Super-Reduced Polyoxometalates: Excellent Molecular Cluster Battery Components and Semipermeable Molecular Capacitors

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• 作者：Yoshio Nishimoto ; Daisuke Yokogawa ; Hirofumi Yoshikawa ; Kunio Awaga ; Stephan Irle
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：June 25, 2014
• 年：2014
• 卷：136
• 期：25
• 页码：9042-9052
• 全文大小：666K
• ISSN：1520-5126

文摘

Theoretical investigations are presented on the molecular and electronic structure changes that occur as 伪-Keggin-type polyoxometalate (POM^{3鈥?/sup>) clusters [PM₁₂O₄₀]3鈥?/sup> (M = Mo, W) are converted toward their super-reduced POM27鈥?/sup> state during the discharging process in lithium-based molecular cluster batteries. Density functional theory was employed in geometry optimization, and first-principles molecular dynamics simulations were used to explore local minima on the potential energy surface of neutral POM clusters adorned with randomly placed Li atoms as electron donors around the cluster surface. On the basis of structural, electron density, and molecular orbital studies, we present evidence that the super-reduction is accompanied by metal鈥搈etal bond formation, beginning from the 12th to 14th excess electron transferred to the cluster. Afterward, the number of metal鈥搈etal bonds increases nearly linearly with the number of additionally transferred excess electrons. In 伪-Keggin-type POMs, metal triangles are a prominently emerging structural feature. The origin of the metal triangle formation during super-reduction stems from the formation of characteristic three-center two-electron bonds in triangular metal atom sites, created under preservation of the POM skeleton via 鈥渟queezing out鈥?of oxygen atoms bridging two metal atoms when the underlying metal atoms form covalent bonds. The driving force for this unusual geometrical and electronic structure change is a local Jahn鈥揟eller distortion at individual transition-metal octahedral sites, where the triply degenerate t₂ d orbitals become partially filled during reduction and gain energy by distortion of the octahedron in such a way that metal鈥搈etal bonds are formed. The bonding orbitals show strong contributions from mixing with metal鈥搊xygen antibonding orbitals, thereby 鈥渟huffling away鈥?excess electrons from the cluster center to the outside of the cage. The high density of negatively charged yet largely separated oxygen atoms on the surface of the super-reduced POM27鈥?/sup> polyanion allows the huge Coulombic repulsion due to the presence of the excess electrons to be counterbalanced by the presence of Li countercations, which partially penetrate into the outer oxygen shell. This 鈥渟emiporous molecular capacitor鈥?structure is likely the reason for the effective electron uptake in POMs.}