Crystal structure and spin crossover studies on bis(2,6-bis(benzimidazol-2-yl)pyridine) iron(II) perchlorate

The structure of the [Fe(bzimpy)₂](ClO₄)₂·xH₂O system (x = 0.25) was determined by single crystal X-ray structure analysis. The Fe(II) ion is hexacoordinated by six donor nitrogen atoms. The magnetic properties of the complex were investigated by powder magnetic susceptibility measurements and ESR. The freshly prepared sample does not show any traces of iron(III) impurities but these are formed as a function of time. After 1 year the sample contains 8.2% iron(III) as shown by UV spectroscopy and indicated by g_eff = 4.3 and 2.0 in its ESR spectrum. This explains the recorded ξ versus T behaviour at low temperature: with increasing temperature the ξ value decreases according to the Curie-Weiss law for a S = 5/2 system having an effective g = 4.3. Above 220 K a continuous increase in the ξ value is observed and a spin crossover applies. The spin transition is not complete at room temperature. A pronounced hysteresis is observed upon heating/cooling the sample between 220 and 414 K on the basis of magnetic data and infrared spectra.     

Two new Fe(II) spin crossover complexes with tetrazol-1-yl-cycloalkane ligands

Two new 1-(tetrazol-1-yl)cycloalkanes [C_ntz] with n = 5 and 6 were synthesised as ligands for iron(II) spin crossover complexes. Just recently, the [Fe(C₃tz)₆](BF₄)₂ showed that the rigid cyclopropyl-substituent of the tetrazole yielded a rather abrupt and complete spin transition at T_½ ≈ 190 K [1]. Aiming for a deeper insight into the factors governing the spin transition behavior such as abruptness and spin transition temperature we synthesized the two new homologous complexes [Fe(C₅tz)₆](BF₄)₂ and [Fe(C₆tz)₆](BF₄)₂ which were characterized by XRPD, magnetic susceptibility measurements, DSC, ⁵⁷Fe-Mössbauer, UV-Vis-NIR and MIR spectroscopy. The magnetic and structural properties of both [Fe(C_ntz)₆](BF₄)₂ with n = 5 and 6 are also compared with the [Fe(C₃tz)₆](BF₄)₂ and its structural peculiarities are discussed.     

Muon spin relaxation study of manganese hydroxy squarate

Muon spin relaxation has been used to study the magnetic phase transition and spin dynamics above T_c in Mn₂(OH)₂(C₄O₄). These studies confirm previous findings of a transition to long range order, providing a more accurate determination of T_c = 12.5 K as well as indicating short range order at temperatures above the phase transition. The muon measurements are insensitive to the presence of a small quantity of manganese carbonate in the sample that tend to dominate bulk magnetic susceptibility experiments.     

Conformational control of spin state in iron(II) tripodal imidazole complexes

Three new iron(II) N₆ tripodal complexes provide information on the role of ligand conformation on spin crossover behavior. The ligands (generated in situ) are the Schiff base condensate of tris(2-aminoethyl)amine (tren) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, H₃(1), and the condensates of tris(2-aminoethyl)methylammonium ion (N(Me)tren⁺) with three equivalents of 4-methyl-5-imidazolecarboxaldehyde, N(Me)H₃(1)⁺, or with 2-imidazole carboxaldehyde, N(Me)H₃(3)⁺. The structures of [FeH₃(1)](ClO₄)₂, [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃(1)](ClO₄)₃ are reported. The central tren nitrogen atom in these complexes exhibits three different geometries, pyramidal with the nitrogen pointed toward the iron (“N in”, Fe-N distance of 3.050 Å), planar (Fe-N distance of 3.527 Å), and pyramidal with the nitrogen pointed away from the iron atom (“N out”, Fe-N distance of 3.921 Å). With iron(II) the “N in” geometry is high spin while the planar and “N out” geometries are low spin. [FeH₃(1)](ClO₄)₂ exhibits spin crossover behavior between room temperature and 77 K as determined by Mössbauer spectroscopy and also exhibits a conformational change from “N in” to planar over this same temperature range. The structures of [FeN(Me)H₃(1)](ClO₄)₃ and [FeN(Me)H₃ (3)](ClO₄)₃ are locked into the “N out” geometry due to the quaternary nitrogen atom and are low spin even at room temperature. The LS planar and “N out” conformations place a strain on the bond angles of the aliphatic arms of the ligand, which are more pronounced in the “N out” case. The HS “N in” geometry lacks this strain.     

