Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
In general, POMs exhibit a multitude of properties: stability in solution and in the solid
state, solubility in organic and aqueous solvents, electrochemical activity and incorporation of
main group elements, transition metals, rare earths or organic groups. Two of these properties
will be discussed below.
1.5.1 Acid properties of polyoxometalates
The free acids of POMs, in which the proton H + acts as the only counter-cations on the anion,
“heteropoly acids, HPAs” have high Brønsted acidity of the corresponding acids. The
strengths of the common Keggin molybdic and tungstic acids are stronger in a Brønsted sense
than the common mineral acids such as HCl, H 2 SO 4 and HNO 3 . Early methods of
characterization of HPAs involved titration with base to determine stoichiometry. Two endpoints
can be detected, the first corresponding to the neutralization of the acid protons and the
second to the complete dissociation of the anions. 1a,1f As aforementioned the first X-ray
investigation of POMs were made on the acids, started by Keggin in 1933. Keggin structure
was redetermined on various types of heteroatoms by both X-ray and neutron diffraction, the
latter showed and confirmed the number of the protons attached to the polyoxometalate
anions. Furthermore, the acid centers are considered to be at the bridging oxygen atoms on the
surface of the anion. 1a,1f
1.5.2 Redox activity of polyoxometalates
The addenda metal atoms in most POMs are in their highest oxidation states (d 0 ), therefore
many POMs are powerful oxidizing agents and undergo multiple reversible one- or twoelectron
reductions which lead to form intensely colored mixed-valence species “heteropoly
blue”. POMs are known which can accept many electrons up to 32 without major structural
change. Therefore, POMs are unequaled in their “electron reservoir” properties. The reduction
process depends on the acidity of the solution and the polyanion charge where the additional
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