Redox and acidic properties of chalcogenido-substituted mixed-metal polyoxoanions: a DFT study of α-[PW₁₁O₃₉ME]⁴⁻ (M = Nb, Ta; E = O, S, Se)

Abstract

The redox and acidic properties of α-Keggin anions [PW 11 O 39 ME] 4− (M = Nb, Ta; E = O, S, Se) have been investigated by using the density functional theory (DFT) method. The calculated results for the studied clusters are consistent with the hypothesis that the substitution of one O atom by S or Se atoms in polyoxometalates (POMs) modifies the relative energy of the lowest unoccupied molecular orbital, inducing slight changes in the redox properties of POMs. The electronic structures and reduction energies of [PW 11 O 39 ME] 4− (M = Nb, Ta; E = O, S, Se) confirm that the substitution of one O atom by S and Se atoms in POMs enhances the redox properties, which is in good agreement with the cyclic voltammetry characterization of α-[PW 11 NbO 40 ] 4− and α-[PW 11 NbSO 39 ] 4− . The bonding energies for adding the first proton to the anions and for ammonia coordination to the protonated α-[PW 11 O 39 NbE] 4− (E = O, S, Se) were simulated to study the basicity of external oxygen sites and the acidity. The results show that the protonation energy becomes more positive after substitution by S and Se, indicating that the substitution enhances the acidity of POMs. The adsorption energy of ammonia in the case of α-[PW 11 O 39 NbE] 4− (E = S, Se) is more negative than that of α-[PW 11 O 40 NbO] 4− , which also provides the same result as mentioned above.

Modeling of MAl12 Keggin Heteroatom Reactivity by Anion Adsorption

“…POMs are nanoscale cluster compounds that are typically composed of higher-valent transition metals (Mo, W, V, Nb, etc. ). − Adjusting synthesis conditions can result in variable stoichiometries, including the well-known clusters [Nb 10 O 28 ] 6– , [PW 12 O 40 ] 3– , [P 2 W 15 V 3 O 62 ] 9– , [Mo 6 O 19 ] 2– . Each of these stoichiometries can be classified into different POM structure types and display distinct chemical and physical properties that can be controlled by adjusting the constituent atoms .…”

Polyoxometalate‐Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

“…Meanwhile, through the combination of environmental and theoretical analysis, it is found that the electronic structures of [PW 11 O 39 M] n − (M = Nb, Ta) with different doped atoms (O, S, Se) can be modified through the substitution effect (Figure 1I). [ 39 ] The relative energy of the LUMO in [PW 11 O 39 M] n − can be optimized through the substitution of O atoms to S or Se atoms, resulting in enhanced redox properties.…”

“…I) Molecular orbital diagram for [PW 11 O 39 M] n− and corresponding derivatives. Reproduced with permission [39]. Copyright 2015, the Royal Society of Chemistry.…”

Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates

“…To attain a better understanding of the basicity of 1_POMs , theoretical calculations were performed by the density functional theory (DFT) method. 33 The results of natural population analyses of the POMs are summarized in Tables S3–S5 †. To investigate the effect of Nb/Ta-substitution on the electron density distribution within the POMs, we compared the natural charges of atoms at equivalent positions in Nb/Ta-substituted POMs ( SiW 11 Nb and SiW 11 Ta ) and non-substituted POM ( BW 12 ).…”