O2 Activation by Non-Heme Thiolate-Based Dinuclear Fe Complexes.

Iron centers featuring thiolates in their metal coordination sphere (as ligands or substrates) are well-known to activate dioxygen. Both heme and non-heme centers that contain iron-thiolate bonds are found in nature. Investigating the ability of iron-thiolate model complexes to activate O2 is expected to improve the understanding of the key factors that direct reactivity to either iron or sulfur. We report here the structural and redox properties of a thiolate-based dinuclear Fe complex, [FeII2(LS)2] (LS2- = 2,2'-(2,2'-bipyridine-6,6'-iyl)bis(1,1-diphenylethanethiolate)), and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [FeII2(LS)(LSH)]+. When reaction with O2 occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [FeII2(LS)(LSH)]+), unsupported µ-oxo or µ-hydroxo FeIII dinuclear complexes ([FeIII2(LS)2O] and [FeIII2(LS)2(OH)]+, respectively) are generated. [FeIII2(LS)2O], reported previously but isolated here for the first time from O2 activation, is characterized by single crystal X-ray diffraction and Mössbauer, resonance Raman, and NMR spectroscopies. The addition of protons leads to the release of water and the generation of a mixture of two Fe-based "oxygen-free" species. Density functional theory calculations provide insight into the formation of the µ-oxo or µ-hydroxo FeIII dimers, suggesting that a dinuclear µ-peroxo FeIII intermediate is key to reactivity, and the structure of which changes as a function of protonation state. Compared to previously reported Mn-thiolate analogues, the evolution of the peroxo intermediates to the final products is different and involves a comproportionation vs a dismutation process for the Mn and Fe derivate, respectively.

Bioengineering of Cytochrome P450 OleTJE: How Does Substrate Positioning Affect the Product Distributions?

The cytochromes P450 are versatile enzymes found in all forms of life. Most P450s use dioxygen on a heme center to activate substrates, but one class of P450s utilizes hydrogen peroxide instead. Within the class of P450 peroxygenases, the P450 OleTJE isozyme binds fatty acid substrates and converts them into a range of products through the α-hydroxylation, ß-hydroxylation and decarboxylation of the substrate. The latter produces hydrocarbon products and hence can be used as biofuels. The origin of these product distributions is unclear, and, as such, we decided to investigate substrate positioning in the active site and find out what the effect is on the chemoselectivity of the reaction. In this work we present a detailed computational study on the wild-type and engineered structures of P450 OleTJE using a combination of density functional theory and quantum mechanics/molecular mechanics methods. We initially explore the wild-type structure with a variety of methods and models and show that various substrate activation transition states are close in energy and hence small perturbations as through the protein may affect product distributions. We then engineered the protein by generating an in silico model of the double mutant Asn242Arg/Arg245Asn that moves the position of an active site Arg residue in the substrate-binding pocket that is known to form a salt-bridge with the substrate. The substrate activation by the iron(IV)-oxo heme cation radical species (Compound I) was again studied using quantum mechanics/molecular mechanics (QM/MM) methods. Dramatic differences in reactivity patterns, barrier heights and structure are seen, which shows the importance of correct substrate positioning in the protein and the effect of the second-coordination sphere on the selectivity and activity of enzymes.

A Non-Heme Diiron Complex for (Electro)catalytic Reduction of Dioxygen: Tuning the Selectivity through Electron Delivery.

In the oxygen reduction reaction (ORR) domain, the investigation of new homogeneous catalysts is a crucial step toward the full comprehension of the key structural and/or electronic factors that control catalytic efficiency and selectivity. Herein, we report a unique non-heme diiron complex that can act as a homogeneous ORR catalyst in acetonitrile solution. This iron(II) thiolate dinuclear complex, [FeII2(LS)(LSH)] ([Fe2SH]+) (LS2- = 2,2'-(2,2'-bipyridine-6,6'-diyl)bis(1,1-diphenylethanethiolate)) contains a thiol group in the metal coordination sphere. [Fe2SH]+ is an efficient ORR catalyst both in the presence of a one-electron reducing agent and under electrochemically assisted conditions. However, its selectivity is dependent on the electron delivery pathway; in particular, the process is selective for H2O2 production under chemical conditions (up to ∼95%), whereas H2O is the main product during electrocatalysis (less than ∼10% H2O2). Based on computational work alongside the experimental data, a mechanistic proposal is discussed that rationalizes the selective and tunable reduction of dioxygen.

