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.

Nanoblackberries of {W72Fe33} and {Mo72Fe30}: Electrocatalytic Water Reduction

The reversible self-assembly of a {Mo₇₂Fe₃₀} cluster into nanoblackberries in a dilute solution of the relevant crystalline compound [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O ({Mo₇₂Fe₃₀}_cryst) was demonstrated by Liu, Müller, and their co-workers as a landmark discovery in the area of polyoxometalate chemistry. We have described, in the present work, how these ∼2.5 nm nano-objects, {M₇₂Fe₃₀} (M = W, Mo) can be self-assembled into nanoblackberries irreversibly, leading to their solid-state isolation as the nanomaterials Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O ({W₇₂Fe₃₃}_NM) and Na₂[Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₄(OH)₁₆(H₂O)₁₀₈]·180H₂O ({Mo₇₂Fe₃₀}_NM), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM have been established by spectral analysis including Raman spectroscopy, elemental analysis including ICP metal analysis, volumetric analysis (for iron), microscopy techniques, and DLS studies. The thermal stability of the tungsten nanoblackberries {W₇₂Fe₃₃}_NM is much higher than that of its molybdenum analogue {Mo₇₂Fe₃₀}_NM. This might due to the extra three ferric (Fe³⁺) ions per {W₇₂Fe₃₀} cluster in {W₇₂Fe₃₃}_NM, which are not part of the {W₇₂Fe₃₀} cluster cage but are placed between two adjacent clusters (i.e., each cluster has six surrounding 0.5Fe³⁺) to form this self-assembly. The isolated blackberries behave like an inorganic acid, a water suspension of which shows pH values of 3.9 for {W₇₂Fe₃₃}_NM and 3.7 for {Mo₇₂Fe₃₀}_NM because of the deprotonation of the hydroxyl groups in them. We have demonstrated, for the first time, a meaningful application of these inexpensive and easily synthesized nanoblackberries by showing that they can act as electrocatalysts for the hydrogen evolution reaction (HER) by reducing water. We have performed detailed kinetic studies for the electrocatalytic water reduction catalyzed by {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM in a comparative study. The relevant turnover frequencies (TOFs) of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (∼0.72 and ∼0.45 s^-1, respectively), the overpotential values of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (527 and 767 mV, respectively at 1 mA cm^-2), and the relative stability issues of the catalysts indicate that {W₇₂Fe₃₃}_NM is reasonably superior to {Mo₇₂Fe₃₀}_NM. We have described a rationale of why {W₇₂Fe₃₃}_NM performs better than {Mo₇₂Fe₃₀}_NM in terms of catalytic activity and stability.

Visible-light driven catalase-like activity of blackberry-shaped {Mo72Fe30} nanovesicles: combined kinetic and mechanistic studies

Catalase-like activity of blackberry-shaped {Mo₇₂Fe₃₀} nanovesicles was exploited in aqueous solution under visible-light irradiation.

Electronic Structure Studies on the Whole Keplerate Family: Predicting New Members

A comprehensive study of the electronic structure of nanoscale molecular oxide capsules of the type [{M^VI (M^VI )₅ O₂₁ }₁₂ {M'^V₂ O₂ (μ-X)(μ-Y)(L^n- )}₃₀ ]^(12+n)- is presented, where M,M'=Mo,W, and the bridging ligands X,Y=O,S, carried out by means of density functional theory. Discussion of the electronic structure of these derivatives is focused on the thermodynamic stability of each of the structures, the one having the highest HOMO-LUMO gap being M=W, M'=Mo, X=Y=S. For the most well-known structure M=M'=Mo, X=Y=O, [Mo₁₃₂ O₃₇₂ ]^12- , the chemical bonding of several ligands to the {Mo^V₂ O₂ (μ-O)₂ } linker moiety produces negligible effects on its stability, which is evidence of a strong ionic component in these bonds. The existence of a hitherto unknown species, namely W₁₃₂ with both bridging alternatives, is discussed and put into context.

Anions coordinating anions: analysis of the interaction between anionic Keplerate nanocapsules and their anionic ligands

Keplerates are a family of anionic metal oxide spherical capsules containing up to 132 metal atoms and some hundreds of oxygen atoms. These capsules holding a high negative charge of -12 coordinate both mono-anionic and di-anionic ligands thus increasing their charge up to -42, even up to -72, which is compensated by the corresponding counter-cations in the X-ray structures. We present an analysis of the relative importance of several energy terms of the coordinate bond between the capsule and ligands like carbonate, sulphate, sulphite, phosphinate, selenate, and a variety of carboxylates, of which the overriding component is contributed by solvation/de-solvation effects.