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at each portal coordinate to a Gd
3
+
cation along with six water molecules [
46
]. A
guest water molecule resides in the cavity of the Q6 and is disordered between two
positions (Fig.
2.17
d). Under hydrothermal conditions (at 120 °C) Pr(NO
3
)
3
reacts
with Q[6] in water to yield [{Pr(NO
3
)
2
(H
2
O)
3
}{Pr(NO
3
)(H
2
O)
3
(H
2
O@Q[6])}]
(NO
3
)
3
4H
2
O [
47
]. An X-ray diffraction study (Fig.
2.17
e) demonstrated that the
two metal centers in this complex are each linked to the Q[6] through tridentate
coordination of three portal oxygens from the respective portals. Each metal center
is also bound to a bidentate nitrato ligand and three water molecules located exo
to the central cavity. The cavity encapsulates both a nitrate ion and a water ligand
(both disordered over two positions) which are each bound in a monodentate fash-
ion to the respective metal centers, each of which achieves a coordination number
of nine.
Use of a lanthanide bromide salt rather than a nitrate salt has been shown to
result in changes in both the composition and the structure of the corresponding
Ln
3
+
-Q[6] complexes—once again emphasizing the difficulties in predicting the
nature of the product from a given synthetic procedure [
45
]. Thus, complex crys-
tallization from an aqueous solution containing cerium bromide and Q[6] results
in a 2:2 Ce
3
+
-Q[6] sandwich complex incorporating two cucurbituril ligands
linked by two bound [Ce(H
2
O)
5
]
3
+
units (Fig.
2.18
a). Each nine-coordinated Ce
3
+
has four carbonyl oxygens and five aqua ligands in its coordination sphere. The
2:2 structure contrasts with the 1:1 arrangements found in the (Ln
3
+
:Q[6]) com-
plexes (a)-(e) discussed above (see Fig.
2.17
)—all of which employed the corre-
sponding lanthanide nitrate salt as a precursor [
45
]. The trigonal crystal packing
of the above sandwich complex is shown in Fig.
2.18
b. Triangular channels with a
van der Waals diameter of ~4.3 Å are filled with disordered water molecules and
bromide anions (omitted in Fig.
2.18
b for clarity). In contrast, the corresponding
GdBr
3
-Q[6] system results in a triple-decker sandwich (Fig.
2.18
c). The latter is
related to that described above for the Sm
3
+
-Q[6] triple-decker sandwich [
44
]
shown in Fig.
2.16
.
As discussed already for Q[5]-based complex systems [
24
], there are also
examples of Q[6] systems prepared in the presence of added small organic mol-
ecules that appear to play a structure directing role. Thus, Fedin and coworkers
showed that the interaction of aqueous Gd
3
+
nitrate with Q[6] in the presence of
pyridine yielded a hydrogen-bonded (supramolecular) adduct between the aqua
nitrate complex [Gd(NO
3
)(H
2
O)
7
]
2
+
and Q[6] (Fig.
2.19
a) [
48
]. In the above
assembly the water molecules coordinated to the Gd
3
+
center are hydrogen
bonded to carbonyl groups of Q[6], with the cavity of the Q[6] molecule encapsu-
lating a pyridine molecule. These workers also reported a further La
3
+
-Q[6] cap-
sule-like complex of type [La(H
2
O)
6
(X@Q[6])(NO
3
)]
2
+
(X
=
0.5 C
5
H
5
N
+
0.5
H
2
O) [
49
] in which a pyridine molecule occupies the internal cavity of one half of
the Q[6] molecules (Fig.
2.19
b), while a water molecule occupies the other half.
The La
3
+
center is bound to two adjacent carbonyl oxygens, a bidentate nitrato
ligand and six aqua ligands to yield a coordination number of ten. The metal
center is disordered between the two portal faces (with both occupancy positions
shown in Fig.
2.19
b). In a related study Liu and coworkers [
50
] crystallized a 1:1
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