Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine

Reactivity of a heterobinuclear heme-peroxo-Cu complex with para-substituted catechols shows a pK a-dependent change in mechanism

In biological systems, heme-copper oxidase (HCO) enzymes play a crucial role in the oxygen reduction reaction (ORR), where the pivotal O-O bond cleavage of the (heme)FeIII-peroxo-CuII intermediate is facilitated by active-site (peroxo core) hydrogen bonding followed by proton-coupled electron transfer (PCET) from a nearby (phenolic) tyrosine residue. A useful approach to comprehend the fundamental relationships among H-bonding/proton/H-atom donors and their abilities to induce O-O bond homolysis involves the investigation of synthetic, bioinspired model systems where the exogenous substrate properties (such as pK a and bond dissociation energy (BDE)) can be systematically altered. This report details the reactivity of a heme-peroxo-copper HCO model complex (LS-4DCHIm) toward a series of substituted catechol substrates that span a range of pK a and O-H bond BDE values, exhibiting different reaction mechanisms. Considering their interactions with the bridging peroxo ligand in LS-4DCHIm, the catechol substrates are importantly capable of one or two (i) H-bonds, (ii) proton transfers, and/or (iii) net H-atom transfers, thereby making them attractive, yet complex candidates for studying the redox chemistry of the metal-bound peroxide. A combination of spectroscopic studies and kinetic analysis implies that the suitable modulation of pK a and O-H bond BDE values of catechols result in either double proton transfer with the release of H2O2 or double PCET resulting in reductive O-O bond rupture. The distinguishing role of substrate properties in directing the mechanism and outcome of O2 protonation/reduction reactions is discussed in terms of designing O2-reduction catalysts based on biological inspiration.

Monomeric Fe(III)-Hydroxo and Fe(III)-Aqua Complexes Display Oxidative Asynchronous Hydrogen Atom Abstraction Reactivity

This paper presents the synthesis and characterization of a series of novel monomeric aqua-ligated iron(III) complexes, [FeIII(L5R)(OH2)]2+ (R=OMe, H, Cl, NO2), supported by an amide-containing pentadentate N5 donor ligand, L5R [HL5R=2-(((1-methyl-1H-imidazol-2-yl)methyl)(pyridin-2-yl-methyl)amino)-N-(5-R-quinolin-8-yl)acetamide]. The complexes were characterized by various spectroscopic and analytical techniques, including electrochemistry and magnetic measurements. The Fe(III)-hydroxo complexes, [FeIII(L5R)(OH)]1+, were generated in situ by deprotonating the corresponding aqua complexes in a pH ~7 aqueous medium. In another way, adding one equivalent of a base to a methanolic solution of the Fe(III)-aqua complexes also produced the Fe(III)-hydroxo complexes. The study uses linoleic fatty acid as a substrate to explore the hydrogen atom abstraction (HAA) reactivity of both hydroxo and aqua complexes. The investigation highlights the substitution effect of the L5R ligand on reactivity, revealing a higher rate when an electron-withdrawing group is present. Hammett analyses and(or) determination of the asynchronicity factor (η) suggest an oxidative asynchronous concerted proton-electron transfer (CPET) pathway for the HAA reactions. Aqua complexes exhibited a higher asynchronicity in CPET, resulting in higher reaction rates than their hydroxo analogs. Overall, the work provides insights into the beneficial role of a higher imbalance in electron-transfer-proton-transfer (ET-PT) contributions in HAA reactivity.

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

My Vision of Electric-Field-Aided Chemistry in 2050

This manuscript outlines my outlook on the development of electric-field (EF)-mediated-chemistry and the vision of its state by 2050. I discuss applications of oriented-external electric-fields (OEEFs) on chemical reactions and proceed with relevant experimental verifications. Subsequently, the Perspective outlines other ways of generating EFs, e.g., by use of pH-switchable charges, ionic additives, water droplets, and so on. A special section summarizes conceptual principles for understanding and predicting OEEF effects, e.g., the "reaction-axis rule", the capability of OEEFs to act as tweezers that orient reactants and accelerate their reaction, etc. Finally, I discuss applications of OEEFs in continuous-flow setups, which may, in principle, scale-up to molar concentrations. The Perspective ends with the vision that by 2050, OEEF usage will change chemical education, if not also the art of making new molecules.

