Polyoxometalate Molecular Science

To an Inorganic Chemist, the high abundance of oxygen in the Earth’s Crust (55 atom%) is largely manifested in the solution chemistry of aquo-, hydroxo-, and oxo-ions and in the solid-state realm of silicates, clays, and metal oxides. The role of (di)oxygen in the biosphere is of course also not insignificant, but that is another story.

The polyoxometalates are intermediate species between the mononuclear oxo/hydroxometalates [MOx(OH)y]n− and polymeric metallic oxides and are usually known with transition metals in a high oxidation state. They are obtained by condensation reactions that occur when a solution of the metal anion is acidified

In the preceding Chapter, the authors describe the general features of polyoxometalate formation in aqueous media and, in particular, demonstrate that it is possible to exercise subtle control over the wide array of polyoxometalates [XxMmOp]n- generated in pH-dependent equilibria. Characterisation of the species present in solution and elucidation of the factors affecting their interconversion has only been achieved by detailed research over many years. By comparison, much less is known about the formation and interconversion of polyoxometalates in non- aqueous systems, although it is evident that new types of structures and reactivity are accessible in organic solvents.

Complex oxides of the early transition metals in their highest oxidation states display a remarkable variety of properties, including catalytic [1], electrooptic [2], high-K dielectric [3], electromechanical [4], ferroelectric [5], and charge density wave [6] behavior. The structural basis for this behavior, although understood in general terms, is not well understood on the atomic-molecular size scale, and as a result, these properties are difficult or impossible to control chemically. A first step toward addressing this problem is clear definition of structure and bonding in these materials, the subject of this Chapter.

Polyoxometalates (POMs) of vanadium, molybdenum and tungsten and to some extent, niobium and tantalum are very important and interesting classes of complexes with great potential in many applications [1‐4]. Since in many cases their structures prove to be unchanged in solution, many methods are applicable for studying their molecular and electronic structures. These studies are expected to be useful for understanding their role in many processes and for rational design of new compounds that are promising for applications.

Continuous-wave time-resolved electron spin resonance (TRESR) spectroscopy has been successfully employed to elucidate photochemical reaction mechanisms, since chemically induced dynamic electron polarization (CIDEP) spectra observed give information about the spin dynamics of short-lived intermediates such as radical pairs (RPs) and free radicals (FRs) [1‐3]. CIDEP spectra were usually interpreted by two main mechanisms: triplet mechanism (TM) and radical pair mechanism (RPM). In the TM, electron spin polarization (ESP), which existed in the excited triplet state, is transferred to each of the radicals created on its reaction. The intersystem crossing (ISC) process is usually spin-selective and produces the excited triplet state with a non-equilibrated population in the spin sublevels, spin polarized triplet state. When a reaction occurs from such a polarized triplet state before the relaxation, spin-polarized radicals are produced, generating net polarized CIDEP spectra. In this paper we describe CIDEP phenomena for the photoredox reaction between the polyoxometalate (POM) and both electron- and proton-donor (DH), which allows us to detect large emissive ESP of deproto- nated one-electron oxidized species (D•) generated by the electron transfer (with an accompanying transfer of proton) from DH to the oxygen-to-metal charge transfer (O→M LMCT) excited triplet states of POM (³(O→M LMCT)), and to investigate the primary processes of the solution chemistry of POM [4]. If the photoredox reaction between POM and DH occurs rapidly via³(O→M LMCT), ESP in the ³(O→M LMCT) can be expected to be transferred to D• and /or POM- H (one-electron reduced protonated species of POM) to give rise to CIDEP To be observed, the production of D• must rapidly take place before thermal equilibrium of the triplet spin-lattice relaxation of³(O→M LMCT) has been established. The TM seems to be common in the photoredox reactions of POMs which produce emissive ESP in elementary steps.

In its broadest acceptation, functionalization of polyoxometalates may consist of replacing one, or several, metal-oxo function(s) by a new function, where the metal and/or the oxo ligand have been changed. If terminal oxo ligands are mainly replaced, some examples are also known of formal replacement at the bridging sites. Functionalization of polyoxometalates may also consist in the grafting of a functional group at the surface of the polyanion.

Large species containing transition metal and chalcogenide groups are involved in many areas of science and represent useful models for magnetochemistry studies, bio-inorganic chemistry, material science [1, 2]. Polyoxometalates, namely POMs, have applications in homogeneous and heterogeneous catalysis [3‐5], and many studies are devoted to POMs since they are expected to mimic the reactivity of metal oxide surface and their catalytic properties.

