Overview. Studies in our laboratory concern concern the use of metal-oxide cluster-anions (polyoxometalates, or POMs) for addressing fundamental problems in electron transfer, catalysis, host/guest chemistry and nanoscience, and as building blocks for functional supramolecular assemblies. Studies range from fundamental investigations of inorganic reaction mechanisms to explorations of supramolecular structures formed by electrostatic, hydrogen-bonded or hydrophobic interactions.


Metal-oxide cluster science. POMs are typically prepared from early-transition metals (V, Mo, and W) in their highest oxidation states (d0, and sometimes d1, electronic configurations). Many POMs possess extensive and reversible redox chemistries, and as a class, their compositions and structures, which control the physical and chemical properties that impart functionality, can be rationally modified at the atomic level.  Functional systems are prepared through the incorporation of reactive components, such as transition-metal ions, by control over the nanoscale architectures of larger metal-oxide (POM) frameworks, and by use of POM clusters as components of supramolecular and nanoscale assemblies. An introduction to this topic was recently published in a special issue of the Israel Journal of Chemistry. See: “Frontiers in Metal-Oxide Cluster Science”, Israel J. Chem. (Guest Editor, I. A. Weinstock), 2011, 51, 176-178.


Some highlights from recent work. Using these structures and properties, recently reported work involves fundamental studies in three general areas: 1) electron-transfer reactions, 2) molecular host/guest chemistry, and 3) nanoscience.


Electron transfer. For example, we are currently deploying an iso-structural series of Keggin ions (Figure 1) as physicochemical probes for understanding fundamental electron-transfer processes.




Figure 1. Iso-structural series of 1.2-nm diameter one-electron reduced alpha-Keggin anions used as physiochemical probes of electron-transfer processes. The W atoms are located at the center of the coordination octahedra (shown in blue), and the heteroatoms (Al(III), Si(IV) and P(V)) are located in tetrahedral sites at the center of each structure.



For example, we recently discovered a multi-site concerted proton electron transfer (CPET) reaction to dioxygen in water (Figure 2), that advances our understanding of this biologically important class of reactions, and is relevant to numerous O2-based processes.




Figure 2. At modest pH values, electron transfer to dioxygen by the reduced-forms of plenary-Keggin anions occurs via sequential electron- and proton-transfer steps. At larger hydronium-ion concentrations, we observe the emergence of a concerted proton-electron transfer (CPET) pathway. See: Snir, O.; Wang, Y.; Tuckerman, M. E.; Geletii, Y.V.; Weinstock, I. A. “Concerted Proton-Electron Transfer (CPET) to Dioxygen in Water” J. Am. Chem. Soc. 2010, 132, 11678-11691. Copyright by ACS.



Supramolecular and host/guest chemistry. In supramolecular chemistry, we used a porous oxomolybdate macroion to demonstrate that large organic “guests” can negotiate passage through comparatively smaller sub-nanometer apertures (Figure 3; see highlights page in this site).




Figure 3. Large molecules pass through comparatively smaller pores of a porous molecular capsule in water. This unexpected phenomenon likely reflects the greater flexibility of molecular versus solid-state structures, and represents a sharp departure from traditional models for diffusion through porous solid-state (rigid) oxides. Copyright by ACS.


Building on this work, we recently reported the use of these porous molecular capsules as catalysts for organic transformation in water. The reactions, which feature pore-restricted encapsulation, ligand-regulated access to multiple structurally integral metal-centers, and options for modifying the microenvironment within this new type of nanoreactor, suggest numerous additional transformations of organic substrates by this and related molybdenum-oxide based capsules (Figure 4).


Highlighted by Stu Borman in Chemical & Engineering News, in a Science & Technology Concentrate entitled, “Flexible Porous Capsules Catalyze Reaction”.



 


Figure 4. Table of contents graphic showing the title phenomenon: “Catalysis in a Porous Molecular Capsule: Activation by Regulated Access to Sixty Metal Centers Spanning a Truncated Icosahedron” See: J. Am. Chem. Soc, 2012, in press. (Kopilevich, S.; Gil, A.; Garcia-Ratés, M.; Avalos, J. B.; Bo, C.; Müller, A.; Weinstock, I. A.*). Copyright by ACS.



Nanoscience. In this area, we reported the first images ever obtained of anion monolayers on a metal(0) nanoparticle (FIgure 5). We are now studying the formation, structures and reactions of anion-monolayer protected Ag and Au nanoparticles, and monolayer formation on binary-salt nanocrystals.




Figure 5. Synthesis and imaging of the ligand monolayer on an anion-protected silver(0) nanoparticle. See original article with the first images of this kind here: J. Am. Chem. Soc., 2008. Copyright by ACS.



Further investigations revealed that POM-monolayer shells on gold nanoparticles are electrostatically stabilized by interactions between the POM cluster anions and structurally integrated counter cations, as shown on the cover graphic of a recent (July 16, 2012) issue of Inorganic Chemistry), Figure 6.



Figure 6. Role of Alkali-Metal Cation Size in the Self-Assembly of Polyoxometalate-Monolayer Shells on Gold Nanoparticles, See: Inorg. Chem., 2012, 51, 7436-7438. Copyright by ACS.

 

Weinstock Laboratory

Ira A. Weinstock

The Irene Evans Professor of Inorganic Chemistry

(Ph.D., Massachusetts Inst. of Technology, 1990)

iraw@bgu.ac.il

updated: May 5, 2013