The spin-transition properties of Fe(III) complexes with hetarylformazan in an ion-exchange polymer: An EPR study

For the first time, the EPR method was used to reveal and study the special features of spin-transition processes between high-spin and low-spin iron(III) complexes with hetarylformazans immobilized on an ion-exchange polymer. An analysis of completely reversible temperature dependences of EPR line positions, widths, and integral intensities in the spectra of high-spin and low-spin complexes allowed four temperature intervals to be identified. These intervals corresponded to preparative periods of spin-transition processes (450–275 K, intervals I and II), their appearance (275–230 K, interval III), and occurrence (230–100 K, interval IV). Local concentrations and spin exchange frequencies in clusters were estimated. Effects related to high-spin complex EPR signal shifts during temperature changes and to the duration of sample storage were revealed. High-spin complexes were found to be very sensitive to external actions, as distinct from very stable low-spin complexes. Experimental EPR observations obtained for ion-exchange polymer satisfied the concept of the nucleation and growth of domains in the spin-transition process.     

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.     

Tuning photophysical properties with ancillary ligands in Ru(II) mono-diimine complexes

The series of complexes [XRu(CO)(L-L)(L′)₂][PF₆] (X = H, TFA, Cl; L-L = 2,2′-bipyridyl, 1,10-phenanthroline, 5-amino-1,10-phenanthroline and 4,4′-dicarboxylic-2,2′-bipyridyl; L′₂ = 2PPh₃, Ph₂PC₂H₄PPh_2, Ph₂PCHCHPPh₂) have been synthesized from the starting complex K[Ru(CO)₃(TFA)₃] (TFA = CF₃CO₂) by first reacting with the phosphine ligand, followed by reaction with the L-L and anion exchange with NaPF₆. In the case of L-L = phenanthroline and L′₂ = 2PPh₃, the neutral complex Ru(Ph₃P)(CO)(1,10-phenanthroline)(TFA)₂ is also obtained and its solid state structure is reported. Solid state structures are also reported for the cationic complexes where L-L = phenanthroline, L₂ = 2PPh₃ and X = Cl and for L-L = 2,2′-bipyridyl, L₂ = 2PPh₃ and X = H. All the complexes were characterized in solution by a combination of ¹H and ³¹P NMR, IR, mass spectrometry and elemental analyses. The purpose of the project was to synthesize a series of complexes that exhibit a range of excited-state lifetimes and that have large Stokes shifts, high quantum yields and high intrinsic polarizations associated with their metal-to-ligand charge-transfer (MLCT) emissions. To a large degree these goals have been realized in that excited-state lifetimes in the range of 100 ns to over 1 μs are observed. The lifetimes are sensitive to both solvent and the presence of oxygen. The measured quantum yields and intrinsic anisotropies are higher than for previously reported Ru(II) complexes. Interestingly, the neutral complex with one phosphine ligand shows no MLCT emission. Under the conditions of synthesis some of the initially formed complexes with X = TFA are converted to the corresponding hydrides or in the presence of chlorinated solvents to the corresponding chlorides, testifying to the lability of the TFA Ligand. The compounds show multiple reduction potentials which are chemically and electrochemically reversible in a few cases as examined by cyclic voltammetry. The relationships between the observed photophysical properties of the complexes and the nature of the ligands on the Ru(II) is discussed.     

The thermodynamic properties of Fe(II) complexes with 1,2,4-triazoles

The temperature dependences of the heat capacities at 106–330 K of the monoligand Fe(NH₂trz)₃I₂ (I) and mixed-ligand Fe(Htrz)_0.3(NH₂trz)_2.7SiF₆ · H₂O (II) complexes (Htrz is 1,2,4-triazole, and NH₂trz is 4-amino-1,2,4-triazole) were studied by adiabatic vacuum calorimetry. The ¹A₁ ⇔ ⁵T₂ spin transition was observed in these compounds. The thermodynamic parameters of phase transitions in I and II were determined.     

Comparative study on the photophysical properties between carbene-based Fe (II) and Ru (II) complexes

The comparative study on the photophysical properties between cheap metal Fe (II) complexes and noble metal Ru (II) complexes with identical ligand coordination is performed by the combination of density functional theory (DFT) and time-dependent density functional theory (TDDFT) to evaluate the potential alternative applications of Fe (II) complexes. RuBIP (BIP = 2,6-bis (imidazol-2- ylidene)pyridine) is theoretically established that the radiative lifetime of the second lowest triplet state is more consistence with experimental value. However, FeBIP retains nonluminous because of low-lying ³MC originated from weak d orbital splitting. FeBIPC (FeBIP with carboxylic acid groups) has twice longer lifetime than its parent complex FeBIP due to the great decrease of the energy gap between ³MLCT and ³MC. What's more, the lifetimes of Fe (II) complexes detected in the experiments are more accessible to nonradiative decay lifetimes of ³MC. The carboxylic acid groups are beneficial for the improvement of luminescent possibility and controllability of Fe (II) complexes, while there is still a huge challenge for effective material replacement comparing with Ru (II) complexes.     

Synthesis and characterization of para-nitro substituted 2,6-bis(phenylimino)pyridyl Fe(II) and Co(II) complexes and their ethylene polymerization properties

Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R₂-phenylimino)pyridines, LMCl₂ (1: R = Me, M = Fe; 2: R = iPr, M = Fe; 3: R = Me, M = Co; 4: R = iPr, M = Co) have been synthesized and fully characterized, and their catalytic ethylene polymerization properties have been investigated. Among these complexes, the iron(II) pre-catalyst bearing the ortho-isopropyl groups (complex 2) exhibited higher activities and produced higher molecular weight polymers than the other complexes in the presence of methylaluminoxane (MAO). A comparison of 2 with the reference non-nitro-substituted catalyst (2,6-bis(2,6-diisopropylphenylimino)pyridyl)FeCl₂ (FeCat 5) revealed a modest increase of the catalytic activity and longer lifetime upon substitution of the para-positions with nitro groups (activity up to 6.0 × 10³ kg mol⁻¹ h⁻¹ bar⁻¹ for 2 and 4.8 × 10³ kg mol⁻¹ h⁻¹ bar⁻¹ for 5), converting ethylene to highly linear polyethylenes with a unimodal molecular weight distribution around 456.4 kg mol⁻¹. However, the iron(II) pre-catalyst 1 on changing from ortho-isopropyl to methyl groups displayed much lower activities (over an order of magnitude) than 2 under mild conditions. As expected, the cobalt analogues showed relatively low polymerization activities.     

The Fe(II) and Co(II) chelation complexes of 2,6-BIS(4-Ethyl-2-Pyridyl)-4-Phenyl-Pyridinl

A study has been made of the spectrophotometric constants of the new modified terpyridine chelation reagent, 2,6-bis(4-ethyl-2-pyridyl)-4-phenyl-pyridine (trivial name “ethyl-terosole”), as its Fe(II) and Co(II) complex cations. This new Iigand is outstanding for use in the trace quantity determination of iron. No advantage as compared to more sensitive ligands previously studied is provided by Co(II) chelation.     

Preparation and surface photoelectric properties of Fe(II/III) complexes

Four Fe(II/III) supramolecules, {[Fe(Hpdc)₂(H₂O)₂]·2H₂O} (1), [Fe(HImbc)₂(H₂O)₂] (2), [Fe(phen)₂(CN)₂]·CH₃CH₂OH·2H₂O (3), K[Fe(tp)₂]·SO₄ (4) (H₂pdc = 2,5-Pyridinedicarboxylic acid, H₂Imbc = 4,5-Imidazoledicarboxylic acid, phen = 1,10-phenanthroline, tp⁻ = poly(pyrazolyl)borate), were synthesized by hydrothermal and room temperature stirring methods. They were characterized by single crystal X-ray diffraction, surface photovoltage spectroscopy (SPS), field-induced surface photovoltage spectroscopy (FISPS), electron paramagnetic resonance (EPR), UV–Vis absorption spectra (UV–Vis), infrared spectra (IR) and element analysis. The structural analyses indicate that complex (1) is a supramolecule with 2D structure connected by hydrogen bonds. Complex (2) is a supramolecule with hydrogen-bonded 3D structure. Complexes (3) and (4) are both 1D supramolecules connected by hydrogen bonds. The electronic state of central metal Fe(II) ions in complexes (1) and (2) is d⁶ with FeN₂O₄ coordination mode, lying in weaker distorted octahedral field. The electronic state of Fe(II) ion in complex (3) is d⁶ with Fe(CN)₂N₄ mode in the strong distorted octahedral field. The electronic state of Fe(III) ion in complex (4) is d⁵ with FeN₆ mode, lying in the strong octahedral field. The micro-environment of Fe(II/III) ions in the four complexes is further investigated by EPR. The SPS of four complexes all exhibit photovoltage responses in the range of 300–700 nm. This indicates that they all possess certain photoelectric conversion capability. The effects of component, structure, type of ligands of the complexes, valence state and coordination micro-environment of the central metal ions on the SPS were discussed. Furthermore, the SPS and UV–Vis absorption spectra were interrelated.     

Electrophysical properties of polymers based on Ni(II) complexes

Thin-layer organometallic films synthesized from the monomer [NiSalen] have been studied. The presence of the oxidized and reduced forms of a redox polymer has been established by cyclic voltammetry and electronic absorption spectroscopy. The temperature-frequency dispersion of dielectric characteristics has been discovered, and the possible mechanisms of relaxation polarization in poly[NiSalen] on solid supports have been considered. Based on the temperature-frequency dependences of conductivity, the energy of the thermal activation of electrical conductivity is determined. The feasibility of the hopping mechanism of charge transfer in the polymer under study is discussed.     

X-ray absorption spectroscopic investigation of the spin-transition character in a series of single-site perturbed iron(II) complexes

Select ferrous spin-transition complexes with the pentadentate ligand 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine (PY5) were examined using variable-temperature solution solid-state magnetic susceptibility, crystallography, X-ray absorption spectroscopy (XAS), and UV/vis absorption spectroscopy. Altering the single exogeneous ligand, X, of [Fe(PY5)(X)]n)+ is sufficient to change the spin-state of the complexes. When X is the weak-field ligand Cl-, the resultant Fe complex is high-spin from 4 to 300 K, whereas the stronger-field ligand MeCN generates a low-spin complex over this temperature range. With intermediate-strength exogenous ligands (X = N3-, MeOH), the complexes undergo a spin-transition. [Fe(PY5)(N3)]+, as a crystalline solid, transitions gradually from a high-spin to a low-spin complex as the temperature is decreased, as evidenced by X-ray crystallography and solid-state magnetic susceptibility measurements. The spin-transition is also evident from changes in the pre-edge and EXAFS regions of the XAS Fe K-edge spectra on a ground crystalline sample. The spin-transition observed with [Fe(PY5)(MeOH)]2+ appears abrupt by solid-state magnetic susceptibility measurements, but gradual by XAS analysis, differences attributed to sample preparation. This research highlights the strengths of XAS in determining the electronic and geometric structure of such spin-transition complexes and underscores the importance of identical sample preparation in the investigation of these physical properties.     

Tuning the reactivity of osmium(II) and ruthenium(II) arene complexes under physiological conditions

