Synthesis, structure and Mössbauer characterization of polymeric iron(II) complexes with bidentate thiourea ligands

Complexes of FeCl₂ with the known bis(3-methyl-2-thione-imidazolyl)methane (L¹) and the new bis(3-tert-butyl-2-thione-imidazolyl)methane (L²) are reported. For both [L¹FeCl₂]_n (3) and [L²FeCl₂]_n (4) X-ray crystallography reveals that 1D-polymeric chain structures are present in the solid state, with the two mercaptoimidazolyl units of L¹ and L² coordinating to different metal ions. Complexes 3 and 4 are further characterized by Mössbauer spectroscopy and SQUID magnetometry. NMR spectroscopy suggests that the complexes largely dissociate in polar solvents. X-ray structures of L² and its precursor bis(imidazolium) salt are also reported.     

Synthesis and characterization of three mononuclear Fe(III) complexes containing bipodal and tripodal ligands: X-ray molecular structure of the dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate

In this work, we present the synthesis and characterization of three mononuclear iron(III) complexes: dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate (1), trichloro[N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III) (2) and trichloro[bis-(2-pyridylmethyl)amine]iron(III) (3). The complexes were characterized by cyclic voltammetry, conductivimetry, elemental analyses, and by electronic, infrared and Mössbauer spectroscopies. Complex 1 was also characterized by X-ray structural analysis, which showed an iron center coordinated to one amide, one tertiary amine, two pyridine groups and two chloride ions. While for 1 the X-ray molecular structure and the infrared spectrum confirm the coordination of the amide group by the oxygen atom, the infrared spectrum of 2 indicates that the ester group present in the ligand is not coordinated, resulting in a N₃Cl₃ donor set, similar to the one present in 3. However, in 3 there is a secondary amine while in 2 a tertiary amine exists. These structural differences result in distinguishable variations in the Lewis acidity of the iron center, which could be evaluated by the analysis of the redox potential of the complexes, as well as by Mössbauer parameters. Thus, the Lewis acidity decreases in the following order: 1 > 2 > 3. It is important to notice that 1 has the amide group coordinated to the iron center, a feature present in metalloenzymes as lipoxygenase and isopenicillin N synthase, and in a small number of mononuclear iron(III) complexes.     

Isolation of a singly oxo-bridged Fe(III) dinuclear complex: Synthesis, crystal structure, spectroscopic and magnetic studies

The μ-oxo dinuclear complex {Fe₂O(tptz)₂[N(CN)₂]₂(NO₃)₂} (1) (where tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine) has been synthesised and characterised by elemental analysis, FT-IR, UV-vis, cyclic voltammetry, Mössbauer spectroscopy, and variable-temperature magnetic susceptibility measurements and single crystal X-ray diffraction. The iron centres have a pentagonal-bipyramidal geometry. The dimeric neutral complex exhibits typical Fe-μ-O bond lengths of 1.763(1) Å and a bridge angle of 180.00°. The Fe?Fe separation is 3.526(3) Å. The Mössbauer spectrum at room temperature consists of one quadrupole doublet with an isomer shift of 0.41 mm/s and a quadrupole splitting of 1.12 mm/s. Variable-temperature magnetic susceptibility measurements have been measured in the temperature range 300-2 K, revealing an intramolecular antiferromagnetic coupling (J = −211.6 cm⁻¹).     

Reversible structural change of host framework triggered by desorption and adsorption of guest benzene molecules in Fe(NCS)2(bpp)2·2(benzene) (bpp = 1,3-bis(4-pyridyl)propane)

Assembled iron complex Fe(NCS)₂(bpp)₂·2(benzene) (bpp = 1,3-bis(4-pyridyl)propane), enclathrating benzene molecules, has been synthesized and its host framework was 1D chain structure. By releasing benzene molecules, the host framework changed to 2D interpenetrated structure. The desorbed complex enclathrated benzene molecules again and the host framework returned to 1D structure. Although the spin state remained in the high spin upon the desorption and adsorption of the benzene molecules, ⁵⁷Fe Mössbauer spectra revealed that the symmetry around iron atom changed depending on the sorption. The reversible structural change of host framework was tried to explain by two factors: the crystal stability by making up the interstitial space and the conformer stability in coordinated bridging ligand bpp.     

