Inorganic Seminar - Yunho Lee

Transition metal-based activation of small molecules such as CO, CO2 and NOx 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 NOx 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 NOx 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 NOx.  

In this presentation, two stories of low-valent square planar 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 COx, COOR, NHR2, NOx and N2. First, 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. Second, through our recent efforts, a novel NOx conversion catalysis based on the Ni/Co pincer systems 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. A Co catalyst facilitating NO generation via NO2– deoxygenation with CO, concomitant 1-electron oxidation and activates benzyl halides, generating a radical species that undergoes C–N coupling with NO. Comparing a nickel analogue, the open-shell reactivity of the Co-system significantly enhances C–N coupling efficiency, achieving a turnover number of 5000 and turnover frequency of 850 h⁻¹ for oxime production. The oxime intermediate can then be converted into valuable N, O-containing bioactive heterocycles, advancing NCU technology.