Electrosynthesis of ReS₄. XAS Analysis of ReS₂, Re₂S₇, and ReS₄

Solutions of ReS₄^- undergo electro-oxidation at 0.5 V vs Ag|AgCl to give black, amorphous ReS₄ via a process that can be reversed at -0.9 V. The oxidative polymerization can also be effected chemically using either I₂ or ferrocenium as oxidants. X-ray absorption spectroscopy (XAS) suggests that the phases Re₂S ₇ and ReS₄ are closely related structurally. The XAS data were calibrated with studies on [ReS₄]^- and crystalline ReS₂. For both ReS₄ and Re₂S₇ XAS data show the presence of both S^2- and S₂^2-, with S^2- being less abundant than S₂^2-. The presence of terminal sulfides (r_Re=S ∼ 2.14 Å) was ruled out by the EXAFS analysis. Variable temperature EXAFS studies showed that ReS₄ and Re₂S₇ convert to ReS₂ as anticipated from the results of TGA experiments. These considerations, when taken together with literature precedents, led to the reformulation of Re ₂S₇ as Re^V(S)_1.5(S₂) ₁, and correspondingly ReS₄ is formulated as Re ^V(S)₁(S₂)_1.5. These two materials are proposed to be members of a continuum of Re(S)_1.5-1(S ₂)_1-1.5 structures that differ in terms of their S ₂^2-/S^2- ratio. Chain structures accommodate these considerations and are consistent with a largely reversible electrochemistry that regenerates ReS₄^-.