Isolating Fe-O2 Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O2) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O2 is a significant challenge because of the thermodynamic stability of O2 in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O2 by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O2 activation, wherein Fe-O2 species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O2 intermediates have been synthesized by activating O2 at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O2 intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.


Introduction
Dioxygen (O 2 ) is a primary component of the atmosphere and is essential in life processes. The biochemical processes that mediate the highly exergonic four-electron reduction of oxygen to water with an enthalpy change of −80 kcal/mol provide the energy for sustaining aerobic organisms [1]. With a redox potential of 0.815 V vs. NHE in water at pH 7 and 25 • C, the overall four-electron reduction of O 2 is thermodynamically favorable [2]. However, the activation of O 2 is kinetically hindered because of its triplet ground state, which makes direct combination of O 2 with typical organic compounds spinforbidden [3][4][5]. To overcome this difficulty, nature has evolved a wide variety of enzymes, which contain transition metals (e.g., Fe, Mn, Cu, and Co) with unpaired d-electrons facilitating the activation of O 2 by metal coordination, to carry out various vital biological processes [6]. The function of transition metals is not only to serve as O 2 binding sites for transport, but also to serve as active centers where they can carry out multi-electron redox process that ultimately result in the incorporation of oxygen in organic substrates [7].
The binding of O 2 to ferrous porphyrins constitutes the first step in the biological function of a majority of heme enzymes. Processes involving cleavage of the O-O bond require proteins with specific structural and chemical features designed to facilitate O 2 activation [7]. Cytochrome P450, one of the important dioxygen-activating heme enzymes, has thrown light on the catalytic cycles of O 2 activation and oxygen transfer reactions [8][9][10][11][12]. The consensus mechanism for O 2 activation by cytochrome P450 is shown in Figure 1 [9]. The spin-state of the six-coordinate ferric complex at the active site in cytochrome P450 is low-spin, which turns into a high-spin five-coordinate ferric complex upon binding of the substrate to remove the aqua ligand. One-electron reduction of this high-spin fivecoordinate ferric species generates a high-spin ferrous compound, which is quite activated  Metal-dioxygen species are crucial intermediates in the O 2 activation cycle of metalloenzymes, and their redox reactivities are modulated by the nature of metal ions and ligands in metalloenzymes [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. In living organisms, the metalloporphyrin sites are sur-rounded by the protein superstructure, which is conducive to avoiding the involvement of metalloporphyrin active sites in unwanted side reactions [13,15,[27][28][29][30]. However, the complexity of the surrounding protein superstructure restricts access to the metalloporphyrin active sites, resulting in the big challenge in studying the intermediates in O 2 activation by metalloenzymes. To deal with this big challenge, the last several decades have seen numerous researchers turn towards the development of molecular model complexes with heme and nonheme ligands that mimic the local coordination environment of the active site in enzymes [31][32][33][34][35][36][37]. Due to the simplicity of molecular model complexes, the metal-dioxygen intermediates can be stabilized and directly characterized using current techniques.
The heme cofactor, being made of an iron-bound porphyrin, is at the active site of many metalloproteins and carries out oxygen transport, storage, electron transfer, and oxygenation reactions [38][39][40]. Thus, due to the bioavailability of iron porphyrins in nature, iron porphyrins have been well-studied. The successfully isolated and characterized metal-dioxygen intermediates in iron porphyrin complexes can make us understand physicochemical properties of the metalloenzymes and study the correlation of the structural properties of the active sites and their reactivities in well-controlled environments. In this review, we focus on a few examples of these advances with emphasis in each case on the particular reaction conditions and design of iron porphyrin complexes to stabilize and isolate Fe-O 2 intermediates in O 2 activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes for the design of effective catalysts.

