Rational Design of Azoles and Group 14 element as SCO-ligand

Bistable magnetic materials being switchable between two magnetic states are currently in the focus of research for possible applications in the next generation of miniaturised electronic devices based on magneto-optic technologies. One class of materials being appropriate for such applications is the class of spin-switchable (iron) compounds. They allow the switching of their spin state by external stimuli such as temperature, light, pressure, etc. This effect is called spin crossover being a feature of these molecules thus being the prerequisite for the design of a molecular switch. Due to the fact that still there does not exist a general valid structure-property relationship to rationally design a desired spin crossover behaviour (e.g. at which temperature and how abrupt such a system switches its spin state), in literature it was tried to derive building principles via comparative studies of rather similar compounds to achieve a rational design. However, it is rather difficult to separately investigate the different factors governing the spin transition behaviour (donicity of the ligands, stereochemistry of the ligands, weak-coordinating anions used, inclusion of non-stoichiometric amounts of solvent, intermolecular interactions, etc). Therefore, we have chosen a new approach by selecting 3 different nitrogen-containing five- membered ring systems (tetrazole, imidazole and pyrazole) with always the same set of substituents to keep the steric orientation and interaction of the ligands similar. In one task we start with well-known tetrazole-systems and their iron(II)-spin crossover-complexes synthesizing the analogue imidazol- and pyrazole-ligands and their iron(II)-complexes, whereas in a second task we start from well-known pyrazole-complexes preparing the analogue imidazole and tetrazole-complexes. In both cases we expect the deconvolution of steric factors and intermolecular interactions from the properties merely due to the donor strength of the coordinating azole-rings thus shedding light on the structure-property relationships of spin crossover-complexes. The most promising ligand systems will be substituted by group 14 elements (Si, Ge, Sn, Pb) instead of C next to the azole ring to upshift the spin transition temperature into the ambient range allowing for possible technological applications.

By modification of nitrogen-containing ligand systems iron(II) complexes could be synthesized, which form novel tetranuclear redox-active iron-oxygen cluster. These cluster could be interesting for future applications in the field of electron shuttle reactions and catalysis. This was achieved via stepwise replacement of well-known tetrazole rings - being investigated by the group of Associate Prof. Peter Weinberger for many years - by imidazole and pyridine rings keeping the overall molecular structure unchanged. In a first step octahedral iron(II) complexes of these new ligands are formed as expected. But totally unexpected and novel was the further reaction yielding tetranuclear Fe4O4 cubane structures. These new tetranuclear iron(II) compounds can be reversibly and stepwise oxidized to iron(III) and stepwise reduced back to iron(II) without destroying the molecular structre and integrity. Analogous Fe4S4 cubane structures are well-known from biological systems, but to the best of our knowledge these redox-active Fe4O4 cubanes prepared by us are absolutely new. They could serve as a new class of chiral catalytic compounds based on earth-abundant elements such as iron. Furthermore, we set a starting point for a new family of photoluminescent ligand molecules based on new carboxylic acid derivatives of tetrazoles developed within this project. This can be a future research perspective on bistable iron(II) complexes based on the extremly versatile tetrazole building blocks with magnetically interesting properties and photoluminescence dependent on these spin states. This was originally not planned within the project, but a new series of tetrazole carboxylic acids with one to three carbon atoms as spacer between the carboxylic group and the tetrazole moiety have been synthesized. These new ligands still show the desired spin crossover behaviour of its respective iron(II) complexes. In future these compounds may represent an interesting group of precursor molecules to be combined with the well-known fluorescent molecule BODIPY. Therefore, we could establish the basis for a new research field towards photoluminescent iron(II) spin crossover compounds based on the extremly versatile tetrazole ligand system. A new project application based on these findings is currently under preparation.