X-ray and Fe Mössbauer study on a Fe(III) complex of 2-acetyl-1,3-indandione

Single-crystal X-ray structure of an iron(III) complex of 2-acetyl-1,3-indandione is resolved which reveals that a racemic mixture, composed of Δ-fac and Λ-fac stereoisomers, is formed. Both species have octahedral geometry, but slightly distinguishable. Detailed Mössbauer data indicate a high-spin electronic structure of the Fe(III) centres Fe1 and Fe2 with spin-spin magnetic relaxation process. Two different approaches for computer processing of the experimental Mössbauer spectra are considered.     

Spin-crossover iron(II) complexes [Fe(Medpq)(py)2(NCS)2] and [Fe(Medpq)(py)2(NCSe)2]: syntheses, characterization and magnetic properties

Two new spin-crossover complexes, [Fe(Medpq)(py)₂(NCS)₂] · py · 0.5H₂O (1) and [Fe(Medpq)(py)₂(NCSe)₂] · py (2) (Medpq = 2-methyldipyrido[3,2-f:2′,3′-h]-quinoxaline, py = pyridine), have been synthesized. The crystal structures were determined at both room temperature (298 K) and low temperature (110 K). Complexes 1 and 2 crystallize in the orthorhombic space group Pbca and monoclinic space group P2₁/n, respectively. In both complexes, the distorted [FeN₆] octahedron is formed by six nitrogen atoms from Medpq, the trans pyridine molecules and the cis NCX⁻ groups. The thermal spin transition is accompanied by the shortening of the mean Fe–N distances by 0.194 Å for 2. The mononuclear [Fe(Medpq)(py)₂(NCS)₂] and [Fe(Medpq)(py)₂(NCSe)₂] neutral species interact each other via π-stacking, resulting in a one-dimensional extended structure for both 1 and 2. There exist C–HX (X = S, Se) hydrogen bonds for both complexes. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy reveal the occurrence of a gradual spin transition. The transitions are centered at T_1/2 = 120 K for 1 and T_1/2 = 180 K for 2, respectively.     

Zn(II) and Hg(II) complexes of naphthalene based thiosemicarbazone: Structure and spectroscopic studies

Synthesis, structure and spectroscopic characterization of 2-thiophenealdehyde-N(4)-napthylthiosemicarbazone and its complexes with biologically important Zn(II) and toxic Hg(II) metal ions have been reported. The crystal structure of the complexes reveals that both are distorted tetrahedral. In the Hg(II) complex the ligand is neutral and mondented where as in Zn(II) complex the ligand is bidented and anionic. Whereas conductivity measurement study shows both the complexes are molecular species. The beautiful changes in absorption spectra along with isosbestic points upon addition of respective metal salts to the ligand solution convincingly support the formation of metal complexes in solution phase. The other spectroscopic studies also show good correlation with their solid state structures.     

New Fe starting materials: preparation, characterisation and structural features of iron halide complexes with alcohol ligands

The new mononuclear [FeCl₂(HOPrⁱ)₄] (1), polymeric [{Cl₃Fe(μ-Cl)Fe(HOPrⁱ)₄}_n] (2) and binuclear [I₂Fe(μ-I)₂Fe(PrⁱOH)₄] (3) iron(II) complexes have been synthesized in high yields in propan-2-ol or toluene/propan-2-ol mixtures at room temperature. Magnetic moment measurements, ⁵⁷Fe Mössbauer spectroscopy data and the results of semi-empirical quantum mechanical calculations confirmed the high-spin configuration of the iron(II) centres, which were shown to be four- and/or six-coordinate by single crystal X-ray diffraction analyses. Intermolecular hydrogen bonding was observed in the solid state structure of 1, intramolecular interactions in 2, while both intra- and intermolecular association was seen in 3. Long iron-(μ-halide) bonds suggest the possibility of complex dissociation in solution and facile ligand substitution in 2 and 3.     

