Structural Aspects of Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 142

59

( R )- 58

( R , R )- 57

Figure 4.25 Chiral 2-pyridylimine ligands prepared by L. Fabrizzi, M.J. Hannon, and S.G.
Telfer and R. Kuroda that undergo diastereoselective self-assembly to optically pure double-
stranded helicates upon coordination to copper(I) or silver(I) ions, respectively.

4.7.3 2-Pyridylimine Ligands

The use of this ligand motif in helicate chemistry dates back to the very first examples of
G. van Koten [( R , S )- 3 ] in the early 1980s (see Figure 4.3) [4].
Later, chiral versions [( R , R )- 3 and ( S , S )- 3 ] of the initially prepared meso -ligand [( R ,S)-
3 ] were prepared by T.D.P. Stack to demonstrate the phenomenon of self-recognition (see
Figure 4.15) [32]. Varieties of this theme were then explored by the groups of L. Fabrizzi
( 57 ) [59], M.J. Hannon ( 58 ) [60], S.G. Telfer and R. Kuroda ( 59 ) [61], and ourselves (see
ligand 28 in Figure 4.16) [33]. An overview of these ligands is shown in Figure 4.25.
Again, either a rigid central chiral building block was introduced into the ligand
structure to ensure diastereoselectivity, or several stereogenic centres were placed
close to the chelating unit in systems with larger, more flexible linkers, similar to
J.M. Lehn's approach.

4.7.4 Further Hexadentate N-Donor Ligands

In addition to the well known iminopyridines, bi-, ter-, and quaterpyridines, there were
also two other reports on the use of enantiomerically pure hexadentate N -donor ligands
for the diastereoselective self-assembly of helicates. Whereas the pinene-derived ligand
60 of A. von Zelewsky was demonstrated to form an optically pure tetranuclear circular
helicate upon coordination to zinc(II) ions [62], the cyclohexanediamine-derived ligand
61 of E.C. Constable was found to form dinuclear helicates [63]. Interestingly, the forma-
tion of [M 2 61 2 ] nþ complexes was observed not only with iron(II) and zinc(II) ions as
expected, but also with silver(I) ions, which were found to be coordinated by a rather
unusually high number of donor atoms, maybe also including an attractive Ag-Ag inter-
action (Figure 4.26).

4.7.5 Oxazoline Ligands
Oxazolines are also powerful ligands that can easily be prepared in optically pure forms
by the condensation of appropriate aldehydes and chiral aminoalcohols derived from,
for example, a-aminoacids. Using such ligands 62 and 63 derived from phenylalanine
and phenylglycine, A.F. Williams was the first to show that they give rise to linear or