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

The reaction of Na10[α-SiW9O34] with tetravalent metallic cations such as 4f ((NH4)2Ce­(NO3)6), 5d (HfCl4), or 5f (UCl4 and Th­(NO3)4) in a pH 4.7 sodium acetate buffer solution leads to the formation of four sandwich-type polyoxometalates [Ce4(μ3-O)2(SiW9O34)2(CH3COO)2]10– (1), [U4(μ3-O)2(SiW9O34)2(CH3COO)2]10– (2), [Th3(μ3-O)­(μ2-OH)3(SiW9O34)2]13– (3), and [Hf3(μ2-OH)3(SiW9O34)2]11– (4). All four compounds consist of a polynuclear cluster fragment stabilized by two [α-SiW9O34]10– polyanions. Compounds 1 and 2 are isostructural with a tetranuclear core (Ce4, U4), while compound 3 presents a trinuclear Th3 core bearing a μ3-O-centered bridge. It is an unprecedented configuration in the case of the thorium­(IV) cluster. Compound 4 also possesses a trinuclear Hf3 core but with the absence of the μ3-O bridge. The molecules have been characterized by single-crystal X-ray diffraction, 183W and 29Si nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis.