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

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 center dot center dot HCaryl hydrogen bonds. The complexes [Ir(ppy)(2)L][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 H-1 and C-13 NMR spectra. In solution, exchange of [PF6](-)by [Delta-TRISPHAT](-) gives rise to four diastereoisomers and we observed good dispersion of H-1 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]}center dot CHCl3 center dot 3H(2)O reveals that the complex crystallizes in the chiral space group P2(1)2(1)2(1), the asymmetric unit containing crystallographically independent Delta- and Lambda-[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 (lambda(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.