RESEARCH

LEE YUNHO GROUP RESEARCH PROGRAM

We have a responsibility to provide plausible resolutions to the global energy/environmental problem for future generations. Our goal is to investigate fundamental inorganic and bioinorganic chemical principles by learning from Nature and then adapt these concepts to future catalyst designs for energy applications. Our research focuses on understanding various biological catalytic reactions, particularly those based on the activation of energy-related small molecules (such as H2, COx, and NOx,) and then seek to accomplish similar catalytic activation using a biomimetic methodology. We incorporate a systematic approach based on the known active-site structures and design ligand architectures that accommodate the necessary steric and electronic environments to a metal site to achieve efficient and controllable synthetic catalysts. Brief descriptions of our research projects are provided below.

•  C1 Conversion at Mononuclear Metal Center 

Developing a new synthetic carbon fixation methodology is receiving great attention due to its relevance to the global energy and environmental issues. As a C1 source, CO2 and CO can be the promising candidate to run a sustainable carbon cycle. Many transition metal complexes have been utilized in CO2 and CO conversions because of their fundamental bond-forming character with such small molecules. The utilization of earth-abundant transition metals is recently drawing much attention due to its potential economic advantages. In Nature, an efficient catalytic transformation of both CO2 and CO occurs in an enzyme possessing a nickel containing active site. In carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS), CO is generated from the reduction of CO2 by CODH, and then delivered to the active site of ACS where acetyl-CoA is catalytically generated. In both active sites of enzymes important C-O bond activation and C-C bond formation occur at a 4-coordinate nickel center. Geometry and electronic structure of each nickel center are crucial to control the reactivity of corresponding nickel-carbon species. In order to explore the transformation of CO2 and CO at a 4-coordinate metal center, we work with a series of 4-coordinate metal-CO2 (and CO) complexes supported by a PEP pincer-type ligand; E = N, P, Si. 

•  Carbon Dioxide Activation at a Bimetallic Center

Much attention has been devoted to synthetic systems that use carbon dioxide as a potential C1 source, and also reduce CO2 to a useable carbon-based fuel, which are both promising solutions to help the global energy crisis. Biologically, carbon dioxide is reversibly converted to CO and H2O at the hetero-dinuclear site in the Ni,Fe-cluster of carbon monoxide dehydrogenase. Our research focus is to develop asymmetric bimetallic complexes utilizing anionic bridging amido/imido ligands as synthetic CO2 activation catalysts. These constructs should effectively mimic the distinct electronic environments contained in the CODH active site. Such binucleating ligands have previously not been reported with late metals, therefore, key fundamental explorations will be possible. 

•  Metal-to-ligand Multiple Bonds 

Various transition metal complexes involving metal-to-ligand multiple bonds can be utilized for various C-C, C-N and C-O coupling reactions via a group transfer and/or C-H bond activation. The species possessing metal-to-ligand multiple bond(s), such as carbyne, oxo, imido and nitrido are proposed as reactive intermediates. Due to the importance of atom economy in synthesis and environmentally benign chemical industry, such species have attracted much attention in recent years particularly for the group transfer reactions. However, the 1st-row late transition metal complexes possessing metal-to-ligand multiple bond(s) are relatively rare due to the instability of the multiple bonds between an electron-rich metal ion and a π-basic ligand. Stability of such metal complexes can be adjusted by altering/tuning a local geometry and electronics about a metal ion, where the reactive intermediate species can be dramatically stabilized at room temperature. Our research is currently targeting to stabilize and isolate such unstable species with the 1st-row late transition metals and ultimately utilize their reactive tendency for the catalytic group transfer chemistry.

•  Photophysical Property of a Copper Complex  

Recent development of luminescent molecular systems has been rapidly expanded with accompanying photophysical properties for the desired functions such as biological imaging, photochemical catalysis and electroluminescent devices. Especially along with various organic molecular emitters, phosphorescent transition metal complexes have been intensively studied for the OLED application. With the tendency of strong spin-orbit coupling, heavy transition metals are known to enhance the quantum efficiency by utilizing both singlet and triplet excitons. But this is somewhat limited to the few phosphorescent emitters employing expensive and rare noble metals such as iridium and ruthenium. Due to the rapid growing global market price of such rare metals, the emitters utilizing earth abundant metals are currently drawing attraction due to their economic advantage and stable supply. Unusual photophysical properties of a series of copper complexes supported by phosphine containing ligands are recently reported with tunable emission wavelength and reasonably quantum efficiency. Photophysical property of metal complexes can be adjusted by altering a local geometry about a metal center. In this project, we develop a new copper system where we can examine how the geometrical change of a metal ion affects on the emission properties.