Experimental and theoretical studies of the porphyrin ligand effect on the electronic structure and reactivity of oxoiron(iv) porphyrin π-cation-radical complexes

Resonance Raman study of oxoiron(IV) porphyrin π-cation radical complex: Porphyrin ligand effect on ν(Fe=O) frequency

Resonance Raman (rR) spectroscopy has been applied to study the nature of the iron-oxo (Fe=O) moiety of oxoiron(IV) porphyrin π-cation radical complex (CompI). While the axial ligand effect on the nature of the Fe=O moiety has been studied with rR spectroscopy, the porphyrin ligand effect has not been studied well. Here, we investigated the porphyrin ligand effect on the Fe=O moiety with rR spectroscopy. The porphyrin ligand effect was modulated by the electron-withdrawing effect of the porphyrin substituent at the meso-position. This study shows that the frequency of the Fe=O stretching band, ν(Fe=O), hardly change even when the electron-withdrawing effect of the porphyrin substituent changes. This result is further supported by theoretical calculation of CompI. The natural atomic charge analysis reveals that the oxo and axial ligands work to buffer the electron-withdrawing effect of the porphyrin substituent. The electron-withdrawing porphyrin substituent shifts an electron population from the ferryl iron to the porphyrin, but the decreased electron population on the ferryl iron is compensated by the shift of the electron population from the oxo ligand and the axial ligand. The shift of the electron population makes the Fe-axial ligand bond length short, but the Fe=O bond length unchanged, resulting in the invariable ν(Fe=O) frequency.

A Porphyrin Iron(III) π-Dication Species and its Relevance in Catalyst Design for the Umpolung of Nucleophiles

Isoporphyrins have recently been identified as remarkable species capable of turning the nucleophile attached to the porphyrin ring into an electrophile, thereby providing umpolung of reactivity (Inorg. Chem. 2022, 61, 8105-8111). They are generated by nucleophilic attack on an iron(III) π-dication, a class of species that has received scant attention. Here, we explore the effect of the porphyrin meso-substituent and report a iron(III) π-dication bearing the meso-tetraphenylporphyrin (TPP) ligand. We provide an extensive study of the species by UV/Vis absorption, 2 H NMR, EPR, applied field Mössbauer, and resonance Raman spectroscopy. We further explore the system's highly dynamic and tunable properties and address the nature of the axial ligands as well as the conformation of the porphyrin ring. The insights presented are essential for the rational design of catalysts for the umpolung of nucleophiles. Such catalytic avenues could for example provide a novel method for electrophilic chlorinations. We further examine the importance of electronic tuning of the porphyrin by nature of the meso-substituent as a factor in catalyst design.

Electric field effect of positive and negative charges of substituents on electronic structure and reactivity of oxoiron(IV) porphyrin π-cation radical complex

Electric field effect by the positive and negative changes near the active site is an important factor for controlling the reactivity of metalloenzymes. Previously, we reported that the positive charge of the N-methyl-2-pyridinium cation increases the reactivity of oxoiron(IV) porphyrin π-cation radical complex (Compound I), due to the attractive Coulomb interaction with electrons in Compound I. To further investigate the electric field effect, we study here the effect of the negative charge of the sulfonate group on the electronic structure and reactivity using Compound I of meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrin (TMPS-I). Although Compound I has been known as a very unstable complex, TMPS-I is very stable in 0.1 M acetate buffer at pH = 6. The half-life of TMPS-I is estimated to be 6.9 × 10³ s, which is the longest in Compound I previously reported. The redox potential of TMPS-I is estimated to be 0.76 V vs SCE in phosphate buffer, pH = 10. Kinetic analysis with stopped-flow technique indicates TMPS-I is less reactive than Compounds I reported previously. However, ¹H NMR and EPR spectra of TMPS-I are very close to those of Compounds I reported previously. The DFT calculations show that the orbital energy of Compound I is drastically altered by the positive and negative charges on the meso-phenyl group, suggesting the electric field effect. The difference of the reactivity of Compound I can be rationalized with the change of the orbital energy caused by the intramolecular electric field effect of the positive and negative charges.

