Electron paramagnetic resonance and Mössbauer spectroscopy and density functional theory analysis of a high-spin Fe(IV)-oxo complex

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

Spin Polarization: A New Frontier in Efficient Photocatalysis for Environmental Purification and Energy Conversion

As a promising strategy to improve photocatalytic efficiency, spin polarization has attracted enormous attention in recent years, which could be involved in various steps of photoreaction. The Pauli repulsion principle and the spin selection rule dictate that the behavior of two electrons in a spatial eigenstate is based on their spin states, and this fact opens up a new avenue for manipulating photocatalytic efficiency. In this review, recent advances in modulating the photocatalytic activity with spin polarization are systematically summarized. Fundamental insights into the influence of spin-polarization effects on photon absorption, carrier separation, and migration, and the behaviors of reaction-related substances from the photon uptake to reactant desorption are highlighted and discussed in detail, and various photocatalytic applications for environmental purification and energy conversion are presented. This review is expected to deliver a timely overview of the recent developments in spin-polarization-modulated photocatalysis for environmental purification and energy conversion in terms of their practical applications.

Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity

Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy, providing structural insight and allowing the distinction of 6- vs 5-coordinate active sites as shown through application to the Fe(IV)═O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)═O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3dxz/yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.

Influence of the ligand-field on EPR parameters of cis- and trans-isomers in MoV systems relevant to molybdenum enzymes: Experimental and density functional theory study

The electron paramagnetic resonance (EPR) investigation of mononuclear cis- and trans-(L1O)MoOCl2 complexes [L1OH = bis(3,5-dimethylpyrazolyl)-3-tert-butyl-2-hydroxy-5-methylphenyl)methane] reveals a significant difference in their spin Hamiltonian parameters which reflect different equatorial and axial ligand fields created by the heteroscorpionate donor atoms. Density functional theory (DFT) was used to calculate the values of principal components and relative orientations of the g and A tensors, and the molecular framework in four pairs of isomeric mononuclear oxo‑molybdenum(V) complexes (cis- and trans-(L1O)MoOCl2, cis,cis- and cis,trans-(L-N2S2)MoOCl [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine], cis,cis- and cis,trans-(L-N2S2)MoO(SCN), and cis- and trans-[(dt)2MoO(OMe)]2- [dtH2 = 2,3-dimercapto-2-butene]). Scalar relativistic DFT calculations were conducted using three different exchange-correlation functionals. It was found that the use of hybrid exchange-correlation functional with 25% of the Hartree-Fock exchange leads to the best quantitative agreement between theory and experiment. A simplified ligand-field approach was used to analyze the influence of the ligand fields in all cis- and trans-isomers on energies and contributions of molybdenum d-orbital manifold to g and A tensors and relative orientations. Specifically, contributions that originated from the spin-orbit coupling of the dxz, dyz, and dx2-y2 orbitals into the ground state have been discussed. The new findings are discussed in the context of the experimental data of mononuclear molybdoenzyme, DMSO reductase.

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.

New Strategies for Direct Methane-to-Methanol Conversion from Active Learning Exploration of 16 Million Catalysts

Despite decades of effort, no earth-abundant homogeneous catalysts have been discovered that can selectively oxidize methane to methanol. We exploit active learning to simultaneously optimize methane activation and methanol release calculated with machine learning-accelerated density functional theory in a space of 16 M candidate catalysts including novel macrocycles. By constructing macrocycles from fragments inspired by synthesized compounds, we ensure synthetic realism in our computational search. Our large-scale search reveals that low-spin Fe(II) compounds paired with strong-field (e.g., P or S-coordinating) ligands have among the best energetic tradeoffs between hydrogen atom transfer (HAT) and methanol release. This observation contrasts with prior efforts that have focused on high-spin Fe(II) with weak-field ligands. By decoupling equatorial and axial ligand effects, we determine that negatively charged axial ligands are critical for more rapid release of methanol and that higher-valency metals [i.e., M(III) vs M(II)] are likely to be rate-limited by slow methanol release. With full characterization of barrier heights, we confirm that optimizing for HAT does not lead to large oxo formation barriers. Energetic span analysis reveals designs for an intermediate-spin Mn(II) catalyst and a low-spin Fe(II) catalyst that are predicted to have good turnover frequencies. Our active learning approach to optimize two distinct reaction energies with efficient global optimization is expected to be beneficial for the search of large catalyst spaces where no prior designs have been identified and where linear scaling relationships between reaction energies or barriers may be limited or unknown.

