Rafaela A. L. Silva

(α-DT-TTF)2[Au(mnt)2]: a weakly disordered molecular spin-ladder system.

The synthesis and characterization of (α-DT-TTF)2[Au(mnt)2] is reported. The magnetic properties of this new salt show that it is still a rare example of an organic spin-ladder. (α-DT-TTF)2[Au(mnt)2] shares the same ladder structure of the DT-TTF and ETT-TTF analogues, and its room temperature conductivity is ∼2 S/cm. Despite the observed donor orientation disorder associated with the thiophenic sulfur atoms, the intermolecular interactions between donor units, calculated using the extended Hückel approximation and a double-ξ basis set, show that the interaction values do not depend on the configuration of the sulfur atom on the thiophenic ring. The insensitivity of the spin-ladder magnetic properties to the donor molecular disorder in (α-DT-TTF)2[Au(mnt)2] is a direct consequence of the negligible contribution of the disordered thiophenic sulfur atom to the HOMO. In the related donor ETT-TTF, this contribution is significant and destroys the magnetic interactions, and no spin-ladder is observed. This compound not only enlarges the number of organic spin-ladder systems in this series of closely related compounds but also provides an interesting example of weakly disordered molecular spin-ladder system.

Dithiophene-TTF Salts; New Ladder Structures and Spin-Ladder Behavior

(α-DT-TTF)2[Au(i-mnt)2] and (α-DT-TTF)2[Co(mnt)2] are two new salts of the donor α-dithiophene-tetrathiafulvalene with stable diamagnetic anions, both presenting a ladder structure of the donors organized in paired segregated stacks. The first one is isostructural with previously reported closely related compounds and presents a magnetic spin-ladder behavior with J∥= 83.5 K and J⊥ = 110.3 K as estimated from spin susceptibility data in single crystals. (α-DT-TTF)2[Co(mnt)2] presents a new structural type with a different arrangement of pairs of donor stacks, alternating with stacks of dimerized [Co(mnt)2] anions which are however arranged in an uncorrelated fashion perpendicular to the stacking axis. Due to the strong coupling between the disordered anion chains and the donor chains, this compound does not present a magnetic spin-ladder behavior. The low temperature superstructure of (DT-TTF)2[Cu(mnt)2] below the transition at 235 K, previously known to be associated with a lattice doubling along the stacking axis, was solved by synchrotron radiation diffraction in small single crystals. It is found that this dimerization is due to donor charge localization with the spin carriers being associated with fully oxidized donor species alternating with neutral donors.

Gold and nickel alkyl substituted bis-thiophenedithiolene complexes: anionic and neutral forms

The Au and Ni monoanionic complexes of tert-butyl and diisopropyl substituted thiophenedithiolate ligands, [M(α-tb-tpdt)2] and [M(α-dp-tpdt)2], were synthesized and characterized namely by single crystal X-ray diffraction and magnetic susceptibility measurements. These complexes, prepared in a first step as monoanionic species, are easier to oxidize than the related non-substituted thiophenedithiolates and could be obtained also as stable neutral species. As expected, the peripheral alkyl groups in the ligands also confer a high solubility to the complexes in common organic solvents. The neutral gold complex [Au(α-tb-tpdt)2] presents a significant ligand asymmetry indicative of unpaired electron localization in one ligand at variance with [Au(α-dp-tpdt)2] that is within experimental uncertainty fully symmetric illustrating the role of intermolecular interactions in the stabilization of SOMO⋯SOMO interactions. While in [Au(α-tb-tpdt)2] a significant intermolecular interaction between paramagnetic molecules is possible leading to diamagnetic dimers of molecules, in [Au(α-dp-tpdt)2] the bulkier substituents prevent the intermolecular interactions, leading to a regular stacking of molecules in symmetrical configuration. The regular stacks of paramagnetic [Au(α-dp-tpdt)2] units behave, at high temperatures, as antiferromagnetic chains undergoing an AFM transition at ca. 25 K.

Studies on the electrochemical growth of (Per)2[Au(mnt)2].

The first stages of the electrocrystallization of (Per)(2)[Au(mnt)(2)] salt from dichloromethane on gold, platinum, and highly orientated pyrolytic graphite (HOPG) were investigated by cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy in order to understand the determinant factors for nucleation and crystal growth. The crystal growth occurs from adsorbed films of dithiolate on gold or platinum and of perylene on HOPG, after homogeneous nucleation, and it is controlled by the low diffusion of the species toward the growing surface.

Gold and Nickel Extended Thiophenic-TTF Bisdithiolene Complexes

Gold and nickel bisdithiolene complexes with methyl and tert-butyl substituted thiophenetetrathiafulavalenedithiolate ligands (α-mtdt and α-tbtdt) were prepared and characterized. These complexes were obtained, under anaerobic conditions, as tetrabutylammonium salts. The diamagnetic gold monoanion (n-Bu4N)[Au(α-mtdt)2] (3) and nickel dianionic species (n-Bu4N)x[Ni(α-mtdt)2] (x = 1,2) (4) were similar to the related non-substituted extended thiophenic-TTF (TTF = tetrathiafulvalene) bisdithiolenes. However the introduction of the large, bulky substituent tert-butyl, led to the formation of a Au (I) dinuclear complex, (n-Bu4N)2[Au2(α-tbtdt)2] (5). The neutral methyl substituted gold and nickel complexes were easily obtained through air or iodine exposure as polycrystalline or amorphous fine powder. [Au(α-mtdt)2] (6) and [Ni(α-mtdt)2] (7) polycrystalline samples display properties of a metallic system with a room temperature electrical conductivity of 0.32 S/cm and ≈4 S/cm and a thermoelectric power of ≈5 µV/K and ≈32 µV/K, respectively. While [Au(α-mtdt)2] (6) presented a Pauli-like magnetic susceptibility typical of conducting systems, in [Ni(α-mtdt)2] (7) large magnetic susceptibilities indicative of high spin states were observed. Both electric transport properties and magnetic properties for gold and nickel [M(α-mtdt)2] are indicative that these compounds are single component molecular conductors.