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

Abstract

The physico-and electrochemical behaviors of a series of [WZn 3 (H 2 O) 2 (ZnW 9 O 34 ) 2 ] 12− (Zn−WZn 3 ) and its first-row transition-metalsubstituted analogues [WZn(TM) 2 (H 2 O) 2 (ZnW 9 O 34 ) 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) bandgap energy in these transition-metal-substituted POM (TMSP) complexes wrt Zn−WZn 3 , as confirmed by diffuse reflectance spectroscopy and DFT study. Cyclic voltammetry reveals that the electrochemistry of these sandwich POMs (Zn−WZn 3 and TMSPs) is highly dependent on the pH of the solution. Moreover, the dioxygen binding/activation studies of these polyoxometalates show that Zn−WZn 3 and Zn−WZnFe 2 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.

Modulation of the Band Gap and Redox Properties by Mixed Addenda in Sandwich Polyoxometalates

“…As sandwich POMs have specific binding sites for the binding of molecular oxygen, it imparts selectivity in the product. Recently, our group has shown dioxygen activation of Zn-based sandwich POMs, i.e., [{ZnWM 2 (H 2 O) 2 (ZnW 9 O 34 ) 2 } n − ], where M = Mn, Co, Fe, Ni, and Cu have been found to be active for catalytic imine synthesis . Here, the binding ability of O 2 has been investigated using mixed addenda sandwich POMs toward the formation of the POM-O 2 adduct.…”

Transition Metal-Based Polyoxometalates for Oxygen Electrode Bifunctional Electrocatalysis

“…Thus, they have found applications as electrocatalysts for ORR [9][10][11][12] and OER [11,[13][14][15] in fuel cells, batteries [9,11,13,16], and electrolytic cells [14]. Adjusting the electrochemical and mechanical properties of POMs can be accomplished by varying the composition of their framework by introducing different metal redox centers [17,18] or incorporating the whole POM structure into porous materials or conductive substrates [7]. Moreover, the effect of intrinsic structure adjustment in terms of anion structure regulation on POM electrocatalytic activity has been explored [19].…”

A series of isopolymolybdate–viologen hybrids with photo-, thermo- and electro-chromic properties

“…Polyoxometalates (POMs), as a large class of anionic clusters with diverse structures, are polynuclear complexes composed of {MO x } polyhedra with classical V V , Nb V , Ta V , Mo VI or W VI atoms sharing edges and angles. [1][2][3] The size, shape and configuration of POMs are controllable, and they have a precise chemical composition and topological structure. [4][5][6] In the past few decades, many POMs with a novel structure and attractive performance have been developed and studied in many application fields.…”