Andrew G. Maher

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

Significance The conversion of solar energy into fuels involves coupled multiproton–multielectron reactions. Because of different length scales for electron transfer and proton transfer, efficient catalysts must couple proton hops to redox events. We have developed a hangman motif where placing a pendant acid–base group over the face of a redox macrocycle ensures coupling of the electron to the proton. We now show that the presence of the pendant acid relay of a Ni hangman porphyrin facilitates proton-coupled electron transfer (PCET) during turnover of the hydrogen evolution reaction (HER). By comparing HER activity of the Ni hangman catalyst to its Co analog, a unified HER mechanism begins to emerge for complexes that use a secondary coordination sphere to manage PCET. The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer–electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Significance Global energy sustainability requires the development of effective energy conversion processes. In the hydrogen evolution reaction, electrons and protons are combined to generate molecular hydrogen, which stores energy in its chemical bond. Molecular electrocatalysts have been designed to facilitate this reaction by making it occur faster with lower energy input, often utilizing proton-coupled electron transfer (PCET), which couples the motions of electrons and protons to avoid high-energy intermediates. Examination of a nickel porphyrin electrocatalyst indicates that the active intermediate stores electrons in the C–H bond of the modified porphyrin generated by PCET, rather than in the traditional metal-hydride bond. The ability to store electrons in the ligand rather than in the metal has significant implications for the design of electrocatalysts. The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. Herein nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C–H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H2. Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.

Electronic Structure of Copper Corroles

The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled Cu(II) corrole radical cation ground state. X-ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, Cu(II) isocorrole and [TBA][Cu(CF3)4] were studied as authentic Cu(II) and Cu(III) samples, respectively. In addition, the one-electron reduction and one-electron oxidation processes are both ligand-based, demonstrating that the Cu(II) centre is retained in these derivatives. These observations underscore ligand non-innocence in copper corrole complexes.

Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

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)3(2+) and the peroxide dianion occurs with a rate constant of 2.0 × 10(10) 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 × 10(10) 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)3(2+) 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.

Halogen Photoelimination from SbV Dihalide Corroles

Main-group p-block metals are ideally suited for mediating two-electron reactions because they cycle between M n and M n+2 redox states, as the one-electron state is thermodynamically unstable. Here, we report the synthesis and structure of an SbIII corrole and its SbVX2 (X = Cl, Br) congeners. SbIII sits above the corrole ring, whereas SbV resides in the corrole centroid. Electrochemistry suggests interconversion between the SbIII and SbVX2 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 2 and 3 in THF is consistent with the computational results, as steady-state photolysis at wavelengths coincident with the Soret absorption of SbVX2 corrole lead to its clean conversion to the SbIII corrole. This ability to photoactivate the Sb-X bond reflects the proclivity of the pnictogens to rely on the PnIII/V couple to drive the two-electron photochemistry of M-X bond activation, an essential transformation needed to develop HX-splitting cycles.

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 N-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.