The Equatorial Ligand Effect on the Properties and Reactivity of Iron(V) Oxo Intermediates.

High-valent metal oxo oxidants are common catalytic-cycle intermediates in enzymes and known to be highly reactive. To understand which features of these oxidants affect their reactivity, a series of biomimetic iron(V) oxo oxidants with peripherally substituted biuret-modified tetraamido macrocyclic ligands were synthesized and characterized. Major shifts in the UV/Vis absorption as a result of replacing a group in the equatorial plane of the iron(V) oxo species were found. Further characterization by EPR spectroscopy, ESI-MS, and resonance Raman spectroscopy revealed differences in structure and the electronic configuration of these complexes. A systematic reactivity study with a range of substrates was performed and showed that the reactions are affected by electron-withdrawing substituents in the equatorial ligand, which enhance the reaction rate by almost 1016 orders of magnitude. Thus, the long-range electrostatic perturbations have a major influence on the rate constant. Finally, computational studies identified the various electronic contributions to the rate-determining reaction step and explained how the equatorial ligand periphery affects the properties of the oxidant.

Hydrogen by Deuterium Substitution in an Aldehyde Tunes the Regioselectivity by a Nonheme Manganese(III)-Peroxo Complex.

Mononuclear nonheme MnIII -peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side-on MnIII -peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C-H by C-D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.

Solvent- and Halide-Induced (Inter)conversion between Iron(II)-Disulfide and Iron(III)-Thiolate Complexes.

Disulfide/thiolate interconversion controlled by Cu is proposed to be involved in relevant biological processes. In analogy to Cu, it can be envisaged that Fe also participates in the control of similar biological processes. We describe here Fe complexes that undergo FeIII -thiolate/FeII -disulfide (inter)conversion mediated by halide (de)coordination, and by the nature of the solvent. The dinuclear FeII -disulfide complex [FeII2 (LSSL)]2+ ((LS)2- =2,2'-(2,2'-bipyridine-6,6'-diyl)bis(1,1-diphenylethanethiolate), (LSSL)2- =the corresponding disulfide ligand) shows solvent-dependent properties. Whereas in a non-coordinating solvent (CH2 Cl2 ) the dinuclear FeII -disulfide complex is the only stable form, in the presence of coordinating solvents like MeCN or DMF it is partly or fully converted into mononuclear FeIII -thiolate species having a bound solvent molecule ([FeIII (LS)(Solv)]+ , Solv=DMF, MeCN). Addition of Cl- to a CH2 Cl2 solution containing the FeII -disulfide dinuclear complex leads to the fast and quantitative formation of a mononuclear FeIII -thiolate species with a bound Cl- , that is, ([FeIII (LS)Cl]). The reverse reaction can be achieved by addition of Li[[B(C6 F5 )4 ]. In relation to the metal-sulfur electronic distribution, the comparison between the redox properties of the Fe, Mn and Co complexes involved in these MIII -thiolate/MII -disulfide interconversion processes allow one to rationalize their respective efficiency.

Keto-Enol Tautomerization Triggers an Electrophilic Aldehyde Deformylation Reaction by a Nonheme Manganese(III)-Peroxo Complex.

