Coordination Polymers of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 50

could be ascribed to these weak intermolecular interactions (Fig. 3.11 e) [ 31 - 33 ].
On the other hand, recent works show that numerous supramolecular polymers are
accompanied by polychloride anion complexes ([MCl x ] n − ) which are formed in
HCl solution or in a medium with excess chloride anions. These [MCl x ] n − ani-
ons seem to play the role of structure inducers. The driving force behind the for-
mation is the interaction between chloride anions and methenyl or methylene on
the back of the Q[ n ] molecules, as well as the ion-dipole interaction between the
[M trans Cl x ] n − anion and the carbonyl carbon atom of Q[ n ] molecules (Fig. 3.11 f)
[ 34 , 35 ].
The alkyl-substituted Q[5]s present the characteristics of direct coordination
with metal ions and spontaneous formation of polydimensional coordination poly-
mers. These characteristics indicate a likely increased electron density as a con-
sequence of the electron donating effect of the alkyl substituents. Many attempts
to synthesize similar polydimensional coordination polymers by using Q[5] under
the similar synthetic conditions have failed. However, when we introduced Hyb
into the Q[5]-KI system, a template or structure inducer, we obtained a stable 2D
network-like coordination polymer with accessible channels [ 36 ]. The solid-state
structure was found to have channels similar to those derived from the aforemen-
tioned alkyl-substituted Q[5]s and Sr 2 + or K + [ 17 ]. The channels are created by
Q[5] “beaded” six-membered rings [ 36 ], which are stacked upon one another to
form channels (Fig. 3.12 b, c). The six-membered rings are joined together to form
a netting layer (Fig. 3.12 b) through the trigonal-planar branches (Fig. 3.12 a). Each
of the branches is constructed from three Q[5]molecular “beads” whose portals are
bound to three K + ions (Fig. 3.12 a). The trigonal branches link a layer of molecu-
lar netting with hexagonal holes, and each netting layer is stacked upon the next
layer below it (Fig. 3.12 c, d).
In the above trigonal-planar branch, the radii of the metal ions are gener-
ally larger than 1 Å, and the portals of the Q[5]s are generally fully covered by
metal ions such as K + , Sr 2 + , and so forth. Recent research revealed that in the
presence of inorganic structure inducers, such as some transition metal salts, a
series of Q[5]-Ln(NO 3 ) 3 systems could yield novel 2D-network coordination
polymers. For example, X-ray structural analysis has shown the Q[5]/Sm 3 + pair
(Fig. 3.13 a) [ 30 ]. The pairs are not discrete, but they are essential building blocks
for constructing the Q[5]/Sm 3 + pair-based 2D-network coordination polymer
(Fig. 3.13 b). In the Q[5] pair, two Q[5] molecules are linked by a [Sm 2 (H 2 O) 4 ] 3 +
cation through direct coordination of the Sm2 cation to two portal carbonyl oxy-
gens of the Q[5]s (O6, O7, and O11, O12). The other portal of the two Q[5]
molecules in the pair is fully capped by Sm1 and Sm3 cations. Moreover, Sm1
and Sm3 also coordinate to the neighboring portal oxygens (O14 or O9, respec-
tively). This additional coordination leads to the formation of the novel network
constructed from the Q[5] pair (Fig. 3.13 b, c). Although the inorganic salt CuCl 2
acts as a supramolecular assembly inducer in this case, Cu 2 + cations do not appear
in the crystal structure of the Q[5]/Sm 3 + complexes. The Sm-Q[5] porous mate-
rial could have similar or even better absorption properties compared with the KI-
Q[5]-Hyb hexagonal netting sheets.