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