Oxygen atom transfer by high-valent enzymatic intermediates remains an enigma in chemical catalysis. In particular, manganese is an important first-row metal involved in key biochemical processes, including the biosynthesis of molecular oxygen (through the photosystem II complex) and biodegradation of toxic superoxide to hydrogen peroxide by superoxide dismutase. Biomimetic models of these biological systems have been developed to gain understanding on the structure and properties of short-lived intermediates but also with the aim to create environmentally benign oxidants. In this work, we report a combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands. Both manganese(III)-peroxo complexes are characterized by UV-vis and mass spectrometry techniques, and their reactivity patterns with aldehydes was investigated. We find a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reaction, which enables a keto-enol tautomerization in the substrate. This is an essential step in the mechanism that makes an electrophilic attack on the olefin bond possible as the attack on the aldehyde carbonyl is too high in energy. Kinetics studies determine a large kinetic isotope effect for the replacement of the transferring hydrogen atom by deuterium, while replacing the transferring hydrogen atom by a methyl group makes the substrate inactive and hence confirm the hypothesized mechanism. Our new mechanism is confirmed with density functional theory modeling on the full mechanism and rationalized through valence bond and thermochemical cycles. Our unprecedented new mechanism may have relevance to biological and biomimetic chemistry processes in general and gives insight into the reactivity patterns of metal-peroxo and metal-hydroperoxo intermediates in general.

Oxygen Atom Transfer Using an Iron(IV)-Oxo Embedded in a Tetracyclic N-Heterocyclic Carbene System: How Does the Reactivity Compare to Cytochrome P450 Compound†I?

N-Heterocyclic carbenes (NHC) are commonly featured as ligands in transition metal catalysis. Recently, a cyclic system containing four NHC groups with a central iron atom was synthesized and its iron(IV)-oxo species, [FeIV (O)(cNHC4 )]2+ , was characterized. This tetracyclic NHC ligand system may give the iron(IV)-oxo species unique catalytic properties as compared to traditional non-heme and heme iron ligand systems. Therefore, we performed a computational study on the structure and reactivity of the [FeIV (O)(cNHC4 )]2+ complex in substrate hydroxylation and epoxidation reactions. The reactivity patterns are compared with cytochrome P450 Compound I and non-heme iron(IV)-oxo models and it is shown that the [FeIV (O)(cNHC4 )]2+ system is an effective oxidant with oxidative power analogous to P450 Compound I. Unfortunately, in polar solvents, a solvent molecule will bind to the sixth ligand position and decrease the catalytic activity of the oxidant. A molecular orbital and valence bond analysis provides insight into the origin of the reactivity differences and makes predictions of how to further exploit these systems in chemical catalysis.

Modulation of Antimalarial Activity at a Putative Bisquinoline Receptor In Vivo Using Fluorinated Bisquinolines.

Antimalarials can interact with heme covalently, by π⋅⋅⋅π interactions or by hydrogen bonding. Consequently, the prototropy of 4-aminoquinolines and quinoline methanols was investigated by using quantum mechanics. Calculations showed mefloquine protonated preferentially at the piperidine and was impeded at the endocyclic nitrogen because of electronic rather than steric factors. In gas-phase calculations, 7-substituted mono- and bis-4-aminoquinolines were preferentially protonated at the endocyclic quinoline nitrogen. By contrast, compounds with a trifluoromethyl substituent on both the 2- and 8-positions, reversed the order of protonation, which now favored the exocyclic secondary amine nitrogen at the 4-position. Loss of antimalarial efficacy by CF3 groups simultaneously occupying the 2- and 8-positions was recovered if the CF3 group occupied the 7-position. Hence, trifluoromethyl groups buttressing the quinolinyl nitrogen shifted binding of antimalarials to hematin, enabling switching from endocyclic to the exocyclic N. Both theoretical calculations (DFT calculations: B3LYP/BS1) and crystal structure of (±)-trans-N1 ,N2 -bis-(2,8-ditrifluoromethylquinolin-4-yl)cyclohexane-1,2-diamine were used to reveal the preferred mode(s) of interaction with hematin. The order of antimalarial activity in vivo followed the capacity for a redox change of the iron(III) state, which has important implications for the future rational design of 4-aminoquinoline antimalarials.

A High-Valent Non-Heme µ-Oxo Manganese(IV) Dimer Generated from a Thiolate-Bound Manganese(II) Complex and Dioxygen.

