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

Abstract

Solutions of ReS 4 - undergo electro-oxidation at 0.5 V vs Ag|AgCl to give black, amorphous ReS 4 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 Re 2 S 7 and ReS 4 are closely related structurally. The XAS data were calibrated with studies on [ReS 4 ] - and crystalline ReS 2 . For both ReS 4 and Re 2 S 7 XAS data show the presence of both S 2 - and S 2 2 -, with S 2 - being less abundant than S 2 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 ReS 4 and Re 2 S 7 convert to ReS 2 as anticipated from the results of TGA experiments. These considerations, when taken together with literature precedents, led to the reformulation of Re 2 S 7 as Re V (S) 1.5 (S 2 ) 1, and correspondingly ReS 4 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 - /S 2 - ratio. Chain structures accommodate these considerations and are consistent with a largely reversible electrochemistry that regenerates ReS 4 - .