Redox and acidic properties of chalcogenido-substituted mixed-metal polyoxoanions: a DFT study of α-[PW11O39ME]4− (M = Nb, Ta; E = O, S, Se)
The redox and acidic properties of α-Keggin anions [PW11O39ME]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 [PW11O39ME]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-[PW11NbO40]4− and α-[PW11NbSO39]4−. The bonding energies for adding the first proton to the anions and for ammonia coordination to the protonated α-[PW11O39NbE]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-[PW11O39NbE]4− (E = S, Se) is more negative than that of α-[PW11O40NbO]4−, which also provides the same result as mentioned above.