Solar Light CO ₂ Photoreduction Enhancement by Mononuclear Rhenium(I) Complexes: Characterization and Mechanistic Insights

The catalytic efficacy of a novel mononuclear rhenium(I) complex in CO₂ reduction is remarkable, with a turnover number (TON_CO) of 1517 in 3 h, significantly outperforming previous Re(I) catalysts. This complex, synthesized via a substitution reaction on an aromatic ring to form a bromo-bipyridine derivative, <b>L1</b> = 2-bromo-6-(1<i>H</i>-pyrazol-1-yl)pyridine, and further reacting with [Re(CO)₅Cl], results in the facial-tricarbonyl complex [ReL1(CO)₃Cl] (<b>1</b>). The light green solid was obtained with an 80% yield and thoroughly characterized using cyclic voltammetry, nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy. Cyclic voltammetry under CO₂ atmosphere revealed three distinct redox processes, suggesting the formation of new electroactive compounds. The studies on photoreduction highlighted the ability of the catalyst to reduce CO₂, while NMR, FTIR, and electrospray ionization (ESI) mass spectrometry provided insights into the mechanism, revealing the formation of solvent-coordinated complexes and new species under varying conditions. Additionally, computational studies (DFT) were undertaken to better understand the electronic structure and reactivity patterns of <b>1</b>, focusing on the role of the ligand, the spectroscopic features, and the redox behavior. This comprehensive approach provides insights into the intricate dynamics of CO₂ photoreduction, showcasing the potential of Re(I) complexes in catalysis.