This study deals with the unprecedented reactivity of dinuclear non-heme MnII -thiolate complexes with O2 , which dependent on the protonation state of the initial MnII dimer selectively generates either a di-µ-oxo or µ-oxo-µ-hydroxo MnIV complex. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction and mass spectrometry. Oxygenation reactions carried out with labeled 18 O2 unambiguously show that the oxygen atoms present in the MnIV dimers originate from O2 . Based on experimental observations and DFT calculations, evidence is provided that these MnIV species comproportionate with a MnII precursor to yield µ-oxo and/or µ-hydroxo MnIII dimers. Our work highlights the delicate balance of reaction conditions to control the synthesis of non-heme high-valent µ-oxo and µ-hydroxo Mn species from MnII precursors and O2 .

Singlet versus Triplet Reactivity in an Mn(V)-Oxo Species: Testing Theoretical Predictions Against Experimental Evidence.

Discerning the factors that control the reactivity of high-valent metal-oxo species is critical to both an understanding of metalloenzyme reactivity and related transition metal catalysts. Computational studies have suggested that an excited higher spin state in a number of metal-oxo species can provide a lower energy barrier for oxidation reactions, leading to the conclusion that this unobserved higher spin state complex should be considered as the active oxidant. However, testing these computational predictions by experiment is difficult and has rarely been accomplished. Herein, we describe a detailed computational study on the role of spin state in the reactivity of a high-valent manganese(V)-oxo complex with para-Z-substituted thioanisoles and utilize experimental evidence to distinguish between the theoretical results. The calculations show an unusual change in mechanism occurs for the dominant singlet spin state that correlates with the electron-donating property of the para-Z substituent, while this change is not observed on the triplet spin state. Minimum energy crossing point calculations predict small spin-orbit coupling constants making the spin state change from low spin to high spin unlikely. The trends in reactivity for the para-Z-substituted thioanisole derivatives provide an experimental measure for the spin state reactivity in manganese-oxo corrolazine complexes. Hence, the calculations show that the V-shaped Hammett plot is reproduced by the singlet surface but not by the triplet state trend. The substituent effect is explained with valence bond models, which confirm a change from an electrophilic to a nucleophilic mechanism through a change of substituent.

A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I: A Low-Pressure Mass Spectrometry and Computational Study.

Cytochrome P450 enzymes are heme-containing mono-oxygenases that mainly react through oxygen-atom transfer. Specific features of substrate and oxidant that determine the reaction rate constant for oxygen atom transfer are still poorly understood and therefore, we did a systematic gas-phase study on reactions by iron(IV)-oxo porphyrin cation radical structures with arenes. We present herein the first results obtained by using Fourier transform-ion cyclotron resonance mass spectrometry and provide rate constants and product distributions for the assayed reactions. Product distributions and kinetic isotope effect studies implicate a rate-determining aromatic hydroxylation reaction that correlates with the ionization energy of the substrate and no evidence of aliphatic hydroxylation products is observed. To further understand the details of the reaction mechanism, a computational study on a model complex was performed. These studies confirm the experimental hypothesis of dominant aromatic over aliphatic hydroxylation and show that the lack of an axial ligand affects the aliphatic pathways. Moreover, a two-parabola valence bond model is used to rationalize the rate constant and identify key properties of the oxidant and substrate that drive the reaction. In particular, the work shows that aromatic hydroxylation rates correlate with the ionization energy of the substrate as well as with the electron affinity of the oxidant.

Substrate Sulfoxidation by an Iron(IV)-Oxo Complex: Benchmarking Computationally Calculated Barrier Heights to Experiment.

High-valent metal-oxo oxidants are common reactive species in synthetic catalysts as well as heme and nonheme iron enzymes. In general, they efficiently react with substrates through oxygen atom transfer, and for a number of cases, experimental rate constants have been determined. However, because these rate constants are generally measured in a polar solution, it has been found difficult to find computational methodologies to reproduce experimental trends and reactivities. In this work, we present a detailed computational study into para-substituted thioanisole sulfoxidation by a nonheme iron(IV)-oxo complex. A range of density functional theory methods and basis sets has been tested for their suitability to describe the reaction mechanism and compared with experimentally obtained free energies of activation. It is found that the enthalpy of activation is reproduced well, but all methods overestimate the entropy of activation by about 50%, for which we recommend a correction factor. The effect of solvent and dispersion on the barrier heights is explored both at the single-point level and also through inclusion in geometry optimizations, and particularly, solvent is seen as highly beneficial to reproduce experimental free energies of activation. Interestingly, in general, experimental trends and Hammett plots are reproduced well with almost all methods and procedures, and only a systematic error seems to apply for these chemical systems. Very good agreement between experiment and theory is found for a number of different methods, including B3LYP and PBE0, and procedures that are highlighted in the paper.