C-H Bond Activation by a Seven-Coordinate Bipyridine-Bipyrazole Ruthenium(IV) Oxo Complex

Ruthenium-oxo species with high coordination numbers have long been proposed as active intermediates in catalytic oxidation chemistry. By employing a tetradentate bipyridine-bipyrazole ligand, we herein reported the synthesis of a seven-coordinate (CN7) ruthenium(IV) oxo complex, [RuIV(tpz)(pic)2(O)]2+ (RuIVO) (tpz = 6,6'-di(1H-pyrazol-1-yl)-2,2'-bipyridine, pic = 4-picoline), which exhibits high activity toward the oxidation of alkylaromatic hydrocarbons. The large kinetic isotope effects (KIE) for the oxidation of DHA/DHA-d4 (KIE = 10.3 ± 0.1) and xanthene/xanthene-d2 (KIE = 17.2 ± 0.1), as well as the linear relationship between log (rate constants) and bond dissociation energies of alkylaromatics, confirmed a mechanism of hydrogen atom abstraction.

Local Electric Fields: From Enzyme Catalysis to Synthetic Catalyst Design

This Mini-Review Article outlines recent advances in the study of local electric field (LEF) governed enzyme catalysis and the application of the LEF principle in synthetic catalyst design. We start by discussing the electrostatics principles that drive enzyme catalysis, and its experimental verifications through vibrational Stark spectroscopy. Subsequently, we describe aspects of LEFs other than catalysis, i.e., induction of mechanistic crossovers, among others. Here, we focus on the early work done using computational tools, along with some recent contributions. Following an in-depth discussion of the role of LEFs in enzyme catalysis, we then highlight some recent works on designed local electric fields (D-LEF) and their applications in organic synthesis. Subsequently, we turn to D-LEFs in synthetic enzymes and supramolecular systems (cf. the work by the Head-Gordon group). We end by discussing some of the software packages that have been developed to analyze local electric fields computationally. Overall, the present Mini-Review Article paints an insightful picture of the current state of the art using LEF in enzyme catalysis and its application for further bioengineering and synthetic organic frameworks in a broad perspective.

Mechanistic Studies for Pd(II)(O2) Reduction Generating Pd(0) and H2O: Formation of Pd(OH)2 as a Key Intermediate

Molecular oxygen (O₂) remains to be an ideal yet underutilized feedstock for the oxidative transformation of organic substrates and renewable energy systems such as fuel cells. Palladium (Pd) has shown particular promise in enabling these applications. The present study describes a Pd-mediated O₂ reduction to water via C-H activation of 9,10-dihydroanthracene (DHA) by a Pd(II) η²-peroxo complex 1O₂. The reaction yields stoichiometric anthracene and Pd(0) product 1 and is notable in two respects. First, plots of concentrations of the reaction participants over time have distinctly sigmoidal shapes, indicating that conversion accelerates over time and implying autocatalysis. Second, the reaction proceeds via a genuine monometallic Pd(II) dihydroxide 1(OH)₂ directly observed to grow and decay as an intermediate. Confirming its role as an intermediate, the dihydroxide 1(OH)₂ was found to mediate C-H oxidation of DHA on par in activity with the peroxo compound 1O₂. Mechanistic studies with density functional theory (DFT) calculations suggested that both 1O₂ and 1(OH)₂ react with DHA by hydrogen atom transfer (HAT) and that autocatalysis in the 1O₂ reaction results from oxidative addition of the initial Pd(II) complex 1O₂ to the Pd(0) product 1. This reaction forms a transient bis(μ-oxo) Pd(II) dimer 1O₂1 that is more active in the HAT oxidation of DHA than the initial 1O₂.

Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

The power of oriented electrostatic fields (ESFs) to influence chemical bonding and reactivity is a phenomenon of rapidly growing interest. The presence of strong ESFs has recently been implicated as one of the most significant contributors to the activity of select enzymes, wherein alignment of a substrate's changing dipole moment with a strong, local electrostatic field has been shown to be responsible for the majority of the enzymatic rate enhancement. Outside of enzymology, researchers have studied the impacts of "internal" electrostatic fields via the addition of ionic salts to reactions and the incorporation of charged functional groups into organic molecules (both experimentally and computationally), and "externally" via the implementation of bulk fields between electrode plates. Incorporation of charged moieties into homogeneous inorganic complexes to generate internal ESFs represents an area of high potential for novel catalyst design. This field has only begun to materialize within the past 10 years but could be an area of significant impact moving forward, since it provides a means for tuning the properties of molecular complexes via a method that is orthogonal to traditional strategies, thereby providing possibilities for improved catalytic conditions and novel reactivity. In this perspective, we highlight recent developments in this area and offer insights, obtained from our own research, on the challenges and future directions of this emerging field of research.

Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)-Aqua Complexes

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq^5NO2)Mn^III(OH₂)]²⁺ (1; dpaq^5NO2 = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamidate with an NO₂ substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal-aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by 1 in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by 1, the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in 1 on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to 1. The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to 1 on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal(III)-aqua complexes.

Cationic Effects on the Net Hydrogen Atom Bond Dissociation Free Energy of High-Valent Manganese Imido Complexes

Local electric fields can alter energy landscapes to impart enhanced reactivity in enzymes and at surfaces. Similar fields can be generated in molecular systems using charged functionalities. Manganese(V) salen nitrido complexes (salen = N,N'-ethylenebis(salicylideneaminato)) appended with a crown ether unit containing Na+ (1-Na), K+, (1-K), Ba2+ (1-Ba), Sr2+ (1-Sr), La3+ (1-La), or Eu3+ (1-Eu) cation were investigated to determine the effect of charge on pKa, E1/2, and the net bond dissociation free energy (BDFE) of N-H bonds. The series, which includes the manganese(V) salen nitrido without an appended crown, spans 4 units of charge. Bounds for the pKa values of the transient imido complexes were used with the Mn(VI/V) reduction potentials to calculate the N-H BDFEs of the imidos in acetonitrile. Despite a span of >700 mV and >9 pKa units across the series, the hydrogen atom BDFE only spans ∼6 kcal/mol (between 73 and 79 kcal/mol). These results suggest that the incorporation of cationic functionalities is an effective strategy for accessing wide ranges of reduction potentials and pKa values while minimally affecting the BDFE, which is essential to modulating electron, proton, or hydrogen atom transfer pathways.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

C(sp3)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO4)

The reaction of osmium tetroxide (OsO₄) and carboxylate anions (acetate: X^- = AcO^- and benzoate: X^- = BzO^-) gave 1 : 1 adducts, [OsO₄(X)]^- (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(VIII) complex. The carboxylate adducts show a negative shift of the redox potentials (E_1/2) and a red shift of the ν_OsO stretches as compared to those of tetrahedral OsO₄ itself. Despite the negative shift of E_1/2, the reactivity of these adduct complexes 1X was enhanced compared to that of OsO₄ in benzylic C(sp³)-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (k₂), which exhibits a linear correlation with the C-H bond dissociation energy (BDE_C-H) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 (k₂(xanthene-h₂)/k₂(xanthene-d₂)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.

Rapid Iron(III)-Fluoride-Mediated Hydrogen Atom Transfer

We anticipate high-valent metal-fluoride species will be highly effective hydrogen atom transfer (HAT) oxidants because of the magnitude of the H-F bond (in the product) that drives HAT oxidation. We prepared a dimeric FeIII (F)-F-FeIII (F) complex (1) by reacting [FeII (NCCH3 )2 (TPA)](ClO4 )2 (TPA=tris(2-pyridylmethyl)amine) with difluoro(phenyl)-λ3 -iodane (difluoroiodobenzene). 1 was a sluggish oxidant, however, it was readily activated by reaction with Lewis or Brønsted acids to yield a monomeric [FeIII (TPA)(F)(X)]+ complex (2) where X=F/OTf. 1 and 2 were characterized using NMR, EPR, UV/Vis, and FT-IR spectroscopies and mass spectrometry. 2 was a remarkably reactive FeIII reagent for oxidative C-H activation, demonstrating reaction rates for hydrocarbon HAT comparable to the most reactive FeIII and FeIV oxidants.

Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

The enzymes manganese lipoxygenase (MnLOX) and manganese superoxide dismutase (MnSOD) utilize mononuclear Mn centers to effect their catalytic reactions. In the oxidized Mn^III state, the active site of each enzyme contains a hydroxo ligand, and X-ray crystal structures imply a hydrogen bond between this hydroxo ligand and a cis carboxylate ligand. While hydrogen bonding is a common feature of enzyme active sites, the importance of this particular hydroxo-carboxylate interaction is relatively unexplored. In this present study, we examined a pair of Mn^III-hydroxo complexes that differ by a single functional group. One of these complexes, [Mn^III(OH)(PaPy₂N)]⁺, contains a naphthyridinyl moiety capable of forming an intramolecular hydrogen bond with the hydroxo ligand. The second complex, [Mn^III(OH)(PaPy₂Q)]⁺, contains a quinolinyl moiety that does not permit any intramolecular hydrogen bonding. Spectroscopic characterization of these complexes supports a common structure, but with perturbations to [Mn^III(OH)(PaPy₂N)]⁺, consistent with a hydrogen bond. Kinetic studies using a variety of substrates with activated O-H bonds, revealed that [Mn^III(OH)(PaPy₂N)]⁺ is far more reactive than [Mn^III(OH)(PaPy₂Q)]⁺, with rate enhancements of 15-100-fold. A detailed analysis of the thermodynamic contributions to these reactions using DFT computations reveals that the former complex is significantly more basic. This increased basicity counteracts the more negative reduction potential of this complex, leading to a stronger O-H BDFE in the [Mn^II(OH₂)(PaPy₂N)]⁺ product. Thus, the differences in reactivity between [Mn^III(OH)(PaPy₂Q)]⁺ and [Mn^III(OH)(PaPy₂N)]⁺ can be understood on the basis of thermodynamic considerations, which are strongly influenced by the ability of the latter complex to form an intramolecular hydrogen bond.

Semiempirical method for examining asynchronicity in metal-oxido-mediated C-H bond activation

The oxidation of substrates via the cleavage of thermodynamically strong C-H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal-oxido species, which are directly responsible for cleaving the C-H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal-oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal-oxido species, C-H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M-oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric MnIV-oxido complex with a series of external substrates was studied, resulting in a spread of over 104 in their second-order rate constants that tracked with the acidity of the C-H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M-oxido units needs to be considered in C-H functionalization.

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.

Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, for HAT from the C-H bonds of 56 substrates to CumO•, spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C-H bond dissociation enthalpies (BDEs) spanning 27 kcal mol-1 has been calculated using the (RO)CBS-QB3 method. The log kH' vs C-H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C-H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG⧧HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol-1, indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi's principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)-H donors.

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 FeIII-OH and MnIII-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.

Modulating the radical reactivity of phenyl radicals with the help of distonic charges: it is all about electrostatic catalysis

This manuscript reports the modulation of H-abstraction reactivity of phenyl radicals by (positive and negative) distonic ions. Specifically, we focus on the origins of this modulating effect: can the charged functional groups truly be described as "extreme forms of electron-withdrawing/donating substituents" - implying a through-bond mechanism - as argued in the literature, or is the modulation mainly caused by through-space effects? Our analysis indicates that the effect of the remote charges can be mimicked almost perfectly with the help of a purely electrostatic treatment, i.e. replacing the charged functional groups by a simple uniform electric field is sufficient to recover the quantitative reactivity trends. Hence, through-space effects dominate, whereas through-bond effects play a minor role at best. We elucidate our results through a careful Valence Bond (VB) analysis and demonstrate that such a qualitative analysis not only reveals through-space dominance, but also demonstrates a remarkable reversal in the reactivity trends of a given polarity upon a rational modification of the reaction partner. As such, our findings demonstrate that VB theory can lead to productive predictions about the behaviour of distonic radical ions.

Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

A series of tetradentate tris(phosphinimine) ligands (^R3P₃tren) was developed and bound to Cu^I to form the trigonal pyramidal, C_3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F₄] (1PR3) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, 1PMe3, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (−780 mV vs. Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of 1PR3via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy-plane that destabilize 2e (d_xy/d_x²−y²) orbital sets and uniquely stabilized a₁ (d_z²) orbitals. Support is provided that the a₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z-axes of 1PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1PMe3 to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (2PMe3), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo-C_s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal C_s-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' properties via a method orthogonal to classic ligand field tuning.

Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials.

Electric-Field Mediated Chemistry: Uncovering and Exploiting the Potential of (Oriented) Electric Fields to Exert Chemical Catalysis and Reaction Control

This Perspective discusses oriented external-electric-fields (OEEF), and other electric-field types, as "smart reagents", which enable in principle control over wide-ranging aspects of reactivity and structure. We discuss the potential of OEEFs to control nonredox reactions and impart rate-enhancement and selectivity. An OEEF along the "reaction axis", which is the direction whereby electronic reorganization converts reactants' to products' bonding, will accelerate reactions, control regioselectivity, induce spin-state selectivity, and elicit mechanistic crossovers. Simply flipping the direction of the OEEF will lead to inhibition. Orienting the OEEF off the reaction axis enables control over stereoselectivity, enantioselectivity, and product selectivity. For polar/polarizable reactants, the OEEF itself will act as tweezers, which orient the reactants and drive their reaction. OEEFs also affect bond-dissociation energies and dissociation modes (covalent vs ionic), as well as alteration of molecular geometries and supramolecular aggregation. The "key" to gaining access to this toolbox provided by OEEFs is microscopic control over the alignment between the molecule and the applied field. We discuss the elegant experimental methods which have been used to verify the theoretical predictions and describe various alternative EEF sources and prospects for upscaling OEEF catalysis in solvents. We also demonstrate the numerous ways in which the OEEF effects can be mimicked by use of (designed) local-electric fields (LEFs), i.e., by embedding charges or dipoles into molecules. LEFs and OEEFs are shown to be equivalent and to obey the same ground rules. Outcomes are exemplified for Diels-Alder cycloadditions, oxidative addition of bonds by transition-metal complexes, H-abstractions by oxo-metal species, ionic cleavage of halogen bonds, methane activation, etc.

C-H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C-H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt-imido bond. It can cleanly cleave strong C-H bonds with a bond dissociation energy of up to 92 kcal mol-1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)2 (NHt Bu)]- . Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.

Oriented (Local) Electric Fields Drive the Millionfold Enhancement of the H-Abstraction Catalysis Observed for Synthetic Metalloenzyme Analogues

This contribution follows the recent remarkable catalysis observed by Groves et al. in hydrogen-abstraction reactions by a) an oxoferryl porphyrin radical-cation complex [Por^⋅+ Fe^IV (O)L_ax ] and b) a hydroxoiron porphyrazine ferric complex [PyPzFe^III (OH)L_ax ], both of which involve positively charged substituents on the outer circumference of the respective macrocyclic ligands. These charge-coronated complexes are analogues of the biologically important Compound I (Cpd I) and synthetic hydroxoferric species, respectively. We demonstrate that the observed enhancement of the H-abstraction catalysis for these systems is a purely electrostatic effect, elicited by the local charges embedded on the peripheries of the respective macrocyclic ligands. Our findings provide new insights into how electrostatics can be employed to tune the catalytic activity of metalloenzymes and can thus contribute to the future design of new and highly efficient hydrogen-abstraction catalysts.

Hydroxo Iron(III) Sites in a Metal-Organic Framework: Proton-Coupled Electron Transfer and Catalytic Oxidation of Alcohol with Molecular Oxygen

Metalloenzymes are powerful biocatalysts that can catalyze particular chemical reactions with high activity, selectivity, and specificity under mild conditions. Metal-organic frameworks (MOFs) composed of metal ions or metal clusters and organic ligands with defined cavities have the potential to impart enzyme-like catalytic activity and mimic metalloenzymes. Here, a new metal-organic framework implanted with hydroxo iron(III) sites with the structural and reactivity characteristics of iron-containing lipoxygenases is reported. Similar to lipoxygenases, the hydrogen atoms and electrons of the substrate can transfer to the hydroxo iron(III) sites, showing typical proton-coupled electron transfer behavior. In the reactivity mimicking biology system, similar to alcohol oxidase, the material also catalyses the oxidation of alcohol into aldehyde by using O₂ with a high yield and 100% selectivity under mild conditions, without the use of a radical cocatalyst or photoexcitation. These results provide strong evidence for the high structural fidelity of enzymatically active sites in MOF materials, verifying that MOFs provide an ideal platform for designing biomimetic heterogeneous catalysts with high conversion efficiency and product selectivity.

Photoinduced charge flow inside an iron porphyrazine complex

Tracking inorganic photochemistry with high resolution poses considerable challenges. Here, sub-picosecond electronic and structural motions and MLCT/d-d intersystem crossing in a cationic iron-porphyrazine are probed using ultrafast transient absorption, stimulated Raman spectroscopy, and quantum calculations. By delineating photoinduced energy relaxation, strategies for extending the lifetime of MLCT state are discussed.

