Intermediate-spin Iron Research Articles

Mononuclear high-spin iron(III) phthalocyanines

2,9(or 10),16(or 17), 23(or 24)-Tetradecyloxycarbonylphthalocyaninatoiron, FeTDPc, and 2,3,9,10,16,17,23,24-octadecyloxycarbonylphthalocyaninatoiron, FeODPc, were synthesized and characterized. These compounds seem to be in trivalent iron high-spin state in solvents such as chloroform, dichloromethane, benzene, and chlorobenzene, although their counter anion could not be detected by elemental analyses. They react with strong bases such as pyridine and imidazoles to form their mono- and subsequently their di-base complexes with formation constant of >106 and < 200 dm3 mol−1, respectively, in dichloromethane at 20 °C. The resultant mono-adducts appear to be trivalent iron low-spin while the di-base adducts are bivalent iron low-spin state complexes. The addition of ca. 10–30 equivalent of tetrabutylammonium-chloride or -bromide (electrolyte) to the solution containing FeTDPc or FeODPc, causes their spin-state change from iron(III) high to low-spin state. In a solid power state, however, both FeTDPc and FeODPc exist as a mixture of high-spin iron(III)- and intermediate-spin iron(II) species. Strangely, when these compounds are dissolved in polystyrene, i.e. each molecules are isolated from each other, the signals originated from the iron(II) component disappear.

An S=1 Iron(IV) Intermediate Revealed in a Non‐Heme Iron Enzyme‐Catalyzed Oxidative C−S Bond Formation

AbstractErgothioneine (ESH) and ovothiol A (OSHA) are two natural thiol‐histidine derivatives. ESH has been implicated as a longevity vitamin and OSHA inhibits the proliferation of hepatocarcinoma. The key biosynthetic step of ESH and OSHA in the aerobic pathways is the O2‐dependent C−S bond formation catalyzed by non‐heme iron enzymes (e.g., OvoA in ovothiol biosynthesis), but due to the lack of identification of key reactive intermediate the mechanism of this novel reaction is unresolved. In this study, we report the identification and characterization of a kinetically competent S=1 iron(IV) intermediate supported by a four‐histidine ligand environment (three from the protein residues and one from the substrate) in enabling C−S bond formation in OvoA from Methyloversatilis thermotoleran, which represents the first experimentally observed intermediate spin iron(IV) species in non‐heme iron enzymes. Results reported in this study thus set the stage to further dissect the mechanism of enzymatic oxidative C−S bond formation in the OSHA biosynthesis pathway. They also afford new opportunities to study the structure‐function relationship of high‐valent iron intermediates supported by a histidine rich ligand environment.

An S=1 Iron(IV) Intermediate Revealed in a Non-Heme Iron Enzyme-Catalyzed Oxidative C-S Bond Formation.

Ergothioneine (ESH) and ovothiol A (OSHA) are two natural thiol-histidine derivatives. ESH has been implicated as a longevity vitamin and OSHA inhibits the proliferation of hepatocarcinoma. The key biosynthetic step of ESH and OSHA in the aerobic pathways is the O2 -dependent C-S bond formation catalyzed by non-heme iron enzymes (e.g., OvoA in ovothiol biosynthesis), but due to the lack of identification of key reactive intermediate the mechanism of this novel reaction is unresolved. In this study, we report the identification and characterization of a kinetically competent S=1 iron(IV) intermediate supported by a four-histidine ligand environment (three from the protein residues and one from the substrate) in enabling C-S bond formation in OvoA from Methyloversatilis thermotoleran, which represents the first experimentally observed intermediate spin iron(IV) species in non-heme iron enzymes. Results reported in this study thus set the stage to further dissect the mechanism of enzymatic oxidative C-S bond formation in the OSHA biosynthesis pathway. They also afford new opportunities to study the structure-function relationship of high-valent iron intermediates supported by a histidine rich ligand environment.

Taking Solution Proton NMR to Its Extreme: Prediction and Detection of a Hydride Resonance in an Intermediate-Spin Iron Complex.

Guided by DFT based modeling the chemical shift range of a hydride resonance in the proton nuclear magnetic resonance (NMR) spectrum of the intermediate-spin, square planar iron complex tBu(PNP)Fe-H was predicted and detected as a broad resonance at -3560 ppm (295 K) with a temperature dependent shift of approximately 2000 ppm between 223 and 383 K. The first detection of a metal-bonded hydrogen atom by solution NMR in a complex with a paramagnetic ground state illustrates the interplay of theory and experiment for the characterization of key components in paramagnetic base metal catalysis.

Synthesis and Electronic Structure Diversity of Pyridine(diimine)iron Tetrazene Complexes.

A series of pyridine(diimine)iron tetrazene compounds, (iPrPDI)Fe[(NR)NN(NR)] [iPrPDI = 2,6-(ArN = CMe)2C5H3N; Ar = 2,6-iPr2C6H3] has been prepared either by the addition of 2 equiv of an organic azide, RN3, to the corresponding iron bis(dinitrogen) compound, (iPrPDI)Fe(N2)2 or by the addition of azide to the iron imide derivatives, (iPrPDI)FeNR. The electronic structures of these compounds were determined using a combination of metrical parameters from X-ray diffraction, solution and solid-state magnetic measurements, zero-field 57Fe Mössbauer and 1H NMR spectroscopies, and density functional theory calculations. The overall electronic structure of the iron tetrazene compounds is sensitive to the nature of the tetrazene nitrogen substituent with three distinct classes of compounds identified: (i) overall diamagnetic ( S = 0) compounds arising from intermediate-spin iron(II) centers ( SFe = 1) engaged in antiferromagnetic coupling with both pyridine(diimine) and tetrazene radical anions ( SPDI = -1/2 and Stetrazene = -1/2; R = 2-adamantyl, cyclooctyl, benzyl); (ii) overall S = 1 compounds best described as intermediate-spin iron(III) ( SFe = 3/2) derivatives engaged in antiferromagnetic coupling with a pyridine(diimine) radical anion ( SPDI = -1/2; R = 3,5-Me2C6H3, 4-MeC6H4); (iii) overall S = 2 compounds best described as high-spin iron(III) centers ( SFe = 5/2) engaged in antiferromagnetic coupling to a pyridine(diimine) radical anion ( SPDI = -1/2; R = 1-adamantyl). For both the intermediate- and high-spin ferric cases, the tetrazene ligand adopts the closed-shell, dianionic form, [N4R2]2-. For the case where R = SiMe3, spin-crossover behavior is observed, arising from a spin-state change from intermediate- to high-spin iron(III).

Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent Macrocyclic Tetraamido Ligand.

An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM4-), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2'-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was assigned on the basis of time-dependent density functional theory calculations, to indicate that the complex is in a class IIA mixed-valence ligand-radical regime with Hab = 884 cm-1. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)3N•+](SbCl6) in CH2Cl2 afforded two-electron-oxidized species of 1, [Fe(DTTM)Cl] (3), which was in the S = 1/2 spin state; complex 3 exhibited a distorted square-pyramidal structure. X-ray absorption near-edge structure spectra of 1-3 were measured in both CH3CN solutions and BN pellets to observe comparable rising-edge energies for the three complexes, and Mössbauer spectra of 1-3 showed almost identical isomer shifts and quadruple splitting parameters, indicating that the iron centers of the three complexes are intact to be in the intermediate-spin iron(III) state. Thus, in complexes 2 and 3, it is evident that antiferromagnetic coupling is operating between the unpaired electron(s) of the ligand radical(s) and those of the iron(III) center.

Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes.

The complexes [FeLN2S2X] [in which LN2S2=2,2′‐(2,2′‐bipryridine‐6,6′‐diyl)bis(1,1′‐diphenylethanethiolate) and X=Cl, Br and I], characterized crystallographically earlier and here (Fe(L)Br), reveal a square pyramidal coordinated FeIII ion. Unusually, all three complexes have intermediate spin ground states. Susceptibility measurements, powder cw X‐ and Q‐band EPR spectra, and zero‐field powder Mössbauer spectra show that all complexes display distinct magnetic anisotropy, which has been rationalized by DFT calculations.

Effective intermediate-spin iron in O2-transporting heme proteins

Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O2 management. The nature of Fe-O2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray Kβ emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20-260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of Kβ main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O2 bonding, tuned toward reversible oxygen transport.

Mononuclear Nonheme High-Spin (S=2) versus Intermediate-Spin (S=1) Iron(IV)-Oxo Complexes in Oxidation Reactions.