Following Nature's Footprint: Mimicking the High-Valent Heme-Oxo Mediated Indole Monooxygenation Reaction Landscape of Heme Enzymes

Pathways for direct conversion of indoles to oxindoles have accumulated considerable interest in recent years due to their significance in the clear comprehension of various pathogenic processes in humans and the multipotent therapeutic value of oxindole pharmacophores. Heme enzymes are predominantly responsible for this conversion in biology and are thought to proceed with a compound-I active oxidant. These heme-enzyme-mediated indole monooxygenation pathways are rapidly emerging therapeutic targets; however, a clear mechanistic understanding is still lacking. Additionally, such knowledge holds promise in the rational design of highly specific indole monooxygenation synthetic protocols that are also cost-effective and environmentally benign. We herein report the first examples of synthetic compound-I and activated compound-II species that can effectively monooxygenate a diverse array of indoles with varied electronic and steric properties to exclusively produce the corresponding 2-oxindole products in good to excellent yields. Rigorous kinetic, thermodynamic, and mechanistic interrogations clearly illustrate an initial rate-limiting epoxidation step that takes place between the heme oxidant and indole substrate, and the resulting indole epoxide intermediate undergoes rearrangement driven by a 2,3-hydride shift on indole ring to ultimately produce 2-oxindole. The complete elucidation of the indole monooxygenation mechanism of these synthetic heme models will help reveal crucial insights into analogous biological systems, directly reinforcing drug design attempts targeting those heme enzymes. Moreover, these bioinspired model compounds are promising candidates for the future development of better synthetic protocols for the selective, efficient, and sustainable generation of 2-oxindole motifs, which are already known for a plethora of pharmacological benefits.

Mechanistic insights into the selectivity of norcarane oxidation by oxoMn(V) porphyrin complexes

OxoMn(V) porphyrin complexes perform competitive hydroxylation, desaturation, and radical rearrangement reactions using diagnostic substrate norcarane. Initial C-H cleavage proceeds through the two hydrogen abstraction steps from the two adjacent carbon on the norcarane, then the selective reaction is performed to generate various products. Using density functional theory calculations, we show that the hydroxylation and desaturation reactions are triggered by a rate-determining H-abstraction step, whereas the rate-determining step for the radical rearrangement is located at the rebound step ( TS2 ). We find that the  endo- 2  reaction is favorable over other reactions, which is consistent with the experimental result. Furthermore, the competitive pathways for norcarane oxidation depend on the non-covalent interaction between norcarane and porphyrin-ring, and orbital energy gaps between donor and acceptor orbitals because of stable or unstable acceptor orbital. The stereo- and regio-selectivities of norcarane oxidation are hardly sensitive to the zero-point energy and thermal free energy corrections.

Heme compound II models in chemoselectivity and disproportionation reactions

Heme compound II models bearing electron-deficient and -rich porphyrins, [Fe^IV(O)(TPFPP)(Cl)]⁻ (1a) and [Fe^IV(O)(TMP)(Cl)]⁻ (2a), respectively, are synthesized, spectroscopically characterized, and investigated in chemoselectivity and disproportionation reactions using cyclohexene as a mechanistic probe. Interestingly, cyclohexene oxidation by 1a occurs at the allylic C–H bonds with a high kinetic isotope effect (KIE) of 41, yielding 2-cyclohexen-1-ol product; this chemoselectivity is the same as that of nonheme iron(iv)-oxo intermediates. In contrast, as observed in heme compound I models, 2a yields cyclohexene oxide product with a KIE of 1, demonstrating a preference for C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C epoxidation. The latter result is interpreted as 2a disproportionating to form [Fe^IV(O)(TMP⁺˙)]⁺ (2b) and Fe^III(OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a, 1a does not disproportionate under the present reaction conditions. DFT calculations confirm that compound II models prefer C–H bond hydroxylation and that disproportionation of compound II models is controlled thermodynamically by the porphyrin ligands. Other aspects, such as acid and base effects on the disproportionation of compound II models, have been discussed as well.

Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.

Rate-Limiting Step of Epoxidation Reaction of the Oxoiron(IV) Porphyrin π-Cation Radical Complex: Electron Transfer Coupled Bond Formation Mechanism

Epoxidation reactions catalyzed by high-valent metal-oxo species are key reactions in various biological and chemical processes. Because the redox potentials of alkenes are higher than those of most high-valent metal-oxo species, the electron transfer (ET) from the alkene to the high-valent metal-oxo species in the epoxidation reaction is endergonic and must be coupled with another exergonic process. To reveal the mechanism of the ET, we performed a Marcus plot analysis for the epoxidation reaction of the oxoiron(IV) porphyrin π-cation radical complex (compound I) with alkene. The Marcus plots can be simulated with a linear line with the gradient of 0.50 when the redox potential of compound I varies and 0.07 when the redox potential of alkene varies. These results indicate that the ET process is involved in the rate-limiting step and coupled with the following O-C bond formation process: ET coupled bond formation mechanism. The DFT calculations support this conclusion and disclose the details of the mechanism. As the alkene comes close to the oxo ligand, the energy of the highest occupied molecular orbital (HOMO) of the alkene increases and the energy for the ET becomes small enough to allow the ET. Finally, the ET occurs from the HOMO of the alkene to the porphyrin π-radical orbital. The shift of one electron from the HOMO of the alkene by the ET simultaneously results in the O-C half bond formation between the oxo ligand and the alkene. The ET process itself is still endergonic and reversible, but the bond formation coupled with the ET changes the overall process to exergonic and irreversible. We also discuss the similarity with the aromatic hydroxylation reaction and the relevance to the epoxidation reactions of other metal-oxo complexes and peracid.