The Os(II) arene ethylenediamine (en) complexes [(eta(6)-biphenyl)Os(en)Cl][Z], Z = BPh(4) (4) and BF(4) (5), are inactive toward A2780 ovarian cancer cells despite 4 being isostructural with an active Ru(II) analogue, 4R. Hydrolysis of 5 occurred 40 times more slowly than 4R. The aqua adduct 5A has a low pK(a) (6.3) compared to that of [(eta(6)-biphenyl)Ru(en)(OH(2))](2+) (7.7) and is therefore largely in the hydroxo form at physiological pH. The rate and extent of reaction of 5 with 9-ethylguanine were also less than those of 4R. We replaced the neutral en ligand by anionic acetylacetonate (acac). The complexes [(eta(6)-arene)Os(acac)Cl], arene = biphenyl (6), benzene (7), and p-cymene (8), adopt piano-stool structures similar to those of the Ru(II) analogues and form weak dimers through intermolecular (arene)C-H...O(acac) H-bonds. Remarkably, these Os(II) acac complexes undergo rapid hydrolysis to produce not only the aqua adduct, [(eta(6)-arene)Os(acac)(OH(2))](+), but also the hydroxo-bridged dimer, [(eta(6)-arene)Os(mu(2)-OH)(3)Os(eta(6)-arene)](+). The pK(a) values for the aqua adducts 6A, 7A, and 8A (7.1, 7.3, and 7.6, respectively) are lower than that for [(eta(6)-p-cymene)Ru(acac)(OH(2))](+) (9.4). Complex 8A rapidly forms adducts with 9-ethylguanine and adenosine, but not with cytidine or thymidine. Despite their reactivity toward nucleobases, complexes 6-8 were inactive toward A549 lung cancer cells. This is attributable to rapid hydrolysis and formation of unreactive hydroxo-bridged dimers which, surprisingly, were the only species present in aqueous solution at biologically relevant concentrations. Hence, the choice of chelating ligand in Os(II) (and Ru(II)) arene complexes can have a dramatic effect on hydrolysis behavior and nucleobase binding and provides a means of tuning the reactivity and the potential for discovery of anticancer complexes.     

Structure of Cu(II) complexes of pyridine-2,6-dithiocarbomethylamide

The crystal structure of the paramagnetic pyridine-2,6-dithiocarbomethyl-amide—copper(II) chloride (CuPDTA-Cl₂) is described: C₉H₁₁N₃S₂CuCl₂·H₂O, monoclinic, P2₁/n,a=9.163 (2),b=8.925 (5),c=17.590 (9) Å, =102.08 (1)°,Z=4,d _x =1.784g cm^–3. The structure was refined to a residual ofR=0.059, The copper is coordinated in a square—pyramidal arrangement by the pyridine nitrogen, the two thioamide sulfur atoms and the two chloride ions. The shortest Cu—Cu distance in the crystal is 5.02 Å, leading to only very weak antiferromagnetism. Spectroscopic and magnetic data are given for additional members of the CuPDTA-X [X=Cl₂, Br₂, I₂, (NO₃)₂, (SCN)₂,Py] complex type, which suggest a large degree of structural resemblance with the CuPDTA-Cl₂ complex.

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Struktur von Kupfer(II)-Komplexen des Pyridin-2,6-dithiocarbomethylamids

Zusammenfassung Die Kristallstruktur des paramagnetischen Pyridin-2,6-dithiocarbomethyl-amid Kupfer(II)-chlorids wurde bestimmt: C₉H₁₁N₃S₂CuCl₂·H₂O, monoklin, P2₁/n,a=9,163 (2),b=8,925 (5),c=17,590 (9) Å, =102,08 (1)°,Z=4,d _x =1,784g cm^–3. Die Struktur wurde bis zu einemR-Faktor vonR=0,059 verfeinert. Das Kupfer-Zentralatom zeigt quadratisch-pyramidale Koordination durch den Pyridin-Stickstoff, die beiden Thioamid Schwefelatome sowie zwei Chloridionen. Der kürzeste Cu—Cu-Abstand im Kristall beträgt 5,02 Å, was zu einem nur sehr schwachen Antiferromagnetismus führt. Spektroskopische und magnetische Größen werden für weitere Derivate vom CuPDTA-X Typ [X=Cl₂, Br₂, I₂, (NO₃)₂, (SCN)₂, Pyridin] angegeben, und im Sinne einer großen strukturellen Ähnlichkeit mit dem CuPDTA-Cl₂ interpretiert.

     

Cu(I) and Fe(II) complexes with N,N-dimethylthioformamide and Fe(II) complexes with N,N-dimethylformamide

The synthesis of the following Cu(I) and Fe(II) complexes with N,N-dimethylthioformamide (DMTF) and N,N-dimethylformamide (DMF) are described: Cu(DMTF)₄ClO₄, Cu(DMTF)₂Cl, Cu(DMTF)₂Br, Cu(DMTF)₂I, Fe(DMTF)₆(ClO₄)₂, Fe(DMTF)₂Cl₂, Fe(DMTF)₂Br₂, Fe(DMTF)₂I₂, Fe(DMF)₆(ClO₄)₂, Fe(DMF)₂Cl₂, Fe(DMF)Br₂ and Fe(DMF)₃I₂. Electronic absorption spectra, IR spectra, magnetic susceptibilities of the solids and in solution as well as conductivities have been measured of these compounds in order to obtain information on the nature of the interaction between the cations and the ligands, the coordination in the crystalline state, in solution and the dissociation of these compounds in the respective solvents.     

Monomeric, two-coordinate Mn, Fe and Co(II) complexes featuring 2,6-(2,4,6-trimethylphenyl)phenyl ligands

The synthesis and characterization of the monomeric, two-coordinate transition-metal complexes (2,6-Mes(2)C(6)H(3))(2)M (Mes = mesityl, 2,4,6-Me(3)C(6)H(2), M = Mn, Fe, Co) are reported; (2,6-Mes(2)C(6)H(3))(2)Co is the first structurally authenticated two-coordinate, homoleptic cobalt(II) complex featuring sigma-bonded aryl ligands.     

2,6-Diacetyl- and 2,6-diformylpyridine bis(N4-substituted thiosemicarbazones) and their copper(II) and nickel(II) complexes

2,6-Diformylpyridine bis(N4-methylthiosemicarbazone) and bis(N4-dimethylthiosemicarbazone), H₂2,6Fo4M and H₂2,6Fo4DM, respectively, and 2,6-diacetylpyridine bis(N4-methylthiosemicarbazone) and bis(N4-dimethylthiosemicarbazone), H₂2,6Ac4M and H₂2,6Ac4DM, and their copper(II) and nickel(II) complexes have been synthesized. The ¹H-n.m.r. spectra of the free bis(thiosemicarbazones) show that, most often, one of the thiosemicarbazone moieties is hydrogen bonded to the pyridine nitrogen, and in [²H₆]-DMSO there is interaction with solvent oxygen. Golden yellow H₂2,6Ac4DM has a bifurcated hydrogen bonding interaction by one of the thiosemicarbazone moieties resulting in conjugation. Coordination to copper(II) and nickel(II) centers is via the pyridine nitrogen, amine nitrogen and thiolato sulfur and most of the complexes formed are polynuclear with thiosemicarbazone moieties from the same ligand coordinating to different metal centers.     

Characterisation of some polyamino complexes of Fe(II) and Fe(III) imides

Complexes of the types [Fe(imide)₂(Am)₂] and [Fe(imide)₃(Am)(H₂O)](imide = deprotonated malonimide and phthalimide; Am = dipyridyl and o-phenanthroline) have been prepared and characterised on the basis of IR, electronic spectra, conductance, magnetic and Mössbauer studies. The bonding powers of phthalimide and malonimide are almost the same and all the complexes have a distorted octahedral structure.     

Effect of alkylaluminum on ethylene polymerization catalyzed by 2,6‐bis(imino)pyridyl complexes of Fe(II)

In the presence of tetraethylaluminoxane (TEAO), iron complexes were used to catalyze ethylene polymerizations with extremely high activities and generally produced polyethylene with a bimodal molecular weight distribution (MWD). This bimodal MWD of polyethylene was mainly derived from residual triethylaluminum in TEAO and was produced through a mechanism of chain transfer to aluminum. Ethylaluminoxane and tetraisobutylaluminoxane also were used to polymerize ethylene with high activities in the presence of iron complexes, and only polyethylene with a unimodal MWD was produced. The ratio of the rate constant of chain transfer to aluminum (k_trA) to the rate constant of chain propagation (k_p) was determined to be 0.12 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐diisopropylphenyl ( 1 ) and 2.48 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐dimethylphenyl ( 2 ); these values are far larger than those for metallocene‐based catalysts. This explains why an iron complex usually produces polyethylene with a broader MWD than metallocene‐based catalysts. Additionally, it can be concluded from the great difference between 1 and 2 with respect to k_trA/k_p that an iron complex with less congested aryl substituents is subjected to chain transfer to aluminum. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1599–1606, 2005