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 I-2 or ferrocenium as oxidants. X-ray absorption spectroscopy (XAS) suggests that the phases Re2S7 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 S2- and S-2(2-) with S2- being less abundant than S-2(2-). The presence of terminal sulfides (r(Re=S) similar to 2.14 Angstrom) 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 Re2S7 as Re-V(S)(1.5)(S-2)(1), and correspondingly ReS4 is formulated as Re-V(S)(1)(S-2)(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-)/S2- ratio. Chain structures accommodate these considerations and are consistent with a largely reversible electrochemistry that regenerates ReS4-.