Heteroanionic Materials Based on Copper Clusters, Bisphosphonates, and Polyoxometalates: Magnetic Properties and Comparative Electrocatalytic NO x Reduction Studies

Three compounds associating for the first time polyoxotungstates, bisphosphonates and copper ions have been 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)2.5Cl0.5} and Dawson {(P2W15O56)2Cu4(H2O)2} complexes are found in Na8Li29[{(SbW9O33)2Cu3(H2O)2.5Cl0.5}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 has allowed to fully quantify the CuII superexchange interactions either for the POM and non-POM subunits, evidencing that while the paramagnetic centers are antiferromagnetically coupled in the polyoxometalate units, both antiferromagnetic 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 pre-catalyst. Moreover, comparison of the activities of P2W15CuAle and [(P2W15O56)2Cu4(H2O)2]16- evidenced that if the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts, it act as a co-factor, significantly enhancing the catalytic efficiency of the active POM