Anoxic oxidation of As(III) during Fe(II)-induced goethite recrystallization: Evidence and importance of Fe(IV) intermediate

Under anoxic conditions, aqueous Fe(II) (Fe(II)_aq)-induced recrystallization of iron (oxyhydr)oxides changes the speciation and geochemical cycle of trace elements in environments. Oxidation of trace element, i.e., As(III), driven by Fe(II)_aq-iron (oxyhydr)oxides interactions under anoxic condition was observed previously, but the oxidative species and involved mechanisms are remained unknown. In the present study, we explored the formed oxidative intermediates during Fe(II)_aq-induced recrystallization of goethite under anoxic conditions. The methyl phenyl sulfoxide-based probe experiment suggested the featured oxidation by Fe(IV) species in Fe(II)_aq-goethite system. Both the Mössbauer spectra and X-ray absorption near edge structure spectroscopic evidenced the generation and quenching of Fe(IV) intermediate. It was proved that the interfacial electron exchange between Fe(II)_aq and Fe(III) of goethite initiated the generation of Fe(IV). After transferring electrons to goethite, Fe(II)_aq was transformed to labile Fe(III), which was then transformed to Fe(IV) via a proton-coupled electron transfer process. This highly reactive transient Fe(IV) could quickly react with reductive species, i.e. Fe(II) or As(III). Considering the ubiquitous occurrence of Fe(II)-iron (oxyhydr)oxides reactions under anoxic conditions, our findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.

A Reactive, Photogenerated High-Spin (S = 2) FeIV(O) Complex via O2 Activation

Addition of dioxygen at low temperature to the non-heme ferrous complex Fe^II(Me₃TACN)((OSi^Ph2)₂O) (1) in 2-MeTHF produces a peroxo-bridged diferric complex Fe₂^III(μ-O₂)(Me₃TACN)₂((OSi^Ph2)₂O)₂ (2), which was characterized by UV-vis, resonance Raman, and variable field Mössbauer spectroscopies. Illumination of a frozen solution of 2 in THF with white light leads to homolytic O-O bond cleavage and generation of a Fe^IV(O) complex 4 (ν(Fe=O) = 818 cm^-1; δ = 0.22 mm s^-1, ΔE_Q = 0.23 mm s^-1). Variable field Mössbauer spectroscopy measurements show that 4 is a rare example of a high-spin S = 2 Fe^IV(O) complex and the first synthetic example to be generated directly from O₂. Complex 4 is highly reactive, as expected for a high-spin ferryl, and decays rapidly in fluid solution at cryogenic temperatures. This decay process in 2-MeTHF involves C-H cleavage of the solvent. However, the controlled photolysis of 2 in situ with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of 4 as the active oxidant.

A six-coordinate high-spin FeIVO species of cucurbit[5]uril: a highly potent catalyst for C-H hydroxylation of methane, if synthesised

DFT and ab initio DLPNO-CCSD(T) calculations predict a stable S = 2 six-coordinate Fe^IVO species with cucurbit[5]uril (CB[5]) as a ligand ([(CB[5])Fe^IVO(H₂O)]²⁺(1)). The strong oxidising capability of 1 far exceeds even that of metalloenzymes such as sMMOs in activating inert substrates such as methane, setting the stage for a new generation of biomimetic catalysts.

C-H Bond Cleavage by Bioinspired Nonheme Metal Complexes

The functionalization of C-H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C-H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C-H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C-H bond functionalization. Avenues for future research are also examined.

Spontaneous Formation of an Fe/Mn Diamond Core: Models for the Fe/Mn Sites in Class 1c Ribonucleotide Reductases

