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Title:Synthesis, characterization, and reactivity of high and low valent group 10 metal complexes supported by pyridine dithiolate ligands
Author(s):Tran, Giang Ngoc Huong
Director of Research:Mirica, Liviu M.
Doctoral Committee Chair(s):Mirica, Liviu M.
Doctoral Committee Member(s):Suslick, Kenneth S.; Fout, Allison R.; Olshansky, Lisa
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Pd(I) and Pd(II) complexes
mononuclear Pd(I)
C–C bond formation
C–H bond activation
organometallic Pd(III) complexes
hydrogen evolution reaction (HER)
Ni(I) complexes
organometallic Ni complexes
bioinspired catalysts
Abstract:Group 10 transition metals, especially palladium (Pd) and nickel (Ni), play an important role serving as catalysts for different transformations with a wide range of applications. However, beside the most commonly proposed M(0)/M(II) (M = Pd or Ni) catalytic cycles, there is still a limited understanding of the mechanisms that involve other less common key intermediates. Recent works have shown substantial evidence of the role of high valent M(III) and M(IV) species in catalytic organic transformations. Inspired by these new findings, our goal is to investigate the rarer oxidation states of these metal complexes to gain better insights into the different mechanisms. This study can potentially serve as a tool to design more efficient catalysts. Herein, we’ll discuss the +1 and +3 oxidation states of these metal centers, with a heavy focus on the low valent species. A series of dithiolate pyridinophane ligands were employed in these studies. With the presence of S atoms as the soft atom donor, we propose that these ligands can allow us to stabilize MI species based on the Hard Soft Acid Base theory. In our first study, we attempted to isolate Pd(I) intermediates with our multidentate ligands. To further enhance the stabilizing effect, we also incorporated ancillary ligands with soft P atoms. To our delight, two monomeric Pd(I) complexes – [(N2S2)Pd(I)(PtBu3)]+ and [(NCHS2)Pd(I)(PtBu3)]+ – were structurally characterized, and both of them showed superior catalytic reactivity for the Csp2-Csp3 Kumada cross-coupling reaction vs. their Pd(0) or Pd(II) analogs. These sulfur-based ligands were also employed to design different bioinspired Ni electrocatalysts for the study of electrochemical hydrogen evolution reaction (HER) in a non-aqueous electrolyte. These complexes are able to achieve stable catalytic currents with high turnover numbers for proton reduction, even at low acid concentration. Additionally, in contrast to the Ni(0)/Ni(II) process employed by most other reported Ni-based HER catalysts, one of our Ni species – [(NCHS2)NiOTf2 – is proposed to go through a Ni(I)/Ni(III) HER mechanism, which is similar to the natural process in [NiFe] hydrogenases. Extensive electrochemical experiments and EPR (Electron Paramagnetic Resonance) spectroscopic studies were carried out to provide evidence for the alternate Ni(I)/Ni(III) catalytic cycle. Lastly, preliminary results also showed that the multidentate pyridine dithiolate ligands can support organometallic Pd(III) species. Excitingly, the solid state structure of complex [(NCS2)Pd(III)(PPh3)]2+ was obtained, which showed a rare four-coordinate geometry at a Pd(III) center. Prior to this, no monomeric Pd(III) complex supported by S-based ligand(s) was structurally characterized. This is an important finding as it is uncommon for a ligand system to be able to stabilize both low and high valent metal species.
Issue Date:2021-06-29
Rights Information:Copyright 2021 Giang Tran
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08

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