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

Using machine learning methods to predict the diabatic bond dissociation energy of non-heme iron complexes

Bond dissociation energy (BDE) is an important property in chemical research. In the process of non-heme iron complex catalytic reactions, diabatic BDE has a significant impact on the selectivity of halogenation and hydroxylation reactions. Measuring or calculating BDE by using traditional experimental or theoretical methods is often expensive and complex, so we propose the first application of machine learning on non-heme iron complexes to predict and rationalize the diabatic BDEs of Fe-X and Fe-OH bonds in order to assist in the study of selectivity in non-heme iron complex catalytic reactions. We built a reliable and representative dataset containing over 600 types of non-heme iron complexes and used density functional theory (DFT) to calculate nearly 900 diabatic BDE for machine learning. In terms of model training, we used 2D molecular fingerprints and 3D descriptors as inputs to train the regression model. The results indicate that the ensemble algorithm combined with Morgan fingerprints can effectively predict the diabatic BDEs of non-heme iron complexes. Using the Gradient Boosting Regressor (GBR) model and Morgan fingerprints can achieve an accurate prediction of R2 = 0.791 and the mean absolute error (MAE) = 10.23 kcal mol-1. The incorporation of 3D descriptors significantly improves the predictive performance of molecular fingerprints other than Morgan fingerprints. Notably, the SOAP descriptor effectively captures key 3D molecular information, making it particularly advantageous for predicting isomers with large ΔBDE. However, when the ΔBDE of isomers in the dataset is small, Morgan fingerprints remain the more efficient choice.

Influence of the second coordination sphere on O2 activation by a nonheme iron(II) thiolate complex

The synthesis and characterization of a new ligand, 1-(bis(pyridin-2-ylmethyl) amino)-2-methylpropane-2-thiolate (BPAMe2S-) and its nonheme iron complex, FeII(BPAMe2S)Br (1), is reported. Reaction of 1 with O2 at -20 °C generates a high-spin iron(III)-hydroxide complex, [FeIII(OH)(BPAMe2S)(Br)] (2), that was characterized by UV-vis, 57Fe Mössbauer, and electron paramagnetic resonance (EPR) spectroscopies, and electrospray ionization mass spectrometry (ESI-MS). Density functional theory (DFT) calculations were employed to support the spectroscopic assignments. In a previous report (J. Am. Chem. Soc.2024, 146, 7915-7921), the related iron(II) complex, FeII(BNPAMe2S)Br (BNPAMe2S- = (bis((6-(neopentylamino)pyridinyl) methyl)amino)-2-methylpropane-2-thiolate) was reported and shown to react with O2 at low temperature to give a rare iron(III)-superoxide intermediate, which then converts to an S‑oxygenated sulfinate as seen for the nonheme iron thiol dioxygenases. This complex includes two hydrogen bonding neopentylamino groups in the second coordination sphere. Complex 1 does not include these H-bonding groups, and its reactivity with O2 does not yield a stabilized Fe/O2 intermediate or S‑oxygenated products, although the data suggest an inner-sphere mechanism and formation of an iron‑oxygen species that is capable of abstracting hydrogen atoms from solvent or weak CH bond substrates. This study indicates that the H-bond donors are critical for stabilizing the FeIII(O2-•) intermediate with the BNPAMe2S- ligand, which in turn leads to S‑oxygenation, as opposed to H-atom abstraction, following O2 activation by the nonheme iron center.

Nonheme Mononuclear and Dinuclear Iron(II) and Iron(III) Fluoride Complexes and Their Fluorine Radical Transfer Reactivity

