Not updated. See list of publications (Chronological) and other topics of Research.
The information below highlights one of our key continuing research projects (see Selected Papers), and is presently our main one. For authors' names of the given papers, see list of publications. Some earlier reviews on basic chemistry: In the below listed original papers, it was argued that only the molybdate system allows the described versatile chemistry.

From Porous Capsules/Artificial Cells to Sphere Surface and Super-Supramolecular Chemistry: Perspectives for Chemistry under Confined Conditions, Artificial Cell Environment Interactions as well as Nanotechnology

Mo132 pore
One of the 20 Mo9O9 pores with crown ether function
{Mo132}

Structurally well-defined porous spherical metal-oxide based nanocapsules/artificial cells of the type {Pentagon}12{Linker}30 allow unprecedented chemistry (Figure). The area shows revolutionary routes to different disciplines, like chemistry (e.g. modelling passive ion transport through membranes), materials science (e.g. construction of a nano ion chromatograph), physics (especially regarding confined matter properties), and mathematics (concerning the tiling problem of spherical surfaces).

This is based on the following related facts:

  1. the size of the capsules and their pores/channels can be tuned, while the latter can be opened and shut,

  2. the internal cavity's shell functionality can be tuned, e.g. from hydrophilic to hydrophobic, and

  3. the twenty(!) abundant pores of the nanosponge have crown-ether functions with respect to cations allowing sphere-surface as well as super-supramolecular chemistry including in principle the study of allosteric effects.

Options for the following basic/new type of research topics are evident:

  1. Encapsulation Chemistry: This refers to studies of encapsulated nanomaterials of different types - including water with and without electrolytes - taken up through capsule pores. The capsules themselves can separate/position cations like a nano ion chromatograph in the sense of structure directed vectorial-type translocations.

  2. Capsule-Coordination Chemistry: This allows a new type of spectroscopic and magnetic studies of encapsulated discrete/shielded mono- and polynuclear complexes, while encapsulated water shells can act as polydentate ligands.

  3. Artificial Cell Environment Interactions: In this respect some of Nature's pathways can be modelled, like ion uptake-and-release processes (Figure) as well as cell response to stimuli, since pore closing by molecules like corks - formally considered as "stimuli" - influences significantly encapsulates' (like water) structures; this refers to confined condition phase transfer. Generally speaking, model cell environment interactions can be studied on a wide range.

Highlighted, e.g. in:


Nanoscaled Objects: Characteristic Internal and External Surface Functionalities Show Routes for a New Type of Chemistry


ring keggin
{Mo12} Ì {Mo72Fe30}

Nanoscaled species with well-defined surface-structured landscapes have properties unknown for small molecules and can give rise (if they can be dissolved) to a versatile chemistry depending on their special type of surface functionalities as well as on their molecular shapes. In the present case this refers to our two important consequential(!) forms, the spherical hollow- and wheel-type kind.

Options include:

Highlighted, e.g. in:


Dynamical Library Building Units Number-Increase Can Lead to Ever Larger Nanoscaled Assemblies: Symmetry Breaking at a Giant Cluster Surface



Mo368

{Mo368}

The nanocosmos as such, does not show the variety-limiting translational symmetry restriction of macroscopic crystalline materials but offers - in contrast to the "microcosmos" with its small molecules - the possibility that (several) larger arrays with local symmetries differing from the overall symmetry occur, a situation well known for spherical viruses and which increases the option for the generation of an extreme structural variety tremendously. Thus the appropriate building units must display a certain type of flexibility as prerequisite for linking, a condition well-fulfilled by molybdenum-oxide based fragments/aggregates under reducing conditions (i.e., those occurring in molybdenum blue solutions). These fragments abundant as potential ingredients of a "dynamic library", can, by a type of "split-and-link process", adapt their size and shape dependent on slight alterations of the relevant boundary conditions which can lead to the formation of a variety of unusual giant molybdenum-oxide based clusters. A unique example corresponds to the formation of a cluster with the size of hemoglobin (diameter approximately 6 nm), which contains 1880 non-hydrogen (368 metal) atoms. It is formed by the linking of 64 {Mo1}-, 32{Mo2}-, and 40{Mo(Mo5)}-type units (exhibiting different stoichiometries) according to a remarkable symmetry-breaking process, which is nicely recognizable at the cluster surface (see Figure; Mo blue, O red, S yellow).

