Polyoxometalate Chemistry: Some Recent Trends

Some Recent Trends

Francis Sécheresse

• Publisher: WSPC
• Year: 2013
• ISBN:
  9789814458993
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

The book highlights recent prominent results in the domain of the synthesis of new polyoxometalates with a specific attention to polyoxothioanions, and provides some novelties and perspectives in selected domains such as magnetism, luminescence and nanochemistry, and macroions self-assembly in solutions. The case of “one-pot” syntheses often used and reported in POMs synthesis is studied in terms of more complex solution speciation processes related to highly dynamical situation connected to factors such as pH, ionic strength, reaction time, temperature, counterion nature, concentration of starting materials, presence of electron donors and redox potentials. The behavior of macroions (2nm-6nm size range) in solution is shown to be quite different from the simple ionic solution or colloidal systems (Debye–Huckel model). Their self-assembling into a single-layered, spherical, hollow vesicle structure, namely the “blackberry” structure, is clearly described. Examples of spin clusters with tunable interactions are given and single molecule magnets based on POMs are specifically tackled. Besides paramagnetic transition metal centres and lanthanoid ions encapsulated in archetypal lacunary polyoxoanions, magnetically functionalized Kleperates are described, their discovery tracing back nearly 15 years.

Contents:

• Polyoxometalate-Protected Metal Nanoparticles: Synthesis, Structure and Catalysis (Yifeng Wang and Ira A Weinstock)
• When Giants Meet Dwarves in the Same Pond — Unique Solution Physical Chemistry Opportunities Offered by Polyoxometalate Macroions (Dong Li, Panchao Yin and Tianbo Liu)
• Directed Assembly of Polyoxometalates Across Length Scales: From Macro-Molecules to Microsystems and iChells (Antoine G Boulay, Geoffrey J T Cooper and Leroy Cronin)
• Magnetic Polyoxometalates (Juan M Clemente-Juan, Eugenio Coronado and Alejandro Gaita-Ariño)
• Magnetism of Keplerates (Paul Kögerler)
• Polyoxometalates as Ligands for Functional Lanthanoid Complexes (Chris Ritchie and Colette Boskovic)
• Polyoxothiometalates POTM (Francis Sécheresse and Emmanuel Cadot)

Readership: Graduate students and researchers of nanoscience and nanotechnology and chemistry (physical and inorganic) — inter-disciplinary.

Chapter 1

POLYOXOMETALATE-PROTECTED METAL NANOPARTICLES: SYNTHESIS, STRUCTURE AND CATALYSIS

YIFENG WANG

School of Chemistry and Chemical Engineering, Shandong University,
Jinan 250199, PR China

IRA A. WEINSTOCK

Department of Chemistry, Ben Gurion University,
POB 653, Beer Sheva, 84105 Israel

1.Introduction

Metal-oxygen cluster–anions (polyoxometalates, or POMs)^1,2 constitute a large and rapidly growing class of discrete molecular structures with applications ranging from catalysis^3,4 to functional materials.⁵ POMs are inexpensive, minimally or non-toxic, negatively-charged clusters com-prised of early-transition metals, usually in their d⁰ or d¹ electronic configurations (e.g., V(V), Mo(VI) and Mo(V), or W(VI)), bridged by oxygen atoms (formally O^2-, or occasionally HO^-, ions). Representative POMs range from small isopolyanions, such as

(<1 nm in diameter), slightly larger Keggin or Wells-Dawson heteropolyanions (e.g.,

and

), and partially reduced “wheel-like” oxomolybdate nanoclusters^6,7 that contain up to 176 Mo atoms (4.1 nm in diameter⁸). These and related polyoxoanions, and derivatives prepared by incorporation of main-group, transition-metal,^9,10 or f-block^11–14 cations, are used as molecular models of magnetic oxides,¹⁵ and in applications from catalysis^3,16–20and electron transport in fuel cells,^21,22 to the design of functional-nanocomposite materials.²³These and thousands of other POMs can be prepared in quantity via either traditional (serial and kinetically controlled) or thermodynamic (one-pot) syntheses in water. As a class, POMs possess extensive and reversible redox chemistries,^24–26 which is central to their use in many applications.^25,27–30Their reduction potentials, acidities, and other key properties relevant to catalysis and materials science, can be extensively yet readily altered by the elemental composition of the POM cluster itself. In addition, because many redox-active POMs are well defined and stable in solution, they are deployed as physico-chemical “probes” of electron-transfer processes.^31–34

As molecular anions, it is not surprising that polyoxometalates can stabilize colloidal metal nanoparticles in solution. And, given the ease with which POMs can be converted from water-soluble to organic-solvent soluble forms (as a function of their counter cations), they have been used with considerable success to stabilize metal nanoparticles in both media. While individual bonds between the oxide ligands of polyoxometalates and metal(0)-nanoparticle surfaces appear to be considerably weaker than those, for example, between alkanethiol ligands and gold(0) surfaces, data from numerous publications demonstrate that POMs can be far more effective than simple organic or inorganic ions such as citrate or phosphate.

