Fe-N Bond Lengths Research Articles

Chitosan-covered spherical natural graphite: A perfect carbon source for efficient non-PGM catalysts for electrocatalysis

Developing non-precious metal catalysts to replace Pt-based catalysts for oxygen reduction reaction (ORR) is a hotspot presently, and transition metal-nitrogen co-doped carbon (TMNC) materials have been deemed as the most promising candidates. In this work, chitosan-derived-curved-carbon covered natural spherical graphite (SG) with a well-defined core-shell structure were prepared via hydrothermal carbonization (HTC) process followed by activation under NH3 atmosphere. A series of active transition metals (M=Fe, Co, Mn) and nitrogen atom were embedded into the curved carbon to form various MNC moieties on SG surface as active sites for ORR. Among the atomic precise MNC@NSG hybrid catalysts, the FeNC@NSG shows the best ORR performance and the half-wave potential is 0.95 V (vs. RHE), which are 108 % as high as that obtained by commercial Pt/C in alkaline environment. Besides, the electrochemical stability and methanol tolerance of the catalyst is also superior to commercial Pt/C. DFT calculations reveals that the high-performance stems from the curved FeNC units in the shell and high electrical conductivity of natural SG in the core for the typical core-shell structures. Moreover, the sluggish ORR kinetics can be efficiently accelerated by the increased FeN bond length, more positive-shifted d-band center and more positive surface charge of active Fe atoms in the curved FeNC units on spherical SG. This provides an enlightenment for developing efficient non-platinum group metal (PGM) catalysts using cheap and volume natural spherical graphite as building brick for energy storage and conversion devices.

Quantification of the thermodynamic effects of the low-spin - high-spin interaction in molecular crystals of a mononuclear iron(II) spin crossover complex.

A method is proposed to estimate the energetic and entropic effects of spins of neighbouring molecules on the spin transition of a mononuclear spin crossover (SCO) complex in a molecular crystal. Density functional theory (DFT) methods have been used to model the SCO material [FeII(Lnpdtz)2(NCS)2] (Lnpdtz = 2-naphthyl-5-pyridyl-1,2,4-thiadiazole) exhibiting numerous π-π interactions using a 2D arrangement of 15 molecules. The modelling considers only the effects in the crystallographical ac plane with a particularly pronounced stacking but paves the way for future work with 3D arrangements which are computational much more costly. It involves the optimisation and normal mode calculation of the molecules in a rigid matrix of both low-spin (LS) and high-spin (HS) neighbours. This procedure has been used to calculate the previously defined cooperativity parameter Hcoop (S. Rackwitz, W. Klopper, V. Schünemann and J. A. Wolny, Phys. Chem. Chem. Phys., 2013, 15, 15450). For [FeII(Lnpdtz)2(NCS)] we obtain Hcoop = 11 kJ mol-1, a value which is comparable to those found for 3D polynuclear spin crossover materials. A normal mode analysis of the optimised centrally located molecule indicates that the vibrational entropy of the spin transition is somewhat higher (5 J K-1 mol-1) for the LS to HS transition in the LS matrix than in the HS one. The calculations show that the interactions with the neighbours influence the low-frequency modes with wave numbers <65-70 cm-1. These cause the main difference in the vibrational entropy of the spin transition for the vicinity of high- and low-spin molecules. Furthermore, a deformation of the coordination sphere of the central molecule is observed when the spins of the surrounding centres are switched. This deformation is accompanied by a change in the equatorial Fe-N bond lengths.

Tuning Active Site Flexibility by Defect Engineering of Graphene Ribbon Edge-hosted Fe-N3 Sites.

