Transition Metal Complexes for Challenging Reductive Transformations: From Nitrogen Fixation Catalysts to Photoreductants

Citation

Fajardo, Javier, Jr. (2020) Transition Metal Complexes for Challenging Reductive Transformations: From Nitrogen Fixation Catalysts to Photoreductants. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0fgd-j835. https://resolver.caltech.edu/CaltechTHESIS:06082020-141438135

Abstract

Transition metal complexes are routinely employed as catalysts for the reductive cleavage of a diverse array of strong chemical bonds. Two notable research areas that exemplify such utility are nitrogen fixation, involving cleavage of the notoriously unreactive triple bond of dinitrogen (N₂) to form ammonia (NH₃), and photoredox catalysis, wherein powerful photoreductants generated by visible light excitation facilitate challenging reduction steps in a host of synthetic organic transformations. This thesis focuses on a number of structure-function studies conducted on group 8 transition metal complexes that catalyze N₂-to-NH₃ conversion, commonly referred to as the nitrogen reduction reaction (N₂RR), and on homoleptic tungsten(0) arylisocyanides that, among their many attractive qualities, possess highly reducing electronically excited states. These comparative studies provide fundamental insight into critical design features which can guide efforts to improve existing N₂RR or photocatalysts or rationally tailor them for specific applications.

Chapter 2 details the effect apical Lewis acidic atom substitution in P₃^XFe platforms (X = B, Al, Ga) has on structure, bonding, and N₂RR activity. Structural, spectroscopic, electrochemical, and computational studies reveal that all three P₃^XFe systems possess similar electronic structures, degrees of N₂ activation, and geometric flexibility, but P₃^AlFe and P₃^GaFe display significantly lower N₂RR efficiencies than P₃^BFe when treated with HBAr^F₄/KC₈ or [H₂NPh₂][OTf]/Cp*₂Co at –78 °C in Et₂O.

Chapter 3 reports on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N₂RR. The study of the homologous, isostructural series of complexes P₃^SiM (M = Fe, Ru, Os) helps delineate important factors for N₂RR catalyst design. Low-temperature protonation of P₃^SiOs–N₂⁻ yields P₃^SiOs=NNH₂⁺, representing the first instance of an Os–N₂ species being converted to a protonated Os–N_xH_y product.

Chapter 4 communicates a novel series of homoleptic tungsten(0) photoactive complexes supported by fused-ring (CN-1-(2-ⁱPr)-Naph) or alkynyl-bridged (CNDipp^CCAr) arylisocyanide ligands. Systematic studies establish facile electronic variation of the CNDipp^CCAr platform as a straightforward method by which to rationally modulate the ground- and excited-state properties of W(CNDipp^CCAr)₆ complexes. The photophysical properties of W(CN-1-(2-ⁱPr)-Naph)₆ reveal potential benefits of utilizing fused-ring arylisocyanide ligands in the design of this class of photosensitizers.

Thesis Files