Utilizing metal ligand cooperativity to activate small molecules.

Author

Christine Burgan, University of LouisvilleFollow

Date on Master's Thesis/Doctoral Dissertation

5-2024

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Grapperhaus, Craig

Committee Co-Chair (if applicable)

Buchanan, Robert

Committee Member

Ramezanipour, Farshid

Committee Member

Zamborini, Francis

Committee Member

Spurgeon, Joshua

Author's Keywords

fossil fuels; alternate energy; CO2 chemistry; atmospheric chemistry; metal ligand cooperativity (MLC)

Abstract

The problems created from the excessive use of fossil fuels can be approached from two directions: capture the CO₂ from the atmosphere or develop alternative energy sources. The current industrial standards for these are environmental and/or health hazards and utilize precious metals, respectively. Metal ligand cooperativity (MLC) has emerged as a promising alternative to using precious metals to activate small molecules. In this dissertation, we focus on a series of metal complexes based on the ligand diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine) (H₂L¹) for either hydrogen evolution or CO₂ capture. The novel complexes were characterized by ¹H NMR, ¹³C NMR, UV-Visible spectroscopy, FT-IR, pK_a, and single crystal X-ray diffraction. The NiL¹ and PdL¹ are explored for the hydrogen evolution reaction across a range of pK_as. The overpotential, turnover frequency, Faradaic efficiency, and proposed mechanism of hydrogen evolution are discussed. After finding the ligand to be unstable under reducing conditions, the CO₂ chemistry was explored. The Co(III), Ni(II), Pd(II), Cu(II), and Zn(II) complexes of H₂L¹ were studied for CO₂ capture chemistry and only the Cu(II) and Zn(II) complexes showed CO₂ chemistry by UV-Visible spectroscopy and ¹H NMR. Interestingly, the Zn(II) complexes capture dilute CO₂ from the atmosphere and the pendent amine was systematically changed to understand how the pendent amine influences CO₂ chemistry. The CO₂ equilibrium constant (K₁) is quantified and used to extrapolate the metal Lewis acidity. The CO₂ chemistry is driven by the mismatch of the BrØnsted basicity of the non-coordinated hydrazino nitrogen and the metal center Lewis acidity. The pendent amine functions to modify the Lewis acidity of the metal center with a small impact on the BrØnsted basicity of the pendent nitrogen. The CO₂ equilibrium constant has an exponential dependency on the Lewis acidity where the more Lewis acidic the metal center, the more CO₂ chemistry. There is a counterintuitive linear dependency on the BrØnsted basicity where the less basic the nitrogen, the more CO₂ chemistry is observed. The most active complex is ZnL⁸(MeOH) with a pK_a,MeOH = 11.13 ± 0.08 and K₁ = 73,700 ± 2,000.

Recommended Citation

Burgan, Christine, "Utilizing metal ligand cooperativity to activate small molecules." (2024). Electronic Theses and Dissertations. Paper 4378.
https://doi.org/10.18297/etd/4378