Organic spin ladders from tetrathiafulvalene (TTF) derivatives


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)[Au(2)(i-mnt)(2)](2) (6) as the most abundant phase and (DT- TTF)(2)[Au(2)(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 Delta/k(B) = 71 K, J(parallel to)/k(B) = 86 K, and J(perpendicular to)/k(B) = 142 K, indicating a J(perpendicular to)/J(parallel to) 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-T'FF)(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 Delta/k(B) = 123 K, J(parallel to)/k(B) = 121 K, and J(perpendicular to)/k(B) = 218 K, corresponding to a J(perpendicular to)/J(parallel to) ratio of 1.75, similar to that of 5 and close to that of an ideal spin ladder.




Chemistry; Science & Technology - Other Topics; Materials Science; Physics


Ribas, X; Mas-Torrent, M; Perez-Benitez, A; Dias, JC; Alves, H; Lopes, EB; Henriques, RT; Molins, E; Santos, IC; Wurst, K; Foury-Leylekian, P; Almeida, M; Veciana, J; Rovira, C

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