Simple Coordination Complexes of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 13

Fig. 2.6 X-ray crystal structures of molecular capsules of CyH 5 Q[5] with alkali metal ions:
a Li

+ + + + +

; b Na ; c K ; d Rb ; and e Cs

of assemblies obtained in the absence of such agents. For example, we cannot
obtain all complexes of CyH 5 Q[5] with all alkali metal ions by simple combina-
tion of CyH 5 Q[5] with alkali metal salts. However, in the presence of ZnCl 2 in an
acidic medium (HCl), the formed [ZnCl 4 ] 2 − anions behave as “structure direct-
ing agent”, and facilitate the formation of all five CyH 5 Q[5]/M alkali complexes
(Fig. 2.6 a-e). Among these complexes are those with the smallest (Li + , 0.59 Å)
and largest (Cs + , 1.67 Å) metal radii (except Fr + , 1.80 Å) [ 23 ]. All four alkali
metal ions, Na + (0.97 Å), K + (1.33 Å), Rb + (1.43 Å), and Cs + (1.67 Å) which
have larger metal radii, coordinate to the five carbonyl oxygen atoms and fully
cover the portals of the CyH 5 Q[5] molecules. An exception to this trend is the
Li + cation, which has the smallest metal radius and can only coordinate to two
portal carbonyl oxygen atoms. The CyH 5 Q[5]-based complexes with the fully
capped portals initially appear to crystallize with similar molecular structures,
such as opened molecular capsules or molecular capsules. The distances of the
capped metal ions to the mean planes of the five portal carbonyl oxygens appear to
increase with increasing atomic numbers of the alkali metals. But a close inspec-
tion shows that the capsules contain different core molecules: a chloride anion
for the CyH 5 Q[5]/Na + and CyH 5 Q[5]/K + complexes, a water molecule for the
CyH 5 Q[5]/Rb + complex, and none for the CyH 5 Q[5]/Cs + complex. Additionally,
the interaction between the capped alkali metal ions and the included ion or mol-
ecule could affect the distances of the capped metal ions from the portal plane.
An example of the Me 10 Q[5]-M alkali system can show this trend better [ 24 ].
Figure 2.7 shows four Me 10 Q[5]/M alkali molecular bowls or capsules obtained by
reactions of Me 10 Q[5] with the corresponding alkali metal salts (NaCl, KCl, RbCl,
and CsCl) in the presence of hydroquinone (Hyq) as a structure directing agent.