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O
O
HN
NH
HO
OH
HO
OH
6
Ga
3+
Λ
Λ
Δ
Λ
Δ
Δ
(
Λ
,
Λ
)-[Ga
2
(6)
3
]
6-
(
Λ
,
Δ
)-[Ga
2
(6)
3
]
6-
(
Λ
,
Λ
)-[Ga
2
(6)
3
]
6-
Figure 4.3 Interconversion of stereoisomeric helicates from ligand 6.
D,D- and L,L-configured dinuclear triple-stranded gallium(III) and iron(III) helicates and
investigated the interconversion of the two enantiomeric gallium complexes by NMR
techniques (Figure 4.3).
Interestingly they found that once dinuclear helicates are formed epimerization of the
individual stereogenic metal centres happens rather through a Bailar twist mechanism
than through a dissociation-reassociation process because the mechanical coupling
between the two metal ion sites is only weak. Hence, the transition state effectively
involves the twisting of only one metal centre at a time, leading to the heterochiral
D,L-intermediate. However, the lifetime of this intermediate is short enough and its
energy high enough compared with its homochiral diastereomers to prevent its detection.
Another result of these studies was that changing the surroundings by for example low-
ering the pH value has of course a major influence on the outcome of the equilibrium-
driven processes and may also result in other reaction pathways, such as a proton-assisted
mechanism.
Another interesting study concerning the formation of two very different types of met-
allosupramolecular aggregates from a single ligand
7
was reported by J.M. Lehn and col-
leagues (Figure 4.4) [11]. They found that mixing ligand
7
with an appropriate amount of
silver(I) ions results in the simultaneous formation and interconversion of a quadruple-
stranded helicate [Ag
10
(
7
)
4
](OTf)
10
and a [4
5]-Ag(I) grid structure [Ag
20
(
7
)
9
](OTf)
20
which could both be characterized by X-ray crystallography.
Similarly, M.J. Hannon reported on bis(pyridylimine) ligands that were found to form a
very complicated and rapidly interconverting mixture of aggregates upon coordination to
silver(I) ions from which two - a triple-stranded helicate and a planar dimer - could be
characterized by X-ray crystallography [12]. These two studies might act as examples to
show that it becomes more and more difficult to design and predict the exclusive
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