Coordination of rare-earth elements in complexes with monovacant Wells-Dawson polyoxoanions

The α-1 and β-2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P₂W₁₇O₆₁]^10- are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with α-1-[P₂W₁₇O₆₁]^10- and α-2-[P₂W₁₇O₆₁]^10- were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(α-1-P₂W₁₇O₆₁)]^7- and [Ln(α-2-X₂W₁₇O₆₁)₂] ^7- (for Ln ≡ Sm, Eu, Lu; X ≡ P, As). The crystal and molecular structures of [(H₂O)₄-Lu(α-1-P ₂W₁₇O₆₁)]^7- (1) and [Lu(α-2-P₂W₁₇O₆₁)₂] ^17- (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/α-1-[P₂W₁₇O₆₁]^10- complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as [(H₂O)₄Ln(α-1-P₂ W₁₇O₆₁)]^7-. This structure (1) is the first of its kind for any metal complex of α-1-[P₂W₁₇O₆₁]^10-, and the data indicate that the general stoichiometry [(H₂O)₄Ln(α-1-P₂W ₁₇O₆₁)]^7- is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/α-2-[X₂W₁₇O₆₁]^10- complexes, no water molecules are in the Ln-O₈ coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(α-2-X₂W₁₇O₆₁)₂] ^17-. The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.