Electronic and Magnetic Properties of α-Keggin Anions:  A DFT Study of XM12O40n-, (M = W, Mo; X = AlIII, SiIV, PV, FeIII, CoII, CoIII ) and SiM11VO40m- (M = Mo and W)

Calculations based on density functional theory (DFT) have been carried out to investigate the electronic and magnetic properties of the alpha-Keggin anions mentioned in the title. The atomic populations and the distribution of the electron density computed for the studied clusters support the hypothesis that an oxidized Keggin anion is an XO(4)(n-) clathrate inside a neutral M(12)O(36) cage. The energy gap between the band of occupied orbitals, formally delocalized over the oxo ligands, and the unoccupied d-metal orbitals, delocalized over the addenda, has been found to be independent of the central ion. However, substitution of a W or a Mo by V modifies the relative energy of the LUMO and then induces important changes in the redox properties of the cluster. In agreement with the most recent X-ray determination of Co(III)W(12)O(40)(5-) and with the simplicity of the (183)W NMR and (17)O NMR spectra observed for this anion the calculations suggest that Co(III)W(12)O(40)(5-) has a slightly distorted T(d ) geometry. For the parent cluster CoW(12)O(40)(6-) the quadruplet corresponding to the anion encapsulating a Co(II) was found to be approximately 1 eV more stable than the species formed by a Co(III) and 1 e delocalized over the sphere of tungstens. The one-electron reduction of Co(II)W(12)O(40)(6-) and Fe(III)W(12)O(40)(5-) leads to the formation of the 1 e blue species Co(II)W(12)O(40)(7-) and Fe(III)W(12)O(40)(6-). The blue-iron cluster is considerably antiferromagnetic, and in full agreement with this behavior the low-spin state computed via a Broken Symmetry approach is 196 cm(-1) lower than the high-spin solution. In contrast, the cobalt blue anion has a low ferromagnetic coupling with an S-T energy gap of +20 cm(-1). This blue species is more stable than the alternative reduction product Co(I)W(12)O(40)(7-) by more than 0.7 eV.