The nonheme iron(II) complexes containing a fluoride anion, FeII(BNPAPh2O)(F) (1) and [FeII(BNPAPh2OH)(F)(THF)](BF4) (2), were synthesized and structurally characterized. Addition of dioxygen to either 1 or 2 led to the formation of a fluoride-bridged, dinuclear iron(III) complex [Fe2III(BNPAPh2O)2(F)2(μ-F)]+ (4), which was characterized by single-crystal X-ray diffraction, 1H NMR, and elemental analysis. An iron(II)(iodide) complex, FeII(BNPAPh2O)(I) (3), was prepared and reacted with O2 to give the mononuclear complex cis-FeIII(BNPAPh2O)(OH)(I) (5). Addition of excess fluoride to 5 led to the formation of the oxo-bridged, dinuclear iron(III) complex [Fe2III(BNPAPh2O)2(F)2(μ-O)] (6), while the mononuclear iron(III)(fluoride) complex cis-FeIII(BNPAPh2O)(F)(Cl) (7) was prepared from the addition of excess F- to FeIII(BNPAPh2O)Cl2. The dinuclear complexes 4 and 6 were unreactive to fluorine radical transfer, but mononuclear 7 reacts with the radical substrate (p-MeO-C6H4)3C• to give the fluorine radical transfer products FeII(BNPAPh2O)(Cl) and (p-OMe-C6H4)3CF. These results show that a mononuclear FeIII(F) complex is capable of mediating fluorine radical transfer, even in the presence of second coordination sphere hydrogen bonds to the F- ligand. These findings are placed in context with what is known about the nonheme iron halogenases and related synthetic catalysts regarding their ability, or lack thereof, to mediate fluorine radical transfer reactions.

Proton-triggered chemoselective halogenation of aliphatic C-H bonds with nonheme FeIV-oxo complexes

Halogenation of aliphatic C-H bonds is a chemical transformation performed in nature by mononuclear nonheme iron dependent halogenases. The mechanism involves the formation of an iron(IV)-oxo-chloride species that abstracts the hydrogen atom from the reactive C-H bond to form a carbon-centered radical that selectively reacts with the bound chloride ligand, a process commonly referred to as halide rebound. The factors that determine the halide rebound, as opposed to the reaction with the incipient hydroxide ligand, are not clearly understood and examples of well-defined iron(IV)-oxo-halide compounds competent in C-H halogenation are scarce. In this work we have studied the reactivity of three well-defined iron(IV)-oxo complexes containing variants of the tetradentate 1-(2-pyridylmethyl)-1,4,7-triazacyclononane ligand (Pytacn). Interestingly, these compounds exhibit a change in their chemoselectivity towards the functionalization of C-H bonds under certain conditions: their reaction towards C-H bonds in the presence of a halide anionleads to exclusive oxygenation, while the addition of a superacid results in halogenation. Almost quantitative halogenation of ethylbenzene is observed when using the two systems with more sterically congested ligands and even the chlorination of strong C-H bonds such as those of cyclohexane is performed when a methyl group is present in the sixth position of the pyridine ring of the ligand. Mechanistic studies suggest that both reactions, oxygenation and halogenation, proceed through a common rate determining hydrogen atom transfer step and the presence of the acid dictates the fate of the resulting alkyl radical towards preferential halogenation over oxygenation.

Radical ligand transfer: mechanism and reactivity governed by three-component thermodynamics

Here, we demonstrate that the relationship between reactivity and thermodynamics in radical ligand transfer chemistry can be understood if this chemistry is dissected as concerted ion-electron transfer (cIET). Namely, we investigate radical ligand transfer reactions from the perspective of thermodynamic contributions to the reaction barrier: the diagonal effect of the free energy of the reaction, and the off-diagonal effect resulting from asynchronicity and frustration, which we originally derived from the thermodynamic cycle for concerted proton-electron transfer (cPET). This study on the OH transfer reaction shows that the three-component thermodynamic model goes beyond cPET chemistry, successfully capturing the changes in radical ligand transfer reactivity in a series of model FeIII-OH⋯(diflouro)cyclohexadienyl systems. We also reveal the decisive role of the off-diagonal thermodynamics in determining the reaction mechanism. Two possible OH transfer mechanisms, in which electron transfer is coupled with either OH- and OH+ transfer, are associated with two competing thermodynamic cycles. Consequently, the operative mechanism is dictated by the cycle yielding a more favorable off-diagonal effect on the barrier. In line with this thermodynamic link to the mechanism, the transferred OH group in OH-/electron transfer retains its anionic character and slightly changes its volume in going from the reactant to the transition state. In contrast, OH+/electron transfer develops an electron deficiency on OH, which is evidenced by an increase in charge and a simultaneous decrease in volume. In addition, the observations in the study suggest that an OH+/electron transfer reaction can be classified as an adiabatic radical transfer, and the OH-/electron transfer reaction as a less adiabatic ion-coupled electron transfer.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C-H Oxidation

