Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
proposed a structure for the 12:1 complexes based on a central XO 4 tetrahedron surrounded
by twelve MO 6 octahedra, where he noted that Mo 6+ and W 6+ had crystal radii appropriate for
octahedral coordination with oxygens. In order to minimize the electrostatic repulsions, all
polyhedra were believed to be linked via only shared corners. 7
In 1933, Keggin solved the structure of H 3 [PW 12 O 40 ] by powder X-ray diffraction and
reported the first polyoxoanion “the parent structure” which is now known as Keggin
polyanion. 8
Keggin showed that anion was based on WO 6 octahedral units and these
octahedra were linked by shared edges as well as corners. Thereafter other types of structure
were reported such as the Anderson-Evans, Lindqvist and Wells-Dawson structures in 1948,
1950, and 1953, respectively. 9-11 The development of modern experimental techniques such as
single-crystal X-ray diffraction and NMR spectroscopy was the turning point for the
determination of structures in POM chemistry. Furthermore, these developments in
combination with the use of modern analytical techniques such as electrochemistry or
electrospray ionization (ESI) mass spectrometry have allowed establishing important links
between solid and solution structures of polyanions. 1b Hence, in the past fifty years, hundreds
of structures have been reported and new classes of structures with unexpected reactivity and
application are still being studied.
1.2 Structural principles of polyoxometalates
POMs are composed of metallic centers or addenda atoms (M) surrounded by oxygen atoms.
Oxygen atoms are the common ligands in polyanions, hence the term “polyoxo”, and are
usually present as bridging and terminal oxo ligands. In some cases protonation of in
particular bridging oxygen atoms can also be observed.
POMs are composed of MO n units, where ‘n’ indicates the coordination number of M (n = 4,
5, 6 or 7). Usually the distorted octahedral coordination (n = 6) is observed. Two main types
of POMs are known, based on their chemical composition: isopoly and heteropoly anions.
2