One-step and two-step spin crossover binuclear iron(III) complexes bridged by 4,4′-bipyridine

Two binuclear iron(III) complexes, [L¹Fe^III(bpy)Fe^IIIL¹](BPh₄)₂ (1) and [L²Fe^III(bpy)Fe^IIIL²](BPh₄)₂ (2), were synthesized and characterized, where H₂L¹ and H₂L² denote bis(salicylicdeneaminopropyl)methylamine and bis(3-methoxysalicylideneaminopropyl)methylamine, respectively, and bpy denotes 4,4′-bipyridine and BPh₄⁻ denotes tetraphenylborate. Complexes 1 and 2 consist of one and two crystallographically unique Fe sites, respectively, while they have a similar binuclear complex-cation [LⁿFe^III(bpy)Fe^IIILⁿ]²⁺ (n = 1, 2) bridged by 4,4′-bipyridine and two tetraphenylborate ions as the counter anions. The magnetic susceptibility measurements of 1 and 2 showed one-step and two-step spin crossover (SCO), respectively. The four saturated six-membered chelate rings at the aminopropyl moieties of 1 exhibit disorder throughout one-step SCO. The two chelate rings of one Fe site of 2 exhibit disorder but the other two of another Fe site do not. The different SCO behaviors of 1 and 2 were ascribed to one and two crystallographically unique Fe sites and the order/disorder at the saturated six-membered chelate rings of aminopropyl moieties.     

Intermediate spin protoporhyrin(IX) iron(III) complexes

‘Intermediate’ spin iron(III) has been identified, using Mössbauer spectroscopy, in materials containing protoporphyrin(IX) iron(III). These materials were all precipitated from acid pH in the presence of a variety of ligands. The implications of the results are discussed both in comparison to other known ‘intermediate’ spin porphyrins and for their relevance to haem proteins.     

Electron spin resonance and magnetic relaxation studies of gadolinium(III) complexes with human transferrin

A human serum transferrin complex was prepared in which Gd(III) was substituted for Fe(III) at the two metal-binding sites. Characteristic changes upon metal binding in both the UV absorption of ligated tyrosines and the solvent proton longitudinal magnetic relaxation rates demonstrated 2/1 metal stoichiometry and pH-dependent binding constants. Binding studies were complicated both by binding of Gd(III) to nonspecific sites on transferrin at pH less than or equal to 7 and by complexation of the Gd(III) by the requisite bicarbonate anion at pH greater than or equal to 6.0. A unique Gd(III) electron spin resonance spectrum, with a prominent signal at g = 4.96, was observed for the specific Gd(III)-transferrin complex. The major features of this spectrum were fit successfully by a model Hamiltonian which utilized crystal field parameters similar to those determined for Fe(III) in transferrin [Aasa, R. (1970) J. Chem. Phys. 52, 3919-3924]. The magnetic field dependence of the solvent proton relaxation rate was measured as a function of both pH and metal ion concentration. An observed biphasic dependence of the relaxation rate on metal concentration is attributed to either sequential metal binding to the two iron-binding sites with different relaxation properties or random binding to two sites that are similar but show conformationally induced changes in relaxation properties as the second metal is bound. The increase in the solvent proton relaxation rate with pH is consistent with a model in which a proton of a second coordination sphere water molecule is hydrogen bonded to a metal ligand which becomes deprotonated at pH 8.5.     

