Superoxide Moieties Research Articles

Selective Adsorption of Oxygen from Humid Air in a Metal-Organic Framework with Trigonal Pyramidal Copper(I) Sites.

High or enriched-purity O2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O2-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O2 over N2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework CuI-MFU-4l (CuxZn5-xCl4-x(btdd)3; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which binds O2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal CuI sites, and we show that this material reversibly captures O2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, CuI-MFU-4l retains a constant O2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N2, differences in O2 and N2 desorption kinetics allow for the isolation of high-purity O2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O2 binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that CuI-MFU-4l is a promising material for the separation of O2 from ambient air, even without dehumidification.

Proton-coupled electron transfer reactivities of electronically divergent heme superoxide intermediates: a kinetic, thermodynamic, and theoretical study.

Heme superoxides are one of the most versatile metallo-intermediates in biology, and they mediate a vast variety of oxidation and oxygenation reactions involving O2(g). Overall proton-coupled electron transfer (PCET) processes they facilitate may proceed via several different mechanistic pathways, attributes of which are not yet fully understood. Herein we present a detailed investigation into concerted PCET events of a series of geometrically similar, but electronically disparate synthetic heme superoxide mimics, where unprecedented, PCET feasibility-determining electronic effects of the heme center have been identified. These electronic factors firmly modulate both thermodynamic and kinetic parameters that are central to PCET, as supported by our experimental and theoretical observations. Consistently, the most electron-deficient superoxide adduct shows the strongest driving force for PCET, whereas the most electron-rich system remains unreactive. The pivotal role of these findings in understanding significant heme systems in biology, as well as in alternative energy applications is also discussed.

Ferroptosis contributes to developmental cell death in rice blast.

Ferroptosis, an iron-dependent cell death process, was found to occur in Magnaporthe oryzae, and plays a key role in infection-related development therein. Ferroptosis in the rice-blast fungus was confirmed based on five basic criteria. We confirmed the dependence of ferroptosis on ferric ions, and optimized ratio-fluorescence imaging of C11-BODIPY581/591 as a precise sensor for lipid peroxides that mediate ferroptosis in M.oryzae. We uncovered an important regulatory function for reduced glutathione and NADPH oxidases in modulating the superoxide moieties required for ferroptotic cell death. We found ferroptosis to be necessary for the developmental cell death of conidia during appressorium maturation in rice blast. Such ferroptotic cell death initiated first in the terminal cell and progressed sequentially to the entire conidium. Iron chelation or chemical inhibition of ferroptosis caused conidial cells to remain viable, and led to strong defects in host invasion by M.oryzae. Ferroptosis induction exclusively in the host severely constrained the invasive growth of M.oryzae. We found inter-reliant and independent roles for ferroptosis and autophagy in controlling such precise cell death in M.oryzae during pathogenic differentiation. Our study provides significant molecular insights into the role of developmental cell death and iron homeostasis in fungal pathogenesis.

Routes to Heterotrinuclear Metal Siloxide Complexes for Cooperative Activation of O2.

The assembly of heterometallic complexes capable of activating dioxygen is synthetically challenging. Here, we report two different approaches for the preparation of heterometallic superoxide complexes [PhL2CrIII-η1-O2][MX]2 (PhL = -OPh2SiOSiPh2O-, MX+ = [CoCl]+, [ZnBr]+, [ZnCl]+) starting from the CrII precursor complex [PhL2CrII]Li2(THF)4. The first strategy proceeds via the exchange of Li+ by [MX]+ through the addition of MX2 to [PhL2CrII]Li2(THF)4 before the reaction with dioxygen, whereas in the second approach a salt metathesis reaction is undertaken after O2 activation by adding MX2 to [PhL2CrIII-η1-O2]Li2(THF)4. The first strategy is not applicable in the case of redox-active metal ions, such as Fe2+ or Co2+, as it leads to the oxidation of the central chromium ion, as exemplified with the isolation of [PhL2CrIIICl][CoCl]2(THF)3. However, it provided access to the hetero-bimetallic complexes [PhL2CrIII-η1-O2][MX]2 ([MX]+ = [ZnBr]+, [ZnCl]+) with redox-inactive flanking metals incorporated. The second strategy can be applied not only for redox-inactive but also for redox-active metal ions and led to the formation of chromium(III) superoxide complexes [PhL2CrIII-η1-O2][MX]2 (MX+ = [ZnCl]+, [ZnBr]+, [CoCl]+). The results of stability and reactivity studies (employing TEMPO-H and phenols as substrates) as well as a comparison with the alkali metal series (M+ = Li+, Na+, K+) confirmed that although the stability is dependent on the Lewis acidity of the counterions M and the number of solvent molecules coordinated to those, the reactivity is strongly dependent on the accessibility of the superoxide moiety. Consequently, replacement of Li+ by XZn+ in the superoxides leads to more stable complexes, which at the same time behave more reactive toward O-H groups. Hence, the approaches presented here broaden the scope of accessible heterometallic O2 activating compounds and provide the basis for further tuning of the reactivity of [RL2CrIII-η1-O2]M2 complexes.

