Jürgen Ensling

A polymeric two-dimensional mixed-metal network. Crystal structure and magnetic properties of {[P(Ph)4][MnCr(ox)3]}

Abstract The mixed-metal ferromagnet {[P(Ph) 4 ][MnCr(ox) 3 ]} n , where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R 3 c , a = b =18.783(3), c =57.283(24) A, α=β=90, γ=120°, Z =24 (C 30 H 20 O 12 PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at T c =5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM′(ox) 3 ] n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO 6 ’ chromophores. Their polarization patterns reflect the electric dipole selection rules for D 3 symmetry. A strong site selective luminescence from the chromium(III) 2 E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.

Hybrid Molecular Magnets Obtained by Insertion of Decamethylmetallocenium Cations into Layered, Bimetallic Oxalate Complexes: [ZIIICp*2][MIIMIII(ox)3] (ZIII=Co, Fe; MIII=Cr, Fe; MII=Mn, Fe, Co, Cu, Zn; ox=oxalate; Cp*=pentamethylcyclopentadienyl)

A new series of hybrid organometallic - inorganic layered magnets with the formula [Z(III)Cp*2][M(II)M(III)(ox)3] (Z(III) = Co, Fe; M(III) = Cr, Fe; M(II) = Mn, Fe, Co, Cu, Zn; ox = oxalate; Cp* = pentamethylcyclopentadienyl) has been prepared. All of these compounds are isostructural and crystallize in the monoclinic space group C2/m, as found by X-ray structure analysis. Their structure consists of an eclipsed stacking of the bimetallic oxalate-based extended layers separated by layers of organometallic cations. These salts show spontaneous magnetization below To, which corresponds to the presence of ferro-, ferri-, or canted antiferromagnetism. Compounds in which the paramagnetic deca-methylferrocenium is used instead of the diamagnetic decamethylcobaltocenium are good examples of chemically constructed magnetic multilayers with alternating ferromagnetic and paramagnetic layers. The physical properties of this series have been thoroughly studied by means of magnetic measurements and ESR and Mossbauer spectroscopy. We have found that the two layers are electronically quasiindependent. As a consequence, the bulk properties of these magnets have not been significantly affected by the insertion of a paramagnetic layer of S = 1/2 spins in between the extended layers. In fact, the critical temperatures remain unchanged even when comparing [MCp*2]+ derivatives with [XR4]+ compounds (X = N, P; R = Ph, nPr, nBu). Nevertheless, the presence of the paramagnetic layer has been shown to have some influence on the hysteresis loops of these compounds. In the same context, the spin polarization of the paramagnetic units (which arises from the internal magnetic field created by the bimetallic layers in the ordered state) has been observed by Mossbauer and ESR spectroscopy.

Mössbauer effect study on low-spin 1A1 ⇌ high-spin 5T2 transition in tris(2-picolylamine) iron chloride I. Dilution effect in [FexZn1-x(2-pic)3]Ch2·C2H5OH

Abstract The 57 Fe Mossbauer effect of the solid solutions [Fe x Zn 1- x (2-pic) 3 ]Ch 2 ·EtOH ( x = 1.0, 0.8, 0.6 and 0.2) has been studied in detail between 10 and 300 K (2-pic = 2 picolylamine). All specimens exhibited a cooperative phase transition due to low-spin ( 1 A 1 ) XXX high-spin ( 5 T 2 ) transition. The coexistence of two quadrupole doublets arising from the high-spin and low-spin states, respectively, near the transition temperature T C indicates a first-order nature for the spin transition. The quadrupole splitting Δ E Q and the isomer shift b are independent of x at all temperatures, whereas T C varies linearly with the iron concentration in the range 0.2 ⩽ x ⩽ 1.0: T C ( x /K = 40x + 82. This concentration dependence of T C is different from those observed in usual magnetic random systems showing a phase transition due to spin ordering. An explanation for this as well as the possible mechanism of the spin transition are discussed.

