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Synthesis and Structural Characterization of ... - Jacobs University

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Chapter 4<br />

Unpublished Results<br />

This polyanion is composed <strong>of</strong> three units <strong>of</strong> P 2 W 12 fused via the respective caps leading to<br />

the open trimeric P 6 W 36 cluster which has no extra tungsten atoms but two extra dangling<br />

phosphate groups instead. Furthermore, the parent P 8 W 48 is known to pick up tungsten atoms<br />

during reaction with transition metal ions. 8a,8f,9<br />

However, the cyclic tungsten-oxo skeleton <strong>of</strong> P 8 W 48 has been known to be chemically robust<br />

<strong>and</strong> to date no reaction in relatively mild acidic pH such the one used in this synthesis has led<br />

to such dramatic altering in the P 8 W 48 skeleton. It is still not clear whether this ring opening is<br />

a result <strong>of</strong> interaction with the lanthanide centers or hydrogen peroxide or both. Several<br />

attempts to obtain the open-ring Fe 16 P 8 W 49 in the absence <strong>of</strong> lanthanide ions using different<br />

synthetic pathways are so far unsuccessful. Therefore, we believe that the presence <strong>of</strong><br />

lanthanide cations in the solution seems to be a crucial factor in the successful synthesis <strong>and</strong><br />

isolation <strong>of</strong> polyanions 24 <strong>and</strong> 25. Moreover, the extra WO 6 group comes most probably from<br />

in situ partial or complete decomposition <strong>of</strong> some P 8 W 48 precursors in solution. The formation<br />

pathway <strong>of</strong> 24 <strong>and</strong> 25 from Fe 16 P 8 W 48 can be postulated as follows:<br />

- Ring opening in [Fe 16 (OH) 28 (H 2 O) 4 (P 8 W 48 O 184 )] 20− through cleavage <strong>of</strong> four Fe–O–<br />

Fe <strong>and</strong> two W–O–W <strong>and</strong> insertion <strong>of</strong> a tungsten center to give<br />

[Fe 16 O 2 (OH) 23 (H 2 O) 9 (P 8 W 49 O 189 )] 23- (Fe 16 P 8 W 49 ).<br />

- Coordination <strong>of</strong> four lanthanide centers to Fe 16 P 8 W 49 to give the title polyanions 24<br />

<strong>and</strong> 25.<br />

<strong>Structural</strong> changes arising from the ring opening are reflected in the interatomic distance<br />

between W1 <strong>and</strong> W47 as well as W2 <strong>and</strong> W48, with an average value <strong>of</strong> ca. 8.4 Å for 24 <strong>and</strong><br />

25 in comparison with 3.7 Å for Fe 16 P 8 W 48 . In addition, the average <strong>of</strong> the W23–O–W25 <strong>and</strong><br />

W24–O–W26 angles is around 133˚ for 24 <strong>and</strong> 136˚ for 25, in comparison with 144˚ for<br />

Fe 16 P 8 W 48 .<br />

160

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