FeIV Complex Research Articles

Isostructural Series of a Cyclometalated Iron Complex in Three Oxidation States.

An isostructural series of FeII, FeIII, and FeIV complexes [Fe(ImP)2]0/+/2+ utilizing the ImP 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene) ligand, combining N-heterocyclic carbenes and cyclometalating functions, is presented. The strong donor motif stabilizes the high-valent FeIV oxidation state yet keeps the FeII oxidation state accessible from the parent FeIII compound. Chemical oxidation of [Fe(ImP)2]+ yields stable [FeIV(ImP)2]2+. In contrast, [FeII(ImP)2]0, obtained by reduction, is highly sensitive toward oxygen. Exhaustive ground state characterization by single-crystal X-ray diffraction, 1H NMR, Mössbauer spectroscopy, temperature-dependent magnetic measurements, a combination of X-ray absorption near edge structure and valence-to-core, as well as core-to-core X-ray emission spectroscopy, complemented by detailed density functional theory (DFT) analysis, reveals that the three complexes [Fe(ImP)2]0/+/2+ can be unequivocally attributed to low-spin d6, d5, and d4 complexes. The excited state landscape of the FeII and FeIV complexes is characterized by short-lived 3MLCT and 3LMCT states, with lifetimes of 5.1 and 1.4 ps, respectively. In the FeII-compound, an energetically low-lying MC state leads to fast deactivation of the MLCT state. The distorted square-pyramidal state, where one carbene is dissociated, can not only relax into the ground state, but also into a singlet dissociated state. Its formation was investigated with time-dependent optical spectroscopy, while insights into its structure were gained by NMR spectroscopy.

Iron-Catalyzed Intermolecular C-H Amination Assisted by an Isolated Iron-Imido Radical Intermediate.

Here we report the use of a base metal complex [(tBu pyrpyrr2 )Fe(OEt2 )] (1-OEt2 ) (tBu pyrpyrr2 2- =3,5-tBu2 -bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of Csp3 -H bonds of 9,10-dihydroanthracene (2 a) using 2,4,6-trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1-OEt2 with control in selectivity for single C-H amination versus double C-H amination. Catalytic C-H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(tBu pyrpyrr2 )Fe{=N(2,6-Me2 -4-tBu-C6 H2 )] (1-imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 FeIV complex. Complex 1-imido reacts with 2 a to produce the ferrous aniline adduct [(tBu pyrpyrr2 )Fe{NH(2,6-Me2 -4-tBu-C6 H2 )(C14 H11 )}] (1-aniline) in 45 % yield. Lastly, it was found that complexes 1-imido and 1-aniline are both competent intermediates in catalytic intermolecular C-H amination.

Iron‐Catalyzed Intermolecular C−H Amination Assisted by an Isolated Iron‐Imido Radical Intermediate

AbstractHere we report the use of a base metal complex [(tBupyrpyrr2)Fe(OEt2)] (1‐OEt2) (tBupyrpyrr22−=3,5‐tBu2‐bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of Csp3−H bonds of 9,10‐dihydroanthracene (2 a) using 2,4,6‐trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1‐OEt2 with control in selectivity for single C−H amination versus double C−H amination. Catalytic C−H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(tBupyrpyrr2)Fe{=N(2,6‐Me2‐4‐tBu‐C6H2)] (1‐imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 FeIV complex. Complex 1‐imido reacts with 2 a to produce the ferrous aniline adduct [(tBupyrpyrr2)Fe{NH(2,6‐Me2‐4‐tBu‐C6H2)(C14H11)}] (1‐aniline) in 45 % yield. Lastly, it was found that complexes 1‐imido and 1‐aniline are both competent intermediates in catalytic intermolecular C−H amination.

Synthesis of FeIII and FeIV Cyanide Complexes Using Hypervalent Iodine Reagents as Cyano‐Transfer One‐Electron Oxidants

AbstractWe disclose a new reactivity mode for electrophilic cyano λ3‐iodanes as group transfer one‐electron oxidants to synthesize FeIII and FeIV cyanide complexes. The inherent thermal instability of high‐valent FeIV compounds without π‐donor ligands (such as oxido (O2−), imido (RN2−) or nitrido (N3−)) makes their isolation and structural characterization a very challenging task. We report the synthesis of an FeIV cyanide complex [(N3N′)FeCN] (4) by two consecutive single electron transfer (SET) processes from FeII precursor [(N3N′)FeLi(THF)] (1) with cyanobenziodoxolone (CBX). The FeIV complex can also be prepared by reaction of [(N3N′)FeIII] (3) with CBX. In contrast, the oxidation of FeII with 1‐cyano‐3,3‐dimethyl‐3‐(1H)‐1,2‐benziodoxole (CDBX) enables the preparation of FeIII cyanide complex [(N3N′)FeIII(CN)(Li)(THF)3] (2‐LiTHF). Complexes 4 and 2‐LiTHF have been structurally characterized by single crystal X‐ray diffraction and their electronic structure has been examined by Mössbauer, EPR spectroscopy, and computational analyses.

Synthesis of FeIII and FeIV Cyanide Complexes Using Hypervalent Iodine Reagents as Cyano-Transfer One-Electron Oxidants.

We disclose a new reactivity mode for electrophilic cyano λ3‐iodanes as group transfer one‐electron oxidants to synthesize FeIII and FeIV cyanide complexes. The inherent thermal instability of high‐valent FeIV compounds without π‐donor ligands (such as oxido (O2−), imido (RN2−) or nitrido (N3−)) makes their isolation and structural characterization a very challenging task. We report the synthesis of an FeIV cyanide complex [(N3N′)FeCN] (4) by two consecutive single electron transfer (SET) processes from FeII precursor [(N3N′)FeLi(THF)] (1) with cyanobenziodoxolone (CBX). The FeIV complex can also be prepared by reaction of [(N3N′)FeIII] (3) with CBX. In contrast, the oxidation of FeII with 1‐cyano‐3,3‐dimethyl‐3‐(1H)‐1,2‐benziodoxole (CDBX) enables the preparation of FeIII cyanide complex [(N3N′)FeIII(CN)(Li)(THF)3] (2‐LiTHF ). Complexes 4 and 2‐LiTHF have been structurally characterized by single crystal X‐ray diffraction and their electronic structure has been examined by Mössbauer, EPR spectroscopy, and computational analyses.

A Stable Homoleptic Organometallic Iron(IV) Complex.

A homoleptic organometallic FeIV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)2](PF6)2 (phtmeimb=[phenyl(tris(3‐methylimidazolin‐2‐ylidene))borate]−). This FeIV N‐heterocyclic carbene (NHC) complex was characterized by 1H NMR, HR‐MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)2](PF6)2 is a triplet FeIV low‐spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)2](PF6)2 in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand‐to‐metal charge transfer) excitation. For the first time the excited state dynamics of a FeIV complex was studied and revealed a ≈0.8 ps lifetime of the 3LMCT excited state of [Fe(phtmeimb)2](PF6)2 in acetonitrile.

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count.

Metallocorroles wherein the metal ion is MnIII and formally FeIV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The MnIII corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm-1, |E| = 0.023(5) cm-1. This result and those for other MnIII tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm-1 for 6-coordinate complexes, the ground state description is [MnIII(Cor3-)]0 or [MnIII(P2-)]+ (Cor = corrole, P = porphyrin). The situation for formally FeIV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [FeIV(Cor3-)]+, but when X = halide, the ground state corresponds to [FeIII(Cor•2-)]+, wherein an intermediate spin (S = 3/2) FeIII is antiferromagnetically coupled to a corrole radical dianion (S = 1/2) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm-1, while that with axial chloride gives D = 14.6(1) cm-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other FeIV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an FeIV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Preparation and characterization of μ-nitrido diiron phthalocyanines with electron-withdrawing substituents: application for catalytic aromatic oxidation

Mu-nitrido-bis [tetra-(hexyl-sulfonyl)phthalocyaninatoiron] (3a) and mu-nitrido-bis [tetra-(tert-butylsulfonyl) phthalocyaninatoiron] (3b) complexes have been prepared and fully characterized by electrospray ionization mass spectrometry, UV-Vis, FTIR, EPR, Mössbauer techniques as well as by X-ray photoelectron and Fe K-edge X-ray absorption spectroscopies. Small changes at the periphery of the phthalocyanine ligand introduce a difference in the iron oxidation state. While 3b with tert-butyl substituents is a neutral complex with a mixed-valence Fe(3.5)-N-Fe(3.5) structural unit, 3a having n-hexyl substituents is an oxidized cationic Fe(IV)-N-Fe(IV) complex. The structural parameters of N-bridged diiron phthalocyanine with a Fe(3.5)-N-Fe(3.5) unit were determined for the first time. Iron atoms in 3b are displaced out of plane by 0.24 A and the Fe-N bond distance of the linear Fe-N-Fe fragment is equal to 1.67 A. Both complexes selectively catalyze benzylic oxidation of alkyl aromatic compounds by tBuOOH. Toluene was oxidized to benzoic acid with 80% selectivity, and the total turnover number was as high as 197. p-Toluic acid was the principal product of p-xylene oxidation. In this case the turnover number achieved 587 substrate molecules per molecule of catalyst. The described catalytic system is complementary to the recently reported system based on mu-nitrido diiron tetrabutylphthalocyanine-H2O2 which effectively oxidizes the benzene ring.

