Determination of Spin State in Dinuclear Iron(II) Coordination Compounds Using Applied Field Mössbauer Spectroscopy

So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high spin (HS) and low spin (LS) species have been deduced from magnetic susceptibility and zero field Mössbauer spectroscopy data irrespective of whether they belong to LS-LS, LS-HS and HS-HS pairs. However, the distinction of pairs becomes possible if Mössbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.

     

Stoichiometry of LiNiO2 Studied by Mössbauer Spectroscopy

From the ⁶¹Ni and ⁵⁷Fe Mössbauer spectroscopy data follows the cationic site assignment in Li_1–x Ni_1+x O₂. Our data explain the ferromagnetic properties of this material because of the appearance of Ni²⁺ (S=1) among Ni³⁺ (S=1/2) in Ni³⁺O₂ hexagonal planes. We have no evidence for the ferromagnetic interaction between the NiO₂ layers through the excess Ni²⁺ ions substituting the Li⁺ ions. The presence of Ni²⁺ found in the Ni³⁺O₂ planes explains the absence of the Jahn–Teller distortions probably because of the electronic transfer between the Ni³⁺ and Ni²⁺ ions.     

Magnetic phase transitions and iron valence in the double perovskite Sr 2 FeOsO 6

The insulating and antiferromagnetic double perovskite Sr₂FeOsO₆ has been studied by ⁵⁷Fe Mössbauer spectroscopy between 5 and 295 K. The iron atoms are essentially in the Fe^3?+? high spin $( {t_{2\mathrm{g}}{^3} e_\mathrm{g}{^2} } )$ and thus the osmium atoms in the Os $^{5+} ( {t_{2\mathrm{g}}{^3} } )$ state. Two magnetic phase transitions, which according to neutron diffraction studies occur below T _N?= 140 K and T ₂?= 67 K, give rise to magnetic hyperfine patterns, which differ considerably in the hyperfine fields and thus, in the corresponding ordered magnetic moments. The evolution of hyperfine field distributions, average values of the hyperfine fields, and magnetic moments with temperature suggests that the magnetic state formed below T _N is strongly frustrated. The frustration is released by a magneto-structural transition which below T ₂ leads to a different spin sequence along the c-direction of the tetragonal crystal structure.     

Application of 57Fe Mössbauer spectroscopy as a tool for mining exploration of bornite (Cu5FeS4) copper ore

Nuclear resonance methods, including Mössbauer spectroscopy,are considered as unique techniques suitable for remote on-line mineralogical analysis. The employment of these methods provides potentially significant commercial benefits for mining industry. As applied to copper sulfide ores, Mössbauer spectroscopy method is suitable for the analysis noted. Bornite (formally Cu₅FeS₄) is a significant part of copper ore and identification of its properties is important for economic exploitation of commercial copper ore deposits. A series of natural bornite samples was studied by ⁵⁷Fe Mössbauer spectroscopy. Two aspects were considered: reexamination of ⁵⁷Fe Mössbauer properties of natural bornite samples and their stability irrespective of origin and potential use of miniaturized Mössbauer spectrometers MIMOS II for in-situ bornite identification. The results obtained show a number of potential benefits of introducing the available portative Mössbauer equipment into the mining industry for express mineralogical analysis. In addition, results of some preliminary ^63,65Cu nuclear quadrupole resonance (NQR) studies of bornite are reported and their merits with Mössbauer techniques for bornite detection discussed.     

Pressure-Induced Semiconductor-Semimetal Transition in Rb0.8Fe1.6S2

JETP Letters - The transport and magnetic properties of the Rb0.8Fe1.6S2 crystal have been studied under quasihydrostatic compression to a pressure of 40.5 GPa in diamond anvil cells. A...     

A Novel-Type Microporous Heteropolyhedral Framework in Crystal Structure of the Natural Sulfate Philoxenite

Crystallography Reports - The crystal structure of a new mineral philoxenite (K,Na,Pb)4(Na,Ca)2(Mg,Cu)3(Fe $$_{{0.5}}^{{3 + }}$$ Al0.5)(SO4)8 from fumarole exhalations of the Tolbachik Volcano...     

