Establishing catalytic formate oxidation and CO2 reduction with models for Mo and W dependent formate dehydrogenases

• Applicant: Professorin Dr. Carola Schulzke
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548994584

The reactivity of metal-dependent formate dehydrogenases (FDHs) are of particular interest since their oxidation of formate to CO2 is generally reversible; i.e. they can be used to transform CO2 into formate. Employing this reaction to CO2 collected from the atmosphere or even directly form processes leading to its emission would be beneficial with regard to measures addressing global warming. The product of CO2 reduction, formate or formic acid, is in addition a chemical of industrial importance and serves various roles (e.g. as preservative, in tanning processes, as neutralizer of limescale etc.). The metal-dependent FDHs belong to the family of molybdenum- and tungsten dependent oxidoreductases. The mechanism of metal dependent FDHs was, until recently, widely believed to be a simple hydride abstraction from formate. We have shown in a collaborative effort, that, in fact, also the FDHs of this enzyme class carry out the typical oxygen atom transfer (OAT) with the formation of bicarbonate as immediate product which is then likely protonated and automatically dehydrates to CO2. Since FDH-model chemistry was previously based on the wrong presumption of hydride abstraction, OAT modelling of formate oxidation has not really been considered and is, hence, un(der)explored. This proposal aims to remedy this fact by using known mono-oxido bis-dithiolene complexes of molybdenum and tungsten as catalysts for the oxidation of formate with co-substrates such as DMSO and amine oxides in organic solvents. The reverse reaction (reduction of CO2) will be similarly explored employing oxygen atom acceptors such as phosphines. Since we have complexes, which are stable and soluble in water, biological conditions as previously used for the enzyme study will be employed as well (water as source or sink of the oxygen atom, NAD+ as oxidant for formate and NADH as reductant for CO2/bicarbonate, buffer solution). Further efforts will be dedicated to not only study readily available, simple model complexes, but also those which would bear the inherent ability to buffer electrons and protons similar to the NAD+/NADH pair, by e.g. employing quinone moieties. Also two approaches towards model ligands more closely related to the natural ligand system molybdopterin shall be explored. All complexes with these more sophisticated ligand systems will be investigated in the same catalytic tests as the simpler complexes are being taken through (organic/non-biological conditions as well as aqueous/biological conditions). In the last part of the project the most successful complexes will be focused on in order to optimise the catalytic procedures in particular for CO2 reduction. The proposed work will provide further support for the OAT mechanism of the FDHs, and thereby contribute to the much needed detailed understanding of this mechanism, and it will pave the way to potent and industrially feasible CO2/bicarbonate reduction catalysts.

• DFG Programme: Research Grants