Enhanced Rates of C-H Bond Cleavage by a Hydrogen-Bonded Synthetic Heme High-Valent Iron(IV) Oxo Complex

Secondary coordination sphere interactions are critical in facilitating the formation, stabilization, and enhanced reactivity of high-valent oxidants required for essential biochemical processes. Herein, we compare the C-H bond oxidizing capabilities of spectroscopically characterized synthetic heme iron(IV) oxo complexes, F8Cmpd-II (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a 2,6-lutidinium triflate (LutH+) Lewis acid adduct involving ferryl O-atom hydrogen-bonding, F8Cmpd-II(LutH+). Second-order rate constants utilizing C-H and C-D substrates were obtained by UV-vis spectroscopic monitoring, while products were characterized and quantified by EPR spectroscopy and gas chromatography (GC). With xanthene, F8Cmpd-II(LutH+) reacts 40 times faster (k2 = 14.2 M-1 s-1; -90 °C) than does F8Cmpd-II, giving bixanthene plus xanthone and the heme product [F8FeIIIOH2]+. For substrates with greater C-H bond dissociation energies (BDEs) F8Cmpd-II(LutH+) reacts with the second order rate constants k2(9,10-dihydroanthracene; DHA) = 0.485 M-1 s-1 and k2(fluorene) = 0.102 M-1 s-1 (-90 °C); by contrast, F8Cmpd-II is unreactive toward these substrates. For xanthene vs xanthene-(d2), large, nonclassical deuterium kinetic isotope effects are roughly estimated for both F8Cmpd-II and F8Cmpd-II(LutH+). The deuterated H-bonded analog, F8Cmpd-II(LutD+), was also prepared; for the reaction with DHA, an inverse KIE (compared to F8Cmpd-II(LutH+)) was observed. This work originates/inaugurates experimental investigation of the reactivity of authentic H-bonded heme-based FeIV═O compounds, critically establishing the importance of oxo H-bonding (or protonation) in heme complexes and enzyme active sites.

Regulating the Basicity of Metal-Oxido Complexes with a Single Hydrogen Bond and Its Effect on C-H Bond Cleavage

The functionalization of C-H bonds is an essential reaction in biology and chemistry. Metalloenzymes that often exhibit this type of reactivity contain metal-oxido intermediates that are directly involved in the initial cleavage of the C-H bonds. Regulation of the cleavage process is achieved, in part, by hydrogen bonds that are proximal to the metal-oxido units, yet our understanding of their exact role(s) is still emerging. To gain further information into the role of H-bonds on C-H bond activation, a hybrid set of urea-containing tripodal ligands has been developed in which a single H-bond can be adjusted through changes in the properties of one ureayl N-H bond. This modularity is achieved by appending a phenyl ring with different para-substituents from one ureayl NH group. The ligands have been used to prepare a series of Mn^III-oxido complexes, and a Hammett correlation was found between the pK_a values of the complexes and the substituents on the phenyl ring that was explained within the context of changes to the H-bonds involving the Mn^III-oxido unit. The complexes were tested for their reactivity toward 9,10-dihydroanthracene (DHA), and a Hammett correlation was found between the second-order rate constants for the reactions and the pK_a values. Studies to determine activation parameters and the kinetic isotope effects are consistent with a mechanism in which rate-limiting proton transfer is an important contributor. However, additional reactivity studies with xanthene found a significant increase in the rate constant compared to DHA, even though the substrates have the same pK_a(C-H) values. These results do not support a discrete proton-transfer/electron-transfer process, but rather an asynchronous mechanism in which the proton and electron are transferred unequally at the transition state.

