At a Glance: The Berkefeld Research Group
Our main research objective is the design of transition-metal based structures that convert electric and chemical bond energy in an efficient manner. More specifically, we address the mechanistic details how multiple electrons (or holes) can be stored in a molecular coordination compound and how charge storage couples to moving multiple electrons to or from a substrate such as a H+ or H2. We pursue this objective by a mechanism-inspired strategy that relies on the principal triad: platform design – electron-precise metal structures – understanding structure-function-relationships.
A.B. is indepted to the
Stipendien-Fonds der Chemischen Industrie
Eliteprogram for Postdocs of the Baden-Württemberg Stiftung
Deutsche Forschungsgemeinschaft (DFG)
for financial support of my research projects.
Current Research Projects
At present, we make use of the unique physical and chemical properties of metal-sulphur bonds of late transition metal thiolate complexes for the named purpose. Following our pincipal strategy, we designed a structurally and electronically flexible thiolate-arene-thiolate platform that stabilizes a series of electron-precise nickel-sulphur structures that perform hydrogen oxidation and formation, just depending on their redox-state. This unique [Ni2S2] system is ideal for studying the mechanistic aspects of two-electron storage and reactivity. The same platform allows us to explore the intruiging electronic and reactivity properties of radical-ligand structures of the group 10 metals that perform complementary reactivity: free radical chemistry.
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 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. How do 2M-2S structures assemble and how does this relate to function? We studied redox-induced skeletal (dis)assembly of a [Ni2(µ-thiolate)2] core quantitatively as shown in case A) below. And what factors contribute to modulating electronic coupling in 2M-2S structures? Charged coligands do! We probed their impact by (pulse)EPR spectroscopy and comprehensive DFT modelling as shown in case B) below. According to our model, 2M-2S sites perform like a molecular transistor: Charge flows from source to drain as long as intracore coupling does not prevail, which is achieved due to charge donation from peripheral donors (equal to applying a base voltage). Otherwise, charge flow through the 2M-2S site would be blocked. Our model platform provided detailed insights regarding 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.
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.