Chemistry Reference
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Fig. 3.35
X-ray crystal structure of
a
Q[8]/UO
2
2
+
-based 2D coordination network;
b
Coordina-
tion of a Q[8] molecule with four UO
2
2
+
cations and
c
the UO
2
/SO
4
-based trinuclear motif
which are coordinated on both sides. Q[8] is thus doubly monodentate at each por-
tal, with the two oxygen donors far apart from one another; whereas it is chelat-
ing, bidentate, and doubly bidentate at each portal (Fig.
3.35
b). The three sulfate
ions are bridging, with two of them (S1 and S2) connecting three uranium atoms
(U1, U2, and U3 for S2; U1, U2, and U3 in the neighboring unit), and the third
(S3) bridging U1 and U3, resulting in the formation of a UO
2
/SO
4
-based trinu-
clear motif (Fig.
3.35
c). This UO
2
/SO
4
-based polymeric subunit constructed from
these motifs is an undulated ribbon running along the
b
axis. Each of these trinu-
clear motifs is bound to two Q[8] molecules and, conversely, each portal of Q[8] is
bound to two such motifs.
Very recently, we reported a series of Q[8]-M
alkali
or M
alkaline
earth
-based 2D
coordination networks (M
alkali
and M
alkaline earth
are alkali or alkaline earth metal
ions, respectively), one of which was first found by chance in 2009. More recently,
they have been readily and robustly prepared with the aid of some inorganic struc-
ture inducers [
59
]. Q[8] is the least soluble member in the Q[
n
]-family, but it
could made more soluble through host-guest interaction with polar organic mol-
ecules or in acidic medium such as HCl. Metal ions do not significantly enhance
the solubility of Q[8]. Generally, Q[8] does not readily coordinate to M
alkali
or
M
alkaline earth
, and upon mixing Q[8] with salts of these metals, metal-free Q[8]-
based crystals are easily obtained. However, when a second metal ion, in particu-
lar, a transition metal ion, is introduced into the Q[8]-M
(alkali or alkaline earth metal)
systems, crystals with novel 2D network features are formed through direct coor-
dination of Q[8] with M
(alkali or alkaline earth metal)
cations. Different metal ions could
lead to the formation of Q[8]/M-based networks with different coordination and
stacking features.
For example, when transition metal ions, such as Cu
2
+
, Co
2
+
, Ni
2
+
, and
Mn
2
+
are introduced into a Q[8]-Rb
+
-HCl system [
59
], novel 2D Q[8]/Rb
+
net-
works with the same crystal feature are always obtained; the crystals are isomor-
phous and belong to the p
(R-3)
space group. X-ray structural analysis has shown
an overall view of the 2D Q[8]/Rb
+
network formed through direct coordination
(Fig.
3.36
a). A six-Q[8]-membered ring from the 2D network, wherein each of the
two Q[8] molecules are linked by a pair of Rb
+
ions at the two portals of each
Q[8] molecule, could be observed (Fig.
3.36
b). Moreover, a trigonal-planar branch
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