The research of my group focuses primarily on the relationship between electronic structure and chemical or physical properties of transition-metal coordination compounds. As the main research objective, we address the mechanistic details how multiple electrons or holes transfer effectively to, within, and from a metal complex in the context of a chemical reaction. One goal is to identify 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 main group radical species that merge with state-of-the-art protocols for catalytic 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 mimicking structure-property relationships of systems found in nature. Starting from a structurally and electronically flexible dithiophenolate ligand scaffold, we have studied the mechanistic aspects of two-electron storage and reactivity of binuclear structures of nickel for hydrogen evolution and oxidation. Studies of radical-ligand complexes of group 10 metals establish a complementary research topic that focusses on the properties of metal complex near-infrared chromophores in addition to 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 applied in studying electronic structure-property relationships. Expertise of collaborators on computational studies of electronic structure-property relationships of open-shell coordination compounds has complemented and expanded the scope of our experimental efforts.
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.
We currently pursue research on three primary topics. First, we investigate the phenomenon of cooperative reactivity between late transition-metal atoms in binuclear structures, with special emphasis on multiple electron reactivity. In this context, we carry out mechanistic work on energy efficient evolution and oxidation of dihydrogen. Second, we explore the electronic structure-property relationship of radical-ligand metal complexes. Because of their unique electronic structures, complexes featuring open-shell ligands possess intriguing chemical and physical properties. Examples include tuneable and electrochemically switchable absorptivity in the NIR and IR spectral region and H-atom abstraction reactivity that is of interest for catalytic bond formation protocols. 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, at present, 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 properties of novel 1,4-terphenyldithiophenols that we have developed as a ligand platform. More precisely, the alignment of two thiophenols across a 1,4-disubstituted p-system creates a structurally and electronically unique donor environment that suits to support a single as well as multiple metal sites in various oxidation states. In addition, combining thiolates and π-systems enables the ligand framework to store and shuttle electrons and protons. The following three examples briefly highlight the strategy that we pursue to devise defined electronic structures for detailed studies of reactivity and physical properties.
The motivation for the work presented here 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 cooperative behaviour of an ensemble of redox active metal atoms is relevant to the search for strategies toward the efficient interconversion of electrical and chemical bond energies.
Novel bimetallic complexes of nickel of a dithiophenol scaffold mediate both formation and oxidation of dihydrogen, just depending on the oxidation state of the bimetallic metal-sulphur core. Essential properties of the ligand include its capability to couple the metals electronically in different formal redox-states and shuttle protons to and from the reactive 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 metal-sulphur cores. In this context, we have reported on quantitative mechanistic studies of the redox-induced skeletal (dis)assembly of a [Ni2(µ-thiolate)2] core from, or into, isomeric congeners in which the nickel ions are geometrically independent. Pertinent results are summarized schematically below. The system provides detailed insights into the properties of natural CuA 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 four complex cations of a group 10 metal drawn below features an open-shell thiolate ligand, which is why these compounds exhibit intense low-energy electronic transitions. The radical-ligand cations of platinum feature divergent chromophore properties although chemical compositions are very similar or even identical. Inter- and intraligand steric interactions result in subtle differences between molecular structures, rendering the three [(S-)Pt(●S)]+ chromophores electronically distinct. In addition, these radical-ligand fragments show promising reactivity toward polarizable element-hydrogen bonds that is of interest with regard to synthetic applications.