Trispyrazolylmethane piano stool complexes of iron(II) and cobalt(II)

A series of cationic trispyrazolylmethane complexes of the general form [Tm^RM(CH₃CN)₃]²⁺ (Tm = tris(pyrazolyl)methane, 1, R = 3,5-Me₂, M = Fe(II); 2, R = 3-Ph, M = Fe(II); 3, R = 3,5-Me₂, M = Co(II); 4, R = 3-Ph, M = Co(II)) with ‘piano-stool’ structures was prepared by the reaction of the N₃tripodal ligands (Tm^R)with [(CH₃CN)₆M](BF₄)₂ in a 1:1 stoichiometric ratio. Magnetic susceptibility measurements indicate that all four complexes with BF₄⁻ counter anions are paramagnetic, high-spin systems in the solid state with μ_eff at high temperatures of 5.2 (1, S = 2), 5.4 (2, S = 2), 4.9 (3, S = 3/2) and 4.6 (4, S = 3/2) BM, respectively. Comparisons of bond lengths from the metal centre to the Tm^R nitrogen donors, and from the metal centre to the acetonitrile nitrogen donors indicate that the neutral tripodal ligands appear to be more weakly coordinated to the metal centre than are the acetonitrile ligands. Reactions of these tripodal complexes with bidentate phosphine ligands, such as 1,2-diphosphinoethane or 1,2-bis(diallylphosphino)ethane leads to displacement of the tripodal ligand, or to the formation of more thermally stable bis-ligand complexes M(Tm^R)₂ (R = 3,5-dimethyl).     

The effect of molecular packing on the occurrence of spin crossover phenomena in one-dimensional Fe(II)-bis-Schiff base complexes

Two new one-dimensional Fe(II)-bis-Schiff base complexes, [Fe(L1)(pyz)] · CH₂Cl₂ (1) and [Fe(L2)(pyz)] · 2CH₂Cl₂ (2) (H₂L1 = bis(O-vanillin)-O-phenylenediimine, H₂L2 = bis(O-vanillin)-2,3-naphthalenediimine, pyz = pyrazine) are reported with their crystal structures and magnetic property. Compound 1 shows a two-step SCO behavior while 2 shows HS at all the temperature range measured. Although the extension of aromatic moiety from benzene (L1) to naphthalene (L2) was introduced for the purpose of strengthening the cooperativity, it leads to the absence of SCO, due to the unanticipated π–π interaction, which leads to the longer Fe–N bond lengths and a weak ligand field around Fe(II) ion.     

Inductive influence of 4′-terpyridyl substituents on redox and spin state properties of iron(II) and cobalt(II) bis-terpyridyl complexes

A series of iron and cobalt bis-terpyridine (terpy) complexes were prepared with the general formula [M(R-terpy)₂](PF₆)₂, where M represents Co(II) and Fe(II), and R is the following terpyridine substituents in order of increasing electron-withdrawing behavior [(C₄H₈)N, (C₄H₉)NH, HO, CH₃O, CH₃-phenyl, H, Cl, CH₃SO, CH₃SO₂]. The complexes were prepared to investigate the extent of redox and spin state control that is attainable by simply varying the electron donating/withdrawing influence using a single substituent site on the terpyridine ligand. Cyclic voltammetry was used to assess the substituents influence on the M(III/II) redox couple. A plot of the M(III/II) redox potential (E_1/2) versus the electron donating/withdrawing nature of the substituents (Hammett constants), shows a strong linear trend for both metals; however, the substituents were observed to have a stronger influence on the Fe(III/II) couple. Solution magnetic susceptibility measurements at room temperature were carried out using standard NMR methodology (modified Evans method) where all of the Fe(II) complexes exhibited a diamagnetic, low spin (S = 0) behavior. In the cobalt series where R = H for [Co(R-terpy)₂]²⁺, the complex is known to be near the spin cross-over where the room temperature effective magnetic moment (μ_eff) in solution is ≈3.1 Bohr magnetons; however, in this study the μ_eff is observed to vary between 2.7 and 4.1 Bohr magnetons depending on the R-substituent.     

