Pulse radiolysis of organic halogen compounds. II. Transient bromine-atom charge-transfer complexes observed by pulse radiolysis

Spectral Evidence of Bevel-Gear-Type Rotation of Benzene around Br in Solid p-H2: Infrared Spectrum of the C6H6Br Radical

Whether the structure of C₆H₆X (X = halogen), an intermediate in the halogenation of benzene, is an open or a bridged form has been debated. We produced Br to react with C₆H₆ upon photolysis in situ of a Br₂/C₆H₆/p-H₂ matrix at 3.2 K. In contrast to the C₆H₆Cl σ-complex reported previously, the observed infrared spectrum indicates that C₆H₆Br is an open-form π-complex. Furthermore, lines of the two CH out-of-plane bending modes associated mainly with even- and odd-numbered carbons, predicted near 672 and 719 cm^-1, merged into a broad line at 697.3 cm^-1, indicating that these modes become nearly equivalent as Br migrates from one carbon atom to another. Quantum-chemical calculations support that the benzene ring performs a bevel-gear-type rotation with respect to Br. Observation of only trans-ortho- and trans-para-C₆H₆Br₂ suggests that this gear-type motion allows the additional Br atom to attack C₆H₆Br only from the opposite side of the Br atom in C₆H₆Br.

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, ΔE°'(Χ^•/-), where X = Br and I, that results from the noncovalent interaction with stabilizing groups. The [Ir(dF-(CF₃)-ppy)₂(tmam)]³⁺ photocatalyst features a dicationic ligand tmam [4,4'-bis[(trimethylamino)methyl]-2,2'-bipyridine]²⁺ that is shown by ¹H NMR spectroscopy to associate a single halide ion, K _eq = 7 × 10⁴ M^-1 (Br^-) and K _eq = 1 × 10⁴ M^-1 (I^-). Light excitation of the photocatalyst in halide-containing acetonitrile solutions results in competitive quenching by the stabilized halide and the more easily oxidized diffusing halide ion. Marcus theory is used to relate the rate constants to the electron-transfer driving forces for oxidation of the stabilized and unstabilized halide, the difference of which provides the increase in reduction potentials of ΔE°'(Br^•/-) = 150 ± 24 meV and ΔE°'(I^•/-) = 67 ± 13 meV. The data reveal that K _eq is a poor indicator of these reduction potential shifts. Furthermore, the historic and widely used assumption that Coulombic interactions alone are responsible for stabilization must be reconsidered, at least for polarizable halogens.

On the Determination of Halogen Atom Reduction Potentials with Photoredox Catalysts

The standard one-electron reduction potentials of halogen atoms, E°'(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 E°'(X^•/-) in propylene carbonate, acetonitrile, butyronitrile, and dichloromethane. The approximations commonly used to determine diffusional rate constants in water gave rise to serious disagreements with the experiment, particularly in high-ionic-strength dichloromethane solutions, indicating the need to utilize the exact Debye expression. The Fuoss equation was adequate for determining photocatalyst-halide association constants with photocatalysts that possessed +2, +1, and 0 ionic charges. Similarly, the work term contribution in the classical Rehm-Weller expression, necessary for E°'(X^•/-) determination, accounted remarkably well for the stabilization of the charged reactants as the solution ionic strength was increased. While a sensitivity analysis indicated that the extracted reduction potentials were all within experimental error the same, use of fixed parameters established for aqueous solution provided the periodic trend expected, E°'(I^•/-) <E°'(Br^•/-) <E°'(Cl^•/-), in all of the organic solvents investigated; however, the potentials were more closely spaced than what would have been predicted based on gas-phase electron affinities or aqueous reduction potentials. The origin(s) of such behavior are discussed that provide new directions for future research.

Halide Photoredox Chemistry

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-to-ligand 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.

Ultrafast photoinduced dynamics of halogenated cyclopentadienes: observation of geminate charge-transfer complexes in solution

The photoinduced dynamics of the fully halogenated cyclopentadienes C5Cl6 and C5Br6 have been investigated in solution and gas phase by femtosecond time-resolved spectroscopy. Both in solution and in gas phase, homolytic dissociation into a halogen radical and a C5X5 (X = Cl, Br) radical was observed. In liquid phase, solvent-dependent formation of charge transfer complexes between geminate radicals was observed for the first time. These complexes were found to be surprisingly stable and offered the opportunity to follow the dynamics of specific radical pairs. In the case of C5Cl6 in trichloroethanol, a reaction of the chlorine radical with molecules from the solvent cage was observed.