Chemistry Reference
In-Depth Information
Figure 4.10
Schematic illustration of the guanine-based square [Pd
4
(
N3
,
N7
-G-prop-en)
4
]
8+
are 86.5 and 93.5 °. This structure is also described as a one-dimensional polymer
with
SO
2−
acting as bridging ligands (see Section 4.3.1).
Tetrameric molecular boxes are generally formed through
cis
- ligand orienta-
tion at the metal centre or with the N1,N7-bridging mode of the purines that also
present a 90 ° angle. However, in [Cd
4
(G - Et - en)
4
(H
2
O)
4
(SO
4
)
2
] N7 and the chelating
tether effectively providing orthogonal bond vectors
10
(Figure 4.9 ).
Another example of a tetrameric complex was isolated from the reaction of
Pd(II) ions with en - Prop - G (G - Prop - en = ethylenediamine - N9 - propylguanine
11
). In
this case a distorted molecular box structure is observed, which features coordina-
tion of both N7 and N3 of the guanine moiety, as well as the ethylenediamine group
(Figure 4.10). The complex may be considered as an octacationic metallomacrocycle.
The Pd···Pd edge distances exhibit a narrow range between 7.237 and 7.269 Å. The
diagonal distances are Pd1 · · · Pd3 9.871 Å and Pd2 · · · Pd4 10.062 Å and the four palla-
dium ions lie in a plane. As in the Cd(II) tetramer, the guaninyl residues act to form
approximately linear bridges, though in this case coordination also involves N3. This
is a rare metal binding site for guanine with a very limited number of examples
reported.
11
A unique aspect of these guanine-based tetramers is that, in contrast to the
natural tetrads (telomeres), which are assembled through hydrogen bonding and
metal ion stabilization, these structures are formed through metal-ligand bonding
alone. Since the sites on the nucleobase involved in coordination are N7 or N7 + N3,
this leaves the Watson-Crick face available for base pairing (Figure 4.11). This
feature presents numerous opportunities for preparing supramolecular metal-DNA
hybrid structures assembled via complementary H-bonding interactions. It also
raises the interesting question of how these compounds might interact with natural
DNA strands.
11
Pyrimidines
The self-assembly of pyrimidine ligands as angular building blocks (120 °) to metal
fragments with binding angles at 90 ° gives rise to metallocalix[4]arene formation.
These are metal analogues to the classical calix[4]arenes with a typical geometry
