Synthesis, characterization, and laser flash photolysis reactivity of a carbonmonoxy heme complex

Catalytic Hydrogenolysis by Atomically Dispersed Iron Sites Embedded in Chemically and Redox Non-innocent N-Doped Carbon

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

Fe-N-C catalyst with Fe-NX sites anchored nano carboncubes derived from Fe-Zn-MOFs activate peroxymonosulfate for high-effective degradation of ciprofloxacin: Thermal activation and catalytic mechanism

Developing high-efficient catalysts is crucial for activating peroxymonosulfate (PMS). Fe-N-C catalysts exhibit excellent performance for PMS activation because of the contribution of doped N, Fe-Nx and Fe₃C sites. In our work, a series of Fe-N-C catalysts with high-performance was obtained by pyrolyzing Fe-Zn-MOFs precursors. During pyrolysis process, the change of chemical bonds and formation of active sites in the precursor were elucidated by characterization analysis and related catalytic experiments. Graphitic N, Fe-Nx and Fe₃C were confirmed to activate PMS synergistically for ciprofloxacin (CIP) degradation. Besides, the catalytic performance was proportional to the amount of doped iron and calcination temperature. Moreover, the Fe-N-C-3-800/PMS system not only displayed good recycling performance, but also had high anti-interference ability. Integrated with quenching and electron paramagnetic resonance (EPR) experiments, a non-radical pathway dominated by ¹O₂ was proposed. Furthermore, PMS could bond to Fe-N-C-3-800 to form intermediate for charge transfer, thus accelerate electron transfer between CIP and PMS to realize degradation of CIP. Six main pathways of CIP degradation were proposed, which include bond fission of N-C on piperazine ring and direct oxidation of CIP. This study provided a new idea for the design of heterogeneous carbon catalysts in advanced oxidation field.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)Fe(II)], [(P(Py))Fe(II)], [(P(Im))Fe(II)], and [(P(ImH))Fe(II)], where F8 = tetrakis(2,6-difluorophenyl)-porphyrinate and P(Py), P(Im), and P(ImH) are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; P(ImH) is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)Fe(II)-(DIMPI)2] in the case of [(F8)Fe(II)], while for the other hemes, mono-DIMPI compounds are obtained, [(P(Py))Fe(II)-(DIMPI)] [(2)-DIMPI], [(P(Im))Fe(II)-(DIMPI)] [(3)-DIMPI], and [(P(ImH))Fe(II)-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. (19)F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)Fe(II)-(NO)], or low-spin six-coordinate compounds [(P(Py))Fe(II)-(NO)], [(P(Im))Fe(II)-(NO)], and [(P(ImH))Fe(II)-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV-Vis, IR, (1)H-NMR, and EPR spectroscopies.

Dehydrogenative α-oxygenation of ethers with an iron catalyst

Selective α-oxidation of ethers under aerobic conditions is a long-pursued transformation; however, a green and efficient catalytic version of this reaction remains challenging. Herein, we report a new family of iron catalysts capable of promoting chemoselective α-oxidation of a range of ethers with excellent mass balance and high turnover numbers under 1 atm of O2 with no need for any additives. Unlike metalloenzymes and related biomimetics, the catalyst produces H2 as the only byproduct. Mechanistic investigations provide evidence for an unexpected two-step reaction pathway, which involves dehydrogenative incorporation of O2 into the ether to give a peroxobisether intermediate followed by cleavage of the peroxy bond to form two ester molecules, releasing stoichiometric H2 gas in each step. The operational simplicity and environmental friendliness of this methodology affords a useful alternative for performing oxidation, while the unique ability of the catalyst in oxygenating a substrate via dehydrogenation points to a new direction for understanding metalloenzymes and designing new biomimetic catalysts.

