Independent Mutation of Two Bridging Carboxylate Ligands Stabilizes Alternate Conformers of the Photosynthetic O2-Evolving Mn4CaO5 Cluster in Photosystem II

The O2-evolving Mn4CaO5 cluster in photosystem II is ligated by six carboxylate residues. One of these is D170 of the D1 subunit. This carboxylate bridges between one Mn ion (Mn4) and the Ca ion. A second carboxylate ligand is D342 of the D1 subunit. This carboxylate bridges between two Mn ions (Mn1 and Mn2). D170 and D342 are located on opposite sides of the Mn4CaO5 cluster. Recently, it was shown that the D170E mutation perturbs both the intricate networks of H-bonds that surround the Mn4CaO5 cluster and the equilibrium between different conformers of the cluster in two of its lower oxidation states, S1 and S2, while still supporting O2 evolution at approximately 50% the rate of the wild type. In this study, we show that the D342E mutation produces much the same alterations to the cluster's FTIR and EPR spectra as D170E, while still supporting O2 evolution at approximately 20% the rate of the wild type. Furthermore, the double mutation, D170E + D342E, behaves similarly to the two single mutations. We conclude that D342E alters the equilibrium between different conformers of the cluster in its S1 and S2 states in the same manner as D170E and perturbs the H-bond networks in a similar fashion. This is the second identification of a Mn4CaO5 metal ligand whose mutation influences the equilibrium between the different conformers of the S1 and S2 states without eliminating O2 evolution. This finding has implications for our understanding of the mechanism of O2 formation in terms of catalytically active/inactive conformations of the Mn4CaO5 cluster in its lower oxidation states.

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.

High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*4 W4 Fe13 S12 , displaying a Fe13 core with M-M bonds that has precedent only in main group and late metal chemistry. M13 clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*4 W4 Fe13 S12 displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.

Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S2 State of the Oxygen Evolving Complex

The S2 state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate MnIII centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a MnIIIMnIV3O4 cuboidal complex featuring five-coordinate MnIII. This cluster displays a spin ground state of S = 5/2, while conversion to a six-coordinate Mn upon treatment with water results in a spin state change to S = 1/2. These results demonstrate that coordination number, without dramatic changes within the Mn4O4 core, has a substantial effect on spectroscopy.

Photogenerated Ni(I)-Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp2)-Cl Oxidative Addition and Dimerization Reactivity Pathways

We report the facile photochemical generation of a library of Ni(I)-bpy halide complexes (Ni(I)(^Rbpy)X (R = t-Bu, H, MeOOC; X = Cl, Br, I) and benchmark their relative reactivity toward competitive oxidative addition and off-cycle dimerization pathways. Structure-function relationships between the ligand set and reactivity are developed, with particular emphasis on rationalizing previously uncharacterized ligand-controlled reactivity toward high energy and challenging C(sp²)-Cl bonds. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition is found to proceed through an S_NAr-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z²) orbital and the C_aryl-Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp²)-Br/I bonds. The bpy substituent provides a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occurs. Here, we elucidate the origin of this substituent influence as arising from perturbations to the effective nuclear charge (Z_eff) of the Ni(I) center. Electron donation to the metal decreases Z_eff, which leads to a significant destabilization of the entire 3d orbital manifold. Decreasing the 3d(z²) electron binding energies leads to a powerful two-electron donor to activate strong C(sp²)-Cl bonds. These changes also prove to have an analogous effect on dimerization, with decreases in Z_eff leading to more rapid dimerization. Ligand-induced modulation of Z_eff and the 3d(z²) orbital energy is thus a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C-X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles.