Nitrogen-doped, carbon-supported transition metal catalysts are excellent for several reactions. Structural engineering of M-Nx sites to boost catalytic activity is rarely studied. Here, we demonstrate that the structural flexibility of Fe-N3 site is vital for tuning the electronic structure of Fe atoms and regulating the catalytic transfer hydrogenation (CTH) activity. By introducing carbon defects, we construct Fe-N3 sites with varying Fe-N bond lengths distinguishable by X-ray absorption spectroscopy. We investigate the CTH activity by density-functional theory and microkinetic calculations and reveal that the vertical displacement of the Fe atom out of the plane of the support, induced by the Fe-N3 distortion, raises the Fe orbital and strengthens binding. We propose that the activity is controlled by the relaxation of the reconstructed site, which is further affected by Fe-N bond length, an excellent activity descriptor. We elucidate the origin of the CTH activity and principles for high-performing Fe-N-C catalysts by defect engineering.

Iron-histidine bonding in bishistidyl hemoproteins-A local vibrational mode study.

We investigated the intrinsic strength of distal and proximal FeN bonds for both ferric and ferrous oxidation states of bishistidyl hemoproteins from bacteria, animals, human, and plants, including two cytoglobins, ten hemoglobins, two myoglobins, six neuroglobins, and six phytoglobins. As a qualified measure of bond strength, we used local vibrational force constants k (FeN) based on local mode theory developed in our group. All calculations were performed with a hybrid QM/MM ansatz. Starting geometries were taken from available x-ray structures. k (FeN) values were correlated with FeN bond lengths and covalent bond character. We also investigated the stiffness of the axial NFeN bond angle. Our results highlight that protein effects are sensitively reflected in k (FeN), allowing one to compare trends in diverse protein groups. Moreover, k (NFeN) is a perfect tool to monitor changes in the axial heme framework caused by different protein environments as well as different Fe oxidation states.

Synthesis and crystal structure of bis-(tert-butyl isocyanide-κC)[5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinato-κ4N]iron(II).

In the title compound, [FeII(C44H24Cl4N4)(C5H9N)2] or [FeII(TClPP)(t-BuNC)2] [where TClPP and t-BuNC are 5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinate and tert-butyl isocyanide ligands, respectively], the metal ion lies on an inversion center and is octa-hedrally coordinated by the N atoms of the porphyrin ring in the equatorial plane and by carbon atoms of the trans t-BuNC ligands in the axial sites. The Fe-N bond length of 2.0074 (14) Å suggests a low-spin complex (S = 0). The crystal packing of the title compound is sustained by C-H⋯Cl, C-H⋯N and C__H⋯Cg (Cg = the centroid of a pyrrole ring of the TClPP porphyrinate) inter-actions, leading to a three-dimensional network. The Hirshfeld surface (HS) analysis indicates that 61.4% of the inter-molecular inter-actions are from H⋯H contacts while other contributions are from C⋯H/H⋯C, O⋯H/H⋯O and N⋯H/H⋯N inter-actions, which comprise 21.3%, 13.3% and 3.6% of the HS, respectively.

Origin of the Distinctive Electronic Structure of Co- and Fe-Porphyrin-Nitrene and Its Effect on Their Nitrene Transfer Reactivity