Five-coordinate complexes [FeX(depe)(2)]BPh(4), X = Cl,Br: electronic structure and spin-forbidden reaction with N(2)

The bonding of N(2) to the five-coordinate complexes [FeX(depe)(2)](+), X = Cl (1a) and Br (1b), has been investigated with the help of X-ray crystallography, spectroscopy, and quantum-chemical calculations. Complexes 1a and 1b are found to have an XP(4) coordination that is intermediate between square-pyramidal and trigonal-bipyramidal. M?ssbauer and optical absorption spectroscopy coupled with angular overlap model (AOM) calculations reveal that 1a and 1b have (3)B(1) ground states deriving from a (xz)(1)(z(2))(1) configuration. The zero-field splitting for this state is found to be 30-35 cm(-1). In contrast, the analogous dinitrogen complexes [FeX(N(2))(depe)(2)](+), X = Cl (2a) and Br (2b), characterized earlier are low-spin (S = 0; Wiesler, B. E.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W. Angew. Chem, Int. Ed. 1998, 37, 815-817). N(2) bonding and release in these systems are thus spin-forbidden. It is shown by density functional theory (DFT) calculations of the chloro complex that the crossing from the singlet state (ground state of 2a) to the triplet state (ground state of 1a) along the Fe-N coordinate occurs at r(C) = 2.4 A. Importantly, this intersystem crossing lowers the enthalpy calculated for N(2) release by 10-18 kcal/mol. The free reaction enthalpy Delta G degrees for this process is calculated to be 4.7 kcal/mol, which explains the thermal instability of N(2) complex 2a with respect to the loss of N(2). The differences in reactivity of analogous trans hydrido systems are discussed.     

Cover Picture

The cover picture shows how both, fine arts and science, avail themselves of a system of intertwined symbolic and iconic languages. They make use of a common set of abstracted signs to report on their results. Thus, already in 1925, Wassily Kandinsky painted a masterpiece (bottom), which now, 75 years later, might be regarded as a blueprint for a scientific project. In his painting, Kandinsky pictured a grid-shaped sign that resembles in effect an actual molecular switch. Apparently following an enigmatic protocol, the groups of Lehn and Gütlich (see p. 2504 ff. for more details) constructed a grid-type inorganic architecture that operates as a three-level magnetic switch (center) triggered by three external perturbations (p, T, hnu). The switching principle is based on the spin-crossover phenomenon of Fe(II) ions and can be monitored by M?ssbauer spectroscopy (left) and magnetic measurements (rear). Maybe not by chance, the English translation of the title of the painting "signs" is a homonym of "science", since both presented works are a product of the insatiable curiosity of man and his untiring desire to recognize his existence.     

A Heteroleptic Push–Pull Substituted Iron(II) Bis(tridentate) Complex with Low‐Energy Charge‐Transfer States

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]²⁺ with a strongly electron‐withdrawing ligand (dcpp, 2,6‐bis(2‐carboxypyridyl)pyridine) and a strongly electron‐donating tridentate tripyridine ligand (ddpd, N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reported. Both ligands form six‐membered chelate rings with the iron center, inducing a strong ligand field. This results in a high‐energy, high‐spin state (⁵T₂, (t_2g)⁴(e_g*)²) and a low‐spin ground state (¹A₁, (t_2g)⁶(e_g*)⁰). The intermediate triplet spin state (³T₁, (t_2g)⁵(e_g*)¹) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low‐energy π^* orbitals of dcpp allow low‐energy MLCT absorption plus additional low‐energy LL′CT absorptions from ddpd to dcpp. The directional charge‐transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]³⁺ and [Fe(dcpp)(ddpd)]⁺, augmented by density functional calculations. The combined effect of push–pull substitution and the strong ligand field paves the way for long‐lived charge‐transfer states in iron(II) complexes.