Studies of POMs on planar metal and graphite surfaces, by Anson,³⁵ Nadjo and Keita,^36–38 Klemperer,³⁹ Gewirth^40–42 and Barteau,^43,44 and others,⁴⁵ demon-strated the formation of POM monolayers with well-defined packing geometries. During that time, Finke^46–49 demonstrated that POMs stabilized Ir(0) and Rh(0) nanaoparticles in organic solvents through a combination of the electrostatic mechanisms common to anion-stabilized colloids and the steric mechanisms typical of, for example, alkanethiol-protected gold(0) nanoparticles. The combined electrostatic and steric mechanism was proposed in response to data indicating direct bonding between POMs and the metal(0) nanoparticle surface. These direct interactions are entirely consistent with the formation of highly stable POM monolayers on planar surfaces.

More recently, Weinstock used cryogenic transmission electron microscopy ( cryo-TEM) to image POM-stabilized Ag(0) and Au(0) nanoparticles in water.^50,51Well-defined POM monolayers were observed covering the nanoparticles. The close distances between POMs on the metal(0) surfaces suggested that POM counter-cations must be extensively incorporated as critical structural components of the POM monolayer. This role is analogous to that of counter-cations in POM monolayers on planar surfaces, and between oxomolybdate macro-ions in single-walled POM vesicles.⁵² Hence, the stability of POM-protected metal(0) nanoparticles likely derives from several phenomena, namely, electrostatic forces typical of anion-stabilized colloids, direct bonding between POMs and the metal(0) surfaces, and the formation of numerous ionic and hydrogen bonds between POMs bound closely to one another on the metal(0) surface. This unique class of metal-nanoparticle structures is the subject of this chapter.

The chapter is divided into three sections, which concern the synthesis, structure and reactivity of POM-stabilized metal(0) nanoparticles. Unlike most nanoparticle-protecting ligands, POMs are redox active, and their reduced forms can reduce metal cations to colloidal metal(0), which are then stabilized by the oxidized POM anions. These methods are described, along with reductions of metal cations to metal(0) nanoparticles by traditional reducing agents, but in the presence of POM ligands. The second section, on the structures of POM-stabilized metal(0) nanoparticles, includes important observations from many groups, along with a detailed summary of new findings obtained with the aid of cryo-TEM imaging. Unlike alkanethiol-Au(0) nanoparticles, the POM-protected metal(0) particles are soluble analogs for the metal(0)/metal-oxide interfaces in heterogeneous metal-oxide supported metal(0) catalysts. Because of this, and given the redox behavior of the POMs themselves, POM-protected metal(0) nanoparticles have been investigated for use as catalysts for selective oxidations and other reactions. Results from those studies, reviewed in the last section of this chapter, indicate that POM-protected metal(0) nanoparticles can serve as active catalysts with unique selectivities, and in some cases, unprecedented rates and stabilities. Recent information concerning the detailed structures of POM-protected metal(0) nanoparticles provide new options for establishing structure–reactivity relationships for this versatile clas...

Table of contents

• Front Cover
• Half Title
• Series Title
• Title Page
• Copyright
• Preface
• Contents
• List of Color Plates
• Chapter 1 Polyoxometalate-Protected Metal Nanoparticles: Synthesis, Structure and Catalysis Yifeng Wang and Ira A. Weinstock
• Chapter 2 When Giants Meet Dwarves in the Same Pond — Unique Solution Physical Chemistry Opportunities Offered by Polyoxometalate Macroions Dong Li, Panchao Yin and Tianbo Liu
• Chapter 3 Directed Assembly of Polyoxometalates Across Length Scales: From Macro-Molecules to Microsystems and iChells Antoine G. Boulay, Geoffrey J. T. Cooper and Leroy Cronin
• Chapter 4 Magnetic Polyoxometalates Juan M. Clemente-Juan, Eugenio Coronado and Alejandro Gaita-Ariño
• Chapter 5 Magnetism of Keplerates Paul Kogerler
• Chapter 6 Polyoxometalates as Ligands for Functional Lanthanoid Complexes Chris Ritchie and Colette Boskovic
• Chapter 7 Polyoxothiometalates POTM Francis Sécheresse and Emmanuel Cadot
• Index