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Keywords:

  • Magnetic materials, organic;
  • Molecular magnets;
  • Spin ladders;
  • Tetrathiafulvalenes (TTFs)

Abstract

Starting from the first organic spin ladder reported, a dithiophene-tetrathiafulvalene salt ((DT-TTF)2[Au(mnt)2]) (mnt = maleonitriledithiolate), two different approaches to enlarge the family of organic spin-ladder systems are described. The first approach consists of a molecular variation of the donor; to that purpose, the new TTF derivative ethylenethiothiophene-tetrathiafulvalene (ETT-TTF, 3), is synthesized and structurally characterized. From this donor a new ladder-like structure compound, (ETT-TTF)2[Au(mnt)2] (4), isostructural with (DT-TTF)2[Au(mnt)2], is obtained. However, the magnetic properties of 4 do not follow the known spin-ladder behavior owing to orientational disorder exhibited by the ETT-TTF molecules in the crystal structure. In the second approach, the acceptor complex is changed, either in the nature of the ligand or in the metal. With the [Au(i-mnt)2] salt (i-mnt = iso-maleonitriledithiolate), the new ladder-like compound (DT-TTF)2[Au(i-mnt)2] (5), isostructural with 4, is obtained, but only as a minority product. Two other compounds with a different anion generated in situ, bearing a Au(I) dimeric core, were also isolated; (DT-TTF)9[Au2(i-mnt)2]2 (6) as the most abundant phase and (DT-TTF)2[Au2(i-mnt)2] (7) as another minority phase. Salt 7 is characterized by X-ray crystallography as a chiral compound, due to the torsion of the ligands around the central Au–Au bond. The magnetic properties of (DT-TTF)2[Au(i-mnt)2] (5) indicate that it follows a spin-ladder behavior and the electron paramagnetic resonance (EPR) data is fitted to the Troyer and Barnes and Riera equations with the parameters Δ/kB = 71 K, J/kB = 86 K, and J/kB = 142 K, indicating a J/J ratio of 1.65. The change of the gold complex [Au(mnt)2] for its copper analogue, [Cu(mnt)2] also leads to a ladder-like structure, (DT-TTF)2[Cu(mnt)2] (8), which is isostructural with the gold analogue and with salts 4 and 5. The fully ionic salt (DT-TTF)[Cu(mnt)2] (9) is also obtained. The magnetic properties demonstrated that compound 8 is the third organic spin-ladder system of this family, and the values found by a fitting to the ladder equations were Δ/kB= 123 K, J/kB = 121 K, and J/kB = 218 K, corresponding to a J/J ratio of 1.75, similar to that of 5 and close to that of an ideal spin ladder.