Leroy Cronin

Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices

Polyoxometalates represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form structures that can bridge several length scales. The new building block principles that have been discovered are beginning to allow the design of complex clusters with desired properties and structures and several structural types and novel physical properties are examined. In this critical review the synthetic and design approaches to the many polyoxometalate cluster types are presented encompassing all the sub-types of polyoxometalates including, isopolyoxometalates, heteropolyoxometalates, and reduced molybdenum blue systems. As well as the fundamental structure and bonding aspects, the final section is devoted to discussing these clusters in the context of contemporary and emerging interdisciplinary interests from areas as diverse as anti-viral agents, biological ion transport models, and materials science.

Integrated 3D-printed reactionware for chemical synthesis and analysis

Three-dimensional (3D) printing has the potential to transform science and technology by creating bespoke, low-cost appliances that previously required dedicated facilities to make. An attractive, but unexplored, application is to use a 3D printer to initiate chemical reactions by printing the reagents directly into a 3D reactionware matrix, and so put reactionware design, construction and operation under digital control. Here, using a low-cost 3D printer and open-source design software we produced reactionware for organic and inorganic synthesis, which included printed-in catalysts and other architectures with printed-in components for electrochemical and spectroscopic analysis. This enabled reactions to be monitored in situ so that different reactionware architectures could be screened for their efficacy for a given process, with a digital feedback mechanism for device optimization. Furthermore, solely by modifying reactionware architecture, reaction outcomes can be altered. Taken together, this approach constitutes a relatively cheap, automated and reconfigurable chemical discovery platform that makes techniques from chemical engineering accessible to typical synthetic laboratories.

Self-Assembly of Organic-Inorganic Hybrid Amphiphilic Surfactants with Large Polyoxometalates as Polar Head Groups

Mn-Anderson-C6 and Mn-Anderson-C16, A type of inorganic-organic hybrid molecules containing a large anionic polyoxometalate (POM) cluster and two C6 and C16 alkyl chains, respectively, demonstrate amphiphilic surfactant behavior in the mixed solvents of acetonitrile and water. The amphiphilic hybrid molecules can slowly assemble into membrane-like vesicles by using the POM clusters as polar head groups, as studied by laser light scattering and TEM techniques. The hollow vesicles have a typical bilayer structure with the hydrophilic Mn-Anderson cluster facing outside and long hydrophobic alkyl chains staying inside to form the solvent-phobic layer. Due to the rigidity of the POM polar heads, the two alkyl tails have to bend significantly for the vesicle formation, which makes the vesicle formation more difficult compared to some conventional surfactants. This is the first example of using hydrophilic POM macroions as polar head groups for a surfactant system.

Polyoxometalate‐Mediated Self‐Assembly of Single‐Molecule Magnets: {[XW9O34]2[MnIII4MnII2O4(H2O)4]}12−

Last night of the POMs: The title compound (X=GeIV) exhibits slow relaxation of magnetization and quantum tunneling with a single-molecule magnetic behavior. Significant structural differences in the [MnIII4MnII2O4(H2O)4]8+ cluster core of the X=SiIV analogue modify the magnetic properties, thereby illustrating how polyoxometalate (POM) ligands can help in the systematic construction of nanoscale magnets.

Observation of Fe(V)=O using variable-temperature mass spectrometry and its enzyme-like C–H and C=C oxidation reactions

Oxo-transfer chemistry mediated by iron underpins many biological processes and today is emerging as synthetically very important for the catalytic oxidation of C-H and C=C moieties that are hard to activate conventionally. Despite the vast amount of research in this area, experimental characterization of the reactive species under catalytic conditions is very limited, although a Fe(V)=O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)=O species within a synthetic non-haem complex at -40 °C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H(2)O(2)/H(2)O through Fe(V)=O to the products and to probe the reactivity as a function of temperature. This study pioneers the implementation of variable-temperature mass spectrometry to investigate reactive intermediates.

