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(La
3
+
:Q[6]) cationic inclusion complex of type {[La(H
2
O)
6
Cl](THF@Q[6])}
2
+
(THF
=
tetrahydrofuran) through slow diffusion of THF vapor into a solution con-
taining Q[6] and lanthanum nitrate in 3 M HCl (Fig.
2.19
c). The La
3
+
coordina-
tion sphere in this case consists of two carbonyl oxygens, six water molecules and
a chloride ion.
Fedin and coworkers have synthesized a series of polynuclear lanthanide com-
plexes by reacting Q[6] with aqueous solutions of lanthanide(III) nitrates [
51
],
chlorides [
52
] or bromides [
39
], Q[6], and 4-cyanopyridine under hydrothermal
conditions (130 °C). These
ʼ
-hydroxo polynuclear lanthanide complexes are
formed in aqueous solution and their formation involves hydrolysis of the cor-
responding Ln
3
+
salts with, under the conditions employed, the 4-cyanopyridine
also being hydrolyzed to isonicotinic acid. Thus, for example, using lanthanide
nitrates, a series of tetranuclear aqua-hydroxo-carboxylate complexes of type
{[Ln
4
(
ʼ
3
-OH)
4
(
ʼ
2
-OH)
2
(C
5
NH
4
COO)
2
(H
2
O)
4
(Q[6])
2
][Ln(H
2
O)
8
]
1.5
[Ln(H
2
O
)
6
(NO
3
)
2
]
0.5
}(NO
3
)
9
nH
2
O (Ln
=
Ho, Gd, or Er) was obtained and character-
ized by X-ray diffraction analysis, elemental analysis, infrared spectrophotometry,
and electrospray ionization (ESI) mass spectrometry [
51
]. The sandwich struc-
ture involves coordination of a central tetrahedral lanthanum cluster to two Q[6]
via four oxygens (two per Ln
3
+
cent) at each portal (Fig.
2.20
a). The presence
of bridging bidentate carboxylate groups in the tetranuclear cluster was postulated
to stabilize the observed structure by inhibiting formation of infinite polymeric
species.
The above products are generally similar to a range of complexes formed from
chloride [
52
] and bromide [
39
] lanthanide salts (Fig.
2.20
b) in that sandwich com-
plexes are generated; although, in particular complexes, some structural differences
do occur—reflecting variation in the synthetic conditions and/or due to the intrinsic
differences in the properties of the lanthanide cations themselves. Some variations
on the basic structure are illustrated in Fig.
2.20
c-f. All these complexes incor-
porate a tetranuclear lanthanide cluster (Ln1-Ln4) core positioned between two
neighboring Q[6] molecules. Ln1 and Ln2 each coordinate to two portal carbonyl
oxygens of a Q[6] molecule while Ln3 and Ln4 coordinate to two portal carbonyl
oxygens of the second Q[6] molecule in the sandwich complex, with the carboxy-
late groups (O1, O2 and O3, O4) from two isonicotinates coordinating to Ln1, Ln2
and Ln3, Ln4, respectively. The aromatic ring of each bridging bidentate isonicoti-
nate is included in a Q[6] cavity. The four Ln
3
+
cations are linked by four
ʼ
3
-OH
groups and from one to four
ʼ
2
-OH groups (depending on the particular lanthanide
involved). Thus, for example, there are four
ʼ
3
-OH groups and one
ʼ
2
-OH group
in the structure shown in Fig.
2.20
c. In particular complexes, one or two addi-
tional isonicotinate ligands also bind to particular Ln
3
+
sites (Fig.
2.20
c-f) while
in other cases the carbonyls from the second (“outside”) portal of each Q[6] also
bind to further Ln
3
+
cations (Fig.
2.20
d, e). Selected compounds from the above
series have been employed as precursors for the synthesis of lanthanide-silver
heterometallic coordination polymers [
39
]. In these heteronuclear compounds
the affinity of the isonicotinate pyridyl nitrogen for Ag
+
has been exploited to
produce chain polymers in which La
3
+
, Pr
3
+
and Dy
3
+
sandwich complexes are
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