Electronic Structure of High‐Spin Iron(IV) Complexes

AbstractHigh‐spin (S = 2) iron(IV) species are rare but increasingly recognized as reactive intermediates in the catalytic cycles of several nonheme iron enzymes. A question of some interest, therefore, concerns how much higher in energy the low‐spin (S = 1) state is for these species. With the use of density functional theory (DFT) and high‐level ab initio calculations [CASPT2 and CCSD(T)], we have attempted to answer this question for the so‐called Collins complex, a square‐pyramidal FeIV complex with a tetraamido‐N equatorial ligand set, a chloride axial ligand, and an S = 2 ground state. The calculations suggest that relative to the ground state, the low‐spin state is higher in energy by at least 0.3 eV and possibly as much as 0.7 eV. Using DFT calculations, a broad quantum chemical survey of high‐spin FeIVO intermediates was also undertaken. A key finding is that the Fe−O distance and O spin population are quite similar across all mononuclear FeIVO species studied, regardless of the heme versus nonheme environment and of the S = 1 versus 2 spin state, reflecting the essential similarity of the Fe(dπ)−O(pπ) orbital interactions in all the species studied. However, the spin density profiles of high‐spin FeIVO species, currently believed to be known only as a nonheme iron enzyme (TauD) intermediate, are predicted to be very different from that of Collins’ high‐spin FeIV complex. Our calculations further suggest that with the help of sterically hindered ligands such as 6‐me3‐tpa, it might be possible to generate synthetic high‐spin FeIVO models of the unique TauD intermediate. Finally, our calculations confirm the aptness of describing the [(6‐me3‐tpa)FeIII(μ‐O)2FeIV(6‐me3‐tpa)]3+ cation as a flexible diamond core and indicate the presence of a fairly discrete high‐spin FeIVO unit within the dinuclear core. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Cover Picture: Electronic Structure of High‐Spin Iron(IV) Complexes (Eur. J. Inorg. Chem. 23/2004)

AbstractThe cover picture shows DFT‐optimized models of a nonheme high‐spin iron(IV)−oxo intermediate and of Collins′ stable high‐spin FeIV complex. A. Ghosh, P. R. Taylor and co‐workers show in their article on p. 4555 ff. that these species, although both are high‐spin FeIV, exhibit radically different electronic configurations and spin density profiles (green). Moreover, the authors present some of the first high‐level ab initio calculations aimed at benchmarking the triplet‐quintet energy difference for FeIV species.

Spectroscopic Characterization of an FeIV Intermediate Generated by Reaction of XO− (X = Cl, Br) with an FeII Complex Bearing a Pentadentate Non‐Porphyrinic Ligand − Hydroxylation and Epoxidation Activity

AbstractMononuclear FeIV intermediates have been generated in MeOH upon reaction of sodium hypochlorite or hypobromite with a ferrous complex bearing the pentadentate ligands N‐methyl‐N,N′,N′‐tris(2‐pyridylmethyl)ethane‐1,2‐diamine (L52) or N‐methyl‐N,N′,N′‐tris(2‐pyridylmethyl)propane‐1,3‐diamine (L53). These highly unstable green complexes are characterized by an absorption band at ca. 750 nm. Mössbauer data indicate that the iron center is low spin (S = 1) with an axial electronic structure, allowing identification of the mononuclear FeIV complexes as [L5FeIVO]2+ or [L5FeIVOCH3]3+. In acetonitrile/dichloromethane solutions, the L53FeIV system exhibits very selective activities toward hydroxylation of cyclohexane or epoxidation of cyclooctene and cis‐stilbene. Computational studies performed on [L5FeIVO]2+ and on the model compound [(NH3)5FeIVO]2+ reveal that the ground state possesses two unpaired electrons in the two π* orbitals, as known for O2. Electronic spectra computed by time‐dependent DFT exhibit only one band in the visible region that is essentially due to d‐d transitions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Mössbauer spectroscopy on Fe impurities in germanium

