Chemistry Reference
In-Depth Information
Fig. 31 Crystal structures of (a) the nickel(II) complex of the monoprotonated form of 18:
[Ni
II
(18H)]
3+
[
65
]; C-H hydrogen atoms omitted for clarity; and (b) the [Cu
II
(18)]
2+
complex
[
66
]; all hydrogen atoms omitted for clarity
ammonium group deprotonates and the [Ni
II
(18)]
2+
complex forms, which reaches
100% concentration at pH
10. [Ni
II
(18)]
2+
undergoes demetallation in acidic
solution, a process that has to be monitored using a stopped-flow spectrophotomet-
ric technique: in particular, the lifetime of the [Ni
II
(L)]
2+
complex in 0.5 M HClO
4
is 3 s. In the case of Cu
II
, three main complex species are present at equilibrium,
along the investigated pH range:
¼
[Cu
II
(18H
2
)]
4+
, which forms at pH
¼
3;
[Cu
II
(18H)]
3+
, present at 100% over the 4-8 pH interval; [Cu
II
(18)]
2+
, the
dominating species at pH
10 [
65
]. The crystal structure of the latter
complex has been determined and is shown in Fig.
31b
[
66
]. The Cu
II
centre is
five-coordinate, profiting from the coordination of a full tren subunit and from that
of a secondary nitrogen atom of the other tren subunit. Three nitrogen atoms remain
available for metal-ligand interaction, but Cu
II
maintains its well-defined prefer-
ence for five-coordination.
At this stage, one could wonder whether the flexible bistren derivative 18 could
accommodate two metal ions in its cavity. Indeed, on reaction of 18 with two
equivalents of Cu
II
(NO
3
)
2
, a salt of formula [Cu
2
(18)](NO
3
)
3
was obtained, which
should formally contain a Cu
II
and a Cu
I
cation [
67
]. One electron reduction is
operated by the solvent (MeOH or EtOH). The crystal structure, shown in Fig.
32a
,
indicates that the two copper centres are equivalent. Electron spin resonance (ESR)
studies revealed that the unpaired electron is delocalised over the short Cu-Cu bond
(2.36
˚
), disclosing the occurrence of a
bonding interaction between the metal
centres. Thus, each copper centre exhibits the average valence of 1.5. Each Cu
1.5
centre experiences a trigonal bipyramidal five-coordination and protrudes from the
equatorial triangle towards the other Cu
1.5
metal centre, in order to establish
metal-metal covalent interaction (Fig.
32b
). It is probably this interaction that
stabilises the dimetallic cryptate. On the other hand, stronger electrostatic
repulsions should destabilise the Cu
II
-Cu
II
s
system with respect to the average
valence dimetallic complex.
Bistren cryptands have been deservedly more successful in anion coordination
chemistry than in traditional coordination chemistry. In particular, the bistren
framework has provided the basis for the design of a variety of receptors that are
suitable for the inclusion of polyatomic anions of varying size and shape. A number
