Exploring the Photophysics and Reactivity of Nickel–Bipyridine Cross-Coupling Catalysts

Citation

Cagan, David Abraham (2024) Exploring the Photophysics and Reactivity of Nickel–Bipyridine Cross-Coupling Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n3xz-6v34. https://resolver.caltech.edu/CaltechTHESIS:05092024-225449838

Abstract

Ni(II)–bipyridine (bpy) aryl halide complexes have been prized for nearly a decade for their catalytic potency to facilitate cross-coupling reactions. To achieve these transformations, the energy from light is leveraged to drive the key catalytic processes. Thus, Ni-mediated photoredox catalysis provides an attractive and sustainable means to replace precious metal catalysts. However, precise mechanistic information regarding how these transformations occur is limited. This thesis thus focuses on a dual experimental and computational analysis of Ni(II)–bpy aryl halide complexes and their photoproducts to provide insight into the specific photophysical and chemical pathways that these catalysts undertake for cross-coupling reactions. The first chapter is a review of the proposed mechanisms presented for Ni-mediated photoredox catalysis. Therein, certain portions of this work are also summarized. The second chapter provides a computational description of the Ni(II) excited states. The third chapter expands on this analysis with experiment, elucidating the photophysical pathway that grants entry into dark Ni(I)/Ni(III) catalytic cycles. Together, chapters two and three show that Ni(II)–bpy aryl halide complexes form low-valent Ni(I)–bpy halide species by an aryl-to-Ni ligand-to-metal charge transfer. Chapter four outlines a method to generate and study these reactive Ni(I)–bpy halide intermediates, identifying their mechanism of C(sp²)–Cl bond activation as nucleophilic aromatic substitution, tunable via the energies of the 3d-orbitals and the effective nuclear charge of Ni. The final chapter finds that these low-valent Ni species are competitive light-absorbers, and it presents a study into their ultrafast photophysics, marking the first of its kind on any Ni(I) complex. The excited-state relaxation dynamics of Ni(I)–bpy halide complexes are well described by vibronic Marcus theory, spanning the normal and inverted regions as a result of simple changes to the bpy substituents. Altogether, these studies have provided a framework to gain electronic structural control over Ni-meditated photoredox catalysis and, thus, guides the use of photonic energy as a sustainable alternative to precious metal catalysis.

Thesis Files