High-spin Schiff-base dinuclear iron(III) complexes bridged by N-oxide ligands

Synthesis and structure of dinuclear complexes [{Fe^III(L⁵)}b{Fe^III(L⁵)}](BPh₄)₂, where L⁵ is a pentadentate Schiff-base ligand, b is a bidentate N-oxide bridging ligand based on bipyridine, is reported. Magnetic behavior is investigated in terms of the magnetic susceptibility, magnetization, and Mössbauer spectroscopy revealing that the complexes are high-spin over the whole temperature region.     

A novel dinuclear nickel (II) complex: spectroscopic and magnetic studies

A novel dinuclear [(MeOH)(L) Ni (di-μ-CCl₃CO₂) Ni (L)(CCl₃CO₂)] 1 [where L={(O⁻)(C₆H₄CH=NCH₂CH₂NMe₃)}] complex has been synthesized and characterized. Characterization based on elemental analyses, infrared and electronic spectra, thermal studies, variable temperature magnetic susceptibility measurement and crystal structure analysis. The complex crystallizes in the monoclinic system, space group P2(1)/n with a=16.818(11), b=20.386(11), c=20.589(13), β=90.94(5) and Z=8. The complex shows di-μ-trichloroacetate bridging with concatenated methanol molecule hydrogen bonded with another trichloroacetate. Magnetic susceptibility measurements of complex 1 reveal an antiferromagnetic intra-dimer interaction with the J value deduced from the spin Hamiltonian H=−JS₁S₂.     

Biotechnological application of homo- and heterotrinuclear iron(III) furoates for cultivation of iron-enriched Spirulina

New trinuclear iron(III) furoates with the general formula [Fe₃O(α-fur)₆(R-OH)₃]X, where α-fur C₄H₃OCOO, R = CH₃ (1), C₂H₅ (2), n-C₃H₇ (3), n-C₄H₉ (4), X = NO₃⁻ (1-4); [Fe₃O(α-Fur)₆(DMF)(CH₃OH)₂]NO₃ (5); [Fe₃O(α-Fur)₆(H₂O)(CH₃OH)₂]Cl (6); [Fe₂MO(α-Fur)₆(L)(H₂O)₂], where L = THF (7-9), DMF (10-12), M = Mn²⁺ (7, 10), Co²⁺ (8, 11), Ni²⁺ (9, 12) and [Fe₂MO(α-Fur)₆(3Cl-Py)₃], where M = Mn²⁺ (13), Co²⁺ (14), Ni²⁺ (15); have been prepared and investigated by Mössbauer and IR spectroscopy. The X-ray crystal structure for the 1·2CH₃OH complex indicates that it crystallizes in the monoclinic crystal system (P2₁/n) and has a structure typical of μ₃-O-bridged trinuclear iron(III) compounds. Coordination compounds 1, 4, 7, 8 can be used as regulators of the biochemical composition of cyanobacterium Spirulina platensis biomass. The supplementation of these compounds, in concentrations exceeding 5-10 mg/l, increases the content of iron, amino acids, peptides and carbohydrates in Spirulina.     

Synthesis, characterization and structural studies of new palladium(II) complexes including non-symmetric phosphorus ylides

The reaction of the non-symmetric phosphorus ylides, Ph₂P(CH₂)_nPPh₂C(H)C(O)PhR [Y₁-Y₄: n = 1, R = Cl, Br, NO₂, OCH₃ and Y₅-Y₈: n = 2, R = Cl, Br, NO, OCH₃] with dichloro(1,5-cyclooctadiene)palladium(II) in dichloromethane under mild conditions afford the monomeric P-C chelated complexes, [(Y)PdCl₂] (Y = Y₁-Y₈). These complexes were fully characterized by elemental analysis and spectroscopic techniques such as IR, ¹H, ³¹P, and ¹³C NMR. In addition, the identity of complexes [(Y₅)PdCl₂] (1b) and [(Y₈)PdCl₂] (4b) was unequivocally determined by single crystal X-diffraction techniques, both structures consisting of six-membered rings formed by coordination of the ligands through the phosphine group and the ylidic carbon atom to the metal center. The coordination geometry around the Pd atoms in both these complexes be defined as slightly distorted square planar. Furthermore, their electrochemical behavior was also investigated by cyclic voltammeters, thus the cyclic voltammetry of complex [(Y₁)PdCl₂], in dichloromethane solution with Pt electrode, shows that the redox reaction of the pair Pd(II)/Pd(0) is irreversible with the cathodic peak potential at −1.08 V versus Ag wire.     

