Density functional study on Keggin heteropolyanions containing fifth period main group heteroatoms

Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn^III ₃ Mn^IV O₃ (CH₃ COO)₃ (A-α-SiW₉ O₃₄ )]^6- . Theoretical results indicated that catalytic active species 1 (Mn₄ ^{III,III,IV,IV} ) was formed after O₂ formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn^IV -oxyl radical 4 (Mn₄ ^IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn₂ O₂ core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn^IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn₄ ^III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol^-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn₄ ^III,IV,IV,IV -O₂ ⋅) enabled the O₂ release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.

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.