Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 408

An important feature of a metallofoldamer is the conformational tunability of the
organic foldamer; most examples in the literature indeed used metal ions to modulate
the conformation of the foldamer chain. Since many transition metal ions have different
oxidation states and these different oxidation states tend to have their own preferred coor-
dination motifs, electrochemistry may be used to control the conformation of the metal-
lofoldamer by tuning the oxidation state of the metal. With such a goal in mind, Fox and
coworkers designed ligand 6 , which contains a central square planar metal-binding salo-
phen unit [30]. The rest of the foldamer backbone has hydrogen-bonding and aromatic
groups. As previously described in Chapter 9, ligand 6 was designed to fold upon binding
of metal ions. Indeed, the addition of Ni(II) and Cu(II) gave rise to the metallofoldamers
7a and 7b , respectively.
The copper complex 7b was found to go through reversible redox Cu(II)/Cu(I) changes
by cyclic voltammetry but the reactions became irreversible when 5% N , N -4-(dimethyla-
mino)-pyridine (DMAP) was added to the THF solution or when the redox was performed
in acetonitrile, a more strongly coordinating solvent. The authors proposed that the Cu(I)
complex initially generated was tetracoordinate but, in the presence of a stronger ligand
(DMAP or acetonitrile), became pentacoordinate and nonhelical. The mechanism was
supported by semiempirical (AM1) calculations. The Fox group also demonstrated in a
following work that chiral end groups and steric interactions could be used to control the
absolute helicity of a similar Ni complex ( 8 ) [31]. When the salophen group was replaced
with a chiral salen, competition came into play as the chiral end groups and the central
chiral diamine biased toward different helicity [32,33]. The chiral salen ligand, which has
extensive applications in asymmetric catalysis, turned out as only a weak director for the
absolute helicity [32].