A Heteroleptic Push–Pull Substituted Iron(II) Bis(tridentate) Complex with Low‐Energy Charge‐Transfer States

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]²⁺ with a strongly electron‐withdrawing ligand (dcpp, 2,6‐bis(2‐carboxypyridyl)pyridine) and a strongly electron‐donating tridentate tripyridine ligand (ddpd, N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reported. Both ligands form six‐membered chelate rings with the iron center, inducing a strong ligand field. This results in a high‐energy, high‐spin state (⁵T₂, (t_2g)⁴(e_g*)²) and a low‐spin ground state (¹A₁, (t_2g)⁶(e_g*)⁰). The intermediate triplet spin state (³T₁, (t_2g)⁵(e_g*)¹) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low‐energy π^* orbitals of dcpp allow low‐energy MLCT absorption plus additional low‐energy LL′CT absorptions from ddpd to dcpp. The directional charge‐transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]³⁺ and [Fe(dcpp)(ddpd)]⁺, augmented by density functional calculations. The combined effect of push–pull substitution and the strong ligand field paves the way for long‐lived charge‐transfer states in iron(II) complexes.     

Synthesis,Crystal Structure and Magnetic Properties of [Fe(bpe)4(H2O)2](TCNQ)2 (bpe = trans‐1,2‐bis(4‐pyridyl)ethane and TCNQ = tetracyanoquinodimethane)

The synthesis, structure and magnetic properties of [Fe(bpe)₄(H₂O)₂](TCNQ)₂ ( 1 ) are reported. 1 crystallizes in the triclinic P space group, a = 13.481(5), b = 14.887(3), c = 16.663(4) Å, α = 101.048(18), β = 112.84(2), γ = 90.92(2)°, V = 3009.6(14) ų, Z = 2. The iron atom defines a compressed octahedron with the equatorial positions occupied by the bpe molecules which act as monodentate ligands and the two axial positions occupied by water molecules. The TCNQ^? radical counterions are uncoordinated and interact by pairs defining (TCNQ)₂^2? units strongly coupled antiferromagnetically. The iron(II) atoms are in the high spin state and its magnetic behaviour indicates the occurrence of zero‐field splitting of the S = 2 ground state.     

SINGLET-TRIPLET SPIN TRANSITION IN AN IRON(II) COMPLEX OF 1,10-PHENANTHROLINE-2-CARBOTHIOAMIDE

Abstract

The ⁵⁷Fe Mössbauer effect for the solid complex FeLCl₂·H₂O (L=10-phenanthroline-2-carbothioamide) has been studied between 4.2°K and 298°K. Two overlapping doublets (I and II) are observed, their relative intensity being strongly temperature dependent. The doublets are characterized by δE_Q(I)=0.53 mm sec^?1, δ^IS(I)=+ 0.22 mm sec^?1 and δE_Q(II)=1.33 mm sec^?1, δ^IS(II)=+0.23 mm sec^?1 at 4.2°K. In conjunction with magnetic data, the Mössbauer spectra are interpreted in terms of a singlet-triplet spin transition of the central iron(II) ion. In an applied magnetic field at 4.2°K, H_int=0, V_ZZ(I)>0, and V_ZZ(II)>0. The results are consistent with optical electronic and infrared vibrational spectra.     

Search for new iron single-molecule magnets

The synthesis, X-ray structure, and magnetic properties of a trinuclear iron complex with the formulation [Fe₃O₂Cl₂(4,7-Me-phen)₆](BF₄)₃ (complex 1) are reported. DC magnetic susceptibility measurements show the Fe atoms are antiferromagnetically coupled, yielding an S=5/2 ground state. An investigation as to whether complex 1 exhibits the properties associated with single-molecule magnetism was undertaken. Detailed high frequency EPR experiments were carried out to determine the spin Hamiltonian parameters associated with the S=5/2 spin ground state. Analysis of the temperature dependence of the transitions seen with the magnetic field oriented along the easy axis (z axis) of the Fe₃ complex confirm that the molecule has a positive D value. A fit of the frequency dependence of the resonances afforded the following spin Hamiltonian parameters: S=5/2, g_z=1.95, g_x=g_y=2.00, D=0.844 cm⁻¹, E=±0.117 cm⁻¹, and B₄ ⁰=−2.7×10⁻⁴ cm⁻¹. Low temperature magnetization versus magnetic field data confirm that complex 1 has no barrier towards magnetization reversal. Thus, it is concluded that, due to the positive D-value, complex 1 is not a single-molecule magnet.     

