Earth Abundant Molecular HX-Splitting Photocatalysts for Solar Energy Storage

Abstract

Photochemical splitting of hydrohalic acids (HX) into their elemental constituents H2 and X2 (2 HX  H2 + X2) represents a chemical approach to solar-to-fuels energy conversion. To this end, this thesis has been directed towards developing new chemical platforms that manage both of the requisite two-electron half-reactions of HX splitting: proton reduction and halide oxidation. The thesis work has been devoted to probing the intimate mechanistic details of these two-electron photoreactions. In previous work in the field, the halide oxidation half cycle remains the kinetic bottleneck of a HX photocycle and chemical traps are required to promote halogen extrusion. This heavily mitigates the utility of the photocatalysis for energy storing applications. In addition, heretofore, little progress has been made toward energy storing halogen elimination chemistry with earth-abundant 3d metal complexes because of their short excited state lifetimes. Finally, the photochemical M–X bond activation step has not been directly observed in the solid state and its mechanism remains unclear. Each of the aforementioned challenges has been addressed with this thesis work. A major objective this thesis is to explore new design strategies for efficient halogen photoelimination reactions with earth abundant transition metal complexes. We begin in Chapter 2 by studying diaryl phosphines as photoredox mediator to demonstrate that H2 evolution catalysis can be achieved via a tandem approach that combines a non-basic photoredox mediator and transition metal catalysts. Next, we examine oxidizing Ni(III)trihalide complexes for authentic photohalogen elimination via LMCT excited states. We continue in Chapter 3 by exploring the use of ancillary ligands to promote elimination, offering a strategy to circumvent the inherently short-lived excited states of 3d metal complexes. We demonstrate HCl-splitting photocatalysis with two closely related bimetallic Rh2 and Ni2 catalysts in Chapter 4. Importantly, we directly probe the structural nature of these excited states using low-temperature steady-state photo-crystallography experiments that establish a common chloride-bridged intermediate in halogen elimination reaction. The results of these studies led to the development in Chapter 4 of second generation Rh2 and Ni2 photocatalysts, which are more active. We further expand photoreduction chemistry toward Fe(III) metal complexes in Chapter 5 with the synthesis of two families of five-coordinate iron complexes with pyridine diimine (PDI) and phosphine ligand platforms. Through the synthesis and preparation of these compounds, we establish the utility of secondary sphere interactions in efficiently assisting metal-halogen bond activation and sustaining persistent halogen atom radical generation. Photocrystallography experiments on these iron compounds provide the first structural characterization of a Cl•|arene interaction, which is η1 in nature. Lastly, in Chapter 6, magneto-structural correlation studies of the mononuclear intermediate spin Fe(III) phosphine complexes of Chapter 5 demonstrate the influence of local symmetry on magnetic anisotropy.