BAR-ILAN UNIVERSITY (R.A.)
)‫אילן (ע"ר‬-‫אוניברסיטת בר‬
Department of Chemistry
‫המחלקה לכימיה‬
‫גן‬-‫רמת‬52900
Ramat-Gan 52900, Israel
:‫ טלפון‬Phone: 03-5318309/10
:‫ פקס‬Fax: 972-3-7384053
SEMINAR
Wednesday, April 1, 2009
at 14.00
Chemistry Departmental Lecture Hall (112)
SPEAKER
Ira A. Weinstock
a
Department of Chemistry, Ben Gurion University,
TOPIC
MOLECULAR- AND NANO-SCIENCE OF
POLYOXOMETALATE CLUSTER ANIONS
ABSTRACT
The topics of this lecture concern fundamental studies of electron-transfer reactions,
supramolecular (host/guest) chemistry and nanoscience.
Electron transfer. Recent findings point to superoxide formation as the first step in the
oxygen-reduction reaction at fuel cell electrodes. In the first part of the lecture, an
isostructural series of small (1.2 nm diameter) one-electron reduced heteropolyanions, aXn+W12O40(9-n)–, Xn+ = Al3+, Si4+, P5+, are used to show that the outer-sphere reduction of
dioxygen to superoxide at acidic pH values typical of those used in fuel cells may occur via
proton-coupled electron transfer (i.e., a PCET mechanism).
Host/guest chemistry. Next, a larger (3 nm diameter) oxomolybate macroion is used to
address an issue fundamental to host/guest chemistry: the size-restricted passage of molecules
through subnanometer-scale apertures of open-framework structures. In zeolites and rigid
functional materials, substrates whose sizes exceed those of the pore dimensions are
rigorously excluded. Meanwhile, cavities within flexible metal-organic frameworks (MOFs)
can accommodate substrates by reversible, stepwise structural expansion (“breathing”). Using
a “capsule”-like molybdenum-oxide-based framework as a soluble analog of porous solidstate (rigid) oxides—e.g., 3 Å molecular sieves—we find that branched-alkane carboxylate
“guests” (RCO2–, R = iso-Pr and tert-Bu) negotiate passage through flexible sub-nanometer
Mo9O9 apertures whose geometrical dimensions (ca. 3 Å based on the van der Waals radii of
opposed O atoms) are smaller than the entering species [1].
Nanoscience. 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 deploy a 1.2-nm sized heteropolytungstate cluster anion, -AlW11O399–
(1), which features 11 W atoms (Z = 183) for effective imaging by electron microscopy, and a
high negative charge to enhance binding to Ag(0) and Au(0) nanoparticles. Cryogenic
methods are then used to “trap” the 1-protected 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 colloidal metal(0)
nanoparticles. Results from ongoing kinetic/mechanistic investigations of ligand exchange
and monolayer formation on metal nanoparticles and binary-salt nanocrystals, and other
reactions will be included [2].
____
[1] Ziv, A.; Grego, A.; Kopilevich, S.; Zeiri, L.; Miro, P.; Bo, C.; Müller, A.; Weinstock, I. A. “Flexible Pores
of a Metal-Oxide-Based Capsule Permit Entry of Comparatively Larger Organic Guests” J. Am. Chem. Soc.
2009, 131, ASAP: March 17, 2009. To be highlighted on the cover of JACS, Vol. 131, Issue 18, May 13, 2009.
[2] Neyman, A.; Meshi, L.; Zeiri, L.; Weinstock, I. A. “Direct imaging of the ligand monolayer on an anionprotected metal nanoparticle through cryogenic trapping of its solution-state structure” J. Am. Chem. Soc. 2008,
130, 16480-16481.