New heme-dioxygen and carbon monoxide adducts using pyridyl or imidazolyl tailed porphyrins

Inspired by the chemistry relevant to dioxygen storage, transport and activation by metalloproteins, in particular for heme/copper oxidases, and carbon monoxide binding to metal-containing active sites as a probe or surrogate for dioxygen binding, a series of heme derived dioxygen and CO complexes have been designed, synthesized, and characterized with respect to their physical properties and reactivity. The focus of this study is in the description and comparison of three types heme-superoxo and heme-CO adducts. The starting point is in the characterization of the reduced heme complexes, [(F₈)Fe^II], [(P^Py)Fe^II] and [(P^Im)Fe^II], where F₈, P^Py and P^Im are iron(II)-porphyrinates and where P^Py and P^Im possess a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of these complexes vary with solvent. The low temperature reaction between O₂ and these reduced porphyrin Fe^II complex yield distinctive low spin heme-superoxo adducts. The dioxygen binding properties for all three complexes are shown to be reversible, via alternate argon or O₂ bubbling. Carbon monoxide binds to the reduced heme-Fe^II precursors to form low spin heme-CO adducts. The implications for future investigations of these heme O₂ and CO adducts are discussed.

Second sphere control of redox catalysis: selective reduction of O2 to O2- or H2O by an iron porphyrin catalyst

"Click" reaction has been utilized to synthesize porphyrin ligands possessing distal superstructures functionalized with ferrocenes, carboxylic acid esters, and phenols. Both structural and spectroscopic evidence indicate that hydrogen bonding interaction between the triazole residues resulting from the "click" reaction promotes axial ligand binding into the sterically demanding distal pocket in preference to the open proximal side. An iron porphyrin complex with four ferrocene groups is found to bind O(2) and quantitatively reduce it by one electron to O(2)(-) in apolar organic solvents. However the same complex electro-catalytically reduces O(2) by four electrons to H(2)O in aqueous medium under fast, moderate, and slow electron fluxes. This selectivity for O(2) reduction is governed by the reduction potential of the electron transfer site (i.e., ferrocene) which in turn is governed by the solvent. This catalyst mimics control of catalysis of an enzyme active site by a second sphere electron transfer residue which is often encountered in naturally occurring metallo-enzymes.

Reactivity of coordinatively unsaturated iron complexes towards carbon monoxide: to bind or not to bind?

An overview of the reactivity of coordinatively unsaturated iron complexes (in most cases Fe(II)) towards carbon monoxide is presented. Unsaturated iron complexes are known with coordination numbers (CN) of two to five adopting linear or slightly bent (CN = 2), trigonal (CN = 3), tetrahedral, square planar or trigonal pyramidal (CN = 4), and square-pyramidal or trigonal-bipyramidal geometries (CN = 5), respectively. The binding of CO depends strongly on the number and the nature of co-ligands (overall ligand field strength), the charge of the complex, the complex geometry, and the spin state of the unsaturated metal center. In many cases, CO addition to high-spin iron complexes takes place with concomitant spin state changes forming compounds in the lowest possible spin state, i.e., with S = 0. In several other cases, however, the addition of CO is reversible or is even totally rejected altogether for either thermodynamic or kinetic reasons. In the case of the latter such reactions are termed "spin-blocked" or "spin forbidden".

Binding and photodissociation of CO in iron(II) complexes for application in positron emission tomography (PET) radiolabelling

(R-DAB)FeI(2) complexes containing bidentate diimide ligands (R-DAB = RN=CH-CH=NR; R = (i)Pr, c-C(6)H(11)) have been investigated for their ability to react with carbon monoxide to form iron(II) dicarbonyl complexes, (R-DAB)FeI(2)(CO)(2). Solution IR spectroscopy revealed two νCO stretches between 2000 and 2040 cm(-1) corresponding to a cis-arrangement of the carbonyl ligands around the iron. Photochemical decarbonylation was achieved by UV irradiation (365 nm), which occurred within 5 min as evidenced by solution IR spectroscopy. (c-C(6)H(11)-DAB)FeI(2) has been characterised by X-ray crystallography. Reactions using (11)C-labelled carbon monoxide were investigated and revealed that both (R-DAB)FeI(2) species were not effective as trapping complexes due to the low concentrations of [(11)C]CO used in these experiments. A Fe(TPP)(THF)(x) (TPP = tetraphenylporphyrin) complex was investigated with unlabelled CO and the monocarbonyl adduct Fe(TPP)(THF)CO was formed in situ as identified by IR spectroscopy (νCO = 1966 cm(-1)) yet was stable to CO loss upon UV irradiation. Carbonylation reactions of in situ-generated Fe(TPP)(THF)(x) using [(11)C]CO revealed that 97% of the [(11)C]CO stream could be trapped in one pass of the gas at room temperature and at atmospheric pressure.