A handful of oxygen-activating enzymes has recently been found to contain Fe/Mn active sites, like Class 1c ribonucleotide reductases and R2-like ligand-binding oxidase, which are closely related to their better characterized diiron cousins. These enzymes are proposed to form high-valent intermediates with Fe-O-Mn cores. Herein, we report the first examples of synthetic Fe/Mn complexes that mimic doubly bridged intermediates proposed for enzymatic oxygen activation. Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis has been used to characterize the structures of each of these compounds. Linear compounds accurately model the Fe···Mn distances found in Fe/Mn proteins in their resting states, and doubly bridged diamond core compounds accurately model the distances found in high-valent biological intermediates. Unlike their diiron analogues, the paramagnetic nature of Fe/Mn compounds can be analyzed by EPR, revealing S = 1/2 signals that reflect antiferromagnetic coupling between the high-spin Fe(III) and Mn(III) units of heterobimetallic centers. These compounds undergo electron transfer with various ferrocenes, linear compounds being capable of oxidizing diacetyl ferrocene, a weak reductant, and diamond core compounds being capable of oxidizing acetyl ferrocene. Diamond core compounds can also perform HAT reactions from substrates with X-H bonds with bond dissociation free energies (BDFEs) up to 75 kcal/mol and are capable of oxidizing TEMPO-H at rates of 0.32-0.37 M^-1 s^-1, which are comparable to those reported for some mononuclear Fe^III-OH and Mn^III-OH compounds. However, such reactivity is not observed for the corresponding diiron compounds, a difference that Nature may have taken advantage of in evolving enzymes with heterobimetallic active sites.

Temperature-Dependent Reactivity of a Non-heme FeIII(OH)(SR) Complex: Relevance to Isopenicillin N Synthase

Non-heme iron complexes with cis-Fe^III(OH)(SAr/OAr) coordination were isolated and examined for their reactivity with a tertiary carbon radical. The sulfur-ligated complex shows a temperature dependence on ^•OH versus ArS^• transfer, whereas the oxygen-ligated complex does not. These results provide the first working model for C-S bond formation in isopenicillin N synthase and indicate that kinetic control may be a key factor in the selectivity of non-heme iron "rebound" processes.

EPR spectroscopy elucidates the electronic structure of [FeV(O)(TAML)] complexes

The complete hyperfine tensor of ¹⁷O of the Fe^V-oxo moeity was probed by ENDOR spectroscopy. The EPR spectroscopic results reported here provide a conclusive experimental basis for elucidating the electronic structure of the Fe^V-oxo complex.

Effects of Noncovalent Interactions on High-Spin Fe(IV)-Oxido Complexes

High-valent nonheme Fe^IV-oxido species are key intermediates in biological oxidation, and their properties are proposed to be influenced by the unique microenvironments present in protein active sites. Microenvironments are regulated by noncovalent interactions, such as hydrogen bonds (H-bonds) and electrostatic interactions; however, there is little quantitative information about how these interactions affect crucial properties of high valent metal-oxido complexes. To address this knowledge gap, we introduced a series of Fe^IV-oxido complexes that have the same S = 2 spin ground state as those found in nature and then systematically probed the effects of noncovalent interactions on their electronic, structural, and vibrational properties. The key design feature that provides access to these complexes is the new tripodal ligand [poat]^3-, which contains phosphinic amido groups. An important structural aspect of [Fe^IVpoat(O)]^- is the inclusion of an auxiliary site capable of binding a Lewis acid (LA^II); we used this unique feature to further modulate the electrostatic environment around the Fe-oxido unit. Experimentally, studies confirmed that H-bonds and LA^II s can interact directly with the oxido ligand in Fe^IV-oxido complexes, which weakens the Fe═O bond and has an impact on the electronic structure. We found that relatively large vibrational changes in the Fe-oxido unit correlate with small structural changes that could be difficult to measure, especially within a protein active site. Our work demonstrates the important role of noncovalent interactions on the properties of metal complexes, and that these interactions need to be considered when developing effective oxidants.

Fe2+/HClO Reaction Produces FeIVO2+: An Enhanced Advanced Oxidation Process

The reaction between Fe²⁺ and HClO constitutes a promising advanced oxidation process (AOP) for removing pollutants from wastewater, and ^•OH has been considered the dominant reactive oxidant despite limited evidence for this. Herein, we demonstrate that the Fe²⁺/HClO reaction enables the production of Fe^IVO²⁺ rather than ^•OH in acid medium, a finding that is strongly supported by multiple lines of evidence. Both X-ray absorption near-edge structure spectroscopic tests and Mössbauer spectroscopic tests confirmed the appearance of Fe^IVO²⁺ as the reactive intermediate in the reaction between Fe²⁺ and HClO. The determination of Fe^IVO²⁺ generation was also derived from the methyl phenyl sulfoxide (PMSO)-based probe experiments with respect to the formation of PMSO₂ without ^•OH adducts and the density functional theory studies according to the lower energy barrier for producing Fe^IVO²⁺ compared with ^•OH. A dual-anode electrolytic system was established for the in situ generation of Fe²⁺ and HClO that allows the production of Fe^IVO²⁺. The system exhibits an enhanced capacity for oxidizing a model pollutant (e.g., phosphite) from industrial wastewater, making it an attractive and promising AOP for the abatement of aqueous contaminants.

