Stabilization of Tetravalent 4f (Ce), 5d (Hf), or
5f (Th, U) Clusters by the [α-SiW<sub>9</sub>O<sub>34</sub>]<sup>10–</sup> Polyoxometalate
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Abstract
The
reaction of Na<sub>10</sub>[α-SiW<sub>9</sub>O<sub>34</sub>]
with tetravalent metallic cations such as 4f ((NH<sub>4</sub>)<sub>2</sub>Ce(NO<sub>3</sub>)<sub>6</sub>), 5d (HfCl<sub>4</sub>), or
5f (UCl<sub>4</sub> and Th(NO<sub>3</sub>)<sub>4</sub>) in a pH 4.7
sodium acetate buffer solution leads to the formation of four sandwich-type
polyoxometalates [Ce<sub>4</sub>(μ<sup>3</sup>-O)<sub>2</sub>(SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>(CH<sub>3</sub>COO)<sub>2</sub>]<sup>10–</sup> (<b>1</b>), [U<sub>4</sub>(μ<sup>3</sup>-O)<sub>2</sub>(SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>(CH<sub>3</sub>COO)<sub>2</sub>]<sup>10–</sup> (<b>2</b>), [Th<sub>3</sub>(μ<sup>3</sup>-O)(μ<sup>2</sup>-OH)<sub>3</sub>(SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>]<sup>13–</sup> (<b>3</b>), and [Hf<sub>3</sub>(μ<sup>2</sup>-OH)<sub>3</sub>(SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>]<sup>11–</sup> (<b>4</b>). All four compounds consist of a polynuclear cluster
fragment stabilized by two [α-SiW<sub>9</sub>O<sub>34</sub>]<sup>10–</sup> polyanions. Compounds <b>1</b> and <b>2</b> are isostructural with a tetranuclear core (Ce<sub>4</sub>, U<sub>4</sub>), while compound <b>3</b> presents a trinuclear Th<sub>3</sub> core bearing a μ<sup>3</sup>-O-centered bridge. It
is an unprecedented configuration in the case of the thorium(IV) cluster.
Compound <b>4</b> also possesses a trinuclear Hf<sub>3</sub> core but with the absence of the μ<sup>3</sup>-O bridge. The
molecules have been characterized by single-crystal X-ray diffraction, <sup>183</sup>W and <sup>29</sup>Si nuclear magnetic resonance (NMR) spectroscopy,
infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and
scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis
