Coordination Polymers of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 48

dihedral angles between each K1 junction plane (the plane through the three K1
atoms defining the triangle) and the corresponding adjacent K2 junction plane are
identical (70.53°); in addition, the dihedrals between any two K1 junction planes
are the same (70.53°) in the trigonal-planar branch (Fig. 3.10 e). The constancy
of these angles is important for the generation of the observed 3D, 10-membered
bracelet frameworks. Figure 3.10 f shows part of the 3D framework, illustrating the
fused trigonal-planar branches in relation to the unit cell axes. Each of the cat-
enated 3D frameworks is thus constructed from multifused, 10-membered 1,2,4-
HMeQ[5] bracelets. Figure 3.10 g shows an isolated 10-membered 1,2,4-HMeQ[5]
bracelet consisting of a string of 1,2,4-HMeQ[5] “beads” linked by direct coordi-
nation to potassium ions. The size of the bracelet is partly dictated by the size of
each 1,2,4-HMeQ[5] bead, which has a diameter of ~12 Å (taking into account the
presence of the methyl substituents). The 10-membered bracelets have an average
diameter of around 19 Å, which is more than enough to fit a 1,2,4-HMeQ[5] unit
in the cavity when forming the catenated structure. Figure 3.10 h shows two equiv-
alent catenated (noncovalently linked) 10-membered bracelets, while Fig. 3.10 i
shows part of the two multicatenated 3D frameworks, each composed of multi-
fused 10-membered 1,2,4-HMeQ[5] bracelets (the unit cell axes are also shown).
As mentioned previously, there has been a trend toward the use of a third spe-
cies as structure inducer in Q[ n ]-metal systems. These species could result novel
Q[ n ]-based supramolecular coordination polymers and assemblies with unusual
properties, structural features. In that case, the positive outer surface of Q[ n ]s may
also play an important role in such supramolecular architectures (Scheme 1.2 ) . For
instance, we have not seen polydimensional coordination polymers of Q[5] with
metal ions other than potassium cation in the absence of a third species, such as an
aromatic organic molecule or polychloridematollate anions as structure directing
agents. However, in the presence of p -hydroxybenzoic acid (Hyb), Q[5] behaves
as a tetradentate ligand and coordinates with two Ca 2 + cations on both portals,
resulting in the formation of a 1D helical coordination polymer. The Hyb mole-
cules surround the helical polymer and form a Hyb-based helix (Fig. 3.11 a) [ 27 ].
Similar 1D coordination polymers have been assembled from Q[5] and various
lanthanide ions (Ln 3 + ) in the presence of Hyq as an organic structure inducer. It
should be noted that the coordination of Q[5] with light lanthanide cations leads
to the formation of zigzag 1D coordination polymers (Fig. 3.11 b) and the coor-
dination of heavy lanthanide cations to Q[5] leads to the formation of homochiral
1D helical coordination polymers (Fig. 3.11 c) [ 28 ]. Furthermore, the Hyq mol-
ecules are distributed around the 1D coordination polymers. In these cases, the
radii of the related cations are around 1 Å; generally, such cations cannot fully
cover both portals of Q[5]. In order to effectively coordinate to the portals of Q[5]
molecules, these metal ions prefer to coordinate to two carbonyl oxygens for each
portal of Q[5] molecules, resulting in the formation of the alternating arrange-
ments of metal ions and Q[5] molecules. Figure 3.11 d shows a CyH 5 Q[5]/K + -
based 1D coordination polymer assembled in the presence of an inorganic species,
[ZnCl 3 H 2 O] − . Both portals of each CyH 5 Q[5] molecule in the polymer are fully
covered by a potassium cation, which further coordinates to a carbonyl oxygen