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

A Nonheme Iron(III) Superoxide Complex Leads to Sulfur Oxygenation

A new alkylthiolate-ligated nonheme iron complex, FeII(BNPAMe2S)Br (1), is reported. Reaction of 1 with O2 at -40 °C, or reaction of the ferric form with O2•- at -80 °C, gives a rare iron(III)-superoxide intermediate, [FeIII(O2)(BNPAMe2S)]+ (2), characterized by UV-vis, 57Fe Mössbauer, ATR-FTIR, EPR, and CSIMS. Metastable 2 then converts to an S-oxygenated FeII(sulfinate) product via a sequential O atom transfer mechanism involving an iron-sulfenate intermediate. These results provide evidence for the feasibility of proposed intermediates in thiol dioxygenases.

Functional Model of Compound II of Cytochrome P450: Spectroscopic Characterization and Reactivity Studies of a FeIV-OH Complex

Herein, we show that the reaction of a mononuclear FeIII(OH) complex (1) with N-tosyliminobenzyliodinane (PhINTs) resulted in the formation of a FeIV(OH) species (3). The obtained complex 3 was characterized by an array of spectroscopic techniques and represented a rare example of a synthetic FeIV(OH) complex. The reaction of 1 with the one-electron oxidizing agent was reported to form a ligand-oxidized FeIII(OH) complex (2). 3 revealed a one-electron reduction potential of -0.22 V vs Fc+/Fc at -15 °C, which was 150 mV anodically shifted than 2 (Ered = -0.37 V vs Fc+/Fc at -15 °C), inferring 3 to be more oxidizing than 2. 3 reacted spontaneously with (4-OMe-C6H4)3C• to form (4-OMe-C6H4)3C(OH) through rebound of the OH group and displayed significantly faster reactivity than 2. Further, activation of the hydrocarbon C-H and the phenolic O-H bond by 2 and 3 was compared and showed that 3 is a stronger oxidant than 2. A detailed kinetic study established the occurrence of a concerted proton-electron transfer/hydrogen atom transfer reaction of 3. Studying one-electron reduction of 2 and 3 using decamethylferrocene (Fc*) revealed a higher ket of 3 than 2. The study established that the primary coordination sphere around Fe and the redox state of the metal center is very crucial in controlling the reactivity of high-valent Fe-OH complexes. Further, a FeIII(OMe) complex (4) was synthesized and thoroughly characterized, including X-ray structure determination. The reaction of 4 with PhINTs resulted in the formation of a FeIV(OMe) species (5), revealing the presence of two FeIV species with isomer shifts of -0.11 mm/s and = 0.17 mm/s in the Mössbauer spectrum and showed FeIV/FeIII potential at -0.36 V vs Fc+/Fc couple in acetonitrile at -15 °C. The reactivity studies of 5 were investigated and compared with the FeIV(OH) complex (3).

How Do Differences in Electronic Structure Affect the Use of Vanadium Intermediates as Mimics in Nonheme Iron Hydroxylases?

We study active-site models of nonheme iron hydroxylases and their vanadium-based mimics using density functional theory to determine if vanadyl is a faithful structural mimic. We identify crucial structural and energetic differences between ferryl and vanadyl isomers owing to the differences in their ground electronic states, i.e., high spin (HS) for Fe and low spin (LS) for V. For the succinate cofactor bound to the ferryl intermediate, we predict facile interconversion between monodentate and bidentate coordination isomers for ferryl species but difficult rearrangement for vanadyl mimics. We study isomerization of the oxo intermediate between axial and equatorial positions and find the ferryl potential energy surface to be characterized by a large barrier of ca. 10 kcal/mol that is completely absent for the vanadyl mimic. This analysis reveals even starker contrasts between Fe and V in hydroxylases than those observed for this metal substitution in nonheme halogenases. Analysis of the relative bond strengths of coordinating carboxylate ligands for Fe and V reveals that all of the ligands show stronger binding to V than Fe owing to the LS ground state of V in contrast to the HS ground state of Fe, highlighting the limitations of vanadyl mimics of native nonheme iron hydroxylases.