Reductive coupling of nitrogen monoxide (*NO) facilitated by heme/copper complexes

The interactions of nitrogen monoxide (*NO; nitric oxide) with transition metal centers continue to be of great interest, in part due to their importance in biochemical processes. Here, we describe *NO((g)) reductive coupling chemistry of possible relevance to that process (i.e., nitric oxide reductase (NOR) biochemistry), which occurs at the heme/Cu active site of cytochrome c oxidases (CcOs). In this report, heme/Cu/*NO((g)) activity is studied using 1:1 ratios of heme and copper complex components, (F(8))Fe (F(8) = tetrakis(2,6-difluorophenyl)porphyrinate(2-)) and [(tmpa)Cu(I)(MeCN)](+) (TMPA = tris(2-pyridylmethyl)amine). The starting point for heme chemistry is the mononitrosyl complex (F(8))Fe(NO) (lambda(max) = 399 (Soret), 541 nm in acetone). Variable-temperature (1)H and (2)H NMR spectra reveal a broad peak at delta = 6.05 ppm (pyrrole) at room temperature (RT), which gives rise to asymmetrically split pyrrole peaks at 9.12 and 8.54 ppm at -80 degrees C. A new heme dinitrosyl species, (F(8))Fe(NO)(2), obtained by bubbling (F(8))Fe(NO) with *NO((g)) at -80 degrees C, could be reversibly formed, as monitored by UV-vis (lambda(max) = 426 (Soret), 538 nm in acetone), EPR (silent), and NMR spectroscopies; that is, the mono-NO complex was regenerated upon warming to RT. (F(8))Fe(NO)(2) reacts with [(tmpa)Cu(I)(MeCN)](+) and 2 equiv of acid to give [(F(8))Fe(III)](+), [(tmpa)Cu(II)(solvent)](2+), and N(2)O((g)), fitting the stoichiometric *NO((g)) reductive coupling reaction: 2*NO((g)) + Fe(II) + Cu(I) + 2H(+) --> N(2)O((g)) + Fe(III) + Cu(II) + H(2)O, equivalent to one enzyme turnover. Control reaction chemistry shows that both iron and copper centers are required for the NOR-type chemistry observed and that, if acid is not present, half the *NO is trapped as a (F(8))Fe(NO) complex, while the remaining nitrogen monoxide undergoes copper complex promoted disproportionation chemistry. As part of this study, [(F(8))Fe(III)]SbF(6) was synthesized and characterized by X-ray crystallography, along with EPR (77 K: g = 5.84 and 6.12 in CH(2)Cl(2) and THF, respectively) and variable-temperature NMR spectroscopies. These structural and physical properties suggest that at RT this complex consists of an admixture of high and intermediate spin states.

Molecular oxygen and sulfur reactivity of a cyclotriveratrylene derived trinuclear copper(I) complex

Our continuing efforts into developing copper coordination chemistry relevant to dioxygen-processing copper proteins has led us to design and synthesize a cyclotriveratrylene (CTV)-based trinucleating ligand, CTV-TMPA, which employs tetradentate tris(2-pyridylmethyl)-amine chelates (TMPA) for their copper ion binding sites. Binding of three copper ions per CTV-TMPA unit was established by various chemical and spectroscopic methods such as UV-vis and resonance Raman (rR) spectroscopies. The following complexes were observed: A tricopper(I) complex [(CTV-TMPA)Cu(I)(3)](3+) (1), a CO adduct [(CTV-TMPA)Cu(I)(3)(CO)(3)](3+) (1-CO; nu(C=O) = 2094 cm(-1)), a triphenylphosphine adduct [(CTV-TMPA)Cu(I)(3)(PPh(3))(3)](3+) (1-PPh(3)), a tricopper(II) complex [(CTV-TMPA)Cu(II)(3)](3+) (1-Ox), and its tris-monochloride or tris-monobromide adducts. Also, introduction of dioxygen to the -80 degrees C solutions of 1 leads to O(2)-adducts, the first example of a synthetic copper complex which can stabilize a mononuclear Cu(II)-superoxo and dinuclear peroxo species simultaneously within one complex {[Cu] = 1.53 mM in THF: (mu-1,2-peroxo complex, lambda(max) = 543 (epsilon 9650) nm): nu(O-O) = 825 ((Delta(18)O(2)) = -47) cm(-1); nu(Cu-O) = 506 ((Delta(18)O(2)) = -26) cm(-1): (superoxo complex, lambda(max) = 427 (epsilon 3150) nm): nu(O-O) = 1129 ((Delta(18)O(2)) = -60) cm(-1); nu(Cu-O) = 463 ((Delta(18)O(2)) = -27) cm(-1)}. Elemental sulfur reacts reversibly with 1 leading to a (proposed) hexanuclear species [{(CTV-TMPA)Cu(II)(3)}(2)(mu-1,2-S(2)(2-))(3)](6+) (1-S) {lambda(max) = 544 (epsilon 7270) nm}, possessing one dicopper(II)-disulfide structural type: {THF solvent) nu(S-S) = 489 ((Delta(34)S) = -10) cm(-1); nu(Cu-S) = 307 ((Delta(34)S) = -5) cm(-1)}. Derivation of spectroscopic, structural, and chemical conclusions were aided by the study of a close mononuclear analogue with one pyridyl group of the TMPA parent possessing a 6-CH(2)OCH(3) substituent, this being part of the CTV-TMPA architecture.

