Halide Photoredox Chemistry

Abstract

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X−X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-toligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights. CONTENTS 1. Introduction, Background, and Motivation

Excited-state proton-coupled electron transfer within ion pairs

“…This has been shown to enhance the formation of ion pairs between cationic ruthenium polypyridyl compounds and anions in acetonitrile (CH 3 CN). 23,44,[55][56][57] The four ruthenium compounds utilized in this study follow the common structure of [Ru(tmam)(LL) 2 ](PF 6 ) 4 , where LL was 4,4 0dimethoxy-2,2 0 -bipyridine (OMe), 4,4 0 -di-tert-butyl-2,2 0 -bipyridine (Dtb), 2,2 0 -bipyridine (Bpy), and 2,2 0 -bipyrazine (Bpz). Herein, the compounds will be denoted Ru-LL where LL is the short name of the derivatized bipyridine or bipyrazine ligand (Scheme 2B).…”

Visible Light–Driven Cascade Carbon–Carbon Bond Scission for Organic Transformations and Plastics Recycling

“…Following our success with 34, we then applied this cascade CC bond cleavage and oxygenation to the poly-ε-caprolactoneblock-polyethyleneglycol-block-poly-ε-caprolactone (PCL-PEG-PCL) triblock co-polymer (35). The reaction proceeded smoothly with complete polymer transformation within 2 days under ambient conditions, with formic acid as the major product ( Figure 5, Entry 2).…”

“…This concept had been pioneered by Meyer and co-workers, who demonstrated its potential for chloride [25] and benzyl alcohol oxidation with simultaneous H 2 evolution. [26] Others, including Sun and co-workers, [27][28][29][30] Meyer and co-workers, [31][32][33][34][35] Berlinguette and co-workers, [36][37][38] Reisner and co-workers, [14,[39][40][41] and Lei and co-workers, [42][43][44] have subsequently shown that the gap between AP and radical catalysis can potentially be bridged via the valorization of biomass, functionalization of amines, oxidation of other halides, photo-electrochemical reduction of CO 2 , and even photoreforming of plastics.…”

“…Cascade CC bond cleavage in macromolecular hydroxyl functionalized biodegradable(34)(35)(36) and non-biodegradable (37 and 38) polymers. In all the polymeric substrates, the sites of CC bond cleavage are highlighted in red, with multiple red colored CC bonds indicating the cascade shortening of the polymer chain.…”

Photoelectrochemical Cells for Artificial Photosynthesis: Alternatives to Water Oxidation

“…Besides alcohol oxidation, T. J. Meyer, G. J. Meyer, and their coworkers have also been investigating halide oxidation reactions as substitutes for water oxidation in a PEC . Water oxidation is a four‐electron process and poses many kinetic challenges, whereas halide oxidation reactions are just two‐electron processes and should in principle be faster.…”