Metal-bound nitrene species are the crucial intermediate in catalytic nitrene transfer reactions exhibited by engineered enzymes and molecular catalysts. The electronic structure of such species and its correlation with nitrene transfer reactivity have not been fully understood yet. This work presents an in-depth electronic structure analysis and nitrene transfer reactivity of two prototypical metal-nitrene species derived from CoII(TPP) and FeII(TPP) (TPP = meso-tetraphenylporphyrin) complexes and tosyl azide nitrene precursor. Parallel to the well-known "cobalt(III)-imidyl" electronic structure of the Co-porphyrin-nitrene species, the formation mechanism and electronic structure of the elusive Fe-porphyrin-nitrene have been established using density functional theory (DFT) and multiconfigurational complete active-space self-consistent field (CASSCF) calculations. Electronic structure evolution analysis for the metal-nitrene formation step and CASSCF-derived natural orbitals advocates that the electronic nature of the metal-nitrene (M-N) core of Fe(TPP) is strikingly different from that of the Co(TPP). Specifically, the "imidyl" nature of the Co-porphyrin-nitrene [(TPP)CoIII-•NTos] (Tos = tosyl) (I1Co) is contrasted by the "imido-like" character of the Fe-porphyrin-nitrene [(TPP)FeIV[Formula: see text]NTos] (I1Fe). This difference between Co- and Fe-nitrene has been attributed to the additional interactions between Fe-dπ and N-pπ orbitals in Fe-nitrene, which is further complemented by the shortened Fe-N bond length of 1.71 Å. This stronger M-N bond in Fe-nitrene compared to the Co-nitrene is also reflected in the higher exothermicity (ΔΔH = 16 kcal/mol) of the Fe-nitrene formation step. The "imido-like" character renders a relatively lower spin population on the nitrene nitrogen (+0.42) in the Fe-nitrene complex I1Fe, which undergoes the nitrene transfer to the C═C bond of styrene with a considerably higher enthalpy barrier (ΔH‡ = 10.0 kcal/mol) compared to the Co congener I1Co (ΔH‡ = 5.6 kcal/mol) possessing a higher nitrogen spin population (+0.88) and a relatively weaker M-N bond (Co-N = 1.80 Å).

Spin-dependent transport properties of a tetra-coordinated Fe(II) spin-crossover complex

Spin-crossover (SCO) complexes are being regarded as one of the most promising components of molecular spintronics, since they can exhibit different spin states (high-spin/HS or low-spin/LS) in response to external stimuli. Here, the electronic structure as well as the transport properties of a tetra-coordinated bis(formazanate) Fe(II) SCO complex have been investigated with the DFT + NEGF approach. From the calculated results, the transition between different spin states of the Fe(II) SCO complex is accompanied by a variation of the Fe-N bond length and the dihedral angle between the planes of the ligands. The I-V curves of the molecular junctions revealed that the SCO complexes in different spin configurations display significantly different bias-dependent characteristics. When the bias is above 0.1 V, the current through the HS-configured molecular junction increases rapidly and is significantly greater as compared to that in the LS configuration. Furthermore, the transport properties of the HS-state SCO complex are dominated by spin-up electrons, indicating a strong spin-filtering effect.

A Spectroscopically Observed Iron Nitrosyl Intermediate in the Reduction of Nitrate by a Surface-Conjugated Electrocatalyst.

We report an iron-based graphite-conjugated electrocatalyst (GCC-FeDIM) that combines the well-defined nature of homogeneous molecular electrocatalysts with the robustness of a heterogeneous electrode. A suite of spectroscopic methods, supported by the results of DFT calculations, reveals that the electrode surface is functionalized by high spin (S = 5/2) Fe(III) ions in an FeN4Cl2 coordination environment. The chloride ions are hydrolyzed in aqueous solution, with the resulting cyclic voltammogram revealing a Gaussian-shaped wave assigned to 1H+/1e- reduction of surface Fe(III)-OH surface. A catalytic wave is observed in the presence of NO3-, with an onset potential of -1.1 V vs SCE. At pH 6.0, GCC-FeDIM rapidly reduces NO3- to ammonium and nitrite with 88 and 6% Faradaic efficiency, respectively. Mechanistic studies, including in situ X-ray absorption spectroscopy, suggest that electrocatalytic NO3- reduction involves an iron nitrosyl intermediate. The Fe-N bond length (1.65 Å) is similar to that observed in {Fe(NO)}6 complexes, which is supported by the results of DFT calculations.

