Chemistry Reference
In-Depth Information
Figure 4.26
Scheme of the structure of the 1D coordination polymer [Cd(6-MP
−
)
2
]
n
adenine molecule acts as bridging ligand in a zigzag manner. The copper ions coor-
dinate via N1 and N9 to propagate the polymeric chain along the crystallographic
b
-axis. Again the structure resembles the expected periodicity of single-stranded
nucleic acids. A water suspension of this insoluble complex exhibits not only facile
chemical nuclease activity for DNA by an oxidative mechanism, but can also be
reused, keeping its scission activity unchanged.
28
The direct reaction between Cd(NO
3
)
2
, and 6 - MP (6 - mercaptopurine) (pH 4.6)
leads to formation of the coordination polymer [Cd(6-MP
−
)
2
· 2H
2
O]
n
(Figure 4.26 ).
29
Its X-ray structure consists of infi nite chains of cadmium atoms octahedrally coor-
dinated by four S6 and two N7 atoms of the m - S6,N7:S6 - mercaptopurinate ligands.
The metal-to-metal distance within the chain is 3.92 Å. Addition of 0.1 M NaOH
and a 0.1 M water solution of Ca(NO
3
)
2
to a suspension of this polymer leads to
further deprotonation of the 6-MP
−
ligand and the subsequent formation of the
coordination polymer [Cd(6-MP
2−
)
2
]
n
[Ca(H
2
O)
6
]
n
.
30
Its structure consists of parallel
anionic chains of
Cd
[
(
)
]
n
n
2
−
n
which are
joined together by means of electrostatic interactions and hydrogen bonds. The 1D
anionic polymer is based on stacked centrosymmetric [Cd(6-MP
2−
)
2
] entities, which
are linked to each other through Cd-S bonds, forming a 1D chain. The ligand coor-
dinates to the cadmium as in the former case. The distance between metal centres
of consecutive entities is 4.14(1) Å. An interesting issue is the different shape of the
two polymeric chains and the prevalence of the
trans
disposition in the [Cd(6-MP
−
)
2
]
entities.
The structural analogy between [Cd(6-MP
−
)
2
] and that proposed for M-DNA
(see Section 4.5), motivated studies of the electrical properties of single molecular
strands. In spite of [Cd(6-MP
−
)
2
] behaving as an insulator, the work opens a new
strategy towards the possible use of DNA-derived coordination polymers as molec-
ular wires.
31
The reaction of tetrakis(m - trifl uoroacetamidato)dirhodium(II) with cytosine in
methanolic/aqueous solution yields a zigzag 1D structure in which the cytosine
molecule bridges the axial positions of the different centrosymmetric
[Rh
2
(CF
3
CONH)
4
] units through N3 and O2 donor atoms (Figure 4.27).
32
The Rh -
N3 axial bonding is accompanied by the formation of two intramolecular hydrogen
bonds, one between N4 of the base and the amidato oxygen and the other between
O2 of the base and the amidato NH. The Rh-O2 coordinative bond is also rein-
forced by a hydrogen bond established between the cytosine N1 atom and the
amidato oxygen atom of a trifl uoroacetamidato bridge.
[
]
2
+
6
-MP
2
−
and cationic chains of
Ca H O
2
(
)
6
2
n
