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Figure 5.18 Molecular structures of the supramolecular aggregates {L,L-[TiMo(13)
3
]
(PNP)D,D-[TiMo(13)
3
]}
7
(left) and {D,D-[TiMo(13)
3
]NaD,D-[TiMo(13)
3
]}
7
(right).
aggregation of helicates of type [TiMo(
13
)
3
]
4
is also controlled by the nature and num-
ber of available alkali metal cations. The addition of only two equivalents of (PNP)Cl to
the methanolic solution containing the components Ti
IV
:Mo
IV
:
13
4
in the stochiometric
ratio 1 : 1 : 3, followed by 12 h of stirring yielded the compound Li
1.5
Na
0.5
(PNP)
2
[TiMo
(
13
)
3
] (Scheme 5.6). Crystals of the composition Li
1.5
Na
0.5
(PNP)
2
[TiMo(
13
)
3
]
6H
2
O
were analyzed by X-ray diffraction. This analysis revealed that the compound crystallized
in the acentric monoclinic space group,
C
2. The geometric parameters of the anions
[TiMo(
13
)
3
]
4
are essentially unchanged when compared to (PNP)
4
[TiMo(
13
)
3
]. Aggre-
gation of the helical [TiMo(
3
)
3
]
4
tetraanions in the solid state is, however, determined by
the cations present. As described above, in (PNP)
4
[TiMo(
13
)
3
], a PNP cation residing on
a crystallographic inversion center bridges two enantiomeric helicates via {Mo(bdt)
3
}/
PNP interactions, leading to {L,L-[TiMo(
13
)
3
]
D,D-[TiMo(
13
)
3
]}
7
(Figure
5.18, left). In Li
1.5
Na
0.5
(PNP)
2
[TiMo[(
13
)
3
], however, two {Ti(cat)
3
} polyhedra interact
with a sodium cation, leading to dimers of the type {D,D-[TiMo(
13
)
3
]
(PNP)
Na
D,D-[TiMo
(
13
)
3
]}
7
, where two homochiral helicates are connected via sodium
O
catecholate
inter-
actions (Figure 5.18, right).
5.4 Subcomponent Self-Assembly Reactions
Previous sections discussed “classical” self-assembly reactions of pre-designed ligands
with suitable metal ions. In spite of its successful application, this strategy is limited by
the often time-consuming synthesis of suitable ligands and their intrinsic stability. As an
alternative the “subcomponent self-assembly” methodology was recently developed. It is
based on the reversible condensation reaction of suitable amines with metal-coordinated
aldehydes under the formation of the thermodynamically most stable product [51]. While
this strategy led to impressive results such as the assembly of an unlockable-relockable
molecular cage [52a] and a self-assembled cage to encapsulate white phosphorus [52b], it
is still limited by the need for stable precursor complexes that allow a reversible imine
formation within the metal coordination sphere [52,53].
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