A rare structural characterisation of the phosphomolybdate lacunary anion, [PMo11O39]7−. Crystal structures of the Ln(iii) complexes, (NH4)11[Ln(PMo11O39)2]·16H2O (Ln = CeIII, SmIII, DyIII or LuIII)

Abstract

The syntheses and the crystal structures of (NH₄)₁₁[Ln^III(PMo₁₁O₃₉)₂]·16H₂O (Ln^III = Ce (1), Sm (2), Dy (3) or Lu (4)) are reported in which an Ln^III cation is sandwiched between two ‘lacunary’ [PMo₁₁O₃₉]⁷⁻ anions to give a complex with eight oxygen atoms coordinated to the lanthanide centre in a twisted square antiprismatic geometry. This analogous series of complexes represents only the second time that the [PMo₁₁O₃₉]⁷⁻ anion has been fully crystallographically characterised. All four compounds are synthesised in high yield and characterised by a range of physical and spectroscopic techniques. ³¹P NMR, Raman and UV/Vis/nIR spectroscopies give a clear indication that the [Ln^III(PMo₁₁O₃₉)₂]¹¹⁻ anion is also stable in solution. As Ln^III contracts across the lanthanide series the Ln–O bond distances decrease and the splitting of the ν_P–O vibrational mode within the [PMo₁₁O₃₉]⁷⁻ unit increases. The relative stability of this species in solution may have importance in elucidating the speciation of Ln(III) and An(III) cations in nuclear waste solutions where phosphopolymolybdate anions are known to form.