Chapter 4 - Jacobs University

Chapter 3 The Wheel-Shaped Cu 20
interest in the synthesis of molecular species with high spin ground states. Lately Christou et al.
reported on the largest molecular magnet known to date composed of 84 manganese atoms. 13
Until then the so called ‘Mn12-acetate’ had probably been the most attractive species in the area
of single molecule magnets. 14 Also polyoxometalate chemistry plays a major role in this field, as
it allows for a bottom-up synthesis of paramagnetic multi-metal-oxo-hydroxo clusters which are
encapsulated and stabilized by diamagnetic polyanion fragments. 5-8,15 The magnetic and EPR
properties of such discrete clusters can be analyzed in great detail, as usually intermolecular
interactions are negligibly small. 12
Nevertheless, it must be realized that routinely
polyoxotungstates with only 3-4 transition metal ions have been synthesized, with only a handful
of systems containing 5 or more paramagnetic centers. 12a,b,16
In our search for highly lacunary polyanion ligands which might be an appropriate
template for significantly larger paramagnetic clusters we have focused our attention on the wellknown
crown heteropolyanion [H 7 P 8 W 48 O 184 ] 33- . 17
This species is composed of four
[H 2 P 2 W 12 O 48 ] 12- fragments which are linked via the cap tungsten atoms resulting in a cyclic
arrangement. The stability of [H 7 P 8 W 48 O 184 ] 33- in aqueous solution over an unusually large pH
range (1-8) and its large central cavity (diameter of around 10 Å) are highly attractive features.
We can consider [H 7 P 8 W 48 O 184 ] 33- as a superlacunary polyanion, but surprisingly this species has
been pretty much neglected as a precursor. 18 This is most likely due to the fact that Tézé and
Contant concluded in 1985 that [H 7 P 8 W 48 O 184 ] 33- ‘does not give complexes with divalent or
trivalent transition-metal ions’. 17a Nevertheless, we decided to investigate in detail the reactivity
of paramagnetic 3d metal ions with [H 7 P 8 W 48 O 184 ] 33- in aqueous medium.
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