The Uptake and Assembly of Alkanes within a Porous Nanocapsule in Water: New Information about Hydrophobic Confinement

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Hofmeister effect in the Keggin-type polyoxotungstate series

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion.

Guest Transition Metals in Host Inorganic Nanocapsules: Single Sites, Discrete Electron Transfer, and Atomic Scale Structure

Host-guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate-type molybdenum oxide-based porous, spherical clusters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here, we demonstrate the synthetically controlled encapsulation of first-row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate anions, {Mo132PO4}. The resulting M2+x⊂{Mo132PO4} host-guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encapsulation of up to 10 equiv showed little magnetic interaction between the encapsulated metals, which indicated that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal were possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light-induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future, such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photoinitiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.

Ligand-Regulated Uptake of Dipolar-Aromatic Guests by Hydrophobically Assembled Suprasphere Hosts

The selective uptake of guests by capsules, cages, and containers, and porous solid-state materials such as zeolites and metal-organic frameworks (MOFs), is generally controlled by pore size and by the dimensions and chemical properties of interior host domains. For soluble and solid-state structures, however, few options are available for modifying their outer pores to impart chemoselectivity to the uptake of similarly sized guests. We now show that by using alkane-coated gold cores as structural building units (SBUs) for the hydrophobic self-assembly of water-soluble suprasphere hosts, ligand exchange can be used to tailor the chemical properties at the pores that provide access to their interiors. For polar polyethylene glycol functionalized ligands, occupancies after equal times increase linearly with the dipole moments of chloro-, nitro- dichloro-, and dinitro- (o-, m-, and p-) benzene guests. Selectivity is reversed, however, upon incorporation of hydrophobic ligands. The findings demonstrate how self-assembled gold-core SBUs, with replaceable ligands, inherently provide for rationally introducing finely tuned and quantitatively predictable chemoselectivity to host-guest chemistry in water.

Sequential Isomerization of a Macrocyclic Polyoxometalate Archetype

Controlled isomerization of individual {α-P₂W₁₂O₄₈} polyoxotungstate building blocks under the constricted conditions of the macrocyclic [P₈W₄₈O₁₈₄]^40- archetype ({P₈W₄₈}) is linked to site-specific Cu^II coordination. The derivatives [αγαγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₂]^36- (1), [γγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)_0.5}₄]^32- (2), and [αγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₃]^34- (3) feature the {αγαγ-P₈W₄₈} and the hitherto unknown {γγγγ-P₈W₄₈} and {αγγγ-P₈W₄₈} isomers based on {α-P₂W₁₂} and/or Cu^II-stabilized {γ-P₂W₁₂} units and form from the reactions of the classical {P₈W₄₈} (={αααα-P₈W₄₈}) and CuCl₂ in sodium acetate medium (pH 5.2). All products were thoroughly characterized in both the solid state and aqueous solutions, including electrocatalysis assessments.

Evaluation of the chemo- and shape-selective association of a bowl-type dodecavanadate cage with an electron-rich group

The host-guest interaction between a half spherical-type dodecavanadate (V12) and a neutral molecule guest was evaluated by monitoring the flip of a VO₅ unit caused by the presence or absence of a guest in the cavity of V12. In N,N-dimethylformamide (DMF), V12 adopted the guest-free form (V12-free). By the addition of several guest molecules, such as acetonitrile, nitromethane, and dichloromethane, the structural conversion to the guest-inserted form (V12(guest)) was observed with the affinity constants of 137 ± 10 M^-1, 0.14 ± 0.1 M^-1, and 0.15 ± 0.1 M^-1, respectively. In the case of 1,2-dichloroethane, 1,2-dibromoethane, and 1,2-diiodoethane, the constants were 35 ± 5 M^-1, 114 ± 5 M^-1, and 2.1 ± 0.5 M^-1, respectively, suggesting that the bromo group is the best fit to the cavity of the bowl. A cyclic carbonate, 5- and 6-membered lactones, cyclobutanone, and hexanal were inserted into the V12 host, while a non-cyclic carbonate, non-cyclic and 7-membered cyclic ester, a ketone with a 5-membered ring, and benzaldehyde showed no effect on the guest insertion. The V12 host preferred to hold a guest with an electron-rich group, and the bowl-type structure showed the unique shape-selective interaction with the guest.

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.

Host-guest chemistry with water-soluble gold nanoparticle supraspheres

The uptake of molecular guests, a hallmark of the supramolecular chemistry of cages and containers, has yet to be documented for soluble assemblies of metal nanoparticles. Here we demonstrate that gold nanoparticle-based supraspheres serve as a host for the hydrophobic uptake, transport and subsequent release of over two million organic guests, exceeding by five orders of magnitude the capacities of individual supramolecular cages or containers and rivalling those of zeolites and metal-organic frameworks on a mass-per-volume basis. The supraspheres are prepared in water by adding hexanethiol to polyoxometalate-protected 4 nm gold nanoparticles. Each 200 nm assembly contains hydrophobic cavities between the estimated 27,400 gold building blocks that are connected to one another by nanometre-sized pores. This gives a percolated network that effectively absorbs large numbers of molecules from water, including 600,000, 2,100,000 and 2,600,000 molecules (35, 190 and 234 g l^-1) of para-dichorobenzene, bisphenol A and trinitrotoluene, respectively.