An artificial nickel chlorinase based on the biotin-streptavidin technology

Herein, we report on an artificial nickel chlorinase (ANCase) resulting from anchoring a biotinylated nickel-based cofactor within streptavidin (Sav). The resulting ANCase was efficient for the chlorination of diverse C(sp3)-H bonds. Guided by the X-ray analysis of the ANCase, the activity of the artificial chlorinase could be significantly improved. This approach opens interesting perspectives for late-stage functionalization of organic intermediates as it complements biocatalytic chlorination strategies.

106-fold faster C-H bond hydroxylation by a CoIII,IV2(µ-O)2 complex [via a CoIII2(µ-O)(µ-OH) intermediate] versus its FeIIIFeIV analog

The hydroxylation of C-H bonds can be carried out by the high-valent CoIII,IV2(µ-O)2 complex 2a supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a CoIII2(µ-O)(µ-OH) intermediate (3a). Complex 3a can be independently generated either by H-atom transfer (HAT) in the reaction of 2a with phenols as the H-atom donor or protonation of its conjugate base, the CoIII2(µ-O)2 complex 1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm-1 associated with the symmetric Co-O-Co stretching mode of the Co2O2 diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for 1a and 2a by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their "diamond core" structural assignments. The independent generation of 3a allows us to investigate HAT reactions of 2a with phenols in detail, measure the redox potential and pKa of the system, and calculate the O-H bond strength (DO-H) of 3a to shed light on the C-H bond activation reactivity of 2a. Complex 3a is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal 2a to be 106-fold more reactive in oxidizing hydrocarbon C-H bonds than corresponding FeIII,IV2(µ-O)2 and MnIII,IV2(µ-O)2 analogs, an unexpected outcome that raises the prospects for using CoIII,IV2(µ-O)2 species to oxidize alkane C-H bonds.

Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, FeIII(BNPAPh2O)(Lax)(Leq) (Lax/eq = N3-, NCS-, NCO-, and Cl-) have been synthesized using the previously reported BNPAPh2O- ligand. The ferrous analogs FeII(BNPAPh2O)(Lax) (Lax = N3-, NCS-, and NCO-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the FeIII complexes are six-coordinate, with one anionic ligand (Lax) in the H-bonding axial site and the other anionic ligand (Leq) in the equatorial plane, cis to the Lax ligand. The reaction of FeIII(BNPAPh2O-)(Lax)(Leq) with Ph3C• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows •N3 is the most favored for transfer in each case to the carbon radical, whereas Cl• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N3 > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPAPh2O-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.

A Guide to Tris(4-Substituted)-triphenylmethyl Radicals

Triphenylmethyl (trityl, Ph3C•) radicals have been considered the prototypical carbon-centered radical since their discovery in 1900. Tris(4-substituted)-trityls [(4-R-Ph)3C•] have since been used in many ways due to their stability, persistence, and spectroscopic activity. Despite their widespread use, existing synthetic routes toward tris(4-substituted)-trityl radicals are not reproducible and often lead to impure materials. We report here robust syntheses of six electronically varied (4-RPh)3C•, where R = NMe2, OCH3, tBu, Ph, Cl, and CF3. The characterization reported for the radicals and related compounds includes five X-ray crystal structures, electrochemical potentials, and optical spectra. Each radical is best accessed using a stepwise approach from the trityl halide, (RPh)3CCl or (RPh)3CBr, by controllably removing the halide with subsequent 1e- reduction of the trityl cation, (RPh)3C+. These syntheses afford consistently crystalline trityl radicals of high purity for further studies.

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)-H Fluorination

[Fe^II(NCCH₃)(NTB)](OTf)₂ (NTB = tris(2-benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ³-iodane (PhIF₂) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally Fe^IV(F)₂ oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F-(Fe^III)₂ product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Mechanistic Study of Chemoselectivity for Carbon Radical Hydroxylation versus Chlorination with FeIII (OH)(Cl) Complexes

