Haralampos N. Miras

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.

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.

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.

The Construction of High-Nuclearity Isopolyoxoniobates with Pentagonal Building Blocks: [HNb27O76]16− and [H10Nb31O93(CO3)]23−†

Big, bigger, biggest: Polyoxoniobate anions [HNb27O76]16− and [H10Nb31O92(CO3)]23−incorporate pentagonal Nb(Nb)5 building blocks; the central Nb ion is seven-coordinate within the clusters. The Nb27 species was observed using ESI-MS, thus demonstrating some solution stability; the Nb31 species is chiral and incorporates a carbonate ligand in the outer section of the cluster. The two species are the largest polyoxoniobates reported to date.

Reversible Redox Reactions in an Extended Polyoxometalate Framework Solid

Back and forth: A concerted reversible redox reaction occurs in a pure metal oxide extended polyoxometalate framework when the accessible pockets are filled with a suitable redox agent. Direct control over the framework properties is demonstrated by repeated reversible switching between an expanded and a contracted structure. Successive recrystallizations from hot water repeatedly destroys and regenerates the framework.

Real-Time Observation of the Self-Assembly of Hybrid Polyoxometalates Using Mass Spectrometry**

Electrospray ionization mass spectrometry (ESI-MS) allows monitoring of the real-time, “in-solution” formation of an organic–inorganic polyoxometalate hybrid system (see picture). This has provided insight into the rearrangement involved in the formation of the manganese-Anderson cluster coordinated with tris(hydroxymethyl)aminomethane ligands, through the rearrangement of the [α-Mo8O26]4− cluster in the presence of Mn(CH3CO2)3.

A Mixed‐Valence Manganese Cubane Trapped by Inequivalent Trilacunary Polyoxometalate Ligands

The title compound contains an embedded mixed-valence {Mn5O6} cubane core, which is structurally similar to the active site in photosystem II. Solid-, solution-, and gas-phase studies indicate the presence of three lacunary Keggin fragments, thereby giving insight into the complex solution chemistry of plenary POM fragments.

Structural Evolution of “S”‐Shaped [H4W22O74]12− and “§”‐Shaped [H10W34O116]18− Isopolyoxotungstate Clusters

Finding the missing species: Two isopolyoxotungstate clusters with unprecedented “S”-shaped [H4W22O74]12− and “§”-shaped [H10W34O116]18− geometries are presented. The development of this structural complexity comes from the structural evolution based on the {W11} fragment and establishes a new building block principle in isopolyoxotungstate chemistry. The evolved clusters can only be isolated in the presence of sulfite ions.

Nanoscale Growth of Molecular Oxides: Assembly of a {V6} Double Cubane Between Two Lacunary {P2W15} Polyoxometalates

POM-in-POM: A Wells–Dawson polyoxometalate sandwich compound with a double cubane core consisting of six vanadium atoms has been synthesized (see structure). Cluster formation was followed by mass spectrometry and the reduction of the double cubane was studied by a novel technique combining mass spectrometry and spectroelectrochemistry.