Importance of non-covalent interactions in a nitrile anion metal-complex based on pyridine ligands: A theoretical and experimental approach

The solvothermal synthesis and crystal structure of [bis(thiocyanato-κN)bis(tris(pyridin-2-yl-κN)amine)iron(II)] (1) are described. The structure obtained was i subjected to theoretical analysis of interaction energies and intra- and intermolecular bonding. This comprehensive study illustrates the richness of non-covalent interactions that are exhibited by nitrile anionmetal complexes. The neutral complex lies on a crystallographic two-fold axis. The iron atom is octahedrally coordinated, with two thiocyanato ligands in cis positions, and the two trispyridine ligands coordinated via the nitrogen atoms of just two of the three pyridine rings. Energy analysis of five significant dimers, supplemented by a detailed analysis of the Hirshfeld surfaces, indicates attractive interaction energies of up to 101 kJ, with the strongest dimer stabilized by bifurcated CH⋯S hydrogen bonds and additional π⋯π interactions. Solid-state and molecular quantum mechanical calculations confirm the high-spin nature of the complex, which is also reflected in the FeN bond lengths, and highlights the relevance of intramolecular ligand-ligand interactions. From a combined analysis using the Quantum Theory of Atoms in Molecules and the Non-Covalent Interaction index it was possible to identify further specific intermolecular interactions contributing the stability of the crystal structure, such as the H⋯H bonds, as well as the presence of several delocalized van der Waals forces, which are mainly of dispersive origin.

Crystal structure of poly[[di-aqua-tetra-μ2-cyanido-platinum(II)iron(II)] methanol 4/3-solvate]: a three-dimensional Hofmann clathrate analogue.

In the title polymeric coordination compound, {[FePt(CN)4(H2O)2]·1.33CH3OH} n , the FeII cation (site symmetry 4/mm.m) is coordinated by the N atoms of four cyanide anions (CN-) and the O atoms of two water mol-ecules, forming a nearly regular [FeN4O2] octa-hedron. According the Fe-N and Fe-O bond lengths, the FeII atom is in the high-spin state. The cyanide anions act in a bridging manner to connect the FeII and PtII atoms. The [Pt(CN)4]2- moieties (Pt with site symmetry 4/mm.m) have a perfect square-planar shape. The latter anion is located perpendicular to the FeN4 plane, thus ensuring the creation of a three-dimensional framework. The crystal structure features methanol solvent mol-ecules of which 4/3 were located per FeII cation. These solvent mol-ecules are located in hexa-gonal pores; they inter-act with coordinating water mol-ecules through weak hydrogen bonds. Other guest mol-ecules could not be modelled in a satisfactory way and their contribution to the scattering was removed by a mask procedure.

Fluorinated click-derived tripodal ligands drive spin crossover in both iron(II) and cobalt(II) complexes.

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently, controlling the spin state in metal complexes is challenging because a precise control of the properties of the secondary coordination sphere is often difficult. It has been shown that non-covalent interactions in the secondary coordination sphere of transition metal complexes can enable spin state control. Here we exploit this strategy for fluorinated triazole ligands and present mononuclear CoII and FeII complexes with "click"-derived tripodal ligands that contain mono-fluorinated benzyl substituents on the backbone. Structural characterization of 1 and 2 at 100 K revealed Co-N bond lengths that are typical of high spin (HS) CoII complexes. In contrast, the Fe-N bond lengths for 3 are characteristic of a low spin (LS) FeII state. All complexes show an intramolecular face-to-face non-covalent interaction between two arms of the ligand. The influence of the substituents and of their geometric structure on the spin state of the metal center was investigated through SQUID magnetometry, which revealed spin crossover occurring in compounds 1 and 3. EPR spectroscopy sheds further light on the electronic structures of 1 and 2 in their low- and high-spin states. Quantum-chemical calculations of the fluorobenzene molecule were performed to obtain insight into the influence of fluorine-specific interactions. Interestingly, this work shows that the same fluorinated tripodal ligands induce SCO behavior in both FeII and CoII complexes.

SCC-DFTB Parameters for Fe-C Interactions.

We present an optimized density-functional tight-binding (DFTB) parameterization for iron-based complexes based on the popular trans3d set of parameters. The transferability of the original and optimized parameterizations is assessed using a set of 50 iron complexes, which include carbonyl, cyanide, polypyridine, and cyclometalated ligands. DFTB-optimized structures predicted using the trans3d parameters show a good agreement with both experimental crystal geometries and density functional theory (DFT)-optimized structures for Fe-N bond lengths. Conversely, Fe-C bond lengths are systematically overestimated. We improve the accuracy of Fe-C interactions by truncating the Fe-O repulsive potential and reparameterizing the Fe-C repulsive potential using a training set of six isolated iron complexes. The new trans3d*-LANLFeC parameter set can produce accurate Fe-C bond lengths in both geometry optimizations and molecular dynamics (MD) simulations, without significantly affecting the accuracy of Fe-N bond lengths. Moreover, the potential energy curves of Fe-C interactions are considerably improved. This improved parameterization may open the door to accurate MD simulations at the DFTB level of theory for large systems containing iron complexes, such as sensitizer-semiconductor assemblies in dye-sensitized solar cells, that are not easily accessible with DFT approaches because of the large number of atoms.

Bis[tris-(pyridin-2-yl)amine]-iron(II) tris-(di-cyano-methyl-idene)methane-diide.

In the title compound, [Fe{(C5H4N)3N}2][C{C(CN)2}3], both ions lie across centres of inversion, with the anion being statistically disordered over two sets of atomic sites having equal occupancy. The cation and anion have approximate and 32 symmetry, respectively, and the Fe-N bond lengths indicate low-spin FeII. A combination of two-centre C-H⋯N and three-centre C-H⋯(N)2 hydrogen bonds link the ions into complex sheets. Several low-occupancy water mol-ecules are present, whose H atoms could not be located: accordingly, the reflection data were subjected to the SQUEEZE procedure [Spek (2015 ▸). Acta Cryst. C71, 9-18].

Theoretical Study on Structure Prediction and Molecular Formula Determination of Polymeric Complexes Comprising Fe(II) and 1,2,4-H-Triazole Ligand

The structure and the molecular formula of Fe(II) 1,2,4-H-triazole complex has been predicted by using Hartree-Fock (HF) and Density Functional Theory (DFT) methods. The distance between the Fe(II) ions and Fe-N bond length show good agreement with experimental measurements at the low-spin state. It has been conducted by using hybrid/basis set functions of B3LYP/6-31G(d), TPSSh/TZVP, and MO6-2x/6-31G(d). The distance between the Fe(II) ions complexes with deprotonated ligands are 3.30–3.75, 3.44–3.74, and 3.46–3.79 A, respectively, and undeprotonated ligands are 3.41–4.04, 3.49–3.90, and 3.52–4.09 A. Meanwhile, the Fe-N bond lengths in the complex with the deprotonated ligand are 1.84–2.07, 1.85–2.04, and 1.89–2.11 A, respectively, while in the complex with undeprotonated ligands they are 1.89–2.20, 1.84–2.12, and 1.96–2.21 A. The molecular formula of Fe(II)-Htrz complex is ([Fe(Htrz)2(trz)]+)n which has been obtained by comparing the energy difference between the complex formation with deprotonated ligands being lower than that with undeprotonated complex. The computational results on the hybrid/basis set function of B3LYP/6-31G(d) induces the difference of energy formation of [Fe2(Htrz)4(trz)2]2+, [Fe2(Htrz)6]4+, [Fe4(Htrz)8(trz)4]4+, [Fe4(Htrz)12]8+, [Fe6(Htrz)12(trz)6]6+, and [Fe6(Htrz)18]12+ complexes to be −5613.38, −3082.67, −11013.19, −147.40, −16101.36, and −6825.09 kJ/mol, respectively.

A five-coordinate iron(III) porphyrin complex including a neutral axial pyridine N-oxide ligand.