Four-coordinate complexes of bis(1-diphenylphosphinoindenyl)iron(II)

Palladium, platinum and rhodium complexes of rac- and meso-bis(1-diphenylphosphinoindenyl)iron(II) (1) are reported. Both rac and meso isomers of {bis(1-diphenylphosphinoindenyl)iron(II)}palladium dichloride (rac- and meso-2) were characterized by X-ray crystallography along with the rac isomer of the Pt analogue (rac-3). NMR analysis of the rhodium complex [{bis(1-diphenylphosphinoindenyl)iron(II)}(cyclooctadiene)rhodium(I)] tetraphenylborate suggests a similar structure in solution. Coupling reactions of n- and sec-BuCl with bromobenzene in THF are catalysed by rac-2 and found to be similar to (PPh₃)₂PdCl₂ but poorer than (dppf)PdCl₂ in diethyl ether.     

Comparative investigations on a series of [hexakis(1-(tetrazol-1-yl)alkane-N4)iron(II)] bis(tetrafluoroborate) spin crossover complexes: Methyl- to butyl-substituted species

A series of 1-(tetrazol-1-yl)alkanes [ntz] with n = 1-4 were synthesised as ligands for iron(II) spin crossover complexes. Within this series 1-(tetrazol-1-yl)butane [4tz] was prepared for the first time, whereas 1-(tetrazol-1-yl)methane [1tz], 1-(tetrazol-1-yl)ethane [2tz], 1-(tetrazol-1-yl)propane [3tz] and the [hexakis(ntz)iron(II)]bis(tetrafluoroborate) complexes were prepared according to the literature. Aiming for a comparative study we characterized all four compounds by XRPD, magnetic susceptibility measurements, ⁵⁷Fe-Moessbauer spectroscopy and IR spectroscopy. [Fe(4tz)₆](BF₄)₂ yielded appropriate single crystals and an X-ray structure of the new compound [Fe(4tz)₆](BF₄)₂ is presented. The magnetic and structural properties of all [Fe(ntz)₆](BF₄)₂ are compared and discussed.     

Model complexes for the high spin iron(II) state in the catalytic cycle of cytochrome P450

Studies using ⁵⁷Fe Mössbauer spectroscopy on frozen concentrated solutions of two iron(II) porphyrins in the presence of a large excess of thiophenol or 2-mercaptoethanol are reported. The iron(II) porphyrins used were photoporphyrin IX iron(II) (PPIXFe(II)) and tetra(p-sulphophenyl)porphinato-iron(II) (TTPPSFe(II)). Evidence for high-spin five-coordinate iron(II) complexes were found for both iron(II) porphyrins with thiophenol, but no reaction was found to occur with 2-mecaptoethanol. In contrast to these findings in solutions dilute in TPPSFe(II) evidence was found (from electronic absorption spectroscopy and spectrophotometric titrations) for both thiophenol and 2-mercaptoethanol acting as axial ligands in high-spin five-coordinate TPPSFe(II) species. Mössbauer data for frozen solutions containing TPPSFe(II), carbon monoxide (CO) and either thiol, are consistent with the presence of only low-spin six-coordinate iron(II) complexes. These latter complexes are deduced to contain both a thiol and a CO molecule as axial ligands> These results are discussed in relation to earlier work on PPIXFe(II)-thiol solutions and also in relation to the high-spin iron(II) state in the catalytic cycle of cytochrome P450.The reactions of TPPSFe(II) with excess of either 2-mercaptoethanol or ethyl 2-mercaptoacetate in air are also reported and their complex nature discussed.     

