Diastereoselective synthesis of bis-cyclometalated Ir(III) stereoisomers Δ/Λ-[Ir(C ∧ N) 2 (D-pro)] and Δ/Λ-[Ir-(C ∧ N) 2 (L-pro)] (where C ∧ N is 2-phenylpyridine (Hppy), (4,6difluorophenyl)pyridine (Hdfppy), and 2-phenylquinoline (Hpq) and pro is proline) with dual stereogenic centers at the metal and auxiliary ligand has been developed. The diastereomers Λ-L and Λ-D, and Δ-L and Δ-D exhibit distinguishable photophysical properties in both solution and the solid state. The thermodynamically stable diastereomers Λ-[Ir(ppy) 2 (L-pro)], Λ-[Ir(dfppy) 2 (Lpro)], and Λ-[Ir(pq) 2 (L-pro)] emit a green emission at 524 nm (Φ = 3.5% and τ = 35 ns), a blue-green emission at 480 nm (Φ = 4.5% and τ = 59 ns), and a red emission at 588 nm (Φ = 6.5% and τ = 200 ns) in DCM solution, respectively, which are blue-shifted accompanied by a large quantum yield and long lifetime relative to the corresponding unstable diastereomers Δ-[Ir(ppy) 2 (L-pro)] at 537 nm (Φ = 2.3% and τ = 29 ns), Δ-[Ir(dfppy) 2 (L-pro)] at 489 nm (Φ = 2.8% and τ = 43 ns), and Δ-[Ir(pq) 2 (L-pro)] at 591 nm (Φ = 5.4% and τ = 192 ns). Similar cases were also observed in crystals, but the signals were significantly red-shifted with respect to those in solution. Single-crystal structural analyses show that the Δ-L and Λ-D diastereomers exhibit larger interligand repulsion and loose molecular packing with respect to the Δ-D and Λ-L diastereomers, resulting in energy and photophysical property differences. In addition, the isomers with Δ and Λ configurations at the metal center exhibit positive and negative circularly polarized luminescence (CPL) signals, respectively, indicating that the effects of the chiral carbon atoms in pro ligands on CPL signals are negligible.

Chiral precursors Λ-[Ir(ppy)2(l-pro)] (Λ-L, where ppy is 2-phenylpyridine; pro is proline), Λ-[Ir(ppy)2(MeCN)2](PF6) (Λ-1), Δ-[Ir(ppy)2(d-pro)] (Δ-D), and Δ-[Ir(ppy)2(MeCN)2](PF6) (Δ-1) were synthesized from rac-[(Ir(ppy)2)2Cl2] and l-pro or d-pro by means of the auxiliary ligand strategy with 99% de values. The enantiopure mono complexes Λ/Δ-[Ir(ppy)2(L)](PF6) (L is 2,2'-bipyridine, Λ/Δ-2; L is 2,2'-dipyrimidine (dpm), Λ/Δ-3; L is 2,2'-bibenzimidazole (H2bbim), Λ/Δ-4) with 99% ee values and binuclear complexes ΛΛ/ΔΔ-[(Ir(ppy)2)2(dpm)](PF6)2 (ΛΛ-5 and ΔΔ-5) and ΛΛ/ΔΔ-[(Ir(ppy)2)2(bbim)] (ΛΛ-6 and ΔΔ-6) with 99% de values were synthesized in one step using the corresponding chiral precursors. The absolute configurations at Ir(III) centers of precursor Δ-1, mononuclear Λ-3, and binuclear ΔΔ-6 were confirmed by single-crystal structural analysis and characterized by circular dichroism (CD) spectroscopy. The correlation between the absolute configuration at Ir(III) center and CD spectra was established. The configurations at Ir(III) centers are stable during the reactions, and the chiral precursors can be used for the asymmetric synthesis of enantiomerically pure mono- and polynuclear Ir(III) complexes. Moreover, meso ΛΔ-[(Ir(ppy)2)2(dpm)](PF6)2 (meso-5) and ΛΔ-[(Ir(ppy)2)2(bbim)] (meso-6) were also synthesized using these precursors.

The optically active Ir(III) complex Λ-[Ir(ppy)(MeCN)](PF) (ppy is 2-phenylpyridine) with a chiral-at-metal was first demonstrated to preferentially react with (R)-configuration sulfoxides 2-(alkylsulfinyl)phenol (HLO-R, R = Me, Et, Pr, and Bn) rather than (S)-configuration sulfoxides under thermodynamic equilibrium due to the hydrogen-bonding interaction and the differences in the steric interference, and thus act as a highly efficient enantioreceptor for resolution of sulfoxide enatiomers. Treatment of Λ-[Ir(ppy)(MeCN)](PF) with 2 equiv of rac-HLO-R offered (S)-HLO-R in yields of 46-47% with 97-99% enantiomeric excess (ee) values and Λ-[Ir(ppy){(S)-LO-R}] complex in yields of 89-93% with 98% diastereomeric excess (de). The (R)-HLO-R chiral sulfoxides were obtained by the acidolysis of Λ-[Ir(ppy){(S)-LO-R}] complexes with trifluoroacetic acid (TFA) in the presence of coordinated solvent MeCN in yields of 45-47% with 98-99% ee values. Moreover, the enantioreceptor Λ-[Ir(ppy)(MeCN)](PF) can be recycled in a yield of 86-91% with complete retention of the configuration at metal center and can be reused in a next reaction cycle without loss of reaction activity and enantioselectivity. The absolute configurations at the metal centers and sulfur atoms were determined by X-ray crystallography.

No abstract

No abstract