Synthesis and Structural Characterization of ... - Jacobs University

Chapter 3
Unpublished Results
3.B.3. Lanthanide Substituted Polyoxoanions Containing Bridging
Acetate Ligand: [{Ln(‐CH3COO)(H2O)2(GeW11O39)}2] 12− (Ln = Eu, Gd, Lu)
3.B.3.1. Introduction
Polyoxometalates (POMs) represent a unique class of inorganic compounds due to the
structural variety as well as interesting and often unexpected properties in fields as diverse as
catalysis, medicine and magnetochemistry. 1 Although the first synthetic POM is known since
1826, the mechanism of POM formation is still not well understood and is commonly
described as a self-assembly. Nevertheless the synthesis of polyoxometalates is mostly rather
simple and straightforward, once the proper reaction conditions have been identified.
Lacunary polyoxometalates are usually synthesized from complete precursor ions by loss
of one or more MO 6 octahedra. It has been shown that interaction of rare-earth metals with
lacunary polyoxoanion precursors has been investigated predominantly by Pope and
Francesconi and some structures have been identified. 2
Most lanthanide-containing
polyoxoanions are composed of monolacunary Keggin and Wells–Dawson fragments (e.g.,
[La(-SiW 11 O 39 )(H 2 O) 3 ] 5− , [{Ce(H 2 O) 4 ([ 1 -P 2 W 17 O 61 )} 2 ] 14− , [{Eu(H 2 O) 3 ([ 2 -
P 2 W 17 O 61 )} 2 ] 14− ). The monolacunary Keggin and Wells–Dawson derivatives can be
considered as pentadentate ligands, but close inspection of the structures of the above species
indicates that the lanthanide ions are too big to enter the respective vacancies fully. They
usually sit somewhat above the lacunary hole and are coordinated to the four equatorial oxodonors
of the polyoxoanion ligands. The coordination sphere is usually completed in solution
by terminal water molecules, but these monomeric species have a strong tendency to dimerize
upon crystallization. 2c
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