Nature of Iron-Dioxygen Bond
The structural and electronic properties of iron-dioxygen adduct are crucial for understanding the O 2 activation in heme proteins and metalloporphyrins. However, a controversy exists over the nature of Fe-O 2 bonds in hemoproteins and metalloporphyrins. As early as 1936, Pauling and Coryell reported that oxygenated hemoglobin (oxy-Hb) was diamagnetic and proposed that dioxygen was bound to iron in a bent, end-on mode [41,42]. This geometric feature was confirmed by an oxy−ferrous model compound prepared by Collman and co-workers and by crystal structures of oxy-Hb reported by Shaanan, respectively [43,44]. These pioneering works suggest that the electronic structure of oxy-Hb is described as a Fe II -O 2 adduct, where complete electron transfer from O 2 to iron has not taken place ( Figure 2) [45]. Metal-dioxygen species are crucial intermediates in the O2 activation cycle of metalloenzymes, and their redox reactivities are modulated by the nature of metal ions and ligands in metalloenzymes [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. In living organisms, the metalloporphyrin sites are surrounded by the protein superstructure, which is conducive to avoiding the involvement of metalloporphyrin active sites in unwanted side reactions [13,15,[27][28][29][30]. However, the complexity of the surrounding protein superstructure restricts access to the metalloporphyrin active sites, resulting in the big challenge in studying the intermediates in O2 activation by metalloenzymes. To deal with this big challenge, the last several decades have seen numerous researchers turn towards the development of molecular model complexes with heme and nonheme ligands that mimic the local coordination environment of the active site in enzymes [31][32][33][34][35][36][37]. Due to the simplicity of molecular model complexes, the metal-dioxygen intermediates can be stabilized and directly characterized using current techniques.
The heme cofactor, being made of an iron-bound porphyrin, is at the active site of many metalloproteins and carries out oxygen transport, storage, electron transfer, and oxygenation reactions [38][39][40]. Thus, due to the bioavailability of iron porphyrins in nature, iron porphyrins have been well-studied. The successfully isolated and characterized metal-dioxygen intermediates in iron porphyrin complexes can make us understand physicochemical properties of the metalloenzymes and study the correlation of the structural properties of the active sites and their reactivities in well-controlled environments. In this review, we focus on a few examples of these advances with emphasis in each case on the particular reaction conditions and design of iron porphyrin complexes to stabilize and isolate Fe-O2 intermediates in O2 activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes for the design of effective catalysts.

