title: Octa- and nonanuclear nickel(II) polyoxometalate clusters: synthesis and electrochemical and magnetic characterizations.
abstract: Three high-nuclearity NiII-substituted polyoxometalate compounds functionalized by exogenous ligands have been synthesized and characterized. The octanuclear complexes in Na15[Na{(A-R-SiW9O34)Ni4(CH3COO)3(OH)3}2] . 4NaCl . 36H2O (1) and Na15[Na{(A-R-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 mu-hydroxo ligands present in 1. The nonanuclear complex K7Na7[(A-R-SiW9O34)2Ni9(OH)6(H2O)6(CO3)3] . 42H2O (3) exhibits a double cubanestructure with two [(A-R-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-R-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.
authors.affiliation: Institut LaVoisier de Versailles, UMR 8180, UniVersité de Versailles Saint-Quentin, 45 AVenue des Etats-Unis, 78035 Versailles Cedex, France.
authors.forename: Céline
authors.initials: C
authors.lastname: Pichon
authors.forename: Pierre
authors.initials: P
authors.lastname: Mialane
authors.forename: Anne
authors.initials: A
authors.lastname: Dolbecq
authors.forename: Jérôme
authors.initials: J
authors.lastname: Marrot
authors.forename: Eric
authors.initials: E
authors.lastname: Rivière
authors.forename: Bassem S
authors.initials: BS
authors.lastname: Bassil
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kortz
authors.forename: Bineta
authors.initials: B
authors.lastname: Keita
authors.forename: Louis
authors.initials: L
authors.lastname: Nadjo
authors.forename: Francis
authors.initials: F
authors.lastname: Sécheresse