Stereochemistry controlled by an asymmetric sulfur atom, and a rare example of a kryptoracemate

Abstract

The ligands 4-methylthio-6-phenyl-2,2′-bipyridine (1) and the corresponding sulfoxide (2) and sulfone (3) have been synthesized and characterized in solution, and in the solid state by single crystal X-ray diffraction. Compounds 2 and 3 crystallize in the same space group (C2/c) with similar unit cell parameters; a small increase in the unit cell volume allows for the presence of the extra oxygen atom in 3. The sulfoxide and sulfone groups adopt conformations that permit intramolecular O⋯HC_aryl hydrogen bonds. The complexes [Ir(ppy)₂L][PF₆] with L = 1, 2 or 3 have been prepared and characterized. The asymmetric sulfur atom in ligand 2 gives rise to pairs of diastereoisomers of the complex which can be distinguished in the ¹H and ¹³C NMR spectra. In solution, exchange of [PF₆]⁻ by [Δ-TRISPHAT]⁻ gives rise to four diastereoisomers and we observed good dispersion of ¹H NMR resonances, especially for those assigned to protons close to the asymmetric sulfur atom. A single crystal X-ray diffraction study of 2{[Ir(ppy)₂(3)][PF₆]}·CHCl₃·3H₂O reveals that the complex crystallizes in the chiral space group P2₁2₁2₁, the asymmetric unit containing crystallographically independent Δ- and Λ-[Ir(ppy)₂(3)]⁺ cations. This provides a rare example of a so-called kryptoracemate in the solid state. In MeCN solution, [Ir(ppy)₂(1)][PF₆], [Ir(ppy)₂(2)][PF₆] and [Ir(ppy)₂(3)][PF₆] are weakly emissive (λ_em = 600, 647 and 672 nm, respectively) and preliminary studies of the electroluminescent properties of [Ir(ppy)₂(2)][PF₆] indicate that the complexes are not suitable candidates for LECs.