Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10- Polyoxometalate

The reaction of Na-10[alpha-SiW9O34] with tetravalent metallic cations such as 4f ((NH4)(2)Ce(NO3)(6)), 5d (HfCl4), or Sf (UCl4 and Th(NO3)(4)) in a pH 4.7 sodium acetate buffer solution leads to the formation of four sandwichtype polyoxometalates [Ce-4(mu(3)-O-2 (SiW9O34)(2) (CH3COO)(2)](10-) (1), [Ce-4 (mu(3) -O)(2)(SiW9O34)(2)(CH3COO)(2)](10-) (2), [Th-3(mu(3)-O)(mu(2) - OH)(3)(SiW9O34)(2)](13-) (3), and [Hf-3(mu(2)-OH)(3) (SiW9O34)(2)](11-) (4). All four compounds consist of a polynudear cluster fragment stabilized by two [alpha-SiW9O34](10-) oi polyanions. Compounds 1 and 2 are isostructural with a tetranuclear core (Ce-4, U-4), while compound 3 presents a trinudear Th-3 core bearing a mu(3)-O-centered bridge. It is an unprecedented configuration in the case of the thorium(IV) cluster. Compound 4 also possesses a trinudear Hf-3 core but with the absence of the mu(3)-O bridge. The molecules have been characterized by single-crystal X-ray diffraction, W-183 and Si-29 nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis.