Structural and electronic properties of hetero-transition-metal Keggin anions: a DFT Study of alpha/beta-[XW12O40]n- (X = CrVI, VV, TiIV, FeIII, CoIII, NiIII, CoII, and ZnII) relative stability

Butyltin Keggin Ion with a Rare Four-Coordinate Ca Center

Alkyltin clusters are exploited in nanolithography for the fabrication of microelectronics. The alkyltin Keggin family is unique among Keggin clusters across the periodic table; its members appear to favor the lower-symmetry β and γ isomers rather than the highly symmetrical α and ε isomers. Therefore, the alkyltin Keggin family may provide important fundamental information about the formation and isomerization of Keggin clusters. We have synthesized and structurally characterized a new butyltin Keggin cluster with a tetrahedral Ca²⁺ center, fully formulated [(BuSn)₁₂(CaO₄)(OCH₃)₁₂(O)₄(OH)₈]²⁺ (β-CaSn₁₂). The synthesis is a simple one-step process. Extensive solution characterization including electrospray ionization mass spectrometry, small-angle X-ray scattering, and multinuclear (¹H, ¹³C, and ¹¹⁹Sn) nuclear magnetic resonance shows β-CaSn₁₂ is essentially phase-pure and stable. This differs from the previously reported Na-centered analogues that always form a mixture of β and γ isomers, with facile interconversion. Therefore, this study has clarified prior confusion over complex spectroscopic and crystallographic characterization of the Na-centered analogues. Density functional theory calculations showed the following stability order: γ-CaSn₁₂ < γ-NaSn₁₂ < β-CaSn₁₂ < β-NaSn₁₂. The β analogue is always more stable than the γ analogue, consistent with experiment. Notable outcomes of this study include a rare tetrahedral Ca coordination, a Na-free alkyltin cluster (important for microelectronics manufacturing), and a better understanding of Keggin families built of different metal cations.

Oxygen Adsorption and Activation on Cobalt Center in Modified Keggin Anion-DFT Calculations

The influence of the cobalt cation geometric environment on catalytic activity, namely, oxygen adsorption and its activation, was investigated by exploring two groups of systems. The first group was formed by cobalt cation complexes, in which the Co2+ was surrounded by water-H2O or acetonitrile-CH3CN solvent molecules. This represents heteropolyacids salts (ConH3-nPW(Mo)12O40), where the Co2+ acts as a cation that compensates for the negative charge of the Keggin anion and is typically surrounded by solvent molecules in that system. The second group consisted of tungsten or molybdenum Keggin anions (H5PW11CoO39 and H5PMo11CoO39), having the Co2+ cation incorporated into the anion framework, in the position of one addenda atom. Detailed NOCV (Natural Orbitals for Chemical Valence) analysis showed that, for all studied systems, the σ-donation and σ-backdonation active channels of the electron transfer were responsible for the creation of a single Co-OO bond. Depending on the chemical/geometrical environment of the Co2+ cation, the different quantities of electrons were flown from the Co2+ 3d orbital to the π* antibonding molecular orbitals of the oxygen ligand, as well as in the opposite direction. In molybdenum and tungsten heteropolyacids, modified by Co2+ in the position of the addenda atom, activation of O2 was supported by a π-polarization process. Calculated data show that the oxygen molecule activation changed in the following order: H5PMo11CoO39 = H5PW11CoO39 > Co(CH3CN)52+ > Co(H2O)52+.

Effect of vanadium valence state on the solution chemistry and the stability of vanadium substituted polyoxometalates

ESI-MS combined with DFT calculation revealed that the vanadium valence state and the vanadium substitution degrees can substantially affect the stabilities of vanadium-substituted POMs.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.