Modeling Tryptophan/Indoleamine 2,3-Dioxygenase with Heme Superoxide Mimics: Is Ferryl the Key Intermediate?

Tryptophan oxidation in biology has been recently implicated in a vast array of paramount pathogenic conditions in humans, including multiple sclerosis, rheumatoid arthritis, type-I diabetes, and cancer. This 2,3-dioxygenative cleavage of the indole ring of tryptophan with dioxygen is mediated by two heme enzymes, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), during its conversion to N-formylkynurenine in the first and rate-limiting step of kynurenine pathway. Despite the pivotal significance of this enzymatic transformation, a vivid viewpoint of the precise mechanistic events is far from complete. A heme superoxide adduct is thought to be the active oxidant in both TDO and IDO, which, following O-O bond cleavage, presumably generates a key ferryl (FeIV=O) reaction intermediate. This study, for the first time in model chemistry, demonstrates the potential of synthetic heme superoxide adducts to mimic the bioinorganic chemistry of indole dioxygenation by TDO and IDO, challenging the widely accepted categorization of these metal adducts as weak oxidants. Herein, an electronically divergent series of ferric heme superoxo oxidants mediates the facile conversion of an array of indole substrates into their corresponding 2,3-dioxygenated products, while shedding light on an unequivocally occurring, putative ferryl intermediate. The oxygenated indole products have been isolated in ∼31% yield, and characterized by LC-MS, 1H and 13C NMR, and FT-IR methodologies, as well as by 18O2(g) labeling experiments. Distinctly, the most electron-deficient superoxo adduct is observed to react the fastest, specifically with the most electron-rich indole substrate, underscoring the cruciality of electrophilicity of the heme superoxide moiety in facilitating the initial indole activation step. Comprehensive understanding of such mechanistic subtleties will benefit future attempts in the rational design of salient therapeutic agents, including next generation anticancer drug targets with amplified effectivity.

Ligand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond.

A superoxide-bridged dicopper(II) complex, [CuII2(XYLO)(O2•-)]2+ (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [CuII2(XYLO)(O22-)]+ (2), using ferrocenium (Fc+) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me10Fc+, a 1 ⇆ 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs Fc+/0. Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [CuII2(XYLO)(OOH)]2+ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pKa of 3 was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [CuII2(XYLO)(O2•-)]2+ (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M-1 s-1. Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O2-derived reduced species, such as pKa, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.

The Influence of Alkali Metal Ions on the Stability and Reactivity of Chromium(III) Superoxide Moieties Spanned by Siloxide Ligands.

In recent years, it has become clear that the presence of redox-inactive Lewis acidic metal ions can decisively influence the reactivity of metal-dioxygen moieties that are formed in the course of O2 activation, in molecular complexes, and metalloenzymes. Superoxide species are often formed as the primary intermediates but they are mostly too unstable for a thorough investigation. We report here a series of chromium(III) superoxide complexes [L2 Cr]M2 O2 (THF)y (L=- OSiPh2 OSiPh2 O- , M+ =Li+ , Na+ , K+ and y=4, 5), which could be accessed, studied spectroscopically and partly crystallized at low temperatures. They only differ in the two incorporated Lewis acidic alkali metal counterions (M+ ) and it could thus be shown that the nature of M+ determines considerably its interaction with the superoxide ligand. This interaction, in turn, has a significant influence on the stability and reactivity of these complexes towards substrates with OH groups. Furthermore, we show that stability and reactivity are also highly solvent dependent (THF versus nitriles), as donor solvents coordinate to the alkali metal ions and thus also influence their interaction with the superoxide moiety. Altogether, these results provide a comprehensive and detailed picture concerning the correlation between spectroscopic properties, structure, and behavior of such superoxides, that may be exemplary for other systems.

Shedding Light on the Oxygen Reduction Reaction Mechanism in Ether-Based Electrolyte Solutions: A Study Using Operando UV-Vis Spectroscopy.

Using UV-vis spectroscopy in conjunction with various electrochemical techniques, we have developed a new effective operando methodology for investigating the oxygen reduction reactions (ORRs) and their mechanisms in nonaqueous solutions. We can follow the in situ formation and presence of superoxide moieties during ORR as a function of solvent, cations, anions, and additives in the solution. Thus, using operando UV-vis spectroscopy, we found evidence for the formation of superoxide radical anions during oxygen reduction in LiTFSI/diglyme electrolyte solutions. Nitro blue tetrazolium (NBT) was used to indicate the presence of superoxide moieties based on its unique spectral response. Indeed, the spectral response of NBT containing solutions undergoing ORR could provide a direct indication for the level of association of the Li cations with the electrolyte anions.

A DFT/B3LYP study of the mechanisms of the O2 formation reaction catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 complex: A paradigm for photosystem II

We present a theoretical study of the reaction pathway for dioxygen molecular formation catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2) (terpy)](NO3)3 (terpy=2,2′:6′,2″-terpyridine) complex based on DFT-B3LYP calculations. In the initial state of the reaction, a partial oxido radical (0.44 spins) is formed ligated to Mn. This radical is involved in a nucleophylic attack by bulk water in the OO bond reaction formation step, in which the oxido fractional unpaired electron is delocalized toward the outermost Mn of the μ-oxo bridge, instead of the ligated Mn center. The reaction then follows with a series of proton-coupled electron transfer steps, in which the oxidation state, as well as the bond strength of the OO moiety increase, while the OOMn(1) bond gets weaker until O2 is released. In this model, basic acetate ions from the buffer solution capture protons in the proton-transfer steps. In each step there is reduction of the OOMn(1) binding strength, with concomitant increase of the OO bond strength, which culminates with the release of O2 in the last step. This last step is entropy driven, while formation of hydroperoxide and superoxide moieties is enthalpy driven. According with experiments, the rate-limiting step is the double oxidation of Mn(IV,III) or peroxymonosulfate binding, which occur prior to the OO bond formation step. This supports our findings that the barriers of all intermediate steps are below the experimental barrier of 19–21kcal/mol. The implications of these findings for understanding photosynthetic water-splitting catalysis are also discussed.

The Mechanistic Role of Lithium Salts in Aprotic Li-O2 Batteries

Lithium oxygen (Li-O2) research generates much interest and many expectations among electrochemists. The high theoretical specific energy, the simplicity of preparation and operation of electrochemical cells, the sizable commercial possibilities, and the ever-increasing number of new publications, has directed many groups to Li-O2 battery research. Over the past decade, significant progress in the study of possible Li-O2 battery systems has also prompted interest from the chemical and automotive industries. Nevertheless, practical Li-O2 batteries are far from realization. Many scientific challenges are related to the oxygen reduction reactions (ORR). In aprotic Li-O2 cells the general assumption is that during discharge, oxygen is reduced on the cathode to form insoluble lithium oxide compounds. Experimental evidence support that the major final product is lithium peroxide (Li2O2). Nevertheless, the electrolyte solution and the carbon cathode instability toward reduced oxygen species lead to intrinsic instability and irreversible formation of many side products. Electrochemical formation of Li2O2 can proceed in two ways: (i) One electron transfer to molecular oxygen to form LiO2, which continues to react by receiving another electron and Li ion to form Li2O2. (ii) Two electron transfer pathway that skip over the formation of LiO2, and forms directly Li2O2. Besides the electrochemical routes, Li2O2 can be formed chemically by disproportionation of Li-superoxide to form molecular oxygen as the second product. In fact, electrochemically formed LiO2 is unstable, its further reaction to form Li2O2 is fast1. The possibility of isolating lithium superoxide and to assess its lifetime in different aprotic media are still under investigation. The rout which Li2O2 is being formed can affect the growth mechanism. Li2O2 is insoluble in aprotic solvents, thus during ORR, crystalline Li2O2 is deposit on the cathode surface. When fully covered by ORR products, the continuation of the process depends on electrons crossing the product layer to reach fresh oxygen molecules. Due to poor electronic conductivity of the oxygen reduction product layer, the cathode is deactivated and the ORR is terminated after the deposition of a few layers of Li-peroxide. The stabilization of superoxide intermediate can change the growth mechanism of the Li2O2 layer. In contrast to lithium peroxide, superoxide radical can be soluble in aprotic solvents that contain lithium cations. During discharge, oxygen is reduce to superoxide through a free cathode site. Then, the charged radical can diffuse in the solution and transfer charge or to disproportionate in all dimensions, leading to a thick deposits of Li2O2. This mechanism can be considered as a top down growth, as superoxide can disproportionate on top of a Li2O2 particle. Using aprotic solvents with high Guttman donor number such as DMSO, DMF, and DMA in Li-oxygen cells can help to extend the superoxide lifetime. The presence of high DN solvent reduces the electrophilicity (Lewis acidity) of the Li cations and thereby helping to extend the stability of O2 -· before disproportionating to Li2O2. In practice using high donor number solvents in Li-O2 cells is challenging due to their reactivity toward the lithium metal anode and reduced oxygen species. Recently it was reported that superoxide radicals can be stabilized indirectly by the electrolytes’ counter anions. In highly associated electrolytes the counter anions are coordinated to the solvated lithium cations, these interactions practically reduce the lithium cations Lewis acidity and thereby temporarily preventing them to bind to the reduced oxygen moieties. As a result, soluble species such as superoxide can take part in the growth mechanism of Li-peroxide deposits on the cathode surface. In this manner we can stabilize superoxide moieties, lead to massive Li2O2 deposition by in a top-down mechanism (i.e. high ORR capacity) although using relatively low DN number solvents such as glymes - CH3O(CH2CH2O)nCH3. Glymes are suggested to be relatively stable towered lithium metal, lithiated carbons and reduced oxygen species.

Hydrogen Storage and Selective, Reversible O2 Adsorption in a Metal-Organic Framework with Open Chromium(II) Sites.

A chromium(II)-based metal-organic framework Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT(3-) =1,3,5-benzenetristetrazolate), featuring coordinatively unsaturated, redox-active Cr(2+) cation sites, was synthesized and investigated for potential applications in H2 storage and O2 production. Low-pressure H2 adsorption and neutron powder diffraction experiments reveal moderately strong Cr-H2 interactions, in line with results from previously reported M-BTT frameworks. Notably, gas adsorption measurements also reveal excellent O2 /N2 selectivity with substantial O2 reversibility at room temperature, based on selective electron transfer to form Cr(III) superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O2 binding.

Hydrogen Storage and Selective, Reversible O2 Adsorption in a Metal–Organic Framework with Open Chromium(II) Sites

AbstractA chromium(II)‐based metal–organic framework Cr3[(Cr4Cl)3(BTT)8]2 (Cr‐BTT; BTT3−=1,3,5‐benzenetristetrazolate), featuring coordinatively unsaturated, redox‐active Cr2+ cation sites, was synthesized and investigated for potential applications in H2 storage and O2 production. Low‐pressure H2 adsorption and neutron powder diffraction experiments reveal moderately strong Cr–H2 interactions, in line with results from previously reported M‐BTT frameworks. Notably, gas adsorption measurements also reveal excellent O2/N2 selectivity with substantial O2 reversibility at room temperature, based on selective electron transfer to form CrIII superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O2 binding.

Unravelling the Molecular Origin of the Regiospecificity in Extradiol Catechol Dioxygenases.