Theoretical Identification of the Factors Governing the Reactivity of C-H Bond Activation by Non-Heme Iron(IV)-Oxo Complexes

Selective functionalization of C-H bonds provides a straightforward approach to a large variety of well-defined derivatives. High-valent mononuclear iron(IV)-oxo complexes are proposed to carry out these C-H activation reactions in enzymes or in biomimetic syntheses. In this Minireview, we aim to highlight the features that delineate the distinct reactivity of non-heme oxo-iron(IV) motifs to cleave strong C-H bonds in hydrocarbons, primarily focusing on the hydrogen atom transfer (HAT) process. We describe how the structural and electronic properties of supporting ligands modulate the oxidative property of the iron(IV)-oxo complexes. Furthermore, we highlight the decisive role played by spin-state in these biomimetic reactions. We also discuss how tunneling and external perturbations like electric field influence the transfer of hydrogen atoms. Lastly, we emphasize how computations could work as a practical guide to sketch and develop synthetic models with greater efficacy.

Determining the Inherent Selectivity for Carbon Radical Hydroxylation versus Halogenation with FeIII(OH)(X) Complexes: Relevance to the Rebound Step in Non-heme Iron Halogenases

The first structural models of the proposed cis-Fe^III(OH)(halide) intermediate in the non-heme iron halogenases were synthesized and examined for their inherent reactivity with tertiary carbon radicals. Selective hydroxylation occurs for these cis-Fe^III(OH)(X) (X = Cl, Br) complexes in a radical rebound-like process. In contrast, a cis-Fe^III(Cl)₂ complex reacts with carbon radicals to give halogenation. These results are discussed in terms of the inherent reactivity of the analogous rebound intermediate in both enzymes and related catalysts.

Characterization of Terminal Iron(III)-Oxo and Iron(III)-Hydroxo Complexes Derived from O2 Activation

O₂ activation at nonheme iron centers is a common motif in biological systems. While synthetic models have provided numerous insights into the reactivity of high-valent iron-oxo complexes related to biological processes, the majority of these complexes are synthesized using alternative oxidants. This report describes O₂ activation by an iron(II)-triflate complex of the imino-functionalized tris(pyrrol-2-ylmethyl)amine ligand framework, H₃[N(pi^Cy)₃]. Initial reaction conditions result in the formation of a mixture of oxidation products including terminal iron(III)-oxo and iron(III)-hydroxo complexes. The relevance of these species to the O₂ activation process is demonstrated through reactivity studies and electrochemical analysis of the iron(III)-oxo complex.

Universal scaling relations for the rational design of molecular water oxidation catalysts with near-zero overpotential

A major roadblock in realizing large-scale production of hydrogen via electrochemical water splitting is the cost and inefficiency of current catalysts for the oxygen evolution reaction (OER). Computational research has driven important developments in understanding and designing heterogeneous OER catalysts using linear scaling relationships derived from computed binding energies. Herein, we interrogate 17 of the most active molecular OER catalysts, based on different transition metals (Ru, Mn, Fe, Co, Ni, and Cu), and show they obey similar scaling relations to those established for heterogeneous systems. However, we find that the conventional OER descriptor underestimates the activity for very active OER complexes as the standard approach neglects a crucial one-electron oxidation that many molecular catalysts undergo prior to O-O bond formation. Importantly, this additional step allows certain molecular catalysts to circumvent the "overpotential wall", leading to enhanced performance. With this knowledge, we establish fundamental principles for the design of ideal molecular OER catalysts.