New cobalt(II) coordination polymer based on carboxyphenyl-tpy ligand: Photoluminescence,crystal structures and magnetic properties,without orbital contribution

One coordination polymer, [Co(3-ctpy)₂(H₂O)₂]_n*2nEtOH (1), was synthesized by a solvothermal method mixing the complex precursor cobalt(II) acetylacetonate (Co(acac)₂*2H₂O) with 4′-(4-carboxyphenyl)-3,2′:6′,3″-terpyridine (3-Hctpy). This paramagnetic complex has been isolated as a neutral, air and thermal stable solid and has been characterized by elemental analysis, FT-IR and single-crystal X-ray diffraction analysis. The thermal stability and photoluminescence properties were studied in the solid state. The magnetic behavior presents an interesting challenge since the Co(II) ion has a spin 3/2, which leads to multiple possibilities under the influence of an energy crystal field, from the identification of the ground state to possible transitions to excited states. The usual orbital contribution of the magnetic moment has been extinguished, due to an axially distorted octahedral environment for cobalt, changing the O_h symmetry (with ⁴T_1g ground state) to D_4h (with ⁴A_2g ground state). This decrease in symmetry is responsible for the quenching of the orbital contribution, as will be discussed in this article.     

Ein Kobalt(II)-Corrin-Komplex als reversibler Sauerstoffträger. Eine ESR.-spektroskopische Untersuchung

The diamagnetic crystalline Jodo-Co(II)-cobyrinicacid-heptamethylester dissolves in benzene or toluene forming solutions which contain the monomeric paramagnetic complex unit. From anisotropic ESR.-parameters, measured in frozen solutions, follows a ground state configuration (d_xz,d_yz)⁴(d_xy)²(d_z²)¹ for the Co(II). The unpaired electron is highly delocalized to the axially coordinated iodide ion, giving rise to a strong hyperfine coupling with the iodine nucleus. The complex forms in a very fast and completely reversible reaction a 1:1 adduct with molecular oxygen in toluene solutions below ca. 280° K. ESR.-parameters of the oxygen adduct are presented and discussed. Thermodynamic data for the formation reaction of the adduct are estimated.     

A Strongly Spin‐Frustrated FeIII7 Complex with a Canted Intermediate Spin Ground State of S=7/2 or 9/2

A disk‐shaped [Fe^III₇(Cl)(MeOH)₆(μ₃‐O)₃(μ‐OMe)₆ (PhCO₂)₆]Cl₂ complex with C₃ symmetry has been synthesised and characterised. The central tetrahedral Fe^III is 0.733 Å above the almost co‐planar Fe^III₆ wheel, to which it is connected through three μ₃‐oxide bridges. For this iron‐oxo core, the magnetic susceptibility analysis proposed a Heisenberg–Dirac–van Vleck (HDvV) mechanism that leads to an intermediate spin ground state of S=7/2 or 9/2. Within either of these ground state manifolds it is reasonable to expect spin frustration effects. The ⁵⁷Fe Mössbauer (MS) analysis verifies that the central Fe^III ion easily aligns its magnetic moment antiparallel to the externally applied field direction, whereas the other six peripheral Fe^III ions keep their moments almost perpendicular to the field at stronger fields. This unusual canted spin structure reflects spin frustration. The small linewidths in the magnetic Mössbauer spectra of polycrystalline samples clearly suggest an isotropic exchange mechanism for realisation of this peculiar spin topology.     

Tris [2-(2′-pyridyl)benzimidazole]iron(II) complexes. Some new examples of 5T2 — 1A1 spin equilibria

Mössbauer spectra and magnetic susceptibilities for three iron (II) complexes of 2-(2′-pyridyl)benzimidazole have been measured in the temperature range 80–300°K. The results are consistent with the existence of ⁵T₂ — ¹A₁ spin equilibria in all three complexes.     

Electronic ground state of iron(II)porphyrin. Ab initio SCF and CI calculations and computed electron deformation densities

Ab initio SCF and CI calculations for the ³A_2g and ³E_g states of the iron(II)porphyrin both predict a ³A_2g ground state. The calculated electron density distribution around the iron atom and the deformation density maps are reported since they represent an important reference with regard to the determination of the ground state of FeP.     

Mössbauer spectra of the tetrakis-(1,8-naphthyridine) iron(II)perchlorate in external magnetic fields. Evidence of slow relaxation ln paramagnetic iron(II)

⁵⁷Fe Mössbauer spectra of [FeL₄] (ClO₄)₂ where L = 1,8-naphthyridine have been measured at 4.2°K in external magnetic fields up to 55 kG parallel to the direction of the γ-rays. The spectra have been fitted in the spin hamiltonian approximation assuming an orbital singlet ground state of the ⁵D multiplet of Fe²⁺. The fit of the spectra is not unique, yet the possible spin hamiltonian parameter sets found lead to a spin doublet ground state split by less than 1 cm^?1. The transition probabilities for spin-lattice relaxation have been calculated for those ground states. Orbach processes via excited spin hamiltonian states cannot be neglected. The results explain the fluctuations observed in the spectra in low external magnetic fields (10 kG).The spin hamiltonian parameters provide information on the orbital energy levels. Therefrom the reduction of the quadrupole splitting by spin—orbit coupling results to be small thus explaining the extremely large quadrupole splitting of 4.54 mm/sec.     

The resonance Raman spectra of intermediate-spin ferrous porphyrin

The resonance Raman and absorption spectra of unligated ferrous octaethylporphyrin [Fe²⁺ (OEP)] and its adduct with 2-MeIm or THF were observed. The resonance Raman spectrum of Fe²⁺ (OEP) in CH₂Cl₂ displayed characteristic features of iron porphyrin with three d_π electrons, suggesting ³E_g as the intermediate-spin (S = 1) ground state. Fe²⁺(OEP) in THF exhibited the ferrous high spin feature like Fe²⁺ (OEP) (2-MeIm) in CH₂Cl₂.     

Anomalous spin state of iron(II) in 57Fe Mössbauer emission spectra of 57Co-Labeled tris(2-methylphenanthroline) cobalt(II) perchlorate

⁵⁷Fe Mössbauer emission spectra of the ⁵⁷Co labeled complex compound [⁵⁷Co(2-CH₃-phen)₃] (ClO₄)₂·2H₂O have been measured as a function of temperature between 293 and 4.6 K. The spectra exclusively show high-spin iron(II) resonances beside a small fraction of an high-spin iron(II) species, whereas the corresponding iron(II) compound is known to exhibit thermally induced high-spin ⁵T_2g(O_h) ? low spin ¹A_1g(O_h) transition. The electronic nature of the anomalous spin state has been found to be ⁵A₁(D₃) by a theoretical treatment of the temperature dependence of the quadrupole splitting. The results are in good agreement with those obtained from Mössbauer absorption measurements on [⁵⁷Fe_0.01Co_0.99(2-CH₃-(phen)₃] (ClO₄)₂·2H₂O.     

Thermal low spin-high spin equilibrium of Fe(II) in thiospinels CuFe0.5(Sn(1−x)Tix)1.5S4 (0≤x≤1)

A series of spinel compounds with composition CuFe_0.5(Sn_(1−x)Ti_x)_1.5S₄ (0≤x≤1) is analysed by X-ray diffraction, measurements of magnetic susceptibilities and ⁵⁷Fe Mössbauer spectroscopy. All samples show a temperature-dependent equilibrium between an electronic low spin 3d(t_2g)⁶(e_g)⁰ and a high spin 3d(t_2g)⁴(e_g)² state of the Fe(II) ions. The spin crossover is of the continuous type and extends over several hundred degrees in all samples. The Sn/Ti ratio influences the thermal equilibrium between the two spin states. Substitution of Sn(IV) by the smaller Ti(IV) ions leads to a more compact crystal lattice, which, in contrast to many metal-organic Fe(II) complexes, does not stabilise the low spin state, but increases the residual high spin fraction for T→0 K. The role played by antiferromagnetic spin coupling in the stabilisation of the high spin state is discussed. The results are compared with model calculations treating the effect of magnetic interactions on spin state equilibria.     

Kinetic Magnetic‐Field Effect Involving the Small Biologically Relevant Inorganic Radicals NO and O2.−

Oxidation of dihydrorhodamine 123 (DHR) to rhodamine 123 (RH) by oxoperoxonitrite (ONOO^?), formed through recombination of NO and O₂^.? radicals resulting from thermal decomposition of 3‐morpholinosydnonimine (SIN‐1) in buffered aerated aqueous solution at pH 7.6, represents a kinetic model system of the reactivity of NO and O₂^.? in biochemical systems. A magnetic‐field effect (MFE) on the yield of RH detected in this system is explored in the full range of fields between 0 and 18 T. It is found to increase in a nearly linear fashion up to a value of 5.5±1.6 % at 18 T and 23 °C (3.1±0.7 % at 40 °C). A theoretical framework to analyze the MFE in terms of the magnetic‐field‐enhanced recombination rate constant k_rec of NO and O₂^.? due to magnetic mixing of T₀ and S spin states of the radical pair by the Δg mechanism is developed, including estimation of magnetic properties (g tensor and spin relaxation times) of NO and O₂^.? in aqueous solution, and calculation of the MFE on k_rec using the theoretical formalism of Gorelik at al. The factor with which the MFE on k_rec is translated to the MFE on the yield of ONOO^? and RH is derived for various kinetic scenarios representing possible sink channels for NO and O₂^.?. With reasonable assumptions for the values of some unknown kinetic parameters, the theoretical predictions account well for the observed MFE.     

