Prof. Dr. Annie K. Powell

During my PhD research I had the chance to develop skills in the coordination chemistry of iron(III) complexes formed in aqueous media. The ready hydrolysis of the ferric ion in such media poses a number of synthetic challenges and it was necessary to develop methods allowing for control or suppression of the hydrolysis to enable pure solid compounds, often as single crystals, to be obtained. One goal of the project was to produce well-defined single ion complexes which might be photo-reducible in ways akin to ferric oxalate and citrate systems. The idea behind the work was to find candidates which could be used in such bimetallic non silver-based systems where the photo-reduced iron system provides electrons to noble metals such as platinum, silver and gold in order to form images of exquisite resolution and, in the case of gold, various hues depending on particle size. My PhD supervisor, Mike Ware, has since concentrated full-time on developing such alternative systems most of which still are based on the photoreduction of iron caboxylates.

A significant hindrance to the characterisation of the compounds at that time was the lack of access to single crystal X-ray diffraction facilities within the Faculty of Science at the University of Manchester. Consequently, the compounds were investigated using a range of the available spectroscopic techniques which provided a solid foundation in these physical methods and the opportunity to develop a thorough background in crystal and ligand field theory. Of particular importance to describing the electronic structures of the compounds were solid state studies using vis-nir spectroscopy and EPR spectroscopy at X- and Q- bands. In collaboration with David Collison, a simulation program was improved upon which made it possible to determine zero-field splitting parameters from the 2 frequency approach. The results were published not only in the thesis but also in the form of a paper in Inorganic Chemistry as well as constituting a chapter in the book published a few years later by Elsevier (1993) authored by Mabbs and Collison entitled “Electron Paramagnetic Resonance of d Transition Metal Compounds”.

Subsequent to my PhD research I had the opportunity to work as a postdoctoral associate in the group of Professor Heinrich Vahrenkamp at the University of Freiburg, Germany. Here I had the chance to learn how to handle the synthesis of carbonyl cluster compounds as well as the opportunity to learn single crystal X-ray structure analysis. Towards the end of my stay I also became involved in the new field of research for the group on the bioinorganic chemistry of Zn(II). I profited enormously from the opportunity to learn crystallography and solved a large (by the standards of the day) number of structures during this time.

I returned to the UK and took up a lectureship at the University of Kent at Canterbury where I stayed for one year, during which time I continued to develop my skills in X-ray structure determination and was assigned two final year project students. They were given projects aiming to produce iron(III) aggregates formed by trapping hydrolytic species within ligand shells. The basis for the idea arose from observations made during my PhD work and the hope was that the ability to apply single crystal structural analysis to these systems would give the handle needed for further characterisation including measuring the magnetic properties of the systems. At this time I was also able to raise sponsorship from ICI Paints Division to contribute to a CASE PhD award with an emphasis on surveying ligands able to suppress rust formation. This PhD award along with the former project student from Kent, Sarah Heath, was transferred when I moved to UEA at the start of the second year of my independent research career.

An early success at UEA was the identification of what was at the time the largest Fe(III) molecule discovered, with 19 iron centres, which Sarah Heath synthesised and crystallised during her PhD project and we published as a communication in Angewandte Chemie in 1992. In response to this publication, I was contacted by Dante Gatteschi of the University of Florence asking to be supplied a sample for magnetic measurements to be undertaken by his then PhD student, Roberta Sessoli. This resulted in establishing the largest spin ground state so far discovered for a molecular species and in further work also establishing that the system behaves as what is now termed a single molecule magnet (SMM). The discovery of this molecule led to developing synthetic routes to other such system where controlling the extent of hydrolysis/solvolysis using chelating ligands is used to obtain aggregated species containing many metal centres which we now term “Coordination Clusters” in recognition of the fact that they represent the next generation in coordination chemistry by going beyond Werner’s concepts developed for single ion complexes. The fact that the coordination cluster contains interacting metal centres means that its properties arise from cooperativity amongst these centres. Such an entity thus displays properties not possible for single metal ions and these may be manifested through high spin states, optical properties arising from the ensemble, catalytic properties relying on the presence of several metal ions and so on. Furthermore, metal ions from different parts of the periodic table can be incorporated and combined in essentially limitless ways with the only constraint being identifying the elements constituting the coordination cluster under study.

During my ten years at UEA not only was this chemistry developed further but also it was possible to use the good collaborations with Biologists within the framework of the BBSRC supported Centre for Metalloprotein Structure and Biology to investigate biological aspects. For example, the composition and structure of the Fe₁₉ compound turn out to provide an excellent miniature model for loaded ferritin. In addition, this molecule and other hydrolytically produced iron coordination clusters were tested as iron(III) sources for anaerobic iron reducing bacteria in work supported by the NERC. Work on the synthesis and magnetic behaviour of such systems was supported through a number of grants from the SERC/EPSRC and a significant boost to the X-ray diffraction facilities at UEA was made through an equipment grant from the Wellcome Trust which made it possible to purchase state-of-the-art equipment and ultimately to establish a platform for protein crystallography at UEA. The synthetic chemistry was developed further by investigating hydrothermal and solvothermal reaction conditions as a means for accessing further metastable systems and investigating the magnetic properties of these (supported by the EPSRC)

My move to Karlsruhe, Germany, in 1999 was prompted by the opportunities there in terms of equipment and infrastructure support. In the past 15 years the concept of the coordination cluster has been developed to the point where we can produce species containing mixed metal ions and also mixed oxidation states. In order to increase the versatility of these species we have discovered both how to produce clusters containing lanthanide ions – these can show fascinating magnetic properties – and also mixed 3d/4f clusters. Our group is probably at the forefront of research into the synthesis and properties of 3d/4f clusters in terms of variety and scope of the systems we have investigated so far. An important underlying theme for us to date has been the investigation of the magnetic properties which we achieve in house using routine SQUID measurements as well as in collaboration for more exotic measurements. This work has been supported over the years through an EU RTN, an EU centre of excellence, a DFG collaborative “Schwerpunkt Programm” and a DFG centre of excellence (the Center for Functional Nanostructures). In addition to my chair at the South Campus of KIT (formerly the University) I have a position and a group at the North Campus at the Institute for Nanotechnology where research is supported through the Helmholtz Society.

In the specific case of iron-containing coordination clusters we have developed Mössbauer methodology for investigating the effect of incorporation of highly anisotropic 4f ions into coordination clusters. Our technique uses the iron centres to report on the anisotropy as gauged from the Mössbauer spectra and we are also able to propose spin structures by deconvoluting spectra obtained in applied fields.

Further highlights include expanding the synthetic approach to allow us to place coordination clusters on surfaces as well as introducing chirality into the systems and exploring spintronic and relaxivity phenomena based on these molecules.