Mononuclear nonheme high-spin (S=2) iron(IV)-oxo species have been identified as the key intermediates responsible for the C-H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C-H bond activation of hydrocarbons by a synthetic mononuclear nonheme high-spin (S=2) iron(IV)-oxo complex occurs through an oxygen non-rebound mechanism, as previously demonstrated in the C-H bond activation by nonheme intermediate (S=1) iron(IV)-oxo complexes. We also report that C-H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high-spin (HS) and intermediate-spin (IS) iron(IV)-oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)-oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.

Mononuclear Nonheme High‐Spin (S=2) versus Intermediate‐Spin (S=1) Iron(IV)–Oxo Complexes in Oxidation Reactions

AbstractMononuclear nonheme high‐spin (S=2) iron(IV)–oxo species have been identified as the key intermediates responsible for the C−H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C−H bond activation of hydrocarbons by a synthetic mononuclear nonheme high‐spin (S=2) iron(IV)–oxo complex occurs through an oxygen non‐rebound mechanism, as previously demonstrated in the C−H bond activation by nonheme intermediate (S=1) iron(IV)–oxo complexes. We also report that C−H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high‐spin (HS) and intermediate‐spin (IS) iron(IV)–oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)–oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.

Proton NMR Investigations of Intermediate Spin Iron(III) Complexes with Macrocyclic N42– Chelate Ligands

AbstractProton NMR spectra of five macrocyclic iron(III) intermediate‐spin complexes are investigated. The signal assignment was accomplished for all five complexes. The temperature dependence of the NMR spectra reveals that for complexes of the type Fe1 an ideal Curie behaviour is observed indicating the absence of excited states. This is not the case for complexes of the type Fe2 and Fe3. A significant deviation from the Curie straight line is observed for Fe3‐Cl indicating low‐lying excited states. Single crystals suitable for X‐ray structure analysis were obtained for the complexes Fe2‐Cl and Fe3‐I and the X‐ray structure of those two complexes is presented.

Spin state of iron in Fe3O4magnetite andh-Fe3O4

The high-pressure behavior of magnetite has been widely debated in the literature. Experimental measurements have found conflicting high-pressure transitions: a charge reordering in magnetite from inverse-spinel to normal-spinel [Pasternak et al., J. Phys. Chem. Solids 65, 1531 (2004); Rozenberg et al., Phys. Rev. B 75, 020102 (2007)], iron high-spin to intermediate-spin transition in magnetite [Ding et al., Phys. Rev. Lett. 100, 045508 (2008)], electron delocalization in magnetite [Baudelet et al., Phys. Rev. B 82, 140412 (2010); Glazyrin et al., Am. Mineral. 97, 128 (2012)], and a structural phase transition from magnetite to $h$-Fe${}_{3}$O${}_{4}$ [Dubrovinsky et al., J. Phys.: Condens. Matter 15, 7697 (2003); Fei et al., Am. Mineral. 84, 203 (1999); Haavik et al., Am. Mineral. 85, 514 (2000)]. We present ab initio calculations of iron's spin state in magnetite and $h$-Fe${}_{3}$O${}_{4}$, which help resolve the high-pressure debate. The results of the calculations find that iron remains high spin in both magnetite and $h$-Fe${}_{3}$O${}_{4}$; intermediate-spin iron is not stable. In addition, magnetite remains inverse-spinel but undergoes a phase transition to $h$-Fe${}_{3}$O${}_{4}$ near 10 GPa. Magnetite has a complex magnetic ordering, multiple valence states (Fe${}^{2+}$ and Fe${}^{3+}$), charge ordering, and different local Fe site environments, all of which were accounted for in the calculations. The lack of intermediate-spin iron in magnetite helps resolve the spin state of iron in perovskite, the major mineral in the lower mantle. In both magnetite and perovskite, x-ray emission spectroscopy (XES) measurements in the literature show a drop in satellite peak intensity by approximately half, which is interpreted as intermediate-spin iron. In both minerals, calculations give no indication of intermediate-spin iron and predict high-spin iron to be stable for defect-free crystals. The results question the interpretation of a nonzero drop in XES satellite peak intensities as intermediate-spin iron.

