Isomerism in copper(I)-induced folding of homoditopic macrocyclic ligands with bis(dithiadiimine) donor sets

The structural properties of the dicopper(I) compounds of the two large macrocyclic Schiff base ligands para-222 and meta-222 with two dithiadiimine coordination sites, where the two sites are linked by two para- or two meta-xylylene spacer groups and all bridges between the donors of each site are ethylene groups (32- and 30-membered macrocycles for the para- or meta-xylylen-bridged para-222 and meta-222 species, respectively) are studied, in solution and by computer modeling. Solid-state structural data have been reported elsewhere. The coordination of the 32-membered macrocycle para-222 to two copper(I) centers leads to a helical figure-of-eight-shaped structure. Two diastereomeric pairs of enantiomers have been observed for this type of compound in solution. The crystallized form is more stable than the second isomer by approximately 6-10 kJ/mol (solution-NMR spectroscopy and force-field calculations). Both experimentally detected isomers are more than 15 kJ/mol more stable than the other five possible configurations (force-field calculations). For the meta-222 macrocycle (30-membered ring) an achiral structure has been observed in the solid while two of the 21 possible isomers have been detected in solution (3:1, NMR spectroscopy). The computed structures and isomer distributions (force-field calculations) are compared with the solid-state structures and with observed isomer distributions from NMR experiments, and dynamic processes are analyzed in detail.