A redox-active inorganic crown ether based on a polyoxometalate capsule.

Cation-uptake has been long researched as an important topic in materials science. Herein we focus on a molecular crystal composed of a charge-neutral polyoxometalate (POM) capsule [MoVI72FeIII30O252(H2O)102(CH3CO2)15]3+ encapsulating a Keggin-type phosphododecamolybdate anion [α-PMoVI12O40]3-. Cation-coupled electron-transfer reaction occurs by treating the molecular crystal in an aqueous solution containing CsCl and ascorbic acid as a reducing reagent. Specifically, multiple Cs+ ions and electrons are captured in crown-ether-like pores {MoVI3FeIII3O6}, which exist on the surface of the POM capsule, and Mo atoms, respectively. The locations of Cs+ ions and electrons are revealed by single-crystal X-ray diffraction and density functional theory studies. Highly selective Cs+ ion uptake is observed from an aqueous solution containing various alkali metal ions. Cs+ ions can be released from the crown-ether-like pores by the addition of aqueous chlorine as an oxidizing reagent. These results show that the POM capsule functions as an unprecedented "redox-active inorganic crown ether", clearly distinguished from the non-redox-active organic counterpart.

Integrating molecular design and crystal engineering approaches in non-humidified intermediate-temperature proton conductors based on a Dawson-type polyoxometalate and poly(ethylene glycol) derivatives.

Anionic metal-oxygen clusters known as polyoxometalates (POMs) have been widely researched as components of proton conductors. While proton conduction under non-humidified intermediate-temperature (100-250 °C) conditions is advantageous from the viewpoint of kinetics, few solid-state materials, not to mention POM-based crystals, show truly effective proton conduction without the aid of water vapor. In this context, non-volatile proton-conductive polymers have been confined into POM-based frameworks, while fast proton conduction was infeasible. Herein, we demonstrate a new strategy to synthesize POM-polymer composites exhibiting fast proton conduction under non-humidified intermediate-temperature conditions. Specifically, a molecular design approach utilizing poly(ethylene glycol)s (PEGs) of different terminal groups or chain lengths controls the proton carrier density, and a crystal engineering approach using a large Dawson-type POM ([α-P2W18O62]6-) with an anisotropic molecular shape and alkali metal ions as counter cations fine-tunes the mobility of the confined PEGs as proton carriers. By integrating these approaches, proton conductivity over 10-4 S cm-1 at 150 °C, comparable to the well-known highly proton-conductive solid-state materials, is achieved. The proton conduction mechanism is discussed with alternative current impedance spectroscopy jointly with specific heat capacity measurements and solid-state NMR spectroscopy.

Dimensional Control in Polyoxometalate Crystals Hybridized with Amphiphilic Polymerizable Ionic Liquids.

Ionic liquids are an important component for constructing functional materials, and polyxometalate cluster anion is a promising partner for building inorganic-organic hybrid materials comprising ionic liquids. In such hybrid materials, the precise control of the molecular arrangement in the bulk structures is crucial for the emergence of characteristic functions, which can be realized by introducing an amphiphilic moiety into the ionic liquids. Here, an amphiphilic polymerizable imidazolium ionic liquid with a methacryloyl group was firstly hybridized with polyoxometalate anions of octamolybdate ([Mo8O26]4-, Mo8) and silicotungstate ([SiW12O40]4-, SiW12) to obtain inorganic-organic hybrid crystals. The polymerizable ionic liquid with a octyl chain (denoted as MAImC8) resulted in the formation of anisotropic molecular arrangements in the bulk crystal structure, which was compared with the hybrid crystals composed from the polymerizable ionic liquid without a long alkyl chain (denoted as MAIm). Rather densely packed isotropic molecular arrangements were observed in the hybrid crystals of MAIm-Mo8 and MAIm-SiW12 due to the lack of the amphiphilic moiety. On the other hand, using the amphiphilic MAImC8 cation gave rise to a honeycomb-like structure with the Mo8 anion and a layered structure with the SiW12 anion, respectively.

Polymerizable Ionic Liquid Crystals Comprising Polyoxometalate Clusters toward Inorganic-Organic Hybrid Solid Electrolytes.

Solid electrolytes are crucial materials for lithium-ion or fuel-cell battery technology due to their structural stability and easiness for handling. Emergence of high conductivity in solid electrolytes requires precise control of the composition and structure. A promising strategy toward highly-conductive solid electrolytes is employing a thermally-stable inorganic component and a structurally-flexible organic moiety to construct inorganic-organic hybrid materials. Ionic liquids as the organic component will be advantageous for the emergence of high conductivity, and polyoxometalate, such as heteropolyacids, are well-known as inorganic proton conductors. Here, newly-designed ionic liquid imidazolium cations, having a polymerizable methacryl group (denoted as MAImC1), were successfully hybridized with heteropolyanions of [PW12O40]3- (PW12) to form inorganic-organic hybrid monomers of MAImC1-PW12. The synthetic procedure of MAImC1-PW12 was a simple ion-exchange reaction, being generally applicable to several polyoxometalates, in principle. MAImC1-PW12 was obtained as single crystals, and its molecular and crystal structures were clearly revealed. Additionally, the hybrid monomer of MAImC1-PW12 was polymerized by a radical polymerization using AIBN as an initiator. Some of the resulting inorganic-organic hybrid polymers exhibited conductivity of 10-4 S·cm-1 order under humidified conditions at 313 K.