Chemistry Reference
In-Depth Information
Figure 4.13
Representation of the tetranuclear cation {[Pt(en)]
4
(UH-N1,N3)
4
]}
4+
and the
molecular boxes formed by coordination of metal ions
cis
- confi gured metal complexes, e.g. Pt(II) and Pd(II), as metal building blocks and
pyrimidine nucleobases as bridging ligands, it is possible to obtain new multinuclear
complexes. The most common architectures of metallocalixarenes with nucleobases
reported so far have been cyclic tri- and hexanuclear supramolecules.
The combination of the 90 ° binding angle provided by the [Pt(en)]
2+
fragments
and the angle of 120 ° of the N1,N3 coordination of uracil allows the formation of
tetranuclear metallocycles, which may be considered as special metallocalix[4]arenes.
These complexes are able to interact with cations, via the exocyclic donating oxygen
atoms located at the 2- and 4-positions, and to act as receptors of organic anions in
the central cavity. The X-ray analysis shows four [Pt(en)]
2+
entities, which occupy
the corners of a square (Pt···Pt 5.86 Å), bridged by four uracil rings such that each
pyrimidine molecule is bound through N1 to a platinum atom and through N3 to a
second one (Figure 4.13). The coordination geometry about the Pt(II) is normal
square planar with slight angle deviations. The addition of
cis
- [(NH
3
)
2
Pt]
2+
, [Pt(en)]
2+
,
[Pd(en)]
2+
, Ni(II), Cu(II) and Ag(I) to [(en)Pt(UH-
N1,N3
)]
4
(NO
3
)
4
yields octanu-
clear complexes. In all the octanuclear compounds the basic structure of the open
box is retained, but instead of a single [Pt(en)]
2+
residue, two metal units, Pd
II
(en)
or M
II
, represent the corners of the boxes (Figure 4.13).
12 - 13
4.2.3 Larger Assemblies
Purines
The reaction between
cis
- [(PMe
3
)
2
Pt(NO
3
)
2
] and 9 - MeGH (9 - methylguanine), in
aqueous solution, produced the fi rst example of a cyclic hexanuclear platinum-
guanine derivative,
cis
- [(PMe
3
)
2
Pt(9 - MeG)]
6
(NO
3
)
6
in which six
cis
- (PMe
3
)
2
Pt units
are bridged by deprotonated guanine ligands through their N1 and N7 atoms (Figure
