# Achim Müller

Achim Müller (14 February 1938 – 28 February 2024) was a German chemist. He was Professor Emeritus at the Faculty of Chemistry, University of Bielefeld.
His research involved mainly the chemistry of transition metals, especially with relation to nanochemistry.
His later research related mainly to the synthesis of spherical porous metal oxide nanocapsules Mo$_{132}$ Keplerates. Müller also worked on simple transition metal sulphur compounds, including related hydrodesulfurization catalysis and a new type of host guest chemistry based on polyoxovanadates He had also strong interest in history and philosophy of science.

## Academic career

Achim Müller studied chemistry and physics at the University of Göttingen and received there his PhD degree (1965) and the Habilitation (1967). In 1971, he became professor at the University of Dortmund and in 1977 professor of Inorganic Chemistry at the University of Bielefeld. In 2006 he was awarded the Manchot-Forschungsprofessur of the Technische Universität München.
Müller was a member of the German Academy of Sciences Leopoldina, Polish Academy of Sciences, The Indian National Science Academy, National Academy of Exact Physical and Natural Sciences in Argentina, and Academia Europaea. He has received honorary degrees from the Russian Academy of Sciences (RAS) and the Université Pierre et Marie Curie, Paris as well as the “Profesor Honorario“ of the National University of La Plata. He has also received the Alfred Stock Memorial Prize 2000, Prix Gay-Lussac/Humboldt 2001, Sir Geoffrey Wilkinson Prize 2001, and the Centenary Medal of the Royal Society of Chemistry 2008/9, London. In 2012 he was awarded the prestigious Advanced Grant by the European Research Council (ERC). He was an Honorary Fellow of the Chemical Research Society of India.

## Research

His later research related mainly to bottom-up pathways towards tailor-made spherical porous metal oxide nanocapsules Mo$_{132}$ Keplerates. Müller's discovery of the molecular giant spheres (Keplerates) of the type Mo$_{132}$ (diameter ca. 3 nm) and their derivatives, of the wheel shaped cluster Mo$_{154}$ (Refs. and) and hedgehog shaped cluster Mo$_{368}$ (as large as 6 nm) has caused a paradigm shift due to the unique structural features and huge application range of these molecular nanoclusters.
The capsule has 20 well-defined pores with crown-ether functions which may be reversibly opened and closed. Substrates enter through these pores, and they can react to form a variety of species depending on the internal tapestry of the nanocontainer. This inorganic nanocell also allows ion transport through the cavity. Several new phenomena under confined conditions can be studied by variation of the internal ligands with hydrophilic and/or hydrophobic character, based on a variety of reactive substrates inside (for the interdisciplinary character of the whole compound class see Ref.). These single molecules are quite large; this can be shown by taking the length of an oxygen molecule with two atoms (length 0.12 nm) as a unit, then considering Mo$_{368}$ which is 50 times larger.
All the mentioned nanomaterials belong to a class commonly termed polyoxometalates and some special ones to the molybdenum blue family; the elucidation by Müller of the chemical nature of molybdenum blue was a real tour de force.
There are two older synthetic topics where Achim Müller did pioneering work. This refers to basic simple transition metal sulphur compounds, including related hydrodesulfurization catalysis and a new type of host guest chemistry based on polyoxovanadates (for both topics see especially the three Honorary Issues under Bibliography). One paper about polyoxometalates was cited ca. 2000 times. The publications of Achim Müller were highlighted many times in the media and related magazines. Important last sentences: “Giant polyoxometalates (POMs) are of particular interest, as they are the largest inorganic molecules ever made, combined with fascinating structures and manifold applications in catalysis, medicine and material sciences. Typically, Müller‘s group has achieved great success in creating giant polyoxomolybdates (POMos) during the past two decades, establishing a series of incredibly large POMos with hundreds of Mo centers, such as {Mo$_{132}$}, {Mo$_{154}$}, {Mo$_{176}$}, {Mo$_{248}$} and {Mo$_{368}$}. Especially {Mo$_{368}$}, with more than 360 Mo atoms, still is the largest POM to date...“

### Applications and impact

These are soluble in water and are very probably one of the most versatile materials regarding applications in nanoscience. It is evident that such applications are of interest not only for chemistry but also for biology, supramolecular chemistry and materials science as well as discrete mathematics. The applications include the following topics:
* Investigating processes, including catalytic ones, under confined conditions, especially in capsules with stepwise tunable pores and tunable internal functions
* Coordination chemistry at surfaces, in pores and in cavities of nanocapsules: sphere-surface supramolecular chemistry, controlled exchange of guests at different internal sites with each other and with the outside, nano ion-chromatography, nucleation processes under confined conditions
* Modelling ion transport across transmembrane channels
* New insights into hydrophobic interactions, e.g. thermodynamics of hydrophobic clustering, sequestration of hydrophobic organic pollutants, and nano-dewetting
* Structure and dynamics of encapsulated high and low density water
* Towards multifunctionality and hierarchical complexity: examples for chemical adaptability and for a supramolecular chemical Darwinism, self-assembly of capsules into various patterns in different phases (e.g. blackberry-like hollow spherical assemblies as well as two- and three-dimensional structures)
Müller's related work shows many applications (see above), for example, how cellular processes like ion-transport can be modelled based on the spherical porous capsules and how the latter can be used to remove toxic compounds from water.
The compounds are studied worldwide by many groups especially in relation to potential applications in Materials Science (see Ref. and below). One aspect is modelling of the Lotus effect, another one is chemical adaptability as a new phenomenon.
An interesting mathematical treatment of the Keplerates could be developed in relation to spherical viruses and Buckminster Fuller Domes based on Archimedean and Platonic solids. The unique range of potential applications of the Mo$_{132}$ Keplerates has also been highlighted by several other authors, e.g.: "Thus, Keplerate-type capsules represent unique supramolecular objects offering a tunable spatially-restricted environment and promising in many domains such as catalysis, electric conductivity, non-linear optics, liquid crystals, vesicles and "blackberry" aggregates. ...a key point to promote confined space engineering." In another publication it is written: "Initiated and led by Müller and co-workers, the synthesis and structural characterization of protein-sized metal oxide clusters (2-6 nm) have generated great interest in the areas of physics, biology, chemistry, and materials science"; see also regarding a similar comment Ref. Some of the clusters can be obtained by planned synthesis, while the related derivative (also because of the structure of a Keplerate) Mo$_{72}$Fe$_{30}$ has unique magnetic properties.

## Personal life and death

Müller enjoyed ancient Greek philosophy, classical music, and mountain hiking. He had a passion for woodland birds from his early childhood, a pastime which his father cherished also.
Achim Müller died on 28 February 2024, at the age of 86.