Nature of Iron-Dioxygen Bond
The structural and electronic properties of iron-dioxygen adduct are crucial for understanding the O2 activation in heme proteins and metalloporphyrins. However, a controversy exists over the nature of Fe-O2 bonds in hemoproteins and metalloporphyrins. As early as 1936, Pauling and Coryell reported that oxygenated hemoglobin (oxy-Hb) was diamagnetic and proposed that dioxygen was bound to iron in a bent, end-on mode [41,42]. This geometric feature was confirmed by an oxy−ferrous model compound prepared by Collman and co-workers and by crystal structures of oxy-Hb reported by Shaanan, respectively [43,44]. These pioneering works suggest that the electronic structure of oxy-Hb is described as a Fe II -O2 adduct, where complete electron transfer from O2 to iron has not taken place ( Figure 2) [45]. Reprinted with permission from Ref. [45]. Copyright 2013 American Chemical Society.
In contrast, Weiss and co-workers propose that the Fe−O2 species would be best described as a ferric superoxide complex, where the observed diamagnetism of oxy-Hb is In contrast, Weiss and co-workers propose that the Fe−O 2 species would be best described as a ferric superoxide complex, where the observed diamagnetism of oxy-Hb is explained by the antiferromagnetic coupling between a doublet superoxide anion O 2 •− (S = 1/2) and a low-spin Fe III (S = 1/2) to result in an overall spin of S = 0 ( Figure 2) [45]. Mössbauer studies have lent support to the Weiss model. The Mössbauer parameters of oxy-Hb with isomer shift δ = 0.2−0.3 mm/s and quadrupole splitting ∆E Q = 2.00-2.20 mm/s, first measured by Lang and co-workers, are in good agreement with a low-spin ferric center [46,47]. Sharrock and co-workers have given close parameters for oxygenated reduced P450cam, which are δ = 0.3159 mm/s and ∆E Q = 2.15 mm/s [48]. Collman and co-workers present infrared data on Fe-O 2 adducts in "picket-fence" porphyrin (α,α,α,αtetrapivalamidophenylporphyrin, α,α,α,α-TpivPP) complexes with 1-methylimidazole (1-MeIm) and 1-tritylimidazole (1-tritylIm) as axial bases. The O 2 vibration of Fe-O 2 adducts are achieved using difference techniques between 16 [49]. The Fe III -O 2 •− model obtained further support by resonance Raman spectroscopic studies, where a ferric center was indicated by the oxidation state marker band of oxy-Hb (ν = 1377 cm −1 ) [50][51][52].
The third model ("Ozone model"), initially suggested by McClure and refined by Goddard, presents the oxy-form electronic structure as an antiferromagnetically coupled intermediate spin ferrous center (S = 1) and a triplet dioxygen ligand (S = 1) [53][54][55][56]. In a recent report, Haumann and co-workers apply the X-ray spectroscopic techniques and computations on O 2 -bound hemes in myoglobin (Mb) and hemoglobin (Hb) and in porphyrin complexes at 20-260 K [57]. Based on the results, they describe an essentially ferrous, intermediate-spin iron in an ozone-like configuration in O 2 -bound heme compounds, supporting the McClure-Goddard model.
Using generalized valence bond (GVB) wave function, Shaik and co-workers have presented Fe-O 2 bonding that includes a σ (Fe−O) bond and a π (Fe-O 2 ) bond between d yz (Fe) and π ⊥ * (O 2 ) with quite small delocalization tails [58]. Thus, they propose one electron transferred from Fe II center to O 2 to form Fe III center and superoxo O 2 •− , that is, the Weiss bonding model. Furthermore, H-bonding capability and the polarity of the protein is able to modulate the O 2 affinity to consolidate the character of Fe III -O 2 •− . Although Fe−O 2 follows the Fe III -O 2 •− bonding model from the GVB wave function, the pure VB analysis showed a mixing of the other two models with the Pauling model being the least important, and axial ligands and protein host determined the nature of the Fe−O 2 bonding. In addition, an iron L-edge X-ray absorption spectroscopy study of a ferric-superoxo "picket fence" porphyrin with 1-methylimidazole (1-MeIm) axial ligand, [Fe(O 2 )(1-MeIm)(α,α,α,α-TpivPP)], also strongly supports the key impact of local environment on the nature of Fe−O 2 bonding [45]. Interestingly, in a recent work of Sarangi and co-workers, the solutionstate structure of oxy-Hb is best described as Fe III -O 2 •− (Weiss model); however, the crystalline oxy-Hb favors (Fe II −O 2 ) (Pauling model) through theoretical and spectroscopic studies. This surprising change of electronic configuration with environment is in favor of the multiconfigurational description [59]. Even though the current findings of oxyheme intermediates are unable to give a simple explanation for their electronic structures, notable recent work has presented the necessity of electron transfer from the iron center to the dioxygen moiety so as to produce oxy-heme-type intermediates, emphasizing the unambiguous validity of the Weiss model [60][61][62][63][64].

Secondary Coordination Sphere Interactions
The secondary coordination sphere interactions are another promising strategy for building discrete molecules with improved O 2 activation properties and increasing the stability of the O 2 intermediates. "Single-coronet" model system (6) designed by Naruta and co-workers is such a compound, which is constitutive of highly sterically bulky substituted dinaphthalene moieties that establish a hydrophobic environment around the Fe center, while providing -OH for secondary coordination sphere interactions ( Figure 4). As presented in biological heme centers [87,88], this uniquely designed architecture has been reported to mediate the Fe-O 2 affinities to increase the stabilities of Fe-O 2 adducts [89,90], wherein hydrogen-bonding interactions on Fe-O 2 adducts result in the huge change in affinity ( Figure 5) [89,91]. formation of dioxygen adduct (5-O2) upon exposure [Fe II (THT)(α,α,α,α-TpivPP)] (5; THT: tetrahydrothiophene) to O2. The Mössbauer spectrum of 5-O2 was found to be quite similar to that of 1-O2.

