Weak Ferromagnetism and Strong Spin−Spin Interaction Mediated by the Mixed-Valence Ethynyltetrathiafulvalene-Type Ligand

A new chromium complex with ethynyltetrathiafulvalene (TTF)-type ligands, [CrCyclam(CC-5-methyl-4′5′-ethylenedithio-TTF)2]OTf ([1]OTf), was synthesized. The cyclic voltammetry of the complex shows two reversible oxidation waves owing to the first and second oxidation of the TTF unit. The electrochemical oxidation of [1]OTf in a Bu4NClO4 or Bu4NBF4 solution of a 1:1 acetonitrile−chlorobenzene mixture gave isostructural crystals of [1][ClO4]2(PhCl)2(MeCN) and [1][BF4]2(PhCl)2(MeCN), where two mixed-valence TTF units of adjacent complexes form a dimer radical cation. The crystal structures are characterized by an alternating chain of S = 3/2 Cr3+Cyclam units and S = 1/2 (TTF)2+ dimers. These two paramagnetic components are connected directly by an ethynyl group, resulting in a strong intrachain spin−spin interaction of 2J/kB = −30 and −28 K for [ClO4]− and [BF4]− salts, respectively (H = −2J∑iSi·Si+1). Both salts show a weak ferromagnetic transition at 23 K thanks to interchain antiferromagnetic interaction between TTF dimers. The remanent magnetizations and coercive forces of nonoriented samples at 1.8 K are 0.016 μB and 90 mT for the [ClO4]− salt and 0.010 μB and 50 mT Oe for the [BF4]− salt, respectively. The weak ferromagnetism is attributed to the Dzyaloshinsky−Moriya interaction between adjacent TTF dimers and/or the single-ion anisotropy of [1]2+.