Two-Electron HCl to H₂ Photocycle Promoted by Ni(II) Polypyridyl Halide Complexes

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni­(II) chloride complex that provides a route to H₂ evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni­(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni­(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni­(I) intermediate affords Ni­(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H₂ and the polypyridyl Ni­(II) dichloride, closing the photocycle for H₂ generation from HCl

Deciphering Radical Transport in the Large Subunit of Class I Ribonucleotide Reductase

Incorporation of 2,3,6-trifluorotyrosine (F₃Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the <i>C</i>-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F₃Y radical from the [Re]–F₃Y−βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y–Y established, radical transport occurs with a rate constant of 3 × 10⁵ s^–1. Point mutations that disrupt the Y–Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the colinear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme

Deciphering Radical Transport in the Large Subunit of Class I Ribonucleotide Reductase

Incorporation of 2,3,6-trifluorotyrosine (F₃Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the <i>C</i>-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F₃Y radical from the [Re]–F₃Y−βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y–Y established, radical transport occurs with a rate constant of 3 × 10⁵ s^–1. Point mutations that disrupt the Y–Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the colinear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme

Solvent-Induced Spin-State Change in Copper Corroles

The electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. The ground state of these compounds is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. In coordinating solvents, these molecules become paramagnetic, and this is often accompanied by a color change. The underlying chemistry of these solvent-induced properties is currently unknown. Here, we show that a coordinating solvent, such as pyridine, induces a change in the ground spin state from an antiferromagnetically coupled Cu(II) corrole radical cation to a ferromagnetically coupled triplet. Over time, the triplet reacts to produce a species with spectral signatures that are characteristic of the one-electron-reduced Cu(II) corrole. These observations account for the solvent-induced paramagnetism and the associated color changes that have been observed for copper corroles in coordinating solvents

Fluoride Complexes of Cyclometalated Iridium(III)

Many electroluminescent devices rely on cyclometalated iridium­(III). Their advancement depends on access to reactive starting materials because of the inertness of Ir­(III). Notably, fluoride complexes of bis­(cyclometalated) Ir­(III) are scarce. Syntheses of bridged and terminal fluorides are reported here. New compounds are luminescent and thermally reactive; they are characterized by ground-state and optical methods. Crystal structures were determined for one bridging and one terminal fluoride complex. The terminal fluoride shows intramolecular hydrogen bonding to an adjacent 3,5-dimethylpyrazole ligand; a lesser interaction may occur between F and a nearby aromatic C–H bond. Terminal fluoride complexes react with carbon-, silicon-, and sulfur-based electrophiles. The new complexes phosphoresce with microsecond lifetimes at 77 and 298 K. Density-functional theory calculations indicate triplet states with little contribution from fluoride. The compounds herein are versatile phosphors having the ground-state reactivity of late transition metal fluorides

Fluoride Complexes of Cyclometalated Iridium(III)

Many electroluminescent devices rely on cyclometalated iridium­(III). Their advancement depends on access to reactive starting materials because of the inertness of Ir­(III). Notably, fluoride complexes of bis­(cyclometalated) Ir­(III) are scarce. Syntheses of bridged and terminal fluorides are reported here. New compounds are luminescent and thermally reactive; they are characterized by ground-state and optical methods. Crystal structures were determined for one bridging and one terminal fluoride complex. The terminal fluoride shows intramolecular hydrogen bonding to an adjacent 3,5-dimethylpyrazole ligand; a lesser interaction may occur between F and a nearby aromatic C–H bond. Terminal fluoride complexes react with carbon-, silicon-, and sulfur-based electrophiles. The new complexes phosphoresce with microsecond lifetimes at 77 and 298 K. Density-functional theory calculations indicate triplet states with little contribution from fluoride. The compounds herein are versatile phosphors having the ground-state reactivity of late transition metal fluorides

Fluoride Complexes of Cyclometalated Iridium(III)

Many electroluminescent devices rely on cyclometalated iridium­(III). Their advancement depends on access to reactive starting materials because of the inertness of Ir­(III). Notably, fluoride complexes of bis­(cyclometalated) Ir­(III) are scarce. Syntheses of bridged and terminal fluorides are reported here. New compounds are luminescent and thermally reactive; they are characterized by ground-state and optical methods. Crystal structures were determined for one bridging and one terminal fluoride complex. The terminal fluoride shows intramolecular hydrogen bonding to an adjacent 3,5-dimethylpyrazole ligand; a lesser interaction may occur between F and a nearby aromatic C–H bond. Terminal fluoride complexes react with carbon-, silicon-, and sulfur-based electrophiles. The new complexes phosphoresce with microsecond lifetimes at 77 and 298 K. Density-functional theory calculations indicate triplet states with little contribution from fluoride. The compounds herein are versatile phosphors having the ground-state reactivity of late transition metal fluorides

Stereoelectronic Effects in Cl₂ Elimination from Binuclear Pt(III) Complexes

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt­(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt­(III) complexes featuring meridionally coordinated Pt­(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt–X bond strength. Conversely, when compared to an isomeric facial Pt­(III) trichloride, a much more efficient photoelimination is observed for the <i>fac</i> isomer than would be predicted based on thermochemistry. This difference in the <i>fac</i> vs <i>mer</i> isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing