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

Three high-nuclearity NiII-substituted polyoxometalate compounds functionalized by exogenous ligands have been synthesized and characterized. The octanuclear complexes in Na15[Na{(A-α-SiW9O34)Ni4(CH3COO)3(OH)3}2]·4NaCl·36H2O (1) and Na15[Na{(A-α-SiW9O34)Ni4(CH3COO)3(OH)2(N3)}2]·32H2O (2) can be described as two {Ni4} 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 μ-hydroxo ligands present in 1. The nonanuclear complex K7Na7[(A-α-SiW9O34)2Ni9(OH)6(H2O)6(CO3)3]·42H2O (3) exhibits a double-cubane structure with two [(A-α-SiW9O34)Ni4(OH)3]5− subunits linked by three carbonato ligands. A ninth NiII 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-α-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.