Short Fe-N Research Articles

Crystalline bilayer formation in homoleptic low-spin Fe(II) compounds with alkyl chain substituents.

A series of asymmetric, homoleptic Fe(II) compounds based on the facially-binding tridentate ligand N-methyl-1,1-di(pyridin-2-yl)Cn-amine (LCn) (Cn = butyl, hexyl, octyl, decyl, dodecyl, tetradecyl and hexadecyl alkyl chains) with formula [FeII(LCn)2](X)2·solvate, where n = 4, 14 and X = BF4 (1C4 and 1C14) or n = 6, 8, 10, 12, 16 and X = CF3SO3 (1C6-1C12 and 1C16), are reported. Complexes 1C6 to 1C16 pack in crystalline bilayers in the solid state, forming hydrophobic and hydrophilic regions between adjacent layers of complexes. The combination of short Fe-N bond distances (∼2.00 Å) and SQUID magnetic susceptibility measurements show that the complexes are in the low-spin state across all measured temperature ranges. Differential scanning calorimetry confirmed phase transitions occur in compounds 1C6, 1C12, 1C14 and 1C16 upon heating from room temperature. The lack of any spin transitions and thermal stability conferred by thermogravimetric analysis over this temperature range suggest that these transitions are crystallographic in nature. 1H NMR studies show that the low-spin Fe(II) centres undergo partial conversion to paramagnetic species in solution. UV-vis spectroscopy in a range of common organic solvents show that the central Fe ions remain in the +2 oxidation state, suggesting that the increase in magnetic susceptibility in solution is likely due to partial spin-crossover, or due to speciation, in which a proportion of the compounds are high-spin Fe(II) complexes.

Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials.

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.

Heterometallic iron(IV) μ-nitrido complexes supported by a tetradentate Schiff base ligand

A nitrido-bridged heterometallic iron(IV) chloride complex supported by a tetradentate Schiff base ligand has been synthesized and its chloride substitution reactions have been studied. Treatment of the Fe(II) Schiff base complex [Fe(Br4saloph)(py)] (H2Br4saloph = N,N’-bis(3,5-dibromosalicylidene)-1,2-phenylenediamine; py = pyridine) with the Ru(VI) nitride [Ru(N)Cl2(LOEt)] (1, LOEt− = [(η5-C5H5)Co{P(O)(OEt)2}3]−) afforded the μ-nitrido complex [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)Cl] (2-Cl). The short Fe-N(nitrido) [1.668(4) Å] and Ru-N(nitrido) [1.709(4) Å] distances and the roughly linear Fe-N-Ru linkage [172.9(3)o] in diamagnetic 2-Cl are indicative of the Fe(IV)=N=Ru(IV) bonding picture. Salt metathesis of 2-Cl with [Ag(CF3CO2)] and [Ag(ReO4)] afforded [{(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)(CF3CO2)}⋅Ag(CF3CO2)] (2-CF3CO2⋅Ag(CF3CO2)) and [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)(ReO4)] (2-ReO4), respectively. Chloride abstraction of 2-Cl with TlPF6, followed by recrystallization in air led to isolation of the μ-oxo complex [{2}2(μ3-O)Tl](PF6)2 (3), which is paramagnetic with a magnetic moment of 2.0 μB. Complex 3 features a [Ru-N-Fe-O(Tl)-Fe-N-Ru]2+ unit, the Tl atom of which is coordinated to the μ-oxo ligand and the oxygen atoms of the Schiff base ligands. The Ru-N(nitrido) (av. 1.709 Å), Fe-N(nitrido) (av. 1.659 Å) and Fe-O(oxo) (av. 1.991 Å) distances in 3 are consistent with M = N double bonds and Fe-O single bonds, respectively. Treatment of 2-Cl with [Tl(SC6F5)] afforded the Fe(III)/Ru(IV) nitrido complex [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)] (4) that has a longer Fe-N(nitrido) distance [1.753(4) Å] than the Fe(IV) congener 2-Cl does. The crystal structures of complexes 2-Cl, 2-CF3CO2⋅Ag(CF3CO2), 2-ReO4, 3, and 4 have been determined.

Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform.

Iron complex [FeIII(N3)(MePy2tacn)](PF6)2 (1), containing a neutral triazacyclononane-based pentadentate ligand, and a terminally bound azide ligand has been prepared and spectroscopically and structurally characterized. Structural details, magnetic susceptibility data, and Mössbauer spectra demonstrate that 1 has a low-spin (S = 1/2) ferric center. X-ray diffraction analysis of 1 reveals remarkably short Fe-N (1.859 Å) and long FeN-N2 (1.246 Å) distances, while the FT-IR spectra show an unusually low N-N stretching frequency (2019 cm-1), suggesting that the FeN-N2 bond is particularly weak. Photolysis of 1 at 470 or 530 nm caused N2 elimination and generation of a nitrido species that on the basis of Mössbauer, magnetic susceptibility, EPR, and X-ray absorption in conjunction with density functional theory computational analyses is formulated as [FeV(N)(MePy2tacn)]2+ (2). Results indicate that 2 is a low-spin (S = 1/2) iron(V) species, which exhibits a short Fe-N distance (1.64 Å), as deduced from extended X-ray absorption fine structure analysis. Compound 2 is only stable at cryogenic (liquid N2) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N2. However, the high-valent compound could be generated in the gas phase, and its reactivity against olefins, sulfides, and substrates with weak C-H bonds studied. Compound 2 proved to be a powerful two-electron oxidant that can add the nitrido ligand to olefin and sulfide sites as well as oxidize cyclohexadiene substrates to benzene in a formal H2-transfer process. In summary, compound 2 constitutes the first case of an octahedral FeV(N) species prepared within a neutral ligand framework and adds to the few examples of FeV species that could be spectroscopically and chemically characterized.

Oxidative Group Transfer to a Triiron Complex to Form a Nucleophilic μ3-Nitride, [Fe3(μ3-N)]−

Utilizing a hexadentate ligand platform, a high-spin trinuclear iron complex of the type ((tbs)L)Fe(3)(thf) was synthesized and characterized ([(tbs)L](6-) = [1,3,5-C(6)H(9)(NPh-o-NSi(t)BuMe(2))(3)](6-)). The silyl-amide groups only permit ligation of one solvent molecule to the tri-iron core, resulting in an asymmetric core wherein each iron ion exhibits a distinct local coordination environment. The triiron complex ((tbs)L)Fe(3)(thf) rapidly consumes inorganic azide ([N(3)]NBu(4)) to afford an anionic, trinuclear nitride complex [((tbs)L)Fe(3)(μ(3)-N)]NBu(4). The nearly C(3)-symmetric complex exhibits a highly pyramidalized nitride ligand that resides 1.205(3) Å above the mean triiron plane with short Fe-N (1.871(3) Å) distances and Fe-Fe separation (2.480(1) Å). The nucleophilic nitride can be readily alkylated via reaction with methyl iodide to afford the neutral, trinuclear methylimide complex ((tbs)L)Fe(3)(μ(3)-NCH(3)). Alkylation of the nitride maintains the approximate C(3)-symmetry in the imide complex, where the imide ligand resides 1.265(9) Å above the mean triiron plane featuring lengthened Fe-N(imide) bond distances (1.892(3) Å) with nearly equal Fe-Fe separation (2.483(1) Å).

ChemInform Abstract: X-Ray Diffraction Study of the Electronic Ground State of (meso- Tetraphenylporphinato)iron(II).

The charge distribution in crystalline (meso-tetraphenylporphinato)iron(II) has been derived from 10,154 intensity data measured at 120 (5) K. Crystal data are a = 14.992 (2), {angstrom}, c = 13.778 {angstrom}, Z = 4, space group I{bar 4}2d. The data have been analyzed with the aspherical-atom multipole formalism including anharmonic thermal parameters for the iron atom. The deformation density maps indicate preferential occupancy of the d{sub z}2 and d{sub xy} orbitals of the iron atom, a conclusion confirmed by the d-orbital populations derived from the aspherical-atom multipole refinement. The results are in agreement with an SCF-CI calculation by Rohmer and an INDO-CI calculation by Edwards, Weiner, and Zerner and supports the {sup 3}H{sub 2g} state as the leading contributor to the ground state of the complex. The ground-state assignment is in contrast to results on the intermediate spin complex iron(II) phthalocyanine, for which the electron density indicates a {sup 3}E{sub g} ground state. The difference is attributed to the effect of short intermolecular Fe-N contacts in crystalline iron(II) phthalocyanine on the relative order of closely spaced energy levels.

Crystals of an Iron Nitride

CHEMISTRY Both industrial and enzymatic nitrogen reduction catalysts rely on iron centers. However, high valent molecular iron nitride complexes (FeN) have stubbornly eluded crystallographic characterization, in contrast to analogous terminal oxo structures (Fe=O). Vogel et al. now find that a tridentate ligand comprising three coordinating N-heterocyclic carbene moieties offers the solution. Reaction of the ligand (bearing either xylyl or mesityl groups for steric protection) with ferrous chloride followed by reduction with sodium amalgam and treatment with trimethylsilylazide yields a cationic Fe-N3 complex that loses dinitrogen under xenon lamp photolysis to afford the terminal iron nitride. Air-stable purple crystals of the compound were characterized by x-ray diffraction, revealing a short FeN bond length of 1.53 . Mssbauer spectroscopy further suggested an iron center more electron-rich than a previously prepared phosphorus-coordinated iron nitride characterized in solution; the authors attribute the difference to -donation from the carbene ligands. JSY Angew. Chem. Int. Ed. 47 , 10.1002/anie.200800600 (2008).

Electronic Structure and FeNO Conformation of Nonheme Iron−Thiolate−NO Complexes: An Experimental and DFT Study

Reactions of NO and CO with Fe(II) complexes of the tripodal trithiolate ligands NS3 and PS3* yield trigonal-bipyramidal (TBP) complexes with varying redox states and reactivity patterns with respect to dissociation of the diatomic ligand. The previously reported four-coordinate [Fe(II)(NS3)](-) complex reacts irreversibly with NO gas to yield the S = 3/2 {FeNO}(7) [Fe(NS3)(NO)](-) anion, isolated as the Me(4)N(+) salt. In contrast, the reaction of NO with the species generated by the reaction of FeCl(2) with Li(3)PS3* gives a high yield of the neutral, TBP, S = 1 complex, [Fe(PS3*)(NO)], the first example of a paramagnetic {FeNO}(6) complex. X-ray crystallographic analyses show that both [Fe(NS3)(NO)](-) and [Fe(PS3*)(NO)] feature short Fe-N(NO) distances, 1.756(6) and 1.676(3) A, respectively. However, whereas [Fe(NS3)(NO)]- exhibits a distinctly bent FeNO angle and a chiral pinwheel conformation of the NS3 ligand, [Fe(PS3*)(NO)] has nearly C(3v) local symmetry and a linear FeNO unit. The S = 1 [Fe(II)(PS3)L] complexes, where L = 1-MeIm, CN(-), CO, and NO(+), exhibit a pronounced lengthening of the Fe-P distances along the series, the values being 2.101(2), 2.142(1), 2.165(7), and 2.240(1) A, respectively. This order correlates with the pi-backbonding ability of the fifth ligand L. The cyclic voltammogram of the [Fe(NS3)(NO)](-) anion shows an irreversible oxidation at +0.394 V (vs SCE), apparently with loss of NO, when scanned anodically in DMF. In contrast, [Fe(PS3*)(NO)] exhibits a reversible {FeNO}(6)/{FeNO}(7) couple at a low potential of -0.127 V. Qualitatively consistent with these electrochemical findings, DFT (PW91/STO-TZP) calculations predict a substantially lower gas-phase adiabatic ionization potential for the [Fe(PS3)(NO)](-) anion (2.06 eV) than for [Fe(NS3)(NO)](-) (2.55 eV). The greater instability of the {FeNO}(7) state with the PS3* ligand results from a stronger antibonding interaction involving the metal d(z(2)) orbital and the phosphine lone pair than the analogous orbital interaction in the NS3 case. The antibonding interaction involving the NS3 amine lone pair affords a relatively "stereochemically active" dz2 electron, the z direction being roughly along the Fe-N(NO) vector. As a result, the {FeNO}(7) unit is substantially bent. By contrast, the lack of a trans ligand in [Fe(S(t)Bu)3(NO)](-), a rare example of a tetrahedral {FeNO}(7) complex, results in a "stereochemically inactive" d(z(2)) orbital and an essentially linear FeNO unit.

