An <i>S</i> = ¹/₂ Iron Complex Featuring N₂, Thiolate, and Hydride Ligands: Reductive Elimination of H₂ and Relevant Thermochemical Fe–H Parameters

Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N₂ binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated Fe^III(H)­(N₂) complex, which releases H₂ upon warming to yield an Fe^II–N₂–Fe^II complex. Bimolecular reductive elimination of H₂ from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe–H bond with a computationally determined BDFE of 55.6 kcal/mol

An <i>S</i> = ¹/₂ Iron Complex Featuring N₂, Thiolate, and Hydride Ligands: Reductive Elimination of H₂ and Relevant Thermochemical Fe–H Parameters

Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N₂ binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated Fe^III(H)­(N₂) complex, which releases H₂ upon warming to yield an Fe^II–N₂–Fe^II complex. Bimolecular reductive elimination of H₂ from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe–H bond with a computationally determined BDFE of 55.6 kcal/mol

An <i>S</i> = ¹/₂ Iron Complex Featuring N₂, Thiolate, and Hydride Ligands: Reductive Elimination of H₂ and Relevant Thermochemical Fe–H Parameters

Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N₂ binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated Fe^III(H)­(N₂) complex, which releases H₂ upon warming to yield an Fe^II–N₂–Fe^II complex. Bimolecular reductive elimination of H₂ from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe–H bond with a computationally determined BDFE of 55.6 kcal/mol

Ammonia Binds to the Dangler Manganese of the Photosystem II Oxygen-Evolving Complex

High-resolution X-ray structures of photosystem II reveal several potential substrate binding sites at the water-oxidizing/oxygen-evolving 4MnCa cluster. Aspartate-61 of the D1 protein hydrogen bonds with one such water (W1), which is bound to the dangler Mn4A of the oxygen-evolving complex. Comparison of pulse EPR spectra of ¹⁴NH₃ and ¹⁵NH₃ bound to wild-type Synechocystis PSII and a D1-D61A mutant lacking this hydrogen-bonding interaction demonstrates that ammonia binds as a terminal NH₃ at this dangler Mn4A site and not as a partially deprotonated bridge between two metal centers. The implications of this finding on identifying the binding sites of the substrate and the subsequent mechanism of dioxygen formation are discussed

Highly Activated Terminal Carbon Monoxide Ligand in an Iron–Sulfur Cluster Model of FeMco with Intermediate Local Spin State at Fe

Nitrogenases, the enzymes that convert N2 to NH3, also catalyze the reductive coupling of CO to yield hydrocarbons. CO-coordinated species of nitrogenase clusters have been isolated and used to infer mechanistic information. However, synthetic FeS clusters displaying CO ligands remain rare, which limits benchmarking. Starting from a synthetic cluster that models a cubane portion of the FeMo cofactor (FeMoco), including a bridging carbyne ligand, we report a heterometallic tungsten–iron–sulfur cluster with a single terminal CO coordination in two oxidation states with a high level of CO activation (νCO = 1851 and 1751 cm–1). The local Fe coordination environment (2S, 1C, 1CO) is identical to that in the protein making this system a suitable benchmark. Computational studies find an unusual intermediate spin electronic configuration at the Fe sites promoted by the presence the carbyne ligand. This electronic feature is partly responsible for the high degree of CO activation in the reduced cluster

Structural Effects of Ammonia Binding to the Mn₄CaO₅ Cluster of Photosystem II

The Mn₄CaO₅ oxygen-evolving complex (OEC) of photosystem II catalyzes the light-driven oxidation of two substrate waters to molecular oxygen. ELDOR-detected NMR along with computational studies indicated that ammonia, a substrate analogue, binds as a terminal ligand to the Mn4A ion <i>trans</i> to the O5 μ₄ oxido bridge. Results from electron spin echo envelope modulation (ESEEM) spectroscopy confirmed this and showed that ammonia hydrogen bonds to the carboxylate side chain of D1-Asp61. Here we further probe the environment of OEC with an emphasis on the proximity of exchangeable protons, comparing ammonia-bound and unbound forms. Our ESEEM and electron nuclear double resonance (ENDOR) results indicate that ammonia substitutes for the W1 terminal water ligand without significantly altering the electronic structure of the OEC

Pulse Electron Paramagnetic Resonance Studies of the Interaction of Methanol with the S₂ State of the Mn₄O₅Ca Cluster of Photosystem II

