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(ppyCO 2 H) 3 (ppy: 2-phenylpyridine)), fac - 6 ( fac -Ir(tpyCO 2 H) 3 (tpy: 2-(4'-tolyl)pyridine)), and fac - 13 ( fac -Ir(mpiqCO 2 H) 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 (1 R, 2 R )-1,2-diaminocyclohexane or (1 R, 2 R )-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.