Overview. Studies in our laboratory concern fundamental problems in molecular and supramolecular chemistry and nanoscience. Numerous intellectually substantive phenomena and societally important issues can be understood or addressed using metal-oxygen cluster anions (polyoxometalates, or POMs), and much of our work involves their use as physicochemical probes of molecular processes and as well defined components for self-assembly.
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.
Our current program of research 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 to address the role of protons in oxygen reduction reactions (ORRs) that occur at low pH values such as those in fuel cells.
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.
Supramolecular and host/guest chemistry. In supramolecular chemistry, we recently used a porous oxomolybdate macroion to demonstrate that large organic “guests” can negotiate passage through comparatively smaller sub-nanometer apertures (Figure 2; see HIghlights page in this site). In other areas of supramolecular chemistry, we are currently studying the formation and reactions of multi-walled vesicles that self-assemble in water from remarkably small and reactive cluster anions (e.g., potentially responsive inorganic vesicles).
Figure 2. 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 Large molecules pass through comparatively smaller pores. In zeolites and other rigid solid-state oxides, substrates whose sizes exceed the pore dimensions of the material are rigorously excluded. Using a porous 3 nm-diameter capsule-like oxomolybdate complex (prepared and characterized by Achim Mueller and co-workers), as a water-soluble analog of solid-state oxides (e.g., as a soluble analog of 3 angstrom molecular sieves), we show that carboxylates (RCO2–) can negotiate passage through flexible Mo9O9 pores in the surface of the capsule, and that rates follow the general trend: R = primary >> secondary > tertiary >> phenyl (no reaction). Surprisingly, the branched alkanes (R = iso-Pr and tert-Bu) enter the capsule even though they are larger than the crystallographic dimensions of the Mo9O9 pores. Four independent lines of spectroscopic and kinetic evidence demonstrate that no irreversible changes in the metal-oxide framework are involved. 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.
Nanoscience. In nanoscience, we recently reported the first images ever obtained of anion monolayers on a metal(0) nanoparticle (FIgure 3). We are now studying the formation, structures and reactions of anion-monolayer protected Ag and Au nanoparticles, and monolayer formation on binary-salt nanocrystals.
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Figure 3. Synthesis and imaging of the ligand monolayer on an anion-protected metal nanoparticle. According to Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, anions stabilize solutions of metal(0) nanoparticles by binding to the metal surface. However, the structure of the metal-anion interface in these colloidal systems has eluded direct observation. To address this, we deployed a 1.2-nm sized heteropolytungstate cluster anion, -AlW11O399- (1), which features 11 W atoms (Z = 74) for effective imaging by electron microscopy, and a high negative charge to enhance binding to a prototypical Ag(0) nanoparticle. Cryogenic methods were then used to “trap” the 1-protected Ag(0) nanoparticles at liquid-N2 temperatures within a water-glass matrix. The “solution-state” structures thus captured provide the first reported TEM images of self-assembled monolayers (SAMs) of anions on a colloidal metal(0) nanoparticle.
