Synthesis and structural characterization of rhenium(I) tricarbonyl complexes with 2,6-bis(pyrazol-1-yl)pyridine ligands functionalized with carboxyl (-COOH), methoxycarbonyl (-COOCH3), and amino (-NH2) groups

Abstract

Rhenium(I) tricarbonyl complexes with 2,6-bis(pyrazol-1-yl)pyridine ligands functionalized with -COOH, -COOCH<inf>3</inf>, and -NH<inf>2</inf> were synthesized via UV-induced substitution of Re(CO)<inf>5</inf>Br in acetone under 365 nm irradiation. Single-crystal X-ray diffraction revealed distorted octahedral geometries, with Re-N bond lengths of 2.133–2.239 Å, N-Re-N angles of 73.30–73.98°, and Re-Br distances of 2.627–2.642 Å, reflecting the influence of ligand substituents. ATR-IR spectroscopy demonstrated that electron-withdrawing -COOH and -COOCH<inf>3</inf> groups caused slight blue shifts in ν(CO), whereas -NH<inf>2</inf> induced red shifts, consistent with modulation of metal-to-ligand backbonding. UV–Vis studies showed that acetone preserved complex stability, whereas acetonitrile caused significant destabilization. Variable-temperature ¹H/¹³C NMR confirmed stable bidentate coordination in solution. DFT calculations revealed that electron-withdrawing groups lowered LUMO energies, while electron-donating group raised HOMO levels, tuning charge-transfer behavior. ESI-MS and elemental analyses confirmed molecular purity. These results indicate that functionalization of ligands enables precise control over rhenium(I) structural and electronic properties.