Observation of a Photogenerated Rh₂ Nitrenoid Intermediate in C–H Amination

Rh₂-catalyzed C–H amination is a powerful method for nitrogenating organic molecules. While Rh₂ nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh₂ nitrenoid that participates in C–H amination. The developed approach to Rh₂ nitrenoids, based on photochemical cleavage of N–Cl bonds in <i>N</i>-chloroamido ligands, has enabled characterization of a reactive Rh₂ nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C–H amination catalysis by providing tools to directly study the structures of these critical intermediates

Direct Characterization of a Reactive Lattice-Confined Ru₂ Nitride by Photocrystallography

Reactive metal–ligand (M–L) multiply bonded complexes are ubiquitous intermediates in redox catalysis and have thus been long-standing targets of synthetic chemistry. The intrinsic reactivity of mid-to-late M–L multiply bonded complexes renders these structures challenging to isolate and structurally characterize. Although synthetic tuning of the ancillary ligand field can stabilize M–L multiply bonded complexes and result in isolable complexes, these efforts inevitably attenuate the reactivity of the M–L multiple bond. Here, we report the first direct characterization of a reactive Ru₂ nitride intermediate by photocrystallography. Photogeneration of reactive M–L multiple bonds within crystalline matrices supports direct characterization of these critical intermediates without synthetic derivatization

Gold Corroles as Near-IR Phosphors for Oxygen Sensing

The triplet state of gold­(III) corroles is exploited for optical oxygen sensing. We report intense phosphorescence for gold­(III) corroles in the near-IR, an optical window that is ideal for tissue transparency. Moreover, the triplet excited-state emission exhibits significant changes in intensity and lifetime over the 0–160 Torr O₂ pressure range. This renders these compounds sensitive at biologically relevant pressures and overcomes the spectral limitations of palladium and platinum porphyrins for oxygen sensing in biology

Halogen Photoelimination from Sb^V Dihalide Corroles

Main-group p-block metals are ideally suited for mediating two-electron reactions because they cycle between M^<i>n</i> and M^<i>n</i>+2 redox states, as the one-electron state is thermodynamically unstable. Here, we report the synthesis and structure of an Sb^III corrole and its Sb^VX₂ (X = Cl, Br) congeners. Sb^III sits above the corrole ring, whereas Sb^V resides in the corrole centroid. Electrochemistry suggests interconversion between the Sb^III and Sb^VX₂ species. TD-DFT calculations indicate a HOMO → LUMO+2 parentage for excited states in the Soret spectral region that have significant antibonding character with respect to the Sb–X fragment. The photochemistry of <b>2</b> and <b>3</b> in THF is consistent with the computational results, as steady-state photolysis at wavelengths coincident with the Soret absorption of Sb^VX₂ corrole lead to its clean conversion to the Sb^III corrole. This ability to photoactivate the Sb–X bond reflects the proclivity of the pnictogens to rely on the Pn^III/V couple to drive the two-electron photochemistry of M–X bond activation, an essential transformation needed to develop HX-splitting cycles

Ag(III)···Ag(III) Argentophilic Interaction in a Cofacial Corrole Dyad

Metallophilic interactions between closed-shell metal centers are exemplified by d10 ions, with Au(I) aurophilic interactions as the archetype. Such an interaction extends to d8 species, and examples involving Au(III) are prevalent. Conversely, Ag(III) argentophilic interactions are uncommon. Here, we identify argentophilic interactions in silver corroles, which are authentic Ag(III) species. The crystal structure of a monomeric silver corrole is a dimer in the solid state, and the macrocycle exhibits an atypical domed conformation. In order to evaluate whether this represents an authentic metallophilic interaction or a crystal-packing artifact, the analogous cofacial or “pacman” corrole was prepared. The conformation of the monomer was recapitulated in the silver pacman corrole, exhibiting a short 3.67 Å distance between metal centers and a significant compression of the xanthene backbone. Theoretical calculations support the presence of a rare Ag(III)···Ag(III) argentophilic interaction in the pacman complex

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H₂ by protonation of a formally Co­(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the <i>meso</i> carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co­(II) species is reduced to a monoanionic singlet Co­(I) species. Subsequent reduction leads to a dianionic doublet, formally a Co(0) complex in which substantial mixing of Co and porphyrin orbitals indicates ligand redox noninnocence. The partial reduction of the ligand disrupts the aromaticity in the porphyrin ring. As a result of this ligand dearomatization, protonation of the dianionic species is significantly more thermodynamically favorable at the <i>meso</i> carbon than at the metal center, and the ET-PT mechanism leads to a dianionic phlorin species. According to the proposed mechanism, the carboxylate group of this dianionic phlorin species is reprotonated, the species is reduced again, and H₂ is evolved from the protonated carboxylate and the protonated carbon. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms

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

Water Oxidation Catalysis by Co(II) Impurities in Co(III)₄O₄ Cubanes

The observed water oxidation activity of the compound class Co₄O₄(OAc)₄(Py–X)₄ emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co₄O₄ molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis

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

Water Oxidation Catalysis by Co(II) Impurities in Co(III)₄O₄ Cubanes

The observed water oxidation activity of the compound class Co₄O₄(OAc)₄(Py–X)₄ emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co₄O₄ molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis