Effective Storage of Electrons in Water by the Formation of Highly Reduced Polyoxometalate Clusters.

Aqueous solutions of polyoxometalates (POMs) have been shown to have potential as high-capacity energy storage materials due to their potential for multi-electron redox processes, yet the mechanism of reduction and practical limits are currently unknown. Herein, we explore the mechanism of multi-electron redox processes that allow the highly reduced POM clusters of the form {MO3}y to absorb y electrons in aqueous solution, focusing mechanistically on the Wells-Dawson structure X6[P2W18O62], which comprises 18 metal centers and can uptake up to 18 electrons reversibly (y = 18) per cluster in aqueous solution when the countercations are lithium. This unconventional redox activity is rationalized by density functional theory, molecular dynamics simulations, UV-vis, electron paramagnetic resonance spectroscopy, and small-angle X-ray scattering spectra. These data point to a new phenomenon showing that cluster protonation and aggregation allow the formation of highly electron-rich meta-stable systems in aqueous solution, which produce H2 when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [P5W30O110]15- and [P8W48O184]40- anions, which can be charged to 23 and 27 electrons per cluster, respectively.

Elucidating the paramagnetic interactions of an inorganic-organic hybrid radical-functionalized Mn-Anderson cluster.

A family of six polyoxometalate-based magnetic compounds were synthesized by anchoring N-oxide type TEMPO radicals onto an Anderson type polyoxometalate cluster. The complexes were structurally characterised by single crystal X-ray diffraction and the intramolecular paramagnetic interactions between TEMPO radicals and Mn ions of the resulting hybrids were investigated in detail by electron paramagnetic resonance and the Evans NMR method.

Encapsulation of a {Cu16} cluster containing four [Cu4O4] cubanes within an isopolyoxometalate {W44} cluster.

We report a {Cu16} embedded within a {W44} cluster containing four cubane-like [Cu4O4] units within an isopolyoxotungstate (isoPOT) in a {Na4Cu4[(H2W11O38) (CH3COO)(OH)3]}4·88H2O (1) and a polyanion Cu-linked {W11} chain Na6Cu2[(H2W11O38)(CH3COO)(OH)]·26H2O (2). Electronically, the redox properties show that both compounds 1 and 2 undergo irreversible reductions resulting in the demetalation of the compounds, whilst the magnetic behavior of 1 and 2 shows a weak antiferromagnetic and a stronger ferromagnetic coupling, respectively.

Following the Reaction of Heteroanions inside a {W18O56} Polyoxometalate Nanocage by NMR Spectroscopy and Mass Spectrometry.

By incorporating phosphorus(III)-based anions into a polyoxometalate cage, a new type of tungsten-based unconventional Dawson-like cluster, [W18O56(HP(III)O3)2(H2O)2](8-), was isolated, in which the reaction of the two phosphite anions [HPO3](2-) within the {W18O56} cage could be followed spectroscopically. As well as full X-ray crystallographic analysis, we studied the reactivity of the cluster using both solution-state NMR spectroscopy and mass spectrometry. These techniques show that the cluster undergoes a structural rearrangement in solution whereby the {HPO3} moieties dimerize to form a weakly interacting (O3PH⋅⋅⋅HPO3) moiety. In the crystalline state the cluster exhibits a thermally triggered oxidation of the two P(III) template moieties to form P(V) centers (phosphite to phosphate), commensurate with the transformation of the cage into a Wells-Dawson {W18O54} cluster.

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.

Towards polyoxometalate-cluster-based nano-electronics.

We explore the concept that the incorporation of polyoxometalates (POMs) into complementary metal oxide semiconductor (CMOS) technologies could offer a fundamentally better way to design and engineer new types of data storage devices, due to the enhanced electronic complementarity with SiO2, high redox potentials, and multiple redox states accessible to polyoxometalate clusters. To explore this we constructed a custom-built simulation domain bridge. Connecting DFT, for the quantum mechanical modelling part, and mesoscopic device modelling, confirms the theoretical basis for the proposed advantages of POMs in non-volatile molecular memories (NVMM) or flash-RAM.

A redox-triggered structural rearrangement in an iodate-templated polyoxotungstate cluster cage.

The new tungstatoiodate, α-[H5W18O59(IO3)](6-), containing I(V)O3(-) within a {W18O54} metal oxide framework has been prepared and shown by X-ray crystallography and mass spectrometry to be derived from the fully oxidised [H3W18O56(IO6)](6-) by two-electron reduction accompanied by a redox-triggered structural rearrangement where three I-O covalent bonds are broken.

Naphthoxanthenyl, a new stable phenalenyl type radical stabilized by electronic effects.

Naphthoxanthenyl 1 is a new stable phenalenyl-type radical. Electrochemical studies indicate that 1 has two reversible redox processes that occur on comparatively short time scales. Crystals containing 1 can be grown by electrocrystallization, suggesting that they are conductive.

Synthesis and characterisation of a lanthanide-capped dodecavanadate cage.

The synthesis of a series of discrete lanthanide-capped polyoxovanadate cages is presented along with magnetic and electrochemical measurements which reveal a redox active dodecavanadate cluster with potential as a new functional building unit in polyoxovanadate chemistry.

Nanoscale control of polyoxometalate assembly: a {Mn8W4} cluster within a {W36Si4Mn10} cluster showing a new type of isomerism.

Two near isomeric clusters containing a novel {Mn(8)W(4)} Keggin cluster within a [W(36)Mn(10)Si(4)O(136)(OH)(4)(H(2)O)8](24-) cluster are reported: K(10)Li(14)[W(36)Si(4)O(136)Mn(II)(10)(OH)(4)(H(2)O)(8)] (1) and K(10)Li1(3.5)Mn(0.25)[W(36)Si(4)O(136)Mn(II)(10)(OH)(4)(H(2)O)(8) ] (1'). Bulk characterization of the clusters has been carried out by single crystal X-ray structure analysis, ICP-MS, TGA, ESI-MS, CV and SQUID-magnetometer analysis. X-ray analysis revealed that 1' has eight positions within the central Keggin core that were disordered W/Mn whereas 1 contained no such disorder. This subtle difference is due to a differences is how the two clusters assemble and recrystallize from the same mother liquor and represents a new type of isomerism. The rapid recrystallization process was captured via digital microscopy and this uncovered two "intermediate" types of crystal which formed temporarily and provided nucleation sites for the final clusters to assemble. The intermediates were investigated by single crystal X-ray analysis and revealed to be novel clusters K(4)Li(22)[W(36)Si(4)Mn(7)O(136)(H(2)O)(8)]·56H(2)O (2) and Mn(2)K(8)Li(14)[W(36)Si(4)Mn(7)O(136)(H(2)O)(8)]·45H(2)O (3). The intermediate clusters contained different yet related building blocks to the final clusters which allowed for the postulation of a mechanism of assembly. This demonstrates a rare example where the use X-ray crystallography directly facilitated understanding the means by which a POM assembled.

Imaging the Belousov-Zhabotinsky reaction in real time using an ion sensitive array.

We show how an array of ion-sensitive-field-effect-transistors can be used to both spatially and temporally image the oscillating pH/ion waves produced by the Belousov-Zhabotinsky (BZ) reaction with high resolution.

Real-time ion-flux imaging in the growth of micrometer-scale structures and membranes.

Real-time ion flux imaging: an ion-sensitive field-effect transistor (ISFET) array is coupled with optical microscopy to image the growth of, and ion flux through, micrometer-scale tubes and membranes built from polyoxometalate clusters. The correlation between the optical and ionic imaging data is excellent, showcasing the use of ISFET arrays for high-resolution spatial and temporal mapping of ionic movements.

A supramolecular heteropolyoxopalladate {Pd15} cluster host encapsulating a {Pd2} dinuclear guest: [Pd(II)2⊂{H7Pd(II)15O10(PO4)10}](9-).

A high-nuclearity polyoxopalladate compound, [Pd(II)(2)⊂{H(7)Pd(II)(15)O(10)(PO(4))(10)}](9-) {Pd(II)(17)}, comprising a {Pd(15)} host occupied by a {Pd(2)} guest and the parent pristine "empty" [H(7)Pd(II)(15)O(10)(P(V)O(4))(10)](13-) {Pd(15)} cluster have both been prepared and characterized by single-crystal X-ray crystallography, (31)P NMR, CSI-MS, and XPS. The encapsulated {Pd(2)} has a short Pd(II)-Pd(II) distance within the {Pd(15)} host. Solution studies indicate that the empty host and filled guest complex are in equilibrium with each other, and UV titrations revealed a binding constant of ca. 10(3) for the guest Pd(II) ions, with a binding stoichiometry of almost 2.

Trinuclear {M1}CN{M2}2 complexes (M1 = Cr(III), Fe(III), Co(III); M2 = Cu(II), Ni(II), Mn(II)). Are single molecule magnets predictable?

The reaction of the hexacyanometalates K3[M(1)(CN)6] (M(1) = Cr(III), Fe(III), Co(III)) with the bispidine complexes [M(2)(L(1))(X)](n+) and [M(2)(L(2))(X)](n+) (M(2) = Mn(II), Ni(II), Cu(II); L(1) = 3-methyl-9-oxo-2,4-di-(2-pyridyl)-7-(2-pyridylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; L(2) = 3-methyl-9-oxo-7-(2-pyridylmethyl)-2,4-di-(2-quinolyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; X = anion or solvent) in water-methanol mixtures affords trinuclear complexes with cis- or trans-arrangement of the bispidine-capped divalent metal centers around the hexacyanometalate. X-ray structural analyses of five members of this family of complexes (cis-Fe[CuL(2)]2, trans-Fe[CuL(1)]2, cis-Co[CuL(2)]2, trans-Cr[MnL(1)]2, trans-Fe[MnL(1)]2) and the magnetic data of the entire series are reported. The magnetic data of the cyanide bridged, ferromagnetically coupled cis- and trans-Fe[ML]2 compounds (M = Ni(II), Cu(II)) with S = 3/2 (Cu(II)) and S = 5/2 (Ni(II)) ground states are analyzed with an extended Heisenberg Hamiltonian which accounts for anisotropy and zero-field splitting, and the data of the Cu(II) systems, for which structures are available, are thoroughly analyzed in terms of an orbital-dependent Heisenberg Hamiltonian, in which both spin-orbit coupling and low-symmetry ligand fields are taken into account. It is shown that the absence of single-molecule magnetic behavior in all spin clusters reported here is due to a large angular distortion of the [Fe(CN)6](3-) center and the concomitant quenching of orbital angular momentum of the Fe(III) ((2)T2g) ground state.