Unveiling the Transient Template in the Self-Assembly of a Molecular Oxide Nanowheel

A Hidden Template The entropic challenge inherent in forming a ring-shaped molecule generally increases considerably with the size of the ring. Assuming that a linear precursor must bind its ends together, extending its length diminishes the likelihood of the opposite ends approaching one another. In the absence of an external force, how then can a family of molybdenum oxide rings, several nanometers in diameter (quite large at the molecular scale), self-assemble? Miras et al. (p. 72, see the cover; see the Perspective by Whitmire) have now uncovered an internal template guiding the process. By carefully controlling conditions in a flow reactor, they were able to halt the assembly process partway through and characterize a smaller molybdenum oxide core cluster, around which the larger ring was forming. Ejection of this template then yielded the hollow finished product. Use of a flow reactor reveals a key intermediate in the formation of a molybdenum oxide nanostructure. Self-assembly has proven a powerful means of preparing structurally intricate nanomaterials, but the mechanism is often masked by the common one-pot mixing procedure. We employed a flow system to study the steps underlying assembly of a previously characterized molybdenum oxide wheel 3.6 nanometers in diameter. We observed crystallization of an intermediate structure in which a central {Mo36} cluster appears to template the assembly of the surrounding {Mo150} wheel. The transient nature of the template is demonstrated by its ejection after the wheel is reduced to its final electronic state. The template’s role in the self-assembly mechanism is further confirmed by the deliberate addition of the template to the reaction mixture, which greatly accelerates the assembly time of the {Mo150} wheel and increases the yield.

Unravelling the complexities of inorganic and supramolecular self-assembly in solution with electrospray and cryospray mass spectrometry

Electrospray (ESI) and cryospray mass spectrometry (CSI-MS) are proving to be exceptionally versatile tools when used in conjunction with high resolution time-of-flight (TOF) systems to investigate the self-assembly of supramolecular architectures, inorganic coordination and organometallic compounds, labile molecules and clusters both from a structural and mechanistic point of view. In this feature article, we review very recent progress where mass spectrometry is being applied to highly labile and complex coordination and polyoxometalate (POM) cluster systems and we present some highlights from our initial electrospray and cryospray studies, which probe the self-assembly of inorganic cluster architectures. We discuss the major contributions of ESI and CSI-MS to labile and self-assembling inorganic architectures with great emphasis on future potential and ramifications for inorganic chemistry and the area of self-assembly as a whole.

Synthesis of Modular “Inorganic–Organic–Inorganic” Polyoxometalates and Their Assembly into Vesicles†

-capped Wells–Dawson-type clusters linkedtogether by the bis(Tris) ligands. Initial investigations intotheir thermal stability revealed that these compounds arestable up to approximately 2258C (see the SupportingInformation, Figure S1 and S2).Crystallization of1 was carried out by diffusion of diethylether into an acetonitrile solution of 1 and yellow blockcrystals were collected after one week. Single-crystal X-rayanalysis

Design and fabrication of memory devices based on nanoscale polyoxometalate clusters

Flash memory devices—that is, non-volatile computer storage media that can be electrically erased and reprogrammed—are vital for portable electronics, but the scaling down of metal–oxide–semiconductor (MOS) flash memory to sizes of below ten nanometres per data cell presents challenges. Molecules have been proposed to replace MOS flash memory, but they suffer from low electrical conductivity, high resistance, low device yield, and finite thermal stability, limiting their integration into current MOS technologies. Although great advances have been made in the pursuit of molecule-based flash memory, there are a number of significant barriers to the realization of devices using conventional MOS technologies. Here we show that core–shell polyoxometalate (POM) molecules can act as candidate storage nodes for MOS flash memory. Realistic, industry-standard device simulations validate our approach at the nanometre scale, where the device performance is determined mainly by the number of molecules in the storage media and not by their position. To exploit the nature of the core–shell POM clusters, we show, at both the molecular and device level, that embedding [(Se(iv)O3)2]4− as an oxidizable dopant in the cluster core allows the oxidation of the molecule to a [Se(v)2O6]2− moiety containing a {Se(v)–Se(v)} bond (where curly brackets indicate a moiety, not a molecule) and reveals a new 5+ oxidation state for selenium. This new oxidation state can be observed at the device level, resulting in a new type of memory, which we call ‘write-once-erase’. Taken together, these results show that POMs have the potential to be used as a realistic nanoscale flash memory. Also, the configuration of the doped POM core may lead to new types of electrical behaviour. This work suggests a route to the practical integration of configurable molecules in MOS technologies as the lithographic scales approach the molecular limit.

