Structural and Electronic Properties of Aqueous Transition−Metal−Substituted Polyoxotungstates

Abstract

Polyoxometalates (POMs) are a discrete class of polyanionic metal−oxo aggregates with a large diversity of topologies, structural flexibility, and functionality at the nanoscale. Controlled base treatment of these compounds produces monolacunary polyanions that behave as multi−dentate ligands, generating a class of compounds known as transition metal substituted POMs. These compounds demonstrate excellent redox stability, making them are integral precursors for electrochemical processes. In this work, Density Functional Theory (DFT) calculations have been employed to systematically studied the accuracy of various exchange−correlation functions in describing the properties of transition−metal−substituted polyoxotungstates which are integral precursors for electrochemical processes. Molecular geometries were optimized using several generalized gradient approximations (GGA), hybrid, and long−range−corrected GGA (LC−GGA) with whom all agreed well with experimental crystallographic results. We report the standard reduction potentials, U0red vs NHE, for the pH−independent redox processes present in Keggin [M(H2O)PW11O39]5−; M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II)) anions calculated as a function of Hartree−Fock (HF) exchange and provide insight into the electronic properties controlling it.

Ion-Pair Proximity in Cyclic Voltammetry and DFT studies of Aqueous Transition-Metal-Substituted Polyoxotungstates

“…This general positive shifting in ΔU 0 Red was attributed to the over-stabilization of the ion-pairs, reported in earlier work. 23 An important aspect of this investigation was to observe counterion trends on ΔU 0 Red. Herein, ΔU 0 were inverted, in which ΔU 0 Exp was negatively shifted as the ionic radii of the counterion decreases.…”

“…In 2023, we explored the accuracy of various exchange-correlation functionals reproducing experimental literature potentials, U 0 Red, in anionic [M(H2O)PW11O39] q− compounds where M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II). 23 The x-c functionals employed in our study were chosen to explore the influence of Hartree-Fock (HF) exchange (0% PBE, 15% B3LYP*, 20% B3LYP, 25% PBE0, and 50% BH&H) for reproducing potentials. Our work involved explicitly located K + counterions, see Figure 1, which induced a positive shift of potentials by > 500 mV which improved the accuracy of computed potentials where M = Mn(III/II)/Co(III/II); however, this was not the case where M= Fe(III/II)/Ru(III/II)…”

“…Figure 1 Schematic representations of 4-fold pockets (A-F) on the surface of [Co(H2O)PW11O39] 5anions (top). The corresponding work focused on the two lowest energy isomers: A and A,D, established in our previous work 23. …”