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 dinner radical cation. The crystal structures are characterized bi an alternating chain of S = 3/2 Cr(3+)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/k(B) = -30 and -28 K for [ClO4](-) and [BF4](-) salts, respectively (H = 2J Sigma S-i(i)center dot 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 mu(B) and 90 mT for the [ClO4](-) salt and 0.010 mu(B) 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+).