Effect of Cyanato, Azido, Carboxylato, and Carbonato Ligands on the Formation of Cobalt(II) Polyoxometalates: Characterization, Magnetic, and Electrochemical Studies of Multinuclear Cobalt Clusters

• Published in: Chemistry : a European journal 2007-01, Vol.13 (12), p.3525-3536
• Main Authors: Lisnard, Laurent, Mialane, Pierre, Dolbecq, Anne, Marrot, Jérôme, Clemente-Juan, Juan Modesto, Coronado, Eugenio, Keita, Bineta, de Oliveira, Pedro, Nadjo, Louis, Sécheresse, Francis
• Format: Article
• Language: English
• Subjects: Chemical Sciences; cobalt; electrochemistry; Inorganic chemistry; ligand effects; magnetic properties; polyoxometalates
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200601252

Five CoII silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K20[{(B‐β‐SiW9O33(OH))(β‐SiW8O29(OH)2)Co3(H2O)}2Co(H2O)2]⋅47 H2O (1) consists of two {(B‐β‐SiW9O33(OH))(β‐SiW8O29(OH)2)Co3(H2O)} units connected by a {CoO4(H2O)2} group. In the chiral species K7[Co1.5(H2O)7)][(γ‐SiW10O36)(β‐SiW8O30(OH))Co4(OH)(H2O)7]⋅36 H2O (2), a {γ‐SiW10O36} and a {β‐SiW8O30(OH)} unit enclose a mononuclear {CoO4(H2O)2} group and a {Co3O7(OH)(H2O)5} fragment. The two trinuclear CoII clusters present in 1 enclose a μ4‐O atom, while in 2 a μ3‐OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co‐L‐Co (L=μ4‐O (1), μ3‐OH (2)) bridging angles in 2 (θav(Co‐L‐Co)=99.1°) than in 1 (θav(Co‐L‐Co)=92.8°). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co‐L‐Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of CoII centers, and indicated that two types of CoII centers in the structure were oxidized in separate waves. Redox activity of the seventh CoII center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K5Na3[(A‐α‐SiW9O34)Co4(OH)3(CH3COO)3]⋅18 H2O (3) and K5Na3[(A‐α‐SiW9O34)Co4(OH)(N3)2(CH3COO)3]⋅18 H2O (4) were characterized: in both, a tetrahedral {Co4(L1)(L2)2(CH3COO)3} (3: L1=L2=OH; 4: L1=OH, L2=N3) unit capped the [A‐α‐SiW9O34]10− trivacant polyanion. The octanuclear complex K8Na8[(A‐α‐SiW9O34)2Co8(OH)6(H2O)2(CO3)3]⋅52 H2O (5), containing two {Co4O9(OH)3(H2O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed. Has anyone seen the like before? The successful grafting of acetato, azido, or carbonato ligands onto the paramagnetic centers embedded in the polyanionic matrix of CoII polyoxometalates has afforded paramagnetic clusters (an example of which is shown here) unprecedented in the chemistry of these complexes.