Mössbauer experiments have been performed on the 14keV γ-radiation of 57Fe emitted in the decay of ion-implanted (60keV, <1012/cm2), radioactive 57Mn+ ions in germanium single crystals held at temperatures of 77–800K. At temperatures ⩽300K, a broad asymmetric quadrupole-split doublet, assigned to Fe in implantation-damage related sites, dominates the spectra. At ∼350K these defects anneal during the 57Mn lifetime (2.2min) and the Mn atoms are incorporated mainly on substitutional lattice sites. An average recoil energy of 40eV imparted on the 57Fe daughter atoms in the nuclear decay expels the majority of the probe atoms into tetrahedral interstitial sites. A sizable fraction traps the mobile vacancy created in the recoil event, forming Fei-V complexes within the lifetime of the 57mFe Mössbauer state (140ns), which are stable up to ≈700K.

The Preparation ofN-tert-Butyloxycarbonyl-(Boc-)Protected Sulfoximines and Sulfimines by an Iron(II)-Mediated Nitrene Transfer from BocN3 to Sulfoxides and Sulfides

The imidation of sulfides and sulfoxides to the corresponding sulfimides and sulfoximides was carried out with N-tert-butyloxycarbonyl azide (BocN3) in the presence of FeCl2. Sulfoxides 1 reacted at room temperature in CH2Cl2 to give the corresponding sulfoximides 3 in 40–95% yield. The imidation of the sterically congested substrate tert-butyl methyl sulfoxide (1f) proceeded sluggishly (10% yield). The sterospecificity of the reaction was demonstrated with the enantiomerically enriched substrates (R)-(+)-1b and (S)-(–)-1d which yielded the sulfoximides (R)-(+)-3b and (S)-(–)-3d with retention of configuration. Mechanistically, an intermediate (nitrene)FeIV complex is postulated as the reactive nitrene transfer reagent which is formed from FeCl2 and BocN3. The more nucleophilic sulfides 2 reacted more readily in the imidation than sulfoxides. Their conversion to the corresponding sulfimides 4 was conducted with BocN3 and a substoichiometric amount of FeCl2 (0.25 equiv.). Yields ranged between 44 and 92%. In an alternative reaction mode, BocN3 was utilized at 0°C in the presence of FeCl2 and acetyl acetone. The sulfimidation, which did not otherwise occur at this temperature, was accelerated by the ligand (36–90% yield).

Stabilization of Iron Centers in High Oxidation State in the Mononuclear Complex [Fev(I)(′N2S2′)

The first stable molecular Fev complex, [Fev(I)(′N2S2′)], forms in the metal-centered oxidation of the readily accessible FeIV complex [FeIV(pnPr3)(′N2S2′)] with elemental iodine according to Equation (a). The high-valent character of the iron center is proved by structural, spectroscopic, and electrochemical data.

17- and 19-Electron Complexes [FeIII(η5-C5R5)(S2CNMe2)L]n+ (n = 1, 0): Electronic Structure and Substitution and Redox Chemistry. Formation of [FeIV(η5-C5R5)(dtc)2] and Characterization of both 17e and 19e States of a Transition-Metal Complex

