POMzites: a roadmap for inverse design in metal oxide chemistry

Computationally exploring the space generated by the self‐assembly of known molecular metal oxides and the ability to predict new architectures is a challenging task. As a proof of concept, here, we propose narrowing it down to a new family of all‐inorganic porous materials named POMzites. Structures with new topologies, but aiming for pure inorganic systems, will be targeted initially. POMzites are composed of ring‐shaped tungsten oxide building blocks connected with transition metal linkers forming zero to three dimensional frameworks. Despite POMzites and zeolites having similar structures, the library of POMzites is an order of magnitude smaller than that of zeolites (14 POMzites vs 213 zeolites). The idea proposed in this perspective article is to accelerate the discovery of new POMzite porous frameworks materials using inverse design approaches

Theoretical view on the origin and implications of structural distortions in polyoxometalates

Structural features of polyoxometalates (POMs) —versatile inorganic clusters of academic and technological interest— are discussed in the present article. POMs are, in general, very regular structures presenting a high symmetry in most cases. Distortions are, however, important for some electronic and magnetic properties. We herein discuss some particular geometric features that are crucial for the theoretical treatment and comprehension of well-known experimental phenomena. For instance, we have been able to understand and rationalize the geometrical distortions present in molybdenum POMs. Moreover, we can affirm that these geometrical distortions are caused by a pseudo Jahn Teller effect. In what concerns NMR chemical shifts, we present a discussion on the importance of geometry for the correct description of the signals and the key role played by the interatomic distances. Finally, a study on the adsorption of Keggin clusters on silver surfaces shows how the POM structure looses its regular shape to adapt to that new situation

Computational study into the effects of countercations on the [P8W48O184]40– polyoxometalate wheel

Porous metal oxide materials have been obtained from a ring-shaped macrocyclic polyoxometalate (POM) structural building unit, [P8W48O184]40–. This is a tungsten oxide building block with an integrated “pore” of 1 nm in diameter, which, when connected with transition metal linkers, can assemble frameworks across a range of dimensions and which are generally referred to as POMzites. Our investigation proposes to gain a better understanding into the basic chemistry of this POM, specifically local electron densities and locations of countercations within and without the aforementioned pore. Through a rigorous benchmarking process, we discovered that 8 potassium cations, located within the pore, provided us with the most accurate model in terms of mimicking empirical properties to a sufficient degree of accuracy while also requiring a relatively small number of computer cores and hours to successfully complete a calculation. Additionally, we analyzed two other similar POMs from the literature, [As8W48O184]40– and [Se8W48O176]32–, in the hopes of determining whether they could be similarly incorporated into a POMzite network; given their close semblance in terms of local electron densities and interaction with potassium cations, we judge these POMs to be theoretically suitable as POMzite building blocks. Finally, we experimented with substituting different cations into the [P8W48O184]40– pore to observe the effect on pore dimensions and overall reactivity; we observed that the monocationic structures, particularly the Li8[P8W48O184]32– framework, yielded the least polarized structures. This correlates with the literature, validating our methodology for determining general POM characteristics and properties moving forward

Controlling the reactivity of the [P8W48O184]40– inorganic ring and its assembly into POMZite inorganic frameworks with silver ions

The construction of pure‐inorganic framework materials with well‐defined design rules and building blocks is challenging. In this work, we show how a polyoxometalate cluster with an integrated pore, based on the [P 8 W 48 O 184 ] 40– (abbreviated as {P 8 W 48 }), can be self‐assembled into an inorganic framework using silver ions which both enable reactions on the cluster, as well as links them together. The {P 8 W 48 } was found to be highly reactive with silver ions resulting in the in‐situ generation of fragments, forming {P 9 W 63 O 235 } and {P 10 W 66 O 251 } in compound ( 1 ) where these two clusters co‐crystallize and are connected into a POMZite framework with 11 Ag + ions as linkers located inside clusters and 10 Ag + linking ions situated between clusters. Decreasing both the concentration of Ag + ions, and the reaction temperature compared to the synthesis of compound ( 1 ), leads to {P 8 W 51 O 196 } in compound 2 where the {P 8 W 48 } clusters are linked to form a new POMZite framework with 9 Ag + ions per formula unit. Further tuning of the reaction conditions yields a cubic porous network compound ( 3 ) where {P 8 W 48 } clusters as cubic sides are joined by 4 Ag + ions to give a cubic array and no Ag + ions were found inside the clusters

Synthesis of polyoxometalate clusters using carbohydrates as reducing agents leads to isomer-selection

By using sugars as the reducing agents, we demonstrate that it is possible to control the self-assembly of polyoxomolybdates through selective preparation of a single heteropolyanion isomer. D-(−)-Fructose has been proved to be an effective reducing sugar compared to the chemically similar carbohydrate D-(+)-glucose. The gentle reduction results in favourable formation of the Wells–Dawson type gamma isomer in 6-fold reduced form at room temperature

Influence of the contact geometry and counterions on the current flow and charge transfer in polyoxometalate molecular junctions: a density functional theory study