While six-coordinate iron(III) porphyrin complexes with pyridine N-oxides as axial ligands have been studied as they exhibit rare spin-crossover behavior, studies of five-coordinate iron(III) porphyrin complexes including neutral axial ligands are rare. A five-coordinate pyridine N-oxide-5,10,15,20-tetraphenylporphyrinate-iron(III) complex, namely (pyridine N-oxide-κO)(5,10,15,20-tetraphenylporphinato-κ4N,N',N'',N''')iron(III) hexafluoroantimonate(V) dichloromethane disolvate, [Fe(C44H28N4)(C5H5NO)][SbF6]·2CH2Cl2, was isolated and its crystal structure determined in the space group P-1. The porphyrin core is moderately saddled and the Fe-O-N bond angle is 122.08 (13)°. The average Fe-N bond length is 2.03 Å and the Fe-ONC5H5 bond length is 1.9500 (14) Å. This complex provides a rare example of a five-coordinate iron(III) porphyrin complex that is coordinated to a neutral organic ligand through an O-monodentate binding mode.

Insight into the biological effects of acupuncture points by X-ray absorption fine structure.

Exploration of the biological effects of transition metal ions in acupuncture points is essential to clarify the functional mechanism of acupuncture treatment. Here we show that in the SP6 acupuncture point (Sanyinjiao) the Fe ions are in a high-spin state of approximately t2g4.5eg1.5 in an Fe-N(O) octahedral crystal field. The Fe K-edge synchrotron radiation X-ray absorption fine structure results reveal that the Fe-N and Fe-O bond lengths in the SP6 acupuncture point are 2.05 and 2.13 Å, respectively, and are 0.05-0.10 Å longer than those in the surrounding tissue. The distorted atomic structure reduces the octahedral symmetry and weakens the crystal field around the Fe ions by approximately 0.3 eV, leading to the high-spin configuration of the Fe ions, which is favorable for strengthening the magnetotransport and oxygen transportation properties in the acupuncture point by the enhanced spin coherence. This finding might provide some insight into the microscopic effect of the atomic and electronic interactions of transition metal ions in the acupuncture point. Graphical Abstract ᅟ.

Spin switch in iron phthalocyanine on Au(111) surface by hydrogen adsorption.

The manipulation of spin states at the molecular scale is of fundamental importance for the development of molecular spintronic devices. One of the feasible approaches for the modification of a molecular spin state is through the adsorption of certain specific atoms or molecules including H, NO, CO, NH3, and O2. In this paper, we demonstrate that the local spin state of an individual iron phthalocyanine (FePc) molecule adsorbed on an Au(111) surface exhibits controllable switching by hydrogen adsorption, as evidenced by using first-principles calculations based on density functional theory. Our theoretical calculations indicate that different numbers of hydrogen adsorbed at the pyridinic N sites of the FePc molecule largely modify the structural and electronic properties of the FePc/Au(111) composite by forming extra N-H bonds. In particular, the adsorption of one or up to three hydrogen atoms induces a redistribution of charge (spin) density within the FePc molecule, and hence a switching to a low spin state (S = 1/2) from an intermediate spin state (S = 1) is achieved, while the adsorption of four hydrogen atoms distorts the molecular conformation by increasing Fe-N bond lengths in FePc and thus breaks the ligand field exerted on the Fe 3d orbitals via stronger hybridization with the substrate, leading to an opposite switching to a high-spin state (S = 2). These findings obtained from the theoretical simulations could be useful for experimental manipulation or design of single-molecule spintronic devices.

A Mononuclear Nonheme Iron(V)-Imido Complex.

Mononuclear nonheme iron(V)-oxo complexes have been reported previously. Herein, we report the first example of a mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), [(TAML)FeV(NTs)]- (1). The spectroscopic characterization of 1 revealed an S = 1/2 Fe(V) oxidation state, an Fe-N bond length of 1.65(4) Å, and an Fe-N vibration at 817 cm-1. The reactivity of 1 was demonstrated in C-H bond functionalization and nitrene transfer reactions.

