Chiral, radical, gold bis(dithiolene) complexes

Abstract

International audienceIn the search for single component conductors, eventually blessed with chirality, we have prepared three novel Au(iii) bis(dithiolene) complexes of the 6,7-dihydro-5H-1,4-dithiepine-2,3-bis(thiolate) ligand exhibiting a fused seven-membered ring. The unsubstituted ligand as well as two chiral enantiopure derivatives, with methyl groups in 5,7 positions or a ketal moiety in 6 position, were used to provide the corresponding anionic d8 Au(iii) bis(dithiolene) complexes (1, 2 and 3) as Bu4N+ salts. The anions adopt a square planar geometry around the gold atom with a pseudo centrosymmetric chair conformation of the outer seven-membered rings. Cyclic voltammetry experiments demonstrate that they can be easily oxidized to the neutral radical state in a 0.40-0.50 V range (vs. SCE). Their electrocrystallization afforded neutral radical species for two of them, which were structurally characterized. An original transformation of the unsubstituted complex 1 upon oxidation is observed in its oxidation product, leading to an unexpected radical complex 1′′, with two different dithiolene ligands. Its peculiar electronic structure and the formation of alternated stacks in the solid state explains its semiconducting character with direct gap. On the other hand, the radical complex 3′ derived from the chiral ketal derivative 3 is organized into face-to-face dimers, isolated from each other in the crystal. © 2016 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique

High Ambipolar Mobility in a Neutral Radical Gold Dithiolene Complex

“…The lack of dispersion along Γ– Z is expected, a result of the omission of interactions between planes along the c axis. Due to the lack of dimerization in the crystal structure the SOMO (and SOMO‐1) bands are not split; large splitting of the SOMO bands has been being provided as the explanation for the band semiconductor behavior in previous gold radical complexes . The calculations instead predict a metallic character for 2 , at odds with the experimentally observed semiconductor behavior, however extended Hückel theory does not take into account strong electron correlation so this is consistent with our Mott insulator interpretation …”

“…Upon cooling, semiconductor behavior is observed ( Figure ) in both the ab plane and along the c axis, with an activation energy of 0.11 eV. This is among the lowest activation energies reported for neutral gold dithiolene complexes that do not contain TTF moieties . Heat capacity measurements (Figure S7, Supporting Information) supported the presence of a nonmetallic state at low temperatures.…”

“…Metal dithiolene complexes have received a great deal of attention in recent years as potential molecular based functional materials, due to their high stability, magnetic, optical and conductive properties, which can be tuned through variation of the metal center or substitution pattern of the dithiolene ligands . In particular, neutral nickel and gold bis(1,2‐dithiolene) complexes have been studied as single component conductors, due to attractive features such as their square planar geometry and delocalized electronic structure, which leads to favorable intermolecular stacking interactions (a requirement for solid state conductivity) and reversible and tunable redox chemistry. Additionally, neutral gold dithiolene complexes can behave as Mott insulators, with one radical per site, resulting in an activation barrier for conductivity behavior that can be overcome with pressure .…”

“…Additionally, neutral gold dithiolene complexes can behave as Mott insulators, with one radical per site, resulting in an activation barrier for conductivity behavior that can be overcome with pressure . Since the first molecular superconductor based on a metal dithiolene complex was reported, a charge transfer salt (TTF)[Ni(dmit) 2 ] 2 , a large number of neutral complexes containing dithiolene ligands with a TTF backbone have been synthesized, such as Au(ptdt) 2 , and Au(tmdt) 2 , showing metallic or near metallic behavior at ambient temperature and pressure. Neutral gold dithiolene complexes without TTF containing ligands have also been reported, such as Au(α‐tpdt) 2 and Au(bdt) 2 , showing a wide variety of conductive properties, from semiconductor to metallic .…”

“…Since the first molecular superconductor based on a metal dithiolene complex was reported, a charge transfer salt (TTF)[Ni(dmit) 2 ] 2 , a large number of neutral complexes containing dithiolene ligands with a TTF backbone have been synthesized, such as Au(ptdt) 2 , and Au(tmdt) 2 , showing metallic or near metallic behavior at ambient temperature and pressure. Neutral gold dithiolene complexes without TTF containing ligands have also been reported, such as Au(α‐tpdt) 2 and Au(bdt) 2 , showing a wide variety of conductive properties, from semiconductor to metallic . Several complexes in the thiazole dithiolate family have been reported showing activated conductivity under ambient conditions, with some showing metal‐like conductivity under pressure, and one showing metallic behavior under ambient conditions …”