XAS characterization of a nitridoiron(IV) complex with a very short Fe-N bond.

X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBPR3]Fe(N) complexes (R=iPr and CyCH2) and obtain direct spectroscopic evidence for a very short Fe-N distance. The distance of 1.51-1.55 A reflects the presence of an FeN triple bond in accord with the observed FeN vibration observed for one of these species (nuFeN=1034 cm(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with FeI-N2-FeI, FeII-NPh2, and FeIIINAd motifs, and those of recently described six-coordinate FeVN and FeVIN complexes. The observation that the FeIV-N distances of two [PhBPR3]Fe(N) complexes are shorter than the FeIV-O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater pi basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex.

The Challenge of Being Straight: Explaining the Linearity of a Low-Spin {FeNO}7 Unit in a Tropocoronand Complex

We have carried out a density functional theory study of the S = 1/2 [FeNO]7 tropocoronand complex, Fe(5,5-TC)NO, as well as of some simplified models of this compound. The calculations accurately reproduce the experimentally observed trigonal-bipyramidal geometry of this complex, featuring a linear NO in an equatorial position and a very short Fe-N(NO) distance. Despite these unique structural features, the qualitative features of the bonding turn out to be rather similar for Fe(5,5-TC)NO and [FeNO]7 porphyrins. Thus, there is a close correspondence between the molecular orbitals (MOs) in the two cases. However, there is a critical, if somewhat subtle, difference in the nature of the singly occupied MOs (SOMOs) between the two. For square-pyramidal heme-NO complexes, the SOMO is primarily Fe d(z)2-based, which favors sigma-bonding interactions with an NO pi orbital, and hence a bent FeNO unit. However, for trigonal-bipyramidal Fe(5,5-TC)(NO), the SOMO is best described as primarily Fe d(x2-z2) in character, with the Fe-N(NO) vector being identified as the z direction. Apparently, such a d orbital is less adept at sigma bonding with NO and, as such, pi bonding dominates the Fe-NO interaction, leading to an essentially linear FeNO unit and a short Fe-N(NO) distance.

Ground-State Singlet L3Fe-(μ-N)-FeL3 and L3Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.

Preparation, Structure, and Electronic Properties of a Low-Spin Iron(II) Hexaamine Compound.

[Fe(trans-diammac)](2+) (trans-diammac) = exo-6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazatetradecane) is one of the very few fully characterized examples of a low-spin iron(II) compound with saturated amine ligands. The crystal structure analysis (monoclinic, P2(1)/c; a = 9.547(8), b = 14.631(13), c = 16.91(2) Å; beta = 98.92(7) degrees; Z = 4) defines the iron(II) coordination geometry as distorted octahedral with very short in-plane Fe-N distances (average of 2.01 Å) to the secondary amines of the macrocyclic ligand and slightly longer distances to the pendant primary amine donors (2.03 Å); there is a considerable tilt of the vector involving the axial donors with respect to the plane defined by the secondary amines and the metal center (theta = 11.5 degrees ). The metal-donor distances are shorter than those for other low-spin iron(II) hexaamines, and consequently, the redox potential (Fe(3+/2+)) is very small (0.45 V vs SHE) and the ligand field splitting is very large (Dq = 1785 cm(-)(1)). The structural, magnetic, and spectroscopic properties are discussed on the basis of the experimental data in comparison with model studies.

X-ray absorption studies of the ferrous active site of isopenicillin N synthase and related model complexes.

