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complex mixture of linear and cyclic aggregates that were formed without any diaster-
eoselectivity [52].
Other approaches focussed on the synthesis and investigation of bis(bipyridine) ligands
using chiral central building blocks like trans -1,2-diaminocyclohexane ( 52 ) [53], 1,2-
diphenyl- or 1,2-dimethylethylenediamine ( 53 ) [54], 2,2 0 -dihydroxy-1,1 0 -binaphthyl
derivatives ( 36 - 39 ) [36,55], Tr
oger's base derivatives ( 29 - 33 ) [33-35], or isomannite
( 54 ) [56], which turned all out to be successful in terms of achieving at least very high
degrees of diastereoselectivity (Figures 4.16, 4.17, 4.23). Hence, this approach can be
regarded as a very general solution to the problem of controlling the stereochemistry of
oligonuclear coordination compounds as long as the ligand strands provide an appropriate
degree of rigidity and at the same time ensure a properly adjusted relative orientation of
the bipyridine units to allow them to be engaged in the formation of a tetrahedral and/or
octahedral coordination sphere around the individual metal centres.
4.7.2 2,2 0 :6 0 ,2 00 -Terpyridine and 2,2 0 :6 0 ,2 00 :6 00 ,2-Quaterpyridine Ligands
These classes of ligands were very successfully investigated by the group of E.C. Consta-
ble again preparing a number of terpyridine [57] and quaterpyridine ligands [58] that
carry terpene moieties in their periphery, which ensure diastereoselective helicate forma-
tion (Figure 4.24).
2 Cu +
55
(M)-[Cu 2 ( 55 ) 2 ] 2+
2 Cu +
56
(P)-[Cu 2 ( 56 ) 2 ] 2+
Figure 4.24 Enantiomerically pure terpyridine ligand 55 and quaterpyridine ligand 56
explored by the group of E.C. Constable that were demonstrated to undergo diastereoselective
self-assembly to optically pure double-stranded helicates (M)-[Cu 2 (55) 2 ] and (P)-
[Cu 2 (56) 2 ] upon coordination to copper(I) or silver(I) ions, respectively (X-ray crystal struc-
ture analyses, H atoms, solvent molecules, and counterions omitted).
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