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SFB 813:  Chemistry at Spin Centres - Concepts, Mechanisms, Functions

Subject Area Chemistry
Biology
Term from 2009 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 60803019
 
Final Report Year 2018

Final Report Abstract

The Collaborative Research Center 813 (CRC 813) was devoted to the Chemistry at Spin Centers. Spin centers were understood as molecules, ions, (bio)polymers and aggregates thereof, all of which featuring a finite number of unpaired electrons. The spin centers that were studied included Systems with Unpaired Electrons in their electronic ground state and also those, which can be prepared in one of their electronically excited states such as through resonant optical excitation. In comparison to the ubiquitous systems exhibiting exclusively paired electrons, the chemical reactivity, the physico-chemical properties, and the spectroscopy of spin centers is of substantially higher complexity. The goal of the CRC was to prepare such spin centers in the laboratory, to control their rapid, often unselective chemical reaction patterns, to elucidate their molecular and electronic structures by means of state-of-the-art spectroscopy combined with quantum-chemical calculations, and – if possible – to exploit the attained knowledge in developing novel materials with fascinating electrical and/or magnetic properties. In the area of Organometallic Chemistry and Catalysis, the CRC 813 was able to develop new concepts for atom-economical and hence, sustainable catalytic transformations based on oxidative additions and reductive eliminations in single-electron transfer steps with spin density translocations between transition metal centers and main group elements. In this spirit, a bifunctional titanocene(III) catalysts was designed that facilitates an elegant epoxide ring opening while at the same time serving as an hydrogen-atom transfer reagent. This unique dual functionality presents also an important step forward in controlling catalyst enantioselectivity in the context of radical chemistry. A spectacular new entry into the rich Chemistry of Silicon was discovered through the CRC by the successful synthesis of a low-valent disilicon species that was stabilized by N-heterocyclic carbenes. This species allowed for the isolation of a transition metal complex featuring a metal-silicon triple bond as well as for the preparation of a stable silanone exhibiting a silicon-oxygen double bond. These important discoveries are of tremendous relevance for the chemical industry, e.g. for the advancement of catalysts or the development of new silicon-based polymers with unique property combinations. A true breakthrough has been achieved in the field of Radiation Chemistry through the CRC’s research activities devoted to understanding the optical spectroscopy and the chemical reactivity of the “Mother of all Spin Centers”, the Solvated Electron. Specifically, by implementing tunable-multiphoton-ionization-probe spectroscopy the CRC was able to monitor the fate of the negative charge carriers following their initial optical preparation in Sir Humphry Davy’s discovery system, the solvated electron in liquid ammonia. In general, such experiments provide important benchmarks for electronic structure theory of condensed matter systems with strong dynamical disorder. Transient, highly reactive transition metal spin centers were also generated by photochemical means. In particular, azidoiron(III) precursors were used to prepare through an ultrafast homolytic N-N bond cleavage and dinitrogen elimination very fascinating nitridoiron(V) species. Importantly, these High-Valent Iron complexes feature a four-fold symmetrical coordination sphere, which makes them inherently unstable and non-isolable. Nonetheless, by exploiting the power of ultrafast time-resolved vibrational spectroscopy, the CRC was able to explore in detail the molecular and electronic structure of such species and it succeeded in disclosing, for the first time and in situ, the chemical reactivity of “superoxidized” iron directly in liquid solution. This research has tremendous impact for an understanding of the role of high-valent iron in nature where it contributes to enzymatic oxidation processes. Finally, in the area of Materials Sciences the CRC 813 designed novel solid state systems that are based on purely inorganic π-systems, which in turn are built from tellurium and bismuth-containing main-group salts. These compounds show electrical conductivity characteristics of either a semiconductor, or a one-dimensional metallic conductor, or even a superconductor depending upon the composition and the temperature.

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