Physico- and Electrochemical Properties of First-Row Transition-Metal-Substituted Sandwich Polyoxometalates

The physico- and electrochemical behaviors of a series of [WZn₃(H₂O)₂(ZnW₉O₃₄)₂]^12- (Zn-WZn₃) and its first-row transition-metal-substituted analogues [WZn(TM)₂(H₂O)₂(ZnW₉O₃₄)₂]^12- (Zn-WZn(TM)₂; TM = Mn^II, Co^II, Fe^III, Ni^II and Cu^II) are reported. Various spectroscopic studies, including Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, electrospray ionization (ESI)-mass spectrometry, and Raman spectroscopy, show similar spectral patterns in all sandwich polyoxometalates (POMs) because of their isostructural geometry and constancy of the overall negative charge (-12). However, the electronic properties highly depend on the transition metals at the "sandwich core" and correlate well with the density functional theory (DFT) study. Further, depending on the substituted TM atoms, there is a decrease in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) band-gap energy in these transition-metal-substituted POM (TMSP) complexes wrt Zn-WZn₃, as confirmed by diffuse reflectance spectroscopy and DFT study. Cyclic voltammetry reveals that the electrochemistry of these sandwich POMs (Zn-WZn₃ and TMSPs) is highly dependent on the pH of the solution. Moreover, the dioxygen binding/activation studies of these polyoxometalates show that Zn-WZn₃ and Zn-WZnFe₂ have better efficiency toward dioxygen binding, as confirmed by FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA), which is also reflected in their catalytic activity toward imine synthesis.