Long-range stereocontrol in the self-assembly of two-nanometer-dimensioned triple-stranded dinuclear helicates

A series of bisimine-bridged dicatechol ligands 2-Hr(4)-5-H-4 were synthesized and were used to prepare triple-stranded dinuclear helicate-type complexes with a length of up to more than 2 nm. X-ray structural analyses of Na-4[(2)(3)V-2], Na-4[(3)(3)Ti-2], Na-4[(4)(3)Ti-2], and Na-4[(5)(3)Ti-2], as well as temperature-dependent NMR investigations of Na-4[(4)(3)Ti-2] and Na-4[(5)(3)Ti-2] show that, in the case of the rigid linear ligands 2 and 3, and of the ligand 5, which possesses C-2h symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4, on the other hand, with idealized C-2v symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso-helicate. This is attributed to the ability of ligand 4 to adopt a less-restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na-4[(4)(3)Ti-2] invert (LambdaDelta -->DeltaLambda) simultaneously, while in the case of Na-4[(5)(3)Ti-2] a stepwise racemization proceeds.