The Berkefeld research group focuses primarily on studies of the correlation of the electronic structures and chemical properties of coordination compounds. The main objective is to address the mechanistic details how multiple electrons or holes transfer to, within, and from a metal complex in the context of a chemical reaction. One goal is to identify functional structures that mediate the interconversion of electrical and chemical bond energies in an efficient manner. Another goal is to devise atom economic redox-chemical methodologies for the generation of reactive synthons that merge with state-of-the-art protocols for bond formation and functionalization. At present, we make use of the distinct physical and chemical properties of metal-sulphur bonds of late transition metal thiolate complexes for the named purposes. We aim for devising metal-thiolate structures based on a bottom-up approach rather than adopting structure-property relationships known from natural systems. Starting from a structurally and electronically flexible dithiophenolate scaffold, we have studied the mechanistic aspects of two-electron storage and reactivity using binuclear structures of nickel. Studies of radical-ligand complexes of the group 10 metals address a complementary topic that focusses on the properties of metal complex near-infrared chromophores and redox mechanisms for bond activation. Single-crystal X-ray diffraction, variable temperature magnetic spin resonance, vibrational and electronic absorption spectroscopy, and electrochemistry are routine experimental techniques used in studying electronic structure-property relations. Computational data from collaborators increasingly contribute to complement experimental work.
A.B. is indepted to the Stipendien-Fonds der Chemischen Industrie and the Eliteprogram for Postdocs of the Baden-Württemberg Stiftung for the financial support of research projects.
Studying the correlation of electronic structure and chemical reactivity or physical properties requires devising model compounds that show functional behaviour. The following properties are of general interest. First, the cooperative reactivity of binuclear structures of redox active metals is investigated in the context of multiple-electron reactions. Second, the electronic structure-property relationship of radical-ligand metal complexes is explored. Third, we devise strategies for the controlled switching of structural and electronic properties of mono- and bimetallic coordination compounds by electro- and photochemical stimuli. Notably, all areas of research rely on the use of the same chemical entity: the transition metal-sulphur bond. This is possible because of the distinct coordinative and electronic properties of a family of 1,4-terphenyldithiophenols that we have developed as a ligand platform. More precisely, the alignment of two thiophenols at a 1,4-disubstituted π-system creates a structurally and electronically flexible donor environment that suits to support a single and multiple metal sites in different oxidation states. The following three examples briefly highlight the strategy that we pursue to devise defined electronic structures for detailed studies of reactivity and physical properties.
Example 1. The motivation for the work presented in this example is to put the phenomenon of synergistic reactivity of two redox active metal sites on a mechanistic basis. With regard to the importance of structures in Nature that comprise multiple metals, understanding the mechanistic details of redox-cooperativity of an ensemble of metal sites is relevant to the search for strategies toward the efficient interconversion of electrical and chemical bond energies. In recent work, we showed that bimetallic complexes of nickel of named dithiophenol scaffold mediate both the formation and oxidation of H2, just depending on the oxidation state of the [2Ni-2S] core. Essential properties of the ligand include its capability to couple the metals electronically in different formal redox-states and to shuttle protons to and from the bimetallic core.
Example 2. Sulphur-bridged di- and multinuclear structures play a pivotal role in a variety of processes that are essential to life. We aim at understanding the effect of the electronic properties of the sulphur linkage on the properties of the [2M-2S] structure. In this context, we reported a quantitative mechanistic study of the redox-induced assembly and disassembly of [2Ni-2S] units from structurally and electronically independent nickel-thiolate sites. The system provides detailed insights into the properties of the bimetallic sites in Nature such as the CuA site and 2Fe-ferredoxins, and contributes to advancing the general understanding of one of Nature’s functionally multifaceted structures.
Example 3. Metal complexes of radical-ligands feature exciting physical and chemical properties that relate to their intriguing electronic structures. Each of the three complex cations of platinum drawn below features an open-shell thiolate ligand, which is why these compounds exhibit intense low-energy electronic transitions. The spectroscopic properties of these radical-ligand complex cations are different although their chemical compositions are very similar or even identical. The reason is that inter- and intraligand interactions result in subtle differences between molecular structures, rendering the [(areneS-)Pt(●Sarene)]+ chromophores electronically distinct. In addition, these radical-ligand fragments show promising reactivity toward polarizable element-hydrogen bonds that may be employed for synthetic applications.