Isopenicillin N synthase (IPNS) from Cephalosporium acremonium (M(r) 38,400) is an iron-containing enzyme that aerobically catalyzes the four-electron oxidative ring closure reactions of delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine (ACV), forming the beta-lactam and thiazolidine rings of isopenicillin N. Here, we report Fe K-edge X-ray absorption studies that provide insight into the iron coordination environment and the effect of substrate and nitric oxide binding. Our analysis reveals an iron(II) coordination environment consisting of two N/O-containing ligands at 2.01 +/- 0.02 A, three N/O ligands at 2.15 +/- 0.02 A, and one C/O scatterer at approximately 2.6-2.7 A. Three His ligands are associated with the 2.15-A shell, while an unsymmetrically chelated carboxylate is associated with a scatterer at 2.01 and at 2.6-2.7 A, a combination which is consistent with the ligand environment deduced from 1H NMR studies [Ming, L.-J., Que, L., Jr., Kriauciunas, A., Frolik, C. A., & Chen, V. J. (1991) Biochemistry 30, 11653-11659]. The remaining scatterer at 2.01 A is assigned to a coordinated solvent molecule, most likely hydroxide, which can act as the proton acceptor for the incoming substrate. ACV binding to Fe(II)IPNS evinces an Fe-S interaction at 2.35 +/- 0.02 A, indicative of the coordination of substrate cysteine thiolate to the metal center. Analysis of the Fe(II)IPNS-ACV-NO data reveals one Fe-N at 1.71 +/- 0.02 A, three Fe-(N,O) at 2.04 +/- 0.02 A, one Fe-S at 2.32 +/- 0.02 A, and one Fe-(C,O) at 2.61 +/- 0.02 A, the short Fe-N bond being derived from the binding of NO. Our EXAFS conclusions, supported by corresponding analysis of relevant model complexes, corroborate and refine the working model for the Fe(II) coordination environment developed from previous spectroscopic studies.

X-ray absorption studies of intermediates in peroxidase activity

The structures of the enzyme-substrate compounds of peroxidases and catalase determined by X-ray absorption spectroscopy are presented. The valence state of the iron in Compounds I and II is determined from the edge to be higher than Fe +3. A short Fe-N e (proximal histidine) distance is observed in all forms except Compound II, forcing the Fe-N p average distance to be long, a result which differentiates the peroxidases from the oxygen transport hemoproteins and plays a pivotal role in the mechanism. A correlation is shown between the ratio of peaks in the low k (ligand field indicator ratio) region, the Fe-N p (heme pyrrole nitrogen) average distance, and the magnetic susceptibility, which provides a sensitive indicator of spin state. The mechanism of H 2O 2 reduction is shown by analysis of the structural changes observed in the intermediates. Possible relationship of these compounds to that of the peroxidatic form of cytochrome oxidase is suggested by these results.

Low-frequency vibrations of ferroprotoporphyrin-substituted imidazole complexes. A resonance Raman study

This article reports the low-frequency regions of resonance Raman spectra of five- and six-coordinated ferroprotoporphyrin complexes in aqueous solution with or without detergent. For high-spin complexes having their iron atom monocoordinated to variously substituted imidazoles or to dimethylformamide, the frequency of a band observed between 194 and 237 cm −1 (labelled band II) primarily depends on the pK a of the axial ligand. In the absence of steric effects from the axial ligand, the lower is the pK a of ligand, the higher the frequency of band II. We previously assigned band II to a mode essentially involving the Fe-N(pyrrole) bonds. The above pK a dependence is readily explained, in the frame of this assignment, in terms of a decrease in the Fe-N(pyrrole) bond strength (and of an increase in bond length) when the basicity of the axial ligand increases. On the other hand, the alternative assignment of band II to a stretching mode of Fe-N(axial ligand) is inconsistent with the observed pK a dependence. As far as hexacoordinated complexes are concerned, specific bands are observed at 203, 194 and 176 cm −1 for imidazole, 1-methylimidazole and pyridine, respectively. These bands are assigned, on the basis of isotopic substitutions, to a summetric stretching mode of the axial ligands [ν(N-Fe-N)]. Band II is observed at 265 cm −1 for these low-spin complexes, a frequency expected from the short Fe-N(pyrrole) bond lengths of nearly planar ferroporphyrins.