Electrochemical Synthesis and Crystallization of a Novel Tetraarylaurate Anion: Synthesis, Structure, and Physical Properties of (BEDT-TTF)Au(C₆Cl₅)₄

The homoleptic tetraarylaurate anion, Au(C6Cl5)4-, which has never been prepared via traditional chemical methods, has been synthesized by use of an electrochemical technique and crystallized in situ with the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, or ET) electron-donor molecule. This salt, (C10H8S8)Au(C6Cl5)4, crystallizes having the monoclinic space group P21/c, with a = 14.7835(9) Å, b = 20.2270(12) Å, c = 16.7303(10) Å, β = 101.396(1)°, Z = 4. The crystal structure of this (ET)Au(C6Cl5)4 salt contains the Au(C6Cl5)4- anion in an essentially square-planar coordination with the pentachlorophenyl groups arranged in a propeller-like D4 symmetry, consistent with the formal oxidation state +3 of the central metal atom. The ET+ cations are completely surrounded by the pentachlorophenyl rings of the anions, thereby precluding the formation of a S···S network with concomitant conducting properties. Analysis of the Raman spectrum of (ET)Au(C6Cl5)4 confirms that the ET molecule in this salt is in the +1 oxidation state. The variable temperature spin susceptibility data obtained from ESR spectroscopy indicate a weak ferromagnetic coupling between the radical ET+ cations. This salt provides the unique opportunity to investigate the properties of a highly isolated ET+ cation in the solid state.