The rare-earth derivant of mixed-polyoxoniobate clusters with high proton release capacity

Abstract

A family of rare-earth derivant mixed-polyoxoniobate clusters K₁₂(NH₄)₁₀[{Nb₁₂P₄W₂₄O₁₂₂}₂{Ln(H₂O)₅}₄{Nb₄O₄(OH)₆}]·xH₂O (Ln = Sm, Eu, Tb, Dy, Er, Tm and Yb for 1–7, abbreviated as {Ln₄Nb₂₈}) were synthesized and structurally characterized by single-crystal X-ray diffraction, elemental analyses, IR spectroscopy and TG analyses. Containing four Wells–Dawson [Nb₆P₂W₁₂O₆₂]¹²⁻, a {Nb₄O₆} core, and four Ln^III ions, the polyanions in 1–7 are a group of rare-earth derivants of phosphoniobotungstates. These water-soluble clusters behave as weak acids with good stability and high proton release capacity depending on the pH. Each cluster carries ∼22 negative charges in the aqueous solution without any deprotonation with the pH the same as that used for deionized water. Upon the introduction of bases, they get deprotonated gradually and each anion cluster can release up to 20 protons from its 20 coordinated water ligands. The pK_a values of these acidic clusters with different degrees of deprotonation range from ∼8.3 to ∼10.5. Moreover, these clusters demonstrate an increasing deprotonation efficiency with the decreasing ionic radius of incorporated Ln^III ions, which could be attributed to the lanthanide contraction. In other words, the {Ln₄Nb₂₈} macroanion clusters with smaller Ln^III centers are easier to deprotonate due to their shorter and stronger Ln–O bonds. This is the first study that focuses on the effect of lanthanide contraction on proton release in polyoxometalate chemistry. The {Ln₄Nb₂₈} clusters with good stability, high proton release capacity, and controllable deprotonation efficiency provide models for the understanding of protonated polyelectrolyte solutions, and the design and applications of polyoxometalate-based protonated materials.