Oxidation of [FeCp*(η1-dtc)(CO)2], 1 (Cp* = η5-C5Me5, dtc = S2CNMe2), or [FeCp*(η2-dtc)(CO)], 2, using [FeIIICp2]+X- (X- = PF6- or BF4-, Cp = η5-C5H5) in THF cleanly gives [FeIIICp*(η2-dtc)(CO)]+X-, 2+X-, as microcrystalline green, thermally stable, but substitution labile, salts. The substitution of CO in 2+PF6- by various solvents (CH2Cl2, THF, CH3COCH3, CH3CN) (visible spectroscopy) follows pseudo-first-order kinetics but shows clearly the influence of the incoming solvent ligand on the substitution rate and, hence, is in good agreement with an associative mechanism. Displacement of the labile solvent ligand in these complexes by a phosphine results in the 17-electron (17e) cations [FeIIICp*(η2-dtc)(L)]+PF6-, L = PPh3 (7+PF6-) or η1-dppe (8+PF6-). The same reaction in the presence of the anionic ligands CN-, SCN-, and Cl- affords the corresponding neutral 17e FeIII complexes (respectively compounds 11, 13, and 14). All these 17e complexes were characterized by IR, ESR, and Mössbauer spectroscopies and elemental analysis. The cations were reduced to isostructural neutral FeII complexes using 1 equiv of [FeICp(C6Me6)] in THF or oxidized to the robust green 18e FeIV complex [FeIV(η5-C5Me5)(η2-S2CNMe2)2]+PF6-, 9+PF6-, using Na+dtc-·2H2O. [FeIV(η5-C5(CH2C6H5)5)(η2-S2CNMe2)2]+PF6-, 16+PF6-, was structurally characterized, and the dihapto mode of coordination of both dtc ligands was established. The 19e FeIII species 9 was shown to be an intermediate which further reduced H2O. It could be alternatively synthesized by reduction of the 18e precursor 9+PF6- using 1 equiv of [FeICp(C6Me6)] or by addition of anhydrous Na+dtc- to 3+PF6- in MeCN at −40 °C. The 19e complex 9 showed an ESR spectrum indicating an axial symmetry (two g values) in contrast with the ESR spectra of all the 17e species (2+−14) which show three g values characteristic of a rhombic distortion (for instance, the very close model 13). The Mössbauer doublet of 9 very slowly evolved to the new doublet of the thermally stable 17e complex 9‘. In MeCN solution, the transformation of the blue complex 9 to the purple 17e complex 9‘ was much more rapid (above −40 °C) as indicated by the rhombic spectrum of 9‘ in frozen solution and by low-temperature 13C NMR. In toluene, however, the 19e complex 9 showed a remarkable stability up to room temperature, which allowed recording of the 13C spectrum in d8-toluene. MO calculations have been performed on models for the 17e and 19e bis-dtc FeIII complexes. They suggest that the 17e species should have some significant sulfur spin density. The 19e species is found to have its odd electron occupying an antibonding metal-centered orbital. The cyclic voltammogram of 9+PF6- under continuous scanning for the monoelectronic reduction and the two monoelectronic reductions showed the decrease of the waves of 9+PF6- and the concomitant increase of those due to the partially decoordinated dtc complexes formed upon reduction. This permits an interpretation of the CV in terms of a triple-square scheme involving 9+/0/-, 9‘+/0/-, and solvent (DMF) adducts in 18- and 19e states.

The production of meta-tritiotyrosine from [formula omitted]-tritiophenylalanine by phenylalanine hydroxylase

The study of high-valent-iron enzyme intermediates began in the mid-1900s with the discovery of compounds I (or ES) and II in the heme peroxidases, progressed to non-heme-diiron enzymes in the 1990s with the detection and characterization of the FeIII–FeIV complex, X, and the FeIV–FeIV complex, Q, in O2 activation by ribonucleotide reductase R2 (RNR-R2) and soluble methane monooxygenase (sMMO), respectively, and was most recently extended to mononuclear non-heme-iron oxygenases with the trapping and spectroscopic characterization of the FeIV–oxo intermediate, J, in the reaction of taurine:α-ketoglutarate dioxygenase (TauD). Individually, each of these landmark studies helped reveal the chemical logic of that particular enzyme system. Collectively, they have significantly advanced our understanding of Nature’s strategies for oxidative transformation of biomolecules (both natural and “xenobiotic”). With high-valent complexes now having been described in representatives of three major classes of iron enzymes, it is an appropriate time to ask whether and what additional insights might be gleaned from further stalking of related intermediates in other systems. In this review, we advocate that there is still much to be learned from this pursuit and summarize the insight provided by two of the landmark discoveries mentioned above (the latter two) and the subsequent studies that they spurred to support our contention. In addition, we attempt to provide, to the extent that it is possible to do so, a “how-to” guide for detection and characterization of such intermediates, focusing primarily on enzymes in which they form by activation of molecular oxygen. In this latter objective, we have drawn from an earlier review by Johnson (Enzymes, third ed. vol. 20, 1992, pp. 1–61) covering, more generally, dissection of enzyme reaction pathways by transient-state kinetic methods. That work elegantly illustrated that, although it may be impossible to develop a true algorithm for the process, a synthesis of guidelines and general principles can be of considerable value.