Effects of Heteroatoms on Electronic States of Divanadium-Substituted γ-Keggin-type Polyoxometalates

Abstract

Effects of heteroatoms on electronic states of divanadium-substituted γ-Keggin-type polyoxometalates (TBA)4[γ-XV2W10O38(μ-OH)(μ-OR)] (X = Ge, Si; R = Me, Et, n-Pr, H; TBA = tetra(n-butyl)ammonium) and (TBA)4[γ-XV2W10O38(μ-O)] (X = Ge, Si) were investigated, using a combination of nuclear magnetic resonance spectroscopy and density functional theory (DFT) calculations. Both the substitution of SiO4 heteroatom units with larger GeO4 ones and the introduction of more electronegative alkoxo groups in place of hydroxo groups resulted in deshielding of the vanadium nuclei. DFT calculations using the Def2-SVP basis set at TPSSh level of theory could well-reproduce the anionic moieties of a series of divanadium-substituted γ-Keggin-type polyoxometalates, and the estimated chemical shifts approximately reproduced the experimental ones with the individual gauge localized orbital method (SO-IGLO) taking the spin-orbit interaction into account. The magnetic shielding (σ) consists of σd + σp + σSD + σFC, where σd, σp, σSD, and σFC are diamagnetic, paramagnetic, spin-dipolar, and Fermi contact terms, respectively. The σp changed much among (TBA)4[γ-XV2W10O38(μ-OH)2], (TBA)4[γ-XV2W10O38(μ-OH)(μ-OR)], and (TBA)4[γ-XV2W10O38(μ-O)], while σd, σSD, and σFC did not change much. Therefore, the σp largely contributed to the magnetic shielding. Moreover, σp consisted of the occupied-occupied transitions (s-terms) and the occupied-virtual ones (u-terms), and the u-terms were predominant for σp. The most contributing occupied localized orbital consisted of the dz(2) orbital of vanadium, the pz orbital of terminal oxygen related to the V═O bond, and the pz orbital of oxygen of the XO4 unit, whereas the two virtual localized orbitals consisted of the dyz orbital of vanadium and the py orbital of terminal oxygen. Analysis of the structural and electronic characteristics of a series of divanadium-substituted γ-Keggin-type POMs revealed a linear correlation between both (51)V{H} chemical shifts and the reciprocal values of the energy gaps between the corresponding XO4-predominant orbital HOMOs-X and the LUMOs+X (X = 0, 1, or 2). All these results indicate that neighboring XO4 units weakly interact with the addenda atoms and control the electronic states of polyoxometalates and the magnetic shielding of their addenda atoms.