Two Compartmentalized Inner Receptors for the Tetramethylammonium Guest within a Keplerate-Type Capsule

Collective Effects of Multiple Fluorine Atoms Causing π-philic Characteristic within a Caged Polyoxometalate Framework

Perfluorination brings about distinctive properties arising from the unusual nature of the F element, which have been extensively developed in materials science and chemistry. Herein we report that the construction of F-rich inner space within a hollowed Mo132 O372 cage ([Mo132 O372 (OCOR)30 (H2 O)72 ]42- ) leads to the emergence of unique guest binding activities in encapsulation. Prominently, the trifluoroacetate-modified cage (R=CF3 , 2) having as many as 90 F groups inside favors trapping cyclopentadiene (Cp), which is hardly trapped by the non-fluorinated counterpart (R=CH3 , 1). Systematic studies using related hydrocarbons show that the amount of the encapsulated guest is correlated with the unsaturation degree of the guests, implying the involvement of the attractive interaction of the CF3 -modified interior wall with the guest π-electron clouds. Control experiments using the semi-fluorinated analogues (R=CF2 H, CFH2 ) reveal that the perfluorination is a critical factor to facilitate the Cp encapsulation by 2, indicating that collective effects of polar C-F bonds spreading over the interior surface, rather than the polarity of the individual C-F bonds, are responsible. We also provide a successful example of the physical molecular confinement within the cage through the "ship-in-a-bottle" Diels-Alder reaction between trapped diene and dienophile.

Enantioselective Recognition of Racemic Amino Alcohols in Aqueous Solution by Chiral Metal-Oxide Keplerate {Mo132 } Cluster Capsules

Determining the relative configuration or enantiomeric excess of a substance may be achieved using NMR spectroscopy by employing chiral shift reagents (CSRs). Such reagents interact noncovalently with the chiral solute, resulting in each chiral form experiencing different magnetic anisotropy; this is then reflected in their NMR spectra. The Keplerate polyoxometalate (POM) is a molybdenum-based, water-soluble, discrete inorganic structure with a pore-accessible inner cavity, decorated by differentiable ligands. Through ligand exchange from the self-assembled nanostructure, a set of chiral Keplerate host molecules has been synthesised. By exploiting the interactions of analyte molecules at the surface pores, the relative configuration of chiral amino alcohol guests (phenylalaninol and 2-amino-1-phenylethanol) in aqueous solvent was establish and their enantiomeric excess was determined by 1 H NMR using shifts of ΔΔδ=0.06 ppm. The use of POMs as chiral shift reagents represents an application of a class that is yet to be well established and opens avenues into aqueous host-guest chemistry with self-assembled recognition agents.

Embedding alkenes within an icosahedral inorganic fullerene {(NH4)42[Mo132O372(L)30(H2O)72]} for trapping volatile organics

Eight alkene-functionalized molybdenum-based spherical Keplerate-type (inorganic fullerene) structures have been obtained via both direct and multistep synthetic approaches. Driven by the opportunity to design unique host-guest interactions within hydrophobic, π-electron rich confined environments, we have synthesised {(NH₄)₄₂[Mo₁₃₂O₃₇₂(L)₃₀(H₂O)₇₂]}, where L = (1) acrylic acid, (2) crotonic acid, (3) methacrylic acid, (4) tiglic acid, (5) 3-butenoic acid, (6) 4-pentenoic acid, (7) 5-hexenoic acid, and (8) sorbic acid. The compounds, which are obtained in good yield (10-40%), contain 30 carboxylate-coordinated alkene ligands which create a central cavity with hydrophobic character. Extensive Nuclear Magnetic Resonance (NMR) spectroscopy studies contribute significantly to the complete characterisation of the structures obtained, including both 1D and 2D measurements. In addition, single-crystal X-ray crystallography and subsequently-generated electron density maps are employed to highlight the distribution in ligand tail positions. These alkene-containing structures are shown to effectively encapsulate small alkyl thiols (1-propanethiol (A), 2-propanethiol (B), 1-butanethiol (C), 2-butanethiol (D) and 2-methyl-1-propanethiol (E)) as guests within the central cavity in aqueous solution. The hydrophobically driven clustering of up to 6 equivalents of volatile thiol guests within the central cavity of the Keplerate-type structure results in effective thermal protection, preventing evaporation at elevated temperatures (ΔT ≈ 25 K).

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.

Investigating the Formation of Giant {Pd72}Prop and {Pd84}Gly Macrocycles Using NMR, HPLC, and Mass Spectrometry

The formation of giant polyoxometalate (POM) species is relatively underexplored, as their self-assembly process is complex due to the rapid kinetics. Polyoxopalladates (POPds) are a class of POMs based on Pd, the largest of which is the {Pd₈₄}^Ac wheel, and its slower kinetics mean the system is more amenable to systematic study. Here, we show that it is possible to follow the assembly of two types of Pd wheels, {Pd₈₄}^Gly and the smaller {Pd₇₂}^Prop, formed using glycolate and propionate ligands, respectively. We analyzed the formation of {Pd₇₂}^Prop and {Pd₈₄}^Gly using mass spectrometry (SEC-HPLC-MS and preparative desalting followed by MS). This was accompanied by studies that followed the chemical shift differences between the outer/inner ligands and the free ligand in solution for the {Pd₈₄}^Ac, {Pd₇₂}^Prop, and {Pd₈₄}^Gly species using NMR, which showed it was possible to track the formation of the wheels. Our findings confirm that the macrocycles assemble from smaller building blocks that react together to form the larger species over a period of days. These findings open the way for further structural derivatives and exploration of their host-guest chemistry.

High-field 95 Mo and 183 W static and MAS NMR study of polyoxometalates

The potential of high-field NMR to measure solid-state ⁹⁵ Mo and ¹⁸³ W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low-gamma NMR nuclei, due mainly to the low spectral resolution and poor signal-to-noise ratio, the observed global trends compare well with the solution-state NMR data. This would open an avenue for application of solid-state NMR to POMs, especially when liquid-state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.