Dithiol-Dithione Tautomerism of 2,3-Pyrazinedithiol in the Synthesis of Copper and Silver Coordination Compounds

Abstract

A promising strategy for new electrically conductive coordination polymers is the combination of d¹⁰ metal ions, which tolerate short metal···metal distances, with dithiolene linkers, known for their "non-innocent" redox behavior. This study explores the coordination chemistry of 2,3-pyrazinedithiol (H₂pdt) toward Cu⁺ and Ag⁺ ions, highlighting similarities and differences. The synthetic approach, starting with the fully protonated ligand, allowed the isolation of a homoleptic bis(dithiolene) complex with formal Cu^I atoms, [Cu(H₂pdt)₂]Cl (1). This complex was further transformed to a 1D coordination polymer with short metal···metal distances, ^1D[Cu(Hpdt)] (2^Cu). The larger Ag⁺ ion directly built up a very similar coordination polymer, ^1D[Ag(Hpdt)] (2^Ag), without any appearance of an intermediate metal complex. The coordination polymer ^1D[Cu(H₂pdt)I] (4), like complex 1, bears fully protonated H₂pdt ligands in their dithione form. Upon heating, both compounds underwent auto-oxidation coupled with a dehydrogenation of the ligand to form the open-shell neutral copper(II) complex [Cu(Hpdt)₂] (3) and the coordination polymer ^1D[Cu₂I₂(H₂pdt)(Hpdt)] (5), respectively. For all presented compounds, crystal structures are discussed in-depth. Furthermore, properties of 1, 3, and those of the three 1D coordination polymers, 2^Ag, 2^Cu, and 4, were investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and variable-temperature magnetic susceptibility, and direct current (dc)-conductivity measurements. The experimental results are compared and discussed with the aid of DFT simulations.