Synthesis and Electronic Structure of Bis(imino)pyridine Iron Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions

The bis(imino)pyridine iron dinitrogen compound, ((iPr(TB))PDI)Fe(N2)2 ((iPr(TB))PDI = 2,6-(2,6-(i)Pr2-C6H3-N═C-(CH2)3)2(C5H1N)) is an effective precatalyst for the [2π + 2π] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to ((iPr(TB))PDI)Fe(N2)2 resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Mössbauer spectroscopic and computational studies established S = 1 iron compounds that are best described as intermediate-spin iron(III) (SFe = 3/2) antiferromagnetically coupled to a chelate radical anion (SPDI = 1/2). Catalytically competent bis(imino)pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)-Fe(III) catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.

Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen Complexes: Evidence for Formation of a Chelate Trianion.

Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, ((iPr)PDI)FeN(2) ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N) has been examined to determine whether the redox events are metal or ligand based. Treatment of ((iPr)PDI)FeN(2) with [Cp(2)Fe][BAr(F)(4)] (BAr(F)(4) = B(3,5-(CF(3))(2)-C(6)H(3))(4)) in diethyl ether solution resulted in N(2) loss and isolation of [((iPr)PDI)Fe(OEt(2))][BAr(F)(4)]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field (57)Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (S(Fe) = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (S(PDI) = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][((iPr)PDI)FeN(2)], was isolated following addition of sodium naphthalenide to ((iPr)PDI)FeN(2) in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, ((iPr)BPDI)Fe(N(2))(2) resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [((iPr)BPDI)FeN(2)](-) and [((iPr)BPDI)FeN(2)](2-), highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.

Spin–spin interactions in iron(III) porphyrin radical cations with ruffled and saddled structure

Oxidation of essentially pure intermediate-spin iron(III) porphyrinates such as ruffled Fe(TiPrP)ClO4 and saddled Fe(OETPP)ClO4 produces the corresponding six-coordinate iron(III) porphyrin(por) radical cations [Fe(Por)(ClO4)2], where TiPrP and OETPP are dianions of 5,10,15,20-tetraisopropylporphyrin and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, respectively.Spin–spin interactions in these complexes are very much different; while ruffled [Fe(TiPrP)(ClO4)2] exhibits no antiferromagnetic coupling, saddled [Fe(OETPP)(ClO4)2] does exhibit it. The difference in magnetic behaviors has been explained in terms of the deformation mode and electron configuration of these complexes.

Synthesis, Structural, Redox and Mössbauer Characterization of Four‐Electron‐Oxidized Tetrakis(cyclohexyl)iron(II)porphodimethene with Different Axial Ligations

AbstractMonocationic FeII porphodimethene complexes, [LΔΔFe–X][Y] {1, X = I– and Y = I3–; 2, X = Br– and Y = [FeIIIBr4]–; LΔΔ = tetrakis(cyclohexyl)porphodimethene}, were achieved by induced electron‐transfer reactions between the square‐planar, intermediate‐spin, iron(III)porphyrinogen complex [Et4N][LFeIII] [L = tetrakis(cyclohexyl)porphyrinogen tetraanion] and different oxidants. Single‐crystal X‐ray diffraction analysis reveals that 1 and 2 have a square‐pyramidal geometry in which the iodide and bromide ligands occupy the axial position, respectively. Compound 1 exhibits an extended two‐dimensional solid‐state structure that comprises porphodimethene cation and triiodide anion chain columns in a 1:1 ratio, which are packed alternately. Electrochemical assessment with cyclic voltammetry reveals reversibly accessible FeII/III oxidation states; however, the redox potential is nearly one volt more positive than that for a typical heme cofactor, which suggests the highly oxidizing nature of the tetrapyrrole framework in the four–electron‐oxidized LΔΔ moiety. In addition, definitive experimental oxidation state and spin state assignments for the intermediate‐spin [Et4N][LFeIII] and high‐spin FeII states of 1 and 2 were afforded by Mössbauer characterization.

Spin states and hyperfine interactions of iron in (Mg,Fe)SiO 3 perovskite under pressure

With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U). Our calculation supports the notion that the transition observed in recent Mössbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase.