A high-resolution fMRI approach to characterize functionally distinct neural pathways within dopaminergic midbrain and nucleus accumbens during reward and salience processing.

Processing of reward and salience without reward association are known to critically rely on the dopamine system. A growing body of evidence from animal studies suggests that both functions may be subserved by distinct subregions in midbrain and ventral striatum, specifically nucleus accumbens (NAcc). Yet in vivo investigation of these brain structures in humans has been rare. Here we examined blood oxygen level dependent signals in response to frequently presented rewarding events and infrequently presented neutral events in 20 healthy subjects using high-resolution functional magnetic resonance imaging (fMRI) for imaging the human midbrain and NAcc. The present findings revealed distinct activation patterns in brain regions of interest, namely increased activation in substantia nigra pars compacta (SNc) and dorsolateral NAcc in response to neutral events, while the VTA and both the ventromedial and dorsolateral NAcc were significantly activated due to rewarding events. Moreover, psychophysiological interaction analyses demonstrated regionally specialized processing pathways, such as a dorsolateral pathway when processing salience per se, i.e. increased functional interactions between SNc, dorsolateral NAcc and dorsolateral and medial prefrontal cortex (PFC); and a ventromedial pathway during reward processing, i.e. increased functional coupling between VTA and ventromedial NAcc. Thus, these findings may not only accelerate the integration of animal models of brain function with human neuroscience but may also improve diagnosis and treatment in patients with neuropsychiatric disorders such as schizophrenia and depression in which dopaminergic dysfunction and aberrant attribution of salience have been implicated.

Sluggish reactivity by a nonheme iron(iv)-tosylimido complex as compared to its oxo analogue.

High-valent iron-nitrido intermediates have been postulated as reactive intermediates in various enzymes, including the nitrogenases and the cytochromes P450, but so far few have been trapped and characterized. As little is known about their oxidative and spectroscopic properties, we decided to create biomimetic models of iron(iv)-imido complexes and compare their structure and reactivity with analogous iron(iv)-oxo systems. In this work we report the synthesis and spectroscopic characterization of a novel [FeIV(NTs)(Bntpen)]2+ complex (Bntpen = N1-benzyl-N1,N2,N2-tris(pyridine-2-ylmethyl)ethane-1,2-diamine) and study its reactivity patterns with respect to hydrogen atom abstraction and nitrogen atom transfer reactions. The work is compared with analogous pentadentate ligand systems as well as with iron(iv)-oxo species with the same ligand features and highlights the differences in chemical properties and reactivity patterns. It is shown that the reactivity is dependent on the metal ligand system that affects the physicochemical properties of the oxidant such as the redox potential, which is the main driving force for the reaction mechanism with substrates.

Equatorial ligand plane perturbations lead to a spin-state change in an iron(iii) porphyrin dimer.

A complete reversal of the spin state of iron(iii) is observed upon a small change to the diporphyrin bridge from ethane to ethene by keeping all other factors intact. Combined analysis using single crystal X-ray structure determination, Mössbauer, variable-temperature magnetic, 1H NMR and EPR studies has confirmed the spin states of iron(iii) complexes both in solid and solution phases.

Mechanistic Studies of Fatty Acid Activation by CYP152 Peroxygenases Reveal Unexpected Desaturase Activity.