The binding of the substrate analogue methanol to the catalytic Mn₄CaO₅ cluster of the water-oxidizing enzyme photosystem II is known to alter the electronic structure properties of the oxygen-evolving complex without retarding O₂-evolution under steady-state illumination conditions. We report the binding mode of ¹³C-labeled methanol determined using 9.4 GHz (X-band) hyperfine sublevel-correlation (HYSCORE) and 34 GHz (Q-band) electron spin–echo electron nuclear double resonance (ESE-ENDOR) spectroscopies. These results are compared to analogous experiments on a mixed-valence Mn­(III)­Mn­(IV) complex (2-OH-3,5-Cl₂-salpn)₂­Mn­(III)­Mn­(IV) (salpn = <i>N</i>,<i>N</i>′-bis­(3,5-dichlorosalicylidene)-1,3-diamino-2-hydroxypropane) in which methanol ligates to the Mn­(III) ion (Larson et al. (1992) J. Am. Chem. Soc., 114, 6263). In the mixed-valence Mn­(III,IV) complex, the hyperfine coupling to the ¹³C of the bound methanol (<i>A</i>_iso = 0.65 MHz, <i>T</i> = 1.25 MHz) is appreciably larger than that observed for ¹³C methanol associated with the Mn₄CaO₅ cluster poised in the S₂ state, where only a weak dipolar hyperfine interaction (<i>A</i>_iso = 0.05 MHz, <i>T</i> = 0.27 MHz) is observed. An evaluation of the ¹³C hyperfine interaction using the X-ray structure coordinates of the Mn₄CaO₅ cluster indicates that methanol does not bind as a terminal ligand to any of the manganese ions in the oxygen-evolving complex. We favor methanol binding in place of a water ligand to the Ca²⁺ in the Mn₄CaO₅ cluster or in place of one of the waters that form hydrogen bonds with the oxygen bridges of the cluster

Biophysical Characterization of Fluorotyrosine Probes Site-Specifically Incorporated into Enzymes: <i>E. coli</i> Ribonucleotide Reductase As an Example

Fluorinated tyrosines (F_<i>n</i>Y’s, <i>n</i> = 2 and 3) have been site-specifically incorporated into <i>E. coli</i> class Ia ribonucleotide reductase (RNR) using the recently evolved <i>M. jannaschii</i> Y-tRNA synthetase/tRNA pair. Class Ia RNRs require four redox active Y’s, a stable Y radical (Y·) in the β subunit (position 122 in <i>E. coli</i>), and three transiently oxidized Y’s (356 in β and 731 and 730 in α) to initiate the radical-dependent nucleotide reduction process. F_<i>n</i>Y (3,5; 2,3; 2,3,5; and 2,3,6) incorporation in place of Y₁₂₂-β and the X-ray structures of each resulting β with a diferric cluster are reported and compared with wt-β2 crystallized under the same conditions. The essential diferric-F_<i>n</i>Y· cofactor is self-assembled from apo F_<i>n</i>Y-β2, Fe²⁺, and O₂ to produce ∼1 Y·/β2 and ∼3 Fe³⁺/β2. The F_<i>n</i>Y· are stable and active in nucleotide reduction with activities that vary from 5% to 85% that of wt-β2. Each F_<i>n</i>Y·-β2 has been characterized by 9 and 130 GHz electron paramagnetic resonance and high-field electron nuclear double resonance spectroscopies. The hyperfine interactions associated with the ¹⁹F nucleus provide unique signatures of each F_<i>n</i>Y· that are readily distinguishable from unlabeled Y·’s. The variability of the abiotic F_<i>n</i>Y p<i>K</i>_a’s (6.4 to 7.8) and reduction potentials (−30 to +130 mV relative to Y at pH 7.5) provide probes of enzymatic reactions proposed to involve Y·’s in catalysis and to investigate the importance and identity of hopping Y·’s within redox active proteins proposed to protect them from uncoupled radical chemistry

Manganese–Cobalt Oxido Cubanes Relevant to Manganese-Doped Water Oxidation Catalysts

Incorporation of Mn into an established water oxidation catalyst based on a Co­(III)₄O₄ cubane was achieved by a simple and efficient assembly of permanganate, cobalt­(II) acetate, and pyridine to form the cubane oxo cluster MnCo₃O₄(OAc)₅py₃ (OAc = acetate, py = pyridine) (<b>1-OAc</b>) in good yield. This allows characterization of electronic and chemical properties for a manganese center in a cobalt oxide environment, and provides a molecular model for Mn-doped cobalt oxides. The electronic properties of the cubane are readily tuned by exchange of the OAc^– ligand for Cl^– (<b>1-Cl</b>), NO₃^– (<b>1-NO</b>_<b>3</b>), and pyridine (<b>[1-py]</b>⁺). EPR spectroscopy, SQUID magnetometry, and DFT calculations thoroughly characterized the valence assignment of the cubane as [Mn^IVCo^III₃]. These cubanes are redox-active, and calculations reveal that the Co ions behave as the reservoir for electrons, but their redox potentials are tuned by the choice of ligand at Mn. This MnCo₃O₄ cubane system represents a new class of easily prepared, versatile, and redox-active oxido clusters that should contribute to an understanding of mixed-metal, Mn-containing oxides