac-13), respectively, by the condensation with (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-2-aminocyclohexanol. The resulting diastereomers were separated by HPLC (with a nonchiral column) or silica gel column chromatography, and their absolute stereochemistry was determined by X-ray single-crystal structure analysis and CD (circular dichroism) spectra. Spectra of all diastereomers of the Ir­(III) complexes are reported. Hydrolysis of the ester moieties of Δ- and Λ-forms of fac-10, fac-11, and fac-14 gave both enantiomers of the corresponding carboxylic acid derivatives in the optically pure forms, Δ-fac and Λ-fac-4, -6, and -13, respectively.