Topics

Bio-inspired Carbon Dioxide Activation on Heterobimetallic Complexes

The efficient activation and direct conversion of CO2 into CO, acetic acid, or methanol is a current challenge and need of modern society. If CO2 could be selectively converted into these molecules, the current feedstock of modern chemistry could entirely be changed from oil to CO2. The use of the greenhouse gas CO2, as opposed to oil, would result in a decrease of this pollutant in our atmosphere. In addition, combustion of the generated fuels would lead to an environmentally neutral balance of the CO2 level in air and is therefore a highly desirable venture.
Recent catalysts solely provide the possibility to transform CO2 to CO by a stepwise reduction. The reduction intermediate, CO2 radical anion, leads to numerous unfavorable side products and is accessible from CO2 with high kinetic activation barriers. Only few catalysts are known that bypass this reactive intermediate. This radical, however, has to be avoided to afford a catalyst with high turn over numbers (TON) and high selectivity in product formation.
The herein proposed project aims at the generation of novel bio-inspired bimetallic complexes that can activate CO2 and allow selective reduction to CO or methanol. The focus of the proposed work will be on the development of heterobimetallic low-valent Fe,Ni- and Ni,Mo-complexes with a sulfur-rich ligand environment. Inspired by the structure and activity of the enzyme CO-dehydrogenase, the proposed complexes should enable a 2-electron electrochemical reduction of CO2 to afford CO, thereby lowering the overpotential and increasing the rate constants for this important process. Additionally, spectroelectrochemical investigations will provide insight and allow tuning of electronic and structural properties of the complexes.

F. Möller, S. Piontek, R. G. Miller, U.-P. Apfel, Chem. Eur. J., 2018, 24, 1471–1493

L.Iffland, A. Khedkar, A. Petuker, M.Lieb, F. Wittkamp, M. van Gastel, M. Roemelt, U.-P. Apfel, Organometallics, 2019, 38(2) , 289–299

F. Möller, L. Castañeda-Losada, J.R.C. Junqueira, R.G. Miller, M. L. Reback, B. Mallick, M. van Gastel, U. P. Apfel, Dalton Trans., 2017, 46, 5680-5688



SOLID STATE ELECTRODES

The need for sustainable catalysts for an efficient hydrogen evolution reaction as well as for CO2 reduction is of significant interest for modern society. Inspired by structural properties of natural enzymes, we design low-cost and robust non-precious metal sulfides as a direct ‘rock’ electrode material for the hydrogen evolution reaction and CO2 reduction. The ‘rock’ materials show high catalytic activity and low overpotential and at the same time offer high current densities and long time stability.

B. Konkena, K. j. Puring, I. Sinev, S. Piontek, O. Khavryuchenko, J. P. Dürholt, R. Schmid, H. Tüysüz, M. Muhler, W. Schuhmann, U.-P. Apfel, Nature Commun., 2016, 7, 12269

S. Piontek, C. Andronescu, A. Zaichenko, B. Konkena, K. junge Puring, B. Marler, H. Antoni, I. Sinev, M. Muhler, D. Mollenhauer, B. Roldan Cuenya, W. Schuhmann, U.-P. Apfel, ACS Catalysis, 2018, 8, 987–996

S. Piontek, K. junge Puring, D. Siegmund, M. Smialkowski, I. Sinev, D. Tetzlaff, B. Roldan Cuenya, U.-P. Apfel, Chem. Sci., 2019, 10, 1075-1081



Hydrogen Generation Utilizing Hydrogenase Mimics

The limitations of fossil fuel sources and the polluting environmental impact of those materials promote society to come up with alternative energy forms. Even though renewable energy sources are already accessible in the form of wind-, sun-, geothermal- and water-energy, an effective storage and transportation system is required for the successful application. Hydrogen is a possible candidate for such a storage and transportation system and allow for environmentally friendly combustion with a high specific heating value. An inexpensive and robust catalyst is therefore required that allows for transformation of proton sources (e.g. water, acetic acid) into hydrogen with negligible energy deficiency.
In contrast to current industrial processes, nature uses a powerful enzymatic system to reversibly transform protons into hydrogen. This class of enzymes is called hydrogenase. Based on the catalytic sites of [FeFe]- and [NiFe]-hydrogenases we intend to develop catalytic active heterobimetallics utilizing low-valent Ni and Fe sites.

F. Wittkamp, M. Senger, S. T. Stripp, U.-P. Apfel, Chem. Commun., 2018, 54, 5934-5942

L. Kertess, F. Wittkamp, C. Sommer, J. Esselborn, O. Rüdiger, E. J. Reijerse, E. Hofmann, W. Lubitz, M. Winkler, T. Happe, U.-P. Apfel, Dalton Trans., 2017, 46, 16947-16958



Multifunctional Catalysis by Ni- and Fe-(Cyclam/Isocyclam) Complexes

Unraveling the mechanistic basis and operational principles of activation of small molecules like dioxygen (O2), carbon dioxide (CO2) or protons (H+) represents a formidable challenge to the chemical sciences. Addressing these challenges is essential for the design and development of efficient catalysts. Because nature mostly uses metal ions to activate these relatively inert molecules and modulate their reactivity, much inspiration for the field has come from bioinorganic chemistry. Amongst the different utilized tetradentate and pentadentate ligand systems, tetraazamacrocyclic cyclams have proved to be versatile ligands in the biomimetic chemistry of O2, CO2, or H+ activations by metal complexes. Many of these complexes show intriguing reactivities, which in turn have substantially supported the conceived enzymatic reaction mechanism. There are nevertheless still significant gaps in our present understanding of the mechanism of small molecule activation in biology. Most often, the reactions exhibited by model complexes are found to be non-catalytic, with higher energy requirements and with activities falling far short of those of the biological catalysts. In our continuous effort to uncover structure-reactivity relationships of biomimetic model complexes, we now propose the directed alteration of the tetraazamacrocyclic framework by consecutive substitution of the nitrogen-donor atoms by oxygen, sulfur, and selenium atoms and investigate the capability of the metal-complexes (Ni and Fe) of the corresponding ligand to perform both the oxygen reduction reaction as well as the CO2 and proton reductions in a proof-of-principle study. While oxygen substitution will be interesting in the context of understanding Nature’s preference for using oxygen based donors in the dioxygen reduction, sulfur and selenium substituents will provide detailed insights into biological CO2/proton reduction processes. This study may allow vital insights into the prerequisites necessary for the design of efficient catalysts for the selective functionalization of unactivated C–H bonds, O2 activation, or CO2/H+ by using cheap and readily available first-row transition metals under ambient conditions.

P. Gerschel, K. Warm, E. R. Farquhar, U. Englert, M. L. Reback, D. Siegmund, K. Ray, U.-P. Apfel, Dalton Trans., 2019, advance Article, DOI: 10.1039/C8DT04740E




NiFe-containing Carbon monoxide dehydrogenase (CODHNi) isolated from M. Thermoacetica [H. Dobbek et al., J. Am. Chem. Soc. 2004, 126, 5382]


CuMo-containing Carbon monoxide dehydrogenase (CODHMo) isolated from O. Carboxidovorans [H. Dobbek et al., PNAS 2002, 99, 15971.]

H-Cluster of the [FeFe] hydrogenase as isolated from D. Desulfuricans [Y. Nicolet et al., Structure 1999, 7, 13–23.]

H-Cluster of the [FeNi] hydrogenase as isolated from Desulfovibrio gigas [Juan C. Fontecilla-Camps et al., Nature 19995, 373, 580-587]