Iron(II) Complexes with Scorpiand-Like Macrocyclic Polyamines: Kinetico-Mechanistic Aspects of Complex Formation and Oxidative Dehydrogenation of Coordinated Amines.

The Fe(II) coordination chemistry of a pyridinophane tren-derived scorpiand type ligand containing a pyridine ring in the pendant arm is explored by potentiometry, X-ray, NMR, and kinetics methods. Equilibrium studies in water show the formation of a stable [FeL]2+ complex that converts to monoprotonated and monohydroxylated species when the pH is changed. A [Fe(H-2L)]2+ complex containing an hexacoordinated dehydrogenated ligand has been isolated, and its crystal structure shows the formation of an imine bond involving the aliphatic nitrogen of the pendant arm. This complex is low spin Fe(II) both in the solid state and in solution, as revealed by the Fe-N bond lengths and by the NMR spectra, respectively. The formation rate of [Fe(H-2L)]2+ in aqueous solutions containing Fe2+ and L (1:1 molar ratio) is strongly dependent on the pH, the process being completed in times that range from months in acid solutions to hours in basic conditions. However, detailed kinetic studies show that those differences are caused, at least in part, by the effect of pH on the rate of formation of the unoxidized [FeL]2+ complex. In this sense, the protonation of the donor atoms in the pendant arm of the scorpiand ligand leads to the formation of protonated species resistant to oxidative dehydrogenation. Complementary studies in acetonitrile solution indicate that the initial stage in the oxidative dehydrogenation process is the oxidation of the starting complex to form a [FeL]3+ complex, which then undergoes disproportionation into [Fe(H-2L)]2+ and [FeL]2+. Experiments starting with Fe(III) have allowed us to determine that disproportionation occurs with first order kinetics both in water and acetonitrile solutions. However, whereas a significant acceleration is observed in water when the pH is increased, no effect of the addition of acid or base on the rate of disproportionation is observed in acetonitrile. Oxidative dehydrogenation of the Fe(II) complex formed in experiments starting with an Fe(III) salt is slower than that occurring when an Fe(II) salt is used, an observation that can be explained in terms of the formation of two different Fe(III) complexes, one of them with a structure unable to evolve directly toward the product of oxidative dehydrogenation.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group.

Two iron(ii) molecular isomers, namely face-Λ-[Fe(L1)3](ClO4)2 (1a) (L1 = R-1-phenyl-N-((1-hexyl-5-methyl-1H-imidazol-2-ylmethylene)ethanamine)) and face-Λ-[Fe(L2)3](ClO4)2 (1b) (L2 = R-1-phenyl-N-(1-hexyl-4-methyl-1H-imidazol-2-ylmethylene)ethanamine), are obtained via a well-designed strategy based on the tautomerization of the 4(5)-methylimidazole group. Structural investigations reveal that the two isomers are extremely similar with only differences in the methyl group position of the ligands. The iron(ii) center is surrounded by three bidentate imidazole Schiff-base ligands in the face-Λ conformation, which affords a distorted [FeN6] octahedral coordination sphere. The average Fe-N bond length of 1a (1.971 Å) is shorter than that of 1b (2.185 Å). Spectroscopy analyses, X-ray crystal structures and magnetic investigations show that 1a is in the low-spin state at room temperature due to the strong ligand field imparted by the electron-donating methyl groups at the 5-position on the imidazole moieties and undergoes a partial spin transition with an estimated T1/2 = 390 K. In contrast, 1b is stabilized in the high-spin state because of the strong steric hindrance when the three methyl "legs" are changed from the 5-position to the 4-position on the imidazole moieties. In addition, a new complex, 2, without methyl groups attached to the imidazole rings is introduced and characterized to further corroborate the steric influence on the spin state. Complex 2 exhibits a gradual spin-crossover behaviour with T1/2 = 258 K. Moreover, the diverse spin states of these complexes are computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that the competition of the electronic effect and steric crowding influence the spin states.