A high-spin Fe(II)/low-spin Fe(III) redox couple featuring the hydro[tris(4-chloro-3,5-dimethyl-pyrazolyl)]borate ligand: Synthesis, spectroscopic and X-ray crystallographic characterization

The white homoleptic high-spin iron(II) complexes Fe[Tp^Me2,4Cl]₂ (1) was isolated in quantitative yield from reaction mixtures containing 1 equiv of FeCl₂(THF)_1.5 and 2 equiv of K[Tp^Me2,4Cl] (Tp^Me2,4Cl = hydrotris[(4-chloro-3,5-dimethyl-pyrazolyl)]borate). Its purple low-spin iron(III) counterparts 1[O₃SCF₃] and 1[PF₆] were synthesized and isolated in 85% yields upon treatment of 1 with 1 equiv of silver triflate and silver hexafluorophosphate, respectively. The three paramagnetic compounds are air and thermally stable as solids and in solution; they were characterized by elemental analyses, IR, magnetic susceptibility measurements, ¹H NMR, and Mössbauer spectroscopy. In addition, 1[PF₆] was authenticated by a single-crystal X-ray diffraction. The two scorpionate ligands are κ³-N,N′,N′′ ligated to the central Fe^III ion, forming an almost perfect FeN₆ octahedron with an average Fe-N bond distance of 1.9551(18) Å. In addition, complex 1 which oxidizes reversibly at E_1/2 = 0.483 V/SCE (ΔE_p = 94 mV), remains high-spin (S = 2) when the temperature is lowered to 2 K.     

Preparation, structural, spectroscopic, thermal and DFT characterization of cadmium(II) complexes with quinaldic acid

Cadmium(II) complex with quinaldic acid (quinH), [Cd(quin)₂(H₂O)₂] (1), was prepared by the reaction of cadmium(II) acetate and quinaldic acid in water-ethanol mixture, while another cadmium(II) complex, [Cd(quin)₂(DMSO)₂] (2), was prepared by the recrystallization of 1 in DMSO. Both complexes were characterized by IR spectroscopy and TGA/DTA methods. The crystal structure of 2 was determined by X-ray structure diffraction analysis. Cadmium(II) ion is octahedrally coordinated by two N,O-bidentate quinaldate ligands in equatorial and by two DMSO molecules in axial positions. Only weak intermolecular C-H···O hydrogen bonds and π-π stacking interactions as packing forces are present in the crystal structure of 2. The theoretical investigations included geometry optimizations of both complexes at DFT level (B3LYP and mPW1PW91 functionals) and calculations of vibrational frequencies. Calculated and experimental IR spectra were compared and characteristic bands assigned. The electronic properties of the complexes were investigated by the NBO analysis. Thermogravimetric studies showed the initial loss of two coordinated water molecules in 1 and of DMSO in 2 and then complete decomposition of quinaldate ligands for both 1 and 2.     

Formation of iron(VI) in ozonalysis of iron(III) in alkaline solution

Here we report the formation of iron in hexavalent state, in ozonalysis of iron(III) in alkaline medium. The formation of tetrahedral ion is confirmed by UV-Visible and Mössbauer spectroscopic techniques. The value of isomer shift, δ, of the tetra-oxy anion is consistent with known δ values for various salts of iron(VI) ion.     

Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant

Fe(II)-tris(2-pyridylmethyl)amine complexes, Fe(II)-tpa, having different co-existing anions, [Fe(tpa)(MeCN)₂](ClO₄)₂ (1), [Fe(tpa)(MeCN)₂](CF₃SO₃)₂ (2) and [Fe(tpa)Cl₂] (3), were prepared. Effective magnetic moments (evaluated by the Evans method) revealed that while 1-3 in acetone and 3 in acetonitrile (MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN. The aerobic oxidation of 1-3 was monitored by UV-Vis spectral changes in acetone or MeCN under air at 298 K. Only the high-spin Fe(II)-tpa complexes were oxidized with rate constants of k_obs = 0.1-1.3 h⁻¹, while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K = 0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene, ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant. Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.     

