Chapter 4 - Jacobs University

Chapter 3 The Wheel-Shaped Cu 20
Chapter 3
The Wheel-Shaped Cu 20 Tungstophosphate
[Cu 20 Cl(OH) 24 (H 2 O) 12 (P 8 W 48 O 184 )] 25-
3.1 Introduction
Polyoxometalates are a unique class of metal-oxide clusters. This family of compounds
was discovered by Berzelius many years ago, but only recently the full potential of these species
has been realized. 1-2 The search for novel polyanion structures is predominantly driven by the
manifold applications of these compounds in areas as diverse as catalysis, bio- and
nanotechnology, medicine and materials science. 3-9
The structural beauty of polyoxometalates is an additional feature why this class has
attracted much attention. Especially the work of Müller et al. has resulted in discrete molecular
species with spectacular sizes and symmetries. For example, these authors reported on gigantic
mixed-valence polyoxomolybdate rings and spheres containing up to 368 molybdenum atoms. 10
Pope et al. reported on a polyoxotungstate with 148 tungsten atoms,
[As 12 Ce 16 (H 2 O) 36 W 148 O 524 ] 76- . 11
Interestingly all of the above species were synthesized without using any polyanion
precursor. It must be realized that formation of polyanions is a self-assembly process, which
depends more on the reaction conditions (e.g. pH, concentration and ratio of reagents, ionic
strengths) than the type of polyanion precursors used.
We have been interested for some time in transition metal substituted polyoxotungstates,
especially with respect to their magnetic and electrochemical properties. 12 There is enormous
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