Octa- and Nonanuclear Nickel(II) Polyoxometalate Clusters: Synthesis and Electrochemical and Magnetic Characterizations

Three high-nuclearity Ni-II-substituted polyoxometalate compounds functionalized by exogenous ligands have been synthesized and characterized. The octanuclear complexes in Na-15[Na{(A-alpha-SiW9O34)Ni-4(CH3COO)(3) (OH)(3)}(2)]center dot 4NaCl center dot 36H(2)O (1) and Na-15[Na((A-alpha-SiW9O34)Ni-4(CH3COO)(3)(OH)(2)(N-3)}(2)]center dot 32H(2)O (2) can be described as two {Ni-4} subunits connected via a {Na(CH3COO)(6)} group, with the acetato ligands also ensuring in each subunit the connection. between the paramagnetic centers. In 2, two azido groups replace two of the six mu-hydroxo ligands present in 1. The nonanuclear complex K7Na7[(A-alpha-SiW9O34)(2)Ni-9(OH)(6)(H2O)(6)(CO3)(3)]center dot 42H(2)O (3) exhibits a double-cubane structure with two [(A-alpha-SiW9O34)Ni-4(OH)(3)](5-) subunits linked by three carbonato ligands. A ninth Ni-II center connected to one subunit via a carbonato ligand and a O=W group completes this asymmetric polyoxometalate. Electronic spectroscopy and electrochemical studies indicate that, while compounds 1-3 decompose in a pure aqueous medium, these complexes are very stable in a pH 6 acetate medium. The cyclic voltammetry pattern of each complex is constituted by a first eight-electron reduction wave followed by a second large-current intensity wave. The characteristics of the first waves of the complexes are clearly distinct from those obtained for their lacunary precursor [A-alpha-SiW9O34](10-), a feature that is due to the Ni centers in the complexes. Such observations of electroactive, stable, and highly nickel-rich polyoxometalates are not common. Measurements of the magnetic susceptibility revealed the occurrence of concomitant ferromagnetic and antiferromagnetic interactions in 1 and 3. For both of these compounds, the extension of the magnetic exchange has been determined by means of a spin Hamiltonian with three and four J constants, respectively.