Many factors have been suggested to control the selectivity for extradiol or intradiol cleavage in catechol dioxygenases. The varied selectivity of model complexes and the ability to force an extradiol enzyme to do intradiol cleavage indicate that the problem may be complex. In this paper we focus on the regiospecificity of the proximal extradiol dioxygenase, homoprotocatechuate 2,3-dioxygenase (HPCD), for which considerable advances have been made in our understanding of the mechanism from an experimental and computational standpoint. Two key steps in the reaction mechanism were investigated: (1) attack of the substrate by the superoxide moiety and (2) attack of the substrate by the oxyl radical generated by O-O bond cleavage. The selectivity at both steps was investigated through a systematic study of the role of the substrate and the first and second coordination spheres. For the isolated native substrate, intradiol cleavage is calculated to be both kinetically and thermodynamically favored, therefore nature must use the enzyme environment to reverse this preference. Two second sphere residues were found to play key roles in controlling the regiospecificity of the reaction: Tyr257 and His200. Tyr257 controls the selectivity by modulating the electronic structure of the substrate, while His200 controls selectivity through steric effects and by preventing alternative pathways to intradiol cleavage.

Nucleophilic Reactivity of a Metal-Bound Superoxide Ligand

Metal–superoxide species have frequently been implicated as transient intermediates in dioxygen activating metalloenzymes. The metal-bound superoxide moiety has been proposed to react both as an electrophile and a nucleophile. In general, model complexes that mimic metalloenzyme function have been shown to react as electrophiles, and none have displayed nucleophilic reactivity. Herein, we highlight a recent contribution from our group in which we demonstrated the first well-defined example of a metal–superoxide acting as a nucleophile. We have shown the propensity for a copper-superoxide species to perform nucleophilic deformylation of various aldehydes. In doing so, we have provided experimental support for the postulated role of an iron(III)–superoxide reacting as a nucleophile in the α-ketoglutarate-dependent dioxygenases amongst others.

The Catalytic Behavior of Lithium Nitrate in Li-O2 Batteries

Over the last decade extra efforts were invested in the development of aprotic Li-O2 cells. Early research presented optimistic results that showed the great potential of this system. However in recent years more research works observed that it is hard to find suitable cell components that will enable prolong cycling of Li-O2 cells. Many challenges need to be addressed. Two dominant subjects were given special attention: the carbon cathodes and the electrolyte solutions. These factors governed Li-O2 cells’ operation during both the oxygen reduction reaction (ORR) and the oxygen evaluation reaction (OER). Despite many attempts to find solvents that are stable toward active oxygen species formed by oxygen reduction (super-oxide, peroxide moieties) , no solvent was found to be fully stable during ORR & OER. Several sovents were explored and although none of them was found to be stable, they presented difference features that can affect positively the ORR. One parameter is the Guttmann number of the solvent. High donor number solvents like dimethyl sulfoxide (DMSO), dimethylacetamide (DMA) and dimethylformamide (DMF) are able to stabilize the soluble superoxide intermediate during ORR. Stabilization of superoxide moieties in the course of oxygen reaction can help the growth mechanism of the main product of ORR in aprotic media, namely, Li2O2. Similar effect was accomplished by adding water contaminations to the electrolyte solution consequently enhancing formation of soluble species. However, solvents and solution additives that stabilize superoxide species can undergo parasitic, side reactions. Soluble charged species were investigated as red-ox mediators that can reduce the over-potential of OER. Redox mediators used as additives in solution can easily oxidize the Li2O2 by accepting charge at relatively low potentials and transfer it to the carbon cathode. In this work we show that by using lithium nitrate (LiNO3) as an electrolyte, we can positively affect ORR mechanism and reduce OER over-potential. LiNO3 is a well-known passivating agent for lithium metal anodes. This phenomenon can be used in Li metal based batteries such as Li-S systems. In Li-O2 cells LiNO3 was first introduced as a passivating agent for Li metal in cells based on amide solvents like DMA1. We show that LiNO3 by-product NO2 -, from the reaction of NO3 - with lithium metal, can behave as a redox mediator that reduces OER over-potential below 4V. We suggest that the oxidation of NO2 - to NO2 forms a NO2/NO2 - couple which serves as a good red-ox mediator. . NO2 can oxidize the Li2O2 and, as a result, the NO2 is reduced back to NO2 -. As with other redox mediators, only a small amount is needed to complete the process. The ability of the soluble NO2 - anions to be oxidized on the carbon surface and then oxidize thick Li2O2 surface films, enables to reduce pronouncedly the OER over-potential. Recent work2 suggested that anions that can complex with the lithium cations reduce their Lewis acidity. As a result, soluble species like superoxide can take part as intermediates in the Li2O2 formation. Lithium nitrate moieties are associated in poly-ethers more than any other lithium salt used in lithium batteries. This means that nitrate anions can be coordinate strongly to solvated Li cations thus enabling formation of mobile superoxide moieties. We show that even using cathodes with flat area, we can achieve high capacity of ORR, including formation of submicron Li2O2 particles due to the influence of the NO3 - ionson the growth mechanism as presented in Figure. 1. Figure 1. HR-SEM images of gold electrodes discharged to 2 V in 1M LiNO3 diglyme electrolyte solution. References (1) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V; Addison, D. J. Am. Chem. Soc. 2013, 135 (6), 2076. (2) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. a.; Abraham, K. M. J. Electrochem. Soc. 2015, 162 (6), A1055. Figure 1