Metal-Oxyl Species and Their Possible Roles in Chemical Oxidations

Metal-oxyl (Mⁿ⁺-O^•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal-oxo (M⁽ⁿ⁺¹⁾⁺═O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mⁿ⁺-O^• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mⁿ⁺-O^• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mⁿ⁺-O^• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

Dioxygen-Derived Nonheme Mononuclear FeIII(OH) Complex and Its Reactivity with Carbon Radicals

A new tetradentate, monoanionic, mixed N/O donor ligand (BNPA^Ph2O^-) with second coordination sphere H-bonding groups has been synthesized for stabilization of a terminal Fe^III(OH) complex. The complex Fe^II(BNPA^Ph2O)(OTf) (1) reacts with O₂ to give a mononuclear terminal Fe^III(OH) complex, Fe^III(OH)(BNPA^Ph2O)(OTf) (2), both of which were characterized by X-ray diffraction, electrospray ionization mass spectrometry, UV-vis, ¹H and ¹⁹F nuclear magnetic resonance, ⁵⁷Fe Mössbauer, and electron paramagnetic resonance spectroscopies. Treatment of 2 with carbon radicals (Ar₃C·) gives Ar₃COH and the Fe^II complex 1, in direct analogy with the elusive radical "rebound" process proposed for nonheme iron enzymes.

Multiple coordination modes of a new ditopic bis(pyrazolyl)methane-based ligand

A new ditopic ligand, N-(2,2-bis(pyrazolyl)ethyl)-2,2-bis(pyrazolyl)acetamide ((pz)₂CH-C(O)-NH-CH₂-CH(pz)₂, L4Pz, pz = pyrazolyl ring), comprising two bis(pyrazolyl)methane donor groups linked via an amide bridge, has been prepared from the reaction of HOOCCH(pz)₂ and H₂NCH₂CH(pz)₂. The ligand coordinates to various metallic salts (i.e. AgO₃SCF₃, PdCl₂, Re(CO)₅Br, and Fe(BF₄)₂), in either a κ²-μ-κ² or a κ³-μ-κ² fashion, depending on the coordination preferences of the metallic center. These compounds were characterized by NMR, UV-Vis and IR spectroscopy, and in solid state by single crystal X-ray diffraction. In the case of silver(i), a mono-dimensional coordination polymer was obtained, while the others were found to be discrete complexes. The synthesis and characterization of a heterobimetallic complex is also described. In solid state, all compounds are associated into supramolecular architectures via hydrogen bonding and pyrazolyl embrace interactions.

Structural and Electronic Responses to the Three Redox Levels of Fe(NO)N2 S2 -Fe(NO)2

The nitrosylated diiron complexes, Fe₂ (NO)₃ , of this study are interpreted as a mono-nitrosyl Fe(NO) unit, MNIU, within an N₂ S₂ ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N₂ S₂ ⋅Fe(NO)₂ ⁺ complex, 1⁺ , of Enemark-Feltham electronic notation {Fe(NO)}⁷ -{Fe(NO)₂ }⁹ , is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}⁷ -{Fe(NO)₂ }¹⁰ , 1⁰ , and {Fe(NO)}⁸ -{Fe(NO)₂ }¹⁰ , 1^- , were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S= 1 / 2 , complex, 1⁰ , finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe-N-O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe-N-O angle to 171° in 1^- . Compensating shifts in Fe_MNIU distances from the N₂ S₂ plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1^- is a triplet-state {Fe(NO)}⁸ with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment.

Models for Unsymmetrical Active Sites in Metalloproteins: Structural, Redox, and Magnetic Properties of Bimetallic Complexes with MII-(μ-OH)-FeIII Cores

Bimetallic complexes are important sites in metalloproteins but are often difficult to prepare synthetically. We have previously introduced an approach to form discrete bimetallic complexes with M^II-(μ-OH)-Fe^III (M^II = Mn, Fe) cores using the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]^3-). This series is extended to include the rest of the late 3d transition metal ions (M^II = Co, Ni, Cu, Zn). All of the bimetallic complexes have similar spectroscopic and structural properties that reflect little change despite varying the M^II centers. Magnetic studies performed on the complexes in solution using electron paramagnetic resonance spectroscopy showed that the observed spin states varied incrementally from S = 0 through S = 5/2; these results are consistent with antiferromagnetic coupling between the high-spin M^II and Fe^III centers. However, the difference in the M^II ion occupancy yielded only slight changes in the magnetic exchange coupling strength, and all complexes had J values ranging from +26(4) to +35(3) cm^-1.

