Vanadium-substituted polycationic Al-oxo cluster in a porous ionic crystal exhibiting Lewis acidity.

Herein we present the first example of a vanadium-substituted δ-Keggin Al13-based (Al28V4) porous Ionic crystal (PIC) (Al28V4-PW9V3). Structural and theoretical studies suggested that Al28V4 may exhibit Lewis acidity, which was confirmed by acetalization reaction with Al28V4-PW9V3 as a solid catalyst and investigations with pyridine or 2,6-lutidine as basic probe molecules. A comparative study with the δ-Keggin Al13 monomer based PIC indicated that the vanadium-substitution gave rise to the Lewis acidity.

Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates.

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5- (BW12), [SiW11NbO40]5- (SiW11Nb), and [SiW11TaO40]5- (SiW11Ta)) are synthesized. Experimental and theoretical characterizations of the three PICs (1_BW12, 1_SiW11Nb, and 1_SiW11Ta) show that the substitution of Nb/Ta for W in the POMs enhances the basicity of PICs, which increases in the order of 1_BW12 < 1_SiW11Nb < 1_SiW11Ta. These findings clearly show that the increase in basicity is due not only to the increase in negative charge of the POM molecule as is often explained, but also to the character of the substituted element itself.

Oxygen Evolution Reaction Driven by Charge Transfer from a Cr Complex to Co-Containing Polyoxometalate in a Porous Ionic Crystal.

Considerable efforts have been devoted to developing oxygen evolution reaction (OER) catalysts based on transition metal oxides. Polyoxometalates (POMs) can be regarded as model compounds of transition metal oxides, and cobalt-containing POMs (Co-POMs) have received significant interest as candidates. Nanocomposites based on Co-POMs have been reported to show high OER activities due to synergistic effects among the components; however, the role of each component is unclear due to its complex structure. Herein, we utilize porous ionic crystals (PICs) based on Co-POMs, which enable a composition-structure-function relationship to be established to understand the origin of the synergistic catalysis. Specifically, a Keggin-type POM [α-CoW12O40]6- and a Cr complex [Cr3O(OOCCH2CN)6(H2O)3]+ are implemented as PIC building blocks for the OER under nonbasic conditions. The potentially OER-active but highly soluble [α-CoW12O40]6- was successfully anchored in the crystalline PIC matrix via Coulomb interactions and hydrogen bonding induced by polar cyano groups of the Cr complex. The PIC exhibits efficient and sustained OER catalytic activity, while each building block is inactive. The Tafel slope of the linear sweep voltammetry curve and the relatively large kinetic isotope effect value suggest that elementary steps closely related to the OER rate involve single-electron and proton transfer reactions. Electrochemical and spectroscopic studies clearly show that the synergistic catalysis originates from the charge transfer from the Cr complex to [α-CoW12O40]6-; the increased electron density of [α-CoW12O40]6- may increase its basicity and accelerate proton abstraction as well as enhance electron transfer to stabilize the reaction intermediates adsorbed on [α-CoW12O40]6-.

Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals.

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear CrIII carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb3O13] or [Ta3O13] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

Isomeric effects on the acidity of Al13 Keggin clusters in porous ionic crystals.

We demonstrate a facile synthesis method of a porous ionic crystal (PIC) composed of the little-known δ-Keggin-type cationic polyoxoaluminum cluster ([δ-Al13O4(OH)24(H2O)12]7+, δ-Al13) with an oppositely-charged polyoxometalate, which enabled us to investigate the activity as a solid acid. The δ-Al13 based PIC exhibited much higher activity in pinacol rearrangement, a typical acid-catalyzed reaction, than the PIC based on the well-known and thermodynamically stable rotational isomer (ε-Al13). This work is a rare example of rotational isomers of polyoxoaluminum clusters exhibiting remarkably different catalytic activities.

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.

Isostructural mesoporous ionic crystals as a tunable platform for acid catalysis.

Eleven isostructural mesoporous ionic crystals (meso-PICs) are synthesized. The initial activities of the Barbier-Grignard reaction, which is a typical C-C bond formation reaction, catalyzed by the meso-PICs are dependent on the acid dissociation constant of the aqua ions of Mn+ and the types of polyoxometalates, which construct the meso-PICs.

Stabilisation of high-valent Cu3+ in a Keggin-type polyoxometalate.

An unprecedented Keggin-type Cu3+-containing polyoxometalate Cs3H[CuIIIPW11O39]·11H2O (1-Cs) was successfully synthesised through a newly-developed Ag+-promoted chemical oxidation by S2O82- in aqueous medium and systematic characterisation with powder XRD, XPS, IR, UV-vis and 31P NMR spectroscopies proves the effective stabilisation of a high-valent Cu3+ center by a mono-lacunary Keggin type polyoxometalate ligand.

Construction of g-C3N4-mNb2O5 Composites with Enhanced Visible Light Photocatalytic Activity.

A series of composites consisting of g-C3N4 sheet and mesoporous Nb2O5 (mNb2O5) microsphere were fabricated by in situ hydrolysis deposition of NbCl5 onto g-C3N4 sheet followed by solvothermal treatment. The samples were characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). The photocatalytic activity of the composites was studied by degradation of rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) in aqueous solution under visible light irradiation (λ > 420 nm). Compared with g-C3N4 and mNb2O5, g-C3N4-mNb2O5 composites have higher photocatalytic activity due to synergistic effect between g-C3N4 and mNb2O5. Among these composites, 4% g-C3N4-mNb2O5 has the highest efficiency and good recyclability for degradation of both RhB and TC-HCl.

Two mixed-addenda Nb/W polyoxometalate-based hybrid compounds containing multicopper units: synthesis, structures, and electrochemical and magnetic properties.

Two mixed-addenda Nb/W polyoxometalate (POM)-based hybrid compounds, [CuICu(µ3-OH)(H2O)6(trz)3]2(PW9Nb3O40)·13H2O (1) and [CuICu(µ3-OH)(H2O)4(Htrz)(trz)3]2(PW9Nb3O40)·13H2O (2), were synthesized under hydrothermal conditions. Single-crystal X-ray diffraction revealed that compound 1 contains planar triangular tricopper {Cu(µ3-OH)(trz)3} units, and these tricopper units link to each other to form a 1D anti-parallel chain. CuI ions connect the 1D chains by the coordination of trz ligands to form 2D layers in the ab plane. Compound 2 contains the same tricopper {Cu(µ3-OH)(trz)3} units as well as bicopper {Cu(Htrz)2} units. The tricopper units also form 1D chains and the bicopper units connect the 1D chains to form similar 2D layers and finally construct the 3D framework. In both compounds, each Nb/W mixed-addenda Keggin {PW9Nb3O40} unit links eight tricopper units to generate the final 3D structure. Magnetism studies indicate anti-ferromagnetic exchange interactions among the CuII ions within the tricopper units, with JCu-Cu values of -209.3 and -200.8 cm-1 in 1 and 2, respectively.