https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

Polyoxometalates (POMs) are a subset of metal oxides that represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form dynamic structures that can range in size from the nano- to the micrometer scale Herein we present the very latest developments from synthesis to structure and function of POMs We discuss the possibilities of creating highly sophisticated functional hierarchical systems with multiple, interdependent, functionalities along with a critical analysis that allows the non-specialist to learn the salient features We propose and present a "periodic table of polyoxometalate building blocks" We also highlight some of the current issues and challenges that need to be addressed to work towards the design of functional systems based upon POM building blocks and look ahead to possible emerging application areas

Polyoxometalates: Building Blocks for Functional Nanoscale Systems

Polyoxometalates (POMs) are discrete anionic metaloxygen clusters which can be regarded as soluble oxide fragments. They exhibit a great diversity of sizes, nuclearities, and shapes. They are built from the connection of {MOx} polyhedra, M being a d-block element in high oxidation state, usually VIV,V, MoVI, or WVI.1 While these species have been known for almost two centuries, they still attract much interest partly based on their large domains of applications. They play a great role in various areas ranging from catalysis,2 medicine,3 electrochemistry,4 photochromism,5 to magnetism.6 This palette of applications is intrinsically due to the combination of their added value properties (redox properties, large sizes, high negative charges, nucleophilicity...). Parallel to this domain, the organic-inorganic hybrids area has followed a similar expansion during the last 10 years. The concept of organic-inorganic hybrid materials * To whom correspondence should be addressed. E-mail: dolbecq@ chimie.uvsq.fr. Chem. Rev. 2010, 110, 6009–6048 6009

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Recent Advances in Polyoxometalate-Catalyzed Reactions

Functionalization of polyoxometalates: towards advanced applications in catalysis and materials science

Enantiomerically pure polytungstates: chirality transfer through zirconium coordination centers to nanosized inorganic clusters.

The first organic−inorganic hybrid complexes between CB[n] and polyoxometalates not only display a surprisingly high structural complementarity, the right pairing also allows their chemical and physical properties to be coupled, as illustrated by two examples.

Cucurbit[n]uril-polyoxoanion hybrids.

While the chemistry of discrete and networked high-nuclearity metal clusters continues to evolve steadily, understanding the driving forces and underlying principles governing the aggregation processes of such clusters remains a profound challenge. The imperative need for effective control over their chemical and physical properties—exploited in catalysis, bioinspired chemistry, and materials science—has stimulated considerable interest in the development of more rational and controllable synthetic strategies. In this context, methods based on templating principles promise both enhanced control and greater mechanistic predictability: the nuclearity and geometry of the resulting aggregate is strongly dependent on the size, shape, charge, and the stereoelectronic and coordinative geometric preference of the template. The template effect of anions, in particular, is being increasingly studied in supramolecular organic and coordination-chemistry systems because of its relevance to many chemical and biological processes. While such templated systems may be constructed utilizing noncovalent forces ranging from van der Waals and hydrogen bonding to stronger metal–ligand coordinative interactions, kinetic (template can be removed) and thermodynamic (template becomes integral part of the product) templating phenomena can be differentiated. The latter predominates anion templation in the field of polyoxometalate (POM) chemistry. For example, various anions are enclosed within the central cavities of discrete polyoxovanadates to form templated host–guest complexes of striking structural complementarity. Furthermore, the structure-directing role of anionic species in the condensation of polyoxothiometalate rings was illustrated by S cheresse and co-workers. The same group also reported the construction of a copper-based polyoxotungstate cluster using halide templation. Herein, we describe the formation of a large heterochiral POM architecture, [{a-P2W15O56}6{Ce3Mn2(m3-O)4(m2-OH)2}3(m2-OH)2(H2O)2(PO4)] 47 (1), from multiple molecular components based on the trivacant derivative of the Dawson polyoxotungstate [a-P2W18O62] 6 . Remarkably, the construction of such a highly negatively charged, aggregated POM is mediated by a far smaller anion, phosphate. The complex, isolated as K36Na111·106H2O, was prepared in the course of our efforts to modify the magnetic properties of preformed metal carboxylate clusters by introducing polyoxoanions through ligand competition. We have recently demonstrated that organic bridging ligands on a manganese carboxylate cluster may be partially replaced by polyoxoanions without altering the connectivity of the magnetic cluster core. We now extend this strategy and show that complete ligand substitution can be achieved, thereby giving rise to an allinorganic magnetic cluster based on supporting polyanion ligands. More importantly, an unexpected templating event subsequently organizes these preconceived building blocks into a supercluster, that is, a “cluster of clusters”. Synthesis of 1 is based on a heterometallic high-oxidationstate precursor 2 (Scheme 1) recently reported by Christou and co-workers. The central [Ce3Mn IV 2(m3-O)6] 8+ core of 2

