A Series of Weak Ferromagnets Based on a Chromium–Acetylide–TTF Type Complex: Correlation of the Structures and Magnetic Properties and Origin of the Weak Ferromagnetism

The crystal structures and magnetic properties of a series of new weak ferromagnets containing a chromium–acetylide–tetrathiafulvalene (TTF) type complex, [CrCyclam­(CC-5-methyl-4′5′-ethylenedithio-TTF)₂]²⁺ ([<b>1</b>]²⁺), were investigated. The six new isostructural weak ferromagnets [<b>1</b>]­[BF₄]₂(PhF)₂(MeCN), [<b>1</b>]­[ClO₄]₂(PhF)₂(MeCN), [<b>1</b>]­[ReO₄]₂(PhCl)₂(MeCN), [<b>1</b>]­[ClO₄]₂(PhBr)₃, [<b>1</b>]­[ReO₄]₂(PhBr)₃, and [<b>1</b>]­[ClO₄]₂(PhI)₃ contain ferrimagnetic chain structures of [<b>1</b>]²⁺_∞ with different interchain distances that are dependent on the sizes of the anions and solvent molecules. Magnetic measurements of the salts revealed that the weak ferromagnetic transition temperature gradually increases from 14.5 to 26.0 K as the interchain distance decreases from 3.997(2) to 3.803(2) Å, while the remanent magnetization at 2 K decreases from 0.0215 to 0.0079 μ_B. The observed magnetic properties and crystal structures suggest that the weak ferromagnetism originates from the single-ion anisotropy of [<b>1</b>]²⁺, where a stronger interchain antiferromagnetic interaction not only causes a higher transition temperature but also suppresses the noncollinear canted spin alignment

Effect of Nonmagnetic Substitution on the Magnetic Properties and Charge-Transfer Phase Transition of an Iron Mixed-Valence Complex, (<i>n‑</i>C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂)

The iron mixed-valence complex (<i>n-</i>C₃H₇)₄N­[Fe^IIFe^III(dto)₃] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe^II and Fe^III occurs reversibly at ∼120 K, in addition to the ferromagnetic phase transition at <i>T</i>_C = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (<i>n-</i>C₃H₇)₄N­[Fe^II_1–<i>x</i>Zn^II_<i>x</i>Fe^III(dto)₃] (dto = C₂O₂S₂; <i>x</i> = 0–1), and carried out magnetic susceptibility and dielectric constant measurements and ⁵⁷Fe Mössbauer spectroscopy. With increasing Zn^II concentration (<i>x</i>), the CTPT is gradually suppressed and disappears at <i>x</i> ≈ 0.13. On the other hand, the ferromagnetic transition temperature (<i>T</i>_C) is initially enhanced from 7 K to 12 K between <i>x</i> = 0.00 and 0.05, despite the nonmagnetic nature of Zn^II ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn^II concentration. This anomalous dependence of <i>T</i>_C on Zn^II concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT

Anionic Iron Complexes with a Bond between an Ate-Type Pentacoordinated Germanium and an Iron Atom

The first stable anionic iron(0) complexes bearing an ate-type pentacoordinated germanium­(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge–Fe bonds which can be cleaved by oxidation

Anionic Iron Complexes with a Bond between an Ate-Type Pentacoordinated Germanium and an Iron Atom

The first stable anionic iron(0) complexes bearing an ate-type pentacoordinated germanium­(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge–Fe bonds which can be cleaved by oxidation