CO₂ Capture: Exploring Rhenium Complexes as Redox Mediators

The electrochemical and photochemical properties of the rhenium complexes Re(CO)₃Cl(phendione) and Re(CO)₃Cl(AQphen) (phendione = 1,10-phenanthroline-5,6-dione; AQphen = naphtho[2,3-h]dipyrido[3,2-a:2',3'-c]phenazine-8,13-dione) were investigated for their potential in CO₂ capture. UV-vis spectra in dimethyl sulfoxide (DMSO) showed absorption maxima at 370 and 391 nm, attributed to metal-to-ligand charge transfer (MLCT) transitions, as confirmed by time-dependent density functional theory (TD-DFT) calculations. Electrochemical studies revealed two one-electron reduction events: -0.12 and -0.77 V for Re(CO)₃Cl(phendione) and -0.30 and -0.79 V for Re(CO)₃Cl(AQphen) (vs Ag/AgCl). In the presence of CO₂, the second reduction shifted positively by 0.20 and 0.14 V, indicating adduct formation. Association constants (log K) were 5.4 and 3.9 for phendione and AQphen derivatives, respectively. DFT calculations supported formation of [quinone-CO₂]^2- adducts, with bond lengths of 1.447 and 1.406 Å. Under blue LED light (410 nm) and CO₂, photochemical experiments showed spectral changes consistent with reduced quinone-CO₂ species. These results highlight the ability of rhenium-quinone complexes to engage in electrochemical and photochemical CO₂ capture, offering insights into their charge transfer properties and CO₂ binding behavior.