Incorporation of Transition-Metal-Ion Guests (Co2+, Ni2+, Cu2+, Zn2+) into the Ti2-Containing 18-Tungsto-2-arsenate(III) Monolacunary Host

The four 18-tungsto-2-arsenates(III) [MII(H2O)(TiIVO)2(α-AsIIIW9O33)2]12– [M = Co (CoTi2), Ni (NiTi2), Cu (CuTi2), Zn (ZnTi2)] were rationally synthesized by treating the respective divalent metal ions with the monolacunary, dititanium(IV)-containing polyanion precursor [(TiIVO)2(α-AsIIIW9O33)2]14– (Ti2) in aqueous solution. All four polyanions are isostructural and correspond to dimers with two Ti4+ and one M2+ ion sandwiched between two [α-AsIIIW9O33]9– units. The compounds were characterized in the solid state by single-crystal XRD, infrared spectroscopy, thermogravimetric analysis, and elemental analysis and in solution by UV/Vis and NMR spectroscopy as well as electrochemistry. Variable-temperature magnetic susceptibility and variable-field magnetization characterization of the paramagnetic derivatives indicate antiferromagnetic interactions between nearest neighbors in the solid state. The resolved hyperfine structure obtained from multifrequency EPR measurements on CuTi2 (which exhibits a Jahn–Teller effect) in the solid state lead us to assign the ground state mainly to the Cu dx²–y² orbital. Despite the fact that the four polyanions are isostructural and have the same overall negative charge, the cyclic voltammetric patterns featuring their chemically reversible reduction waves display distinct features. Electrocatalytic studies demonstrate that the activities of the polyanions towards nitrate reduction are strikingly influenced by the nature of the substituent metal ion.