Helical Structures Featuring Thiolato Donors - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 184

for the homodinuclear complexes with a weak resonance at d¼ 3.90 ppm appearing for

the heterodinuclear [NiZn(N,S^N,S) 2 ] complex. The intensity of this resonance increases
with time and becomes the dominant resonance in the spectrum after 18 h (Figure 5.20b).
Formation of the heterodinuclear complex is consistent with the MALDI MS data
recorded for this reaction and the analogous ones involving the metal exchanges of 16 or
17 with 18 .
The formation of heterodinuclear species from the reaction of two homodinuclear heli-
cates verifies that the helicates interact and exchange metals while in equilibrium and,
thereby, confirms the reversibility of the metal exchange. This reversibility represents the
essential criterion for self-assembly processes and enables the formation of the thermo-
dynamically most stable product from a number of possible aggregates.

5.5 Summary and Outlook

Multidentate ligands featuring benzene- o -dithiolato donor groups have been shown to be
suitable building blocks for the generation of metallosupramolecular assemblies. Both
symmetric bis(benzene- o -ditholato) ligands and unsymmetric benzene- o -dithiolato/cate-
cholato ligands form dinuclear double- or triple-stranded complexes with suitable metal
ions. Some analogies to homologous dicatechol ligands were observed. Heterobimetallic
complexes can be obtained from the unsymmetrical benzene- o -dithiolato/catecholato
ligand by utilizing the different binding preferences of the benzene- o -dithiolato and cat-
echolato donor groups. The C 3 -symmetrical tris(benzene- o -dithiolato) ligands yield dinu-
clear, double-stranded complexes or tetranuclear, tetrahedral clusters depending on the
spacer between the donor groups.
The coordination chemistry of the benzene- o -dithiolato ligand differs significantly
from that of the analogous catecholato donor. Besides the differences in the metric param-
eters (C OvsC S and M OvsM S bonds), benzene- o -dithiolato complexes often

feature a non-planar S M chelate ring, and the coordination geometry

around the metal center in tris(benzene- o -dithiolato) complexes can fluctuate between
OC and TP depending on the number of d-electrons at the metal center. It can be expected
that these features will lead to new and interesting coordination compounds with benzene-
o -dithiolato donor groups in the future.

C C S

References

1. (a) Lehn, J.-M., Rigault, A., Siegel, J. et al. (1987) Spontaneous assembly of double-stranded
helicates from oligobipyridine ligands and copper(I) cations: Structure of an inorganic double
helix. Proc. Natl Acad. Sci. USA , 84 , 2565-2569; (b) Kramer, R., Lehn, J.-M., and Marquis-
Rigault, A. (1993) Self-recognition in helicate self-assembly: Spontaneous formation of helical
metal complexes from mixtures of ligands and metal ions. Proc. Natl Acad. Sci. USA , 90 , 5394-
5398; (c) Hasenknopf, B., Lehn, J.-M., Baum, G., and Fenske, D. (1996) Self-assembly of a
heteroduplex helicate from two different ligand strands and Cu(II) cations. Proc. Natl Acad.
Sci. USA , 93 , 1397-1400.
2. Lehn, J.-M. (1995) Supramolecular Chemistry: Concepts and Perspectives , VCH, Weinheim.