Acid–responsive rhenium( i ) NHC complexes: pyrazine vs. pyridine

Four rhenium(I) tricarbonyl complexes featuring pyridine- or pyrazine-functionalized N-heterocyclic carbenes were synthesized and fully characterized, including the determination of their molecular structures by single-crystal X-ray diffraction. Electrochemical and spectroelectrochemical measurements were performed. 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 in 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 alkali salts with Brønsted acids revealed that the presence of the latter induces substantially larger electronic perturbations.