Capsules with Highly Active Pores and Interiors: Versatile Platforms at the Nanoscale

Abstract

Spherical porous capsules offer new exciting approaches in chemistry, materials sciences, and in context of physical and biological phenomena. The underlying concepts are reported with particular emphasis on metal oxide based capsules of the {M132 } Keplerate type which display-due to their exceptional structural features and easy variation/derivatization as well as exchange of building units-an unmatched range of properties and offer unique opportunities for investigating a variety of basic aspects of nanoscience, including the discovery of some new phenomena, especially those related to hydrophobicity issues that are of significance for everyday life. This relies in particular on the existence of a large number of flexible crown ether type pores/channels and the possibility of changing the interior from completely hydrophilic to completely hydrophobic due to the presence of numerous easily exchangeable internal ligands/functionalities; the capsules can even be constructed so that they enclose a large number of highly active Lewis and Brønsted acid sites. The manifold of possible applications/uses are outlined as subtitles with reference to results as well as possible future studies. There are, among many others, options to control passing cations under different internal frames allowing also their separations, to conduct studies about hydrophobic recognitions and clustering of biological interest in water, controlled internal ion transport, nanoscale dewetting, and to carry out basic as well as new types of reactions under confined conditions.

Tunable Keplerate Type‐Cluster “Mo132” Cavity with Dicarboxylate Anions

“…Such specific interactions have been observed also in solution between the NMe 4 + ion and the sulfurated Keplerate [Mo 132 O 312 S 60 (H 2 O) 72 (CH 3 COO) 30 ] 42− 31. Such solution studies and related results correspond to complementary insights previously evidenced in the solid state and described as the pore plugging process 32…”

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

“… Structural representation of cation–POM frameworks described in Sections 2.2 and 2.3. Left: ionic zeotype frameworks reported by Mizuno, Uchida, and colleagues; center: Keggin‐net reported by Cronin and colleagues; right: guanidinium‐blocked {Mo 132 } Keplerate capsule reported by Müller and colleagues …”

“…In addition to linking POMs into larger, possibly porous frameworks, POMs themselves can act as anionic, porous architectures. Seminal studies by Müller and others have examined the molybdenum‐based Keplerate “spheres” (e.g. [[Mo 72 VI Mo 60 V O 372 (CH 3 COO) 30 (H 2 O) 72 ] 42− = {Mo 132 } ) with respect to their supramolecular reactivity.…”

“…carboxylate) and neutral (e.g. water) species . Of particular interest is the selective blocking of the C 3 ‐symmetric pore windows, for example, by guanidinium.…”

“…These cations combine electrostatic and hydrogen‐bonding interactions with matching C 3 ‐symmetry and complementary size to fit the metal–oxide pores in {Mo 132 } (Figure ). Based on this guest exchange behavior of {Mo 132 } , the authors describe them as artificial inorganic cells to explore biologically relevant ion‐gating and transport …”