Studies of Rhenium(I) Complexes with Protic Ligand Backbones: Base-Enabled Substitution and CO₂ Reduction

Carbon dioxide is a major contributor to global climate change, and developing efficient electrocatalysts for its reduction to useful products remains an important challenge in renewable energy and sustainable chemistry. Rhenium(I) tricarbonyl complexes have been widely studied for selective CO₂-to-CO conversion, and recent efforts have focused on understanding how ligand structure and proton-coupled electron transfer (PCET) processes govern catalytic activity. In this work, two families of rhenium complexes with protic backbones, fac-Re(PyBim)(CO)₃L and fac-Re(BidmimH)(CO)₃L (PyBimH = 2-(2′-pyridyl)benzimidazole; BidmimH₂ = 2,2′-bis(4,5-dimethylimidazole); L = neutral ligands), were synthesized and characterized to investigate the influence of axial ligand electronics on CO₂ reduction behavior. Infrared spectroelectrochemistry and cyclic voltammetry revealed systematic redshifts in the carbonyl stretching frequencies (Δν = 12–79 cm⁻¹) and cathodic shifts in reduction potentials as axial ligand σ-donor strength increased. Complexes bearing PPh₃ and NH₂(CH₂)₃Ph ligands exhibited the largest catalytic current enhancements under CO₂ in the presence of trifluoroethanol, indicating faster PCET and CO₂ reduction. Bulk electrolysis confirmed that both ligand scaffolds favor CO formation, with PyBim derivatives showing higher stability and CO selectivity. Complementary studies of cobalt(II) and nickel(II) analogues demonstrated similar ligand-dependent reactivity, with the Ni complex SE16 (a nickel(II) bipyridyl derivative studied in collaboration with Fairfield University) achieving the highest CO faradaic efficiency (51%). Together, these results establish clear structure–activity relationships between ligand donor strength, electron density at the metal center, and CO₂ reduction efficiency, providing new insights into how proton-responsive ligands and axial electronic effects can be leveraged to design more effective molecular electrocatalysts for CO₂ conversion.