5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

Spectroscopic studies of water-soluble superstructured iron(III) porphyrin. Interaction with the bovine serum albumin protein

Abstract

Acid-base equilibrium of the “one-face”-hindered sulfonated porphyrin, α5,15-[2,2′(dodecamethyleneoxy),(5-sulfonato)diphenyl]-10,20-bis(2-hydroxy,5-sulfonatophenyl)porphyrinato iron(III), has been studied by paramagnetic ¹H NMR. The isotropically shifted signals change in a fast exchange regime on the NMR time-scale. ¹H longitudinal relaxation times and temperature dependence of the chemical shifts were measured and analyzed. The electronic structure of hydroxo specie is characteristic of a six- or five-coordinate high-spin iron(III) porphyrin with an S = 5/2 ground state. The ¹H NMR titration allowed determination of the acidity constant, pK_a 6.2 (0.1 M KNO₃, 25 °C). In addition, we also report the interaction between the monohydroxo iron(III) porphyrin and the bovine serum albumin protein. From a ¹H NMR titration, we have determined the affinity apparent constant, log K_ap 3.2 (pH 7, KNO₃ 0.1 M, 25 °C). The formation of superstructured iron porphyrin-albumin protein adduct was confirmed by electronic absorption spectroscopy and electron paramagnetic resonance.     

Magnetic superexchange involving the oxygen-sulfur-oxygen pathway of the sulfate ion: A Mössbauer spectroscopy and magnetic susceptibility study of Fe(2,9-di-CH3-phenanthroline) SO4

Zero-field Mössbauer spectra of powder samples of Fe(2,9-di-CH₃-phenanthroline) SO₄ over the range 1.7 to 300°K show a large (～ 3.6 mm/sec) temperature-independent quadrupole splitting corresponding to an orbital singlet ground term. The chemical isomer shift, δ_FE=O, is 1.16 mm/sec (source and absorber at 4.2°K) corresponding to six coordinate high-spin iron (II). Below 4.2°K, the compound exhibits magnetic hyperfine splitting suggesting slow relaxation and the possibility of long-range three-dimensional magnetic ordering with a critical temperature T_c such that 3.5°K < T_c < 4.2°K and an internal hyperfine field H_n = 325 kG at 1.69°K. High-field Mössbauer spectra at 300°K indicate that the principal component of the electric field gradient tensor is positive and axial. Similar spectra at 4.2°K show an absence of nuclear Zeeman splitting for applied fields up to 60 kG, and indicate that at 4.2°K the material is rapidly relaxing but with substantial magnetic polarization and a negative internal hyperfine field. The temperature dependence of the magnetic susceptibility confirms antiferromagnetic interactions with a broad maximum χ′_M at ～ 11.8°K presumably due to low-dimensionality exchange interactions (possibly one) along MOSOM chains. Least-squares fits of χ′_M^?1 versus T for T50°K indicate (Curie-Weiss behavior with C = 3.26 emu/mole, θ = -21.95°K, and μ_eff = 5.11.     

Maximum Spin Polarization in Chromium Dimer Cations as Demonstrated by X‐ray Magnetic Circular Dichroism Spectroscopy

X‐ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr₂⁺. Our results indicate that the removal of a single electron from the 4sσ_g bonding orbital of Cr₂ drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero‐spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d‐4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground‐state candidates, the ferromagnetically coupled ¹²Σ and the low‐spin ²Σ state.     

A linear trinuclear complex of copper(II) with N,N′-bis(2-hydroxy-5-methoxybenzelidene)-1,3-diiminopropane

The synthesis and X-ray crystal structure of a linear phenolate-bridged Cu(II) complex 1 with a Cu–Cu bond distance of 2.9260(5)?Å is reported. The complex consists of three Cu(II) ions with two molecules of the N,N′-bis(2-hydroxy-5-methoxybenzelidene)-l,3-diiminopropane ligand and two nitrate ions in such a manner that one ligand is connected with two Cu(II) ions. The complex is monoclinic, space group P2₁/n, with a?=?10.6305(5), b?=?13.0719(7), c?=?14.6336(8)?Å and β?=?102.549(1)° at 293?K, Z?=?2. The structure shows deprotonation of the phenolate oxygen to form a μ₂ bridge. The magnetic moment (1.627 BM per Cu₃ unit) at 300?K reveals that the spin doublet state is the ground state.     

First principles investigations of Fe,Co, Ni in model honeycomb carbon networks

The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC₆. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μ_B for FeC₆ and 1 μ_B for CoC₆ while no magnetic solution could be identified for NiC₆. In the D_6h point group of the P6/mmm space group l_m decomposition of the d states results with increasing energy into doublet state E_1g with d(x²-y²) and d(xy); singlet state A_1g d(z²) and doublet state E_2g d(xz) and d(yz) lying on E_F and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC₆ compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications.