Helical Figure-of-Eight Loop Dicopper(I) Compounds: Syntheses, Structures, and Dynamics

The synthesis of a series of nine large macrocyclic ligands with two N2S2 (thioether and Schiff-base imine)
binding sites each, with different bridges between the donor atoms of each site (ethylene, o-xylylene, propylene,
butylene) and different spacer groups between the two binding sites (p-xylylene, 2,5-dimethyl-p-xylylene, 2,5-dimethoxy-p-xylylene), and the synthesis of a similar ligand with a preorganized double-helical geometry, based
on a paracyclophane spacer group, are reported, together with the syntheses and characterizations of the
corresponding dicopper(I) compounds. The solid state structures of the dicopper(I) complexes have two tetrahedral
copper(I) sites, separated by ca. 8 Å, and a figure-of-eight loop configuration of the ligand with a parallel
arrangement of the two substituted benzene spacer groups (benzene···benzene distance of ca. 3.5 Å). All the
dicopper(I) compounds have the same double-helical configuration (“twisted ring figure-of-eight loop”). NMR
spectroscopy indicates that the monocyclic metal-free ligands have an open, cyclic structure in solution, while
the dicopper(I) compounds are folded as in the solid. In acetonitrile there is a fast dynamic equilibrium between
two enantiomeric forms of the double-helical dicopper(I) compounds. The fact that copper(I)-donor atom bond
breaking is involved in this process is supported by 1H NMR data and by the X-ray crystal structure analysis of
a putative intermediate with each of the two copper(I) centers coordinated to one acetonitrile and three donors of
the macrocycle. A second fast dynamic, solvent independent process (epimerization) has been identified in
nitromethane and acetonitrile, involving helix inversion with full conservation of the copper(I) coordination.