Mössbauer effect study on low-spin 1A1 ⇌ high spin 5T2 transition in [Fe(2-pic)3]Cl2: II. Influence of non-coordinating solvent molecule in [Fe(2-pic)3] Cl2 · X1 X = C2H5OH, CH3OH, H2O and 2H2O

Abstract The 57Fe Mossbauer effect of solvated tris(2-aminomethylpyridine) iron (II) chloride has been studied in detail between 80 and 310 K. The methanolate exhibits a gradual but complete spin transition between 1A1 and 5T2. At 153 K, both the intensities of 1A1 and 5T2 quadrupole doublets become equal. An extremely large hysteresis (ΔT = 91 K) has been observed for the cooperative 1A1 ⇌5T2 transition in the monohydrate. The critical transition temperature is TC = 295 K for the heating and TC = 204 K for the cooling direction. The spin state of the dihydrate is the low-spin 1A1 state over the whole temperature range studied. A remarkable influence of the non-coordinating solvent molecule on the spin transition behaviour (e.g., TC the quadrupole splitting, cooperative effect) has been found in the present series of complexes. This is accounted for in terms of a combined action of both the hyrdrogen-bond formation between the ligand and the intervening solvent molecule and a change in the relevant phonon system caused by different packing geometry and/or possible crystallographic change. A ligand-field calculation indicates an orbitally non-degenerate 5A1 or a 5A2 ground state under a trigonal or a rhombic distortion in the high-spin state.

[MII(tcne)2]⋅x CH2Cl2 (M=Mn, Fe, Co, Ni) Molecule-Based Magnets withTc Values Above 100 K and Coercive Fields up to 6500 Oe

Remarkable thermal stability is exhibited by [FeII(tcne)2]⋅x CH2Cl2 (57Fe Mossbauer spectra reveal that the oxidation state of iron is II), which was prepared by treating the MeCN solvate of FeI2 with tetracyanoethylene (TCNE). This method, which can also be applied to synthesize the analogous Mn, Co, and Ni compounds, provides a simple route to novel molecule-based magnets with interesting properties.

Chemical characterization of iron in atmospheric aerosols

Abstract The total Fe content, the soluble fractions of Fe(II) and Fe(III), and several species of Fe were determined in various atmospheric aerosol samples by means of atomic absorption spectrometry, ion chromatography and Mossbauer spectrometry.

Spin crossover in hexakis(1-(2-chloroethyl)-tetrazole)iron(II) complexes; synthesis and magnetic properties

Abstract The spin-crossover complexes [Fe(teec)6](X)2 (with teec=1-(2-chloroethyl)-tetrazole and X=BF4, ClO4, PF6) possess two iron(II) sites each having a different spin-crossover behaviour. The magnetic behaviour of the complexes obtained by slow crystallisation differs strongly from those obtained by precipitation. This difference results in either a spin crossover of 50% (X=ClO4, crystalline), two gradual steps (X=BF4, PF6), or two steps, of which one is gradual and the other having a thermal hysteresis (X=ClO4 precipitated). Both 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements have been recorded as a function of T. The crystal structure of [Fe(teec)6](BF4)2 has been determined using X-ray synchrotron powder diffraction. Of [Fe(teec)6](ClO4)2 only the cell parameters could be determined. The ligand field spectra of the low-spin and high-spin states clearly indicate a 1A1→5T2 spin transition with 10 DqLS/10 DqHS=1.72.

A corrosion study of austenitic and martensitic steels under boiler conditions by means of 57Fe conversion electron mössbauer spectroscopy

Abstract The growth of protective oxide layers on a martensitic FeCr (1.4122) and two austenitic FeCrNi steel samples (1.4550 and Incoloy 800) was studied by means of the 57Fe Conversion Electron Mossbauer Spectroscopy (CEMS). Scanning electron microscopy was applied to characterize the topographical nature of the oxide layers. Following a technical procedure of running-in steam generators, the steel specimens were oxidized in oxygen-free water at 295°C, 142 bar and a pH value between 9 and 10 for periods up to 70 h. In the case of the steels 1.4122 and 1.4550 the only oxide phase detected was non-stoichiometric magnetite, whereas nickel ferrite of nonstoichiometric composition was found on Incoloy 800. The study of the kinetics of the oxide layer growth revealed that the oxidation process is most probably controlled by short-circuit diffusion.