The majority of cytochrome P450 enzymes (CYPs) predominantly operate as monooxygenases, but recently a class of P450 enzymes was discovered, that can act as peroxygenases (CYP152). These enzymes convert fatty acids through oxidative decarboxylation, yielding terminal alkenes, and through α- and ß-hydroxylation to yield hydroxy-fatty acids. Bioderived olefins may serve as biofuels, and hence understanding the mechanism and substrate scope of this class of enzymes is important. In this work, we report on the substrate scope and catalytic promiscuity of CYP OleTJE and two of its orthologues from the CYP152 family, utilizing α-monosubstituted branched carboxylic acids. We identify α,ß-desaturation as an unexpected dominant pathway for CYP OleTJE with 2-methylbutyric acid. To rationalize product distributions arising from α/ß-hydroxylation, oxidative decarboxylation, and desaturation depending on the substrate's structure and binding pattern, a computational study was performed based on an active site complex of CYP OleTJE containing the heme cofactor in the substrate binding pocket and 2-methylbutyric acid as substrate. It is shown that substrate positioning determines the accessibility of the oxidizing species (Compound I) to the substrate and hence the regio- and chemoselectivity of the reaction. Furthermore, the results show that, for 2-methylbutyric acid, α,ß-desaturation is favorable because of a rate-determining α-hydrogen atom abstraction, which cannot proceed to decarboxylation. Moreover, substrate hydroxylation is energetically impeded due to the tight shape and size of the substrate binding pocket.

Catalytic Mechanism of Nogalamycin Monoxygenase: How Does Nature Synthesize Antibiotics without a Metal Cofactor?

Nogalamycin monoxygenase (NMO) is a member of a family of enzymes that catalyze a key step in the biosynthesis of tetracycline antibiotics used to treat, for example, breast cancer in humans, using molecular oxygen for substrate oxidation but without an apparent cofactor. As most monoxygenases and dioxygenases contain a transition metal center (Fe/Cu) or flavin, this begs the question how NMO catalyzes this unusual oxygen atom transfer reaction from molecular oxygen to substrate directly. We performed a detailed computational study on the mechanism and catalytic cycle of NMO using density functional theory and quantum mechanics/molecular mechanics on the full protein. We considered the substrate in various protonation states and its reaction with oxidant O2 as well as O2-• through either electron transfer, proton transfer, or hydrogen atom transfer. The lowest energy pathway for the models presented here is a reaction of the neutral substrate with a superoxo anion radical (O2-•). In the absence of available free superoxo anions, however, the alternative neutral pathway between 3O2 and the substrate may be accessible at room temperature, although the barrier is higher in energy by about 20 kcal mol-1 and therefore the reaction will be much slower. In contrast to previous experimental findings for both the enzymatic and uncatalyzed reactions, the mechanisms with the substrate in its deprotonated state were found to be high in energy, and therefore mechanistic suggestions are proposed. A thermodynamic analysis shows that the substrate has a very weak C-H bond that can be activated by a weak oxidant, and hence, a metal cofactor may not be needed for oxidizing this particular substrate. Finally, site-directed mutations were studied where active-site Asn residues were replaced, and the function of these residues in guiding oxygen to the C12-position of the substrate was highlighted. Overall, NMO shows a versatile reactivity pattern, where the substrate can be activated by several low-energy pathways with oxidants and substrates in various oxidation and protonation states.

Dramatic rate-enhancement of oxygen atom transfer by an iron(iv)-oxo species by equatorial ligand field perturbations.

Nonheme iron dioxygenases are efficient enzymes with relevance for human health that regio- and stereospecifically transfer an oxygen atom to substrates. How they perform this task with such selectivity remains unknown, but may have to do with substrate binding, positioning and oxidant approach. To understand substrate approach on a catalytic reaction centre, we investigated the structure and reactivity of a biomimetic oxidant with ligand features that affect the interactions between oxidant and substrate. Thus, we report here the synthesis and characterization of an iron(iv)-oxo complex with pentadentate nonheme ligand, where structurally induced perturbations in the equatorial ligand field affect the spectroscopy and reactivity of the complex. We tested the activity of the complex with respect to oxygen atom transfer to and hydrogen atom abstraction from substrates. This oxidant shows improved reaction rates toward heteroatom oxidation with respect to the nonsubstituted ligand complex by ∼104 fold. The origin of the enhanced reactivity is explained with a series of density functional theory studies that show an enhanced electron affinity of the oxidant through equatorial ligand perturbations.