Coordination Polymers of Cucurbit[n]urils with Metal Ions - Supramolecular Assemblies of Cucurbit[n]urils with Metal Ions - page 42

Fig. 3.7 Electrostatic potential maps (ESPs) for Q[5], Me 10 Q[5], and CyP 5 Q[5], respec-
tively. ESPs are mapped on electron density isosurfaces (0.001 e/au 3 ) for cucurbit[ n ]urils at the
B3LYP/6-311G (d, p) level of theory with Gaussian09
rings (Scheme 3.1 a), which may further fuse into 2D networks (Scheme 3.1 b).
The first SQ[5]-based supramolecular ring was constructed from ʱ , ʱ ′-
dimethylcucurbit[5]uril (DMeQ[5] though direct coordination of metal ions to the
carbonyl oxygen of the substituted glycoluril moieties of DMeQ[5] [ 18 ]. Further
studies have revealed that it is a common feature of the alkyl-substituted SQ[5]s to
coordinate directly with metal ions and to form a SQ[5]-based supramolecular ring
which can fuse into various SQ[5]-based metal-organic networks (Fig. 3.8 ) [ 18 ].
In all of these cases, a common structural motif, i.e., a trigonal-planar branch, can
be observed. Each branch contains 12 metal ions (the metal ion is K + ) coordinated
in a similar way and three SQ[5] moieties; each K + ion not only coordinates to
the portal carbonyl oxygens of an SQ[5], but also directly coordinates to one of
the carbonyl oxygens of the neighboring SQ[5]. The branches can fuse into six-
membered “bracelets,” and these can further fuse into 2D networks (Scheme 3.1 b
and Fig. 3.8 ).
X-ray crystal structures show that these SQ[5]/K + -based networks are assem-
bled from trigonal-planar branches in which each of three SQ[5] “beads” is linked
by K + ions, as shown in Fig. 3.8 . Six-membered rings with different geometric
patterns can be identified in the corresponding 2D SQ[5]/K + -based networks.
These patterns include trigonal (Fig. 3.8 d, q), hexagonal (Fig. 3.8 c), and quadrilat-
eral patterns (Fig. 3.8 h, u). At first glance, all five trigonal-planar branches of the
respective compounds are structurally similar. In reality, however, they have many