Most of the known polyoxometalates (POMs) only contain diamagnetic transition metal ions (d⁰) and therefore are not interesting in magnetism. However these molecular metal-oxides are currently receiving much attention in molecular magnetism due to two important properties: (a) they can act as ligands coordinating groups of paramagnetic ions, like Co(II), Mn(II), Ni(II), Fe(II), Fe(III) and Cu(II); and (b) they can be reversibly reduced to mixed-valence species by injection of variable number of electrons. From this point of view they have shown to provide model systems to study exchange interactions and electron transfer processes at the molecular level. Recent reviews accounting for the state-of-the-art in this area can be found in [1] and [2].

Nature’s evolution from the primordial earth to the present overwhelming variety of macroscopic forms and functionalities, the formation of which is directed by their molecular “counterparts”, i.e. by biomolecules, stimulates thoughts about the potentiality of material systems in general and their related basic first principles [6]. Especially processes that take place in reservoir systems containing appropriate building blocks can lead to a myriad of molecular forms including routes that employ symmetry breaking. This is in particular valid when we enter the nanocosmos size category [7]. The nanocosmos as such does not pose restrictions like the variety-limiting translational symmetry of macroscopic crystalline materials, but offers —in contrast to the microcosmos, defined by its smaller molecular structures— the possibility of larger arrays with local symmetries that differ from the overall symmetry, thereby increasing the number of options for the generation of an extreme structural versatility —a situation well known, e.g. for spherical viruses (Figure 1) [8]. These deliberations are relevant for a nanochemistry based on polyoxometalates, especially polyoxomolybdates under reducing conditions, which form an overwhelming variety of structures.

This chapter will present an overview on the use of polyoxometalates as catalysts in homogeneous reaction media. This is not meant to be a comprehensive review as several such reviews are already available covering material up to the last few years [1], but rather an attempt to describe, also to non-practitioners, the general principles involved in this important field of application of polyoxometalate chemistry. This chapter will cover first more briefly the use of polyoxometalates as acid catalysts and then the use of polyoxometalates in oxidation catalysis. The discussion of oxidation catalysis will emphasize both general principles involved in the activation of oxidants and oxygen donors and the specific use of polyoxometalates in this context. Emphasis will be on oxidative transformations involving hydrocarbons and much of the description and discussion will be concentrated on research carried out in our group (my apologies in advance to those whose research was not completely covered).

Heteropoly compounds (HPC) include polyoxometalates (POM) —nanosized metal-oxygen cluster anions (heteropoly anions)— as the principal building blocks [1]. These anions form by a self-assembly process in solution and can be isolated as as heteropoly acids or salts with appropriate countercations, e.g., H⁺, alkali metal cation, etc.

Photochemistry is the realm of chemical reactions influenced by the absorption of light. In this Chapter we discuss how light initiates and propagates chemical reactions in which POMs are involved. We examine some fundamental aspects of photochemistry, and show how these are influenced by POMs.

Molecular materials with cooperative physical properties constitute one of the most active focus of interest in contemporary materials science. An attractive chemical feature of these materials derives from the possibility of building them from molecular bricks using the advantages provided by molecular chemistry and the knowledge achieved over the last 20 years in the so-called molecular engineering. From the point of view of the physical properties, it is well known that molecular materials can exhibit the properties typically associated with the inorganic network solids, as for example metallic conductivity and superconductivity [1], ferromagnetism [2] and non-linear optics [3].

Molecular self-organization, a universal driving force in Nature, represents an efficient way to combine, position, and orient molecular components in a well-defined supramolecular architecture through weak non-covalent interactions. In the progression of structural hierarchy from the atom to the molecule, the supermolecule, and the supramolecular module (SUMO), characteristic functions emerge that do not exist at lower levels. SUMOs evolve spontaneously from suitably instructed components through a sequence of recognition, growth, and termination steps [1]. Intriguing examples of SUMOs have been reported, exploiting ligand-metal ion coordination [2], π-π interactions [3], or hydrogen-bonding [4]. The modularity of self-assembly provides access to a wide range of structures and functions and permits control thereof from molecular to macroscopic length scales. The ability of SUMOs to accomplish intricate functions provides opportunities that go far beyond current micro-fabrication technology [5]. Applications of such systems are intriguingly diverse, including information storage, signal transduction and amplification, as well as host-guest recognition [6].