PO43−‐Mediated Polyoxometalate Supercluster Assembly

The self-assembly of large metal oxide clusters usually proceeds via condensation steps thermodynamically driven by charge and nucleophilicity of the growing transient cluster fragments. Polyoxometalate (POM) chemistry has accumulated many strategies to interfere with these basic formation principles, aiming at both directed molecular growth and targeted functionalization by selective introduction of metal centers or organic moieties. POM structures integrating 3d or 4d transition-metal ions in particular attest to this approach, and they have led to a rich class of molecular materials ranging from molecular magnets to oxidation catalysts. In this context, polyoxotungstate clusters provide rigid and redox-stable scaffolds based on building-block-type fragments that are frequently derived from archetypal structures such as the Keggin, Dawson, or Lindqvist species. Additional heterometal cations coordinating to or interconnecting these nucleophilic structures are key to the reactivity and the electronic and magnetic characteristics of the resulting adducts. This situation is exemplified by the spherical {M72L30} Keplerate clusters that have been realized both as polymolybdate (M = Mo) and polytungstate (M = W) structures containing a variety of heterometal linkers (L = V, Cr, Fe, etc.). These clusters, comprising unique spin polytopes that represent molecular analogues of Kagom lattices, constitute structural platforms for subsequent reactions, ranging from redox reactions and partial heterometal exchange to condensations to oneand two-dimensional coordination networks—all of which alter the clusters magnetic characteristics while retaining the basic cluster structure. Moreover, recent development of POM-based single-molecule magnets raises the hope that magnetic POMs may find their way into areas such as molecular spintronics or quantum computing. However, the controlled growth of large metal oxide clusters remains elusive: precise prediction of the outcome is very difficult given the involvement of many, often labile, metal–oxygen bonds. The self-assembly mechanisms that underlie the formation of larger POMs in aqueous reaction solutions are barely established and are not compatible with the synthetic controls available in classical coordination chemistry, as exemplified by the use of molecular tectons, characterized by their specific connectivity constraints, in the rational production of supramolecular aggregates or porous metal–organic frameworks. We postulate that this roadblock in controlling the molecular growth steps in polyoxotungstate chemistry can be partially circumvented by kinetic control of competing reactions, as illustrated by the template-induced formation of a 4.3 nm manganese(III) polyoxotungstate cluster anion [Mn40P32W VI 224O888] 144 (1). The preparation of 1 starts from metastable [a-H2P2W12O48] 12 ({P2W12}), a hexavacant phosphotungstate derived from the plenary [a-P2W18O62] 6

Molecular Growth of a Core–Shell Polyoxometalate†

Carbonate-assisted hydrolysis of Y or Yb I I I ions in the presence of the trivacant Wells-Dawson polyoxo-anion, α-[P 2 W 1 5 O 5 6 ] 1 2 - , produced two polyoxometalate-supported Y- or Yb I I I -hydroxo/oxo clusters, which have been characterized by single-crystal X-ray structure determination. The structure of the Y complex consists of a distorted Y 4 (OH) 4 cubane cluster encapsulated by two lacunary α-[P 2 W 1 5 O 5 6 ] 1 2 - units, while the Yb cluster features a hexametallic core centered around a μ 6 -oxo atom with each Yb I I I 3 triangular face capped by an oxo or a hydroxo group. Magnetization measurements of the ytterbium(III) derivative suggested that intermolecular dipolar exchange is present at low temperatures (below 15 K). Despite its absence in the structures themselves, control experiments show that carbonate not only functions in the hydrolysis, it also influences the structure of the complexes by complexation to yttrium and the f-block elements.

Xikui Fang

Papers

Enantiomerically pure polytungstates: chirality transfer through zirconium coordination centers to nanosized inorganic clusters.

Cucurbit[n]uril-polyoxoanion hybrids.

PO43−‐Mediated Polyoxometalate Supercluster Assembly

Molecular Growth of a Core–Shell Polyoxometalate†

Polyoxometalate-supported Y- and YbIII-hydroxo/oxo clusters from carbonate-assisted hydrolysis.