Bio-inspired nitrogen oxide (NOx) interconversion reactivities of synthetic heme Compound-I and Compound-II intermediates

Dioxygen activating heme enzymes have long predicted to be powerhouses for nitrogen oxide interconversion, especially for nitric oxide (NO) oxidation which has far-reaching biological and/or environmental impacts. Lending credence, reactivity of NO with high-valent heme‑oxygen intermediates of globin proteins has recently been implicated in the regulation of a variety of pivotal physiological events such as modulating catalytic activities of various heme enzymes, enhancing antioxidant activity to inhibit oxidative damage, controlling inflammatory and infectious properties within the local heme environments, and NO scavenging. To reveal insights into such crucial biological processes, we have investigated low temperature NO reactivities of two classes of synthetic high-valent heme intermediates, Compound-II and Compound-I. In that, Compound-II rapidly reacts with NO yielding the six-coordinate (NO bound) heme ferric nitrite complex, which upon warming to room temperature converts into the five-coordinate heme ferric nitrite species. These ferric nitrite complexes mediate efficient substrate oxidation reactions liberating NO; i.e., shuttling NO₂^- back to NO. In contrast, Compound-I and NO proceed through an oxygen-atom transfer process generating the strong nitrating agent NO₂, along with the corresponding ferric nitrosyl species that converts to the naked heme ferric parent complex upon warmup. All reaction components have been fully characterized by UV-vis, ²H NMR and EPR spectroscopic methods, mass spectrometry, elemental analyses, and semi-quantitative determination of NO₂^- anions. The clean, efficient, potentially catalytic NO_x interconversions driven by high-valent heme species presented herein illustrate the strong prospects of a heme enzyme/O₂/NO_x dependent unexplored territory that is central to human physiology, pathology, and therapeutics.

Synthesis, characterization, and reactivity of oxoiron(IV) porphyrin π-cation radical complexes bearing cationic N-methyl-2-pyridinium group

Electronic charge near the active site is an important factor for controlling the reactivity of metalloenzymes. Here, to investigate the effect of the cationic charge near the heme in heme proteins, we synthesized new iron porphyrin complexes (1 and 2) having cationic 3-methyl-N-methyl-2-pyrdinium group and N-methyl-2-pyridinium group at one of the four meso-positions, respectively. The N-methyl-2-pyridinium groups could be introduced by Stille coupling used palladium catalysts. Oxoiron(IV) porphyrin π-cation radical complexes (Compound I) of 1 (1-CompI) and 2 (2-CompI) are soluble in most organic solvents, allowing direct comparison of their electronic structure and reactivity with Compound I of tetramesitylporphyrin (3-CompI) and tetrakis-(2,6-dichlorophenyl)porphyrin (4-CompI) under the same conditions. Spectroscopic data for 1-CompI are close to those for 3-CompI, but the redox potential for 1-CompI is close to that of 4-CompI. Kinetic analysis of the epoxidation reactions shows that 1-CompI and 2-CompI are (~250-fold) more reactive than 3-CompI, and comparable to 4-CompI. DFT calculations allow to propose that the positive shift of the redox potential and the enhanced reactivity of 1-CompI and 2-CompI is induced by the intramolecular electric field effect of N-methyl-2-pyridinium cation, not by the electron-withdrawing effect.

Ruthenium-Catalyzed, Chemoselective and Regioselective Oxidation of Polyisobutene

Polyolefins are important commodity plastics, yet their lack of functional groups limits their applications. The functionalization of C-H bonds holds promise for incorporating functionalities into polymers of ethylene and linear α-olefins. However, the selective functionalization of polyolefins derived from branched alkenes, even monobranched, 1,1-substituted alkenes, has not been achieved. These polymers are less reactive, due to steric effects, and they are prone to chain scission that degrades the polymer. We report the chemoselective and regioselective oxidation of a commercially important polymer of a branched olefin, polyisobutene. A polyfluorinated ruthenium-porphyrin catalyst incorporates ketone units into polyisobutene at methylene positions without chain cleavage. The oxidized polymer is thermally stable, yet it undergoes programmed reactions and possesses enhanced wetting properties.

