P. Comba et A. Kuhner, Isomerism in copper(I)-induced folding of homoditopic macrocyclic ligands with bis(dithiadiimine) donor sets, EUR J INORG, (3), 1999, pp. 509-515
The structural properties of the dicopper(I) compounds of the two large mac
rocyclic Schiff base ligands para-222 and meta-222 with two dithiadiimine c
oordination 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-xyly
len-bridged para-222 and meta-222 species, respectively) are studied, in so
lution and by computer modeling. Solid-state structural data have been repo
rted 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-f
ield calculations). Both experimentally detected isomers are more than 15 k
J/mol more stable than the other five possible configurations (force-field
calculations). For the meta-222 macrocycle (30-membered ring) an achiral st
ructure has been observed in the solid while two of the 21 possible isomers
have been detected in solution (3:1, NMR spectroscopy). The computed struc
tures 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.