Effect of cyanato, azido, carboxylato, and carbonato ligands on the formation of cobalt(II) polyoxometalates:: Characterization, magnetic, and electrochemical studies of multinuclear cobalt clusters

Five Co-II silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K-20[{(B-beta-SiW9O33(OH))(beta-SiW8O29-(OH)(2))Co-3(H2O)}(2)Co(H2O)(2)].47 H2O (1) consists of two {(B-beta-SiW9O33(OH))(beta-SiW8O29(OH)(2))Co-3-(H2O)} units connected by a {CoO4- (H2O)(2)} group. In the chiral species K-7-[Co-1,Co-5(H2O)(7))][(gamma-SiW10O36)(beta-SiW8O30(OH))Co-4(OH)(H2O)(7)].36H(2)O (2), a {gamma-SiW10O31} and a {beta-SiW8O30(OH)} unit enclose a mononuclear {CoO4-(H2O)(2)} group and a {Co3O7(OH)(H2O)(5)} fragment. The two trinuclear Co-II clusters present in 1 enclose a mu(4)-O atom, while in 2 a mu(3)-OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co- L-Co (L=mu(4)-O (1), mu(3)-OH (2)) bridging angles in 2 (theta(av(Co-L-Co)) = 99.1 degrees) than in 1 (theta(av(Co-L-Co))=92.8 degrees). 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 Co-II centers, and indicated that two types of Co-II centers in the structure were oxidized in separate waves. Redox activity of the seventh Co-II 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-alpha-SiW9O34)Co-4(OH)(3)(CH3COO)(3)}.18 H2O (3) and K5Na3[(A-alpha-SiW9O34)Co-4(OH)(N-3)(2)-(CH3COO)(3).18H(2)O (4) were characterized: in both, a tetrahedral {Co-4(L-1)(L-2)(2)(CH3COO)(3)} (3: L-1-L-2=OH; 4: L-1=OH, L-2=N-3) unit capped the [A-alpha-SiW9O34](10-) trivacant polyanion. The octanuclear complex K8Na8[(A-alpha-SiW9O34)(2)Co-8(OH)(6)(H2O)(2)(CO3)(3)].52H(2)O (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.