Tracking the FeIV(O) intermediate and O–O bond formation of a nonheme iron catalyst for water oxidation

We describe here a new complex 1 by subtle modulation of the TPA ligand and succeed in capturing the high-valent Fe^IV(O) species, which is responsible for the O–O bond formation and oxygen evolution with higher efficiency.

Reactivity of an FeIV-Oxo Complex with Protons and Oxidants

High-valent Fe-OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme FeIV-OH complexes, we demonstrate the reversible protonation of a synthetic FeIV-oxo species containing a tris-urea tripodal ligand. The same protonated FeIV-oxo species can be prepared via oxidation, suggesting that a putative FeV-oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the FeIV-oxo complex most likely occurs on the tripodal ligand, which undergoes a structural change that results in the formation of a new intramolecular H-bond with the oxido ligand that aids in stabilizing the protonated adduct. We suggest that similar protonated high-valent Fe-oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high-valent Fe-OH species to mechanisms.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

HFEPR and Computational Studies on the Electronic Structure of a High-Spin Oxidoiron(IV) Complex in Solution

Nonheme iron enzymes perform diverse and important functions in biochemistry. The active form of these enzymes comprises the ferryl, oxidoiron(IV), [FeO](2+) unit. In enzymes, this unit is in the high-spin, quintet, S = 2, ground state, while many synthetic model compounds exist in the spin triplet, S = 1, ground state. Recently, however, Que and co-workers reported an oxidoiron(IV) complex with a quintet ground state, [FeO(TMG3tren)](OTf)2, where TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine and OTf = CF3SO3(-). The trigonal geometry imposed by this ligand, as opposed to the tetragonal geometry of earlier model complexes, favors the high-spin ground state. Although [FeO(TMG3tren)](2+) has been earlier probed by magnetic circular dichroism (MCD) and Mössbauer spectroscopies, the technique of high-frequency and -field electron paramagnetic resonance (HFEPR) is superior for describing the electronic structure of the iron(IV) center because of its ability to establish directly the spin-Hamiltonian parameters of high-spin metal centers with high precision. Herein we describe HFEPR studies on [FeO(TMG3tren)](OTf)2 generated in situ and confirm the S = 2 ground state with the following parameters: D = +4.940(5) cm(-1), E = 0.000(5), B4(0) = -14(1) × 10(-4) cm(-1), g⊥ = 2.006(2), and g∥ = 2.03(2). Extraction of a fourth-order spin-Hamiltonian parameter is unusual for HFEPR and impossible by other techniques. These experimental results are combined with state-of-the-art computational studies along with previous structural and spectroscopic results to provide a complete picture of the electronic structure of this biomimetic complex. Specifically, the calculations reproduce well the spin-Hamiltonian parameters of the complex, provide a satisfying geometrical picture of the S = 2 oxidoiron(IV) moiety, and demonstrate that the TMG3tren is an "innocent" ligand.

Redox flexibility of iron complexes supported by sulfur-based tris(o-methylenethiophenolato)amine relative to its tripodal oxygen-based congener

Tripodal ligands designed to generate a local C₃ symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties.