The Fe^III (OH)(Cl) complex resembles the key intermediate proposed for the non-heme iron halogenases. Goldberg and co-workers reported that the Fe^III (OH)(Cl) RC reacts with triphenylmethyl radical 1 to give an exclusive hydroxylation product. To understand the chemoselectivity of the reaction of RC with 1, density functional theory (DFT) calculations have been conducted. From RC, the competing pathways were identified as the OH-transfer, Cl-transfer, and isomerization pathways. The direct Cl-transfer is more favorable than direct OH-transfer by 2.8 kcal/mol. The hydrogen bonding interactions between the hydroxyl group and the pendent amine ligand impede the direct OH-transfer from RC. Compared with the direct Cl-transfer pathway, the isomerization pathways require lower barriers. In isomer RC_iso2 , the equatorial hydroxyl group, which has smaller diabatic bond dissociation energy, prefers to transfer to form the hydroxylation product. In Fe^III (Cl)₂ RC2 and RC2_iso , the equatorial chloride group also prefers to transfer to give the chlorination product.

Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

The new nonheme iron complexes FeII(BNPAPh2O)(N3) (1), FeIII(BNPAPh2O)(OH)(N3) (2), FeII(BNPAPh2O)(OH) (3), FeIII(BNPAPh2O)(OH)(NCS) (4), FeII(BNPAPh2O)(NCS) (5), FeIII(BNPAPh2O)(NCS)2 (6), and FeIII(BNPAPh2O)(N3)2 (7) (BNPAPh2O = 2-(bis((6-(neopentylamino)pyridin-2-yl) methyl)amino)-1,1-diphenylethanolate) were synthesized and characterized by single crystal X-ray diffraction (XRD), as well as by 1H NMR, 57Fe Mössbauer, and ATR-IR spectroscopies. Complex 2 was reacted with a series of carbon radicals, ArX3C· (ArX = p-X-C6H4), analogous to the proposed radical rebound step for nonheme iron hydroxylases and halogenases. The results show that for ArX3C· (X = Cl, H, tBu), only OH· transfer occurs to give ArX3COH. However, when X = OMe, a mixture of alcohol (ArX3COH) (30%) and azide (ArX3CN3) (40%) products was obtained. These data indicate that the rebound selectivity is influenced by the electron-rich nature of the carbon radicals for the azide complex. Reaction of 2 with Ph3C· in the presence of Sc3+ or H+ reverses the selectivity, giving only the azide product. In contrast to the mixed selectivity seen for 2, the reactivity of cis-FeIII(OH)(NCS) with the X = OMe radical derivative leads only to hydroxylation. Catalytic azidation was achieved with 1 as catalyst, λ3-azidoiodane as oxidant and azide source, and Ph3CH as test substrate, giving Ph3CN3 in 84% (TON = 8). These studies show that hydroxylation is favored over azidation for nonheme iron(III) complexes, but the nature of the carbon radical can alter this selectivity. If an OH· transfer pathway can be avoided, the FeIII(N3) complexes are capable of mediating both stoichiometric and catalytic azidation.

Non-Native Anionic Ligand Binding and Reactivity in Engineered Variants of the Fe(II)- and α-Ketoglutarate-Dependent Oxygenase, SadA

Mononuclear non-heme Fe(II)- and α-ketoglutarate-dependent oxygenases (FeDOs) catalyze a site-selective C-H hydroxylation. Variants of these enzymes in which a conserved Asp/Glu residue in the Fe(II)-binding facial triad is replaced by Ala/Gly can, in some cases, bind various anionic ligands and catalyze non-native chlorination and bromination reactions. In this study, we explore the binding of different anions to an FeDO facial triad variant, SadX, and the effects of that binding on HO• vs X• rebound. We establish not only that chloride and bromide enable non-native halogenation reactions but also that all anions investigated, including azide, cyanate, formate, and fluoride, significantly accelerate and influence the site selectivity of SadX hydroxylation catalysis. Azide and cyanate also lead to the formation of products resulting from N3•, NCO•, and OCN• rebound. While fluoride rebound is not observed, the rate acceleration provided by this ligand leads us to calculate barriers for HO• and F• rebound from a putative Fe(III)(OH)(F) intermediate. These calculations suggest that the lack of fluorination is due to the relative barriers of the HO• and F• rebound transition states rather than an inaccessible barrier for F• rebound. Together, these results improve our understanding of the FeDO facial triad variant tolerance of different anionic ligands, their ability to promote rebound involving these ligands, and inherent rebound preferences relative to HO• that will aid efforts to develop non-native catalysis using these enzymes.

