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

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 dz2 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 51V­{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.