Synthesis of redox-active ligands and their use in hydrogenase modeling

Lansing, James

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https://hdl.handle.net/2142/78310 Copy

Description

Title

Synthesis of redox-active ligands and their use in hydrogenase modeling

Author(s)

Lansing, James

Issue Date

2015-02-10

Director of Research (if dissertation) or Advisor (if thesis)

Rauchfuss, Thomas B.

Doctoral Committee Chair(s)

Rauchfuss, Thomas B.

Committee Member(s)

• Girolami, Gregory S.
• Gewirth, Andrew A.
• Mitchell, Douglas A.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Ferrocene
• Redox-Active Ligands
• Cobalt
• Iron
• Hydrogenase

Abstract

In Nature, hydrogen (H2) is used both as a fuel source and as a facile way to control the pH to biological systems. Hydrogen possesses many characteristics that make it desirable as a clean fuel: H2 can ideally be derived from water (or mildly acidic aqueous conditions), and the combustion of hydrogen with air yields water as a byproduct, rather than carbon based wastes (C, CO, CO2, etc.). Thus the storage of energy in the form of hydrogen is a desirable goal. Nature uses the enzymatic system of hydrogenases (H2ases) to reversibly convert protons and electrons into H2. The two primary H2ases are the [FeFe] and [NiFe]-H2ases, aptly named for the metals present at the active sites. Both H2ases possess numerous unique features, including FeS cluster to relay electrons, CN and CO ligands to maintain low spin metal centers, and thiolate ligands and internal bases near the metal active sites. The inclusion of ferrodoxin clusters in both families of H2ases, as well as the fact that a ferredoxin cluster is present in the H-cluster of [FeFe]-H2ase, suggests that the utilization of redox-active ligands may be able to stimulate novel reactivity in model systems. Chapter 1 of this thesis introduces the concept of redox-active ligands, as well as presenting a brief overview of substitutional changes at the cyclopentadiene rings that lead to changes in electrochemical response of ferrocene (Fc). Chapter 1 also provides an overview of current model systems for H2 generation and oxidation derived from H2ase model systems. In Chapter 2, synthetic considerations are taken into account for the design of redox-active ligands. The beauty of the Fc core lies in both its stability and ease of functionalization. Synthetic routes to ferrocenyl phosphine complexes are presented. Routes allow for variability in ligand synthesis, allowing the Fc core electronics to be altered by both the number of methyl groups and the identity of the phosphine linker atom. Furthermore, exploration of these redox active ligands in metal systems was explored. Both Mo and Ir systems were studied to gauge the effect of metal binding on the ligand redox potential. The effect of stereochemistry and the presence of multiple ligands were also explored. Iridium systems were constructed reminiscent of Vaska’s complex, and it was found that the presence of electron holes on the ligands lead to unique oxidative addition reactions. Chapter 3 describes the implementation of highly reducing ferrocenyl phosphine ligands in FeFe models. Addition of ferrocenyl phosphine ligands to [FeFe]-H2ase model systems yielded a complex containing two accessible electrons and an internal base, all of which in turn were able to produce H2 in the presence of excess acid. When additional reductant was present, these [FeFe]-H2ase systems became catalytic. Variations in the catalytic system were implemented and explored. Catalytic behavior was also observed when the redox active ligand was not covalently detached, but no reactivity was observed when the internal base was removed from the system. While catalysis was slow, multiple turnovers were observed, and the system features low overpotentials in catalysis. Chapter 4 describes the synthesis of Fe(dithiolate)(diphosphine)(CO)2 complexes as building blocks to yield bimetallic model systems. When models are constructed with non-rigid disphosphine ligands, the synthesis and isolation of FeFe systems proves challenging. However, the comproportionation reaction of Fe(dithiolate)(diphosphine)(CO)2 with an Fe0 source proved to be a high yielding synthetic route to unsymmetrical FeFe complexes. The utilization of Fe(dithiolate)(diphosphine)(CO)2 units also led to the synthesis of MnFe and CoFe systems. MnFe and CoFe systems were also studied as possible [FeFe]-H2ase models. Chapter 5 describes work with cobalt systems with respect to hydrogen generation. Implementation of redox active ferrocenyl ligands into cyclopentadienylcobalt complexes was investigated, attempting to utilize CoI platforms for hydride formation and subsequent H2 generation. Chapter 5 also describes the synthesis of cobalt systems designed to mimic [FeFe]-H2ase. The Fe(CO)3 moiety of Fe2(xdt)(CO)6 was replaced with CpCo, and in some cases Cp′Co. These CpCo systems were found to reversibly protonate, allowing for the determination of pKa values. The effect of changing the Cp unit and dithiolate were also explored with respect to pKa and oxidation potentials. Furthermore, a one-electron oxidized system was synthesized and investigated by EPR.

Graduation Semester

2015-5

Type of Resource

text

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Copyright and License Information

Copyright 2015 James Lansing

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