An examination of the metal ion content in the active sites of human endonucleases CPSF73 and INTS11

Human cleavage and polyadenylation specificity factor (CPSF)73 (also known as CPSF3) is the endoribonuclease that catalyzes the cleavage reaction for the 3'-end processing of pre-mRNAs. The active site of CPSF73 is located at the interface between a metallo-β-lactamase domain and a β-CASP domain. Two metal ions are coordinated by conserved residues, five His and two Asp, in the active site, and they are critical for the nuclease reaction. The metal ions have long been thought to be zinc ions, but their exact identity has not been examined. Here we present evidence from inductively coupled plasma mass spectrometry and X-ray diffraction analyses that a mixture of metal ions, including Fe, Zn, and Mn, is present in the active site of CPSF73. The abundance of the various metal ions is different in samples prepared from different expression hosts. Zinc is present at less than 20% abundance in a sample expressed in insect cells, but the sample is active in cleaving a pre-mRNA substrate in a reconstituted canonical 3'-end processing machinery. Zinc is present at 75% abundance in a sample expressed in human cells, which has comparable endonuclease activity. We also observe a mixture of metal ions in the active site of the CPSF73 homolog INTS11, the endonuclease for Integrator. Taken together, our results provide further insights into the role of metal ions in the activity of CPSF73 and INTS11 for RNA 3'-end processing.

Boronated Cyanometallates

Thirteen boronated cyanometallates [M(CN-BR₃)₆]^3/4/5- [M = Cr, Mn, Fe, Ru, Os; BR₃ = BPh₃, B(2,4,6,-F₃C₆H₂)₃, B(C₆F₅)₃] and one metalloboratonitrile [Cr(NC-BPh₃)₆]^3- have been characterized by X-ray crystallography and spectroscopy [UV-vis-near-IR, NMR, IR, spectroelectrochemistry, and magnetic circular dichroism (MCD)]; CASSCF+NEVPT2 methods were employed in calculations of electronic structures. For (t_2g)⁵ electronic configurations, the lowest-energy ligand-to-metal charge-transfer (LMCT) absorptions and MCD C-terms in the spectra of boronated species have been assigned to transitions from cyanide π + B-C borane σ orbitals. CASSCF+NEVPT2 calculations including t_1u and t_2u orbitals reproduced t_1u/t_2u → t_2g excitation energies. Many [M(CN-BR₃)₆]^3/4- complexes exhibited highly electrochemically reversible redox couples. Notably, the reduction formal potentials of all five [M(CN-B(C₆F₅)₃)₆]^3- anions scale with the LMCT energies, and Mn(I) and Cr(II) compounds, [K(18-crown-6)]₅[Mn(CN-B(C₆F₅)₃)₆] and [K(18-crown-6)]₄[Cr(CN-B(C₆F₅)₃)₆], are surprisingly stable. Continuous-wave and pulsed electron paramagnetic resonance (EPR; hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, ¹⁴N hyperfine splittings are greater for (Ph₄As)₃[Cr(NC-BPh₃)₆] than for (Ph₄As)₃[Cr(CN-BPh₃)₆].

MnIV4O4 Model of the S3 Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different S_n-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn^IV₄ core corresponding to the S₃ state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued. Herein, we report the synthesis, electrochemistry, SQUID magnetometry, and electron paramagnetic resonance spectroscopy of a stable Mn^IV₄O₄ cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand shifts the reduction potential of the Mn^IIIMn^IV₃/Mn^IV₄ redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated Mn^IV₄O₄ complex matches the acetate and amidate variants, in corroboration with the Mn^IV₄ assignment of the S₃ state of the OEC.

Characterization of a Proposed Terminal Iron(III) Nitride Intermediate of Nitrogen Fixation Stabilized by a Trisphosphine-Borane Ligand

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic iron nitrides have been characterized in high oxidation states, including FeIV , FeV , and FeVI . This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P3 B Fe(N) (P3 B =tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P3 B Fe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a 2 A ground state electronic structure of this C3 -symmetric nitride species, placing the unpaired spin in a sigma orbital along the B-Fe-N vector; this electronic structure is distinct for an iron nitride. The unusual d5 -configuration is stabilized by significant delocalization (≈50 %) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

17O Electron Nuclear Double Resonance Analysis of Compound I: Inverse Correlation between Oxygen Spin Population and Electron Donation

