Hydrogen-Bonding Interactions in a Single-Component Molecular Conductor: a Hydroxyethyl-Substituted Radical Gold Dithiolene Complex

Abstract

The anionic hydroxyethyl-substituted gold dithiolene complex [NEt4][Au(EtOH-thiazdt)2] is synthesized and further oxidized to the neutral radical species [Au(EtOH-thiazdt)2](•) through electrocrystallization. Single-crystal X-ray diffraction studies highlight the existence of the two cis and trans isomers for the monoanionic complex, with involvement of the hydroxy group in intermolecular O-H···S hydrogen-bonding interactions. The neutral radical complex, [Au(EtOH-thiazdt)2](•), is isostructural with its known ethyl analogue, namely, [Au(Et-thiazdt)2](•). It exhibits a semiconducting behavior (σRT = 0.05-0.07 S cm(-1)) at room temperature and ambient pressure with an activation energy of 0.14 eV. Comparison of the crystal structures and transport and magnetic properties with those of the prototypical [Au(Et-thiazdt)2](•) single-component conductor shows that the replacement of ethyl by a slightly bulkier hydroxyethyl substituent affects only weakly the overlap interactions, complemented here by added O-H···S hydrogen-bonding interactions.

High Ambipolar Mobility in a Neutral Radical Gold Dithiolene Complex

“…The susceptibility (χ p ) of 2 is low, only 4 × 10 −4 emu mol −1 at room temperature, far lower than expected for isolated S = 1/2 molecules. This kind of Pauli‐like, small paramagnetic susceptibility with weak temperature dependence is reported in other neutral gold dithiolene complexes with a thermally‐activated conductivity . The decrease below 60 K could be ascribed to a transition to a nonmagnetic ground state, a suggestion supported by solid state 1 H NMR data, which show no critical divergence of T 1 −1 and almost temperature independent signal shapes (Figure S8, Supporting Information), unlike that seen in antiferromagnetic transitions .…”

“…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.…”

“…The isomers can be distinguished by a 0.01 ppm difference between the dithiolene protons in the 1 H NMR, and the isomers do not appear to interconvert (Figure S1, Supporting Information). While rapid cis–trans isomerism in solution has been reported for asymmetric nickel dithiolene complexes, rapid interconversion for asymmetric gold dithiolene complexes has not been observed . In most cases, the isomers are separated after the oxidation to the neutral species by selective crystallization of one isomer over the other.…”

“…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 .…”

“…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 …”