Bimetallic iridium(III) complexes consisting of Ir(ppy)₂ units (ppy = 2-phenylpyridine) and two laterally connected N∧N chelates as bridge: Synthesis, separation, and photophysical properties

Reaction of the precursor complex Ir₂(ppy)₄Cl ₂ (ppy = 2-phenylpyridine) with the bridging ligand 3,8-dipyridyl-4,7-phenanthroline (L) affords, in 94% yield, the cyclometalated iridium dinuclear complex [(ppy)₂Ir(μ-L)Ir(ppy)₂] ²⁺ (1²⁺) as a mixture of three stereoisomers. This mixture consists of a meso form Δ,Δ and a racemic form (enantiomeric pair Δ,Δ and Λ,Λ) in the ratio 1:1.5. Single-crystal X-ray characterization of the perchlorate salt of the meso form reveals (i) the distortion of the bridging ligand from the planarity and (ii) the location of the two iridium subunits above and below the medium plane of the bridging ligand. Ion-pair chromatography with Δ-TRISPHAT anion (TRISPHAT = tris(tetrachlorobenzenediolato)phosphate(V)) as resolving anion permits the separation of the three stereoisomers. The ¹H NMR spectroscopic analysis of each fraction indicates high diastereomeric purity. Electronic circular dichroism properties and comparison with literature data establish their absolute configuration. The absorption and emission properties of the three stereoisomers show only very small variations. The anisotropic properties can be interpreted as distinct interactions of the isomers with the chiral resolving Δ-TRISPHAT anion.