Although the activation of inert C-H bonds by metal-oxo complexes has been widely studied, important questions remain, particularly regarding the role of oxygen spin population (i.e., unpaired electrons on the oxo ligand) in facilitating C-H bond cleavage. In order to shed light on this issue, we have utilized ¹⁷O electron nuclear double resonance spectroscopy to measure the oxygen spin populations of three compound I intermediates in heme enzymes with different reactivities toward C-H bonds: chloroperoxidase, cytochrome P450, and a selenolate (selenocysteinyl)-ligated cytochrome P450. The experimental data suggest an inverse correlation between oxygen spin population and electron donation from the axial ligand. We have explored the implications of this result using a Hückel-type molecular orbital model and constrained density functional theory calculations. These investigations have allowed us to examine the relationship between oxygen spin population, oxygen charge, electron donation from the axial ligand, and reactivity.

Copper(II) Binding to the Intrinsically Disordered C-Terminal Peptide of SARS-CoV-2 Virulence Factor Nsp1

The first encoded SARS-CoV-2 protein (Nsp1) binds to the human 40S ribosome and blocks synthesis of host proteins, thereby inhibiting critical elements of the innate immune response. The final 33 residues of the natively unstructured Nsp1 C-terminus adopt a helix-turn-helix geometry upon binding to the ribosome. We have characterized the fluctuating conformations of this peptide using circular dichroism spectroscopy along with measurements of tryptophan fluorescence and energy transfer. Tryptophan fluorescence decay kinetics reveal that copper(II) binds to the peptide at micromolar concentrations, and electron paramagnetic resonance spectroscopy indicates that the metal ion coordinates to the lone histidine residue.

Chemical control of spin-lattice relaxation to discover a room temperature molecular qubit

The second quantum revolution harnesses exquisite quantum control for a slate of diverse applications including sensing, communication, and computation. Of the many candidates for building quantum systems, molecules offer both tunability and specificity, but the principles to enable high temperature operation are not well established. Spin-lattice relaxation, represented by the time constant T ₁, is the primary factor dictating the high temperature performance of quantum bits (qubits), and serves as the upper limit on qubit coherence times (T ₂). For molecular qubits at elevated temperatures (>100 K), molecular vibrations facilitate rapid spin-lattice relaxation which limits T ₂ to well below operational minimums for certain quantum technologies. Here we identify the effects of controlling orbital angular momentum through metal coordination geometry and ligand rigidity via π-conjugation on T ₁ relaxation in three four-coordinate Cu²⁺ S = ½ qubit candidates: bis(N,N'-dimethyl-4-amino-3-penten-2-imine) copper(ii) (Me₂Nac)₂ (1), bis(acetylacetone)ethylenediamine copper(ii) Cu(acacen) (2), and tetramethyltetraazaannulene copper(ii) Cu(tmtaa) (3). We obtain significant T ₁ improvement upon changing from tetrahedral to square planar geometries through changes in orbital angular momentum. T ₁ is further improved with greater π-conjugation in the ligand framework. Our electronic structure calculations reveal that the reduced motion of low energy vibrations in the primary coordination sphere slows relaxation and increases T ₁. These principles enable us to report a new molecular qubit candidate with room temperature T ₂ = 0.43 μs, and establishes guidelines for designing novel qubit candidates operating above 100 K.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

Investigation of the C-N Bond-Forming Step in a Photoinduced, Copper-Catalyzed Enantioconvergent N-Alkylation: Characterization and Application of a Stabilized Organic Radical as a Mechanistic Probe

