Synthesis, Structure and Electrochemistry of the Dinickel(II)-Containing 30-Tungsto-4-Phosphate [Ni2Na2(H2O)2(P2W15O56)2]18- (2017)

Background: Lacunary (vacant) heteropolyanions are a subclass of polyoxometalates (POMs) and are known to act as inorganic, polydentate ligands with a multitude of oxophilic electrophiles such as 3d metal ions. The family of Weakley-type POM sandwich structures is rather large, as they are known for Keggin- and Dawson-based capping units, for a multitude of 3d metal ions, as well as different tetrahedral heteroatoms. Moreover, several structural modifications of the ‘sandwich core' in terms of metal nuclearity have been achieved, in particular two ‘inner' transition metal ions and two sodium ions in the ‘outer' positions. Objectives: Herein we report on the synthesis and characterization of a di-nickel(II)-containing Weakleytype heteropolytungstate based on the Dawson POM. Methods: Reaction of Ni2+ ions with the trilacunary Dawson ion [P2W15O56]12- in sodium acetate buffer (pH 4.6, Method 1) or slightly basic medium (pH 8, Method 2) resulted in the dinickel(II)-containing sandwichtype 30-tungsto-4-phosphate [Ni2Na2(H2O)2(P2W15O56)2]18-, which crystallized as the hydrated sodium salt Na18[Ni2Na2(H2O)2(P2W15O56)2]·70H2O. We also decided to compare the electrochemical properties of the polyanion with those of [Ni4(H2O)2(P2W15O56)2]16-, which was reported previously. The stability of both polyanions in aqueous solution as a function of pH and time was studied by monitoring the evolution of their UV-visible spectra and their cyclic voltammograms (CVs). Both are sufficiently stable at pH 3 and pH 5 to allow for characterization by cyclic voltammetry (CV). The title compound was also studied by thermal analysis (TGA), FT-IR spectroscopy and elemental analysis. Results: As based on single-crystal X-ray diffraction, the structure of [Ni2Na2(H2O)2(P2W15O56)2]18- corresponds to a Weakley-type sandwich dimer with a pair of nickel(II) ions in the ‘inner' positions and a pair of sodium ions in the ‘outer' positions, encapsulated by two [P2W15O56]12- POM units. The two nickel ions in the rhombic {Ni2Na2} core are structurally and crystallographically equivalent, and are octahedrally coordinated by six oxo ligands. The synthesis can be accomplished by two different procedures, either in sodium acetate buffer at pH 4.6 (Method 1) or in slightly basic aqueous medium by adding oxalate (Method 2). The CV of [Ni2Na2(H2O)2(P2W15O56)2]18- has a composite 2e-reduction wave, These redox waves are due to the reduction of the W centers. As far as the CV of [Ni4(H2O)2(P2W15O56)2]16- is concerned, only two distinct reduction waves are observed. As expected, the entire CV shifts towards negative potentials upon increasing the pH. Conclusion: We have synthesized and structurally characterized the dinickel(II)-containing 30-tungsto-4- phosphate [Ni2Na2(H2O)2(P2W15O56)2]18-, which was prepared by a one-pot reaction of NiII ions with the trilacunary Dawson ion [P2W15O56]12- via two different procedures, either in acetate buffer at pH 4.6 or in water at pH 8.0, both leading to the same hydrated sodium salt in crystalline form. The structure of the polyanion corresponds to a Weakley-type sandwich dimer with two nickel(II) ions occupying the ‘inner’ positions and two sodium ions the ‘outer’ positions of the rhombic core of the polyanion structure. Keywords: Polyoxometalates, nickel, tungsten, x-ray crystallography, electrochemistry.