Interplay between helical Δ/Λ chirality and the ligand-based asymmetric centre in cyclometalated iridium(III) complexes: Possibility for selective sensing of D-serine
Cyclometalated iridium(III) complexes can possess Δ/Λ chirality and incorporate chiral ligands, enabling diverse photochemical applications, including sensing and asymmetric catalysis. Herein, two series of chiral iridium(III) complexes with or without a butyl group in their cyclometalated ligand framework were synthesised and characterised to clarify the role of the ligand framework in modulating the effects of Δ/Λ and ligand chirality on luminescence properties. The complexes were prepared by reacting enantiopure Δ- or Λ-[Ir( n BuBQ) 2 Cl] ( 1 ; n BuBQ = monodeprotonated 2-butyl-benzo[ h ]quinoline) and racemic [Ir(HBQ) 2 (CH 3 CN) 2 ]PF 6 ( 2 ; HBQ = monodeprotonated benzo[ h ]quinoline) with d - or l -serine. Precursor 1 exhibited higher substitutional lability than precursor 2 under comparable conditions because of its coordinatively unsaturated five-coordinate structure. Each precursor yielded four enantiomerically pure stereoisomers—Δ 1 - d , Λ 1 - d , Δ 1 - l , and Λ 1 - l ( 1 -series) and Δ 2 - d , Λ 2 - d , Δ 2 - l , and Λ 2 - l ( 2 -series)—enabling the systematic isolation and comparison of all possible stereoisomers in each series. Molecular structure analysis by single-crystal X-ray crystallography and theoretical calculations suggest intramolecular interactions between the serine ligand and n BuBQ framework in the 1 -series. In dichloromethane, the diastereomeric sets in the 1 -series ( i.e. , Δ 1 - d /Λ 1 - l and Λ 1 - d /Δ 1 - l ) showed much larger differences in emission intensity and radiative rate constant than those in the 2 -series. These results suggest that the butyl group in the ligand framework influences how differences arising from the combination of Δ/Λ chirality and d / l configuration of the coordinated serine ligand are manifested in the luminescence properties of the complexes. This work may provide a useful guideline for the ligand framework design toward selective luminescence sensing of d -serine.