Reaction of the Five-Coordinate O-Nitrito Complex Fe(Por)(ONO) (Por = meso-tetra-arylporphyrinato) with THF Gives Two Six-Coordinate Isomers

The effect of the proximal ligand on the coordination of the nitrite ligand to the heme model systems Fe(Por)(η1-ONO) (Por = meso-tetraarylporphyrinato dianion) was investigated by FTIR and UV-vis spectra in solvent free, low temperature, porous layered solids and by density functional computations. The reaction of the five-coordinate complex Fe(Por)(η1-ONO) with the ether tetrahydrofuran gives a mixture of the O-nitrito and N-nitrito isomers Fe(Por)(THF)(η1-ONO) and Fe(Por)(THF)(NO2), respectively. This observation is in contrast to earlier studies with nitrogen donor Lewis bases where the N-nitrito isomers were clearly the more stable of the six-coordinated complexes. The adduct formation is reversible; the five-coordinate O-nitrito complexes Fe(Por)(η1-ONO) were largely restored upon warming under vacuum pumping.

Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K1 = 267 and K2 approximately 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)2] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930(4) A, beta = 116.246(1) degrees , V = 1992.95(10) A3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-Np distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.

High-spin diimine complexes of iron(II) reject binding of carbon monoxide: theoretical analysis of thermodynamic factors inhibiting or favoring spin-crossover

A new series of Fe(II) complexes, FeCl2[N(R)=C(Me)C(Me)=N(R)], containing diimine ligands with hemilabile sidearms R (R = CH2(CH2)2NMe2, 1, CH2(CH2)2OMe, 2, CH2(CH2)2SMe), 3) were synthesized. The crystal structure of 1 showed 6-coordination where both amine arms were attached, whereas 2 was a 5-coordinate 16e species with one methoxy arm dangling free. Extensive attempts were made to bind CO to these species to synthesize precursors for dihydrogen complexes but were unsuccessful. Reaction of 1 with 1 or 2 equiv of AgOTf under CO atmosphere resulted in isolation of only a 6-coordinate bis(triflate)-containing product [Fe[N(R)=C(Me)C(Me)=N(R)](OTf)2] (R = CH2(CH2)2NMe2), 5. Reaction of 5-coordinate 2 with AgSbF6 under CO did not give a CO adduct but afforded instead a dicationic dinuclear complex [Fe[N(R)=C(Me)C(Me)=N(R)](mu-Cl)]2[SbF6]2 (R = CH2(CH2)2OMe), 4, containing a weakly bound SbF6. Thus coordination of hard-donor anions to iron was favored over CO binding. The unexpected rejection of binding of CO is rationalized by the iron being in a high-spin state in this system and energetically incapable of spin crossover to a low-spin state. Theoretical calculations on CO interaction with Fe(II) centers in spin states S = 0, 1, and 2 for both the 16e complexes and their CO adducts aid further understanding of this problem. They show that interaction of CO with a high-spin 5-coordinate Fe model diimine complex is essentially thermoneutral but is exergonic by about 48 kcal/mol to a comparable but low-spin diphosphine fragment. Spin crossover is thus disfavored thermodynamically rather than kinetically (e.g. a "spin block" effect); i.e., the ligand field strengths of the primarily N-donor groups are apparently insufficient to give a low-spin CO adduct.