Molecular designs for controlling the local environments around metal ions

The functions of metal complexes are directly linked to the local environment in which they are housed; modifications to the local environment (or secondary coordination sphere) are known to produce changes in key properties of the metal centers that can affect reactivity. Noncovalent interactions are the most common and influential forces that regulate the properties of secondary coordination spheres, which leads to complexities in structure that are often difficult to achieve in synthetic systems. Using key architectural features from the active sites of metalloproteins as inspiration, we have developed molecular systems that enforce intramolecular hydrogen bonds (H-bonds) around a metal center via incorporation of H-bond donors and acceptors into rigid ligand scaffolds. We have utilized these molecular species to probe mechanistic aspects of biological dioxygen activation and water oxidation. This Account describes the stabilization and characterization of unusual M-oxo and heterobimetallic complexes. These types of species have been implicated in a range of oxidative processes in biology but are often difficult to study because of their inherent reactivity. Our H-bonding ligand systems allowed us to prepare an Fe(III)-oxo species directly from the activation of O2 that was subsequently oxidized to form a monomeric Fe(IV)-oxo species with an S = 2 spin state, similar to those species proposed as key intermediates in non-heme monooxygenases. We also demonstrated that a single Mn(III)-oxo center that was prepared from water could be converted to a high-spin Mn(V)-oxo species via stepwise oxidation, a process that mimics the oxidative charging of the oxygen-evolving complex (OEC) of photosystem II. Current mechanisms for photosynthetic O-O bond formation invoke a Mn(IV)-oxyl species rather than the isoelectronic Mn(V)-oxo system as the key oxidant based on computational studies. However, there is no experimental information to support the existence of a Mn-oxyl radical. We therefore probed the amount of spin density on the oxido ligand of our complexes using EPR spectroscopy in conjunction with oxygen-17 labeling. Our findings showed that there is a significant amount of spin on the oxido ligand, yet the M-oxo bonds are best described as highly covalent and there is no indication that an oxyl radical is formed. These results offer the intriguing possibility that high-spin M-oxo complexes are involved in O-O bond formation in biology. Ligand redesign to incorporate H-bond accepting units (sulfonamido groups) simultaneously provided a metal ion binding pocket, adjacent H-bond acceptors, and an auxiliary binding site for a second metal ion. These properties allowed us to isolate a series of heterobimetallic complexes of Fe(III) and Mn(III) in which a group II metal ion was coordinated within the secondary coordination sphere. Examination of the influence of the second metal ion on the electron transfer properties of the primary metal center revealed unexpected similarities between Ca(II) and Sr(II) ions, a result with relevance to the OEC. In addition, the presence of a second metal ion was found to prevent intramolecular oxidation of the ligand with an O atom transfer reagent.

Characterization of Paramagnetic Reactive Intermediates: Predicting the NMR Spectra of Iron(IV)-Oxo Complexes by DFT

The relative energies of spin states of several iron(IV)-oxo complexes and related species have been calculated with DFT methods by employing the B3LYP* functional. We show that such calculations can predict the correct ground spin state of Fe(IV) complexes and can then be used to determine the (1) H NMR spectra of all spin states; the spectral features are remarkably different, hence calculated paramagnetic (1) H NMR spectra can be used to support the structure elucidation of numerous paramagnetic complexes. Applications to a number of stable and reactive iron(IV)-oxo species are described.

Quantitative Interpretation of Multifrequency Multimode EPR Spectra of Metal Containing Proteins, Enzymes, and Biomimetic Complexes

Electron paramagnetic resonance (EPR) spectroscopy has long been a primary method for characterization of paramagnetic centers in materials and biological complexes. Transition metals in biological complexes have valence d-orbitals that largely define the chemistry of the metal centers. EPR spectra are distinctive for metal type, oxidation state, protein environment, substrates, and inhibitors. The study of many metal centers in proteins, enzymes, and biomimetic complexes has led to the development of a systematic methodology for quantitative interpretation of EPR spectra from a wide array of metal containing complexes. The methodology is now contained in the computer program SpinCount. SpinCount allows simulation of EPR spectra from any sample containing multiple species composed of one or two metals in any spin state. The simulations are quantitative, thus allowing determination of all species concentrations in a sample directly from spectra. This chapter will focus on applications to transition metals in biological systems using EPR spectra from multiple microwave frequencies and modes.

Lessons from Nature: A Bio-Inspired Approach to Molecular Design

Metalloproteins contain actives sites with intricate structures that perform specific functions with high selectivity and efficiency. The complexity of these systems complicates the study of their function and the understanding of the properties that give rise to their reactivity. One approach that has contributed to the current level of understanding of their biological function is the study of synthetic constructs that mimic one or more aspects of the native metalloproteins. These systems allow individual contributions to the structure and function to be analyzed and also permit spectroscopic characterization of the metal cofactors without complications from the protein environment. This Current Topic is a review of synthetic constructs as probes for understanding the biological activation of small molecules. These topics are developed from the perspective of seminal molecular design breakthroughs from the past that provide the foundation for the systems used today.

