Regio- and diastereo-selective formation of dicopper(I) and disilver(I) double helicates with chiral 6-substituted 2,2′∶6′,2″-terpyridines

Two enantiomeric pairs of chiral terpy ligands (I and II; III and IV) bearing enantiopure bornyloxy substituents at the 6-position were prepared in high yield, stereoretentive reactions from (1R)-endo- or (1S)-endo-borneol; compounds I, II and III were structurally characterised. Dinuclear double helicates were formed upon reaction with copper(I) salts, but solvent-dependent and reversible formation of mononuclear or dinuclear double-helical complexes was observed with silver(I) salts. The double helicates are formed with good to excellent diastereoselectivity for helical chirality. With these 6-substituted ligands, double helicates can exist as head-to-head (HH) or head-to-tail (HT) isomers; in solution, the HT isomers are favored, although solid state interactions can overcome this preference.