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.

What Drives Radical Halogenation versus Hydroxylation in Mononuclear Nonheme Iron Complexes? A Combined Experimental and Computational Study

Nonheme iron halogenases are unique enzymes in nature that selectively activate an aliphatic C–H bond of a substrate to convert it into C–X (X = Cl/Br, but not F/I). It is proposed that they generate an Fe^III(OH)(X) intermediate in their catalytic cycle. The analogous Fe^III(OH) intermediate in nonheme iron hydroxylases transfers OH^• to give alcohol product, whereas the halogenases transfer X^• to the carbon radical substrate. There remains significant debate regarding what factors control their remarkable selectivity of the halogenases. The reactivity of the complexes Fe^III(BNPA^Ph2O)(OH)(X) (X = Cl, Br) with a secondary carbon radical (R^•) is described. It is found that X^• transfer occurs with a secondary carbon radical, as opposed to OH^• transfer with tertiary radicals. Comprehensive computational studies involving density functional theory were carried out to examine the possible origins of this selectivity. The calculations reproduce the experimental findings, which indicate that halogen transfer is not observed for the tertiary radicals because of a nonproductive equilibrium that results from the endergonic nature of these reactions, despite a potentially lower reaction barrier for the halogenation pathway. In contrast, halogen transfer is favored for secondary carbon radicals, for which the halogenated product complex is thermodynamically more stable than the reactant complex. These results are rationalized by considering the relative strengths of the C–X bonds that are formed for tertiary versus secondary carbon centers. The computational analysis also shows that the reaction barrier for halogen transfer is significantly dependent on secondary coordination sphere effects, including steric and H-bonding interactions.

Taming the Chlorine Radical: Enforcing Steric Control over Chlorine-Radical-Mediated C-H Activation

Chlorine radicals readily activate C-H bonds, but the high reactivity of these intermediates precludes their use in regioselective C-H functionalization reactions. We demonstrate that the secondary coordination sphere of a metal complex can confine photoeliminated chlorine radicals and afford steric control over their reactivity. Specifically, a series of iron(III) chloride pyridinediimine complexes exhibit activity for photochemical C(sp³)-H chlorination and bromination with selectivity for primary and secondary C-H bonds, overriding thermodynamic preference for weaker tertiary C-H bonds. Transient absorption spectroscopy reveals that Cl· remains confined through formation of a Cl·|arene complex with aromatic groups on the pyridinediimine ligand. Furthermore, photocrystallography confirms that this selectivity arises from the generation of Cl· within the steric environment defined by the iron secondary coordination sphere.

Mechanistic Studies on the Hexadecafluorophthalocyanine-Iron-Catalyzed Wacker-Type Oxidation of Olefins to Ketones*

The hexadecafluorophthalocyanine-iron complex FePcF16 was recently shown to convert olefins into ketones in the presence of stoichiometric amounts of triethylsilane in ethanol at room temperature under an oxygen atmosphere. Herein, we describe an extensive mechanistic investigation for the conversion of 2-vinylnaphthalene into 2-acetylnaphthalene as model reaction. A variety of studies including deuterium- and 18 O2 -labeling experiments, ESI-MS, and 57 Fe Mössbauer spectroscopy were performed to identify the intermediates involved in the catalytic cycle of the oxidation process. Finally, a detailed and well-supported reaction mechanism for the FePcF16 -catalyzed Wacker-type oxidation is proposed.

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato^3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an Ag^I-pyrrole (η²-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C₆H₄)₃C• and 1 gave k = 1.34(3) × 10³ M^-1 s^-1 at 23 °C and 2.2(2) M^-1 s^-1 at -60 °C, ΔH^⧧ = +9.8(3) kcal mol^-1, and ΔS^⧧ = -14(1) cal mol^-1 K^-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 10⁵ M^-1 s^-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F^- > Cl^- > OH^- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

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.

Synthesis of a series of M(II) (M = Mn, Fe, Co) chloride complexes with both inter- and intra-ligand hydrogen bonding interactions

Hydrogen bonding networks are vital for metallo-enzymes to function; however, modeling these systems is non-trivial. We report the synthesis of metal chloride (M = Mn, Fe, Co) complexes with intra- and inter-ligand hydrogen bonding interactions. The intra-ligand hydrogen bonds are shown to have a profound effect on the geometry of the metal center.

Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase

Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol^-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.

Iron-Catalyzed Wacker-type Oxidation of Olefins at Room Temperature with 1,3-Diketones or Neocuproine as Ligands*

Herein, we describe a convenient and general method for the oxidation of olefins to ketones using either tris(dibenzoylmethanato)iron(III) [Fe(dbm)₃ ] or a combination of iron(II) chloride and neocuproine (2,9-dimethyl-1,10-phenanthroline) as catalysts and phenylsilane (PhSiH₃ ) as additive. All reactions proceed efficiently at room temperature using air as sole oxidant. This transformation has been applied to a variety of substrates, is operationally simple, proceeds under mild reaction conditions, and shows a high functional-group tolerance. The ketones are formed smoothly in up to 97 % yield and with 100 % regioselectivity, while the corresponding alcohols were observed as by-products. Labeling experiments showed that an incorporated hydrogen atom originates from the phenylsilane. The oxygen atom of the ketone as well as of the alcohol derives from the ambient atmosphere.

Characterization and reactivity study of non-heme high-valent iron-hydroxo complexes

A terminal Fe^IIIOH complex, [Fe^III(L)(OH)]²⁻ (1), has been synthesized and structurally characterized (H₄L = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene). The oxidation reaction of 1 with one equiv. of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBAH) or ceric ammonium nitrate (CAN) in acetonitrile at −45 °C results in the formation of a Fe^IIIOH ligand radical complex, [Fe^III(L˙)(OH)]⁻ (2), which is hereby characterized by UV-visible, ¹H nuclear magnetic resonance, electron paramagnetic resonance, and X-ray absorption spectroscopy techniques. The reaction of 2 with a triphenylcarbon radical further gives triphenylmethanol and mimics the so-called oxygen rebound step of Cpd II of cytochrome P450. Furthermore, the reaction of 2 was explored with different 4-substituted-2,6-di-tert-butylphenols. Based on kinetic analysis, a hydrogen atom transfer (HAT) mechanism has been established. A pK_a value of 19.3 and a BDFE value of 78.2 kcal/mol have been estimated for complex 2.

One-electron oxidation of an Fe^III–OH complex (1) results in the formation of a Fe^III–OH ligand radical complex (2). Its reaction with (C₆H₅)₃C˙ results in the formation of (C₆H₅)₃COH, which is a functional mimic of compound II of cytochrome P450.

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.

Hydroxyl Transfer to Carbon Radicals by Mn(OH) vs Fe(OH) Corrole Complexes

The transfer of •OH from metal-hydroxo species to carbon radicals (R•) to give hydroxylated products (ROH) is a fundamental process in metal-mediated heme and nonheme C-H bond oxidations. This step, often referred to as the hydroxyl "rebound" step, is typically very fast, making direct study of this process challenging if not impossible. In this report, we describe the reactions of the synthetic models M(OH)(ttppc) (M = Fe (1), Mn (3); ttppc = 5,10,15-tris(2,4,6-triphenyl)phenyl corrolato^3-) with a series of triphenylmethyl carbon radical (R•) derivatives ((4-X-C₆H₄)₃C•; X = OMe, tBu, Ph, Cl, CN) to give the one-electron reduced M^III(ttppc) complexes and ROH products. Rate constants for 3 for the different radicals ranged from 11.4(1) to 58.4(2) M^-1 s^-1, as compared to those for 1, which fall between 0.74(2) and 357(4) M^-1 s^-1. Linear correlations for Hammett and Marcus plots for both Mn and Fe were observed, and the small magnitudes of the slopes for both correlations imply a concerted •OH transfer reaction for both metals. Eyring analyses of reactions for 1 and 3 with (4-X-C₆H₄)₃C• (X = tBu, CN) also give good linear correlations, and a comparison of the resulting activation parameters highlight the importance of entropy in these •OH transfer reactions. Density functional theory calculations of the reaction profiles show a concerted process with one transition state for all radicals investigated and help to explain the electronic features of the OH rebound process.