Periodic Trends in Intra-ionic Excited State Quenching by Halide.

The preassociation of reactants in a photoinitiated redox reaction through the use of noncovalent interactions can have a significant impact on excited state reactivity. As these noncovalent interactions render some stabilization to the associated species, they impact the kinetics and thermodynamics of photoinitiated electron transfer. Reported herein is a novel iridium(III) photocatalyst, equipped with an anion-sensitive, amide-substituted bipyridine ligand, and its reactivity with the halides (X = I, Br, Cl) in acetonitrile and dichloromethane.

Factors that Impact Photochemical Cage Escape Yields.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them.

Nanosecond Intra-Ionic Chloride Photo-Oxidation.

Transition-metal photocatalysts capable of oxidizing chloride are rare yet serve as an attractive means to controllably generate chlorine atoms, which have continued to garner the interest of researchers for notable applications in photoredox catalysis and solar energy storage. Herein, a new series of four Ir-photocatalysts with different dicationic chloride-sequestering ligands were synthesized and characterized to probe the relationship between chloride binding affinities, ion pair solution structures, and rate constants for chloride photo-oxidation in acetonitrile at room temperature. The substituents on the quaternary amines of dicationic bipyridine ligands had negligible effects on the photocatalyst excited-state reduction potential, yet dramatically influenced the affinity for chloride binding, indicating that synthetic design can be utilized to independently tune these important properties.

Chloride Oxidation by One- or Two-Photon Excitation of -Phenylphenothiazine.

Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C-H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, (Cl), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of -phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH by oxidative quenching that subsequently disproportionates to yield PTH that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH.

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers.

Stabilization of ions and radicals often determines reaction kinetics and thermodynamics, but experimental determination of the stabilization magnitude remains difficult, especially when the species is short-lived. Herein, a competitive kinetic approach to quantify the stabilization of a halide ion toward oxidation imparted by specific stabilizing groups relative to a solvated halide ion is reported. This approach provides the increase in the formal reduction potential, Δ°'(Χ), where X = Br and I, that results from the noncovalent interaction with stabilizing groups.

On the Determination of Halogen Atom Reduction Potentials with Photoredox Catalysts.

The standard one-electron reduction potentials of halogen atoms, °'(X), and many other radical or unstable species, are not accessible through standard electrochemical methods. Here, we report the use of two Ir(III) photoredox catalysts to initiate chloride, bromide, and iodide oxidation in organic solvents. The kinetic rate constants were critically analyzed through a derived diffusional model with Marcus theory to estimate °'(X) in propylene carbonate, acetonitrile, butyronitrile, and dichloromethane.

Examining the role of Rh/Si cooperation in alkene hydrogenation by a pincer-type [P2Si]Rh complex.

A bis(phosphine)/triflatosilyl pincer-type Rh(i) complex can reversibly store one equivalent of H2 across the Si-Rh bond upon triflate migration from silicon to rhodium. The triflatosilyl complex serves as an effective precatalyst for norbornene hydrogenation, but Si-OTf bond cleavage is not implicated in the major catalytic pathway. The combined findings suggest possible strategies for M/Si cooperation in catalytic processes.