Whereas photoinduced, copper-catalyzed couplings of nitrogen nucleophiles with alkyl electrophiles have recently been shown to provide an attractive approach to achieving a variety of enantioselective C-N bond constructions, mechanistic studies of these transformations have lagged the advances in reaction development. Herein we provide mechanistic insight into a previously reported photoinduced, copper-catalyzed enantioconvergent C-N coupling of a carbazole nucleophile with a racemic tertiary α-haloamide electrophile. Building on the isolation of a copper(II) model complex whose EPR parameters serve as a guide, we independently synthesize two key intermediates in the proposed catalytic cycle, a copper(II) metalloradical (L*Cu^II(carb')₂) (L* = a monodentate chiral phosphine ligand; carb' = a carbazolide ligand), as well as a tertiary α-amide organic radical (R·); the generation and characterization of R· was guided by DFT calculations, which suggested that it would be stable to homocoupling. Continuous-wave (CW) and pulse EPR studies, along with corresponding DFT calculations, are among the techniques used to characterize these reactive radicals. We establish that these two radicals do indeed combine to furnish the C-N coupling product in good yield and with significant enantiomeric excess (77% yield, 55% ee), thereby supporting the chemical competence of these proposed intermediates. DFT calculations are consistent with R· initially binding to copper(II) via a dative interaction from the closed-shell carbonyl oxygen atom of the radical, which positions the α-carbon for direct reaction with the copper(II)-bound carbazole N atom, to generate the C-N bond with enantioselectivity, without the formation of an alkylcopper(III) intermediate.

Probing Redox Non-Innocence in Iron-Carbene Complexes {Fe=C(H)Ar}10-11 by 1,2 H and 13 C Pulse Electron Paramagnetic Resonance

We report the synthesis and spectroscopic characterization of a series of iron-carbene complexes in redox states {Fe=C(H)Ar}^10-11 . Pulse EPR studies of the ^1,2 H and ¹³ C isotopologues of {Fe=C(H)Ar}¹¹ reveal the high covalency of the Fe-carbene bonding, leading to a more even spin distribution than commonly observed for reduced Fischer carbenes.

Terminal, Open-Shell Mo Carbide and Carbyne Complexes: Spin Delocalization and Ligand Noninnocence

Open-shell compounds bearing metal-carbon triple bonds, such as carbides and carbynes, are of significant interest as plausible intermediates in the reductive catenation of C₁ oxygenates. Despite the abundance of closed-shell carbynes reported, open-shell variants are very limited, and an open-shell carbide has yet to be reported. Herein, we report the synthesis of the first terminal, open-shell carbide complexes, [K][1] and [1][BAr^F₄] (1 = P2Mo(≡C:)(CO), P2 = a terphenyl diphosphine ligand), which differ by two redox states, as well as a series of related open-shell carbyne complexes. The complexes are characterized by single-crystal X-ray diffraction and NMR, EPR, and IR spectroscopies, while the electronic structures are probed by EPR studies and DFT calculations to assess spin delocalization. In the d₁ complexes, the spin is primarily localized on the metal (∼55-77% Mo d_xy) with delocalization on the triply bonded carbon of ∼0.05-0.09 e^-. In the reduced carbide [K][1], a direct metal-arene interaction enables ancillary ligand reduction, resulting in reduced radical character on the terminal carbide (⩽0.02 e^-). Reactivity studies with [K][1] reveal the formation of mixed-valent C-C coupled products at -40 °C, illustrating how productive reactivity manifolds can be engendered through the manipulation of redox states. Combined, the results inform on the electronic structure and reactivity of a new and underrepresented class of compounds with potential significance to a wide array of reactions involving open-shell species.

CaMn3 IV O4 Cubane Models of the Oxygen-Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn₃ ^IV O₄ and YMn₃ ^IV O₄ complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn₃ ^IV O₄ complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C₃ -symmetric Ca(Y)Mn₃ ^IV O₄ core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn₃ ^IV O₃ (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S₂ state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes.

Hydrazine Formation via Coupling of a Nickel(III)-NH2 Radical

M(NHx ) intermediates involved in N-N bond formation are central to ammonia oxidation (AO) catalysis, an enabling technology to ultimately exploit ammonia (NH3 ) as an alternative fuel source. While homocoupling of a terminal amide species (M-NH2 ) to form hydrazine (N2 H4 ) has been proposed, well-defined examples are without precedent. Herein, we discuss the generation and electronic structure of a NiIII -NH2 species that undergoes bimolecular coupling to generate a NiII 2 (N2 H4 ) complex. This hydrazine adduct can be further oxidized to a structurally unusual Ni2 (N2 H2 ) species; this releases N2 in the presence of NH3 , thus establishing a synthetic cycle for Ni-mediated AO. Distribution of the redox load for H2 N-NH2 formation via NH2 coupling between two metal centers presents an attractive strategy for AO catalysis using Earth-abundant, late first-row metals.

