Chemistry Reference
In-Depth Information
Figure 4.6
Schematic representation of the
trans
-[{(NH
3
)
2
Pt(
µ
-N1-Pu-N7)}
3
]
6+
molecular tri-
angle and the
trans
-[{(NH
3
)
2
Pt(
µ
-N1-Pu-N7)}
4
]
8+
molecular square
condenses at room temperature to form short chains that evolve, after fi ve days, to
give two different closed complexes: the
trans
- [{(NH
3
)
2
Pt( m - N1 - Pu - N7)}
3
]
6+
molecu-
lar triangle and the
trans
- [{(NH
3
)
2
Pt( m - N1 - Pu - N7)}
4
]
8+
molecular square, in a 1 to
0.6 ratio (Figure 4.6). It is noteworthy that the N1-Pt-N7 angles in the trimer deviate
from the ideal 180 ° to 168.8-170.8 ° and the angles between the Pt atoms coordi-
nated to N1 and N7 sites of the purine ligands deviate signifi cantly from 90 ° to
67.8-70.8 °. These substantial geometric deviations allow the formation of the unex-
pected triangular arrangement. The ratio of products can be reversed to 1 : 2.5 (tri-
angle:square) by addition of Na
2
SO
4
and this also has the effect of enhancing the
reaction rate. In the X-ray structures of the compounds, the purine ligands and the
platinum(II) atoms are essentially coplanar and the ammine ligands are oriented
perpendicular to the former plane. In addition, both complexes provide molecular
cavities capable of anion recognition with particular affi nity for sulfate.
Few examples of organometallic complexes with nucleobases are known. Cyclic
trimers resembling hollow truncated cones have been synthesized by the reaction
of trimethylplatinum(IV) acetate with 9 - methyladenine (9 - MeAH) in a 1 : 1 ratio
4
.
The complex presents three PtMe
3
moieties bridged by deprotonated 9-methylade-
nine ligands which are bound to one platinum via N6 and N7 and via N1 to another
platinum (m -
2
N6,N7
) (Figure 4.7). Thus, the platinum centres are octahedrally
coordinated by three methyl ligands and three nitrogen donor atoms of the adenines.
Crystals from different solvents show slight differences in the structural details of
the trimeric unit and in crystal packing.
The reaction between the metal fragments (h
6
- arene)Ru(II) (arene = C
6
H
6
,
p
-
cymene = CH
3
C
6
H
4
CH(CH
3
)
2
) and (h
5
- arene)M (M = Rh(III) and Ir(III); arene =
pentamethylcyclopentadienyl) with hypoxanthine and adenine derivatives provides
attractive water-soluble cyclic trimeric (Figure 4.7) and tetrameric complexes.
5 - 8
The
ability of these types of cyclic supramolecular complexes as receptors have been
studied, and binding of biological molecules has been observed. In particular, the
Rh(III) cyclic trimers with adenine derivatives recognize amino acids by a variety
κ
N1
:
κ
