Step-by-step by Bioinspiration: Mechanistic Understanding of Acetylene Hydratase and Perchlorate Reductase with Tungsten and Molybdenum Model Complexes and Photoactivated Mo(VI)-and W(VI)-Schiff base complexes for biomimetic oxygen atom

"Step-by-step by Bioinspiration: Mechanistic Understanding of Acetylene Hydratase and Perchlorate Reductase with Tungsten and Molybdenum Model Complexes"

Molybdenum and tungsten are the only 2nd and 3rd row transition metals known to be essential in biology. Both enable the transfer of an oxygen atom in various reactions. Tungsten-dependent Acetylene hydratase (AH) catalyzes the net-hydration reaction of acetylene to acetaldehyde. Two diverging mechanisms are discussed in literature: it is unclear which of the two substrates, water or acetylene, is coordinated at tungsten within the active site during the catalytic cycle. Molybdenum-dependent (per)chlorate reductase (PcrAB), a member of the DMSO reductase family, catalyzes the first steps of microbial perchlorate respiration, namely the conversion of perchlorate (ClO4–) to chlorate (ClO3–) and further to chlorite (ClO2–).[1] Anthropogenic activities lead to contamination of the ground water with perchlorate and chlorate. A better understanding of the mechanism of the PcrAB-catalyzed reduction of (per)chlorate is therefore also important for the remediation of drinking water. We develop bioinspired model chemistry which leads us one step further towards mechanistic understanding of the enzymes AH and PcrAB. The tungsten and molybdenum complexes which are supported by sulfur-rich ligands such as pyridine-2-thiolates and pyrimidine-2-thiolates allow the coordination of acetylene and their subsequent nucleophilic attack[1] as well catalytic reduction of ClO4 ̄ to Cl ̄.[2]

"Photoactivated Mo(VI)-and W(VI)-Schiff base complexes for biomimetic oxygen atom"

Molybdenum-based complexes containing oxo-ligands are established functional models of  oxotransferase enzymes, and most reported catalysts are able to achieve high substrate conversions at elevated temperatures. To further mimic the single-electron transfer (SET) mechanisms prevalent in biological systems, recently reported Mo- 1 and W-based 2 catalysts can be excited using UV-visible light, yielding highly active oxygen atom transfer (OAT) agents. During previous investigations in our group, 3-5 it was observed that functional modifications to the ancillary tridentate salicylidene-aminophenol (SAP) ligands can promote photoactivation of cis-MoO 2 and cis-WO 2 based complexes towards phosphine oxidation reactions. In this talk, I will provide an overview of our recent advancements in catalyst development and mechanistic investigations into the photocatalytic OAT cycle. Using established methods, catalysts 1-12 were synthesised and fully characterised, and their photocatalytic activity for benchmark PPh 3 oxidation was tested and monitored by 1 H NMR . The two most active catalysts, 2 and 5, showed respective 78% and 97% PPh 3 conversion after 3 h irradiation (λ = 410 nm). The catalytic cycle of complex 2 was further investigated using TR-FTIR spectroscopic measurements on the second timescale. Distinctive shifts of the Mo=O bands in the 900-1000 cm -1 fingerprint region upon irradiation allowed the detection of a postulated monomeric oxo-Mo(IV) intermediate. Further insights into the kinetic behaviour, solvent coordination effects and the reoxidation of the Mo(IV) intermediate were also obtained. Variable temperature 1 H NMR experiments provided further evidence for monomeric oxo-Mo(IV) species. Computational calculations revealed that upon photoexcitation the C=N bond order is diminished due to the HOMO-LUMO transition. Correspondingly, the N-Mo bond strengthens, whilst the Mo-oxo bonds weaken. Current work is focused on examining the excited states of the catalysts and further catalytic steps using computational simulations, and expanding the catalyst reaction scope towards more complex substrates and products with pharmaceutical or industrial interest.