Conformational effect of a spin crossover iron(II) complex: Bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamineiron(II) perchlorate

A iron(II) complex of the linear hexadentate N₆ ligand H₂L^2-3-2, [Fe(H₂L^2-3-2)](ClO₄)₂, was synthesized and the spin crossover properties were investigated, where H₂L^2-3-2 denotes bis[N-(2-methylimidazol-4-yl)methylidene-2-aminoethyl]propanediamine. The complex showed a gradual and reversible one-step spin crossover (SCO) between the high-spin (S = 2) and low-spin (S = 0) states at T_1/2 = 208 K without hysteresis. The crystal structures were determined at 296 and 250 K (HS state), 230, 210, and 200 K (intermediate between the HS and LS states) and 150 and 110 K (LS state). The spin transition from 296 to 150 K accompanies with the conformation change of the chelate rings at the triamine moiety and the formation of the hydrogen bond network in the same space group of orthorhombic Pbcn (no. 60). However, in the LS state at 110 K, the space group changed from orthorhombic Pbcn at 150 K (Pcan when the same axial setting to 110 K was used) to monoclinic P2₁/a (no. 14) at 110 K, although no spin transition and no change of assembly structure between 150 and 110 K were observed. It give us an idea that the space group transformation is mainly related to the conformational thermodynamic stability of the chelate rings at the triamine moiety and is not directly correlated with the spin transition.     

Different iron(II) complexes of bleomycin A2

By 13C-nmr on iron-bleomycin preparations, an iron-bleomycin-CO complex is found that loses its CO upon standing, as demonstrated using 14CO. Iron-bleomycin, prepared without rigorous exclusion of oxygen, reacts with CO to a stable diamagnetic iron-bleomycin-CO complex.     

Mössbauer studies on protoporphyrin IX iron(II) frozen solutions containing ligands that cause the iron to be in a five coordinate high spin iron(II) environment

Mössbauer parameters are presented for a number of protoporphyrin IX iron(II) complexes containing ligands that allow the iron to be in a five coordinate high spin iron(II) electronic environment. Such environments are characterised by large quadrupole splittings in the range 4.0 to 4.4 mm s^?1. These compounds have characteristic electronic spectra.The implications of catechol type ligands binding protoporphyrin IX iron II/III moieties are discussed.     

Synthetic,chemical spectroscopic (infrared and electronic) and magnetic studies on dizirconiumenneaisopropoxide complexes of iron(II)

Bimetallic alkoxide derivatives of iron(II) with zirconium of the types [Fe{Zr₂(OPrⁱ)₉}₂], [ClFe- {Zr₂(OPrⁱ)₉}], [(RO)Fe{Zr₂(OPrⁱ)₉}], and [(acac)- Fe{Zr₂(OPrⁱ)₉}] have been synthesized and characterized by elemental analyses, infrared and electronic spectral as well as magnetic susceptibility studies.     

EPR studies of copper(II) and cobalt(II) complexes of adriamycin

Cu2+ and Co2+ complexes of adriamycin (ADM) in aqueous solutions have been examined using EPR spectroscopy. An appreciable amount of Cu2+ and Co2+ complexes formed in the solutions were found to be in the EPR silent associated form, where the metal ions are antiferromagnetically coupled. The associated form of the Cu2+ complex may be neither a simple dimer nor coordination polymer but aggregates of a stacked type. Formation of a complex having Cu2+-ADM stoichiometry of 1:2 was observed for the solutions containing excess of ADM as an EPR observable species. The complex having Cu2+-ADM stoichiometry of 1:1 was not observed directly by EPR, but the presence of the complex is undeniable, especially at low pH range so far as large excessive ADM is not present. The Co2+ complex of ADM observed by EPR is in the high-spin (S = 3/2) state and may have a coordination structure of tetragonal symmetry. The EPR spectra of these complexes apparently show that the Cu2+ and Co2+ ions are bound at the carbonyl and phenolate oxygen in the 1,4-dihydroxyanthraquinone moiety and the amino nitrogen in the sugar part does not seem to participate in the coordination to the metal ions.