μ‑Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO₂ Reduction Catalyst

Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO2 reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ2 = 2–4 ps), and finally relaxation of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an FeIII–O bond to generate transient FeIVO and FeII porphyrin species (τ1 = 2 ps) that recombine to the ground-state μ-oxo species (τ2 = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO2 reduction reactions

Anisotropic Covalency Contributions to Superexchange Pathways in Type One Copper Active Sites

Type one (T1) Cu sites deliver electrons to catalytic Cu active sites: the mononuclear type two (T2) Cu site in nitrite reductases (NiRs) and the trinuclear Cu cluster in the multicopper oxidases (MCOs). The T1 Cu and the remote catalytic sites are connected via a Cys-His intramolecular electron-transfer (ET) bridge, which contains two potential ET pathways: P1 through the protein backbone and P2 through the H-bond between the Cys and the His. The high covalency of the T1 Cu–S­(Cys) bond is shown here to activate the T1 Cu site for hole superexchange via occupied valence orbitals of the bridge. This covalency-activated electronic coupling (<i>H</i>_DA) facilitates long-range ET through both pathways. These pathways can be selectively activated depending on the geometric and electronic structure of the T1 Cu site and thus the anisotropic covalency of the T1 Cu–S­(Cys) bond. In NiRs, blue (π-type) T1 sites utilize P1 and green (σ-type) T1 sites utilize P2, with P2 being more efficient. Comparing the MCOs to NiRs, the second-sphere environment changes the conformation of the Cys-His pathway, which selectively activates <i>H</i>_DA for superexchange by blue π sites for efficient turnover in catalysis. These studies show that a given protein bridge, here Cys-His, provides different superexchange pathways and electronic couplings depending on the anisotropic covalencies of the donor and acceptor metal sites

Photocatalysts Based on Cobalt-Chelating Conjugated Polymers for Hydrogen Evolution from Water

Developing photocatalytic systems for water splitting to generate oxygen and hydrogen is one of the biggest chemical challenges in solar energy utilization. In this work, we report the first example of heterogeneous photocatalysts for hydrogen evolution based on in-chain cobalt-chelating conjugated polymers. Two conjugated polymers chelated with earth-abundant cobalt ions were synthesized and found to evolve hydrogen photocatalytically from water. These polymers are designed to combine functions of the conjugated backbone as a light-harvesting antenna and electron-transfer conduit with the in-chain bipyridyl-chelated transition metal centers as catalytic active sites. In addition, these polymers are soluble in organic solvents, enabling effective interactions with the substrates as well as detailed characterization. We also found a polymer-dependent optimal cobalt chelating concentration at which the highest photocatalytic hydrogen production (PHP) activity can be achieved

Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation

Two distinct [Cu–O–Cu]²⁺ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]²⁺ active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu–O–Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu–O–Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites

Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co–O–Co Unit

A dicobalt­(II) complex, [Co₂­(<i>m</i>BDCA-5t)]^2– (<b>1</b>), demonstrates a cofacial arrangement of trigonal mono­pyramidal Co­(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of <b>1</b> with potassium superoxide generates an encapsulated Co–O–Co core in the dianionic complex, [Co₂O­(<i>m</i>BDCA-5t)]^2– (<b>2</b>); to form the linear Co–O–Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both <b>1</b> and <b>2</b>. Multi­reference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of <b>2</b>. Complex <b>2</b> acts as a 2-e^– oxidant toward substrates including CO and H₂, in both cases efficiently regenerating <b>1</b> in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H₂O₂

Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co–O–Co Unit

A dicobalt­(II) complex, [Co₂­(<i>m</i>BDCA-5t)]^2– (<b>1</b>), demonstrates a cofacial arrangement of trigonal mono­pyramidal Co­(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of <b>1</b> with potassium superoxide generates an encapsulated Co–O–Co core in the dianionic complex, [Co₂O­(<i>m</i>BDCA-5t)]^2– (<b>2</b>); to form the linear Co–O–Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both <b>1</b> and <b>2</b>. Multi­reference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of <b>2</b>. Complex <b>2</b> acts as a 2-e^– oxidant toward substrates including CO and H₂, in both cases efficiently regenerating <b>1</b> in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H₂O₂

Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co–O–Co Unit

A dicobalt­(II) complex, [Co₂­(<i>m</i>BDCA-5t)]^2– (<b>1</b>), demonstrates a cofacial arrangement of trigonal mono­pyramidal Co­(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of <b>1</b> with potassium superoxide generates an encapsulated Co–O–Co core in the dianionic complex, [Co₂O­(<i>m</i>BDCA-5t)]^2– (<b>2</b>); to form the linear Co–O–Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both <b>1</b> and <b>2</b>. Multi­reference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of <b>2</b>. Complex <b>2</b> acts as a 2-e^– oxidant toward substrates including CO and H₂, in both cases efficiently regenerating <b>1</b> in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H₂O₂

Resonant Inelastic X‑ray Scattering on Ferrous and Ferric Bis-imidazole Porphyrin and Cytochrome <i>c</i>: Nature and Role of the Axial Methionine–Fe Bond

Axial Cu–S­(Met) bonds in electron transfer (ET) active sites are generally found to lower their reduction potentials. An axial S­(Met) bond is also present in cytochrome <i>c</i> (cyt <i>c</i>) and is generally thought to increase the reduction potential. The highly covalent nature of the porphyrin environment in heme proteins precludes using many spectroscopic approaches to directly study the Fe site to experimentally quantify this bond. Alternatively, L-edge X-ray absorption spectroscopy (XAS) enables one to directly focus on the 3d-orbitals in a highly covalent environment and has previously been successfully applied to porphyrin model complexes. However, this technique cannot be extended to metalloproteins in solution. Here, we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole porphyrin model complex and cyt <i>c</i>. The RIXS data on the model complex are directly correlated to L-edge XAS data to develop the complementary nature of these two spectroscopic methods. Comparison between the bis-imidazole model complex and cyt <i>c</i> in ferrous and ferric oxidation states show quantitative differences that reflect differences in axial ligand covalency. The data reveal an increased covalency for the S­(Met) relative to N­(His) axial ligand and a higher degree of covalency for the ferric states relative to the ferrous states. These results are reproduced by DFT calculations, which are used to evaluate the thermodynamics of the Fe–S­(Met) bond and its dependence on redox state. These results provide insight into a number of previous chemical and physical results on cyt <i>c</i>