Heteroanionic Materials Based on Copper Clusters, Bisphosphonates, and Polyoxometalates: Magnetic Properties and Comparative Electrocatalytic NOx Reduction Studies

Three compounds associating for the first time polyoxotungstates, bisphosphonates, and copper ions were structurally characterized. They consist in heteropolyanionic monodimensional materials where [Cu-6(Ale)(4)(H2O)(4)](4-) (Ale = alendronate = [O3PC(O)(C3H6NH3)PO3](4-)) complexes alternate with polyoxometalate (POM) units. In Na-12[{SiW9O34Cu3(Ale)(H2O)}{Cu-6(Ale)(4)(H2O)(4)}]center dot 50H(2)O (SiWo(9)CuAle), the polyoxometalate core consists in a {SiW9Cu3} monomer capped by a pentacoordinated Ale ligand, while sandwich-type Keggin {( SbW9O33)(2)Cu-3(H2O)(2.5)Cl-0.5} and Dawson {(P2W15O56)(2)Cu-4(H2O)(2)} complexes are found in Na8Li29[{(SbW9O33)(2)Cu-3(H2O)(2.5)Cl-0.5}(2){Cu-6(Ale)(4)(H2O)(4)}(3)]center dot 163H(2)O (SbW(9)CuAle) and Na-20(P2W15O56)(2)Cu-4(H2O)(2)}{Cu-6(Ale)(4)(H2O)(4)}]center dot 50H(2)O (P(2)W(15)CuAle), respectively. A comparative magnetic study of the SiW(9)CuAle and SbW(9)CuAle compounds enabled full quantification of the Cu superexchange interactions both for the POM and non-POM subunits, evidencing that, while the paramagnetic centers are anti-ferromagnetically coupled in the polyoxometalate units, both anti-ferromagnetic and ferromagnetic interactions coexist in the {Cu-6(Ale)(4)(H2O)(4)} cluster. All the studied compounds present a good efficiency upon the reduction of HNO2 or NO2-, the POM acting as a catalyst. However, it has been found that SbW(9)CuAle is inactive toward the reduction of nitrates, highlighting that both the {(SbW9O33)(2)Cu-3} unit and the {Cu-6(Ale)(4)(H2O)(4)} cluster do not act as electrocatalysts for this reaction. In contrast, SiW(9)CuAle and P(2)W(15)CuAle have shown a significant activity upon the reduction of NO3 and thus both at pH 1 and pH 5, evidencing that the chemical nature of the polyoxometalate is a crucial parameter even if it acts as precatalyst. Moreover, comparison of the activities of P(2)W(15)CuAle and [(P2W15O56)(2)Cu-4(H(2)o)(2)](16)- evidenced that if the [Cu-6(Ale)(4)(H2O)(4)](4-) cluster does not act as electrocatalyst, it acts as a cofactor, significantly enhancing the catalytic efficiency of the active POM.