Chemistry Reference
In-Depth Information
Figure 4.17
Schematic representation of the [{Pt(CH
3
NH
2
)
2
(1-MeC)
2
Hg
3
(OH)(NO
3
)}
2
]
(NO
3
)
4
molecular hexagon.
(sides 7 and 5.5 Å) with four Hg
II
and two Pt
II
atoms at the edges and four nucleo-
bases and two OH groups representing the corners (Figure 4.17). All metal ions and
OH groups are almost coplanar. A special structural characteristic of this hexagon
is the arrangement of the six Hg
II
ions. The interactions between the Lewis acidic
Hg centres and nucleophiles could lead to specifi c substrate binding and catalysis.
18
4.3 Infi nite Architectures
As with discrete supramolecules, polymers may also adopt different architectures
(see Figure 4.4) depending on the choice of metal and the nucleobase binding sites.
The design and construction of polymeric compounds are of great interest due to
their ability to act as functional materials, such as nonlinear optical (NLO), electrical
switches or materials which possess interesting magnetic or catalytic properties.
Three-dimensional polymers may also show applications in molecular storage (gases,
anionic interchange etc.) (Figure 4.18). The formation of polymers based on biologi-
cally relevant ligands such as nucleobases, may provide additional applications in
biological fi elds. For instance, some of these structures could be used as models of
specifi c forms of DNA (e.g. M-DNA, G
4
) .
Nucleobase-derived coordination polymers may be classifi ed in three different
groups: (i) the nucleobases act as unique bridging ligands between the metal frag-
ments, (ii) the nucleobases and a second ligand type link the metal centres or (iii)
the nucleobases are terminal ligands coordinated to the metals, which are connected
through other bridging ligands. A general observation is the relatively small number
of polymeric structures based on bridging nucleobases alone, in spite of the great
number of available coordination positions. A feasible explanation for this fact
could be the use of nucleobases in their neutral form, which imposes the presence
of counterions to balance the charge of the metal ions. At the same time, the anions
compete with the nucleobases for binding to the metal centres. This could, in prin-
ciple, be overcome by the deprotonation of the bases, improving their coordination
ability and avoiding the need for counterbalancing anions.