Preparation, Magnetic and Structural Study on Oxido‐Bridged Diiron(III) Complexes with Open‐Chain Tetrapyrrolic 2,2′‐Bidipyrrin Ligands

AbstractFour (μ‐oxido)diiron(III) complexes of differently alkyl‐substituted 2,2′‐bidipyrrin ligands [(L)Fe]2O have been prepared by a one‐pot approach from the respectively substituted 2,2′‐bipyrroledialdehydes and 3,4‐dialkylpyrroles, and from a suitable source of trivalent iron, under alkaline conditions. Four single‐crystal X‐ray diffraction studies were undertaken on three of the new compounds as different solvates. The iron atoms are five‐coordinate in all cases, and the observed Fe–N and Fe–O distances are in the range of 2.024–2.082 Å and 1.761–1.818 Å, respectively. These bonds are (porphyrinato)iron‐like, typical for high spin iron(III) complexes, and clearly distinct from those observed for the respective mononuclear (chlorido)‐, (bromido)‐ and (iodido)iron(III)–2,2′‐bidipyrrins, which contain a central intermediate spin iron(III) ion in the solid state. All complexes contain two helically distorted tetrapyrrolic ligands with the same sense of helicity, but pack quite differently from one case to the other. A cleft is present on one side of the connecting oxygen atom, which is occupied either by a self‐complementary second molecule of the complex itself, or by a small molecule (dichloromethane, or pyrrole with N–H···O bond). The Fe–O‐Fe subunits are bent and can be grouped with respect to the Fe–O–Fe bond angle range of either 153.91–156.89° or 131.89–132.97°. All new compounds were investigated by analytical, spectroscopic and magnetic means and found weakly paramagnetic and dynamic with respect to helix inversion at ambient temperature in solution. SQUID magnetometry revealed antiferromagnetically coupled high spin iron(III) species with exchange coupling constants of J = –111 to –122 cm–1. The exchange coupling appears independent from the Fe–O–Fe angle as described earlier for related porphyrinoid and salen‐type (μ‐oxido)diiron(III) species. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis, thermal stability and structural characterisation of iron(II) phthalocyanine complex with 4-cyanopyridine

A new complex of bis-axially coordinated iron(II) phthalocyanine by 4-cyanopyridine (4-CNpy) has been obtained in crystalline form as an adduct with two 4-CNpy molecules. The [FePc(4-CNpy) 2] · 2(4-CNpy) crystallises in the monoclinic system, space group P2 1/ c with two molecules in the unit cell. The iron(II) coordinates four isoindole nitrogen atoms of the almost planar phthalocyaninato(2−) macroring and axially two nitrogen atoms of 4-CNpy molecules. The coordination polyhedron around the Fe(II) atom approximates to a tetragonal by-pyramid. Four equatorial Fe–N bonds are shorter (1.936(2) Å) than two axial Fe–N bonds (2.027(2) Å). The centrosymmetric FePc(4-CNpy) 2 molecules form alternating sheets parallel to the bc crystallographic plane and solvated 4-CNpy molecules that are anti-parallel oriented by their polar cyano groups are located between the sheets of FePc(4-CNpy) 2 molecules. Ligation of the intermediate-spin iron(II) phthalocyanine by 4-CNpy molecules leads to the low spin Fe(II) complex. The importance of the d(π) → π ∗(Pc) back donation is manifested in the difference between the values of C–N isoindole and C–N azamethine bond lengths of the Pc macrocycle. The thermal analysis of the crystals of [FePc(4-CN) 2] · 2(4-CNpy) shows two steps responsible for a loss of solvated (∼170 °C) and coordinated (∼235 °C) 4-CNpy molecules.

Synthesis and Hydrogenation of Bis(imino)pyridine Iron Imides

Treatment of the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = (2,6-iPr2C6H3N=CMe)2C5H3N), with a series of aryl azides resulted in loss of 3 equiv of N2 and formation of the corresponding four-coordinate iron imide compounds, (iPrPDI)Fe(NAr). These complexes, two of which (Ar = 2,6-iPr2-C6H3 and 2,4,6-Me3-C6H2) have been characterized by X-ray diffraction, are significantly distorted from planarity. The metrical parameters in combination with Mössbauer spectroscopic and SQUID magnetic data suggest an intermediate spin iron(III) center antiferromagnetically coupled to a ligand-centered radical. Nitrene group transfer has been accomplished by addition of 1 atm of CO, yielding aryl isocyanates, ArNCO, and (iPrPDI)Fe(CO)2. Hydrogenation of the more sterically hindered members of the series furnished free aniline and the previously reported iron dihydrogen complex. Catalytic aryl azide hydrogenation has also been achieved, and the observed relative rates are consistent with N-H bond formation as the rate-determining step in aniline formation.