Page 302 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN010H-470 July 16, 2001 16:53

Nanosized Inorganic Clusters 311

In analogy, the cluster correspondingly shows spheri- as well as nondirected) linking of (a huge variety of) basic
cal shells of terminal oxygen and molybdenum atoms and well-defined preorganized (or stepwise organized)
in which an icosahedron spanned by the centers of the fragments. An impressive example of this is the self-
12 {(Mo)Mo 5 } pentagons—the Mo atoms of the cen- aggregation/reconstitution process of the tobacco mosaic
tral MoO 7 bipyramids—is inscribed (see Fig. 11). The virus (TMV) from preorganized units. This process more
{Mo 132 }-type cluster has been the starting point for or less meets the strategy of controlling the linking of
the development of a Keplerate-type chemistry illustrated fragments to form larger units and linking the latter again.
by the fact that these types of species are stable in aque- The linking of building blocks containing 17 metal atoms
ous solution and allow the encapsulation of different types ({Mo 17 } units) to form cluster anions consisting of two
of guests into the cavity. It is possible to replace the or three of these units provides an archetypal example.
V
{Mo }-type linker units by Fe III ions which results in a The resulting 2- or 3-fragment clusters are of the {Mo 36 }
2
{((Mo)Mo 5 ) 12 Fe 30 }-type cluster (a discrete cluster with ([{MoO 2 } 2 {H 12 Mo 17 (NO) 2 O 58 (H 2 O) 2 } 2 ] 12− ∗
= [{Mo } 2
1

the largest number of paramagnetic centers known). Such ({Mo 8 } 2 {Mo }) 2 ]) or of the {Mo 57 M 6 } type (e.g.,
1
exchange reactions also allow resizing of the cluster shells [{VO(H 2 O)} 6 {Mo 2 (µ-H 2 O) 2 (µ-OH)} 3 {Mo 17 (NO) 2 O 58

while keeping the icosahedral symmetry (Fig. 12). (H 2 O) 2 } 3 ] 21− [{VO(H 2 O)} 6 {Mo } 3 ({Mo 8 } 2 {Mo }) 3 ])
2 1
(Fig. 13).
The structure of the {Mo 17 } unit can be reduced to
B. The Route to Wheel and
Giant-Wheel Structures two {Mo 8 }-type groups which are symmetrically linked

by an {Mo }-type unit. The {Mo 8 } building block, found
1
In generating large complex molecular species the corre- in many other large polyoxometalate structures, is itself
sponding natural processes are effected by the (directed (as mentioned above) built up from a densely packed

FIGURE 14 Schematic representation of the general architecture principle for the Giant-Wheel-type clusters, using
the {Mo 154 } cluster (top view left-hand side and side view right-hand side) as an example. The structural building
blocks for all Giant-Wheel clusters can be formulated as comprising {Mo 8 } units that are connected together via the
{Mo 2 } units on the inner side of the wheel. Finally, the {Mo 1 } units form the equatorial plane of the ring linking the
{Mo 8 } units together above and below the equator (the {Mo 1 } units are circled in the side view of the cluster for clarity).