Acid–Responsive Rhenium(I) NHC Complexes: Pyrazine vs Pyridine

Four rhenium(I) tricarbonyl complexes featuring pyridine- or pyrazine-functionalized N-heterocyclic carbenes have been synthesized and fully characterized, including determination of their molecular structures by single-crystal X-ray diffraction. Electrochemical and spectroelectrochemical measurements were also carried out. The development of isosteric Re(I) complexes enabled a direct comparison of how Brønsted and Lewis acids influence their electronic properties. Complexes containing the pyridyl core showed no detectable changes under either electrochemical or spectroscopic studies. In contrast, those bearing the pyrazine fragment exhibited significant shifts at both reductive and oxidative potentials, attributable to the extra nitrogen atom in the 1,4-diazine ring, which can interact with Brønsted (and Lewis) acids. In this regard, comparison of alkaline salts and Brønsted acids revealed that the presence of a Brønsted acid induces substantially larger electronic perturbations.