Understanding Covalent versus Spin-Orbit Coupling Contributions to Temperature-Dependent Electron Spin Relaxation in Cupric and Vanadyl Phthalocyanines

Recent interest in transition-metal complexes as potential quantum bits (qubits) has reinvigorated the investigation of fundamental contributions to electron spin relaxation in various ligand scaffolds. From quantum computers to chemical and biological sensors, interest in leveraging the quantum properties of these molecules has opened a discussion of the requirements to maintain coherence over a large temperature range, including near room temperature. Here we compare temperature-, magnetic field position-, and concentration-dependent electron spin relaxation in copper(II) phthalocyanine (CuPc) and vanadyl phthalocyanine (VOPc) doped into diamagnetic hosts. While VOPc demonstrates coherence up to room temperature, CuPc coherence times become rapidly T₁-limited with increasing temperature, despite featuring a more covalent ground-state wave function than VOPc. As rationalized by a ligand field model, this difference is ascribed to different spin-orbit coupling (SOC) constants for Cu(II) versus V(IV). The manifestation of SOC contributions to spin-phonon coupling and electron spin relaxation in different ligand fields is discussed, allowing for a further understanding of the competing roles of SOC and covalency in electron spin relaxation.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e^- and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e^- transfer which bear some resemblance to a pair of E_n and E_n+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]^-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e^- reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Generating Potent C-H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII-H Complex Demonstrates a Promising Strategy

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. This work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*Fe^III-H species, [Fe^III(η⁵-Cp*)(dppe)H]⁺ (Cp* = C₅Me₅^-, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H-Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [Fe^III(η⁵-Cp*)(dppe)(CO)H]⁺. Herein, pulse EPR spectroscopy (^1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [Fe^I(endo-η⁴-Cp*H)(dppe)(CO)]⁺. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe⁰(exo-η⁴-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFE_C-H ≈ 29 kcal mol^-1 and 25 kcal mol^-1, respectively) cf. BDFE_Fe-H of 56 kcal mol^-1 for [Fe^III(η⁵-Cp*)(dppe)H]⁺. These weak C-H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [Fe^II(η⁵-Cp*)(dppe)CO]⁺ byproduct.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8-10 complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/-. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]- complex, an example of a ls-{FeNO}10 species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8 system, with two additional electrons "stored" on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}9 complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N2 complex, [Fe(TPB)(N2)]-, which is a key intermediate in Fe(TPB)-catalyzed N2 fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

While the icosahedral closo-[B₁₂H₁₂]^2- cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B₁₂(O-3-methylbutyl)₁₂ (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br₂C₆H₃)₃]^•+ afforded the isolable [1]^•+ cluster, which is the first example of an open-shell cationic B₁₂ cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]^•+ with ferrocene resulted in its reduction back to 1. The identity of [1]^•+ is supported by EPR, UV-vis, multinuclear NMR (¹H, ¹¹B), and X-ray photoelectron spectroscopic characterization.

H2 Evolution from a Thiolate-Bound Ni(III) Hydride

Terminal Ni^III hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = 1/2, terminally bound thiolate-Ni^III-H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate-Ni^III-H species is comparable to that of the hydride ligand proposed for the Ni-C hydrogenase intermediate (Ni^III-H-Fe^II). Upon warming, the featured thiolate-Ni^III-H species undergoes bimolecular reductive elimination of H₂. Associated kinetic studies are discussed and compared with a structurally related Fe^III-H species that has also recently been reported to undergo bimolecular H-H coupling.