Tridentate Copper Ligand Influences on Heme−Peroxo−Copper Formation and Properties: Reduced, Superoxo, and μ-Peroxo Iron/Copper Complexes

In cytochrome c oxidase synthetic modeling studies, we recently reported a new mu-eta2:eta2-peroxo binding mode in the heteronuclear heme/copper complex [(2L)Fe(III)-(O2(2-))-CuII]+ (6) which is effected by tridentate copper chelation (J. Am. Chem. Soc. 2004, 126, 12716). To establish fundamental coordination and O2-reactivity chemistry, we have studied and describe here (i) the structure and dioxygen reactivity of the copper-free compound (2L)FeII (1), (ii) detailed spectroscopic properties of 6 in comparisons with those of known mu-eta2:eta1 heme-peroxo-copper complexes, (iii) formation of 6 from the reactions of [(2L)FeIICuI]+ (3) and dioxygen by stopped-flow kinetics, and (iv) reactivities of 6 with CO and PPh3. In the absence of copper, 1 serves as a myoglobin model compound possessing a pyridine-bound five-coordinate iron(II)-porphyrinate which undergoes reversible dioxygen binding. Oxygenation of 3 below -60 degrees C generates the heme-peroxo-copper complex 6 with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) to yield an S = 2 spin system. Stopped-flow kinetics in CH2Cl2/6% EtCN show that dioxygen reacts with iron(II) first to form a heme-superoxide moiety, [(EtCN)(2L)FeIII-(O2-)...CuI(EtCN)]+ (5), which further reacts with Cu(I) to generate 6. Compared to those properties of a known mu-eta2:eta1-heme-peroxo-copper complex, 6 has a significantly diminished resonance Raman nu(O-O) stretching frequency at 747 cm(-1) and distinctive visible absorptions at 485, 541, and 572 nm, all of which seem to be characteristics of a mu-eta2:eta2-heme-peroxo-copper system. Addition of CO or PPh3 to 6 yields a bis-CO adduct of 3 or a PPh(3) adduct of 5, the latter with a remaining FeIII-(O2-) moiety.

Oxidants containing rhodium. X-ray photoelectroc spectra of dicaesium hexachlororhodate(iv), claus' blue and model compounds

The X-ray photoeletron spectra of dicaesium hexachlororhodate(IV), tricaesium hexachlororhodate(III), a hydrated rhodium(III) oxide, and the barium salt of Claus' blue—previously formulated as a rhodate(VI)—suggest that the latter has rhodium(III) ions in an oxygen environment. ESR and Raman spectra of this blue solid confirm that the superoxide moiety is present. Prior work on higher oxidation states of rhodium in various environments is critically re-assessed. The compounds Cs 2[MCl 6] (M = Rh, Pt and Ir) are isomorphous by X-ray powder diffraction.