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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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