S = 3 Ground State for a Tetranuclear MnIV4O4 Complex Mimicking the S3 State of the Oxygen-Evolving Complex

The S₃ state is currently the last observable intermediate prior to O-O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn^IV₄ core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S₃ state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S₃ state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn^IV₄O₄ cuboidal complex as a spectroscopic model of the S₃ state. Results show that this Mn^IV₄O₄ complex has an S = 3 ground state with isotropic ⁵⁵Mn hyperfine coupling constants of -75, -88, -91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer-monomer magnetic coupling model of pseudo-octahedral Mn^IV centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S₂ state to the S₃ state. This same spin state change is observed following oxidation of the previously reported Mn^IIIMn^IV₃O₄ cuboidal complex to the Mn^IV₄O₄ complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.

Mononuclear Fe(I) and Fe(II) Acetylene Adducts and Their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. "Fischer-Tropsch" type coupling of non-native C1 substrates to higher-order C≥2 products is also known for nitrogenase, though potential metal-carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 FeI(η2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3 and the paramagnetic cation S = 1/2 [(SiP3)Fe≡CCH3]+. Both alkylcarbyne compounds possess short Fe-C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH3 unit. The electronic structures of (SiP3)Fe≡CCH3 and [(SiP3)Fe≡CCH3]+ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer

We report the characterization of an S= 1 / 2 iron π-complex, [Fe(η⁶ -IndH)(depe)]⁺ (Ind=Indenide (C₉ H₇ ^- ), depe=1,2-bis(diethylphosphino)ethane), which results via C-H elimination from a transient Fe^III hydride, [Fe(η³ :η² -Ind)(depe)H]⁺ . Owing to weak M-H/C-H bonds, these species appear to undergo proton-coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell π-arene complex to have a BDFE_C-H value of ≈50 kcal mol^-1 , roughly equal to the BDFE_Fe-H of its Fe^III -H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵ -Cp/Cp*) compounds, for which terminal Fe^III -H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring- versus a metal-bound H-atom by way of continuous-wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.

Characterization of the Earliest Intermediate of Fe-N2 Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH2

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N₂-to-NH₃ conversion (N₂RR) mediated by synthetic iron complexes and relatedly as intermediates of N₂RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR₃), and also of further downstream intermediates (e.g., Fe(NNH₂)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N₂^- species, supported by a bulky trisphosphinoborane ligand (^ArP₃^B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The ¹H-hyperfine for ^ArP₃^BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( a_iso = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH₂)]⁺ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH₂)]⁺ intermediate of N₂RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N₂-to-NH₃ conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N₂RR in a synthetic system.

Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center, Three-Electron Polar σ Bonds

Species with 2-center, 3-electron (2c/3e^- ) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹ ) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^- σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X-ray diffraction, continuous wave and pulse electron paramagnetic resonance, density-functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^- bonding framework with a σ*-SOMO of metal 4- or 5d_z ² parentage that is partially stabilized by significant mixing with Si (3p_z ) and metal (5- or 6p_z ) orbitals. Metal-ligand covalency thus buffers the expected destabilization of transition-metal (TM)-silyl σ*-orbitals by d-p mixing, affording well-characterized examples of TM-main group, and hence polar, 2c/3e^- σ "half-bonds".

Electronic Structures of an [Fe(NNR2)]+/0/- Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal-NNH₂ complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N₂) fixation. In this context, we have shown that triphosphine-supported Fe(N₂) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH₂) complexes over an array of charge and oxidation states. Upon exposure to further H⁺/e^- equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N-H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe₂) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the "NNMe₂" moiety. Specifically, although two closed-shell configurations of the "NNR₂" ligand have been commonly considered in the literature-isodiazene and hydrazido(2-)-we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR₂]^•- ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N₂ fixation activity.

Tetranuclear [MnIIIMn3IVO4] Complexes as Spectroscopic Models of the S2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S₂ state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure-electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^IIIMn₃^IVO₄] cuboidal complexes as spectroscopic models of the S₂ state of the OEC. Resembling the oxidation state and EPR spectra of the S₂ state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed.

