Synthesis and Structural Characterization of ... - Jacobs University

Chapter 4
Unpublished Results
Chapter IV
Mixed Lanthanide/d‐Transition Metal Containing POMs
Ring opening in the cyclic Phosphotungstate
[Fe16O2(OH)23(H2O)9(P8W49O189)Ln4(H2O)20] 11−
4.1. Introduction
Polyoxometalates (POMs) represent a class of discrete structurally and chemically diverse
nanosized metal-oxygen molecular anions (hence the ‘oxometalate’ terminology). 1a Due to
this diversity, POMs have acquired interesting chemical and physical properties useful for
applications in fields such as photochemistry, clinical chemistry, magnetism, catalysis and
materials science. 1-2
Phosphotungstates are part of a subclass of POMs known as heteropolytungstates where
the metal is tungsten (VI) and the ‘heterogroup’ is phosphate. The majority of known
phophotungstate structures are based on the classical Keggin [PW 12 O 40 ] 3− and Wells-Dawson
[P 2 W 18 O 62 ] 6− types; the Keggin type having one heterogroup and the Wells-Dawson type
having two. The Wells-Dawson type phosphotungstates are known to have distinct chemical
behaviour compared to the Keggin type ones, although some common properties arise for
both. 1a
Mono-, tri-, and hexa-vacant species of the Wells-Dawson molecule [P 2 W 18 O 62 ] 6−
(P 2 W 18 ) can be formed and isolated in aqueous solution depending on the pH and countercations
present during their synthesis. The monovacant anion [P 2 W 17 O 61 ] 10−
(P 2 W 17 ) is
isolated as a potassium salt by hydrolysis of P 2 W 18 at pH 6-7 using KHCO 3 . The trivacant
anion [P 2 W 15 O 56 ] 12− (P 2 W 15 ) is however formed by hydrolysis of P 2 W 18 using Na 2 CO 3
without the presence of potassium at pH 9-10, and is isolated as a sodium salt. Finally, the
hexavacant anion [H 2 P 2 W 12 O 48 ] 12− (P 2 W 12 ) is formed by interaction of P 2 W 18 with the base
(tris-hydroxymethylaminomethane) and is isolated as a potassium salt. 1f,3 Although P 2 W 12 ,
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