Size complementarity

Indeed, macrobicyclic ligands such as 7-9 form cryptates [Mn+ c cryptand], 10, by inclusion of a metal cation inside the molecule [1.26, 1.27, 2.17, 2.24-2.26]. The optimal cryptates of AC and AEC have stabilities several orders of magnitude higher than those of either the natural or synthetic macrocyclic ligands. They show pronounced selectivity as a function of the size complementarity between the cation... 

The first problems with the new anion-tcmplated rouuane synthesis were encountered when the preparation of rotaxanes with smaller 3,5-dw-butyl phenol stoppers was attempted (Scheme 17). These rot xanca were intended for the investigation of their deslipping behavior 9 48 which should provide insight into the size complementarity of the stopper mid the wheel cavity. For this purpose, smaller stoppers are needed and a yield of 2% to 5%, which was obtained for 44 independent of the length of the axle centerpiece, was thus quite disappointing. 

According to the CPK-model examination, the distance (ca. 8.4 A) of the two crown moieties in the structure of 40 fits the length between two carboxylates in relaxing BzGlu (ca. 8.6 A) better than that of BzAsp (ca. 7.2 A). This size complementarity certainly agrees with the transport rate of BzGlu which is... 

Successful anion complexation was also achieved with larger macrobicyclic ligands. Thus, the macrobicycle 17 (Fig. 17). SI binds halide anions and the linear triatomic azide anion with a very high stability constant. The x-ray structure revealed an efficient shape and size complementarity between N3 and the cavity. A whole series of polyazacryptands was obtained in high yields by a simple Schiff base [2 + 3] condensation between the triaminotriethylamine (tren) and various dicarboxalde-hydes. The hexaimine macrobicycles prepared by this way lead, after reduction, to the polyaza cryptands, which in... 

Figure 2. Complementarity in complexation a) stereoelectronic and conformational sh complementarity b) stereoelectronic and conformational shape noncomplementarity c) size complementarity d) charge complementarity e) Bond complementarity.

Figure 3. Chelate effect and size complementarity in linear polydentate ligands.

Increasing strain energy produces a next level of non-complementarity, in which the conformation changes to a shape that is less sensitive to the hole-size/metal ion-size complementarity the most familar structure involving the extrusion of the metal ion from the plane of the ring so that the macrocycle is somewhere between a belt, shifted... 

Figure 5. Variation in size complementarity of chelate rings in fused ring structures ...

Recently, a third process has been identified which constitutes an example of strict self-assembly.The so-called slippage approach to rotaxane synthesis Figure 5) is based upon size complementarity between the macrocyclic polyether, BPP34C10, containing two 7i-electron rich recognition sites and the stoppers comprised of suitable tetra-arylmethane groups at the ends of dumbbell-shaped molecules incorporating at least one 4,4 -bipyridinium unit. ... 

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