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

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

Ultrafast Photoinduced Electron Transfer from Peroxide Dianion

The encapsulation of peroxide dianion by hexacarboxamide cryptand provides a platform for the study of electron transfer of isolated peroxide anion. Photoinitiated electron transfer (ET) between freely diffusing Ru­(bpy)₃²⁺ and the peroxide dianion occurs with a rate constant of 2.0 × 10¹⁰ M^–1 s^–1. A competing electron transfer quenching pathway is observed within an ion pair. Picosecond transient spectroscopy furnishes a rate constant of 1.1 × 10¹⁰ s^–1 for this first-order process. A driving force dependence for the ET rate within the ion pair using a series of Ru­(bpy)₃²⁺ derivatives allows for the electronic coupling and reorganization energies to be assessed. The ET reaction is nonadiabatic and dominated by a large inner-sphere reorganization energy, in accordance with that expected for the change in bond distance accompanying the conversion of peroxide dianion to superoxide anion

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

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

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