Secondary Coordination Sphere Interactions
The secondary coordination sphere interactions are another promising strategy for building discrete molecules with improved O2 activation properties and increasing the stability of the O2 intermediates. "Single-coronet" model system (6) designed by Naruta and co-workers is such a compound, which is constitutive of highly sterically bulky substituted dinaphthalene moieties that establish a hydrophobic environment around the Fe center, while providing -OH for secondary coordination sphere interactions ( Figure 4). As presented in biological heme centers [87,88], this uniquely designed architecture has been reported to mediate the Fe-O2 affinities to increase the stabilities of Fe-O2 adducts [89,90], wherein hydrogen-bonding interactions on Fe-O2 adducts result in the huge change in affinity ( Figure 5) [89,91].  In a subsequent study, Naruta and co-workers also prepare "twin-coronet" ligand platforms, which introduce a tethered imidazole (7) or pyridine (8) to the porphyrinate periphery ( Figure 6) [90]. The dioxygen adducts (7-O2, 8-O2) of these "twin-coronet" models reveal striking stability with half-lives up to several days in toluene at 25 °C. They suggested that the remarkable stability of the Fe-O2 adducts resulted from the hydrogen bonding to bound O2 moieties in the secondary coordination sphere in 7-O2 and 8-O2. The resonance Raman (rR) spectra of 7-O2 and 8-O2 showed ν(Fe−O) stretching centered at 586 formation of dioxygen adduct (5-O2) upon exposure [Fe II (THT)(α,α,α,α-TpivPP)] (5; THT: tetrahydrothiophene) to O2. The Mössbauer spectrum of 5-O2 was found to be quite similar to that of 1-O2.

Secondary Coordination Sphere Interactions
The secondary coordination sphere interactions are another promising strategy for building discrete molecules with improved O2 activation properties and increasing the stability of the O2 intermediates. "Single-coronet" model system (6) designed by Naruta and co-workers is such a compound, which is constitutive of highly sterically bulky substituted dinaphthalene moieties that establish a hydrophobic environment around the Fe center, while providing -OH for secondary coordination sphere interactions ( Figure 4). As presented in biological heme centers [87,88], this uniquely designed architecture has been reported to mediate the Fe-O2 affinities to increase the stabilities of Fe-O2 adducts [89,90], wherein hydrogen-bonding interactions on Fe-O2 adducts result in the huge change in affinity ( Figure 5) [89,91].  In a subsequent study, Naruta and co-workers also prepare "twin-coronet" ligand platforms, which introduce a tethered imidazole (7) or pyridine (8) to the porphyrinate periphery ( Figure 6) [90]. The dioxygen adducts (7-O2, 8-O2) of these "twin-coronet" models reveal striking stability with half-lives up to several days in toluene at 25 °C. They suggested that the remarkable stability of the Fe-O2 adducts resulted from the hydrogen bonding to bound O2 moieties in the secondary coordination sphere in 7-O2 and 8-O2. The resonance Raman (rR) spectra of 7-O2 and 8-O2 showed ν(Fe−O) stretching centered at 586 In a subsequent study, Naruta and co-workers also prepare "twin-coronet" ligand platforms, which introduce a tethered imidazole (7) or pyridine (8) to the porphyrinate periphery ( Figure 6) [90]. The dioxygen adducts (7-O 2 , 8-O 2 ) of these "twin-coronet" models reveal striking stability with half-lives up to several days in toluene at 25 • C. They suggested that the remarkable stability of the Fe-O 2 adducts resulted from the hydrogen bonding to bound O 2 moieties in the secondary coordination sphere in 7-O 2 and 8-O 2 . The resonance Raman (rR) spectra of 7-O 2 and 8-O 2 showed ν(Fe−O) stretching centered at 586 cm −1 and 583 cm −1 , respectively, which are higher than those of other Fe-oxy species in model systems [92][93][94] and biological examples [95,96] cm −1 and 583 cm −1 , respectively, which are higher than those of other Fe-oxy species in model systems [92][93][94] and biological examples [95,96], resulting from hydrogen-bonding interactions between the O2 moiety and the -OH of the coronet ligand. The hydrogenbonding interactions are further proved by infrared (IR) spectroscopic features of 7-O2 and 8-O2, where the O−H IR stretching frequency of the ligand significantly loses intensity compared to 7 and 8. Thus, the "twin-coronet" model systems are able to mediate O2 affinity and shed light on the importance of secondary coordination sphere interactions, which could explain how O2 activation occurs in enzyme systems. Figure 6. "Twin-coronet" model systems with tethered axial N-based ligands. Reprinted with permission from Ref. [15]. Copyright 2018, American Chemical Society.

Cryogenic Temperature
Designing model systems that can precisely mimic the electronic and geometric characteristics of iron-dioxygen adducts that form within biological heme-containing proteins remains a huge challenge for biomimetic chemists [58,72]. Heme iron(III)-superoxo intermediates can be readily produced when iron(II) porphyrin complexes are exposed to O2. Nevertheless, formed iron(III)-superoxo intermediates are too unstable to be isolated or observed, resulting in only a handful of oxyheme models found to date [97,98]. The cryogenic temperature technique is a very effective way to observe and isolate iron-dioxygen adducts, because the decomposition of iron-dioxygen adducts is retarded and the bimolecular reaction to form μ-peroxo derivatives is remarkably slowed at low temperature [60,73].
Karlin and co-workers report several examples of heme−superoxo complexes, which are characterized at cryogenic temperature [99][100][101][102] (9) at -90 °C leads to the generation of 9-O2 with characteristic absorption band at λmax = 535 nm within 1 ms, which is also observed when 9 alone reacts with O2 in THF. However, the low-temperature stopped-flow kinetic experiments exhibit that 9-O2 is very unstable and transforms into a new species with distinct absorption bands at 560 nm within 1 s. Soon afterward, Karlin and co-workers reported the detailed characterization of 9-O2 under various conditions of solvent and temperature using multinuclear NMR and UV-visible spectroscopies in 2001 [100]. In coordinating solvents, including THF, propionitrile, and acetone, 9 reacts with O2 at cryogenic temperature to generate a quite stable iron(III)superoxo species, [(S)(F8TPP)Fe III (O2 •− )] (9-S-O2) (S = solvent) (Figure 7). A full reversible Figure 6. "Twin-coronet" model systems with tethered axial N-based ligands. Reprinted with permission from Ref. [15]. Copyright 2018, American Chemical Society.

Cryogenic Temperature
Designing model systems that can precisely mimic the electronic and geometric characteristics of iron-dioxygen adducts that form within biological heme-containing proteins remains a huge challenge for biomimetic chemists [58,72]. Heme iron(III)-superoxo intermediates can be readily produced when iron(II) porphyrin complexes are exposed to O 2 . Nevertheless, formed iron(III)-superoxo intermediates are too unstable to be isolated or observed, resulting in only a handful of oxyheme models found to date [97,98]. The cryogenic temperature technique is a very effective way to observe and isolate iron-dioxygen adducts, because the decomposition of iron-dioxygen adducts is retarded and the bimolecular reaction to form µ-peroxo derivatives is remarkably slowed at low temperature [60,73].
Karlin and co-workers report several examples of heme−superoxo complexes, which are characterized at cryogenic temperature [99][100][101][102]. As early as in 1999, when Karlin and co-workers carry out the synthesis of an iron-copper dinuclear active site, [(  (Figure 7). A full reversible phenomenon is observed by benchtop UV-visible spectroscopy, the kinetics and thermodynamics of formation of 9-S-O 2 is determined by stopped-flow spectrophotometry, and Fe/O 2 stoichiometry of 1:1 is determined by dioxygen-uptake manometry at 193 K. The 2 H NMR spectrum of 9-THF-O 2 with deuterated β-pyrrolic positions gives evidence of a diamagnetic low-spin iron center with δ pyrrole = 8.9 ppm, confirming the presence of a 6C Fe III center in the superoxo complex, wherein the diamagnetism results from the antiferromagnetic coupling of the low-spin iron(III) ion with the superoxo radical anion [49,73]. The 19 F-NMR spectrum of 9-THF-O 2 reveals two identical integrations of F resonances at −111 and −113 ppm, respectively, showing two chemically different environments for the 2,6-difluoro substituents of the porphyrinate, presumably giving the credit to the two chemically inequivalent sides of the porphyrinate, wherein one bears the superoxo radical anion [68]. However, due to the instability of 9-THF-O 2 , upon increasing temperature to room temperature, 9-THF-O 2 decomposes to a high-spin iron(III)-hydroxide species, [(F 8 TPP)Fe III (OH)] (Figure 7), with a paramagnetic 2 H NMR shift at δ pyrrole = 125 ppm. phenomenon is observed by benchtop UV-visible spectroscopy, the kinetics and dynamics of formation of 9-S-O2 is determined by stopped-flow spectrophotome Fe/O2 stoichiometry of 1:1 is determined by dioxygen-uptake manometry at 193 K NMR spectrum of 9-THF-O2 with deuterated β-pyrrolic positions gives evidence magnetic low-spin iron center with δpyrrole = 8.9 ppm, confirming the presence of a center in the superoxo complex, wherein the diamagnetism results from the antife netic coupling of the low-spin iron(III) ion with the superoxo radical anion [49,19 F-NMR spectrum of 9-THF-O2 reveals two identical integrations of F resonance and −113 ppm, respectively, showing two chemically different environments for difluoro substituents of the porphyrinate, presumably giving the credit to the tw cally inequivalent sides of the porphyrinate, wherein one bears the superoxo radic [68]. However, due to the instability of 9-THF-O2, upon increasing temperature temperature, 9-THF-O2 decomposes to a high-spin iron(III)-hydroxide [(F8TPP)Fe III (OH)] (Figure 7), with a paramagnetic 2 H NMR shift at δpyrrole = 125 p

Encapsulation of the Metalloporphyrin in Metal-Organic Frameworks (MOFs)
The aforementioned strategies to promote O2 activation and stabilize iron-O2 adducts involve complicated multistep organic synthesis and harsh reaction conditions, which limits the development of O2 activation. One promising approach encapsulating the metalloporphyrin in MOFs has been used as a platform to study iron-dioxygen species. MOFs are particularly well-suited for O2 activation because of their high degree of synthetic tunability, their porous structure, and their amenability to single-crystal diffraction studies [103,104].
Recently, Harris and co-workers reported a five-coordinate heme dioxygen adduct isolated within an MOF (Figure 9) [105]. The Zr-based porphyrinic MOF PCN-224 is metalated with iron(II) to produce a four-coordinate (4C) ferrous heme-containing complex, PCN-224Fe II (10). Even though some heme-containing MOFs have been synthesized [106][107][108][109][110], 10 is the first example of an MOF with coordinatively unsaturated ferrous heme centers. Upon exposure of single crystals of 10 to 1 atm of O2 at −78 °C, an obvious color change was observed. Its X-ray diffraction analysis reveals that an iron(III) center coordinates to superoxo moiety (PCN-224Fe III O2 •− , 10-O2) in an end-on η 1  Mössbauer spectroscopy further demonstrates the presence of a low-spin electronic configuration for iron(III). Temperature-dependent experiments reveal stoichiometry 1:1 of Fe:O2 and also provide insight into the energetics of dioxygen binding, giving a binding enthalpy of −34(4) kJ/mol. The binding enthalpy of 10-O2 is notably lower than that of 6C Fe II centers (−63 to −65 kJ/mol). 10-O2 is the only example with a structurally characterized "base-free" 5C heme-superoxo species. These results certify the solid-state structure of MOFs can provide a platform to enable isolation and thorough characterization of unstable species that can only be observed transiently in molecular form. In their subsequent works, this system is further extended to PCN-224(Co) and PCN-224(Mn), which also demonstrated selective reversible O2 binding to generate Co III -O2 •− adduct and Mn IV -O2 2adduct, respectively [111,112].

Encapsulation of the Metalloporphyrin in Metal-Organic Frameworks (MOFs)
The aforementioned strategies to promote O 2 activation and stabilize iron-O 2 adducts involve complicated multistep organic synthesis and harsh reaction conditions, which limits the development of O 2 activation. One promising approach encapsulating the metalloporphyrin in MOFs has been used as a platform to study iron-dioxygen species. MOFs are particularly well-suited for O 2 activation because of their high degree of synthetic tunability, their porous structure, and their amenability to single-crystal diffraction studies [103,104].

Conclusions
Dioxygen activation to generate the reactive metal-oxygen species is extremely i portant in the context of energy storage and utilization. In nature, a variety of metallo zymes with cheap metal active sites, such as iron, are able to activate dioxygen und ambient conditions. These systems inspire bioinorganic chemists to synthesize biomime small-molecule iron porphyrin complexes, which are designed to carry out the O2 acti tion. However, the isolation and characterization of iron-dioxygen adducts is a big ch lenge because the absence of surrounding protein superstructure makes iron-dioxyg adducts unstable. The last half century has seen the development of biomimetic chemis to overcome this challenge. From a historical standpoint, this review shows the great p gress in the development of a variety of iron porphyrin models of O2-dependent he enzymes to interpret iron-dioxygen intermediates. Owing to increasingly sophistica spectroscopic techniques (e.g., Mössbauer, XAS, EXAFS, EPR, rR), cryogenic temperatu techniques, and porphyrin ligand development, a few iron-dioxygen species are spect scopically observed by reacting iron porphyrin complexes with O2. Significant sterica hindered iron porphyrin complexes with axial ligands could mimic enzymes to ma metal active sites sequestered, promote O2 binding, and increase the stability of ironoxygen species. Secondary coordination sphere interaction is another useful strategy stabilize iron-dioxygen species via H-bond interactions, which helps keep iron-dioxyg species separate from each other. However, there are limits to these strategies becau they involve complicated multistep organic synthesis and harsh reaction conditions. V recently, due to the merit of MOFs-which are well-suited for O2 activation because their high degree of synthetic tunability, their porous structure, and their amenability single-crystal diffraction studies-one promising approach encapsulating and separat the iron porphyrin in MOFs has been used as a platform to study iron-dioxygen spec The synthesis of MOFs is facile, and the encapsulation results in the trapping of Fe− adducts that lack a sixth axial ligand trans to the O2 binding sites.
Metalloenzymes can efficiently catalyze O2-dependent oxidations, but their mec nisms remain unclear. The development of synthetic models of metal-oxygen interme ates help to illustrate the enzyme mechanisms. Meanwhile, progress in the illustration the mechanisms for the metalloenzymes guides chemists to design potent and select catalysts. Although O2 activation by biomimetic heme complexes has always been a foc of biomimetic chemists, there are still relatively few synthesized complexes that are ca ble of activating O2 so far, and the oxidation yields of these few successful complexes main poor compared with that of metalloenzymes. With the great progress being ma

Conclusions
Dioxygen activation to generate the reactive metal-oxygen species is extremely important in the context of energy storage and utilization. In nature, a variety of metalloenzymes with cheap metal active sites, such as iron, are able to activate dioxygen under ambient conditions. These systems inspire bioinorganic chemists to synthesize biomimetic smallmolecule iron porphyrin complexes, which are designed to carry out the O 2 activation. However, the isolation and characterization of iron-dioxygen adducts is a big challenge because the absence of surrounding protein superstructure makes iron-dioxygen adducts unstable. The last half century has seen the development of biomimetic chemistry to overcome this challenge. From a historical standpoint, this review shows the great progress in the development of a variety of iron porphyrin models of O 2 -dependent heme enzymes to interpret iron-dioxygen intermediates. Owing to increasingly sophisticated spectroscopic techniques (e.g., Mössbauer, XAS, EXAFS, EPR, rR), cryogenic temperature techniques, and porphyrin ligand development, a few iron-dioxygen species are spectroscopically observed by reacting iron porphyrin complexes with O 2 . Significant sterically hindered iron porphyrin complexes with axial ligands could mimic enzymes to make metal active sites sequestered, promote O 2 binding, and increase the stability of iron-dioxygen species. Secondary coordination sphere interaction is another useful strategy to stabilize iron-dioxygen species via H-bond interactions, which helps keep iron-dioxygen species separate from each other. However, there are limits to these strategies because they involve complicated multistep organic synthesis and harsh reaction conditions. Very recently, due to the merit of MOFs-which are well-suited for O 2 activation because of their high degree of synthetic tunability, their porous structure, and their amenability to single-crystal diffraction studiesone promising approach encapsulating and separating the iron porphyrin in MOFs has been used as a platform to study iron-dioxygen species. The synthesis of MOFs is facile, and the encapsulation results in the trapping of Fe−O 2 adducts that lack a sixth axial ligand trans to the O 2 binding sites.
Metalloenzymes can efficiently catalyze O 2 -dependent oxidations, but their mechanisms remain unclear. The development of synthetic models of metal-oxygen intermediates help to illustrate the enzyme mechanisms. Meanwhile, progress in the illustration of the mechanisms for the metalloenzymes guides chemists to design potent and selective catalysts. Although O 2 activation by biomimetic heme complexes has always been a focus of biomimetic chemists, there are still relatively few synthesized complexes that are capable of activating O 2 so far, and the oxidation yields of these few successful complexes remain poor compared with that of metalloenzymes. With the great progress being made over the past several decades, the future holds remarkable promise in isolating metal-dioxygen intermediates in O 2 activation by metal porphyrin complexes.

Conflicts of Interest:
The authors declare no conflict of interest.