ABSTRACT:The effectiveness of the interactions between various alkyl ammonium cations and the well-defined spherical Keplerate-type {Mo 132 } capsule has been tracked by 1 H DOSY NMR methodology which reveals a strong dependence of the selfdiffusion coefficient of the cationic guests balancing from the solvated to the plugging situations. Analysis of the data is fully consistent with a two-site exchange regime involving the 20 independent {Mo 9 O 9 } receptors of the capsule. Furthermore, quantitative analysis allowed determining the stability constants associated to the plugging process of the pores. Surprisingly, the affinity of the capsule for a series of cationic guests increases continuously with its apolar character as shown by the significant change of the stability constant from 370 to 6500 from NH 4 + and NEt 4 + , respectively. Such observations, supported by the thermodynamic parameters evidence that the major factor dictating selectivity in the trapping process is mainly the so-called "hydrophobic effect". Computational studies, using molecular dynamics simulations have been carried out in the conjunction of the experimental data. The analysis of the radial distribution functions g(r) reveals that NH 4 + and NMe 4 + ions behave differently in the vicinity of the capsule. The NH 4 + ions do not exhibit well-defined distributions in its close vicinity. In contrast, the NMe 4 + ions were identified as sharp distributions related to different scenario such as firmly trapped or as labile guest facing the {Mo 9 O 9 } pores. These conjugated experimental and theoretical insights should aid the exploitation of these giant polyoxometalates in solution for various applications.

The formation pathway of a closed spherical cluster [Mo132], starting from a library of building blocks of molybdate anions, has been reported. Electrospray ionization mass spectrometry, Raman spectroscopy, and theoretical studies describe the formation of such a complex cluster from a reduced and acidified aqueous solution of molybdate. Understanding the emergence of such an enormous spherical model cluster may lead to the design of new clusters in the future. Formation of such a highly symmetric cluster is principally controlled by charge balance and the emergence of more symmetric structures at the expense of less symmetric ones.

The present paper presents a comprehensive study of the electronic structure of nano-scale molecular oxide capsules of the type [{M O 2 (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 hitherto unknown species, namely W 132 with both bridging alternatives is discussed and put into context with our findings.

Keplerates are a family of anionic metal oxide spherical capsules containing up to 132 metal atoms and some hundreds of oxygens. These capsules holding a high negative charge of -12 coordinate both mono-anionic and di-anionic ligands thus raising its charge till -42 even till -72, which is compensated by the corresponding counter-cations in 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 the contribution due to solvation/de-solvation effects.

Water in different physical states has been attracting great interest for decades for its strong association with the origin of life, the biophysical behaviors of biomacromolecules, and the transportation of matter and energy. Structurally well-defined, monodispersed nanocapsules provide excellent opportunities to understand water under confinement. Herein a group of porous and hollow polyoxometalates with varied inner ligands have been studied to investigate the distributions of water inside them in solution states via contrast variation small angle neutron scattering (CV-SANS) techniques. Meanwhile, molecular dynamics simulations have been conducted to cross-examine fitting results of SANS data and provide structural details of the formation of such water distribution patterns. Our work suggests that the densities of water in the core of nanocapsules rely highly on the dimensions of hydrophobic confinement space (cores inside nanocapsules) and can approach the dewetting state at the critical dimension, ca. 1.1 nm.

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