Chemistry Reference
In-Depth Information
Figure 4.31
Schematic illustration of the polymeric chains in [
-{CdBr(
N3
,
N7
-A-Et-
en)}
2
CdBr
4
], (where X = Cl or Br) generated by adeninyl bridging of trimetallic units
µ
the third terminal. The remaining coordination sites are occupied by the nitrogen
atoms of the diamine group and N3 of adenine (Figure 4.31).
36
Garc í a - Ter án and coworkers have found a strategy for the design of 1D oxalato-
complexes which have been extended to the synthesis of infi nite M
II
- oxalato chains
with pendant purine, [M(m
- o x
)(H
2
O)(pur)]
n
(M = Cu, Co and Zn), and adenine,
{[(M(m - ox)(H
2
O)(ade)] · 2(ade)(H
2
O)]}
n
(M = Co and Zn), nucleobases.
37
The poly-
meric chains consist of
cis
- [M(H
2
O)(L)]
2+
units bridged by bis-bidentate oxalato
ligands with M· · ·M intrachain distances in the range 5.23- 5.57 Å (Figure 4.32 a, b).
The metal atoms are six-coordinate with four oxalato oxygen atoms, one water
molecule and one nitrogen atom from a terminal nucleobase, building distorted
octahedral MO
4
O
w
N surroundings. The purine ligand is bound to the metal atom
through the most basic imidazole N9 atom, whereas for the adenine nucleobase the
minor groove site N3 is the donor atom. The nucleobases are projected outside the
zigzag chain in both structures, setting up a complex network of noncovalent
interactions. The interplanar distance of 7.52A between two consecutive parallel
adenine ligands on the same side of the chain permits the insertion of free adenine
molecules between, and approximately parallel to, the coordinated adenines, with a
mean interplanar distance of 3.22A. This value is somewhat shorter than that found
between adjacent bases in the B-DNA double helix (3.4A). Additionally, a second
free adenine interacts with the coordinated adenine molecule through the donor
and acceptor sites of both Watson-Crick (N1 and N6 atoms) and Hoogsten (N7 and
N6 atoms) edges to give a zigzag array with the intercalated free adenine forming
a strong N9-H· · ·N3 hydrogen bond. Furthermore the cohesiveness of the supramo-
lecular architecture is ensured by three additional hydrogen bonds (N9-H· · · O
w
, O
w
-
H · · ·N3 and C8-H· · · O
w
) (Figure 4.32c). These compounds are rare examples of the
coexistence of high dimensional M-nucleobase covalent and supramolecular
networks.
