Isomerism in Copper(I)-Induced Folding of Homoditopic Macrocyclic Ligands with Bis(dithiadiimine) Donor Sets

Abstract

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.