Ligand Architecture Perturbation Influences the Reactivity of Nonheme Iron(V)-Oxo Tetraamido Macrocyclic Ligand Complexes: A Combined Experimental and Theoretical Study

Iron(V)-oxo complexes bearing negatively charged tetraamido macrocyclic ligands (TAMLs) have provided excellent opportunities to investigate the chemical properties and the mechanisms of oxidation reactions of mononuclear nonheme iron(V)-oxo intermediates. Herein, we report the differences in chemical properties and reactivities of two iron(V)-oxo TAML complexes differing by modification on the "Head" part of the TAML framework; one has a phenyl group at the "Head" part (1), whereas the other has four methyl groups replacing the phenyl ring (2). The reactivities of 1 and 2 in both C-H bond activation reactions, such as hydrogen atom transfer (HAT) of 1,4-cyclohexadiene, and oxygen atom transfer (OAT) reactions, such as the oxidation of thioanisole and its derivatives, were compared experimentally. Under identical reaction conditions, 1 showed much greater reactivity than 2, such as a 10²-fold decrease in HAT and a 10⁵-fold decrease in OAT by replacing the phenyl group (i.e., 1) with four methyl groups (i.e., 2). Then, density functional theory calculations were performed to rationalize the reactivity differences between 1 and 2. Computations reproduced the experimental findings well and revealed that the replacement of the phenyl group in 1 with four methyl groups in 2 not only increased the steric hindrance but also enlarged the energy gap between the electron-donating orbital and the electron-accepting orbital. These two factors, steric hindrance and the orbital energy gap, resulted in differences in the reduction potentials of 1 and 2 and their reactivities in oxidation reactions.

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

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

Enhanced Redox Reactivity of a Nonheme Iron(V)-Oxo Complex Binding Proton

Acid effects on the chemical properties of metal-oxygen intermediates have attracted much attention recently, such as the enhanced reactivity of high-valent metal(IV)-oxo species by binding proton(s) or Lewis acidic metal ion(s) in redox reactions. Herein, we report for the first time the proton effects of an iron(V)-oxo complex bearing a negatively charged tetraamido macrocyclic ligand (TAML) in oxygen atom transfer (OAT) and electron-transfer (ET) reactions. First, we synthesized and characterized a mononuclear nonheme Fe(V)-oxo TAML complex (1) and its protonated iron(V)-oxo complexes binding two and three protons, which are denoted as 2 and 3, respectively. The protons were found to bind to the TAML ligand of the Fe(V)-oxo species based on spectroscopic characterization, such as resonance Raman, extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) measurements, along with density functional theory (DFT) calculations. The two-protons binding constant of 1 to produce 2 and the third protonation constant of 2 to produce 3 were determined to be 8.0(7) × 10⁸ M^-2 and 10(1) M^-1, respectively. The reactivities of the proton-bound iron(V)-oxo complexes were investigated in OAT and ET reactions, showing a dramatic increase in the rate of sulfoxidation of thioanisole derivatives, such as 10⁷ times increase in reactivity when the oxidation of p-CN-thioanisole by 1 was performed in the presence of HOTf (i.e., 200 mM). The one-electron reduction potential of 2 (E_red vs SCE = 0.97 V) was significantly shifted to the positive direction, compared to that of 1 (E_red vs SCE = 0.33 V). Upon further addition of a proton to a solution of 2, a more positive shift of the E_red value was observed with a slope of 47 mV/log([HOTf]). The sulfoxidation of thioanisole derivatives by 2 was shown to proceed via ET from thioanisoles to 2 or direct OAT from 2 to thioanisoles, depending on the ET driving force.

DFT insight into axial ligand effects on electronic structure and mechanistic reactivity of oxoiron(iv) porphyrin

A series of DFT studies on the epoxidation reactions of olefins by oxoiron(iv) porphyrin cation radical complexes are performed in this work, to elucidate the axial ligand effects on the electronic features and reaction mechanism in detail. We analyzed the molecular orbitals, spin populations, and Mulliken charges along the intrinsic reaction coordinate route. From the findings, we confirmed that the interaction between the axial ligand and the oxoiron(iv) porphyrin is strong and the initial changes in the electronic structures occur early during the reaction, which further enhances the reactivity toward olefin epoxidation. More importantly, the patterns of the electron transfer from olefin to oxoiron(iv) porphyrin were impacted by the axial ligand. The pattern of successive electron transfer from Fe-O to porphyrin and then from C[double bond, length as m-dash]C to Fe-O for oxoiron(iv) porphyrin in case of fluorine and acetate axial ligands, whereas the pattern of electron transfer occurs from C[double bond, length as m-dash]C to porphyrin for oxoiron(iv) porphyrin in case of chlorine and nitrate axial ligands during the epoxidation reaction of the olefins. We also determined the intersystem crossing between the quartet and sextet spin states occurring at the second transition state (TS2) by the analysis of the two-dimensional potential energy surface.