Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[ n ]helicenes ( 6a – d, 10a, b, n = 4–7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2′-bipyridine ligands was then coordinated to Ru(bipy) 2 2+ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy) 2 ] 2+ ( 11a, c, L = 6a, c ) or [Ru 2 L′(bipy) 4 ] 4+ ( 12, L′ = 10b ) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.