Catalytic two-electron reduction of dioxygen by ferrocene derivatives with manganese(V) corroles

Electron transfer from octamethylferrocene (Me8Fc) to the manganese(V) imidocorrole complex (tpfc)Mn(V)(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl2C6H3] proceeds efficiently to give an octamethylferrocenium ion (Me8Fc(+)) and [(tpfc)Mn(IV)(NAr)](-) in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn(IV)(NAr)](-) by Me8Fc gives (tpfc)Mn(III) and ArNH2 in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn(V)(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn(V)(OH)](+)) in deaerated MeCN. [(tpfc)Mn(V)(OH)](+) is rapidly reduced by 2 equiv of Me8Fc in the presence of TFA to give (tpfc)Mn(III) in deaerated MeCN. In the presence of dioxygen (O2), (tpfc)Mn(III) catalyzes the two-electron reduction of O2 by Me8Fc with TFA in MeCN to produce H2O2 and Me8Fc(+). The rate of formation of Me8Fc(+) in the catalytic reduction of O2 follows zeroth-order kinetics with respect to the concentrations of Me8Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn(V)(NAr) and O2. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn(III) to O2, followed by further proton-coupled electron transfer from Me8Fc to produce H2O2 and [(tpfc)Mn(IV)](+). Rapid electron transfer from Me8Fc to [(tpfc)Mn(IV)](+) regenerates (tpfc)Mn(III), completing the catalytic cycle. Thus, catalytic two-electron reduction of O2 by Me8Fc with (tpfc)Mn(V)(NAr) as a catalyst precursor proceeds via a Mn(III)/Mn(IV) redox cycle.

Redesigning the blue copper azurin into a redox-active mononuclear nonheme iron protein: preparation and study of Fe(II)-M121E azurin

Much progress has been made in designing heme and dinuclear nonheme iron enzymes. In contrast, engineering mononuclear nonheme iron enzymes is lagging, even though these enzymes belong to a large class that catalyzes quite diverse reactions. Herein we report spectroscopic and X-ray crystallographic studies of Fe(II)-M121E azurin (Az), by replacing the axial Met121 and Cu(II) in wild-type azurin (wtAz) with Glu and Fe(II), respectively. In contrast to the redox inactive Fe(II)-wtAz, the Fe(II)-M121EAz mutant can be readily oxidized by Na2IrCl6, and interestingly, the protein exhibits superoxide scavenging activity. Mössbauer and EPR spectroscopies, along with X-ray structural comparisons, revealed similarities and differences between Fe(II)-M121EAz, Fe(II)-wtAz, and superoxide reductase (SOR) and allowed design of the second generation mutant, Fe(II)-M121EM44KAz, that exhibits increased superoxide scavenging activity by 2 orders of magnitude. This finding demonstrates the importance of noncovalent secondary coordination sphere interactions in fine-tuning enzymatic activity.

Dramatically accelerated selective oxygen-atom transfer by a nonheme iron(IV)-oxo complex: tuning of the first and second coordination spheres

The new ligand N3Py^amideSR and its Fe^II complex [Fe^II(N3Py^amideSR)](BF₄)₂ (1) are described. Reaction of 1 with PhIO at −40 °C gives metastable [Fe^IV(O)(N3Py^amideSR)]²⁺ (2), containing a sulfide ligand and a single amide H-bond donor in proximity to the terminal oxo group. Direct evidence for H-bonding is seen in a structural analogue, [Fe^II(Cl)(N3Py^amideSR)](BF₄)₂ (3). Complex 2 exhibits rapid O-atom transfer (OAT) toward external sulfide substrates, but no intramolecular OAT. However, direct S-oxygenation does occur in the reaction of 1 with mCPBA, yielding sulfoxide-ligated [Fe^II(N3Py^amideS(O)R)](BF₄)₂ (4). Catalytic OAT with 1 was also observed.

Do Spin State and Spin Density Affect Hydrogen Atom Transfer Reactivity?

The prevalence of hydrogen atom transfer (HAT) reactions in chemical and biological systems has prompted much interest in establishing and understanding the underlying factors that enable this reactivity. Arguments have been advanced that the electronic spin state of the abstractor and/or the spin-density at the abstracting atom are critical for HAT reactivity. This is consistent with the intuition derived from introductory organic chemistry courses. Herein we present an alternative view on the role of spin state and spin-density in HAT reactions. After a brief introduction, the second section introduces a new and simple fundamental kinetic analysis, which shows that unpaired spin cannot be the dominant effect. The third section examines published computational studies of HAT reactions, which indicates that the spin state affects these reactions indirectly, primarily via changes in driving force. The essay concludes with a broader view of HAT reactivity, including indirect effects of spin and other properties on reactivity. It is suggested that some of the controversy in this area may arise from the diversity of HAT reactions and their overlap with proton-coupled electron transfer (PCET) reactions.