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836 Inorganic Exotic Molecules

Scandium chemistry is hardly commonplace; the Sc 3 N
core is unexpected.
C 60 is a highly electrophilic molecule and numerous
addition reactions of nucleophiles and electrons (to give
n−
C , n = 1–6) are known. Another surprise was therefore
60
+
the recent report of the protonated [HC 60 ] . To stabilize
this unusual acid, the exceptionally inert [CB 11 H 6 X 6 ] −
carborane anion was used (X = Cl and Br). (Have we be-
come so jaded not to consider the anion “exotic” because
its framework interpolates the “superaromatic” icosahe-
dral [B 12 H 12 ] 2− and its dicarbon isoelectronic relatives,
theisomerico-,m-,and p-carboranes,B 10 C 2 H 12 ,andtheir
numerous and diversely substituted derivatives?)
E. Molecular Grids

The synthesis of molecules of nanoscopic dimensions
by controlled self-assembly is a rapidly growing field in
inorganic chemistry. Based on defined transition metal–
ligand interactions, amazingly complex and exotic struc-
tures have been built, such as one-dimensional helicates
FIGURE 15 A trefoil copper phenanthroline knot (30).
and tubes, two-dimensional polygons, as well as three-
dimensional cages and polyhedra. The 3 × 3 molecular F. Large Cages
9+
grid [26] shown in Fig. 12 was synthesized in 99% yield
by reaction of ligand 27 with 1.5 equiv of [Ag(CF 3 SO 3 )]. Three-dimensional cages have been obtained using rigid
The structure of [26] 9+ in the crystal reveals a rhombus- C 3 symmetric ligands in combination with appropriate
shaped geometry. transition metals. One of the largest assemblies synthe-
Another remarkable example of a grid like molecule sized (29) is depicted in Fig. 14. The nanocage con-
was obtained by palladium(II)-directed self-assembly. sists of six 1,3,5-tris(pyrimid-5-yl)benzene ligands and 18
2+
The self-organization proceeds by coordination of nine [Pd(en)] (en = ethylene diamine) fragments and has di-
mensions of 3 × 2.5 × 2.5 nm. With an internal volume
zinc porphyrins having pyridine side chains to 12 PdCl 2 3
2
units, resulting in the 25-nm grid (28) shown in Fig. 13. of about 0.9 nm , the polynuclear complex 29 is an ideal
Interestingly, the assembly forms columnar clusters on host for the encapsulation of large guest molecules.
silica surfaces that can be visualized by atomic force
G. Large Knots
microscopy.
To make a knot is a task easily accomplished by a child
playing with a rope, or even by a careless adult with
shoelaces. For a chemist playing with molecules this is
an intrinsically difficult problem and so far only a few
of these topologically interesting compounds have been
synthesized. A strategy that finally led to success is based
on the self-assembly of double-helical copper phenanthro-
line complexes. The helix represents the core structure
from which the trefoil knot 30 in Fig. 15 is obtained in a
final cyclization step. It is important to note that a trefoil
knot is chiral. The resolution of the two enantiomers was
recently accomplished by fractional crystallization of the
diastereoisomers obtained with a chiral counterion.

XI. CONCLUSION

In this article we have assembled a collection of ex-
FIGURE 14 A palladium(II) trispyrimidylbenzene nanocage (29). otic species. The definition of the word “exotic” remains