Localization effects in mixed-ligand gold bis(dithiolene) complexes as single-component conductors

Abstract

Mixed-ligand gold bis(dithiolene) complexes involving two non-innocent dithiolene ligands with different electronic characteristics have been developed, involving one TTF dithiolate (BMT-TTFS₂²⁻ = 4′,5′-bis(methylthio)tetrathiafuvalene-4,5-dithiolate) as a highly electron-rich ligand and either a benzene-1,2-dithiolate (bdt) or a pyrazine-2,3-dithiolate (pzdt) as an electron-poor ligand. The monoanionic closed-shell complexes are oxidized (by electrocrystallization) to neutral radical species, which behave as single-component conductors. The notably different electronic properties of the two dithiolene ligands lead to an exacerbated spin density localization on TTF dithiolate, with the resulting SOMO localized on the less electron-rich ligand. The dissymmetry imposed by the presence of two different ligands leads to a head-to-tail arrangement in the solid state and stack dimerization. The solid-state properties of the two radical complexes [Au(BMT-TTFS₂)(bdt)]˙ and [Au(BMT-TTFS₂)(pzdt)]˙ are deduced from transport measurements under pressure (up to 21 GPa) and spin-polarized band structure calculations. The 1D electronic structure with strongly dimerized chains and a direct, large band gap explains the observed semiconducting behavior. At variance with weakly dimerized systems adopting a Mott insulator behavior sensitive to pressure effects (toward a metallic state), [Au(BMT-TTFS₂)(bdt)]˙ and [Au(BMT-TTFS₂)(pzdt)]˙ show a robust gap under pressure, a direct consequence of the reinforced dimerization of BMT-TTFS₂ moieties in the solid state.