Polyoxometalates (POMs) are promising candidates for molecular electronic applications because (1) they are inorganic molecules, which have better CMOS compatibility compared to organic molecules; (2) they are easily synthesized in a one-pot reaction from metal oxides (MOx) (where the metal M can be, e.g., W, V, or Mo, and x is an integer between 4 and 7); (3) POMs can self-assemble to form various shapes and configurations, and thus the chemical synthesis can be tailored for specific device performance; and (4) they are redox-active with multiple states that have a very low voltage switching between polarized states. However, a deep understanding is required if we are to make commercial molecular devices a reality. Simulation and modeling are the most time efficient and cost-effective methods to evaluate a potential device performance. Here, we use density functional theory in combination with nonequilibrium Green’s function to study the transport properties of [W18O54(SO3)2]4–, a POM cluster, in a variety of molecular junction configurations. Our calculations reveal that the transport profile not only is linked to the electronic structure of the molecule but also is influenced by contact geometry and presence of ions. More specifically, the contact geometry and the number of bonds between the POM and the electrodes determine the current flow. Hence, strong and reproducible contact between the leads and the molecule is mandatory to establish a reliable fabrication process. Moreover, although often ignored, our simulations show that the charge balancing counterions activate the conductance channels intrinsic to the molecule, leading to a dramatic increase in the computed current at low bias. Therefore, the role of these counterions cannot be ignored when molecular based devices are fabricated. In summary, this work shows that the current transport in POM junctions is determined by not only the contact geometry between the molecule and the electrode but also the presence of ions around the molecule. This significantly impacts the transport properties in such nanoscale molecular electronic devices

Investigating the transformations of polyoxoanions using mass spectrometry and molecular dynamics

The reactions of [γ-SiW10O36]8– represent one of the most important synthetic gateways into a vast array of polyoxotungstate chemistry. Herein, we set about exploring the transformation of the lacunary polyoxoanion [β2-SiW11O39]8– into [γ-SiW10O36]8– using high-resolution electrospray mass spectrometry, density functional theory, and molecular dynamics. We show that the reaction proceeds through an unexpected {SiW9} precursor capable of undertaking a direct β → γ isomerization via a rotational transformation. The remarkably low-energy transition state of this transformation could be identified through theoretical calculations. Moreover, we explore the significant role of the countercations for the first time in such studies. This combination of experimental and the theoretical studies can now be used to understand the complex chemical transformations of oxoanions, leading to the design of reactivity by structural control

A metamorphic inorganic framework that can be switched between eight single-crystalline states

The design of highly flexible framework materials requires organic linkers, whereas inorganic materials are more robust but inflexible. Here, by using linkable inorganic rings made up of tungsten oxide (P8W48O184) building blocks, we synthesized an inorganic single crystal material that can undergo at least eight different crystal-to-crystal transformations, with gigantic crystal volume contraction and expansion changes ranging from −2,170 to +1,720 Å3 with no reduction in crystallinity. Not only does this material undergo the largest single crystal-to-single crystal volume transformation thus far reported (to the best of our knowledge), the system also shows conformational flexibility while maintaining robustness over several cycles in the reversible uptake and release of guest molecules switching the crystal between different metamorphic states. This material combines the robustness of inorganic materials with the flexibility of organic frameworks, thereby challenging the notion that flexible materials with robustness are mutually exclusive

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

Assembly and properties of polyoxometalates: a theoretical point of view

En la química del Mo i W destaca la formació d’un gran nombre de poliàcids, coneguts amb el nom de polioxometalats. Els polioxometalats (POMs) són clústers de metalls (normalment Mo, W, V) i oxigen. L’objectiu inicial d’aquesta tesi fou l’estudi teòric del procés de formació del polianió més senzill [W6O19]2–. També es van voler identificar les espècies més estables presents en solució. L’èxit obtingut en la primera etapa va fer que ens plantegéssim nous reptes, d’aquesta manera vam voler estudiar la formació d’altres polianions com ara [Mo6O19]2– i els heteropolianions [XM12O40]q– (X=P, As, M=W, Mo). La comprensió dels mecanismes de formació d’aquestes estructures hauria estat impossible sense la informació obtinguda dels experiments mitjançant ESI-MS (Espectrometria de Masses amb Ionització per Electroesprai). En aquesta tesi també s’han estudiat altres propietats rellevants dels POMs com ara els seus isòmers i la predicció de senyals de RMN (Ressonància Magnètica Nuclear).Inorganic metal oxygen clusters, or polyoxometalates (POMs) for short, form a vast class of inorganic compounds that is unequaled in terms of their molecular and electronic structural versatility, reactivity, and relevance. POMs are formed by transition metals in high oxidation states (e.g. WVI, MoVI) surrounded by oxo-ligands. In the present thesis we have studied the formation mechanisms of POMs, initially we have analized the case of the well known Lindqvist anion [M6O19]2– when M= W, Mo. Afterwards, we have considered the effect of the heteroatom in the formation mechanism analyzing the case of the Keggin anion, [XM12O40]n– when M=W, Mo and X=P, As. We have used electrospray-ionization mass spectrometry (ESI-MS) in order to obtain experimental information of molecular oxide clusters in solution. We have also studied the rotational isomerism in the Dawson anion [X2M18O62]n– and finally we present our improvements in the study 183W NMR chemical shifts