Nitric Oxide Modulates Endonuclease III Redox Activity by a 800 mV Negative Shift upon [Fe4S4] Cluster Nitrosylation

Here we characterize the [Fe₄S₄] cluster nitrosylation of a DNA repair enzyme, endonuclease III (EndoIII), using DNA-modified gold electrochemistry and protein film voltammetry, electrophoretic mobility shift assays, mass spectrometry of whole and trypsin-digested protein, and a variety of spectroscopies. Exposure of EndoIII to nitric oxide under anaerobic conditions transforms the [Fe₄S₄] cluster into a dinitrosyl iron complex, [(Cys)₂Fe(NO)₂]^-, and Roussin's red ester, [(μ-Cys)₂Fe₂(NO)₄], in a 1:1 ratio with an average retention of 3.05 ± 0.01 Fe per nitrosylated cluster. The formation of the dinitrosyl iron complex is consistent with previous reports, but the Roussin's red ester is an unreported product of EndoIII nitrosylation. Hyperfine sublevel correlation (HYSCORE) pulse EPR spectroscopy detects two distinct classes of NO with ¹⁴N hyperfine couplings consistent with the dinitrosyl iron complex and reduced Roussin's red ester. Whole-protein mass spectrometry of EndoIII nitrosylated with ¹⁴NO and ¹⁵NO support the assignment of a protein-bound [(μ-Cys)₂Fe₂(NO)₄] Roussin's red ester. The [Fe₄S₄]^2+/3+ redox couple of DNA-bound EndoIII is observable using DNA-modified gold electrochemistry, but nitrosylated EndoIII does not display observable redox activity using DNA electrochemistry on gold despite having a similar DNA-binding affinity as the native protein. However, direct electrochemistry of protein films on graphite reveals the reduction potential of native and nitrosylated EndoIII to be 127 ± 6 and -674 ± 8 mV vs NHE, respectively, corresponding to a shift of approximately -800 mV with cluster nitrosylation. Collectively, these data demonstrate that DNA-bound redox activity, and by extension DNA-mediated charge transport, is modulated by [Fe₄S₄] cluster nitrosylation.

An S = 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.

N2 -to-NH3 Conversion by a triphos-Iron Catalyst and Enhanced Turnover under Photolysis

Bridging iron hydrides are proposed to form at the active site of MoFe-nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N2 via reductive elimination of H2 . This possibility inspires the investigation of well-defined molecular iron hydrides as precursors for catalytic N2 -to-NH3 conversion. Herein, we describe the synthesis and characterization of new P2P'Ph Fe(N2 )(H)x systems that are active for catalytic N2 -to-NH3 conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that photo-elimination of H2 is one means by which the enhanced activity may arise.

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) (1-OAc) 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^- (1-Cl), NO₃^- (1-NO₃), and pyridine ([1-py]⁺). 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.

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

Fluorinated tyrosines (F_nY’s, n = 2 and 3) have been site-specifically incorporated into E. coli class Ia ribonucleotide reductase (RNR) using the recently evolved M. jannaschii 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 E. coli), and three transiently oxidized Y’s (356 in β and 731 and 730 in α) to initiate the radical-dependent nucleotide reduction process. F_nY (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_nY· cofactor is self-assembled from apo F_nY-β2, Fe²⁺, and O₂ to produce ∼1 Y·/β2 and ∼3 Fe³⁺/β2. The F_nY· are stable and active in nucleotide reduction with activities that vary from 5% to 85% that of wt-β2. Each F_nY·-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_nY· that are readily distinguishable from unlabeled Y·’s. The variability of the abiotic F_nY pK_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.

Metal ion oxidation state assignment based on coordinating ligand hyperfine interaction

In exchange-coupled mixed-valence spin systems, the magnitude and sign of the effective ligand hyperfine interaction (HFI) can be useful in determining the formal oxidation state of the coordinating metal ion, as well as provide information about the coordination geometry. This is due to the fact that the observed ligand HFI is a function of the projection factor (Clebsch-Gordon coefficient) that maps the site spin value S i of the local paramagnetic center onto the total spin of the exchange-coupled system, S T. Recently, this relationship has been successfully exploited in identifying the oxidation state of the Mn ion coordinated by the sole nitrogenous ligand to the oxygen-evolving complex in certain states of photosystem II. The origin and evolution of these efforts is described.