Anomalous spin states of iron(II) in 57Fe Mössbauer emission spectra of [57Co(phen)2(NCS)2] and [57Co(bipy)2(NCS)2](cis)

Abstract 57 Fe Mossbauer emission spectra of [ 57 Co(phen) 2 (NCS) 2 ] and [ 57 Co(bipy) 2 (NCS) 2 ] as Mossbauer sources have been measured at various temperatures (down to 4.2 K). Whereas the corresponding synthesized iron complexes [Fe(phen) 2 (NCS) 2 ] and [Fe(bipy) 2 (NCS) 2 ] are known to exhibit temperature dependent high-spin ( 5 T 2g ,O h ) ⇌ low-spin ( 1 A 1g ,O h ) transition, the 57 Fe complexes formed by nuclear decay of 57 Co are strictly high-spin at all temperatures under study. The nature of the “anomalous” spin states has been found to be consistent with 5 A 2 under C 2v symmetry by performing a theoretical analysis of the quadrupole splitting as a function of temperature in conjunction with a ligand field calculation including trigonal and rhombic distortion, spin-orbit coupling, and covalency effects.

K3Cr2(PS4)3: A New Chromium Thiophosphate with a One-Dimensional [Cr2(PS4)]3— Anion Chain

A new chromium thiophosphate, K3Cr2(PS4)3 has been prepared and characterized by single-crystal diffraction, temperature dependent magnetic susceptibility measurements and optical spectroscopy. K3Cr2(PS4)3 crystallizes in the monoclinic space group P21/n (No. 14) with a = 9.731(2) A, b = 11.986(2) A, c = 17.727(4) A, β = 96.52(2)°, V = 2054.2(2) A3, Z = 4, and R = 0.044. The anionic part of the structure consists of dimeric Cr2(μ3-S3PS)2 units which are linked by bidentate PS4 groups to form infinite one-dimensional [S2PS2Cr2(μ3S3PS)2]3— chains separated by K+ cations. The CrIII centers of the Cr2(μ3-S3PS)2 units are antiferromagnetically coupled. The magnetic susceptibility data may be fitted using a D-Heisenberg model for S = 3/2 with g = 2.02 and J/k = 10K. K3Cr2(PS4)3 is semiconducting with an optical band gap of 1.35 eV. K3Cr2(PS4)3: ein neues Chromthiophosphat mit einer eindimensional-unendlichen [Cr2(PS4)3— Anionenkette Das neue Chromthiophosphat K3Cr2(PS4)3 wurde synthetisiert und durch Strukturanalyse an Einkristallen, temperaturabhanige magnetische Suszeptibilitatsmessungen und optische Spektroskopie charakterisiert. K3Cr2(PS4)3 kristallisiert monoklin in der Raumgruppe P21/n (No. 14) mit den Gitterparametern a = 9, 731(2) A, b = 11, 986(2) A, c = 17, 727(4) A, β = 96, 52(2)°, V = 2054, 2(2) A3, Z = 4; die Struktur wurde zu R1/wR2= 0, 044/0, 091 verfeinert. Der anionische Teil der Struktur enthalt dimere [Cr2(μ3S3PS)2]-Baugruppen, die uber zweizahnige [PS43—]-Liganden zu linearen [S2PS2Cr2(μ3S3PS)2]3— Ketten verknupft sind, zwischen denen sich die K+-Kationen befinden. Die Cr3+-Zentren der [Cr2(μ3S3PS)2]-Baugruppen sind antiferromagnetisch gekoppelt. Eine Anpassung an die magnetischen Suszeptibilitatsdaten ist auf Basis eines 1D-Heisenberg-Models fur S = 3/2 und g = 2, 02 sowie J/k = 10K moglich. K3Cr2(PS4)3 ist halbleitend mit einer optischen Bandlucke von 1, 35 eV.