Development & characterization of bio-inspired earth-abundant metal systems and extreme ultraviolet spectroscopy of precious metal photocatalysts

Leahy, Clare Anastasia

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https://hdl.handle.net/2142/116088 Copy

Description

Title

Development & characterization of bio-inspired earth-abundant metal systems and extreme ultraviolet spectroscopy of precious metal photocatalysts

Author(s)

Leahy, Clare Anastasia

Issue Date

2022-07-14

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

• Fout, Alison R
• Vura-Weis, Josh

Doctoral Committee Chair(s)

Fout, Alison R

Committee Member(s)

• Girolami, Gregory
• Olshansky, Lisa

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Bioinorganic chemistry
• secondary coordination sphere hydrogen-bonding
• intramolecular hydrogen-bonding
• ligand synthesis
• dioxygen activation
• cobalt dioxygen activation
• cobalt coordination complex
• electronic structure
• metal electronic structure
• magnetic susceptibility
• extreme ultraviolet spectroscopy
• X-ray absorption near edge structure
• XUV
• XANES
• XPS
• transient XANES
• transient XUV
• M2,3-edge
• O2,3-edge
• O3-edge
• N6,7-edge
• iridium photocatalyst
• platinum photocatalyst
• photodynamics
• excited states
• metal charge transfer

Abstract

Transition metal catalysis plays a vital role in a wide range of chemical transformations. Characterization of these metal catalysts and intermediates provides crucial insight into their structures, electronic manifolds, and reaction mechanisms. Overall, this thesis demonstrates the use of complementary spectroscopic techniques towards understanding the physical and electronic structures of bio-inspired earth abundant metal systems and precious metal photocatalysts. Active sites of metalloenzymes are crucial targets for spectroscopic characterization as they are incredibly effective at performing challenging, multielectron reactions. Primary sphere coordination to the metal center and secondary sphere hydrogen-bonding interactions work together to achieve these transformations by facilitating small molecule binding & activation, stabilize reactive intermediates, and shuttle protons and electrons in the active site. Synthetic mimics integrating these enzymatic motifs can serve as useful tools in mimicking proposed intermediates and reactivity of metalloenzymes. The Fout group designed tripodal and tetrapodal scaffolds, H3(N(piCy)3) and Py2Py(piCy)2, that incorporate biomimetic primary and secondary sphere interactions through tautomerizable arms in the ligand frameworks. The ligand arm incorporates a pyrrole-imine (pi) form that give anionic coordination to a bound metal center while providing a hydrogen-bond acceptor in the secondary sphere. Tautomerization to the azafulvene-amine (afa) form enables dative coordination to the metal while giving a hydrogen-bond donor in the secondary coordination sphere. Early work focused on developing a new tripodal ligand scaffold that contained two tautomerizable arms and introduced a phenol group to act as the third arm, which upon deprotonation gave only anionic coordination to the metal center. A series of late 1st row transition metal(II) chloride complexes were characterized within this new framework that showed the afa arms displaying inter- and intra-ligand hydrogen donation to the bound axial chloride and the oxygen of the phenoxy arm. The latter was found to have a dramatic impact on the geometry of the metal center and donor strength of the oxygen, not only showing the ability of hydrogen bonds from the secondary coordination sphere to stabilize axial ligands but also demonstrates a design principle for tuning geometry & reactivity at the metal center through hydrogen bonding to the ligand framework. Later work returned to the original tripodal and tetrapodal ligand scaffolds containing three or two of the tautomerizable ligand arms, respectively. The latter Py2Py(piCy)2 was used in developing and characterizing a series of octahedral cobalt(II) complexes. The cobalt(II) bis(triflate) complex showed dioxygen activation to form a diamagnetic cobalt(III) hydroxide complex, demonstrating the use of secondary sphere hydrogen-bonds for binding & activating dioxygen. The cobalt(III) species was also found to be stable, ascribed to both hydrogen-bond donation from the afa amines stabilizing the bound axial hydroxide. Further electronic characterization of the cobalt tetrapodal system showed a strong axial ligand influence on the cobalt(II) centers’ electronic structure. To explore this ligand influence, two additional cobalt(II) tetrapodal species were synthesized and characterized by structural and electronic techniques. Additional characterization of late 1st row transition metal complexes in the H3(N(piCy)3) scaffold was undertaken using X-ray absorption spectroscopy to both explore the impact of ligand modification in the primary and secondary coordination spheres on the electronics and further characterize the electronic manifolds of these species. Extreme ultraviolet (XUV) spectroscopy probes the M2,3-edge of 1st row transition metals, which is sensitive oxidation, spin-state, and ligand-field. The Vura-Weis group has used this technique to examine metal-based structures and ultrafast photodynamics in several 1st row metal systems in their in-house, table-top instrument. M2,3-edge spectroscopy of iron(III)-oxo species in the different ligand variants of the tripodal ligand shows minor edge and pre-peak changes due to installation of an electron-withdrawing group in the ligand backbone affecting the primary sphere. The manganese(II)-hydroxo and manganese(III)-oxo tripodal species were also investigated, but the manganese(III)-oxo showed photoreduction to a manganese(II) complex. L2,3-edge spectroscopy of the iron, manganese, and cobalt tripodal species showed good qualitative agreement with original assignment of these complexes as high-spin metal centers of the appropriate oxidation state with weak trigonal bipyramidal ligand fields. Challenges in fitting the M2,3- and L2,3-edges of these trigonal bipyramidal metal species precluded quantitative fitting of the spectra. Application of XUV spectroscopy to characterizing precious metal systems was also investigated. 3rd row transition metals serve as important catalytic centers in organic transformations, particularly as photocatalysts and photosensitizers. The XUV energy region (30-100 eV) contains the O2,3- and N6,7-edges of 3rd row metals, but there has been little published work on the utility of XUV spectroscopy towards probing electronic structure or metal-based dynamics in these systems. Several platinum(II) and iridium(III) species were characterized at their O2,3- and N6,7-edges with XUV, with initial insights into changes in edge shifts from ligand field strength were obtained. Transient N6,7-edge spectroscopy of Ir(III)ppy3 tracking the early photodynamics showed excellent agreement with literature dynamics. Implications for current and future directions in applying XUV spectroscopy to study 3rd row metal complexes and their dynamics were discussed. Overall, using O2,3- and N6,7-edge XUV spectroscopy showed promise as a probe for interrogating metal structure and photodynamics of 3rd row transition metal systems.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Clare Leahy

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