Dirk Baabe

Enhanced magnetisation in nanocrystalline high-energy milled MgFe2O4

The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by means of Mossbauer spectroscopy, magnetisation measurements, and electron microscopy. The observed enhancement of the magnetisation in nanoscale milled MgFe2O4 is discussed with respect to the mechanically induced cation redistribution and spin canting.

Structural disorder in the high-energy milled magnesium ferrite

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mossbauer spectroscopy. It is found that the nanostructural state of the milled MgFe2O4 is characterized by a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites in the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distribution curves of mechanically treated samples show additional components at smaller magnetic fields. The weight of the B(6) field decreases with increasing milling time, and the B(5) field becom...

Mechanically Induced Cation Redistribution and Spin Canting in Nickel Ferrite

The structural and magnetic evolution in nickel ferrite (NiFe2O4) caused by high-energy milling are investigated by Mössbauer spectroscopy. It is found that the nanostructural state of the milled NiFe2O4 is characterized by a reduced concentration of iron ions on tetrahedral sites. The degree of inversion in NiFe2O4 is calculated from the subspectral area ratio of both high- and zero-field Mössbauer spectra. Several interesting features are involved in the work, e.g., superparamagnetic relaxation, mechanically induced cation redistribution, and spin-canting effect.

Weak ferromagnetism with very large canting in a chiral lattice: Fe(pyrimidine)2Cl2

The transition metal coordination compound Fe(pyrimidine) 2 Cl 2 crystallizes in a chiral lattice, space group I4 1 22 (or I4 3 22). Combined magnetization, Mossbauer spectroscopy, and powder neutron diffraction studies reveal that it is a canted antiferromagnet below T N =6.4 K with an unusually large canting of the magnetic moments of 14° from their general antiferromagnetic alignment, one of the largest reported to date. This results in weak ferromagnetism with a ferromagnetic component of ∼1 μ B . The large canting is due to the interplay between the antiferromagnetic exchange interaction and the local single-ion anisotropy in the chiral lattice. The magnetically ordered structure of Fe(pyrimidine) 2 Cl 2 , however, is not chiral. The implications of these findings for the search of molecule based materials exhibiting chiral magnetic ordering are discussed.

Iron 10‐Thiacorroles: Bioinspired Iron(III) Complexes with an Intermediate Spin (S=3/2) Ground State

A first systematic study upon the preparation and exploration of a series of iron 10-thiacorroles with simple halogenido (F, Cl, Br, I), pseudo-halogenido (N3 , I3 ) and solvent-derived axial ligands (DMSO, pyridine) is reported. The compounds were prepared from the free-base octaethyl-10-thiacorrole by iron insertion and subsequent ligand-exchange reactions. The small N4 cavity of the ring-contracted porphyrinoid results in an intermediate spin (i.s., S=3/2) state as the ground state for the iron(III) ion. In most of the investigated cases, the i.s. state is found unperturbed and independent of temperature, as determined by a combination of X-ray crystallography and magnetometry with (1) H NMR-, EPR-, and Mössbauer spectroscopy. Two exceptions were found. The fluorido iron(III) complex is inhomogenous in the solid and contains a thermal i.s. (S=3/2)→high spin (h.s., S=5/2) crossover fraction. On the other side, the cationic bis(pyridine) complex resides in the expected low spin (l.s., S=1/2) state. Chemically, the iron 10-thiacorroles differ from the iron porphyrins mainly by weaker axial ligand binding and by a cathodic shift of the redox potentials. These features make the 10-thiacorroles interesting ligands for future research on biomimetic catalysts and model systems for unusual heme protein active sites.

Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced 89Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

N-Heterocyclic carbene adducts to [Cp′FeI] 2 : synthesis and molecular and electronic structure

Addition of N-heterocyclic carbenes (L = 1,3-di-tert-butylimidazol-2-ylidene (ItBu), 1,3-di-iso-propyl-4,5-dimethylimidazol-2-yildene (IiPr2Me2), 1,3-mesitylimidazol-2-yildene (IMes) and 1,3-di-(2,6-di-isopropylphenyl)imidazol-2-yildene (IPr)) to the iron half-sandwich complex [Cp′FeI]2 (Cp′ = η5-1,2,4-(Me3C)3C5H2, 1) forms the neutral, 16VE adducts [Cp′FeI(L)] (2–5) in moderate to excellent yields. These complexes were structurally characterised. The NHC ligand binds strongly to the Fe(II) atom, so that no exchange is observed on the NMR and chemical time scale. Fe(II) atoms in the starting material 1 adopt a high-spin configuration (S = 2) and are weakly antiferromagnetically coupled at low temperatures. Furthermore, in contrast to previous reports on related [(η5-C5Me5)FeCl(NHC)] systems, in which the Fe(II) atoms assume an intermediate spin (S = 1), no spin state change occurs upon coordination of the NHC ligand; the Fe(II) atoms in complexes 2–5 retain their high-spin state (S = 2) as shown by solid state magnetic susceptibility and zero-field 57Fe Mossbauer spectroscopy investigations. Density functional theory (DFT) studies at the B3LYP level of theory also agree with a well separated S = 2 ground state for compounds 2–5. Surprisingly for Fe(II) high-spin systems, compounds 1–5 exhibit slow paramagnetic relaxation in their Mossbauer spectra; this can be traced to spin–spin and spin–lattice relaxation processes with unusually large spin–lattice relaxation barriers. A structural model is proposed that associates these processes with crystal packing effects.

Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited

The pyrrolyl-based iron pincer compounds [(tBuPNP)FeCl] (1), [(tBuPNP)FeN2] (2), and [(tBuPNP)Fe(CO)2] (3) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field 57Fe Mössbauer and X-band electron paramagnetic resonance spectroscopy. While the iron(II) starting material 1 adopts an intermediate-spin (S = 1) state, the iron(I) reduction products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent with an intermediate-spin configuration for 1, the zero-field 57Fe Mössbauer spectrum shows a characteristically large quadrupole splitting (ΔEQ ≈ 3.7 mm s-1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| ≈ 37 cm-1). The effective magnetic moments observed for the iron(I) species 2 and 3 are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin-orbit coupling in the ground state. The experimental findings are complemented by density functional theory computations, which are in good agreement with the experimental data. Most notably, these calculations reveal a low-lying (S = 2) excited state for complex 1; furthermore, the computed Mössbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings.

Spin Crossover and Valence Tautomerism in Neutral Homoleptic Iron Complexes of Bis(pyridylimino)isoindolines

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe2) at the 4-positions of the pyridine moieties have been prepared and studied with regard to temperature-dependent spin and redox states by a combination of (57)Fe Mössbauer spectroscopy, SQUID magnetometry, single-crystal X-ray diffraction analysis, X-band EPR, and (1)H NMR spectroscopy. While the H-, methyl-, and ethyl-substituted complexes remain in a pure high-spin state irrespective of the temperature, the 4-tert-butyl-substituted derivative shows spin-crossover behavior. The methoxy- and dimethylamino-substituted compounds were found to easily undergo oxidation. In the crystalline state, valence tautomeric behavior was observed for the methoxy derivative as a thermally activated charge-transfer transition, accompanied by a spin crossover above 200 K. The valence tautomerism leads to a chelate with one of the bpi ligands as a dianion radical L(2-·) and with an effective spin of S=2.

A Mössbauer study of mechanically activated MgFe2 O4

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling is investigated by Mössbauer spectroscopy.