Postsynthetic covalent modification of metal-organic framework (MOF) materials.

The ability to reliably design, synthesize, and model porous inorganic materials from zeolites and other extended pure inorganic networks to systems comprised of complex inorganic coordination clusters and metal–organic coordination complexes has allowed a fundamental shift from discovery to design in this area of science. This is particularly true in the area of metal–organic materials (MOMs, e.g., metal– organic polyhedra, metal–organic frameworks (MOFs), or coordination polymers) in which many materials have been synthesized and reported as potential platforms for applications in various fields, such as gas storage, ion exchange, catalysis, magnetism, separation, and nonlinear optical materials. To date, there are two principal ways to prepare porous materials: one is focused on the templating method, and the other is based on metal–organic frameworks constructed from molecular building blocks (MBBs). In the former approach, soft templates, such as triblock copolymers and surfactants, or hard templates, such as porous alumina and porous silica, play a crucial role in directing the formation of porous structure. The other approach, which uses metal– organic frameworks (MOFs) to construct porous materials, is also well documented and has been extraordinarily successful. The presence of structure-directing ligands in MOFs means that they are highly amenable to systematic design, both in terms of the structure formed and the tailoring of the properties, structures, and surfaces of the pores present in the material. For example, high-throughput methodologies have been applied successfully to develop a robust synthesis of a zeolitic imidazol framework (ZIF), and the “secondary spillover technique” is able to increase hydrogen storage capacities markedly with the aid of a catalyst that is capable of dissociating H2 within the materials. [12] Besides exploring new methodologies and searching for new MOFs for effective gas absorption, it is of vital importance and at the same time a great challenge to make MOFs with functionalized pores. The ability to develop well-defined structures with functionalized pores or cavities is of interest, as this will open up the possibility to design hybrid functional materials that will have application in catalysis, as sensors, and as multifunctional materials arising from the incorporation of more than one function within the pore structure. Of course, the key limitation in the development of functional MOFs lies with the introduction of functionality, whilst maintaining the same ability to design and assemble overall framework without unfavorable effects from the presence of the “functional” components. Therefore two possible approaches involve a) the self assembly of the MOF with the functionality already present, and b) the postsynthetic modification of the MOF after self-assembly. In the latter approach, Gamez et al. have recently reported the crystallographic observation of MOF-based postsynthetic covalent modification inside the pores of a new MOF, in which the amino functional groups oriented inside the pores have been covalently functionalized by reaction with other organic molecules in the cavities without modifying the original three-dimensional framework. This was achieved by the judicious selection of organic linkers which allowed the construction of delicate molecular building blocks (MBBs), and the zeolite-like metal–organic frameworks (ZMOFs) with the desired functionality and directionality. In this case, the small building block, 2-amino-1,4benzenedicarboxylic acid (N-H2BDC), which is shown in Scheme 1 and was used in Yaghi>s investigation of novel isoreticular MOFs, was employed. Reaction of this ligand with gadolinium(III) nitrate at 120 8C in DMF leads to a dinuclear gadolinium(III) compound, and both gadolinium(III) ions are connected through four bridging carboxylato moieties of four ligands (Scheme 1). Eight digadolinium building blocks are arranged to form a cube in the crystal structure, and these cubes are further associated to produce an extended three-dimensional octahedral network which features porous channels. Interestingly, the pendant amino groups do not participate in coordination to the dinuclear Gd2O7 cluster in this new MOF, which is similar to Yaghi>s Zn4O tetrahedral cluster. [3c] As such, they are readily available for postsynthetic transformation. The postsynthetic transformation reaction of the amino functionality in this MOF was realized by reaction with ethylisocyanate, and the reaction proceeds as a crystal-to[*] Dr. Y.-F. Song, Prof. Dr. L. Cronin WestChem, Department of Chemistry The University of Glasgow, G128QQ, Glasgow (UK) Fax: (+44)141-330-4888 E-mail: l.cronin@chem.gla.ac.uk Homepage: http://www.chem.gla.ac.uk/staff/lee/ Angewandte Chemie