Chemistry Reference
In-Depth Information
only 26 complexes with coordination found at other sites. For uracil, however, even
though the majority of the reported cases are N3-coordinated, the number of com-
plexes with metal coordinated to other positions is considerable. One of the essen-
tial points in the interpretation of Table 4.1 is related to mechanistic aspects. Direct
coordination of a metal to a nucleobase usually involves the preferred sites: N7 in
purines, N3 in pyrimidines. After coordination, the metal can migrate to other posi-
tions, or the nucleobases can even undergo a p
K
a
alteration, making it easier for
further coordination by another metal ion. Thus, multiple coordination favours the
coordination of a metal ion to an initially nonpreferred site. In Table 4.1 all the cases
of multimetal complexes were counted as individual entities for each site.
Though it is possible to form chelate binding modes for nucleobases, and these
have been proposed in earlier literature, there are remarkably few well-character-
ized examples. The most abundant cases are for the adenine nucleobase, which shows
two different chelating modes:
N1,N6
and
N6,N7
. Two examples, both with molyb-
denum,
56
are known for the
N1,N6
mode, and several examples with different metals
for the latter: platinum (3), copper (2), ruthenium (5), rhodium (3), iridium (2).
57
Metal chelation has also been described for the other nucleobases, with the
exception of thymine. For guanine, only one example has been found, involving zinc
bound through
N1,O6
.
58
This is perhaps an unexpected feature given the large
number of guanine complexes characterized. The number of examples containing
pyrimidine bases is also very small. One example is known for uracil, in which a
copper ion is bound to the base at both N1 and O2.
59
Two chelating modes were
found for cytosine:
O2,N3
and
N3,N4
, comprising six different cases involving
cadmium (4), copper (1), and rhodium (1).
60
The direct utilization of DNA base pairing to control the assembly of metal
complexes has been realized by Houlton. This was achieved by extending ligand
functionalization to include two complementary nucleobases (Figure 4.41).
61
Sequen-
tial alkylation of 1,2 - dithioethane using 2 - chloroethyl - 9 - adenine and 3 - chloropropyl -
1 - thymine yields the AT - containing dithioether. Reaction with either [PdCl
2
(MeCN)
2
]
or K
2
[PtCl
4
] gives the isostructural complex cations, [MCl(
N3
- A - SS - T)]
+
(M = Pd(II)
or Pt(II)), in which the dithioether strand is coordinated through the S-atoms of the
thioether and N3 of adenine. In the solid state the complex cations form infi nite
chains of Hoogsteen base pairs (T-N3· · · A - N7 separation 2.89 Å), which is the most
thermodynamically stable pattern for these bases. It was also found that the effect
of metal ion binding to the adeninyl group is to reduce the hydrogen bonding
between the A-T pair in solution compared to the free ligand.
62
This approach pro-
vides a range of possibilities for the development of supramolecular syntheses,
which take advantage of both coordinate bonding and complementary hydrogen
bonding.
4.5 M - DNA
The use of DNA as an electrically conductive material, with self-organization prop-
erties able to produce molecular nano-architectures, has motivated extensive
