Influence of Steric and Electronic Properties of the Defect Site, Lanthanide Ionic Radii, and Solution Conditions on the Composition of Lanthanide(III) α₁-P₂W₁₇O₆₁^10- Polyoxometalates

This study identifies the principles that govern the formation and stability of Ln complexes of the (α₁-P₂W₁₇O₆₁)^10- isomer. The conditional stability constants for the stepwise formation equilibria, K_1cond and K_2cond, determined by ³¹P NMR spectroscopy, show that the high log K_1cond/log K_2cond ratio predicts the stabilization of the 1:1 Ln/ (α₁-P₂W₁₇O₆₁)^10- species. The value of log K_1cond increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic α₁ defect site. The conditional stability constants, K₂, are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early−mid lanthanide analogues, where the close contact of the (α₁-P₂W₁₇O₆₁)^10- units result in severe steric encumbrance. The electronic properties of the α₁ defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis of lanthanide polyoxometalates.