Structure and Reactivity of (μ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand

Mononuclear Mn^III-hydroxo and dinuclear (μ-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq^5R) contained a substituent R (where R = OMe, Cl, and NO₂) at the 5-position of the quinolinyl group. Generation of the Mn^III complexes was achieved by either O₂ oxidation of Mn^II precursors (for [Mn^II(dpaq^5OMe)]⁺ and [Mn^II(dpaq^5Cl)]⁺ or PhIO oxidation (for [Mn^II(dpaq^5NO₂)]⁺). For each oxidized complex, ¹H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [Mn^III(OH)(dpaq^5R)]⁺ and an antiferromagnetically coupled [Mn^IIIMn^III(μ-O)(dpaq^5R)₂]²⁺ species in acetonitrile, with the addition of water favoring the Mn^III-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [Mn^IIIMn^III(μ-O)(dpaq^5R)₂](OTf)₂ complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 Å, and a Mn···Mn separation of 3.58 Å. X-ray crystallographic information was also obtained for the mononuclear [Mn^III(OH)(dpaq^5Cl)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) Å. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [Mn^III(OH)(dpaq^5R)]⁺ complexes was demonstrated through shifts in a number of ¹H NMR resonances, as well as a steady increase in the Mn^III/II cyclic voltammetry peak potential in the order [Mn^III(OH)(dpaq^5OMe)]⁺ < [Mn^III(OH)(dpaq)]⁺ < [Mn^III(OH)(dpaq^5Cl)]⁺ < [Mn^III(OH)(dpaq^5NO₂)]⁺. These changes in oxidizing power of the Mn^III-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [Mn^III(OH)(dpaq^5R)]⁺ complexes, with the most reactive [Mn^III(OH)(dpaq^5NO₂)]⁺ complex showing a second-order rate constant only 9-fold larger than that of the least reactive [Mn^III(OH)(dpaq^5OMe)]⁺ complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed p K_a values for the corresponding [Mn^II(OH₂)(dpaq^5R)]⁺ complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the Mn^III-hydroxo unit.

O–H and (CO)N–H bond weakening by coordination to Fe(ii)

Coordination of hydroxyl/amide groups to Fe(ii) diminishes BDFEs of O–H and (CO)N–H bonds down to 76.0 and 80.5 kcal mol⁻¹ respectively.

Electrochemical benzylic oxidation of C–H bonds

An electrochemical benzylic C–H oxidation reaction that is mediated by tert-butyl peroxyl radical is reported.

Photoinduced O2-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex

The iron(III) complex [Fe(tpena)]²⁺ (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine- N'-acetate) undergoes irreversible O₂-dependent N-demethylcarboxylation to afford [Fe^II(SBPy3)(MeCN)]²⁺ (SBPy3 = N, N-bis(2-pyridylmethyl)amine- N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO₂ and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O₂ in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [Fe^III(OH)(SBPy₃)]²⁺. Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [Fe^II(SBPy3)(X)]^2+/3+, X = MeCN, [Zn(tpena)]⁺.

Effects of Charged Ligand Substituents on the Properties of the Formally Copper(III)-Hydroxide ([CuOH]2+) Unit

With the goal of understanding how distal charge influences the properties and hydrogen atom transfer (HAT) reactivity of the [CuOH]2+ core proposed to be important in oxidation catalysis, the complexes [M]3[SO3LCuOH] (M = [K(18-crown-6)]+ or [K(crypt-222)]+) and [NMe3LCuOH]X (X = BArF4- or ClO4-) were prepared, in which SO3- or NMe3+ substituents occupy the para positions of the flanking aryl rings of the supporting bis(carboxamide)pyridine ligands. Structural and spectroscopic characterization showed that the [CuOH]+ cores in the corresponding complexes were similar, but cyclic voltammetry revealed the E1/2 value for the [CuOH]2+/[CuOH]+ couple to be nearly 0.3 V more oxidizing for the [NMe3LCuOH]2+ than the [SO3LCuOH]- species, with the latter influenced by interactions between the distal -SO3- substituents and K+ or Na+ counterions. Chemical oxidations of the complexes generated the corresponding [CuOH]2+ species as evinced by UV-vis spectroscopy. The rates of HAT reactions of these species with 9,10-dihydroanthracene to yield the corresponding [Cu(OH2)]2+ complexes and anthracene were measured, and the thermodynamics of the processes were evaluated via determination of the bond dissociation enthalpies (BDEs) of the product O-H bonds. The HAT rate for [SO3LCuOH]- was found to be ∼150 times faster than that for [NMe3LCuOH]2+, despite finding approximately the same BDEs for the product O-H bonds. Rationales for these observations and new insights into the roles of supporting ligand attributes on the properties of the [CuOH]2+ unit are presented.

A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di- tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ∼3 orders of magnitude larger than 4 ( k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di- tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.