Simple Coordination Complexes of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 20

be controlled by pH-dependent complexation and decomplexation of metal ions at
the portals of the Q[6] molecule (Fig. 2.13 d). Their observations suggest that the
sodium ion “lids” are removed in strongly acidic solution because of the protona-
tion of the carbonyl groups at the cucurbituril portals; consequently, the encapsu-
lated THF molecule can readily escape from the cavity (Fig. 2.13 ). Raising the pH
of the solution restores the sodium ion “lids” of cucurbituril and, therefore, the
capability for encapsulating THF. Interestingly, similar supramolecular interaction
behaviors may be observed with other guest molecules such as benzene, cyclopen-
tanone, and furan, as well as with other metal ions such as cesium. Knoche and
Buschmann first introduced metal ions into a Q[6]-G organic molecule system to deter-
mine the stability constants and the mechanism of formation of association and
inclusion complexes of cucurbituril with the 4-methylbenzylammonium ion [ 34 ].
Although they only focused on the solution structure of the Q[6]-G organic molecule -
M alkali systems, their seminal work on Q[ n ]-based host-guest/coordination chem-
istry is a significant contribution. We will further demonstrate more cases in the
succeeding parts of this chapter.
Returning to the coordination features of Q[6]/metal complexes, we note that
they are clearly different from those of Q[5] complexes because of their larger por-
tal size. In typical Q[6]-based complexes, a metal ion with a smaller ionic radius
can only coordinate to a limited number of portal carbonyl oxygens (one to three).
This restriction results in the formation of Q[6]-based capsules or bowls in which
a portal of the Q[6] molecule could be coordinated with multiple metal ions. The
aforementioned Q[6]/THF/Na + molecular capsule is a typical instance, in which
two coordinated Na + cations at each of the two portals of Q[6] coordinates to two
portal carbonyl oxygens. Fedin and coworkers made the greatest contribution to
Q[6]-based coordination chemistry; they studied almost all alkaline earth, tran-
sition, lanthanide, actinide metal ions, as well as their aqueous complexes, clus-
ters, and so on. For example, they demonstrated that Q[6]/Sr 2 + capsules may
be obtained by slow concentration of an aqueous solution of Sr(NO 3 ) 2 and Q[6]
(Fig. 2.14 a) [ 35 ]. Each Q[6] molecule is bound to four strontium cations (two
cations per portal), forming a molecular capsule that includes a nitrate anion. The
coordination of the four Sr 2 + ions in the Q[6] molecular capsule is different, in
that they coordinate two or three portal carbonyl oxygens by sharing a nitrate
anion (N31). This sharing results in the formation of infinite 1D supramolecu-
lar chains through electrostatic interaction. Furthermore, Fedin and coworkers
recently discovered a Q[6]/Mg 2 + complex that involves the interaction between
Q[6] with the amino acid tryptophan and its decarboxylation product tryptamine
in the solid state [ 36 ]. This complex is the first known supramolecular assembly
of cucurbituril with magnesium ion, which has the smallest ionic radius (0.49 and
0.72 Å for hexacoordinated Mg 2 + ) among metals studied for their coordination
to unsubstituted cucurbituril ligands (Fig. 2.14 b). Generally, Q[6] molecules show
affinity to alkali and alkaline earth metal ions, and form molecular bowls or cap-
sules [ 37 - 39 ]. However, their interaction with metal ions in the presence of a third
species could result in the formation of novel coordination polymers and supramo-
lecular assemblies, which will be described in detail later.