Introduction - New Synthetic Routes to Polyoxometalate Containing Ionic Liquids - page 8

chemistry [29] , catalysis [30, 31] and medicine [32-34] and are as novel materials of great
interest to materials science.
1.3.2 Formation of clusters - the self-assembly process
Mimicking " natura naturans " or the self-causing activity of nature is a long cherished
dream of chemists. In nature complex molecular systems like proteins are formed by
molecular growth guarding the gateway to molecular complexity. Attaining extensive
compounds correspondingly in the lab under one-pot conditions without the problem of
repetitive separation and purification of individual products is a fiercely pursued goal.
The world of polyoxometalates is such a kind of chemistry were highly ordered clusters
are spontaneously formed from smaller building blocks by self-assembly, leading to
completely new physical and chemical properties compared to the starting materials.
POMs can range in size from merely six metal centers to the size of many proteins and
show versatile chemical, photochemical and catalytic activity. Their properties can be
further tuned on the molecular level by chemical and structural modification of the
cluster shell. It is this dramatic increase in complexity and functionality without the
need of extensive synthetic efforts which makes self-assembled structures so promising
for materials science and to one of the most fascinating areas of research in modern
inorganic chemistry.
Though the process of self-assembly in polyoxometalates is described very well, [35] and
it has been studied extensively by in-situ 1 H-, 17 O- and 138 W-nuclear magnetic resonance
spectroscopy, the exact reaction mechanism is still unknown. [36-39] However it is clear
that the assembly of large cluster compounds, falling into the field of inorganic
coordination chemistry, is strongly influenced by coordinative bonds owing to their
kinetic lability and variable bond strength. In particular the self-assembly of
polyoxometalates, anionic transition metal-oxygen clusters, is dominated by the
formation of coordinative bonds between the electron deficient metal centre and
oxygen atoms which act as nucleophilic ligands. Assembly of these preformed
complexes into structurally extended systems is driven mainly by intra- and
intermolecular interactions such as Coulomb interactions, hydrogen bonds and
dispersive interactions which guide the self-assembly into complex systems and which