Synthesis and Structural Characterization of ... - Jacobs University

Chapter 4
Unpublished Results
being hexavacant, is regarded as a multi-dentate ligand for interaction with electrophiles, in
particular transition metal cations, very few compounds are known to form from such
interaction due the metastability of P 2 W 12 in solution. 4
Contant and Tézé were able to condense four P 2 W 12 units in lithium acetate buffer,
resulting in the cyclic polyanion [H 7 P 8 W 48 O 184 ] 33− (P 8 W 48 ) which can be isolated as a mixed
potassium/lithium salt. 5 The wheel-shaped phosphotungstate P 8 W 48 polyanion is stable at an
unusually large pH range (1-8) and has D 4h point symmetry.
The multi-vacant P 8 W 48 , having 16 donor oxygens and a large central cavity (diameter of
around 10 Å), can be considered as ‘super-lacunary’, but its stability and inertness were
noteworthy. Although the wheel-shaped polyanion was reported in 1985, 5 the first transition
metal derivative was reported twenty years later by our group. 6 The large, wheel-shaped
[Cu 20 Cl(OH) 24 (H 2 O) 12 (P 8 W 48 O 184 )] 25− was synthesized by direct reaction of Cu(II) ions with
the precursor P 8 W 48 in aqueous medium. This indicated that the terminal oxygens in the
cavity of the tungstophosphate precursor P 8 W 48 can actually interact with transition-metal
ions. This cavity is occupied by a highly symmetrical copper-hydroxo cluster (Cu 20 ) where all
20 copper centers are bridged to adjacent copper ions by protonated µ 3 -oxo ligands. The
Cu 20 -containing polyanion exhibits a highly symmetrical D 4h structure. Furthermore, the
copper cluster is composed of three structurally unique types of Cu II ions with different
coordination numbers and geometries (Jahn-Teller-distorted octahedral, square-pyramidal,
and square-planar geometry). The center of the cluster cavity which has a diameter of around
7 Å is occupied by a chloride ion. The Cu 20 polyanion was further studied by
electrochemistry and magnetism, and proved to form nanosized ‘blackberry’ assemblies in
water; furthermore, other halide derivatives of Cu 20 were synthesized. 7 Since then, more 3d
and 4d transition metal derivatives of the cyclic P 8 W 48 polyanion have been synthesized and
reported. 8
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