Chapter 4 - Jacobs University

Chapter 6
U-peroxo-Polyoxometalate
6.4 Results and discussion
6.4.1 Synthesis and structure
Polyanion [Li(H 2 O)K 4 (H 2 O) 3 {(UO 2 ) 4 (O 2 ) 4 (H 2 O) 2 } 2 (PO 3 OH) 2 P 6 W 36 O 136 ] 25- (4) was synthesized
by reaction of the [H 6 P 4 W 24 O 94 ] 18- (P 4 W 24 ) precursor with UO 2+ 2 ions in a 1:3 ratio in aqueous
medium at pH 4.0. The polyanion was crystallized as a mixed potassium-lithium salt in the
monoclinic space group P2 1 /n. The compound
K 6 Li 19 [Li(H 2 O)K 4 (H 2 O) 3 {(UO 2 ) 4 (O 2 ) 4 (H 2 O) 2 } 2 (PO 3 OH) 2 P 6 W 36 O 136 ]×74H 2 O (4a) has been fully
characterized in the solid state by single-crystal XRD, TGA-DSC, IR, and elemental analysis, as
well as in solution by 31 P NMR. Interestingly, we were unable to obtain 4 by using P 8 W 48 as the
reagent instead of P 4 W 24 .
Polyanion 4 is composed of three units of P 2 W 12 fused via the respective caps and two dangling
phosphates, which are hanging outside of the polyanion where a fourth unit of P 2 W 12 would be
placed in P 8 W 48 (Figure 6.3). In contrast with the various complexes mentioned above, only
three “P 2 W 12 “ units combine in 4 to form a gapped ring encircling the central U 8 -peroxo moiety.
This is the first time that such a U-shaped “(P 2 W 12 ) 3 ” assembly has been seen.
There are also four potassium ions inside the anion, which connect the two independent, neutral
[(UO 2 )(O 2 )] 4 units. The third P 2 W 12 unit and two dangling phosphate groups are apparently
formed in situ by decomposition of some P 4 W 24 precursor. The cavity of the newly generated
polyanion “P 6 W 36 ” is filled by two symmetrical uranium-peroxo clusters. This type of uranium
cluster has been seen once before with the 2,3,6,7-tetrahydroxy-9, 10-dimethyl-9, 10-dihydro-9,
10-ethnoanthacene organic ligand, 24
but this is the first report of a uranium-peroxo cluster
coordinated with an inorganic nucleophile.
113