A promising method to store solar energy consists of transferring the potentials it generates into carbon-neutral fuels. This can be better achieved by employing catalysts to lower the energetic barriers of fuel-forming reactions that utilize carbon dioxide as the “raw material” for carbon-neutral processes. In addition to having specific selectivity and efficiency, these catalysts must be outstandingly cheap and abundant in order to be germane to the energy grid. For instance, transition metal-bipyridine complexes are economic, well-established electrocatalytic candidates for CO₂ reduction, but challenges can come up regarding their stability and solubility. Adding to this family of molecular electrocatalysts, new [fac-Re(tmam)(CO)₃X](PF₆)₂ complexes (tmam = dicationic methylene trimethylamine bipyridine; X = anionic axial ligand) were synthesized and characterized by ¹H NMR, IR, and UV-Vis spectroscopy. The pendant methylene trimethylamine group was used in bipyridine’s 4,4’ positions, expecting the methylene linker to keep the bipyridine electronically isolated from the cationic trimethylamines, and the charged group to contribute electrostatic stabilization to anionic intermediates in the CO₂-reducing catalytic cycle, predicted to promote catalytic activity and reduce overpotentials, as other species have been proven to do. The catalytic activity was explored by cyclic voltammetry, bulk electrolysis, and differential pulse voltammetry. Initial electrochemical characterization suggested axial chloride ligands were more labile than expected. This led to further investigation of the cationic pendant group’s effect on the lability of the halide ligand, which was found to also have implications on the catalytic cycle after performing chloride titrations. Ion pairing was studied to a greater extent through ¹H NMR and IR spectroscopy. By using different counterions with the trimethylamines from hexafluorophosphate to chloride, the electrochemistry and stability in organic and aqueous solution were studied, respectively. The instability of the tmam ligand itself led to minimal change in the catalytic activity of the rhenium complex compared to unfunctionalized complex. Nonetheless, valuable insights were gained regarding the interaction between the cationic group and the axial halide for the future development of catalytic cycles involving axial ligand dissociation.

Brian Sanders

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