Eugenol-Derived Bipyridine Ligand Scaffolds: Design, Metal Coordination, and Catalytic Applications in Cyclic...

This work reports the synthesis of bis-(4-allyl-2-methoxyphenyl) [2,2'-bipyridine]-5,5'-dicarboxylate (bpyEu) (1) derived from the conjugation of 2,2'-5,5'-dicarboxylic acid with the natural product eugenol. This hybrid bis-eugenol ligand integrates π-conjugated bipyridine with a phenolic ether moiety. After successful multigram synthesis of this bipyridine ligand 1, its metalation was explored by reacting with M-(CO)₅X, where M = Mn, Re and X = Br, Cl, resulting in the formation of M-(bpyEu)-(CO)₃X, where M = Mn, X = Br (2); M = Re, X = Cl (3). The hybrid ligand (1) and its transition metal complexes 2 and 3 were thoroughly characterized by different spectroscopic techniques, such as NMR and Fourier transform infrared (FTIR) spectroscopy, which confirmed their formation, and their molecular structures were established using single-crystal XRD in the solid state and mass spectrometry in solution. The electrochemical behaviors of 2 and 3 were further investigated by cyclic voltammetry. Compound 2 exhibited reversible reductions at -0.828 V and -1.493 V with an irreversible oxidation at +1.44 V, whereas complex 3 displayed a reversible reduction at -0.66 V and -1.08 V along with an irreversible metal-centered oxidation at +1.69 V. Further, we used bis-eugenol-based ligand 1 as an organocatalyst for the cycloaddition of CO₂ with epoxides, which showed excellent selectivity for cyclic carbonate synthesis without the need for a cocatalyst. The catalytic activities of complexes 2 and 3 were also evaluated for cyclic carbonate synthesis, which revealed that these metal complexes lacked the catalytic ability for this transformation.