Synthesis and characterization of Pt(II) and Pt(II)/Ru(II) complexes with the bidentate bridging ligand dipyrido(2,3-a:3′,2′-h)phenazine (dpop)

Three new complexes [Pt(dpop)(Cl)₂], [(Cl)₂Pt(dpop)Pt(Cl)₂] and [(bpy)₂Ru(dpop)Pt(Cl)₂](PF₆)₂ (dpop = dipyrido(2,3-a:3′,2′-h)phenazine) were prepared and studied. The electronic absorption spectra of the complexes display Pt dπ → dpop π* and Ru dπ → dpop π* MLCT transitions at longer wavelengths than for previously reported similar complexes. Results of cyclic voltammograms show reversible dpop centered reductions while for the mixed metal [(bpy)₂Ru(dpop)Pt(Cl)₂]²⁺ an irreversible Pt(II) oxidative wave precedes the Ru(II) oxidation/reduction couple. Spectroelectrochemical results show that all oxidative and reductive processes are completely reversible. The [(Cl)₂Pt(dpop)Pt(Cl)₂] complex cleaves in solution with pseudo-first order kinetics resulting in loss of the Pt dπ → dpop π* MLCT transition at 545 nm.     

Coordination chemistry of acrylamide 6: Formation and structural characterization of [Fe(O-OC(NH2)CHCH2)6][Fe2OCl6]

The new acrylamide iron(II)/iron(III) complex [Fe(O-OC(NH₂)CHCH₂)₆][Fe₂OCl₆] (1) was obtained by the reaction of a mixture of anhydrous FeCl₂ and anhydrous FeCl₃ with acrylamide (molar ratio 1:2:6) in 98% pure commercial nitromethane under nitrogen atmosphere. According to an X-ray structural analysis, the acrylamide ligands in the cation are coordinated via the amide-oxygen atoms. The formation of the (μ-oxo)bis[trichloroferrate(III)]²⁻ anion presumably resulted from partial hydrolysis of FeCl₃ or [FeCl₄]⁻ by small amounts of water in the nitromethane and/or by the nitromethane itself.     

Synthesis, structure, spectroscopic and magnetic characterization of a novel spin-crossover iron(II) complex with 1-cyclopropyltetrazole ligands

For the first time a 1-cyclopropyl substituted tetrazole (C₃tz) has been used as a potential ligand for iron(II) spin-transition complexes. The complexation of 1-cyclopropyltetrazol with iron(II) tetrafluoroborate yielded a fine powdered product of [Fe(C₃tz)₆](BF₄)₂ being poorly soluble in most common solvents. Single crystals of complex were grown in situ from a solution of ligand and iron(II) hexafluorophosphate, which yielded a hexagonal prismatic crystalline product of [Fe(C₃tz)₆](PF₆)₂. A comparison of XRPD data of the homologues [Fe(C₃tz)₆](BF₄)₂ and [Fe(C₃tz)₆](PF₆)₂ proves them to be homeotypic. The thermally induced spin-crossover phenomenon of [Fe(C₃tz)₆](BF₄)₂ complex shows very abrupt spin transitions, with a spin-crossover temperature T_1/2 ≈ 180 K which is found to be ≈50 K above the T_1/2 of all known iron(II) complexes with n-alkyltetrazoles as ligands. The T_1/2 was determined by temperature-dependent ⁵⁷Fe-Mössbauer, far FT-IR and UV-Vis-NIR spectroscopy as well as temperature dependent magnetic susceptibility measurements (SQUID).     

Synthesis, characterization and properties of iron(II) complexes with a series of tripodal ligands based on the parent ligand tris(2-pyridylmethyl)amine

Iron(II) complexes of the type [Fe(L)(NCS)₂] with the tripodal ligand apme (apme = N¹-(2-aminoethyl)-N¹-(2-pyridyl-methyl)-1,2-ethanediamine) as well as with its derivatives were prepared and structurally characterized. The bond distances thus obtained showed that all complexes investigated were high-spin at the respective temperature. Furthermore [Fe(Me₄apme)(NCS)₂] was analyzed using Mößbauer spectroscopy that showed that this complex remains in its high-spin state over the entire temperature range.