Highlighted, e.g. in:


Confined Water and the Mystery of the Liquid


H2O100

{H2O}100

The properties and structure of "The Liquid Water" including its behaviour as a function of temperature and/or pressure as well as in the presence of (poly)electrolytes are yet not well understood. An important investigation, particularly for an understanding of several aspects of the role of water in biological processes, would be to study how moderately large assemblies of water molecules - say from 10 to 1000 - respond to their confinement in nanometer-sized cavities. (Note: In biological cells the water between biomolecules consists only of a rather small number of layers of molecules.) In the context of our work on "Encapsulation Chemistry" it is possible to influence encapsulated water-assembly structures on a wide range by the size, charge, internal surface, and encapsulated electrolytes as well as the capsule-hosts. The obtained information also provides general knowledge about the intrinsic properties of water molecules in general as well as about their potentiality to aggregate stepwise under confined conditions. The largest water cluster obtained consists of 100 H2O molecules (Figure).

Highlighted, e.g. in:


Unprecedented Vesicle Formed From Wheel-Shaped-Type Clusters Exhibiting Hydrophilic Surfaces: A New Solute State of Inorganic Ions


Fig.5

Special kinds of molecules abundant in solution can aggregate to different kinds of superstructured hierarchies: surfactants and membrane lipids can assemble into complex structures such as micelles, liposomes, or hollow vesicles due to their amphiphilic character. The wheel-shaped clusters, isolated from molybdenum blue solutions, with their hydrophilic surfaces and related high solubility, like the "classical" Mo154, form in aqueous solution an unprecedented type of (hollow) superstructure/vesicle, where ca. 1200 molecular wheels form a sphere (80 nm diameter) with an encapsulated nanodrop of water (Figure). According to the rather large hydrophilic cluster surface, the interface water gets structured (proven by dielectric spectroscopy), which probably contributes to the vesicle formation, and may be described also by a new inorganic ion solute state. The high solubility prevented the isolation of crystals from Mo blue solutions for more than 200 years, since the first experiments were done by Scheele and Berzelius

Highlighted, e.g. in:


Icosahedral Nanocapsules' Interpenetrating Platonic and Archimedean Solids: Chemistry and Aesthetics


Fig.6

As the capsules of the type {Pentagon}12{Linker}30 have the highest possible (icosahedral) symmetry for molecules, a large number of sets of equivalent integrated as well as encapsulated atoms/species (like water molecules) are automatically generated, which span several more or less distorted Platonic and Archimedean solids. The architectural variety of such interpenetrating solids even fascinates mathematicians. The complex cluster system [{(NH2)3C+}20 + ({H2O}20 Ì {H2O}20 Ì {H2O}60) Ì {(Mo)Mo5O21(H2O)6}12 {Mo2O4(SO4/H2PO2) 2}30]32- shows, e.g. the following solids: an icosahedron (blue), dodecahedron (red), icosidodecahedron (violet), truncated icosahedron (brown), rhombicosidodecahedron (green), and rhombitruncated icosidodecahedron (yellow) spanned by 12 Mo, 20 C, 30 S/P, 30 Mo2, 60 N, and 120 H, respectively (Figure). Surprisingly, the ratio of edges of the 30 rectangles of the {H2O}60 shell is very near to the value of the 'golden section' (i.e., ca. 1.6). In this sense the Platonic philosophy of beauty, truth and good can be referred to - if one also takes into account the importance of water for our, and all organisms' daily life in the context of the statements of Thales and Paracelsus.

Highlighted, e.g. in:


Templates Direct Like Conductors Fragment Linking


Mo132 Halide as template
Azide as template


During the generation of giant polyoxometalate clusters template-directing processes are often involved. A nice text-book example refers to the controlled linking of O=VO4 fragments. The templates are either added to the reaction mixture or are formed in it as "seedlings"; they regulate the mobility of the basic building blocks in solution and guide, like a conductor, the usually freely mobile fragments into a well-defined order. A comparison with the term "slaving principle" in the terminology of Hermann Haken's Synergetics is certainly worthwhile. In the present case, the finally formed system is of the host/guest type (figured) where the host is complementary to the guest/template which is encapsulated.

Unusual Molecular Anion Cages: "The Taming of the Shrew"
(special aspect; see paper below with that title)
The properties of ions in the solid state as well as in solution can in general only be evaluated in the presence of appropriate "perturbing" counterions being necessarily present due to the governing principle of electroneutrality. Therefore, it is very difficult to derive the properties of the free ions from spectroscopic data. However, this dilemma can be avoided by encapsulating anionic species as guests within a rigid host that is also negatively charged, for example a hollow isopolyvanadate, as mentioned above. This leads to relatively weak interactions, and hence, to rather large distances between the anions and the cluster shell. The resulting pseudomechanical fixation of the guests by the host offers the opportunity to have an almost isolated anion in a cryptand as object of scientific curiosity. The "corresponding counterions" in this case are located far away, outside the host-guest system, that is in the cationic lattice and have, therefore, no noticeable effect on the encapsulated anions.

Highlighted, e.g. in: