Redução eletrocatalítica de CO2 utilizando um novo complexo de Re(I) contendo o ligante (z)-n-metil-2-(6-oxo-1,10-fenantrolin-5 (6H)-ilideno) hidrazinacarbotiamida

Abstract

Tricarbonyl rhenium(I) complexes, commonly represented as fac-[Re(CO)₃(NN)Cl], where NN denotes bipyridine-derived ligands, have been extensively investigated as homogeneous catalysts for CO₂ reduction. This study aimed to synthesize a new complex, fac-[Re(CO)₃(Phmt)Cl], where Phmt = (Z)-N-methyl-2-(6-oxo-1,10-phenanthrolin-5(6H)-ylidene)hydrazinecarbothioamide, and to investigate its electrochemical behavior as a potential electrocatalyst for CO₂ reduction. The synthesis of fac-[Re(CO)₃(Phmt)Cl] was carried out by first preparing the Phmt ligand under reflux in toluene, followed by reaction with the metal precursor [Re(CO)₅Cl] under reflux to afford the desired complex. The product was characterized by Fourier-transform infrared (FTIR) spectroscopy and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy to confirm its molecular structure. The ¹H NMR spectrum displayed signals predominantly between 7 and 15 ppm, with the two most downfield resonances assigned to the amine hydrogen atoms, while the remaining signals corresponded to the aromatic protons. The FTIR spectrum provided further structural confirmation, exhibiting characteristic bands assigned to ν(N–H) (≈3268 cm⁻¹), ν(C≡O) (≈2027, 1900, and 1890 cm⁻¹), and ν(Re–N) (≈260 cm⁻¹), indicating the presence of amine groups, the facial geometry of the tricarbonyl complex, and metal–ligand coordination, respectively. The electrochemical properties were investigated by cyclic voltammetry (CV) in anhydrous dimethylformamide (DMF) under argon and CO₂ atmospheres. Under argon, the complex exhibited an irreversible oxidation peak corresponding to the Re(I)/Re(II) redox process. In the cathodic region, three irreversible reduction peaks were observed at −1.0, −1.6, and −2.1 V vs. Fc⁺/Fc, consistent with values reported for structurally related compounds. Under a CO₂ atmosphere, a significant catalytic current enhancement was observed (Ic/Ip = 2.41), with the first reduction peak shifting to −1.93 V vs. Fc⁺/Fc. These electrochemical results indicate that the synthesized complex is a promising candidate for the electrocatalytic reduction of CO₂ to CO.