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 [Cu6(Ale)4(H2O)4](4-) (Ale = alendronate = [O3PC(O)(C3H6NH3)PO3](4-)) complexes alternate with polyoxometalate (POM) units. In Na12[{SiW9O34Cu3(Ale)(H2O)}{Cu6(Ale)4(H2O)4}]·50H2O (SiW9CuAle), the polyoxometalate core consists in a {SiW9Cu3} monomer capped by a pentacoordinated Ale ligand, while sandwich-type Keggin {(SbW9O33)2Cu3(H2O)(2.5)Cl(0.5)} and Dawson {(P2W15O56)2Cu4(H2O)2} complexes are found in Na8Li29[{(SbW9O33)2Cu3(H2O)(2.5)Cl(0.5)}2{Cu6(Ale)4(H2O)4}3]·163H2O (SbW9CuAle) and Na20[{(P2W15O56)2Cu4(H2O)2}{Cu6(Ale)4(H2O)4}]·50H2O (P2W15CuAle), respectively. A comparative magnetic study of the SiW9CuAle and SbW9CuAle compounds enabled full quantification of the Cu(II) 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 {Cu6(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 SbW9CuAle is inactive toward the reduction of nitrates, highlighting that both the {(SbW9O33)2Cu3} unit and the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts for this reaction. In contrast, SiW9CuAle and P2W15CuAle 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 P2W15CuAle and [(P2W15O56)2Cu4(H2O)2](16-) evidenced that if the [Cu6(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.