Join us to hear from Yunho Lee of Seoul National University!

Abstract: 

Transition metal-based activation of small molecules such as CO, CO₂ and NO_x are drawing much attention due to their importance in understanding the active site chemistry of metalloenzymes and biological denitrification processes. This is important because both global carbon and nitrogen cycles have been significantly influenced by anthropogenic perturbation. In order to contribute to re-balance these cycles, new technology to convert CO₂ and NO_x species to safer and useful chemicals is timely needed. Biological CO₂/CO interconversion found in the nickel containing metalloenzyme carbon monoxide dehydrogenase (CODH) and several organometallic reactions involving C–C and C–S coupling are recognized from acetyl CoA synthase (ACS) activity. Furthermore, biological denitrification involving NO_x conversion occurs via a multi-step process to reduce nitrate to dinitrogen (NO₃^– → N₂). Although the enzymatic reactions are attractive because they selectively and efficiently operate under mild conditions, it is fairly challenging to apply into the industrial processes. This is because they are too complicate in structural characteristic of their active sites and sequential enzymatic reactions operated in different active sites. By using our nickel model systems, we have developed a synthetic approach to unravel these issues for producing value-added products from CO₂ and NO_x. 

In this presentation, two stories of low-valent nickel complexes will be delivered. Since the reactivity toward small molecule is controlled in part by the geometry of a L₃Ni scaffold, we have employed a series of pincer systems to work with a four coordinate (PEP)Ni-L complexes (E = N, P or Si), where the L site is occupied by various ligands such as CO_x, COOR, NHR₂, NO_x and N₂. Through our recent efforts, we have established organonickel chemistry for CO₂ conversion to CO. In particular, a selective CO₂ reaction of a nickel carbonyl species will be discussed, which was successfully established by employing a structurally rigidified (^acriPNP)Ni scaffold. Furthermore, a novel NO_x conversion catalysis based on a nickel pincer system will be presented. The catalysis starts with converting Ni–NO₃ to Ni–NO via deoxygenation with CO(g), which is followed by transfer of the in-situ generated nitroso group to organic substrates. The transfer favorably occurs at the flattened Ni(I)–NO site via its nucleophilic reaction. Successful catalytic production of oximes from benzyl halides by using both nitrate and nitrite salts was established under mild conditions. Our nickel catalyst effectively fulfils a dual-purpose, namely deoxygenating NO_x anions and catalyzing C–N coupling.